Imaginary optical constants in near-infrared (NIR) spectral range for the separation and discrimination of adulterated diesel oil binary mixtures

Fuel adulteration is a major factor contributing to environmental pollution. The tendency to adulterate diesel oil with kerosene is becoming prevalent in many countries, which is particularly driven by the lower cost of kerosene than of diesel oil. The objective of this study is to develop a method of separating and discriminating between different compositions of kerosene in binary liquid fuel mixtures. In this study, we have utilized refractive indices of binary fuel mixtures measured using an Abbe refractometer, and the inversion of measured transmittance spectra in the visible and near-infrared (NIR) spectral range by a general method based on Kramers–Kronig dispersion analysis, where the singly subtractive Kramers–Kronig relations have been used to obtain the wavelength-dependent imaginary excess permittivity of a binary liquid mixture. The excess permittivity has considerable importance in the interpretation of different liquid molecule interactions in binary liquid mixtures. It is herein demonstrated that the problematic cases of relatively low adulterations of 5% and 10% can be detected and distinguished from 15% adulteration using imaginary optical properties in the NIR spectral range. We show that imaginary optical constants such as excess imaginary permittivity and the extinction coefficient are useful for not only screening for adulteration, but also discriminating and separating low from high kerosene compositions in mixtures. Moreover, these quantities can also be exploited for the development of practical sensors, especially for sensing diesel oil adulteration under field conditions.