Combined Determination of Specific Surface Area and Surface Charge Properties of Charged Particles from a Single Experiment

Specific surface area and surface charge properties such as surface potential, surface charge density, electrostatic field strength at surface, and surface charge number are important characteristics of charged particles in soil and the environment. Currently there is not a theory or method that could give a combined determination for those parameters from a single experiment. In this paper, an approach is described that employs cation exchange experiments to develop data that can be interpreted using newly a derived set of theoretically-based equations. First we give two modifications, respectively, for the nonlinear Poisson-Boltzmann equation and for the cation exchange equilibrium equation. Second, from the modified Poisson-Boltzmann equation, new equations for describing cation exchange equilibrium as considering the cationic hydration effect were obtained. Finally, the theory for the combined determination of the five surface properties through a single experiment of cation exchange equilibrium was established. In the experimental study, the three constant parameters in the theory have been calibrated through a standard sample, illite, and the calibrated variables can be directly applied to any other materials determination for a given electrolyte system. In addition, Na/Ca exchange equilibrium for three different soils were determined, and then the five surface properties were calculated. The application of the new theory showed that this new method could give reliable determination results of the five surface properties for both permanently and variably charged materials. For the specific surface area determination, the new method could be used in both swelling and nonswelling materials. For the surface potential determination, this method could give the surface potential values under different electrolyte composition and concentration conditions.