Influence of surface hydrophobicity on the layer properties of adsorbed nonionic surfactants

The structure of adsorbed octa(oxyethlyene) n-dodecyl ether (C12E8) layers at a series of gold-alkanethiolate surfaces was investigated by atomic force microscopy. The hydrophobicity of the surfaces was systematically varied by changing the relative amounts of chemisorbed thiohexadecane (CH3(CH2)(15)(-) SH) and thiohexadecanol (CH2OH(CH2)(15)SH) surface groups. This allowed complete control over the hydrophobicity of the surface. Adsorption was studied on five different thiol-modified gold surfaces prepared from solutions containing 0%, 25%, 50%, 75%, and 100% hexadecane thiol, respectively thenceforth referred to as 0% CH3, 25% CH3, 50% CH3, 75% CH3, and 100% CH3). The following general evolution of the adsorbed layer morphology with increasing surface hydrophobicity was observed: diffuse micellar coverage; dense micellar coverage; bilayer; and finally a monolayer structure at the most hydrophobic surface. The adsorbed layer structure observed at the different surfaces was interpreted in terms of the effective interaction between different parts of the surfactant and the solid surface. On the basis of our adsorption data, we also infer that hydrophobic interactions are the main driving force for adsorption of ethylene oxide segments at partially hydroxylated surfaces (such as silica) and that hydrogen bonding reduces the free energy penalty of displacing water.