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Simultaneous Determination of Molecular Orientation and Surface Concentration in Anisotropic Monolayers on Liquid Substrates

Volume 47, Number 10 (Oct. 1993) Page 1643-1650

Tung, Yei-Shin; Gao, Tao; Rosen, Milton J.; Valentini, Jose E.; Fina, Leslie J.

A new method is developed for the simultaneous determination of chain orientation and surface concentration in monolayers on liquid substrates. The method is based on the use of s- and p-polarized reflection-absorption (RA) infrared intensities in conjunction with an optical model that includes anisotropy in the optical constants. The advantages and limitations of the method are explored in detail. A modeling assessment of the RAp/RAs ratio leads to information on how the ratio can be used to qualitatively interpret RA spectra. Spectral simulations are given that address the issues of frequency shifting and band distortion in reflection-absorption due solely to optical considerations. Experimental spectra of sodium dodecyl sulfonate are collected as soluble monolayers at the air/water interface. The spectral intensities are used to calculate the chain orientation, the surface concentration, and the associated errors. The surface concentration is compared with that predicted from surface tension isotherms.