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Hubbe, M. A., Szlek, D. B., and Vera, R. E. (2022). "Detergency mechanisms and cellulosic surfaces: A review," BioResources 17(4), Page numbers to be added.

Abstract

The release of soils and impurities from cellulosic surfaces plays a critical role in such processes as the laundering of clothes and the deinking of wastepaper pulps. This article reviews publications that provide evidence about factors that affect such release and the mechanisms by which such factors operate. In general, cellulosic substrates provide advantages for the release of contaminants due to their hydrophilic nature and due to their permeability, allowing the transport of surfactants to contact interfaces with dirt. However, the same permeability of cellulosic material also provides opportunities for contaminants to work themselves into internal crevices and pores, from which they are difficult to remove. The article also reviews aspects of theory related to detergency and how those theories relate to the laundering, deinking, and purifying of substrates based on cellulose and related plant materials. Cellulose and some of its derivatives also can play a role in detergent formulation, especially as builders or as finishes placed on textile surfaces, which sometimes aid in the release of dirt.


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Detergency Mechanisms and Cellulosic Surfaces: A Review

Martin A. Hubbe,a Dorota B. Szlek,a,b and Ramon E. Vera a

The release of soils and impurities from cellulosic surfaces plays a critical role in such processes as the laundering of clothes and the deinking of wastepaper pulps. This article reviews publications that provide evidence about factors that affect such release and the mechanisms by which such factors operate. In general, cellulosic substrates provide advantages for the release of contaminants due to their hydrophilic nature and due to their permeability, allowing the transport of surfactants to contact interfaces with dirt. However, the same permeability of cellulosic material also provides opportunities for contaminants to work themselves into internal crevices and pores, from which they are difficult to remove. The article also reviews aspects of theory related to detergency and how those theories relate to the laundering, deinking, and purifying of substrates based on cellulose and related plant materials. Cellulose and some of its derivatives also can play a role in detergent formulation, especially as builders or as finishes placed on textile surfaces, which sometimes aid in the release of dirt.

DOI: 10.15376/biores.17.4.Hubbe

 Keywords:  Wettability; Washing; Surfactants; Hydrodynamic forces; Dirt particles; Desorption; Emulsification; Roll-up

 Contact information: a: Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Campus Box 8005, Raleigh, NC, 27695-8005, USA; b: Currently at Department of Human Centered Design, College of Human Ecology, Cornell University, T57, Human Ecology Building, Cornell University, 37 Forest Home Dr., Ithaca, NY 14853

 

INTRODUCTION

The premise of this article is that various characteristic properties of cellulosic materials, such as cotton, rayon, and cellulosic pulp fibers, will have notable effects on the needed formulation and the efficiency of detergent systems and process conditions leading to their efficient cleaning. A detergent can be defined as a formulation that includes a surface-active compound, together with other ingredients, to promote the release of dirt and other contaminants. The main focus of this article is aqueous media. Though laundering has attracted the most research related to detergency, such studies are also relevant for the de-inking of wastepaper, the washing of freshly pulped cellulosic fibers, and other potential technologies aiming to purify cellulose-based materials.

Laundering is big business. It is an unusual business with respect to where most of the automated processing occurs – right within the households of modern citizens. It has been estimated that approximately one hundred billion dollars of detergents will be used per year in the world by 2028 (GlobeNewswire 2022), of which about 44 billion dollars of it will be powder-type detergents. Given the immense scale of the usage of detergents, it is important to focus on such issues as the efficient action of detergents, factors affecting the release of dirt, and efforts to decrease environmental impacts. For example, it has been estimated that the energy required for a typical laundering cycle can be reduced by a factor of 2.5 by decreasing the washing temperature from 50 to 20 °C (Schmitz and Stamminger 2014). The development, over recent decades, of systems capable of washing clothes at lower temperatures can be counted as a great achievement, both in theoretical and practical terms (Phaodee et al. 2019). Despite such progress, there has been a continuing need for better understanding of the mechanism underlying detergency and for more clarity of the factors affecting the release of dirt. In this article, the work “dirt” will be used to denote a broad range of dirty substances, including both solid-like and liquid-like matter.

Various aspects of detergency have been covered in earlier review articles. Table 1 lists such reviews, calling attention to their primary focus. The existence of these review articles can help to justify the focus of the present article on detergency phenomena related to cellulose-based substrates.

Table 1. Published Review Articles Dealing with Detergency

To provide context for such a discussion, focusing on cellulose-based substrates, certain background is provided in the next section. Thus, some essential information about the following topics will be reviewed, with reference to sources with more details: cellulosic surface, surfactants, buffers and salinity, builders, enzymes, bleaches, bio-based detergent systems, characteristics of the dirt often present on cellulose-based surfaces, and typical laundering techniques.

TECHNICAL BACKGROUND

Definitions

Table A (in the Appendix) provides a list of terms used in the field of detergency. Readers who are new to this field might choose to study the list first, before continuing. Of particular interest are some terms related to mechanistic interpretations of detergency. The term “roll-up” merits some priority attention, since it pertains to a mechanism that is believed to be important for hydrophilic surfaces, such as cellulosic surfaces (Dillan et al. 1979; Miller and Raney 1993). One can envision a monolayer of surface-active agent rolling up and separating from the substrate that is being cleaned, thus causing oily or particulate dirt to be released. A contrasting term, snap-off, refers to a mechanism that more often has been applied to explain the release of oily dirt from relatively hydrophobic surfaces (Miller and Raney 1993). Entries in Table A are listed in alphabetical order.

Cellulosic Surfaces

The chemistry and physical structure of cellulose and its fibers have been described in detail elsewhere (Fengel and Wegener 1984), so only relevant highlights will be provided in this section. Briefly stated, cellulose can be escribed as a linear polymer comprised of glucose units. The regularity, simplicity, and the detailed structure of the cellulose chain favor a high degree of crystallinity in the natural polymer (e.g. about 65 to 85%), regardless of its plant source.

Fig. 1. Schematic depiction of the intermittent crystalline nature of cellulose. The fine dotted lines (in black) indicate that organized hydrogen bonds are present within and between the cellulose chains

Cellulose has three hydrophilic –OH groups per repeat unit, when considering anhydroglucose as the repeat unit (French 2017); however most of those –OH groups will be involved in highly regular intra- and inter-chain hydrogen bonding, which contributes to cellulose’s non-solubility in most solvents and its intermediate affinity with water. Figure 1 emphasizes the intermittent crystallinity of cellulose (Nishiyama et al. 2003).

When it is obtained from trees, the cellulose is accompanied by hemicellulose and lignin (Fengel and Wegener 1984). The hemicellulose consists of relatively short copolymer chains comprising at least two types of either five-carbon (pentose) or six-carbon (hexose) groups connected by glycosidic bonds. The irregular nature of hemicellulose provide it with an amorphous (non-crystalline) nature. The lignin can be described as a random, moderately cross-linked copolymer comprised of phenol-propane units, giving it a relatively rigid and hydrophobic character. Kraft pulping, a treatment of wood chips with concentrated NaOH and Na2S under pressure, is intended to remove some or most of the lignin (Fardim and Tikka 2011). Cotton, which is one of the main cellulosic materials used in textiles, is almost completely comprised of cellulose; relatively small amounts of wax on raw cotton fibers are often removed in the course of its processing (Easson et al. 2018). Though the content of wax in cotton is minor, its presence near the surface of the fibers means that it can have a disproportionate effect on such processes as laundering. For instance, Ginn et al. (1961) found that the adsorption of surfactant was primarily determined by the wax content of the cotton. The waxy materials, if still present in cotton cloth, also can be expected to favor the adhesion of oily dirt. Figure 2 shows the structure of waxy material that has been found on the surfaces of raw cotton fibers (Schmutz et al. 1994).

Fig. 2. Reported structure of a waxy compound found on cotton fibers

Likewise, regenerated cellulose fibers, such as rayon (viscose) and lyocell are essentially pure cellulose, though the chemistry may have been modified by chemical derivatization or surface treatment (Sayyed et al. 2019).

In contrast to some other types of fibers used in textile products, cellulose-based fibers, during laundering, can be envisioned as being a water-infused molecular structure. In addition to the water present between the fibers and in the fiber lumens, the swellability of the cellulose will bring water into the mesopores within the cell walls. Water also can be expected to diffuse into and to saturate the non-crystalline regions of the cellulose as well as any hemicellulose domains. In principle, the water that is able to penetrate into these regions also can allow access to alkalinity, some surfactant molecules, and possibly some enzymes, depending on accessibility. Thus, the characteristics of the cellulosic materials offer potential pathways for detergent components to reach interfaces between dirt and substrate, thus being able to influence the washing process.

Easier release from cellulose-based substrates

Several studies have reported easier release of soils from cellulose-based substrates in comparison to various synthetic polymers used in textiles. For example, Kissa (1981) reported much faster and more complete release of soil from cotton in comparison to either nylon of polyester fabrics, when subjected to the same detergent conditions and mild agitation. Likewise, Gotoh (2005) reported that removal efficiency of oily contaminants was higher for cellulose acetate in comparison with nylon and polyethylene in the presence of surfactant and variable amounts of ethanol. Tanthakit et al. (2008) stated that detergency was observed to be more effective with increasing hydrophilic character of the tested fabric. Thus, cotton items were found to be the cleanest after laundering, and polyester was the least clean. Reasons to help explain such differences will be considered in this article.

Ion exchange capacity

An inherent characteristic of most cellulosic materials is an ability to bind with positively charged ions, e.g. divalent metal ions such as Ca2+ (Lambert 1950; O’Connell et al. 2008; Hubbe et al. 2011, 2012). It is well known that increased water hardness, i.e. increased concentration of Ca2+ and Mg2+ in solution, tends to make it more difficult to wash clothes (Tanthakit et al. 2009; Gotoh and Mei 2017). The fact that cellulosic materials are able to bind hardness ions can be viewed in various perspectives. On the one hand, addition of purified cellulosic material to water will tend to reduce the hardness of the water, due to adsorption. On the other hand, cellulose-based articles, after their usage, are likely to remain partly saturated with hardness ions, which have potential to function as binders for various contaminants. As indicated in Fig. 3, the term “builder” often refers to a component of a detergent formulation that has the ability to bind hardness ions.

Fig. 3. Schematic depiction of a builder to be used in a detergent formulation and its ability to sequester hardness ions by an ion exchange process

Cotton

Cotton is a natural form cellulose that can be readily spun into yarns that can be woven into cloth for textile products. Like wood fibers, cotton fibers have a multilayer structure (Li and Hardin 1998). Cotton fibers have only a relatively small lumen space in the fiber’s center, leading to high overall density. In principle, such a dense construction is expected to provide stiffer fibers that are less conformable in their wet state, compared for instance to a kraft pulp fiber from wood. Minor non-cellulose components of cotton include wax, pectic compounds, and proteins (Li and Hardin 1998).

The properties of cotton can be markedly changed by a process called mercerization (Grancarić et al. 1997; Obendorf 2004). This process involves treatment with moderately concentrated NaOH, which greatly swells the material. For instance, Stana-Kleinschek et al. (1990) employed a 24% solution of NaOH at 15 °C for 60 s to mercerize cotton. By temporarily dissolving the cellulose macromolecules, mercerization results in a transformation from the cellulose I to the cellulose II crystal form, usually with a lower degree of crystallinity. This transformation is illustrated in Fig. 4.

Fig. 4. Reversal of the direction of 50% of the cellulose chains during transformation from cellulose I to cellulose II in the course of mercerization

Another effect of the mercerization of cotton is that the lumen becomes less prominent, and the fiber surface becomes smoother (Stana-Kleinschek et al. 1999; Obendorf 2004). Grancarić et al. (1997) reported that mercerization increased the magnitude of the negative zeta potential of cotton threads, also leading to a higher ability to adsorb positively charged substances. Stana-Kleinschek et al. (1999) reported that mercerized cellulose tended to have a greater absorption of moisture and to be more accessible to dye molecules.

Regenerated cellulose

The popular textile fibers rayon (viscose) and lyocell are examples of regenerated cellulose. The term means that the cellulose had been completely dissolved, then precipitated out of solution in the process of returning to the solid phase. The processes for the two types of regenerated cellulose production are shown in Fig. 5. Regeneration processes result in some key differences relative to the starting material. Most importantly, the fibers can be produced as continuous filaments, and the surfaces are often quite smooth. Gotoh et al. (2015) noted, however, that rayon fibers can be sufficiently porous or rough such that particulate dirt can become entrapped in crevices. Penford et al. (2007) observed that the ability of regenerated cellulose to absorb cationic surfactant was much higher than that of the default cellulose surface prepared by monolayer deposition onto a silica substrate.

Fig. 5. Processes by which ordinary cellulose is formed into a solution, drawn into filament form, then returned to cellulose, but as a different crystal form and morphology; A: Viscose (xanthate rayon) process; B: Lyocell (n-morpholine oxide) process

Finishes on yarns and fabrics

The surface nature of textile fibers, regardless of their main composition, can be markedly changed by a process called finishing or sizing. Such processes usually involve passing a yarn or filament through a solution or suspension, allowing it to pick up some material that becomes dried onto its surface. For example, Higgins et al. (2003) studied various changes resulting from the application of a wrinkle-resistant finish onto plain woven cotton fabrics. Rhee et al. (1993a,b) tested the effects of finishing treatments aimed at stain-resistance and antistatic properties. In principle, a surface finish on a textile fiber has the potential to change its properties from hydrophobic to hydrophilic, thus rendering the fabric easier to launder. However, there can be ongoing concerns about the durability of such surface layers and their likely removal in the course of repeated laundering.

Surfactants

The key component of most common detergent systems is a surface-active compound, i.e. a surfactant. A surfactant generally can be described as having a hydrophobic “tail” group connected to a hydrophilic “head” group (Salager et al. 2017). The amphiphilic nature of such compounds helps them to draw various hydrophobic direct materials into an aqueous phase, often in the form of an emulsion or a stabilized suspended solid (Salager et al. 2022). Widely used surfactants in detergent formulations include anionic surfactants, nonionic surfactants, and extended surfactants. In addition, binary mixtures have been widely used, and research has been carried out with cationic surfactants (Salager et al. 2019).

Anionic surfactants

Surfactants having a negatively charged headgroup, i.e. the anionic surfactants, comprise the largest component, on average, of laundry detergents used in the US (Matheson 1996). Figure 6 shows two widely used examples. The headgroup is usually sulfonate or sulfate, both of which become fully dissociated over the whole pH range at which laundering takes place (Scheibel 2004; Ponnusamy et al. 2008; Budhathoki et al. 2016; Phaodee et al. 2020). Schott (1967) may have been the first to quantify the adsorption of surfactants (including sodium dodecyl sulfate) onto cellulose. The tail groups often include alkyl chains, such as the dodecyl group just mentioned. However, technologists have obtained superior detergency results when using products that include an aromatic group as well, i.e. dodecylbenzene sulfonate (Wingrave 1984; Cohen et al. 1992; Paria et al. 2004, 2005a; Samanta et al. 2004; Takahashi et al. 2007; Kalak and Cierpiszewski 2015; Gotoh et al. 2016; Gotoh and Mei 2017; Ou et al. 2021). Because most cellulosic surfaces have a weak negative charge (Hubbe et al. 2012), it is reasonable to assume that the anionic surfactants orient themselves with the tails facing outwards or lying down on the substrate, with the detailed orientation depending on the concentration in solution (Simoncić and Roman 2007).

Fig. 6. Widely used anionic surfactant components for detergent formulations

Nonionic surfactants

Surfactants having an ethylene oxide chain as the headgroup constitute the major group of nonionic surfactants used in detergent formulations. The ratio of oxygen to carbon in the –CH2CH2O– repeating unit is high enough to achieve complete solubility of ethylene oxide in water. Furthermore, ethylene oxide segments can be attached to a variety of hydrophobic groups, including propylene oxide, to make a range of useful surfactants. However, as noted by Miller and Raney (1993), the ethylene oxide groups are less hydrophilic than the anionic groups just considered. The fact that those groups do not interact with hardness ions such as Ca2+ can be regarded as an inherent simplification, relative to the use of anionic surfactants in detergents. The nonylphenol ethoxylates have shown particular promise as components of detergent formulations (Dillan et al. 1980). Another class of nonionic surfactants used in detergents goes by the name of “ethoxylated alcohols” (Scheuing and Hsieh 1988). Polyoxyethylene sorbitol esters, though they are widely used in industry, are less often used in laundry detergent formulations (Seo et al. 2011).

The fact that some surfactants work better than others, often depending on temperature and ionic conditions, is important in terms of formulation and when one is attempting to determine the mechanisms of action. Figure 7 shows two examples of nonionic surfactants (Matheson 1996). However, there is a serious limitation on using ethoxylated nonionics in household formulations due to 1,4-dioxane formation during the ethoxylation reaction. Accordingly, biobased alternatives such as alkylpolyglycosides are being used in several cleaning formulations in Europe and the US (Ortiz et al. 2022).

Fig. 7. Examples of nonionic surfactants

Extended surfactants

An extended surfactant can be briefly described as a surfactant that has a few propylene oxide groups or ethylene oxide groups (having intermediate polarity) interposed between the hydrophilic headgroup and the hydrophobic tail (Salager et al. 1979; Matheson 1996; Do et al. 2015; Salager et al. 2019). Compared to conventional surfactants, extended surfactants have been found to be more effective in the formation of microemulsions. Moreover, these microemulsions have been found to have much higher capacity to solubilize oil and to promote much lower interfacial tension (Dillan et al. 1979; Miñana-Perez et al. 1995; Kaewpukpa et al. 2008; Witthayapanyanon et al. 2008; Phan et al. 2010; Acosta et al. 2012; Hammond and Acosta 2012; Budhathoki et al. 2016; Attaphong and Sabatini 2017; Chanwattanakit and Chavadej 2018; Salager et al. 2019). This ability to form very small phase entities implies that the extended surfactants are able to achieve very low interfacial tensions. It appears that the groups of intermediate affinity (i.e. the ethylene-oxide segments), placed in the middle of this class of surfactant, increase the effectiveness of the extended surfactants in detergency. Two examples of extended surfactants (Matheson 1996; Witthayapanyanon et al. 2008) are shown in Fig. 8. Due to their limited biodegradability, however, such extended surfactants are not regarded as the most suitable for laundry detergents (Salager et al. 2022).

Fig. 8. Schematic examples of extended surfactants

Cationic surfactants

Due to their ability to interact strongly with the negatively charged carboxylate groups present on cellulosic surfaces, cationic surfactants have attracted a lot of research attention for such systems. The orientation often has been described as “head facing the substrate” (Alila et al. 2005; Paria et al. 2005b). Evidence in favor of such an orientation includes findings of increased hydrophobic character, especially at low to moderate dosages of surfactant (Syverud et al. 2011). Further evidence consists of the release of counter-ions (e.g. Cl) into the bulk solution upon adsorption of the cationic surfactant onto cellulose surfaces (Alila et al. 2005, 2007). At higher concentrations of cationic surfactant, double-layers can be expected; the second layer adsorbs with its tails facing or intertwining with the first layer, and the headgroups are left facing the aqueous solution (Muller et al. 1998). Patches of such double-layers of adsorbed cationic surfactant are referred to as admicelles (Alila et al. 2005). Notably, such double-layer structures will present a hydrophilic surface towards the aqueous phase. Cationic surfactants are used extensively as fabric softeners, which can be applied during the drying process (Pucha 1984).

Hydrophobic interactions

A tendency of the tail groups of surfactants to associate into tight films has been attributed to a hydrophobic effect (Tanford 1980; Hubbe et al. 2020). The driving force for such association is the relatively high energy of hydrogen bond interactions in aqueous systems. By self-organizing the hydrophobic entities such that they are pressed together, the system maximizes the amount of hydrogen bonds that can be formed. Figure 9 depicts some of the common ways in which surfactant molecules (using a linear alkyl sulfonate, etc., as an example) in various forms that decrease the extent of blockage of hydrogen bonding within the system as a whole. Biswas and Chattoraj (1997) and Penford et al. (2007) found evidence of self-association between the hydrophobic tails of a cationic surfactant (hexadecyl trimethyl ammonium bromide), such that the molecules were present as hemimicelles, i.e. condensed patches of the cationic surfactants adsorbed as single monolayers. Woods et al. (2011) found related evidence in a mixed surfactant system with a combination of the same cationic surfactant as used in the other cited studies, but in combination with a nonionic surfactant. It will be assumed in this article that such interactions play an important role in detergency.

Fig. 9. Idealized depiction of ways of association among surfactant molecules in aqueous systems

Surfactant adsorption

It is proposed that “elbowing” is a useful analogy when thinking about the likely action of surfactants as they work to release dirt from surfaces during laundering. The effectiveness of the additive may depend on its ability to push things out of its way and make space for itself, thereby opening up a space between dirt and a textile fiber. It follows that the affinity of the surfactant for the interfacial region will play a key role. The energy associated with adsorption can be summarized by focusing on three categories of interactions – with the solid surfaces, with the water, and the self-association – as just considered above.

The tendency of surfactant molecules to form monolayers at surfaces is closely related to their tendency to form into micelles. There can be a close relationship between the critical micelle concentration (CMC) and the concentration at which a maximum adsorption at surfaces has been observed (Ginn et al. 1961; Biswas and Chattoraj 1997). The cited authors found many cases in which specific affinity of the surfactant for substrate led to a critical concentration for maximum adsorption that lay below the CMC. Paria et al. (2005a) proposed that the maximum is associated with the formation of hemimicelles, which can be regarded as patches of self-associated surfactant molecules oriented tightly at the surface. It has been proposed that the removal of dirt from a surface can be similar in principle to the formation of an emulsion (Kalak and Cierpiszewski 2015). Thus, as a first approximation, when the conditions for emulsification are better, one can expect that removal or dirt will be more effective. A later section of this article will consider Winsor conditions at the optimization of detergency.

Cloud point of a surfactant

To carry the elbowing analogy a step further, one can expect that more persistent and pushy commuters will be the most successful in getting into and out of packed subway cars. The situation can be compared to that of a surfactant that wants badly to get out of the bulk phase of solution. The cloud point refers to surfactant-oil-water systems that are formulated with nonionic surfactants. Technologists have long been aware that surfactant systems are often the most effective when they are somewhat higher than the cloud point, which represents the temperature above which they precipitate out of solution (Raney et al. 1987; Miller and Raney 1993).

Fig. 10. General temperature-concentration plot for phases in aqueous surfactant mixtures

For instance, Borchart (1994) observed greatest effectiveness of deinking operations at temperatures somewhat higher than the cloud point. Kalak and Cierpiszewski (2015) estimated that the best detergency was obtained when only about 87 to 92% of the surfactant could be dissolved in the studies systems. Figure 10, based on a version by Nakama (2017), shows the location of the cloud point temperatures on an idealized plot of temperature vs. surfactant concentration.

Foam potential

The generation of foam is often taken as a clue by consumers as to the effectiveness of detergent action. Indeed, the ability of surfactants to lower interfacial tensions can be a factor in the stabilization of foam bubbles (Pugh 1996). Another factor important for foam stabilization in aqueous systems is the presence of water-soluble polymers, which can contribute to increased elasticity of bubble walls. Karthick et al. (2018) found that different classes of surfactants used in detergent formulations had different foaming tendencies. Raney and Miller (1987) found that the calcium salt of oleic acid functioned as a defoaming agent in such systems. The calcium oleate appeared to disrupt the surfactant monolayer structure, thus causing the bubbles to be less persistent.

Surfactant toxicity

In view of the need for more eco-friendly laundering systems, the toxicity of surfactant compounds can be important. Warne and Schifko (1999) found large differences in toxicity for different surfactant types. Scheibel (2004) reported the evolution of anionic surfactant technology over the passage of time, such that it currently tends to employ more eco-friendly options than in the past. A recent review by Ortiz et al. (2021) describes a trend toward the usage of bio-based surfactants, which in many cases have lower toxicity. In addition, as described in the cited article, new regulations related to dioxane are motivating growth in the marked of narrow-range ethoxylates having ultra-low dioxane levels.

Buffers and Salinity

It is tempting to underestimate the importance of aqueous ionic conditions, including the pH and buffering capacity, when thinking about the formulation of a detergent system. According to Matheson (1996), almost the opposite is true in the formulation of some industrial detergent systems. Such systems often place main reliance on the use of higher pH values, higher temperatures, and the use of bleach. The usage of surfactants in some industrial washing systems is minimal. Because of the relatively uniform operations of household washing machines, one may get a false impression that pH, buffering capacity, and salinity are not potent tools for achieving detergent effects.

By mass, sodium carbonate is often the first or second most prominent ingredient in a common laundry detergent formulation, often exceeding the amount of surfactant (Schwuger and Smulders 1987; Matheson 1996). The cited sources mention a combined range of about 5 to 30% of sodium carbonate in traditional and so-called high-density detergent formulations. Sodium carbonate can be described as a safe and relatively inexpensive agent for moderately increasing and buffering the pH in an alkali range. The high pH tends to convert insoluble calcium salts of fatty acids – a common form of soiling – into water-soluble sodium salts, which isa form of saponification (Gotoh 2005). The general reaction is outlined in Fig. 11. As shown, the alkalinity released upon dissolution of the sodium carbonate in water can convert insoluble calcium compounds into emulsifiable sodium compounds. Also shown is a sodium docecylsulfate (SDS) molecule, serving as an example of an anionic surfactant that could be used with the sodium carbonate in a detergent formulation.

Burkinshaw and Anthoulias (1997) found that a concentration of 5 g/L or sodium carbonate was sufficient to remove excess reactive dye after coloring cellulosic fibers, even in the absence of a surfactant. Follow-up work by Burkinshaw and Katsarelias (1997) showed that the addition of sodium carbonate rendered five different surfactants more effective in the removal of excess reactive dyes. Gotoh (2010) observed that increasing the pH aided the release of various soil types from both cotton and polyester. Dillan et al. (1979) attributed the effectiveness of sodium carbonate as a detergent component to its ability to provide soil particles with negative surface charges, due presumably to the dissociation of carboxylic acid groups on the surfaces.

Fig. 11. Suggested role of sodium carbonate in converting insoluble calcium salts of fatty acids into their more soluble sodium forms

Salts, especially sodium sulfate, also have been shown to boost detergency in many cases, but the mechanism appears to be much different from that of sodium carbonate. As will be described in more detail later, the main role of sodium sulfate may be to fine-tune the effective size of anionic head-groups of anionic surfactants (Hammond and Acosta 2012). Powe (1963) found that inorganic salt addition tend to decrease the value of critical micelle concentration (CMC) of anionic surfactants, which is considered to be beneficial. Whatever the reason, an optimum level of the salt addition often has been found to coincide with the lowest interfacial tension and the best detergency effects related to microemulsions (Aveyard et al. 1985; Tongcumpou et al. 2003a,b; Acosta et al. 2012; Phaodee et al. 2020). Azemar et al. (1993) found that the addition of salt made it possible to increase the effectiveness of detergents at lower temperatures. Additionally, Vera et al. (2020) found that the nature of the salt has important effects on the surfactant solubility, interfacial tension, and rheological properties. This may significantly affect detergency performance of anionic surfactants.

Water hardness, i.e. the presence of calcium and magnesium ions, is widely reported to have unfavorable effects on detergent performance (Tanthakit et al. 2009; Gotoh et al. 2016, 2017; Vera et al. 2020). As already mentioned, calcium salts of various fatty acids and other organic acids tend to be insoluble (Harwot and van de Ven 1997). In addition, the divalent ions can adversely affect the anionic surfactants, tending to make them precipitate (Gotoh et al. 2016). These issues call for the use of additives called builders, which are described next.

Builders

Water softening

A primary function of various builder components in detergent formulations is to mitigate the effects of water hardness. Several contrasting types of builders have been found to be effective (Schwuger and Smulders 1987). These include phosphates, acrylates, carboxymethylcellulose (CMC), and zeolites. The listed substances each are able to bind the calcium and magnesium ions in some way, making their concentration in solution appear to be lower.

Phosphates

Various phosphate compounds, especially sodium tripolyphosphate (STPP), are known to be effective for mitigating the effects of hardness ions (Powe 1963; Schwuger and Smulders 1987; Cohen et al. 1992; Leal et al. 1996; Phillips et al. 2001; Tanthakit et al. 2009). Leal et al. (1996) described STPP as having the smallest adverse environmental impact of various phosphate types considered, based on the concentrations that were needed. However, there has been a switch from phosphates to alternative builders due to environmental concerns (Matheson 1996). When wastewater that contains phosphates is discharged to waterways, it supports eutrophication (Awual 2019). Because phosphate is often a limiting factor in the growth of microbes in water, its addition to rivers, lakes, and the ocean should be minimized. Part A of Figure 12 depicts the main interaction whereby STPP binds calcium ions.

Fig. 12. Sequestration of hardness ions by (A) sodium tripolyphosphate (STPP) and sodium poly-acrylate. The dotted lines represent moderately stable chemical complexation.

Acrylates

Since the carboxylate ion is well known to bind divalent metal ions, especially when there are several of them close together in a compound, it is reasonable to consider carboxylated polymers as builders. Sodium acrylate and its copolymers have become widely used for that purpose (Schaffer and Woodhams 1977; Komaki et al. 2002; Milojević et al. 2013). The sequestration of hardness ions by sodium polyacrylate is depicted in Part B of Fig. 12. Milojević et al. (2013) found that the performance of such builders increased with increasing molecular mass up to 70,000 g/mole, after which there was no benefit of further increases in mass. Komaki et al. (2002) studied the ability of a certain type of polyacrylate to form complexes with fatty acid, which can be regarded as being unfavorable for laundering. According to Schaffer and Woodhams (1977), the acrylate polymers can have a superior binding ability for hardness ions in comparison to phosphates, but the acrylate polymers are not biodegradable.

Carboxymethylcellulose (CMC)

As a means to achieve the benefits of a polymer-based carboxylated builder, while attempting to minimize environmental problems, it makes sense to consider water-soluble polymers derived from cellulose. CMC is prepared from chemical-grade cellulose material by reaction of chloroacetic acid under strongly alkaline conditions with the formation of ether bonds (Rahman et al. 2021). When the degree of substitution (DS) becomes higher than about 0.4 per anhydroglucose unit, the material becomes increasingly soluble in water. Likewise, its ability to bind hardness ions increases with increasing DS. The biodegradability of CMC is evidenced by the fact that CMC is widely used in assays of the potency of cellulose-degrading enzymes (Chan et al. 2019). The effectiveness of CMC as a builder in detergent formulations has been reported (Vaughn and Smith 1948; Agarwal et al. 2012).

Microcrystalline cellulose (MCC)

Agarwal et al. (2012) tried something unusual. As an alternative to reacting microcrystalline cellulose (MCC) with chloroacetic acid under strongly alkaline conditions (to make CMC, as just described), they simply used the MCC itself in detergent formulations. Not only was the MCC effective, but it was judged to have performed better than then CMC used as a baseline. The MCC served as a thickener in a liquid detergent formulation. On the other hand, there is no reason to expect that ordinary MCC would affect the availability of hardness ions in aqueous solution, which is often regarded as a main job of builders. Some commercially available MCC products are coated with an adsorbed layer of CMC, which contributes to stability of their aqueous suspensions.

Zeolites

Zeolites can be described as microporous alumino-silicate crystalline materials that have huge internal capacity to bind various ions and small compounds. The complete regularity of their pore dimensions offer zeolites the potential to have very specific adsorption capabilities relative to the size of the adsorbate. A zeolite called Zeolite A has been found to be especially effective for the removal of hardness ions from solutions as part of detergent formulations. Figure 13 shows the crystal structure of Zeolite A, which contains two types of cage structure (Tanney 2017). Notably, the α-cage (supercage) structure appears well suited for accommodating the calcium ion.

Zeolites have been found to perform similarly to STPP (a phosphate compound) as a builder (Cohen et al. 1992; Matheson 1996; Phillips et al. 2001). In addition, zeolites do not contribute to the eutrophication of waterways after the discharging of treated wastewaters, as phosphates do (Matheson 1996). In fact, some positive effects of zeolites on health have been reported (Grancarić et al. 2009). Because of the solid particulate nature of zeolites, it is perhaps unsurprising that some of the solid material can remain with the textile products after laundering (Hurem et al. 1992). However, effects of the residual zeolite on the appearance and other properties of the laundered items were reported as being minor. An additional benefit of zeolites as that they may adsorb colorants that have been released from textiles (Kokol et al. 2018).

Fig. 13. Cage structure of Zeolite A (redrawn from original by Tanney 2017)

EDTA

Certain monomeric chelating agents, notably ethylenediaminetetraacetic acid (EDTA) and related chemicals, are known to be extremely effective for the binding of hardness ions and a range of other divalent metal ions. Such products can be effective as builders in detergent formulations (Eken-Saracoglu and Culfaz 1999; Tanthakit et al. 2009). However, chelating agents are seldom mentioned in literature related to detergency. Possible reasons may include the known lack of biodegradability of chelating agents (Pinto et al. 2014) and concerns about their environmental effects due to exceptionally strong binding of metal ions.

Fabric Softeners

The term “softening” can have a second meaning in addition to the binding of calcium and magnesium ions, as discussed above. The other usage refers to products that confer a softer feel to laundered items after they have been dried. Notably, it has been reported that such effects can be achieved by addition of bentonite to the detergent formulation (Carrion-Fite 2014). Presumably the effect is due to interruption of the bonding between filaments in the yarn as the material is being dried, thus favoring a fluffier, less dense structure that feels softer.

The conventional way to soften fabrics is to add certain cationic surfactants having optimized structures to the laundered items before the drying process (Pucha 1984; Matheson 1996; Levinson 1999; Murphy 2015). Some such agents are added during the rinsing stage of laundering (Levinson 1999; Murphy 2015), and often the material is added during the drying process. Quaternary ammonium compounds involving ester bonds, i.e. “ester quats,” are now widely used, since they are readily biodegradable (Murphy 2015). Such fabric softeners, as shown in Part A of Fig. 14, have a family resemblance to the debonding agents that are sometimes used in papermaking when the goal is to achieve a bulkier, softer structure (Touchette and Jenness 1960; Conte and Bender 1992; Poffenberger et al. 2000). As was noted earlier, many cationic surfactants tend to adsorb as dense patches with their cationic groups facing a cellulose-based surface and their condensed monolayers of hydrophobic tails groups effectively preventing the development of hydrogen bonding between adjacent fibers or filaments of the fabric. This is shown, schematically, in Part B of Fig. 14. A reduction in inter-fiber hydrogen bonding means that the fibers within a cellulose-based textile material, after it has been dried, will slide easily relative to each other, thus decreasing the stiffness of the material. Tests of similar technology related to papermaking suggest that debonding agents will contribute to a softer feel of the dried item (Pawlak et al. 2022).

Fig. 14. Example of a fabric softener (ester quat type) (A) and its possible orientation on cellulose-based filaments after drying (B), leading to the inhibition of intra-fiber hydrogen bond formation

Enzymes

To supplement efforts aimed at the detachment of dirt, it can make sense also to break down its chemical structure. Such a concept might be invoked to explain, for instance, the effect of alkaline conditions, which are able to convert fatty acids and fatty esters to dispersible sodium salts (McBain et al. 1929; Mercantili et al. 2014). Enzymes clearly can play a similar role, since they are able to catalyze the hydrolytic breakdown of a wide range of substances that can be serving as the binders in various kinds of dirt. Several classes of enzymes are of particular interest for detergent formulations, as considered below.

Amongst the main enzymes used in detergents, proteases and lipases belong to the pancreatic enzyme family; cellulases and amylases are glycoside hydrolases (Olsen and Falholt 1998). Most enzymes today are produced by aerobic batch or continuous fermentation, on sterilized nutrients. They are based on such feedstocks as corn starch, sugars, or soy grits with the addition of salts. The enzymes are harvested after the filtering out (or centrifuging) of insoluble products and the produced biomass. Afterwards, enzyme in solution is concentrated through evaporation, membrane processes, and crystallization. After post-treatment, formulations are manufactured; the detergents are prepared either in powder form, as a nondusting granulated enzyme, or as liquid detergents, in liquid or encapsulated form (Olsen and Falholt 1998).

Enzymes are seen as an environmentally-friendly component in detergents not only because they are derived from renewable resources, but also because of their washing time, energy, and water consumption savings due to equivalent performance at lower temperatures, as well as the resulting contributions in “greener” wash-water effluents and lower pH in wash liquor and fabric care (Olsen and Falholt 1998).

Lipases

Fats associated with human skin are often a prominent component of the soiling of clothing (Sonesson et al. 2007; Do et al. 2015). As illustrated in Fig. 15, lipases have evolved to cleave such fats into their component fatty acids and glycerol (Sharma et al. 2001). Lipases added to detergent formulations have been shown to be effective (Fujii et al. 1986; Aaslyng et al. 1990; Sonesson et al. 2007). For example, Fujii et al. 1986) showed that lipase was effective for the removal of olive oil from cotton fabric. Aaslyng et al. (1990) showed that the lipase treatment was much more effective under alkaline pH conditions, which is consistent with the conversion of the fatty acids to their more hydrophilic carboxylate form.

Fig. 15. Role of the lipase enzyme in hydrolysis of triglyceride fats. The reagent (triglyceride fat) in the presence of lipase reacts to form glycerol and oleic acid.

Lipases work by hydrolyzing hydrophobic triglycerides into more hydrophilic mono- and diglycerides, free fatty acids, and glycerol, which are all soluble in alkaline conditions. Interestingly, the optimal activity of lipases was found to be at 20 to 40% moisture concentration on a fabric during line-drying at room temperature. This finding helped explain the delay in cleaning efficacy, whereby the effectiveness of lipase appeared to increase after several washes (Olsen and Falholt 1998).

By breaking down lipid-based stains, lipases can also act as alternatives to surfactants in detergent formulations that are trying to decrease chemical ingredients and increase biobased content (Dybdahl Hede 2020).

Amylases

Starch can be present in a variety of soil types, such as soiling due to foods. The term amylase covers a variety of enzymes, including some of them that are effective in breaking down amylopectins and starch derivatives (van der Maarel et al. 2002). As shown by Tanaka and Hoshino (1999), amylase addition to a detergent formulation can enhance the results of laundering. Such treatment can be valuable if there are food stains.

The preferred amylases in detergent use are α-amylases, as they catalyze the endo-hydrolysis of starch, with excellent performance in lower temperatures and milder chemical detergent formulations. In contrast, β-amylases catalyze exo-hydrolysis of 1,4-α-D-glycosidic linkages, which is not as efficient in terms of stain removal and is therefore not used in detergents (Aehle 2007).

In addition to working to slowly degrade starch via swelling and gelatinization, α-amylases also prevent the swollen starch from adhering to the textile surface. During the enzymatic breakdown, α-1,4-linkages in starch are hydrolyzed, resulting in a conversion to water-soluble dextrins and oligosaccharides (Olsen and Falholt 1998).

One challenge for starch stain laundering has been that the gelatinized starch can form a thin coating layer on the surface of the textile, which can contribute to the increase of particulate soil pick-up, resulting in greying of white-colored fabrics after multiple washes. This film-forming will depend on the amylose content of the particular starch and can make it more difficult to remove the starch stain and particulate soiling combination. However, amylases have been shown to contribute to whiteness and prevention of graying caused by the combination of starches and particulate soiling in laundered fabrics (Olsen and Falholt 1998).

Proteases

Proteins are often present in soiled garments due to the rubbing against human skin during usage. For instance, sebum is a protein-based soil substance that can accumulate in clothing (Murata et al. 1991). Aaslyng et al. (1990) determined the aqueous conditions (e.g. pH and temperature) needed to achieve effective results with selected proteases when used as components in detergents. It was proposed that the proteases converted the original proteins to smaller and more water-soluble fragments. Florescu et al. (2009) reported that protease improved the removal of food stains during laundering in the presence of anionic surfactants.

Proteases are the most widely used enzymes in detergents and they are known as the main enzymes to contribute to environmental savings by helping reduce washing times, temperature, and water consumption. They are usually classified based on their origin (plant, animal, or microbial), their catalytic actions (endopeptidase or exopeptidase), and type of active site. Four different protease families are recognized based on comparison of active site, catalytic residue, and 3D structures: serine, thiol, aspartic, and metalloproteases. Serine proteases are further sub-divided into chymotrypsin-like and into, what is most commonly used in detergents, subtilisin-like proteases (Olsen and Falholt 1998).

Proteases used in commercial detergents are mostly similar in structure, with variation in pH and temperature optimum, bleach sensitivity, and calcium ion demand. Protease performance is influenced by detergent pH and ionic strength, as well as by type of surfactant used, which can also have an impact on its stability in the wash. Also, while proteases need a small amount of calcium in the detergent to maintain their stability, their performance generally decreases with increasing calcium concentration; accordingly, accurate proportions are key (Aehle 2007).

One of the main challenges for proteases used in liquid detergents is their instability in aqueous environments. This challenge is currently addressed by the use of inhibitors, which help prevent activity and autodigestion, as well as potential degradation of other enzymes. Some examples of inhibitors used to stabilize proteases in liquid detergents are polyols in combination with boric acid, amino acids, and protein hydrolysate products. These are all diluted during the wash (Olsen and Falholt 1998).

On the other hand, in powder detergent formulations containing bleach, storage stability can be a potential issue, as the amino acid methionine in the protease molecule can be oxidized by the bleach, leading to the deactivation of the protease enzyme. In newer protease systems, this is addressed by the replacement of methionine in the position closest to active site, with other amino acids that are insensitive to oxidation (Aehle 2007).

Cellulases

Cellulose-degrading enzymes have been widely studied for usage in detergent formulations that are specifically intended for laundering of cotton and other cellulose-based textiles (Obendorf 2004). It is important to balance the effectiveness of such systems for detergency with concerns about excessive hydrolytic breakdown of the fibers. Sufficiently high dosages and durations of treatment will convert the material to monomeric sugars (Eriksson et al. 2002). Nevertheless, cellulases have been introduced as a component in detergent products (Aaslyng et al. 1990).

Hoshino et al. (2000) showed that cellulase by itself was able to achieve equivalent results to treatment with surfactants and builders. The cited authors stated that enzymatic hydrolysis mainly affected the amorphous regions of cellulose. It was proposed that the effectiveness of the treatment was attributable to the lodging of dirt in those same amorphous cellulose regions. In contrast, de Souza Moreira et al. (2016) and Olsen and Falholt (1998) both argued that cellulase targets the crystalline part of cellulose. According to Olsen and Falholt (1998), the cellulase molecule is often composed of up to three functionally unique domains: the core, a linker, and a cellulose-binding domain (CBD). They also mention that CBD is responsible for binding on insoluble and crystalline cellulose for hydrolysis. Similarly, de Souza Moreira et al. (2016) also highlight CBD as the necessary accessory in the breakdown of crystalline cellulose structure (Olsen and Falholt 1998; de Souza Moreira et al. 2016).

Murata et al. (1991, 1993) found that cellulase in a detergent formulation was effective for the removal of sebum from cotton clothing. Figure 16 indicates the three main classes of cellulase enzyme, which respectively cleave randomly, adjacent to the chain ends, and breakup of cellobiose or cellotriose.

Fig. 16. Sites of hydrolytic cleavage be three categories of the cellulase enzyme

In solid phase, endo-glucanases and exo-glucanases (cellobiohydrolases) carry out the primary hydrolysis of cellulose, while β-glucosidases are responsible for secondary hydrolysis in the liquid phase. Cellulases are usually a mixture of endo-glucanases, cellobiohydrolases, and β-glucosidases. Endo-glucanases and cellobiohydrolases tend to only attack the external cellulose chains, without diffusing inside the fibers, which is important when considering the potential extent of degradation of the cellulosic fiber during washing (de Souza Moreira et al. 2016). This translates into the elimination of pilling and fuzz from the surface of the cellulosic fabric during washing, which is a feature specific only to the cellulase enzyme (Olsen and Falholt 1998).

In addition to cleaning the textile through the hydrolysis of glycosidic bonds, cellulases provide the additional benefit of softening and brightening the color of the used cellulosic fabric, hence the name “color clarification cellulase” (Olsen and Falholt 1998).

From a process standpoint, it is important to keep in mind optimal conditions of cellulase use when incorporating in a detergent, due to the concern that temperatures higher than 80 °C might deactivate the enzyme (Rahman et al. 2021).

Mannanases

Although not as widely-used as pectinases, mannanases are enzymes also used to target hard-to-remove stains containing mannanolytic compounds, of such products as gravy, ice cream, ketchup, and mayonnaise, amongst many. Mannan endo-mannosidase degrades the β-1,4-mannose linkage of galactomannans, which is found in neutral pectin-containing food products, as well as in cosmetics, household products, and toiletries. Mannan, the gum polymer, is then broken down into a smaller molecule, making it more water-soluble, so it can be easily removed during laundering. Aside from being an effective stain removal agent, mannanases are also knows to help prevent redeposition of soil (Aehle 2007; David et al. 2018; Dybdahl Hede 2020).

Multi-enzyme systems

While above sections describe distinctive functions of individual enzymes, multi-enzyme systems have become more widely used for several reasons. Firstly, many detergents use multiple enzymes in order to increase the enzyme tolerance for other ingredients, such as surfactants, builders, and bleaching agents. Second, as detergents are more and more often formulated for use in colder temperatures, use of multi-enzyme systems increases their efficiency, especially with regards to starch and fat soiling. Lastly, when looking at increasing environmental savings, they allow for elimination of phosphates without much impact to detergent’s cleaning performance (Dybdahl Hede 2020).

The use of multiple enzymes in a detergent recipe is not new. Between 1967 and 1970, combining amylases, cellulases, and lipases resulted in first advances in energy savings in textile laundering (Olsen and Falholt 1998). Some examples of performance enhancement with regards to stain removal by multiple enzyme formulations includes the common use of amylases and pullulanases to achieve a more complete degradation of starch. Additionally, bacterial α-amylases remain stable in presence of proteases in both, powder and liquid formulations, during storage (Aehle 2007).

Stabilization of enzymes

Some studies have found synergistic effects between surfactants and enzymes (Traore and Buschle-Diller 1999; Iyer and Ananthanarayan 2008; Seo et al. 2011). In the case of cellulase, increased hydrolysis sometimes has been attributed to an ability of the surfactant to mitigate the unproductive binding of the enzymes to lignin, which has a more hydrophobic character than cellulose (Zeng et al. 2014). Ooshima et al. (1986) attributed faster hydrolysis of cellulose to the ability of a surfactant to hold the cellulase in the liquid phase. Lou et al. (2018) showed that nonionic surfactants were able to partly protect cellulase from deactivation in the presence of hydrodynamic shear.

Future opportunities in detergent enzymes research

Recently, a new variety of enzymes, called extremozymes, has appeared as a potential enhancement to current enzyme use in laundry detergents. Extremozymes are essentially optimized enzymes, capable of remaining active and stable under extreme conditions. They are mainly engineered for use in cold water temperatures and in alkaline pH, and are said to be on track to eventually replace the mesophilic enzymes due to their higher efficiency. Currently, some examples of extremozymes available for detergents are cold-active proteases, lipases, cellulases, amylases, pectate lyases, and type I pullulanases; alkaline proteases and lipases; thermophilic and alkalophilic mannanases; thermostable pectinases; thermo-alkaline cutinases; and alkaliphilic xylanases. However, the specific nutrient requirements, environmental factors, and maintenance of the strains is still an issue. Additionally, as extremozymes have low thermal stability and lesser shelf life, more research is needed to allow for their commercial use in detergent products (Al-Ghanayem et al. 2022).

Another process improvement sought after in enzyme use optimization for detergents is the implementation of smart enzyme delivery systems. Such dosing systems would allow the automatic release of the active ingredient only at the stage of washing when it is specifically needed. Such improvement would build on enzymes’ compactness and suitability for concentrated detergent formulations. This would in turn allow for reduced chemical use, cost savings, as well as reduced water and energy consumption, all while optimizing cleaning performance (Olsen and Falholt 1998).

Finally, Aehle (2007) listed a plethora of innovative enzymes being worked on for detergent applications, such as low-allergenicity enzymes, redox enzymes, targeting enzymes, as well as enzymes of psychrophilic and extremophilic organism origin. Redox enzymes, such as peroxidases, haloperoxidases, and laccases, have the potential to offer cost-savings and utilization of less-harmful chemicals thanks to their ability to deliver parallel bleaching performance at lower washing temperatures. Advances in protein engineering, genomics, and proteomics will ultimately drive these new developments in detergent enzyme components.

Bleach Components

Bleaching agents are commonly used in laundering (Matheson 1996; Upadek et al. 1996; Bianchetti et al. 2015). Often the bleaches are added separately, thus allowing the user to make decisions related to the tolerance of different items of clothing to bleaching. In principle, bleaching agents can contribute to laundering in two ways. First, by decolorizing various chromophoric groups, the bleach makes the material seem less soiled (Oakes 2005). Second, many bleaches are oxidizing agents, and the products of oxidation are often more soluble in water (Moskaliuk and Katović 1997). However, as shown by Li et al. (2012), the addition of bleach to a detergent formulation can increase the damage to fabrics during washing. Adverse effects of bleaching in laundering on the environment also need to be considered, since chlorinated aromatic compounds can be formed and discharged to waterways (Moskaliuk and Katović 1997). To address this, researchers have been looking into the use of redox enzymes, such as peroxidases, haloperoxidases, and laccases, to help replace the traditional bleaching systems with equivalent enzyme-based bleaching performance at lower washing temperatures. Mono- and dioxygenases have also been explored as alternative bleaching agents through direct oxidation of the substrate (Aehle 2007).

New Trends in Bio-based Detergents

Surfactants have been conventionally manufactured from fossil sources that are involved in the most significant generation of global warming emissions (Hayes and Smith 2019). In this context, the replacement of petroleum-derived surfactants with bio-based components has been proposed as a means toward increasing progressively their eco-friendly character (Hayes and Smith 2019; Bettenhausen 2022; Ortiz et al. 2022). Besides biodegradability, the decreasing of carbon release during surfactant manufacturing process is nowadays considered as one of the most important socio-economical aspects in their production (Homma et al. 2008; Santos et al. 2016; Alwadani and Fatehi 2018; Hayes and Smith 2019; Jadhav et al. 2019; Ortiz et al. 2022). As a result, several strategies regarding renewable sources to produce bio-based surfactant and their components have arisen, which are reviewed in this section. In addition, there appears to be a popular demand for products that can be listed as “bio-based” (Bettenhausen 2022). Examples shown in the cited article include lauryl glucoside (inherently bio-based), sodium lauryl ether sulfate (semisynthetic), rhamnolipid (biosurfactant), and sophorolid (biosurfactant).

Surfactants from carbohydrate derivatives

Carbohydrates are the most abundant organic compounds worldwide and are classified as monosaccharides and polysaccharides (Santos et al. 2016; Hayes and Smith 2019; Ortiz et al. 2022). Glucose and other sugars are the most representative of the monosaccharides and can be obtained from the enzymatic hydrolysis of cellulose, the most abundant of the polysaccharides (Ortiz et al. 2022; Vera et al. 2022a,b). Glucose is likewise the most abundant monosaccharide in nature. It can serve as a raw material for manufacturing a marriage of value-added building blocks at industrial scale such as succinic acid, levulinic acid, itaconic acid, maleic acid, lactic acid, and 5-hydroxymethyl furfural (5-HMF) (Willke and Vorlop 2001; Delhomme et al. 2009; Saxena et al. 2017; Schmidt et al. 2017; de Carvalho et al. 2018; Ortiz et al. 2022; Vera et al. 2022b). In this sense, the abundance of hydroxyl groups makes monosaccharides and their chemical derivatives feasible to be the polar head of surfactant molecules. The production of these types of surfactants has a low carbon footprint and may potentially have low toxicity, biodegradability and biocompatibility for pharmaceutical, food, and detergency applications when compared to fossil-based surfactants (Hayes and Smith 2019; Jadhav et al. 2019; Koteich Khatib et al. 2020; Ortiz et al. 2022). Surfactants that incorporate monosaccharides as a hydrophilic group have become well known (Saleeb 1970; Fanun et al. 2010; Ortiz et al. 2022).

Ortiz et al. (2022) compiled a marriage of surfactants made from building blocks and chemicals derived from biomass carbohydrates. Bevinakatti and Waite (2012) synthesized, from succinic acid, a sorbitol succinate (oligo laurate) that provides well-proven interfacial properties. The reaction of succinic acid with polyglycerol and a monocarboxylic acid has been reported to form an oligomeric surfactant under acid/bases catalysis at 200 °C (Santos et al. 2016; Hayes and Smith 2019; Ortiz et al. 2022). Moreover, nonionic surfactants have been synthesized from oligoesters, in which the hydrophilic part is formed by succinic acid and glycerol units, whereas lauric acid serves as the hydrophobic part (Agach et al. 2016, 2018). On the other hand, several approaches to synthesize bio-based surfactant components from other carbohydrate derivatives such as succinic acid, fumaric acid, itaconic acid, furfural, and 5-HMF have been reported (Santos et al. 2016; Hayes and Smith 2019; Ortiz et al. 2022).

Surfactants from vegetable oils

The worldwide consumption of vegetable oils and fats for surfactant making is expected to increase considerably due to their inherent biodegradability and renewability (Hayes and Smith 2019; Jadhav et al. 2019). Vegetable oils have always been an important source for making surfactants, where saponification is the oldest production process (Hayes and Smith 2019; Jadhav et al. 2019). Fatty acids and triglycerides from vegetable oils can be chemically modified to provide interfacial properties for several applications, including detergency (Santos et al. 2016; Jadhav et al. 2019). Glycolipids are common biosurfactants that are made from the fermentation of vegetable oils (e.g., Soybean oil, corn oil, coconut oil, sunflower oil, cottonseed oil, palm oil, and olive oil) (Santos et al. 2016; Hayes and Smith 2019; Jadhav et al. 2019). In addition, several detergents and surfactants types (e.g., amphoteric, ionic, and nonionic) can be synthesized via amination, sulfonation, ethoxylation, epoxidation, and esterification processes applied to vegetable oils (Pratap et al. 2011; Hayes and Smith 2019). The properties of surfactants made of vegetable oils can change based on the nature and composition of the oil (e.g., triglycerides, carbon-chain distribution, etc.). Coconut and palm oil are the most used vegetable oils in the production of surfactants due to their unique and large proportion of C12 fatty acids and low levels of unsaturation that provides high foam ability and detergency (Pratap et al. 2011; Santos et al. 2016; Hayes and Smith 2019). Others have used vegetable oil sources such as canola and soybean oil, which have a high proportion of C18 fatty acids with much higher levels of unsaturation that provides less hydrophilic surfactants (Pratap et al. 2011; Santos et al. 2016; Jadhav et al. 2019). Several extracted oils can be used for surfactant synthesis even when they can be produced in a lower ratio compared to the conventional oils (Pratap et al. 2011). For example, a surfactant synthesized using the oil from the cashew nut shell has been reported to have antibacterial and interfacial properties that make it promising for detergency applications (Koteich Khatib et al. 2020). Palm kernel, jatropha, soapnut, olive, cottonseed, and rapeseed have also been reported to be used to synthesize biosurfactants (Hayes and Smith 2019).

Lignin-derived detergents

Lignin is an abundant organic polymer that has been tagged as a potential substitute for conventional surfactants made from fossil sources (Homma et al. 2008; Alwadani and Fatehi 2018). This low-cost polymer is mostly obtained from pulping processes of the pulp and paper industry, where it is usually burned to generate energy (Alwadani and Fatehi 2018; Li and Takkellapati 2018). However, lignin has been reported to be a promising raw material for surfactant making even when having an undefined polymeric structure that challenges the making of defined products (Homma et al. 2008; Alwadani and Fatehi 2018). Soda, kraft, and lignosulfonates are the most common types of lignin, among which the lignosulfonates are highly soluble in water (Jardim et al. 2020). On the other hand, kraft and soda lignins lack water solubility and need an alkaline medium to be solubilized in water (Ou and Zhao 2017; Li and Takkellapati 2018; Jardim et al. 2020; Gong et al. 2022). Regarding surfactants derived from lignin, several processes such as alkylation, amination, carboxylation, acylation, halogenation, methylation, oxidation, reduction, sulfomethylation, sulfonation, and epoxidation have been developed to modify and increase the amphiphilic character of lignin (Hayes and Smith 2019; Jadhav et al. 2019; Ou et al. 2021). Thus, more hydrophilic functional groups can be added in the original lignin structure to produce promising, lignin-derived surfactants (Alwadani and Fatehi 2018). Lignin-poly(ethylene oxide) has been reported to have similar amphiphilic behavior compared to conventional nonyl-phenol, non-ionic surfactants (Homma et al. 2008; Alwadani and Fatehi 2018; Ou et al. 2021). Moreover, the modification of lignin with polyethylene glycol di-glycidyl ethers has been reported to generate modified lignin with interfacial properties able to decrease the water surface tension from 72.8 to 28 mN/m. Hence, those amphiphilic lignin-based surfactants have potential applications as detergents and emulsifiers for oil-in-water emulsions (Alwadani and Fatehi 2018). Other studies report that lignin modified via alkylation, sulfonation, and oxidation produces high quality surfactants able to reduce water surface tension down to 0.32 mN/m (Homma et al. 2008; Alwadani and Fatehi 2018; Ou et al. 2021).

Potential challenges in the use of lignin-derived detergents are the inconsistency in purity and limited efficiency, as well as processing difficulty due to their complex structure, unreactive nature and differences amongst the different lignin types and pulping processes used. As a result, lignin-based surfactants that are surface-active in different systems could offer a viable component for detergent products in the future (Alwadani and Fatehi 2018).

Another challenge is that functional groups in lignin-derived products may be protonated/deprotonated at different pH, causing variance in surface activities. To address this variance, pK and solubility should be considered at the time of lignin derivative selection (Alwadani and Fatehi 2018). Similarly, it is worth noting the electrostatic interactions between certain enzymes and lignin. At high pH, enzymes and lignin become negatively charged, which reduces their affinity. Additionally, the presence of surfactants in detergents further reduces that affinity through its impact on hydrophobic effects, as well as through formation of hydrogen bonds with lignin. In their study, Fritz et al. (2015) found that electrostatic interactions between cellulase and native lignin resulted in ~20% reduction in enzyme affinity at pH 5.5.

Modified lignosulfonates as detergent builders

In principle, by replacing various petroleum-derived components of surfactants with bio-based components there is potential to progressively increase their eco-friendly character. Ou and Zhao (2017) modified lignosulfonate with polypropylene glycol and found that the product was effective as a detergent builder. Due to its composition, one might also classify this product as a surfactant.

Lignosulfonates are also inexpensive materials and, in comparison with kraft lignin, possess a hydrophilic sulfonate group attached to a hydrophobic and aromatic chain (Ou and Zhao 2017; Alwadani and Fatehi 2018; Gong et al. 2022). This characteristic of lignosulfonates makes them suitable to be used as natural surfactants, dispersants, and flocculants (Homma et al. 2008; Xu and Ferdosian 2017; Alwadani and Fatehi 2018; Ou et al. 2021). Sulfonate’s negative charge, in combination with the inherent hydrophobicity of the aromatic groups, means that lignosulfonates are capable of being adsorbed on surfaces, which promotes both surface charge and wettability changes (Ou and Zhao 2017; Gong et al. 2022). However, they do not have a significant effect on reducing interfacial/surface tension, which is an important limitation in their applications (Alwadani and Fatehi 2018; Homma et al. 2008; Ou et al. 2021). Thus, as well as kraft lignin, chemical modification such as sulfonation, oxidation, carboxylation, and epoxidation processes are reported to improve significantly the surfactant performance of lignosulfonates (Ou and Zhao 2017; Gong et al. 2022). Regarding detergency applications, Ou and Zhao (2017) modified lignosulfonate with polypropylene glycol and found that the product was effective as a detergent builder. When compared to commercial dodecyl benzene sodium sulfonate, modified lignosulfonate had 10% higher detergency. On the other hand, lignosulfonate has been reported to significantly improve the prevention of ash deposits on cotton fibers during the laundry process (Gong et al. 2022). Additionally, lignosulfonates can act as chelating agents to remove Ca2+ and Mg2+ from hard water and improve the detergency effect of the main surfactant laundry formulation (Ou and Zhao 2017; Xu and Ferdosian 2017).

Characteristics of the Dirt

The word “dirt” is often used in the field of detergency to denote the contaminant material that is being removed during a washing or laundering process. One of the challenges in understanding detergency is the fact that the character of the dirt is not always the same. To make sense of this, dirt has been categorized based on various criteria, as follows.

Wettability

The degree to which a substance is wettable by water can be assessed by placing a small water droplet on a flat surface and measuring the angle of contact, which is drawn through the water phase (Berg 1993; Hubbe et al. 2015). The geometry is shown in Fig. 17. As indicated in the figure, contact angles also can be determined based on the profiles of droplets on single filaments of a textile (Carroll 1976, 1993).