Effect of concentration and addition of ions on the adsorption of sodium dodecyl sulfate on stainless steel surface in aqueous solutions

Abstract

The adsorption characteristics of sodium dodecyl sulfate (SDS) on stainless steel surface in aqueous solutions as well as the effect of added NaClO4 on adsorption are investigated. The stainless steel surface is hydrophobic when wetted by water and negatively charged in SDS solutions, which was characterized by performing open circuit potential (EOCP) and zero charge potential (EPZC) measurements. The adsorption isotherm of SDS on stainless steel surface in SDS aqueous solutions was measured by quartz crystal microbalance (QCM). The results indicate a four-stage adsorption process according to the micellization of SDS molecules both in bulk solution and on stainless steel surface. With the increase of SDS concentration, the mass of the adsorbed SDS molecules increases, while the structure of the adsorbed layer changes from monomers to hemimicelles. In the presence of NaClO4 as background electrolyte, the adsorption isotherm shifts to lower SDS concentration regime, mainly as a result of changing in electrostatic interactions both by the binding of Na+ on micelles and by the changing of ionic strength of the solution. The ionic strength effect is shown to be of vital importance in the change in electrostatic interactions between adsorbed hemimicelles mimicked by recording force-distance curves between a negatively charged tip and the hemimicelles using atomic force microscope (AFM). All the effects of SDS concentration and addition of NaClO4 on adsorption are clarified based on hydrophobic interactions and electrostatic interactions both between SDS molecules and between SDS molecules and stainless steel surface.