The layer charge of five smectites, one vermiculitic material, and five reduced-charge clays was determined by the alkylammonium and structural formula methods. The two sets of results were found to be linearly correlated (r =.961); however, the values that were determined by the alkylammonium method were 20 to 30% lower than those determined by the structural formula method, and the regression slope for their linear relationship was 1.67. The fact that the structural formula method includes the effects of cations on the lateral edges of the clay particles probably accounted for some of the differences in the magnitude of the results but should not have caused the regression slope to deviate substantially from 1.00. Therefore, inaccurate estimates of the packing density of alkylammonium cations in the interlayer space of 2:1 phyllosilicates were deemed responsible for the systematic divergence of the results of the two methods. To satisfy the need for a relationship between the two methods of determining layer charge, an empirical means of adjusting the alkylammonium values has been proposed and shown to yield values of layer charge that are comparable to those determined by the structural formula method.

Infrared (IR) spectra in the fundamental and near-IR regions were obtained for Na-saturated Wyoming montmorillonite and reduced-charge Na/Li-saturated Wyoming montmorillonites hydrated under water vapor at 50% RH and dehydrated under vacuum. For the Na-montmorillonite, changes in the intensities of the structural OH-bending modes, particularly that of the MgAlOH group, were observed as the clay was dehydrated. This result was interpreted as evidence that exchangeable Na ions lose solvation water and settle into the ditrigonal cavities on the clay surface as it becomes desiccated. For the Na/Li-montmorillonites, the structural OH-bending modes also decreased in IR intensity because of Li+ migration into the octahedral sheet. The fundamental IR spectra of D2O adsorbed by the montmorillonites showed characteristic absorptions at 2685, 2510, 2400, and 1205 cm-1 that decreased in intensity proportionally to the cation-exchange capacity. This result, along with corroborating data from X-ray powder diffractograms and from near-IR diffuse reflectance spectra of H2O adsorbed by the clays, suggest that the exchangeable cations on Na-montmorillonite dissociated from the clay surface as it hydrated and played a significant role in organizing the structure of adsorbed water at low water contents.