Composition of Asphaltene Solvate Shell at Precipitation Onset Conditions and Estimation of Average Aggregate Sizes in Model Oils

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Abstract

Macroscopic properties of petroleum disperse systems are functions of its particle size. Asphaltene particles in model oils are in fact building agents. Compositional changes of disperse medium have an effect on the internal structure of the dispersed system. Optical methods are informative enough for analysis of structural evolutionary processes. Near-infrared (NIR) spectroscopy was used to determine asphaltene precipitation onset conditions in model oils (asphaltenes were dissolved in binary solvent: toluene and n-alkane precipitant n-hexane/n-heptane). Asphaltene precipitation onset was determined as a point where the deviation from Beer’s law takes place. Solvate shells of asphaltenes in binary solvents are formed according to differences between intermolecular interaction potentials of components of the model oil. The model of non-uniform distribution of components in solvate shells of asphaltene aggregates at the moment of aggregation stability loss (first stage of precipitation onset) is proposed. On the basis of experimentally determined asphaltene precipitation onset conditions, average aggregate sizes in toluene solutions, set of assumptions concerning geometry of solvent and precipitant molecules, and interaction energy of molecule pairs, the proposed model allows for estimation of the precipitant fraction threshold value in solvate shells of asphaltene aggregates at the moment of stability loss. Polarization of fluorescence was used for the determination of the concentration dependence of average sizes of aspahltene aggregates in toluene solution. The linear aggregation model was used for characterization of asphaltene solutions in toluene; the fraction of n-mers was determined. The concentration dependence of the asphaltene aggregation rate constant was calculated within this model.

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