PNNL

Abstract

This paper investigates the accuracy of identifying binary liquid mixtures via reflectance spectroscopy (either contact or standoff) using spectra derived from only the complex indices of refraction of the neat materials. Using volume fractions, each mixture’s complex index of refraction was approximated via three different mixing rules. To explore the impact of intermolecular interactions, these predictions are tested by experimental measurements for two representative sets of binary mixtures: 1) Tributyl phosphate and n-dodecane represent mixtures with only weak van der Waals forces whereas 2) tributyl phosphate and 1-butanol mixtures include not only van der Waals forces but also strong hydrogen bonding interactions. The residuals and the root-mean-square error between the experimental and calculated index values are computed and demonstrate that for miscible liquids with only weak interactions, the refractive indices of the mixtures can be modeled using composite n and k values derived from volume fractions of the neat liquids. In spectral regions where strong intermolecular interactions such as hydrogen bonding between the two liquids occur, however, the calculated n and k values vary from the measured values. The utility of calculating the refractive indices is then demonstrated by modeling a thin film of the binary mixtures on an aluminum substrate using both the measured and the mathematically computed indices of refraction.