Diffusion control in thin clay films: tailoring layered clay film structures through control of aqueous electrolyte

Clay-modified electrodes have been used to investigate the effects of varying anions and cations in the bathing solution and clay film history on the transport of probe molecules as controlled by the swelling properties of Wyoming montmorillonite. The cations K+ and Ca2+ have been shown to restrict swelling in clays exposed to mixtures of NaCl and KCl or CaCl2. In dilute NaCl/KCl or NaCl/CaCl2 mixtures, the K+ and Ca2+ appeared to be exchangeable, however in more concentrated mixtures attenuation of the swelling due to K+ or Ca2+ was more persistent. Trivalent anions were more readily incorporated in the interlayer than quadravalent anions in well swollen clay films. However, the opposite trend was observed in films formed with smaller interlayer spacing via bathing in higher concentration electrolytes. Divalent electrolyte anions also affected larger probe molecule transport than monovalent anions in these less-swollen films.