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 he ad groups with solvent molecules and other fatty acid molecules in monolaye rs at the air-water interface. Recent findings indicate the formation of hy drogen 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 investi gated on subphases of pH 5.6, 9, 9.7 and 12.0, that contain a variable amou nt 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-hyd rogen bonded carbonyl and carboxylate. From the change of the concentration s with the pH tentative conclusions are drawn on the bonding situation of f atty acids in monolayers on water and alkaline solution. The degree of diss ociation as derived fi om the peak intensities is compared with calculated results according to the Gouy-Chapman-Stern theory. (C) 2001 Elsevier Scien ce B.V. All rights reserved.