Adsorption of Cationic and Anionic Surfactants on Charged Metal Oxide Surfaces

Abstract

The adsorption of cationic (alkyl pyridinium salts) and anionic (alkyl benzene sulphonates) surfactants on the mineral oxide rutile is studied as a function of surfactant concentration, surfactant chain length, solution pH, and ionic strength in solution. Experimental results are complemented with model calculations based on the self-consistent (mean) field lattice theory for adsorption and association (SCFA). In the theory no ad hoc assumptions are made with respect to the adsorbed layer structure, but local aggregates cannot be considered. Both the experiment and theory show that the adsorption isotherms, plotted as the log of the adsorption versus the log of the equilibrium concentration, can be divided into four characteristic regions. This behavior has been found previously for anionic surfactants adsorbed on metal oxides, but for cationic surfactants on metal oxides this is a novelty. The four-region isotherm vanishes at pH values close to the point of zero charge of the oxide and when the background electrolyte concentration is on the order of 0.1 M. In region I, isolated molecules adsorb. In region II, hemi-micelles, defined as aggregates of "head-on" adsorbed surfactant molecules, are formed. In region III, the hemi-micelles transform into ad-micelles, defined as aggregates of both "head-on" and "head-out" adsorbed surfactant molecules. Region IV occurs once the CMC is reached. The transition from hemi-micelles to ad-micelles is highlighted by an (approximately) common intersection point between isotherms measured at different salt concentrations. Salt concentration and pH govern the Coulombic interactions; their effect observed in praxis is also found with the model calculations. In addition to the similarity in adsorption behavior of the cationic and the anionic surfactant on oppositely charged rutile, specific differences resulting from the different head groups are discussed.