A surface with photocontrollable wetting behavior is introduced. A monolayer of a polymeric material containing 4‘-[trifluormethoxy-4,4‘-dibenzoazo] dyes in the side chains has been transferred on quartz slides and silicon wafer. The azobenzene chromophore possesses two distinct isomers, cis and trans. Transition between these states can be triggered by illumination with light of two different wavelengths. It will be demonstrated that with the use of light and a mask, fine cis−trans patterns on the order of micrometers can be written in the monolayer. The corresponding interface exhibits different wetting behavior. This is visualized by a surface decoration with water droplets. The formation of water microdroplets on the patterned monolayer can be controlled by light. Writing and erasing of patterns is completely reversible. The system has potential for studying wetting behavior on microstructured surfaces.

Equilibrium surface tension (σ) versus bulk concentration (c) isotherms of aqueous, surface-chemically pure solutions of various alkali perfluoro-n-octanoates were measured at 295 K. These 1:1 ionic surfactant systems belong to the pseudo nonionic ones. Evaluating the different σ vs log c isotherms by basic adsorption equations reveals that they follow ideal surface behavior. The novelty of this investigation exists in the fact that the surface area demand per molecule adsorbed calculated from the experimental σ vs log c isotherms is identical to that of the hydrated alkali cation. Thus, as long as the counterion's cross-sectional area is greater than that of its amphiphilic anion, the amphiphile's total surface area demand will exclusively be governed by that of its alkali counterion. This, in turn, means that the counterion is nonrandomly bound to the amphiphilic anion in the adsorption layer. Furthermore, the size of the hydrated alkali counterion in the adsorption layer does not differ from that in the bulk phase.

The unusual behavior of saturation adsorption calculated from experimental equilibrium surface tension (σ(e)) versus logarithm of concentration (c) isotherms within the homologous series of aqueous sodium perfluoro-n-alkanoate solutions represents a particular problem in the adsorption of homologous ionic 1:1 amphiphiles at fluid interfaces. Special precautions were taken to guarantee surface-chemical purity for all solutions, avoiding falsifying effects by surface-active trace impurities. Surprisingly, all homologues' adsorption isotherms reveal ideal surface behavior. The minimal surface area demand per molecule adsorbed for shorter-chain homologues slightly decreases with increasing chain lengths but then goes up steeply after having passed a minimum. A similar feature has been observed with the chemically quite different homologous series of the hydrocarbon surfactants of sodium-n-alkylsulfates. Comparing the corresponding 3D saturation concentrations in the boundary layer and in the bulk, it becomes evident that at high bulk concentrations when boundary layer and bulk concentrations are of the same order of magnitude the adsorption behavior may be treated as that of a pseudononionic surfactant. However, under conditions of the homologues' strongest surface activity, adsorption seems to become increasingly governed by electrostatic repulsion, resulting in increasingly greater cross-sectional areas. Deviation from pseudononionic behavior sets in when the Debye length becomes distinctly greater than the adsorbent's diameter at saturation. Formerly available theories on ionic amphiphiles' adsorption deal either with electrical conditions of surfactant ions and counterions in the adsorption boundary layer or alternatively with pseudononionic behavior neglecting the former theories completely. Warszynski et al.'s novel theoretical model of the "surface quasi-two-dimensional electrolyte" seems to be capable of describing the adsorption of ionic amphiphiles at fluid interfaces in general. We conclude that the conditions of the two alternative approaches may be met within homologous series of ionic amphiphiles as limiting cases only.

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