A surface equation of state is derived and used to interpret the liquid-expanded (LE)−liquid-condensed (LC) phase transition. The monolayer is treated as a two-dimensional mixture consisting of LC domains, disordered molecules in the LE state, and free sites. The free sites are assumed to have two-dimensional size only and to be located at the internal surface between the headgroups and the tails of the surfactant. They are introduced to account for the translational freedom of the domains and of the disordered molecules on the surface. The model can explain the nonhorizontal LE/LC phase transition and its dependence on temperature. In addition, the equilibrium radius and equilibrium area fraction of the domains are calculated as a function of the average molecular surface area (A). In the absence of domains, the surface equation of state reduces to a modified two-dimensional van der Waals equation. The dependence of the surface cohesion pressure on the molecular surface area A is calculated by including three contributions:  the dipole−dipole interactions, the chain van der Waals interactions, and the chain conformation. The theoretical model is compared with the experimental π− A isotherms for six phospholipid surfactants, and good agreement is obtained.

No abstract

A model for the adsorption of ionic surfactants on an oppositely charged solid surface of uniform charge density is developed. The model is based on the consideration that, on the solid surface, adsorbed surfactant monomers, monolayered and bilayered surfactant aggregates of various sizes, and "empty sites" constitute a nonideal two-dimensional mixture. The competition between the entropic and enthalpic contributions to the free energy is responsible for the formation of the monolayered or bilayered surfactant aggregates. The nonideality is treated via the Flory-Huggins equation extended to a two-dimensional solution. The standard free energy change is calculated by considering five contributions: the hydrophobic effect and the conformational change of the surfactant tails, the electrostatic and steric interactions due to the surfactant headgroups, and the surfactant tails-water interfacial free energy. The electrostatic effects are treated in the framework of the Poisson-Boltzmann equation. The model, which does not include empirical parameters, is compared with the experimental adsorption isotherms for three sodium alkylbenzenesulfonates on alumina and kaolinite and for sodium dodecyl sulfate on alumina.

No abstract

No abstract