Forces between Crystalline Alumina (Sapphire) Surfaces in Aqueous Sodium Dodecyl Sulfate Surfactant Solutions

Abstract

Using the surface forces apparatus, we have studied the adsorption of negatively charged sodium dodecyl sulfate (SDS) surfactant onto positively charged surfaces of Al2O3 and the resulting interactions between these surfaces in aqueous solutions. The adsorbed layer thicknesses, adhesion forces, and long range colloidal interactions were measured at SDS concentrations from 0.01 to 5 mM (below the critical micelle concentration, cmc = 8 mM). Our results show that an SDS bilayer of thickness ∼3.2 nm forms at bulk concentrations above 1 mM (≥1/10(cmc)) and that beyond bilayer−bilayer contact the measured forces are well described by the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (with a constant surface potential at low surfactant coverage and constant surface charge density at high coverage). The lack of any hydrophobic monolayer formation well below the cmc was noticed in the force measurements, confirmed indirectly by wettability studies, and attributed to the low surface charge density on the sapphire basal plane (52.7 nm2 per charge). With one exception, the results are in good agreement with previous data on similar systems obtained using other techniques. The results clarify how electrostatic binding interactions determine the stepped adsorption of, and transitions between, hydrophobic and hydrophilic surfactant layers. The results also provide a basis for understanding why certain surfactants (under appropriate solution conditions) can be used as effective additives in the colloidal processing of ceramic materials.