Characterizing solid/ionic liquid interfaces in the presence of water and nonionic surfactants

Abstract

The adsorption of nonionic surfactants on hydrophilic solid surfaces has been characterized using mixtures of an aprotic room-temperature ionic liquid (RT-IL) with water. The surfactants employed in this study were a series of polyoxyethylene alkyl ethers (CnEm, n is the hydrocarbon chain length, and m is the number of oxyethylene units, C10E6, C12E6, C14E6, C12E4, and C12E8), and the RT-IL used was 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4). This RT-IL is completely miscible with water over a whole range of concentrations. Force curve measurements by atomic force microscopy (AFM) demonstrated that the adsorption of the nonionic surfactants occurred at a silica/solution interface even in the presence of added water. The surfactant aggregates formed at the interface grew as a function of the surfactant concentration; an attractive interaction observed below the critical micelle concentration (cmc) changed into a repulsive interaction above the cmc. Interestingly, multi-step repulsive interactions were observed in a few relatively solvophobic surfactant systems (i.e., the surfactants having longer hydrocarbon chain and/or shorter oxyethylene chain) in the presence of added water. It seems likely that these repulsive interactions arise from the repeated structure of the surfactant assemblies formed between the cantilever tip and the silica surface. In contrast, the surfactant adsorption was significantly inhibited on mica because of the absence of hydrogen bonding sites and/or the presence of well-structured IL solvation layers on the mica surface.