Nonelectrostatic interactions between ions with anisotropic ab initio dynamic polarisabilities

Abstract

Ion specific effects are common in colloidal and biological systems. Bubble lifetime before coalescence depends on the electrolyte. The strength of different salts at precipating proteins leads to Hofmeister series. Theories of ion and colloid interactions based on electrostatics alone, including the Debye–Hückel theory of electrolytes or the DLVO theory of colloids, are unable to predict these ion specific effects. Rather, the theories need to be modified to account for quantum mechanically derived nonelectrostatic dispersion interactions [1]. The dynamic polarisability (i) of an ion lies at the heart of its dispersion interactions. In general it may be written as a sum over many quantum modes, each with frequency n. Approximations in the past have reduced it to a single mode with modal frequency derived from the ionisation potential (IP) of the ion [2]. We have calculated the exact dynamic polarisabilities of a wide range of ions [3] using ab initio quantum mechanics and present here comparisons against the single-mode IP approximation. The error in calculated dispersion energies due to the single-mode IP approximation averages around 40%, and reaches as high as 86% error for halide ions. Ionic self-energies, and ion–ion and ion–surface interaction energies are calculated. Applications to activity coefficients in the bulk electrolyte are presented. Hofmeister series in the activity coefficients of alkali halides consistent with experiment are found. We also discuss further development of the theory of dispersion interactions with the aim of obtaining a quantitatively, not merely qualitatively, predictive theory. The steps include (i) taking into account the nonspherical anisotropic features of the ions and (ii) using a nonlocal description of the solvent to include solvent spatial structure. Step (i) should be crucial in resolving the adsorption of anisotropic ions such OH to interfaces, which is currently subject of debate between theory and experiment [4,5].