Microscopic structure of crystalline Langmuir monolayers of hydroxystearic acids by X-ray reflectivity and GID: OH group position and dimensionality effect

Hydroxystearic acid (HSA) molecules at the air-water interface present an interesting bicompetitive adsorption between primary and secondary hydrophilic groups on either end of an alkyl chain, which, depending on the position of the second hydrophilic group, may lead to a sharp transition from an expanded phase to a crystalline condensed morphology as surface pressure is increased. Here we report a set of measurements on a series of hydroxystearic acids in which the position of the secondary competing hydrophilic group position is varied along the whole extent of the alkyl chain from position 2 (i.e., close to the primary hydrophilic group) to positions 7, 9 and 12, the latter being the compounds mostly studied in the literature. We show here direct microscopic evidence, obtained by synchrotron radiation reflectometry and grazing incidence diffraction, that the position of the secondary hydrophilic group not only strongly influences the phase diagram as determined by compression isotherms and ellipsometry but also induces different crystallization patterns in the 2D system of the Langmuir monolayer. In particular, we report for the first time the existence of a turning point in the effects of the hydroxyl position on the monolayers structure at 7-HSA.