Asphaltene Molecular Size by Fluorescence Correlation Spectroscopy

Abstract

Asphaltene molecular size and weight have been of concern since asphaltenes were first isolated from crude oils. Despite previous divergent results on this topic, in recent years, there has been a growing consensus among all mass spectral ionization techniques and all diffusion measurements that asphaltenes are fairly small molecules. In this paper, fluorescence correlation spectroscopy (FCS) is used to determine translational diffusion coefficients of asphaltene and model compounds under a variety of conditions. These FCS studies provide several stringent tests on asphaltene molecular size and architecture. A broad range of concentrations including ultralow concentrations is investigated to ensure the lack of potential aggregation difficulties. Large temperature variations are used to test the application of the simple diffusion equation. FCS results here clearly show the dependence of the diffusion constant on the molecular weight. Finally, FCS results on asphaltenes are in quantitative agreement with t...