Ionic strength effects on hexadecane contact angles on a gold-coated glass surface in ionic surfactant solutions

Abstract

Abstract As aqueous surfactant cleaning becomes increasingly prevalent in industry, an understanding behind surfactant cleaning behavior is needed to improve the systems. Improvements in these aqueous surfactant solution systems can reduce the impact of the industrial cleaning systems on the environment and improve their efficiency. Studying various parameters of the cleaning systems can provide useful insight into surfactant behavior. The contact angle of the soil to the solid surface is strongly related to cleaning system performance and is a relatively simple parameter to measure. For this study, a simplified cleaning system was studied to investigate the effects of modifications to the surfactant system. The contact angle of hexadecane on a gold-coated glass surface in aqueous surfactant solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were determined up to and above the critical micelle concentrations. The effects of modifying the ionic strength of the aqueous surfactant solution by the addition of sodium chloride (NaCl) have been studied. Similar trends in contact angle behavior of the oil droplet to the gold-coated surface were found for both the anionic (SDS) and cationic (CTAB) surfactant systems. The addition of NaCl to the aqueous surfactant systems resulted in a significant increase in the contact angles over the surfactants concentration ranges. The increasing trend in the contact angles was observed for NaCl concentrations ranging from 0.05 to 0.5 mM, while little effect was noted for the higher NaCl concentrations. The observed effect is consistent with literature on electrostatic-charge-induced changes in surfactant adsorption at the solid surface.