Dispersant Adsorption during Asphaltene Aggregation Studied by Fluorescence Resonance Energy Transfer (FRET)

Abstract

Fluorescence resonance energy transfer (FRET) is used to study the adsorption of a commercial dispersant on asphaltenes in a toluene−heptane solution. We induce the asphaltene aggregation process by changing the heptane volumetric fraction, where heptane plays the role of a precipitant agent. The dispersant and asphaltenes are naturally fluorescent, and no special labeling was performed to use this fluorescent technique. Low concentrations of asphaltene and dispersant were used in this work0.007 and 0.055 g/L, respectivelyto avoid self-association of the dispersant and to induce asphaltene aggregation only by the presence of heptane in the solution. The results show different behavior of the energy transfer below and above the precipitation onset of the asphaltenes, corresponding to a heptane volumetric fraction of 0.6. The dispersant adsorption was followed during the induced aggregation by measuring the resonance energy transfer from the dispersant (energy donor) to the asphaltenes (energy acceptor) above a heptane volumetric fraction of 0.7, where aggregates are formed and stabilized by the dispersant. The fluorescent experiments presented here also give information about the asphaltene stability in the solvent below the asphaltene precipitation onset.