Thermodynamic model of surfactant adsorption on soft liquid crystal interfaces.

The Gibbs adsorption isotherm for planar liquid crystal/fluid interfaces is derived using the anisotropic Gibbs-Duhem equation. The Gibbs adsorption isotherm for planar interfaces is used to analyze the adsorption-driven orientation transition in aqueous solutions of anionic surfactants in contact with rodlike uniaxial nematic liquid crystal films. In qualitative agreement with experiments, the model predicts that, as the surfactant concentration increases, the tangential (planar) average molecular orientation of the liquid crystal with respect to the interface undergoes a transition to a normal (homeotropic) orientation. The anchoring coefficient or strength of anisotropic component of the interfacial tension is shown to depend on the surfactant's concentration. Analyzing the response to addition of a co-cation, the model reveals that, as the fractional coverage of the surfactant's chains increases, the interpenetration of liquid crystal molecules between the adsorbed surfactant tails promotes the orientation transition; at even higher surfactant chain concentrations, interpenetration is hindered because of lack of available space and a random surface orientation emerges. Thus, for aqueous surfactant solutions in contact with nematic liquid crystals, increasing the surfactant concentration leads to the following interfacial liquid crystal orientation transition cascade, planar orientation --> homeotropic orientation --> random orientation, which can lead to new sensor capabilities and surface structuring processes.