Supramolecular aggregates in vacuum: positively mono-charged sodium alkanesulfonate clusters

The formation and structural features of positively mono-charged aggregates of sodium bis(2-ethythexyl) sulfosuccinate (AOT) and sodium methane-(MetS), butane-(ButS) and octane-(OctS) sulfonate molecules in the gas phase have been investigated by etectrospray ionization mass spectrometry, energy-resolved mass spectrometry and density functional theory (DFT) calculations. The experimental results show that the center-of-mass collision energy required to dissociate 50% of these mono-charged aggregates scantly depends on the length of the alkyl. chain as well as on the aggregation number. This, together with the large predominance of mono-charged species in the mass spectra, was rationalized in terms of an aggregation pattern mainly driven by the counter ions and head groups electrostatic interactions while minor effects were attributed to the steric hindrance caused by the size of the surfactant head group and alkyl chain. DFT calculations show that the most favored structural arrangement of these aggregates is always characterized by an internal polar core constituted by the sodium counter ions and surfactant head groups surrounded by an external layer composed by the surfactant alkyl chains.