Influence of Cationic Interactions on Fatty Acid Monolayers Studied by Ellipsometry

Abstract

ABSTRACTOptical properties of fatty acid monolayers were measured as a function of surface density and under the influence of cations in the aqueous substrate. The monolayer behavior of pentadecanoic acid (C15) on an aqueous solution of HCl at pH=2.0 was studied. The change in the ellipsometric phase angle δΔ was mildly affected by increases in surface density at constant orientation but strongly influenced by the liquid-expanded/solid phase transition presumably due to a change in conformation.The increase in the ellipsometric phase angle 6Δ with chain length C15, C16, C18, and C22 was attributed to an increase in thickness of fatty acids at the air/water interface. Separating the effect of optical density of head groups, the refractive indices calculated from the chain length data were independent of ionic environment. The magnitude of δΔ for fatty acids on aqueous substrates increased in the order HCl, CdCl2, and PbCl2. This was due to an increase in the refractive index of fatty acid head groups caused by association of divalent cations. Ellipsometry was also found to be a useful technique to monitor in-situ the degree of divalent cation association.