Self-Assembly of Cations in Aqueous Solutions of Hydroxyl-Functionalized Ionic Liquids: Molecular Dynamics Studies

The effect of presence of a hydroxyl-functionalized alkyl chain of varying carbon number on the self-assembly of cations in aqueous solutions of 1-(n-hydroxyalkyl)-3-decylimidazolium bromide (where the alkyl groups are ethyl, butyl, heptyl, and decyl) has been studied using atomistic molecular dynamics simulations. Spontaneous self-assembly of cations to form aggregates with hydrophobic core and hydrophilic surface is observed. The shape of the aggregates changes from quasispherical in the case of cations with hydroxyheptyl or smaller substituent chain, to a thin film like intercalated aggregate in the case of cations with hydroxydecyl chain. Cations with hydroxydecyl substituent chain exhibit long-range spatial correlations, and the anions are associated with cations to a greater extent due to the higher surface charge density of the aggregate. The ordered film like aggregate is stabilized by the dispersion interactions between the intercalated substituent chains and the intermolecular hydrogen bonds formed between the alkoxy oxygen atoms and the hydrogen atoms of the imidazolium ring. The cations form less compact aggregates with lower aggregation number than their nonhydroxyl analogues in the corresponding aqueous solutions. The intracationic and aggregate structures are governed by the length of the hydroxyalkyl chain.