Influence of Strongly and Weakly Interfacially Active Asphaltene Particles on Solubility and Crystallizability of...

Asphaltenes and waxes are two components of crude oil that contribute to flow assurance challenges; however, the interaction between them is rarely studied. This study investigates how asphaltene particles with different physicochemical properties, specifically, remaining asphaltenes (RA) and interfacially active asphaltenes (IAA), influence the crystallization kinetics of C₂₈H₅₈, a model wax compound. IAA fractions are more polar, primarily due to their higher content of heteroatoms such as sulfur and oxygen, and they tend to form larger particles in solution compared to the RA fraction. At a fixed C₂₈-to-asphaltene ratio of 1000:1 (w/w), the introduction of either RA or IAA, especially at low concentrations (10 g/L C₂₈ + 0.01 g/L asphaltene), significantly inhibits crystallization, although crystallization continues to proceed via a progressive nucleation mechanism. Inhibited crystallization was evidenced by the increased solubility temperature (T _e), decreased supersolubility temperature (T _c,l), and a broader metastable zone width. These changes are associated with enhanced supersaturation, an elevated free energy barrier to nucleation, increased interfacial tension (γ_eff), and a higher number of molecules in the critical nucleus (i*). The data supports a 2-step crystallization model: initially, asphaltene particles hinder 3D molecular aggregation (nucleation step), followed by inhibition of crystal growth through adsorption on lateral (h k 0) crystal faces. A key finding of this study is the concentration-dependent inhibitory effect of asphaltene particles on wax crystallization. At low concentrations, small asphaltene particles predominate, providing an abundance of asphaltene molecules and nanoaggregates that can fully interact with C₂₈ molecules, resulting in stronger inhibition. In contrast, at higher concentrations, larger asphaltene particles become dominant, reducing the number of asphaltene molecules and nanoaggregates in solution. Consequently, their interaction with C₂₈ is limited, leading to a weaker inhibitory effect on crystallization. Notably, the inhibitory capacity of IAA is more sensitive to concentration than that of RA. This sensitivity is due to IAA's strong tendency to self-associate and form large particles, which are less able to interact effectively with C₂₈. These findings confirm the significance of asphaltene particle behavior relative to solute concentration in influencing the nucleation pathway and crystallization kinetics of paraffin waxes.