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 to first see if we could generate linear correlations between 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 lifetime of the crude oils increased rapidly with decreasing temperature down to the freezing point and then 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 lifetimetemperature 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 to 19.2 kJmol − 1) implied that in the liquid phase, non-radiative 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 kJmol − 1 and from −67 to −93 JK − 1 mol − 1 respectively. The Eyring model was used to describe the non-radiative decay mechanism arising from vibrational coupling from the fluorophores to the surrounding matrix. This modeling of the temperature dependence of fluorescence lifetime has provided a clearer, quantitative picture of the fluorescence quenching processes in crude petroleum oils.

Evaluating steady-state and time-resolved fluorescence as a tool to study the behavior of asphaltene in toluene

“…Both mechanisms have been previously proposed to be responsible for the shortening of lifetimes with increasing concentration of asphaltenes or crude oils. 7,12,[14][15][16][17] For the emission at 520 nm the average lifetime is constant up to a concentration of 1 g L −1 of AA-5 with continuously shorter lifetimes being observed at higher asphaltene concentrations. The A values at this wavelength remain constant as the AA-5 concentration was raised while all three lifetimes were shortened (Tables S2 and S3 in the ESI †).…”

“…10,11,13 Different fluorophores in crude oils or asphaltene samples have different lifetimes and shorter lifetimes were observed when the concentrations of crude oil or asphaltenes were raised. 7,12,[14][15][16][17] These observations were attributed to energy transfer between separate asphaltene molecules or between asphaltene molecules within aggregates.…”

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid

“…We have calculated the activation energy of non‐radiative decay process in order to understand whether, the viscosity of the media influences the nonradiative decay. We have used the modified Arrhenius equation in order to calculate the activation energy of nonradiative decay : where E nr represents the activation energy of nonradiative decay process, A is the pre exponential factor and τ 0 is the intrinsic lifetime. By plotting reciprocal of lifetime value against reciprocal of temperature we have obtained the activation energy of nonradiative decay process.…”

Characterization of Petroleum Crude Oils using Laser Induced Fluorescence

“…Figures 1A and 1B Figure 2B: Fluorescence spectra of crude oil samples dissolved in methylene chloride using excitation sources at (B) 532 nm. The excitation of diluted crude oil samples using different laser sources in the ultraviolet and visible portion led to florescence emission in the visible portion range from 400 nm-600 nm as observed by many authors [10][11][12][13][14][15][16][17][18][19][20]. Before 450 nm, the emission spectra of oils with high API gravity (light oils) can be described as strong fluorescence emission with relatively narrow bands than that for heavier oils (lower API gravity) where there is a red shifted weaker and broader fluorescence emission.…”

“…Energy transfer and quenching effects are considered the major players for determining the emission behavior of petroleum oils. They are very considerable factors due to the fact of the relatively high abundance of quenchers and fluorophores that are found naturally in crude oils [18].…”