Adsorption Mechanism of Short Hydrophobic Extended Anionic Surfactants at the Quartz–Solution Interface

This study elucidates the interfacial adsorption behavior of a series of short-chain extended anionic surfactants C₈P_xE_yC (sodium ethylhexyl polyoxypropylene x-polyoxyethylene y-carboxylate) on quartz surfaces, with a focus on the regulatory role of polyoxyethylene (EO) and polyoxypropylene (PO) groups. The results indicate that there is a two-stage adsorption process for C₈P_xE_yC molecules on the quartz surface. In the first adsorption stage, C₈P_xE_yC molecules adsorb at the interface primarily via hydrogen bonding with their hydrophobic tails oriented to water and thereby reducing quartz hydrophilicity before the critical micelle concentration (CMC). The second adsorption stage appears at the CMC, and a saturated monolayer forms via hydrogen bonding. When further increasing the concentration, C₈P_xE_yC molecules interact with the pre-adsorbed monolayer by hydrophobic interactions to establish a loose bilayer structure. As the hydrophilic heads of C₈P_xE_yC orient to water, the surface hydrophilicity of quartz enhances after CMC. The PO/EO chain length critically governs the adsorption behavior of C₈P_xE_yC. Specifically, longer PO chains enhance the molecular size and hydrophobicity of C₈P_xE_yC, leading to a decreased saturation adsorption amount. Conversely, longer EO chains improve the hydrophilicity of C₈P_xE_yC, promoting C₈P_xE_yC molecules to stay in water and consequently decreasing the interfacial adsorption amount of C₈P_xE_yC. This structure-activity relationship of C₈P_xE_yC molecules guides the design of extended surfactants for applications requiring precise wettability control.