Wetting Behavior of Cationic and Anionic Surfactants on Hydrophobic Surfaces: Surface Tension and Contact Angle Measurements

Abstract

In this study, cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) are employed to systematically investigate surface and wetting properties on hydrophobic surfaces, specifically in mixed solvents composed of ethylene glycol (EG) and water at 298.15 K. By varying the concentration of each surfactant within the EG–water mixture, both surface tension and contact angle measurements are performed to elucidate how surfactant type and solvent composition influence interfacial behavior and wettability. PTFE and wax surfaces were chosen as model hydrophobic surfaces. Surface tension measurements obtained in pure water and in water–EG mixtures containing 5, 10, and 20 volume percentage EG reveal a consistent decrease in the premicellar slope ( (Formula presented.) ) with increasing EG content. This reduction reflects weakened hydrophobic interactions and less effective surfactant adsorption at the air–solution interface. The corresponding decline in maximum surface excess ( (Formula presented.) ) and increase in minimum area per molecule ( (Formula presented.) ) confirm looser interfacial packing due to EG participation in the solvation layer. Plots of adhesion tension (AT) versus surface tension (γ) exhibit negative slopes, consistent with reduced solid–liquid interfacial tension ( (Formula presented.) ) and greater redistribution of surfactant molecules toward the solid–liquid interface. AOT shows stronger sensitivity to EG compared to CTAB, reflecting structural headgroup-specific adsorption behavior. Work of adhesion (WA) measurements demonstrate enhanced wettability at higher EG concentrations, highlighting the cooperative impact of co-solvent environment and surfactant type on wetting phenomena.