Special solvation effects of mixed water and alcohols revealed by molecular aggregation

Abstract

Owing to their green nature, good miscibility and tunable polarity, alcohols are widely used as popular co-solvents of water in supramolecular assembly. Intriguingly, the solvation properties of water/alcohol mixtures often show a complex dependence on the composition. Herein, the unusual solvation effects of water and alcohol mixtures are elucidated by examining the aggregation motif changes of amphiphilic polycycles bearing more polar side chains. While the molecule assembles into long fibrous J-aggregates in pure water and alcohols, H-aggregates of smaller sizes appear in water-alcohol mixtures. Similar dissociation is found to occur in different low-carbon alcohol-water systems, but H/J switching takes place at different volume ratios, depending on the alcohol structure. It is thus speculated that the aggregate dissociations reflect an enhanced solubilizing ability of the mixed solvents, which is assumed to be related to the microscopic structures of the water-alcohol binary systems. That is, the highly structured networks formed by mixed water and alcohols possess enhanced solvating capacity for the amphiphilic molecules, and alcohols having longer alkyl groups and forming larger structured hydration shells in water disrupt the aggregation more effectively. Molecular dynamics simulations have provided supportive evidence for the special solubilizing properties of mixed water and alcohols.