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 forc e 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 allowe d complete control over the hydrophobicity of the surface. Adsorption was s tudied on five different thiol-modified gold surfaces prepared from solutio ns containing 0%, 25%, 50%, 75%, and 100% hexadecane thiol, respectively th enceforth referred to as 0% CH3, 25% CH3, 50% CH3, 75% CH3, and 100% CH3). The following general evolution of the adsorbed layer morphology with incre asing 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 dif ferent parts of the surfactant and the solid surface. On the basis of our a dsorption data, we also infer that hydrophobic interactions are the main dr iving force for adsorption of ethylene oxide segments at partially hydroxyl ated surfaces (such as silica) and that hydrogen bonding reduces the free e nergy penalty of displacing water.