Interfacial tension and phase properties of water – hydrotrope – oil solutions: water – 2-butoxyethanol – toluene

Abstract Hydrotropes, the substances consisting of small amphiphilic molecules, demonstrate striking duality in aqueous solutions and at the oil-water interface. The question arises, how do specific properties of different hydrotropes affect their generic dual actions? A higher hydrophobicity of 2-butoxyethanol (BEG), with respect to the reference hydrotrope, tert-butanol (TBA), is responsible for the difference in their behavior. The higher hydrophobicity increases the adsorption of hydrotropes at the oil-water interface in the dilute regime and the oil-water co-solvency at higher concentrations of hydrotropes. We investigated phase equilibrium and interfacial phenomena of the system water – BEG – toluene. For the equilibrium phases, we determined the density, compositions, viscosity, and interfacial tension. The critical point in the ternary system water – BEG – toluene is shifted towards the water vertex due to the higher hydrophobicity of BEG when compared with a “perfect amphiphile,” TBA. However, this shift alone does not explain the differences between the properties of the ternary systems containing either BEG or TBA. The higher adsorption of BEG results in the earlier decrease of interfacial tension and phase density difference. The dramatic decrease of the interfacial tension upon the approach to the consolute critical point, observed in both systems, demonstrates a high degree of universality after proper rescaling of the hydrotrope concentration. The viscosity in the oil phase increases due to the higher solubilization ability of BEG; the effect is attributed to the formation of hydrogen bonds with water.