Alternative models for describing the acid/base properties of bleached softwood fibres

To evaluate the capacity of different thermodynamic models to mimic the build-up of charge on fully bleached softwood fibres as a function of -log[H+] and ionic strength (0.01-0.6 M NaCl), the model of an Ideal Monoprotic Acid, the Constant Capacitance Model (CCM), the Basic Stern Model (BSM), and the Diffuse Layer Model (DLM) were compared. The Ideal Monoprotic Acid, IMA, model consistently yielded a too steep increase in the build up of charge with -log[H+] and thereby indicated the need for applying a surface potential model. The "best" of these, with respect to describing the experimental data, was found to be the Constant Capacitance Model, which utilises an individual equilibrium constant and an individual capacitance value at each ionic strength modelled. However, the Basic Stern Model, which uses only two variables over the entire ionic strength interval, was found to describe data almost as well. Therefore, in modelling applications where a variation in the salt concentration is significant, the Basic Stern Model must be judged as being superior to the Constant Capacitance Model. As for the IMA model, the Diffuse Layer Model, finally, was found to yield a too steep increase in the fibre charge with rising -log[H]. Nevertheless, it was found to model the variation of the apparent acidity with ionic strength, relatively well.