Cluster of Asphaltene Nanoaggregates by DC Conductivity and Centrifugation

Abstract

A model of the dominant molecular and stable colloidal structures of asphaltenes has been proposed, the Yen− Mullins model. The formation of clusters of asphaltene nanoaggregates in toluene was reported elsewhere to occur at a concentration of a few grams per liter with a cluster aggregation number of approximately 8 (Mullins, O. C. Energy Fuels 2010, 24, 2179−2207). Here, we measure the DC-conductivity signal of toluene as a function of asphaltene concentration obtaining support for the critical clustering concentration (CCC) of a roughly 1.7 g/L in toluene. In addition, the small change in the Stokes drag at the CCC indicates that the cluster aggregation number is small, less than 10. The temperature variation of the CCC is measured to be small and within error, suggesting that cluster formation is entropically driven. Centrifugation experiments were also performed on asphaltene−toluene solutions at different concentrations. These experiments confirmed that a significant change in asphaltene aggregation occurs in the concentration range of the CCC. The CCC values from centrifugation and DC-conductivity measurements are roughly the same. The centrifugation experiments confirm a cluster size of ∼5 nm corroborating the small aggregation number found in the DC-conductivity experiments. These results add to the growing body of literature validating the Yen−Mullins model.

Advances in Asphaltene Science and the Yen–Mullins Model

“…The latter technique provides a close look at the concentration of the CNAC but only by analysis of a small subset of asphaltenes. 65 These two different techniques are very complementary and support all previous studies on CNAC. Figure 13 shows the DC conductivity and centrifugation results.…”

“…Moreover, the centrifugation experiments were designed to collect small nanoaggregates. 65 Both of these experiments are consistent with small aggregation numbers of asphaltene nanoaggregates.…”

“…36 A recent study showed that the CNAC obtained by DC conductivity matched that obtained by centrifugation for the same asphaltene. 65 Centrifugation provides unassailable evidence that there is an increase in aggregation at the CNAC, the same CNAC obtained by DC conductivity. The latter technique provides a close look at the concentration of the CNAC but only by analysis of a small subset of asphaltenes.…”

“…Comparison of DC conductivity (top) and centrifugation (bottom) applied to a specific Gulf of Mexico (GOM) asphaltene shows excellent agreement of the measured CNAC. 65 At 150 mg/L asphaltene in toluene, the DC conductivity plot shows a reduction of conductivity associated with increased Stokes drag upon aggregate formation. The centrifugation plot shows a significant increase in collected asphaltene at the base of the centrifuge tube at 150 mg/L for this asphaltene.…”

“…Figure 13 shows the DC conductivity and centrifugation results. 65 The small change of the Stokes drag upon aggregation (small reduction in conductivity at CNAC) indicates that the nanoaggregates are small. Moreover, the centrifugation experiments were designed to collect small nanoaggregates.…”