Modeling Effects of pH and Counterions on Surfactant Adsorption at the Oxide/Water Interface

Abstract

A model is presented that describes the effect of pH on the adsorption of an isomerically pure anionic surfactant species at a mineral oxide/water interface. A site-binding model, to account for effects of pH, surface heterogeneities, and counterions, is incorporated into a patchwise, phase-separation modeling approach, making it possible to predict both the surface charge and the counterion association beneath an adsorbed surfactant aggregate. Parameters for the site binding model on α-alumina are obtained from experimental surface charge measurements. The formation of both local monolayers (hemimicelles) and bilayers (admicelles) is allowed, although the isotherms studied in this paper are fit by parameter values that predict admicelle formation only. The model is able to predict experimental measurements of the adsorption of an isomerically pure, anionic surfactant species on α-alumina as a function of pH. It reproduces several previously unexplained experimental observations; in particular, it offers an explanation for the observation of significant adsorption of anionic surfactant above the point of zero charge (pzc) of a mineral oxide surface.

Adsorption of sodium dodecyl sulfate (SDS) at ZnSe and α-Fe2O3 surfaces: Combining infrared spectroscopy and batch uptake studies

“…Below the SDS cmc and the hematite pzc, adsorption is dominated by electrostatic interactions between the negatively charged sulfonate head and the positively charged hematite surface hydroxyls. In contrast to the batch adsorption results, substantial FTIR signal remains even at pH 9.0 (above the pzc of a-Fe 2 O 3 ), likely reflecting adsorption via Na + counterion bridging [30]. In addition, the porous hematite film present in the FTIR experiments, but not in the batch studies, could promote surface-induced formation of hemimicelles or admicelles well below the cmc in the film mesopores.…”

Interactions in Calcium Oxalate Hydrate/Surfactant Systems

“…The subsequent rapid increase of the adsorption at higher surfactant concentrations is generally explained by surfactant aggregation at the solid-liquid interface involving mechanisms similar to those causing the formation of micelles in aqueous solutions, i.e., hydrophobic tail-tail interactions. Finally the leveling off of adsorption starting at the CMC is ascribed to a chemical potential sink caused by the presence of micelles in the bulk solution (24,26).…”

Adsorption of Sodiumn-Decylbenzenesulfonate onto Alumina: Relationships between Macro- and Microscopic Behavior

“…Region 3 for this system showed a smaller slope compared to isotherms obtained with similar surfactants (2,5,8). Although the reason for this was not clear, the acidic solution (pH 4.0) may be related to the isotherm shape (2, 8,12,32). Above the CMC, Region 4 leads to the saturation adsorption amount ca.…”

“…The reverse orientation model is derived from experimental data of adsorption isotherms, electrophoresis, and spectroscopy (5,6). The bilayer model is based mainly on thermodynamic theory with an emphasis on counter ion binding (9,11,12). The reverse orientation model predicts monolayer formation followed by the establishment of bilayers.…”