Changes in b Dimension of Na–Montmorillonite with Interlayer Swelling

Abstract

THE purpose of this letter is to point out an alternative explanation for the results published under this title by Low et al.¹. Briefly, they found that when montmorillonite saturated in sodium is swollen in water, the lateral (b) dimension of the silicate layer increases as a function of water content, even at water thicknesses of 400 Å or more. They ascribe these changes to strong (epitaxial) interaction between montmorillonite and water “which modifies their respective structures”. We propose that there is a solubility equilibrium between some component of the silicate layer (probably octahedral aluminium) and the interlayer water, and that an increase in water content of the clay suspension favours further solution of this component. It is well known that substitution of octahedral Al³⁺ by lower-valent ions results in an increase in b (ref. 2), and also that such substitution occurs rapidly in the presence of hydrogen ion^3,4. (Other possible mechanisms for changing the composition of the silicate layer have been considered in another context by Low himself³—slight hydrolysis of an Al-bentonite, or chemisorption of H⁺ into the silicate lattice.)