DESIGN OF 2-DIMENSIONAL CRYSTALS AS MODELS FOR PROBING THE STRUCTURE OF THE SOLID-LIQUID INTERFACE

Crystals of beta-alanine and the alpha-form of glycine, when grown in the presence of 3% molar CdCl2, display morphologies which are different from those obtained from pure water solution. This effect was interpreted in terms of binding of Cd2+ and Cl- ions to the exposed CO2- and NH3+ moieties at the various faces, so changing their relative rates of growth and leading to the development of new faces. The structure of the crystal face of beta-alanine most affected by the presence of CdCl2 was mimicked by a monolayer containing an equimolar mixture of two different amphiphiles octadecylamine (C18H37NH2) and stearic acid (C17H35COOH) Spread on aqueous solution. Binding of the Cd2+ and Cl- ions to such a monolayer has been investigated. X-ray specular reflectivity measurements reveal that the mixed monolayer is fully bound by Cd2+ and Cl- ions when their concentration reaches 0.1 M. Grazing incidence X-ray diffraction measurements, using synchrotron radiation, of the mixed monolayer on pure water and on a 0.1 M CdCl2 solution provide strong evidence that the monolayer CO2- and NH3+ head groups are arranged in ordered array and the Cd2+ and Cl- ions are bound to the monolayer head groups at ordered sites.