Adsorption of extended anionic surfactants at the water- polymethylmethacrylate interface: The effect of polyoxyethylene groups

Wetting of solid surfaces is a typical phenomenon in daily life and manufacturing. The wettability of solid surfaces can be efficiently altered by certain surfactants. Extended surfactants, a novel class of surfactant with a unique structure, have a potent potential to convert neutral surfaces into hydrophilic surfaces. To deeply understand the influence of EO groups on the adsorption mechanism of surfactants at solid surfaces, the wettability of the extended anionic surfactants sodium cetyl (oxyethylene)₃ carboxylate (C₁₆EO₃C), sodium cetyl (oxyethylene)₅ carboxylate (C₁₆EO₅C), and sodium cetyl (oxyethylene)₇ carboxylate (C₁₆EO₇C) on polymethylmethacrylate (PMMA) surfaces was investigated. Surface tension and contact angle experiments were used to determine adsorption characteristics such as adhesion tension, PMMA-liquid interface tension, and adhesion work. The experimental results show that the adsorption amounts of C₁₆EO_nC at the liquid-air interface are obviously larger than those at the PMMA-liquid interface. Below CMC, all three extended surfactant molecules adsorb at the PMMA-liquid interface via polar interactions and hydrophobic part towards the aqueous phase, which results in an increase of γ_SL. However, the decrease of γ_LV and the increase of γ_SL have opposite effects on the contact angle, resulting in almost constant values below CMC. With an increase in EO number, the hydrophobic modification ability increases because longer EO chains provide stronger polar interactions between C₁₆EOnC molecules and the PMMA surface. Above CMC, C₁₆EOnC molecules continue to adsorb at the PMMA-liquid interface and form a bilayer film. Conclusively, the contact angle abruptly decreased due to the hydrophilic modification of the PMMA surface. With an increase of EO number, the hydrophilic modification ability increases because surfactants with a larger EO number endow the second layer with a more hydrophilic nature.