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.

The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle’s entropy of transfer as high as 82 meV K⁻¹. Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.