Effect of Isopropanol on the Fluorescence of Crude Oil-in-Water Dispersions

Fluorescence spectroscopy is a promising technique to quantify crude oil-in-water because it provides information on both dissolved and dispersed molecules. One of the challenges of measuring properties of dispersions is the creaming of droplets, mainly due to density differences with the continuous phase. The addition of isopropanol to oil-in-water dispersions is an often-recommended trick to prevent the creaming of dispersed oil and obtain more reproducible measurements. Although isopropanol does not in itself fluoresce, the aim of the present work was to investigate if the emission from crude oil-in-water would be affected by the addition of isopropanol. The data set comprised synchronous fluorescence and emission spectra of 193 samples of crude oil-in-water/isopropanol mixtures, with oil concentrations varying from 10 to 1271 mg L-1 covering typical concentrations along the water treatment system offshore and isopropanol mass fractions from 0 to 1. Selected spectral features were used as input for principal component analysis, from which it was observed that the samples clustered into three groups. The isopropanol content in each mixture and, consequently, the dielectric constants were found to be the main underlying factors driving the shape and intensity of the emission bands. In the range of 0.01 to 0.5 w/w of isopropanol, the samples became milky which cleared at higher isopropanol content. Additional experiments showed that significant changes in the spectral shapes and emission intensities were observed more as a function of the isopropanol content than of the actual oil concentration. Furthermore, the quantum yield of crude oil is highly dependent on the isopropanol-to-water fraction, which means that adding isopropanol to crude oil-in-water dispersions is not advisable for oil quantification purposes by fluorescence spectroscopy.