Study of individual Na-montmorillonite particles size, morphology, and apparent charge.

Size, morphology, and apparent charge of individual Na-montmorillonite particles of natural MX-80 sodium montmorillonite were investigated in the present study by the use of three coupling methods. In the first part of this work, natural and synthetic montmorillonite clays were studied with atomic force microscopy (AFM) and photo-correlation spectroscopy (PCS). Both techniques exhibit the presence of two clay populations with a high dispersion of the length distribution. Microscopic analysis of the system revealed that clay particles could be reasonably approximated at low concentrations to ellipsoidal tactoids about 1.2 nm high. Average dimensions of the first population were typically 320-400 nm long/250 nm wide and 200-250 nm long/120 nm wide for natural and synthetic clays, respectively. The second population exhibits smaller sizes: 65 and 50 nm long and 35 and 25 nm wide for natural and synthetic clays, respectively. The statistics obtained for natural clay were then verified by PCS experiments on sodium montmorillonite suspensions. Both techniques reveal an important length dispersion. However, the relative proportions of the two kinds of particles could not be established properly because of both lack of statistics and limitations of the employed techniques. In the following part, conductivity measurements were performed on dilute montmorillonite clay suspensions. Raw data were then interpreted with the sizes and morphological information gained in the first part of the present work. The apparent charge of the clay sheets was found to be 8% of the structural charge.