Spatially ordered surfactant assemblies in the gas phase: negatively charged bis(2-ethylhexyl)sulfosuccinate- alkaline metal ion aggregates

The formation and structural features of negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) surfactant molecules in the gas phase have been investigated by electrospray ionization mass spectrometry (ESI-MS) and density functional theory calculations. Mainly driven by the interactions of alkali metal ions both with the oxygen atoms of the sulfonate group and with the succinate moiety of the AOT(-) anion, spatially ordered supramolecular assemblies, characterized by an internal core composed of surfactant counterions and hydrophilic head groups surrounded by the surfactant alkyl chains pointing outwards, are formed. Calculations have shown that surfactant self-organization in the gas phase is energetically favoured, the energy of formation of negatively charged aggregates from isolated AOTNa and AOT- being linearly related to the aggregation number. Information on the chelating properties of AOTNa towards clusters of inorganic salts was achieved by infusion of solutions at various AOTNa/metal salt (NaCl, NaBr, Nal, LiI, KCl, CsI, RbI) ratios in the ESI source of a mass spectrometer. A wide variety of negatively charged AOT-metal aggregates, some of them also incorporating halide (X(-)) ions, has been observed. Calculations have shown that the capture of a halide anion to give the AOTMX(-) species is favoured but the energetics of the process depends on the alkali metal and halide types. The use of energy-resolved mass spectrometry has allowed us to evaluate the stability of different complexes and to evaluate the role played by the metal ion. Overall, the present investigation supports the idea that, in the gas phase, mainly driven by electrostatic interactions, surfactant molecules are present as molecular aggregates characterized by a reverse micelle-like organization with an internal core formed by the surfactant counterions and head groups surrounded by the surfactant alkyl chains. These peculiar aggregates are able to incorporate ionic clusters in their hydrophilic core.