STRUCTURE AND ORIENTATION OF SURFACTANT MOLECULES AT THE AIR-WATER-INTERFACE

Fourier transform reflection-absorption infrared (RA-IR) spectroscopy is used to probe the structure and properties of sodium dodecyl sulfonate (C12S) monolayers that are self-assembled from dilute solution at an air-water interface. Recent optical models for the interpretation of signal intensity measurements are briefly reviewed. The methylene stretching peaks of C12S monolayers in the RA-IR spectra are used to determine the chain orientation, the surface concentration and the conformational state of the alkyl chains. Comparisons are drawn between monolayers and C12S crystals. A phase transition is found as the concentration of C12S in the subphase below the monolayer is reduced. The effect of salt on the monolayers is presented. The infrared data is interpreted in terms of the surface tension behavior.