Charge/pH isotherms were determined for various cellulose fibres: cotton linters, bleached sulphate, and unbleached sulphate pulp. The chare was determined as a function of pH in 1.0, 10⁻¹, 10⁻², and 10⁻³ mol dm⁻³ NaCl. The effect of various cations on the charge was also investigated. The surface areas of the fibres were determined by BET nitrogen adsorption; the pulps were initially in a ‘never-dried’ state and for the BET work they were specially prepared using solvent-exchange techniques whereby all the water was replaced by dry pentane. The surface areas of the fibres were also obtained using the method of negative adsorption: corrections for low surface potential were applied using Gouy-Chapman theory and the charge/pH isotherms. Surface areas obtained by these two entirely different methods are compared. Once drying the ‘never-dried’ pulps halved the surface areas.
Zeta potentials for cotton linters and bleached sulphate pulp were calculated from measurements of the streaming potential. These measurements were made at the same electrolyte and pH conditions as the charge/pH isotherms and the zeta potentials compared with the surface potentials calculated from Gouy-Chapman theory. It was found that the zeta potential is not a good relative measure of the surface charge and cannot be used for qualitative comparison between such similar materials as bleached sulphate pulp and cotton linters.