Adsorption of anionic surfactant sodium dodecyl sulfate onto alpha alumina with small surface area

Adsorption of sodium dodecyl sulfate (SDS) onto large α-alumina beads was systematically analyzed as functions of pH and NaCl concentration. The maximum adsorption density of SDS onto α-Al₂O₃ increases with decreasing pH due to the increase in equilibrium surface charge. Adsorption isotherms at different salt concentrations demonstrated a common intersection point (CIP) corresponding to charge neutralization. The obtained comprehensive data clarified the applicability of two-step adsorption and four-region models to describe the adsorption isotherms of SDS onto α-Al₂O₃ beads. Also, proton adsorption isotherms upon SDS uptake onto α-Al₂O₃ can be fitted by two-step adsorption model with almost the same parameters of surfactant adsorption for 0.001 and 0.01 M NaCl. Our finding indicates that proton adsorption takes place onto the adsorbed SDS. After passing through CIP, proton adsorption onto α-Al₂O₃ increases at low ionic strength while the amount of proton adsorption does not change for 0.1 M NaCl. Adsorption amount of proton as a function of SDS adsorption shows that only hemimicelles are plausible for case of low ionic strength. The increase in the proton adsorption is not significant at high ionic strength, suggesting the presence of admicelles on the surface of α-Al₂O₃ beads.