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 t heory together with the hypernetted chain formalism based on the primi tive model for electrolyte solutions to examine recently published res ults for extensive crystalline swelling of Li-montmorillonite as revea led by d(001) spacings over the pressure range 0.05-0.9 MPa and the co ncentration range 1-10(-4) M LiCl. The analysis based on Langmuir's us e of elliptical integrals reveals that the theory satisfactorily predi cts surface separation over the range 18-120 Angstrom and that the DLV O theory, with a 5.5 Angstrom Stern layer, indicates Gouy plane potent ials 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 Cm-2, which is 30% of the crystal lattice charge. The anisotropic hypernetted chain (HNC ) analysis, also based on lattice charge, satisfactorily predicts pres sure-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 Angstrom.