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 C8PxEyC (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 C8PxEyC molecules on the quartz surface. In the first adsorption stage, C8PxEyC 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, C8PxEyC molecules interact with the pre-adsorbed monolayer by hydrophobic interactions to establish a loose bilayer structure. As the hydrophilic heads of C8PxEyC orient to water, the surface hydrophilicity of quartz enhances after CMC. The PO/EO chain length critically governs the adsorption behavior of C8PxEyC. Specifically, longer PO chains enhance the molecular size and hydrophobicity of C8PxEyC, leading to a decreased saturation adsorption amount. Conversely, longer EO chains improve the hydrophilicity of C8PxEyC, promoting C8PxEyC molecules to stay in water and consequently decreasing the interfacial adsorption amount of C8PxEyC. This structure-activity relationship of C8PxEyC molecules guides the design of extended surfactants for applications requiring precise wettability control.