Hydrotropes are small amphiphilic molecules that disrupt the interface between two liquids. Due to their nature, they behave as surfactants or co-solvents depending on the concentration in which they are present in solution. We compare the results of molecular dynamics simulations of water-cyclohexane systems in the presence of <i>tert</i>-butanol (TBA), a typical hydrotrope, with experimental measurements of surface tension. The experiments were performed using Wilhelmy plate and spinning drop methods for systems with the same overall concentration of hydrotrope as the simulations. We observed a rapid decrease in surface tension at very low hydrotrope concentrations (&lt; 0.7 TBA mol% in water). Moreover, a clear crossover of hydrotrope behavior is demonstrated through both simulation and experiment. At low concentrations, TBA behaves as a surfactant, adsorbing to the interface and lowering the surface tension. However, as more TBA is added to the solution, it becomes a co-solvent and exhibits critical-like behavior in vanishing the surface tension. Finally, we examined the thickness of the interface and its correlation with surface tension as described by van der Waals theory of smooth interfaces. Simulations evaluated this variable for systems with low to moderate hydrotrope concentration; while dynamic light scattering measurements examined systems with hydrotrope concentrations close to the critical point of the ternary mixture. We show the correlation between the inverse of the thickness and the square root of the reduced surface tension is linear both near the critical point of the mixture as well as far from it. The conclusions of this work can be extended to understand the behavior of other hydrotropes given their relevance in chemistry and biology.

Interfacial behavior of hydrotropes in aqueous solutions

254th National Meeting (2017)

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