Aliphatic alcohols facilitate interfacial reorientation of thiols: correlation with alcohol adsorptivity

The influence of a series of aliphatic alcohols on the reorientation of alkylthiols during their self-assembly has been studied by cathodic stripping voltammetry. The presence of an aliphatic alcohol in the deposition solution is shown to lower the critical reorientational surface concentration of alkylthiols, making it less sensitive to molecular size. The use of a series of alcohols differing in their molecular length or branching reveals that the onset of thiol reorientation correlates well with the extent of alcohol adsorption. A theoretical model is developed to account for this effect, whose crux is the competition between the alcohol molecule and the alkyl chain of the thiol for adsorption sites. The analytical expression derived for the critical reorientational surface concentration reveals that the effect of adding alcohol can be rationalized in terms of an apparent reorientation equilibrium constant, which embodies both the bulk concentration and the adsorption equilibrium constant of the relevant alcohol. Overall, these findings corroborate that the interfacial reorientation of n-alkanethiols is sterically controlled and demonstrate that its onset can be finely tuned by addition of a suitable coadsorbate.