Mesoscale inhomogeneities in aqueous solutions of small amphiphilic molecules

Abstract

Small amphiphilic molecules, also known as hydrotropes, are too small to form micelles in aqueous solutions. However, aqueous solutions of nonionic hydrotropes show the presence of a dynamic, loose, non-covalent clustering in the water-rich region. This clustering can be viewed as “micelle-like structural fluctuations”. Although these fluctuations are short ranged (∼1 nm) and short lived (10 ps–50 ps), they may lead to thermodynamic anomalies. In addition, many experiments on aqueous solutions of hydrotropes show the occasional presence of mesoscale (∼100 nm) inhomogeneities. We have combined results obtained from molecular dynamics simulations, small-angle neutron scattering, and dynamic light-scattering experiments carried out on tertiary butyl alcohol (hydrotrope)–water solutions and on tertiary butyl alcohol–water–cyclohexane (hydrophobe) solutions to elucidate the nature and structure of these inhomogeneities. We have shown that stable mesoscale inhomogeneities occur in aqueous solutions of nonionic hydrotropes only when the solution contains a third, more hydrophobic, component. Moreover, these inhomogeneities exist in ternary systems only in the concentration range where structural fluctuations and thermodynamic anomalies are observed in the binary water–hydrotrope solutions. Addition of a hydrophobe seems to stabilize the water–hydrotrope structural fluctuations, and leads to the formation of larger (mesoscopic) droplets. The structure of these mesoscopic droplets is such that they have a hydrophobe-rich core, surrounded by a hydrogen-bonded shell of water and hydrotrope molecules. These droplets can be extremely long-lived, being stable for over a year. We refer to the phenomenon of formation of mesoscopic droplets in aqueous solutions of nonionic hydrotropes containing hydrophobes, as mesoscale solubilization. This phenomenon may represent a ubiquitous feature of nonionic hydrotropes that exhibit clustering in water, and may have important practical applications in areas, such as drug delivery, where the replacement of traditional surfactants may be necessary.