Optical studies of liquid interfaces in amphiphilic systems: how wetting and adsorption are modified by surfactant aggregation

Abstract

The free liquid surface and the liquid/liquid interface of water + amphiphile systems with an upper miscibility gap has been studied by ellipsometry in a composition and temperature range around the lower critical solution point. The ellipticity of both the liquid/gas ( l/g) and the liquid/liquid ( l/l) interface exhibits a pronounced temperature dependence in this region and has been analysed in terms of scaling theories for the intrinsic composition profile of semicritical and critical interfaces, respectively, and the interface roughness due to capillary waves. According to these theories the width of the interfacial region scales with the correlation length ξ of critical-point composition fluctuations. The temperature dependence of ξ and further data needed for this analysis (surface tension, interfacial tensions and properties of the bulk phases) were determined parallel to the ellipsometric measurements. In aqueous surfactant systems the formation of micelles and of oriented layers of surfactant molecules at interfaces have a pronounced influence on the structure of the entire interfacial region. This article analyses these influences by comparing water + amphiphile systems with increasing degree of molecular shape anisotropy of the amphiphilic component, from small, nearly globular molecules as sec- and tert-butanol to straight-chain oxyethylene alkyi ethers (C 4E 1 and c 10E 4). The influence of added hydrocarbon on the surface of the C 4E 1 + water system, and the structure of the l/l interfaces of the ternary system C 4E 1+ water + octane in the temperature range of three-phase coexistence (where the interfaces of the water- and hydrocarbon-rich phases against the amphiphile-rich middle phase transform gradually from critical to semicritical) has also been studied. Shape anisotropy of C nE m molecules induces a remarkable change of the surface structure of the aqueous solution and a cross-over of the wetting behaviour of the free liquid surface: A water-rich layer develops below the adsorbed monolayer of C nE m molecules at the surface of concentrated aqueous solutions of the amphiphilic compound on approaching the region of liquid-liquid immiscibility. This behaviour is opposite to that observed in the case of weakly anisotropic amphiphilic molecules as sec-/tert-butanol, which exhibit multilay- er adsorption of the amphiphilic compound at the surface of dilute aqueous solutions near two-phase coexistence. The formation of a water-rich prewetting layer is attributed to the hydration of the surfactant head groups which causes a repulsion between micelles and the adsorbed surfactant monolayer. The l/l-interface of simple systems including C 4E 1+water can be represented by a Fisk-Widom intrinsic profile plus a capillary-wave contribution. For the C 10E 4 + water system there are indications that a layer of surfactant is forming in the interface between the two aqueous phases further away from the critical solution temperature. From the low rigidity of this surfactant layer it is inferred that it may consist of micellar aggregates.