Generic Nature of Interfacial Phenomena in Solutions of Nonionic Hydrotropes

Abstract

Nonionic hydrotropes (low-molecular-weight amphiphiles) demonstrate striking dual actions in bulk solutions and interfaces, exhibiting both surfactant-like and co-solvent properties. We report on peculiar, strongly affected by this duality, liquid–liquid and air–liquid–liquid interfacial behavior in aqueous ternary systems, containing hydrotropes and hydrocarbons, in a broad range of compositions and at various temperatures. Phase diagrams of the studied systems, containing tertiary butanol (TBA), as a hydrotrope, are of Type 1: the hydrotrope, at the experimental conditions, is completely miscible with water and with all investigated hydrocarbons [cyclohexane (CHX), toluene (TOL), and n-decane (DEC)], whereas the ternary mixtures exhibit liquid–liquid phase separation terminated at corresponding critical points. The shape and location of the phase separation boundary are only weakly dependent on temperature and the hydrocarbon’s nature; however, the critical point in the water–TBA–DEC system is significantly shifted toward a higher TBA concentration. For the experimentally studied systems and for available data reported in the literature, we confirmed an apparently generic (for nonionic hydrotropes) phenomenon of a dual action at water–oil interfaces (earlier found in water–TBA–CHX [J. Phys. Chem. C 2017, 121, 16423]): at low concentrations, hydrotropes saturate the water–oil interface like a surfactant, whereas at higher concentrations they act as co-solvents, resulting in vanishing interfacial tension at the liquid–liquid critical point. We suggest a universal crossover function that accurately interpolates the two theoretically based limits of interfacial behavior. This crossover function also accounts for earlier deviations from Langmuir–von Szyszkowski limiting behavior in the water–TBA–DEC system, caused by lower solubility (relative to other studied hydrocarbons) of DEC in water. An intriguing correlation between the dual action of hydrotropes at the water–oil interface and the behavior of the liquid–air interfaces is also discussed.

Dataset for the interfacial tension and phase properties of the ternary system water – 2-butoxyethanol – toluene

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Fig. 2 (a) Binodal curves at 293.15 K for ternary systems of H 2 O–TBA–TOL [6] and H 2 O–BEG–TOL (this work) approximated by 9th order polynomial; (b) fitting residuals; (Y is mass fraction of hydrotrope ω h , ); (a) circles and hexagons are experimental data (titration); blue and green curves are polynomial approximations. Data are from Table 2 (X is from Column II, Y is from Column III, residuals Y are from Column IV).

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Fig. 3 Phase diagrams at 293.15 K of the systems W–BEG–TOL (this work) and W–TBA–TOL [6] in mole percentage; circles are experimental points (titration), color lines are 9th order polynomial approximations; red filled markers denote critical points; blue-filled area corresponds to the two-phase region in the W–BEG–TOL system. Data on W–BEG–TOL system are from Table 1 ( x H 2 O is from Column XI; x BEG is from Column XII; x Tol is from Column XIII).

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Fig. 5 Density difference of the equilibrated phases in the systems W–BEG–TOL (this work) and W–TBA–TOL [6] versus the equilibrium hydrotrope mole fraction in water phase at 293.15 K; red filled symbols are critical points, vertical color dashed lines are critical mole fractions of hydrotropes, red dotted lines are linear approximations of density difference for near-critical samples of W–BEG–TOL and W–TBA–TOL. Data on W–BEG–TOL system are from Tables 3 , 4 ( X BEG in water phase is from Table 3 , Column VIII; ∆ ρ is from Table 4 , Column XVI).

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Fig. 6 Interfacial tension isotherms at 293.15 K for the W–BEG–TOL (this work) and W–TBA–TOL [6] systems versus the equilibrium hydrotrope mass fraction in water phase; the symbols are experimental data (green pentagons for TBA, blue hexagons for BEG), the dotted curves are crossover approximations [4] , the vertical dashed lines indicate the critical hydrotrope concentrations. Data on W–BEG–TOL system are from Table 3 ( ω BEG in water phase is from Column X; γ / γ 0 is from Column XII).

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“… (a) Binodal curves at 293.15 K for ternary systems of H 2 O–TBA–TOL [6] and H 2 O–BEG–TOL (this work) approximated by 9th order polynomial; (b) fitting residuals; …”