Adsorption of ionic surfactants to a plasma polymer substrate

The adsorption of an anionic C₁₂ surfactant and cationic C₁₂ and C₁₆ surfactants to a synthetic, anionic radio frequency glow discharge plasma polymer substrate has been investigated. This substrate has a similar charge density to that of amorphous silica but also possesses hydrophobic character, producing a water sessile contact angle of about 74° at pH values between 2 and 7. Both the cationic and anionic C₁₂ surfactants had the same saturation surface excess values on the plasma polymer substrate. Surprisingly, the addition of electrolyte did not affect the saturation surface excess, indicating that hydrophobic interactions between the surfactants and the substrate dominate the adsorption process. All surfactant systems show a change in slope in the isotherm at a surface excess of 0.3 mg·m^-2. The likely conformation of the adsorbed surfactants at this surface excess is with the monomer lying flat on the substrate, to maximize hydrophobic interactions between the hydrocarbon chain and the surface. As the bulk concentration is increased further, adsorption is dominated by hydrophobic interactions. The structure of the adsorbed layer at saturation is discussed.