Effect of Hydrophobization of Gold QCM-D Crystals on Surfactant Adsorption at the Solid-Liquid Interface

The effect of hydrophobization of gold quartz crystal microbalance with dissipation (QCM-D) crystals is investigated in terms of its effect on surfactant adsorption at the solid-liquid interface. Hydrophilic gold crystals were modified with octadecanethiol to produce a hydrophobic surface. The gold surface was characterized prior to and after coating. Water contact angles were obtained to confirm hydrophobicity and atomic force microscopy (AFM) images were acquired to confirm surface modification and alkylthiol monolayer morphology. Modified crystals were tested for QCM-D response uniformity and then exposed to surfactant solutions. The effect of hydrophobization on the adsorption of surfactants is investigated for sodium dodecyl sulfate (SDS), 'dioctyl' sulfosuccinate sodium salt (AOT), hexadecyltrimethylammonium bromide (CTAB), and dihexadecyldimethylammonium bromide (DHDAB). Surfactant solutions were studied above their respective critical micelle concentration (CMC) in the absence of added electrolyte. Adsorption for SDS and AOT on hydrophilic gold was minimal and was below the detection limit for QCM-D at concentrations just above the CMC. For CTAB and DHDAB solutions at concentrations well above the CMC, adsorption on hydrophilic gold was substantial, forming more than one bilayer of coverage. On hydrophobed gold, SDS, AOT, CTAB, and DHDAB all adsorbed to form a surfactant monolayer.