Tracking small heterogeneity in binary mixtures of aliphatic and aromatic hydrocarbons: NIR spectroscopic, 2DCOS and MCR-ALS studies

Abstract

To our best knowledge, this is the first spectroscopic study of small heterogeneity at a molecular level in binary mixtures of aliphatic and aromatic hydrocarbons. The composition-dependent spectra of benzene/toluene, benzene/n‑hexane, benzene/cyclohexane, n‑hexane/n‑heptane and n‑hexane/cyclohexane mixtures were recorded by NIR transmission technique in the whole range of mole fractions. A comprehensive exploration of the experimental data was performed by two-dimensional correlation analysis (2DCOS) and chemometric methods. In addition, we calculated excess NIR molar absorption spectra and parameter of ER, which allows for comparison of the degree of deviation from ideality for various mixtures. Our results reveal that all studied mixtures deviate from the ideal mixture. The extent of these deviations is small for the mixtures were both components are aromatic or aliphatic and increases for aromatic/aliphatic mixtures. In addition, molecular shapes have an important effect on the degree of deviation from the ideal mixture. As expected, benzene/n‑hexane is the most non-ideal mixture, while behavior of n‑hexane/n‑heptane and benzene/toluene mixtures is close to the ideal one. Our results suggest that in the mixtures with the largest deviations from ideality are present homoclusters of both components and heteroclusters. The heteroclusters exist in the whole range of compositions, while the homoclusters appears above a certain concentration limit. If both components of the mixture are similar, the molecules with equal probability form the homo- and heteroclusters. In the case of unlike components, the molecules prefer to form the homoclusters. The homoclusters of both components in the mixture are similar as those observed in bulk constituents. This means that deviation from the ideal mixture results from presence of the heteroclusters.