Abstract

European Journal of Mass Spectrometry
Volume 16 Issue 1, Pages 151–161 (2010)
doi: 10.1255/ejms.1013

Supramolecular aggregates in vacuum: positively monocharged sodium alkanesulfonate clusters

David Bongiorno,^a Leopoldo Ceraulo,^a,* Gianluca Giorgi,^b Serena Indelicato,^a Angela Ruggirello^c and Vincenzo Turco Liveri^a
aDipartimento di Chimica e Tecnologie Farmaceutiche, Università di Palermo, Via Archirafi 32, I-90123 Palermo, Italy
^bDipartimento di Chimica, Università degli Studi di Siena, Via Aldo Moro, I-53100 Siena, Italy
^cDipartimento di Chimica-Fisica “F. Accascina”, Università di Palermo, Viale delle Scienze, I-90128, Palermo, Italy

The formation and structural features of positively monocharged aggregates of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium methane- (MetS), butane- (ButS) and octane- (OctS) sulfonate molecules in gas phase have been investigated by electrospray 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 monocharged aggregates scantly depends on the length of the alkyl chain as well as on the aggregation number. This, together with the large predominance of monocharged 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 favoured 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.

Keywords: supramolecular aggregates, amphiphilic molecules, surfactants, reverse micelles, ESI-MS, ESI-MS/MS, energy resolved mass spectrometry, DFT calculations

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