Study of Asphaltene Nanoaggregation by Nuclear Magnetic Resonance (NMR)

The colloidal nature of asphaltenes impacts various physical properties of crude oils and exhibits a series of aggregation phenomena. Optical diffusion experiments at very low concentrations have measured the size of asphaltene monomers; ultrasound experiments have observed a nanoaggregation transition in toluene. In this work, we use a suite of nuclear magnetic resonance (NMR) methods to study the molecular dynamics and thermodynamics of aggregation in dilute asphaltene solutions. The molecular size of the single asphaltenes and aggregates is determined from NMR diffusion measurements. We observe an abrupt drop of the asphaltene diffusion constant at the onset of aggregation. We also measured the hydrogen index and observed a reduction in the spin-echo signal because of restricted motion of the alkyl chains upon aggregation. The enthalpy of aggregation is found to be negative, most likely because of pi-stacking interactions. The entropy of aggregation is found to be positive, which is unexpected for a nonpolar solution. We also discuss the interpretation of these results.