Interfacial tensions in simple oil-water-surfactant systems near their three-phase region

Abstract

Measurements are reported of the interfacial tension in two-phase ternary systems of water(1) + hydrocarbon(2) + 2-butoxyethanol(3) atT=25°C along a linear path to the critical point (plait point). The two systems studied (with heptane and octane as the hydrocarbon component) exhibit a three-phase region and differ in respect of the distance of the upper critical end pointT _u of this three-phase region from the experimental temperature (T>T _u). Along the chosen linear path, a stepwise decrease of the tensionσ with increasing concentration of the amphiphile,ω ₃, is found: in region (i)σ decreases from its initial value (50 mN m⁻¹) to about 2 mN m⁻¹ due to the adsorption of the amphiphile; in region (ii) which extends over a wide composition range of the upper phase,σ exhibits an inflection point and is only a weak function ofw ₃; on approaching the critical point there is a second inflection point inσ vs.w ₃ and finally, in a rather narrow range of concentration (w ^c₃ -w ₃), a follows the asymptotic critical-point scaling (region iii). In region (ii), the interfacial tension of the octane system (for whichT-T _u is only about 1 K) falls below that of the heptane system and remains nearly constant, as expected for systems near three-phase coexistence.