Structure, Stability, and Fragmentation of Sodiumbis(2-ethylhexyl)Sulfosuccinate Negatively ChargedAggregates In Vacuo by MD Simulations

Negatively charged supramolecular aggregates formed in vacuo by nbis(2-ethylhexyl)sulfosuccinate (AOT–) anions and n + nc sodium counterions (i.e.,[AOTnNan+nc]nc) have been investigated by molecular dynamics (MD) simulationsfor n = 1 to 20 and nc = –1 to –5. By comparing the maximum excess chargevalues of negatively and positively charged AOTNa aggregates, it is found thatthe charge storage capability is higher for the latter systems, the differencedecreasing as the aggregation number increases. Statistical analysis of physicalproperties like gyration radii and moment of inertia tensors of aggregates providesdetailed information on their structural properties. Even for nc = –5, all stableaggregates show a reverse micelle-like structure with an internal core, includingsodium counterions and surfactant polar heads, surrounded by an external layer consisting of the surfactantalkyl chains. Interestingly, the reverse micelle-like structure is retained also in proximity of fragmentation.Moreover, the aggregate shapes may be approximated by elongated ellipsoids whose longer axis increaseswith n and |nc|. The fragmentation patterns of a number of these aggregates have also been examined andhave been found to markedly depend on the aggregate charge state. The simulated fragmentation patternsof a representative aggregate show good agreement with experimental data obtained using low collisionvoltages.