Design of two-dimensional crystals as models for probing the structure of the solid-liquid interface

Crystals of β-alanine and the α-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 β-alanine most affected by the presence of CdCl₂was 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 CdCl₂solution 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.