FORCES BETWEEN CRYSTALLINE ALUMINA (SAPPHIRE) SURFACES IN AQUEOUS SODIUM DODECYL-SULFATE SURFACTANT SOLUTIONS

Using the surface forces apparatus, we have studied the adsorption of negatively charged sodium dodecyl sulfate (SDS) surfactant onto positi vely charged surfaces of Al2O3 and the resulting interactions between these surfaces in aqueous solutions. The adsorbed layer thicknesses, a dhesion forces, and long range colloidal interactions were measured at SDS concentrations from 0.01 to 5 mM (below the critical micelle conc entration, cmc = 8 mM). Our results show that an SDS bilayer of thickn ess similar to 3.2 nm forms at bulk concentrations above 1 mM (greater than or equal to 1/10(cmc)) and that beyond bilayer-bilayer contact t he measured forces are well described by the classical Derjaguin-Landa u-Verwey-Overbeek (DLVO) theory (with a constant surface potential at low surfactant coverage and constant surface charge density at high co verage). The lack of any hydrophobic monolayer formation well below th e cmc was noticed in the force measurements confirmed indirectly by we ttability studies, and attributed to the low surface charge density on the sapphire basal plane (52.7 nm(2) per charge). With one exception, the results are in good agreement with previous data on similar syste ms obtained using other techniques. The results clarify how electrosta tic binding interactions determine the stepped adsorption of, and tran sitions between, hydrophobic and hydrophilic surfactant layers. The re sults also provide a basis for understanding why certain surfactants ( under appropriate solution conditions) can be used as effective additi ves in the colloidal processing of ceramic materials.