Water sorption on Ca-saturated clays: II. Internal and external surfaces of montmorillonite

Water-sorption isotherms and interlamellar spacings were measured for Ca montmorillonite (from Redhill. UK) over the relative pressure range 1 > P / P 0 > 0. By taking account of the change in interlamellar volume with relative pressure, it is shown that internal and external surfaces can be calculated by a modified t -plot analysis. The results also show that sorption on the external surfaces of the montmorillonite is quantitatively similar to that on reference oxide surfaces. Desorption-sorption hysteresis occurred and was apparently caused by an ageing process in which the interlamellar volume increased at the expense of the external surfaces.