Modelling specific ion effects with the continuum solvent

Abstract

Electrolyte solutions play a central role in many processes from industry to biology. Understanding and building predictive models of their properties has therefore been a fundamental goal of physical chemistry from its beginnings. The challenge remains. In this thesis I outline a continuum solvent model of univalent monatomic ions in water. This model calculates the free energy of: 1) a single ion in bulk, 2) of an ion approaching the air–water interface and 3) of two ions approaching each other. Its central advancements are to include quantitatively accurate ionic dispersion interaction energies, missing from classical theories, including the higher order multipole moment contributions to these interactions. It also includes the contribution from the cavity formation energy consistently, including the effect of changes in the cavity’s shape. Lastly, it uses a quantum mechanical treatment of the ions and provides satisfactory values for their size parameters. Because one consistent framework is used with the same assumptions to calculate the free energies in these three different situations the number of parameters can be minimised and the model can be properly tested. These three calculations can be used to reproduce experimental solvation free energies, solvation entropies, partial molar volumes, surface tensions and activity/osmotic coefficients of the alkali-halide electrolyte solutions. A minimum of parameters are used and crucially no salt–specific fitting parameters are necessary. The model is quantitative and predictive and is therefore a satisfactory model of electrolyte solutions. It provides an explanation of several key qualitative puzzles regarding these properties. Namely that ions of the same size can have different solvation energies, that large ions can adsorb to the air–water interface and that ions in solution that have similar solvation energies are more strongly attracted to each other than ions that have dissimilar solvation energies. The continuum solvent model and separate ab initio calculations show that dispersion interactions play a key role in controlling these effects. In particular, dispersion energies explain the attraction of large ions for each other in water and the difference in solvation energy of ions of the same size. The success of the model implies that it is possible to understand the key properties of electrolyte solutions using a continuum solvent model. This is an important conclusion considering the massive computational demands of explicit solvent treatments. Show more