Aqueous solutions of tetraalkylammonium halides: Ion hydration, dynamics and ion-ion interactions in light of steric effects

Molecular simulations have allowed us to probe the atomic details of aqueous solutions of tetramethylammonium (TMA) and tetrabutylammonium (TBA) bromide, across a wide range of concentrations (0.5 to 3-4 molal). We highlight the space-filling (TMA⁺) versus penetrable (TBA⁺) nature of these polyatomic cations and its consequence for ion hydration, ion dynamics and ion-ion interactions. A well-established hydration is seen for both TMA⁺ and TBA⁺ throughout the concentration range studied. A clear penetration of water molecules, as well as counterions, between the hydrocarbon arms of TBA⁺, which remain in an extended configuration, is seen. Global rotation of individual TBA⁺ points towards isolated rather than aggregated ions (from dilute up to 1 m concentration). Only for highly concentrated solutions, in which inter-penetration of adjacent TBA⁺s cannot be avoided, does the rotational time increase dramatically. From both structural and dynamic data we conclude that there is absence of hydrophobicity-driven cation-cation aggregation in both TMABr and TBABr solutions studied. The link between these real systems and the theoretical predictions for spherical hydrophobic solutes of varying size does not seem straightforward.