IONIC INTERACTIONS OF FATTY-ACID MONOLAYERS STUDIED BY ELLIPSOMETRY

Monolayers of fatty acids with chain lengths of 15, 16, 18, and 22 carbons at the air/water interface were studied on aqueous substrates containing hydrochloric acid, cadmium chloride, and lead chloride by using phase-modulatedellipsometry. The change in ellipsometric phase angle 5A, which is the phase difference between the orthogonal s and pwaves, was found to be linearly dependent on chain length with the same coefficient for all three cations whereas the magnitude of 5A was shown to be different. The proportionality constant with respect to chain length was modeled in terms of the refractive index contributions of the hydrocarbon tails, giving thicknesses in agreement with those obtained by other techniques. The magnitudes of ᵟΔ were analyzed in terms of film refractive index anisotropy, whereby our results were shown to be consistent with anisotropic refractive indices obtained by other methods. They were also examined relative to pH values and cation concentration, and the results are discussed in terms of a refractive index increase due to the interaction of carboxylates with divalent cations. © 1990, American Chemical Society. All rights reserved.