Amphiphilic self-assembly of alkanols in protic ionic liquids

Abstract

Strong cohesive forces in protic ionic liquids (PILs) can induce a liquid nanostructure consisting of segregated polar and apolar domains. Small-angle X-ray scattering has shown that these forces can also induce medium chain length n-alkanols to self-assemble into micelle- and microemulsion-like structures in ethylammonium (EA+) and propylammonium (PA+) PILs, in contrast to their immiscibility with both water and ethanolammonium (EtA +) PILs. These binary mixtures are structured on two distinct length scales: one associated with the self-assembled n-alkanol aggregates and the other with the underlying liquid nanostructure. This suggests that EA + and PA+ enable n-alkanol aggregation by acting as cosurfactants, which EtA+ cannot do because its terminating hydroxyl renders the cation nonamphiphilic. The primary determining factor for miscibility and self-assembly is the ratio of alkyl chain lengths of the alkanol and PIL cation, modulated by the anion type. These results show how ILs can support the self-assembly of nontraditional amphiphiles and enable the creation of new forms of soft matter. © 2014 American Chemical Society.