Low Temperature Fluorescence Studies of Crude Petroleum Oils

Abstract

We have studied the low-temperature (133-298 K) fluorescence emission of crude petroleum oils using a combination of steady-state and time-resolved measurements. This was done, first, to see if we could generate linear correlations between the oil composition information and the fluorescence measurements and, second, to better understand how static and dynamic quenching affect fluorescence emission. It was observed that the fluorescence intensity and the lifetime of the crude oils increased rapidly with decreasing temperature down to the freezing point, and then, they either remained constant or, surprisingly, began to decrease slightly. These changes could not be correlated accurately with the compositional data available. However, despite the very large variations in sample composition, it was found that these lifetime temperature changes followed simple Arrhenius and Eyring behavior. For the cold liquid phase, an Arrhenius model enabled the calculation of an intrinsic lifetime, the magnitude of which was inversely related to the degree of static quenching. The low values of the calculated activation energies (4.6-19.2 kJ mol-1) implied that, in the liquid phase, nonradiative decay was primarily diffusion based quenching. At the lowest temperatures, when all samples have solidified, the lifetime data followed Eyring like behavior, giving typical enthalpy and entropy values of -1 kJ mol-1 and from -67 to -93 J K-1 mol-1, respectively. The Eyring model was used to describe the nonradiative decay mechanism arising from the vibrational coupling from the fluorophores to the surrounding matrix. This modeling of the temperature dependence of the fluorescence lifetime has provided a clearer, quantitative picture of the fluorescence quenching processes in crude petroleum oils.