The role of hydroxyl groups in the self-assembly of long chain alkylhydroxyl carboxylic acids on mica

Abstract

The adsorption and self-assembly of hydroxylated long chain fatty acids on mica has been studied by AFM. The presence of secondary midchain hydroxyl groups is found to strongly affect the self-assembly process via the reinforcement of lateral interactions between molecules by -OH·HO-hydrogen bonding. On the other side, the availability of an exposed terminal hydroxyl group in the acid molecule triggers the formation of a second layer due to the strength of the -OH·HOOC-tail-to-head interaction. Surface coverage (θ) data follow a Langmuir-type relationship vs solution concentration (c). The analysis of the slope of 1/θ vs 1/c plots provides a relative estimation of the adsorption enthalpy for the formation of the first and second monolayers. Based on that, both the contribution of molecule-substrate and lateral molecule-molecule energy terms are found to be relevant in the self-assembly of this type of molecule on mica. © 2008 American Chemical Society.