Proceeding Articles

The surface and interfacial properties of fibres and paper are important in the production and end-use of paper and board, and much attention has been paid to the nature of the interface between water-swollen cellulose fibres and the aqueous suspension medium. Much less attention has been paid to the equally important interface between fibres containing water and the atmosphere. The properties of this interface must range from those characteristic of a dry fibre to those of pure water, corresponding to water contents ranging from a fraction of a monolayer up to the fibre saturation value.

The apparent thermal expansion of cellulose immersed in liquid water, d⌀₂/dT, is several times that of dry cellulose. A similar but greater disparity is observed when the temperature dependence of the apparent specific volume of glucose in aqueous solution is compared with the thermal expansion of crystalline glucose. This effect appears to be a general one for polyols such as glucose and glycerol and can be interpreted in terms of a ‘mixture’ theory of water structure. Water is pictured as being made up of small short-lived clusters which may be classified as either solidlike or fluidlike. The solidlike component consists of rigid, hydrogen bonded ring structures (tetramer, pentamer, hexamer). The fluidlike component consists of non-rigid, less hydrogen bonded, chain structures (dimer, trimer, star pentamer). Surfaces rich in hydroxyl groups appear to act as structure breakers by causing an increase in the proportion of the fluidlike component in the water adjacent to the interface. With cellulose, this perturbed layer consists of the non-rigid chain structures, hydrogen bonded to the -OH groups on the surface. The high values for d⌀₂/dT for glucose and cellulose are caused by the high thermal expansion of the perturbed layer.

The sonic tensile modulus (E) of paper decreases with moisture content (m). The slope of the decrease, d In E|dm, is constant up to a moisture content equal to the fibre saturation point of paper. This phenomenon seems consistent with the multilayer adsorption of water and its explanation provided by the polarisation theory of Polanyi. The energies of interactions between layers of adsorbed water indicate that hydrogen bonds between layers may contribute to the load carrying capacity of wet paper. This hydrogen bonding between layers implies a structured organisation throughout the adsorbed water that extends several layers from the cellulose surface. In this way, it appears that water near a cellulose surface is structured by the cellulose surface rather than destructured by it. One such model for structured water is presented.

The parallels between ionic transport, electrophoresis and electro-osmosis are briefly discussed. It is pointed out that the interpretation of the electrophoretic mobility cannot be based on charge and friction constant alone. The flow and the distortion of the ionic atmosphere (electric double layer) cause in nearly all circumstances a large retardation.

The zeta potential is defined as the potential at the surface between the freely mobile liquid and the liquid firmly adhering to the particle surface. It is a useful notion when the particle surface is well defined, much less useful in the case of polyelectrolytes or highly swollen structures. Electrolytes compress ionic atmospheres and double layers and thereby cause a decrease of the electrokinetic mobility and the zeta potential.

The electrokinetic mobility can be used to obtain information on the charge of particles and from that to estimate electrostatic repulsion or attraction. Here again the effect is determined by charge and electrolyte content, not by charge or zeta potential alone. Direct applications of electrokinetic phenomena are drying by electro-osmosis, electrodeposition on electrodes, accumulation on a membrane or filter, orientation of fibres and plates, separations based on differences in mobility.

The electrokinetic euphoria which gripped the paper industry a few years ago gradually subsided when it became clear that the zeta potential is not the panacea many people, particularly instrument manufacturers, had hoped it to be. This may have the salutary effect that electrokinetic phenomena, which undoubtedly occur in papermaking, are seen to be part of a complex of physico-chemical processes to which they make different contributions under different circumstances. It may well become the order of the day in the immediate future to study case histories in the hope that one day some more general rules may emerge. This is one of the reasons why the second half of this session is given to a number of short contributions, unusual for these symposia but a step we decided to take because it seemed the right thing to do.

The purpose of my own short contribution to this first and more academic half of the session is not to provide one of those case studies but to report on a simple observation in the laboratory which, if our interpretation is correct, would be very much at variance with the established concept of the electric double layer and the various models built around it.

Interfacial potential differences can only be interpreted in terms of a physical reality between phases of identical chemical composition. Many substances acquire a charge when immersed in water and migrate under an applied
electric field. This is usually interpreted in terms of an ionic double layer at the surface and the zeta potential is defined as the potential at the plain of shear. It is doubtful if the concept of the zeta potential is of assistance in describing electrokinetic phenomena as interfacial potentials are only physically meaningful in a few carefully proscribed instances. Instead, in electrophoresis, for example, all the necessary observed experimental data to define the system should be recorded and a comparison made of mobilities, other parameters being kept constant.

The use of cationic organic polyelectrolytes as drainage and retention aids in the papermaking process and as flocculants in sewage treatment is steadily increasing. Therefore, there has been a growing interest in a better understanding of the mode of action of these additives.

There are two basic concepts discussed in the current literature which can be briefly described in terms of bridging and of charge neutralisation, respectively.⁽¹⁻⁶⁾ Of particular interest is a variety of the latter, the ‘patch charge model’, which was introduced recently.⁽⁷⁾ In general the discussions are based on the data of electrophoretic mobility and of flocculation, drainage, or retention.⁽⁸⁻¹⁴⁾

Clearly, in all cases the adsorption of the polymers from the bulk of the solution onto the dispersed particles is of prime importance for the discussion of the mechanism of action. There have been, however, only a few attempts to study the adsorption and the adsorption kinetics of cationic polymers in cellulose systems.⁽¹⁵⁻¹⁸⁾

The object of this investigation was to study the influence of molecular weight and of charge density on the adsorption of polyethyleneimines (PEI) and of cationic polyacrylamides (PAA) in cellulose systems, in conjunction with electrokinetic and flocculation measurements.

This investigation was initiated with the intention of defining the mode of action of the chemical retention agents used, especially with regard to their ionic nature and chain length. The efficiency of these agents is often explained in the literature as being due to their action on the electrical charge on the surface of the various substances present in papermaking suspensions.⁽¹⁻¹²⁾ After numerous laboratory studies carried out in media with well defined ionic strengths and on simple mixtures, we tried to see if a relationship existed between retention and zeta potential for a medium as complex as the papermaking water system.

Trials were carried out on the pilot papermachine at the Centre Technique du Papier.

LASER Doppler Electrophoresis (LDE) has appeared recently as an alternative to classical methods of electrophoresis in the study of colloid systems.⁽¹-²⁾ In microelectrophoresis, individual particles are tracked under a microscope, while in LDE the counting of particles of various mobilities is performed automatically by means of optical processes. LDE provides a less time consuming and inherently more objective method than microelectrophoresis for analysing the mobility distribution of a system of particles.

LDE may also have advantages over other classical electrophoretic methods in many cases. Moving boundary electrophoresis is useful for small particles or macromolecules, whereas mass transport electrophoresis is more suited to concentrated suspensions. LDE is applicable to reasonably dilute systems of particles which range in size from molecular dimensions to micrometers. The LDE system described here is designed to study particles in the upper part of this range.

During every minute of operation of a typical paper machine several square kilometers of interface between the solid and liquid phases of the pulp suspension pass down the wire. These interfaces possess a number of special properties. The formation of an electrical double layer on the surface of the solid phase, for instance, influences to a large extent the process of sheet forming, the retention of fibres, fillers and sizing agents and, in turn, the characteristics of the finished sheet. There is understandably, therefore, a strong interest in obtaining measurements of these interfacial characteristics with the intention of optimising the operation of the process and the performance of the products. This interest is demonstrated by the number of publications which have appeared on the subject over the last few years. Melzer,⁽¹⁾ for example, cited as many as 102 papers in 1972 and since then many more have appeared.