Abstract
The continued development and application of surfactant encapsulated polyoxometalates (SEPs) relies on understanding the ordering and organization of species at their interfaces and how these are impacted by the various local environments they are used in. Here, we report on the equilibrium properties of two common SEPs adsorbed to the air-water interface probed with surface specific vibrational sum-frequency generation (SFG) spectroscopy. These results reveal clear shifts in vibrational band positions, the magnitude of which tracks with the charge of the SEP core, which is indicative of a static field effect on the surfactant coating and the associated local chemical environment. Thi static field also induces ordering in surrounding water molecules that is mediated by the surface bound surfactants via charge screening. From these SFG measurements we are able to show that Mo132 based SEPs are more polar than Mo72V30 SEPs. Disorder in the surfactant chain packing at the highly curved SEP surfaces is attributed to large conic volumes that can be sampled without interactions with neighboring chains. Adsorption isotherms obtained using SFG yield free energies of adsorption to the air-water interface of −46.8 ± 0.4 and −44.8 ± 1.2 kJ/mol for Mo132 and Mo72V30 SEPs, respectively, indicating a strong propensity for the surface. The influence of intermolecular interactions on the surface adsorption energies is discussed.
