Mikhail A. Anisimov
Department of Chemical & Biomolecular Engineering and Institute for Physical Science & Technology, University of Maryland, College Park, MD 20742,USA
Hydrotropes are substances containing small amphiphilic molecules, which manifest a striking duality in bulk water and at water/oil interface. Typical examples of nonionic hydrotropes with well-balanced amphiphilicity are tertiary butanol and 3-methyl pyridine. Hydrotropes do not form stable micelles in aqueous solutions, but they may form reversible dynamic clusters (less or about 1 ns lifetime) with water molecules; such clusters can be viewed as “pre-micelles” or as fluctuations of aqueous solution structure. Molecular dynamics simulations demonstrate that the origin of the clustering is the formation of strong hydrogen bonds between the water and nonionic hydrotrope molecules. This molecular clustering causes pronounced thermodynamic anomalies, experimentally observed in dilute and semi-dilute aqueous solutions of nonionic hydrotropes. In particular, the isobaric heat capacity, the measure of entropy fluctuations, exhibits a lambda-like cusp in semi dilute aqueous solutions of tertiary butanol, resembling a second-order phase-transition anomaly. Most recently, experimental and simulation studies of water-hydrotrope-oil ternary solutions have revealed remarkable consequences of the duality of hydrotropes. In bulk, a spectacular phenomenon, “mesoscale solubilization”, intermediate between molecular solubility and macroscopic phase separation, is observed. It has been proved that mesoscale solubilization is a kinetically stabilized phenomenon, though extremely long-lived. Making practically stable colloids from small hydrotrope molecules has an untapped potential for applications in material science and medicine. At water/oil interface, a nonionic hydrotrope at small concentrations (prior the region of pronounced structural fluctuations) acts as a surfactant, while at higher concentrations (after the interface is saturated by the hydrotrope) it acts as a co-solvent toward complete water/oil miscibility. Crossover between these limits is described by a universal function interpolating the Gibbs-Langmuir adsorption theory and scaling theory of a smooth near-critical interface.