Application of double-layer theories to the extensive crystalline swelling of Li-montmorillonite

We have used the Poisson-Boltzmann (PB) theory and DLVO double-layer theory together with the hypernetted chain formalism based on the primitive model for electrolyte solutions to examine recently published results for extensive crystalline swelling of Li-montmorillonite as revealed by d(001) spacings over the pressure range 0.05-0.9 MPa and the concentration range 1-10^-4 M LiCl. The analysis based on Langmuir's use of elliptical integrals reveals that the theory satisfactorily predicts surface separation over the range 18-120 Å and that the DLVO theory, with a 5.5 Å Stern layer, indicates Gouy plane potentials ranging from -58 to -224 mV over the concentration range 0.1-10^-4 M LiCl and a constant Gouy plane charge of 0.038 C m^-2, which is 30% of the crystal lattice charge. The anisotropic hypernetted chain (HNC) analysis, also based on lattice charge, satisfactorily predicts pressure-distance relationships over the range of concentrations. There is no evidence for the operation of the "secondary hydration force" at a surface separation of 18 Å.