ATOMIC FORCE MICROSCOPY INVESTIGATION OF HETEROGENEITY IN SMECTITE SWELLING

The traditional method of measuring swelling in smectites, x-ray diffraction (XRD), provides only an ensemble average of the interlayer spacings and is consequently not well suited to investigations of layer heterogeneity. We present here a method for directly measuring swelling of individual clay quasi-crystals in situ as they swell in an aqueous environment. AFM is used to produce topographical images of the particles at different times throughout the swelling process. By setting a z, or height, value threshold on an AFM image, a plane of the quasi-crystal corresponding to certain layers within the stack can be isolated and compared to subsequent images restricted to the same plane later in the swelling process. The difference in lateral area of the particle in the two images reveals whether swelling or compression has occurred, and to what extent. AFM analyses of Na-exchanged smectites in this study have demonstrated differences in swelling behavior from layer to layer within a single quasi-crystal. Although variation in the crystalline stages of swelling is well known to be correlated with layer charge, this study provides the first direct evidence that the heterogeneity in hydration properties extends to the osmotic stage of swelling.