Self-assembly in aqueous solutions of a non-ionic hydrotrope

Abstract

Hydrotropes are amphiphilic molecules, too small to cause spontaneous self-assembly towards equilibrium mesoscale structures in aqueous solutions, but they form dynamic, noncovalent assemblies, which may create microscopic regions of lowered polarity. This enhances the solubilization of hydrophobic compounds, also known as solubilizates, in aqueous solutions and may cause further aggregation to larger structures. In this work, unusual mesoscopic properties of aqueous solutions of a non-ionic hydrotrope, namely tertiary butyl alcohol (TBA) have been investigated by light scattering, microscopy, and chromatography. Aqueous TBA solutions show anomalous thermodynamic and structural properties in the range of concentrations 3-8 mol % TBA and temperatures 0 - 25 °C. These anomalies appear to be associated with short-lived, short-ranged micelle-like structural fluctuations, distinctly different from usual concentration fluctuations in non-ideal solutions. Molecular dynamics simulations and neutron-scattering experiments show clustering of TBA molecules on a nanometer scale, interacting through hydrogen bonds with a shell of water molecules. In this concentration range, TBA aqueous solutions, although macroscopically homogeneous, occasionally show the presence of "mysterious" inhomogeneities on a 100 nm scale. We have found that the emergence of such inhomogeneities strongly correlates with impurities present in commercial TBA samples. Experiments with controlled addition of a third component, such as propylene oxide, isobutyl alcohol, or cyclohexane, reveal the mechanism of formation of these inhomogeneities through stabilization of micelle-like fluctuations by a solubilizate. These structures are long-lived, i.e., stable from a few days up to many months. We have confirmed that mesoscale structures in aqueous solutions can be generated from self-assembly of small molecules, without involvement of surfactants or polymers. This kind of self-assembly may potentially result in the development of novel nanomaterials.