“…The interlayer expansion, which affects the BET surface area, depends on both the exchangeable cation and the structural charge of the smectite (Olejnik, 1974;Slade, 1991). Analysis using transmission electron microscope and X-ray diffraction (XRD) showed that the size of the tactoids that are formed when dilute smectite suspensions are dried depends on the chemical identity of the interlayer cations: tactoids of Na-montmorillonitic smectites are composed almost entirely of single unit cell particles, Ca-montmorillonitic smectite tactoids, such as the SAz-1 sample used in the present study, consist of 7 to 11 unit layers, and tactoids of H-exchanged smectite consist of around 17 unit layers (Van Olphen, 1970;Vali and Bachmann, 1988;Blum, 1994;Schoonheydt, 1995;Stepkowska et al, 2004). Since smectite particles disperse to varying degrees ranging from fully dispersed TOT sheets, to faceto-face stacks of sheets, the rate law for smectite dissolution might be written as (Ganor et al, 1995a):…”