| Abstract
| - A model for the adsorption of ionic surfactants on an oppositelycharged solid surface of uniform chargedensity is developed. The model is based on the considerationthat, on the solid surface, adsorbed surfactantmonomers, monolayered and bilayered surfactant aggregates of varioussizes, and “empty sites” constitutea nonideal two-dimensional mixture. The competition between theentropic and enthalpic contributionsto the free energy is responsible for the formation of the monolayeredor bilayered surfactant aggregates.The nonideality is treated via the Flory−Huggins equationextended to a two-dimensional solution. Thestandard free energy change is calculated by considering fivecontributions: the hydrophobic effect andthe conformational change of the surfactant tails, the electrostaticand steric interactions due to thesurfactant headgroups, and the surfactant tails−water interfacialfree energy. The electrostatic effectsare treated in the framework of the Poisson−Boltzmann equation.The model, which does not includeempirical parameters, is compared with the experimental adsorptionisotherms for three sodiumalkylbenzenesulfonates on alumina and kaolinite and for sodium dodecylsulfate on alumina.
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