Surface of aqueous solutions of alkali halides: layer by layer analysis

Interfacial layers of both vapour/liquid and crystal/liquid aqueous solutions of sodium and caesium halides have been analysed in detail using four different methods for the identification of interfacial molecules and layers, and both non-polarisable and polarisable models. It is found that each of the methods yields a somewhat different result with the best mutual agreement found between the ITIM (Identification of Truly Interfacial Molecules) and alpha-shape-based USTI (Universal Scheme for Triangulated Interfaces) methods. Concerning the water models, the commonly used JC [I. S. Joung, T. E. Cheatham, J. Phys. Chem. 112 (2008) 9020] model predicts the structural properties of the vapor/liquid interface different from both the non-polarisable MADRID and polarisable AH/BK3 models, and also from experiment. The same applies also to the structural properties of the solid-liquid interface; although the AH/BK3 and JC models yield a surplus of chlorine ions, a segregation of Na and Cl- ions in the interfacial layers predicted by the JC model is not observed in the polarisable model.