Self-organization and solubilization properties of gemini hydrotropic compounds in aqueous solution

Abstract

A series of amidoamine derivatives composed of two amphiphilic units linked with alkyl spacers varying in length was synthesized. The amphiphilic unit consisted of a quaternary ethylammonium headgroup, 14 carbon atoms in the tail and an amide group placed in-between. The aggregation properties of the gemini compounds were examined by tensiometric, conductometric and spectroscopic methods and compared with the properties of their single-tail analogue. In spite of structural relation to gemini surfactants, some significant changes in solution behaviour, in comparison to typical surfactants and previously described gemini surfactants, were found. Most of the compounds within the obtained series aggregated to very small associates rather than to the typical micelles formed by gemini amphiphiles at very low concentrations. The size of aggregates obtained from dynamic light scattering measurements was confirmed by cryo-TEM visualization of the studied assemblies which contained only a few molecules. The size of aggregates and very low aggregation number indicated similarity of the studied compounds to hydrotropes which, despite their amphiphilic structure, do not form micelles. The influence of spacer length on the solution behaviour of geminis was discussed. The solubilization studies of Sudan I showed that the solubility of the hydrophobic dye in water is high at very small concentrations of the studied compounds, significantly smaller than in the case of their single-tail analogue.