Study of the pH dependence of head group bonding in arachidic acid monolayers by polarization modulation infrared reflection absorption spectroscopy

Little information is available on the bonding interaction of fatty acid head groups with solvent molecules and other fatty acid molecules in monolayers at the air–water interface. Recent findings indicate the formation of hydrogen bonds between pairs of neutral fatty acid and carboxylate in the pH range from pH 5.6 to 10. In this work arachidic acid monolayers are investigated on subphases of pH 5.6, 9, 9.7 and 12.0, that contain a variable amount of sodium ions, by means of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The fitted peak intensities are related to surface concentrations of singly and doubly hydrogen bonded and non-hydrogen bonded carbonyl and carboxylate. From the change of the concentrations with the pH tentative conclusions are drawn on the bonding situation of fatty acids in monolayers on water and alkaline solution. The degree of dissociation as derived from the peak intensities is compared with calculated results according to the Gouy–Chapman–Stern theory.