Adsorption of Cationic and Anionic Surfactants on Metal Oxide Surfaces: Surface Charge Adjustment and Competition Effects

Abstract

The adsorption of cationic and anionic surfactants on rutile is studied as a function of surfactant concentration and ionic strength in solution. The pH is kept constant for each series of isotherms. Special attention is given to the extent of adaptation of the surface charge as a function of surfactant adsorption. Experimental results are complemented with model calculations based on the self-consistent (mean) field lattice theory for adsorption and association (SCFA) to confirm the correctness of the given interpretation. The adsorption behavior of sodium nonyl benzene sulfonate (SNBS) and dodecyl or tetradecyl pyridinium chloride (DPC/TPC) on rutile is broadly similar. Isotherms can be divided in two regions by the (approximately) common intersection point (cip) between isotherms measured at different salt concentrations. Before the cip adsorption occurs head-on (hemi-micelles); after the cip both head-on and head-out adsorption (ad-micelles) occur. Intersection points similar to the cip occur in the proton excess and the net surface charge/surfactant charge plots. Although broadly similar in their adsorption behavior, the SNBS and DPC/TPC systems differ markedly if the charge situation near the interface is considered. SNBS follows simple rules; the strongly adsorbing SNBS head groups can compete easily with the salt ions for the surface sites. The point where the surface charge and the surfactant charge balance, the equivalence point, and the cip in the isotherms coincide. Moreover, the cip corresponds with the isoelectric point (iep). The DPC and TPC systems behave in a more complicated way, due to their relatively weak interaction with the surface. The cip in the isotherms and the cip in the net surface/surfactant charge plots occur at much lower adsorption values than the equivalence point. Co-ions thus contribute at the cip to the charge balance in the surface region. In this situation the iep will occur at lower adsorption values than the cip.