The effect of headgroup interactions on structure and morphology of arachidic acid monolayers

The structural and morphological features of arachidic acid monolayers are studied systematically between pH 2 and 13 by X-ray diffraction, Brewster-a ngle microscopy, and thermodynamic measurements on subsolutions free of pol yvalent ions. A monotonic relationship is found between the pH and the dist ortion of the lattice perpendicular to the chain axes. A nonmonotonic pH de pendence is found for structural parameters like lattice spacings, tilt ang les, molecular area, and positional correlation length as well as phase bou ndaries. The results provide evidence that the fatty acid amphiphiles are h ydrogen bonded at the interface and that the bonding strength is affected b y the degree of headgroup dissociation in a discontinuous way. Strongest bo nding is found at pH 9, where 50% of the carboxylic headgroups are dissocia ted. Hydrogen bonding force hence opposes electrostatic headgroup repulsion and stabilizes a particular structure. At higher pH (12-13) a drastic pH d ependence of the domain shapes and domain fusion are observed. This is ascr ibed to a change of line tension and electrostatic repulsion between liquid condensed and liquid expanded phase.