Collision induced fragmentations of multiply charged sodium bis(2-ethylhexyl)-sulfosuccinate aggregates in gas phase: neutral loss versus charge separation

Stability and fragmentation patterns of multicharged aggregates of sodium bis(2-ethylhexyl)-sulfosuccinate (NaAOT) in gas phase have been investigated by ion mobility mass spectrometry (IM-MS) and tandem mass spectrometry (MS-MS). Positively doubly charged NaAOT aggregates show at low collision energy a preference for the loss of NaAOT molecules, whereas fragmentation through charge separation process is favored at higher collision energy. By increasing the charge state of the aggregates, the fragmentation through charge separation tends to predominate especially at low aggregation number and only charge separation fragmentation is observed for positively quadruply charged species. On the other hand, the fragmentation of negatively doubly charged NaAOT aggregates is always characterized by a structural collapse of the entire ensemble leading to the formation of small-size singly charged fragments. The analysis of the collision energy required to fragment the 50% (CE50%) of positively and negatively charged NaAOT aggregates allows to emphasize the role of electrostatic repulsion, degree of freedom and statistical effects as determining factors of the aggregation number and charge state dependence of CE50% values. Overall, the observed behavior of positively and negatively multicharged NaAOT aggregates in gas phase has been rationalized in terms of the electrostatic interactions between sodium counterions and negatively charged surfactant heads mainly responsible of the reverse-micelle like structural arrangement of these non-covalent aggregates and of their stability.