Adsorption, Ordering, and Local Environments of Surfactant-Encapsulated Polyoxometalate Ions Probed at the Air-Water Interface

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Abstract

The continued development and application of surfactant-encapsulated polyoxometalates (SEPs) relies on understanding the ordering and organization of species at their interface and how these are impacted by the various local environments to which they are exposed. Here, we report on the equilibrium properties of two common SEPs adsorbed to the air-water interface and probed with surface-specific vibrational sum-frequency generation (SFG) spectroscopy. These results reveal clear shifts in vibrational band positions, the magnitude of which scales 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. This static field also induces ordering in surrounding water molecules that is mediated by charge screening via the surface-bound surfactants. 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. Measurements of adsorption isotherms yield free energies of adsorption to the air-water interface of -46.8 ± 0.4 and -4.8 ± 1.2 kJ/mol for the Mo132 and Mo72V30 SEPs, respectively, indicating a strong propensity for the fluid surface. The influence of intermolecular interactions on the surface adsorption energies is discussed.

Original languageEnglish
Pages (from-to)8116-8122
Number of pages7
JournalLangmuir
Volume32
Issue number32
DOIs
StatePublished - Aug 16 2016

Funding

Benjamin Doughty and Ying-Zhong Ma were supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Panchao Yin is grateful for the support of the Clifford G. Shull Fellowship from the Neutron Sciences Directorate of Oak Ridge National Laboratory. The sample preparation was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Oak Ridge National Laboratory is supported by the Office of Science of the U.S. DOE under Contract DE-AC05-00OR22725.

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