Kinetics and isotherms of adsorption of alkyldimethylamine oxide on a porous hydrophobic polymer: A thermodynamic approach to a Langmuir-like phenomenon

We studied the adsorption of decyl-, dodecyl- and tetradecyldimethylamine oxide surfactants on porous beads of polystyrene divinylbenzene (Bio-beads SM2). The concentration of free surfactant is followed in situ using an ion-selective electrode. Isotherms for the three surfactants were determined using a batch method at 25 °C. We tested various parameters for their influence on the adsorption kinetics: initial surfactant concentration (below and above the critical micellar concentration (CMC)), initial quantity of bead, temperature and surfactant chain length. The behaviour can be modelled using a generalised Langmuir kinetics approach that assumes systems do not possess adsorption sites. We used the same set of parameters for the isotherm and kinetics results. These parameters are linked to the parameters from the Langmuir equation and we discuss the validity of the model. Although the proposed model does not require any adsorption sites, the results from the model are similar to the Langmuir approach. Adsorption is shown to be due to hydrophobic interactions.