Influence of long-chain alcohols on the adsorption of nonionic surfactantsto silica

The adsorption of surfactant-alcohol mixtures at the silica-water interface was studied by means of ellipsometry. The results show that addition of ev en small amounts of alcohol can have large effects on the characteristics o f the adsorbed layer. For example, a 20% replacement of the octa(ethylene g lycol) dodecyl (C12E8) surfactants by dodecanol results in an increase in t he total surface excess of 80%. The thickness of the adsorbed layer, on the other hand, is virtually unaffected by the alcohol being added. Hence, as the alcohol content increases, the adsorbed surfactant aggregates at the si lica-water interface mainly grow in the surface plane. A surfactant such as C12E5, which forms relatively large surface aggregates from the start, can only solubilize a small fraction of the long-chain alcohols before the sys tem phase separates. This fraction was found not to result in any major str uctural changes in the surface layer. These findings are discussed in terms of surfactant packing and in relation to observations in bulk solutions re ported earlier. Our study also includes measurements of adsorption and deso rption kinetics for both the surfactant and the surfactant-alcohol systems. The main finding is that the effect of alcohol is most obvious in the deso rption kinetics. We conclude that the effects observed are due to differenc es between the surfactant and the alcohol in monomer solubility.