ADSORPTION OF IONIC SURFACTANTS ON VARIABLE-CHARGE SURFACES .2. MOLECULAR ARCHITECTURE AND STRUCTURE OF THE ADSORBED LAYER

The adsorption of alkylbenzenesulfonates on rutile is studied as a function of chain length and branching. The results are compared with theoretical predictions using a self-consistent field lattice theory for adsorption and or association. Both experiment and theory show that the adsorption increases with chain length and decreases if the point of attachment of the aliphatic chain to the hydrophilic part of the molecule is shifted from a terminal to a medial position. The measured and calculated results agree well with each other in the initial part and the upper part of the isotherm. Around the cmc the adsorbed layer resembles a bilayer membrane. In the intermediate part of the isotherm the predicted association is too strong, which is due to the neglect of inhomogeneities parallel to the surface. A simple model is proposed to estimate the average size of the adsorbed surfactant aggregates in this region. Branching decreases the aggregation number whereas an increase in chain length increases this number.