Effect of Low Concentration Salt on Organic Contact Angle in Ionic Surfactant Solutions: Insight from Theory and Experiment

A theoretical approach to predict equilibrium organic/solid-surface contact angles as affected by the addition of electrolytes to an aqueous surfactant solution has been developed. While the effects of electrolytes on surfactant self-assembly and adsorption are extensively documented, there is a noticable gap in the literature for systems where less than 10 mM of electrolyte is added to the solution. This article presents an improved approach, based on our earlier model, that accounts for the dramatic changes observed for previously published hexadecane droplet contact angles data on gold for very low concentration additions of sodium chloride (NaCl) in separate aqueous solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB). In addition to providing insight into changes in interfacial phenomena the model demonstrates that both charge and type of salt ions play a significant role in the extent to which droplet contact angles vary from those of salt free solutions.