Crossover from preferential adsorption to depletion: Aqueous systems of short-chain CnEm amphiphiles at liquid/liquid interfaces

Using ellipsometry we investigate the influence of a gradual change in the amphiphilicity of short-chain oligo-ethyleneglycol monoalkyl ethers (CnEm) on critical adsorption and incipient wetting of an inert liquid perfluoroal kane (F) substrate by the aqueous amphiphile phase (AW) near its upper misc ibility gap. For the shortest molecule, i-C4E1, a pronounced increase of th e adsorption of the amphiphile is observed as the phase separation temperat ure TB is approached at concentrations w below the critical composition w(c ), indicating incipient wetting of the W/F interface by the amphiphile-rich phase A. For the longer-chain molecules C5E2 and C6E3, an opposite behavio r, viz., increasing adsorption of water, is found for T --> T-B for concent rated mixtures (w greater than or equal to w(c)), indicating incipient wett ing of the A/F interface by the water-rich phase W. This crossover from pre ferential adsorption to depletion is attributed to an autophobic effect of the oriented monolayer of the longer-chain CnEm molecules, which is expecte d to be absent for short-chain molecules with a globular tail like i-C4E1. The depletion effect also disappears when perfluoroheptane is replaced with octane as the substrate phase. We discuss these results in terms of critic al adsorption and incipient wetting effects and provide complementary infor mation from interfacial tension measurements.