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Shared Depletion and Restabilization Colloidal Interactions in Phase Diagrams for Silica Nanoparticle and Asphaltene + Polystyrene + Solvent Mixtures

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journal contribution
posted on 27.07.2020, 20:09 by Anupam Kumar, Sourav Chowdhury, John M. Shaw
Nanocolloids (nanoparticle + solvent mixtures) and nanocolloid + non-adsorbing polymer mixtures arise in fields as diverse as pharmaceutics, hydrocarbon production, and environmental science. While there are many parallels with the phase behavior of molecular fluids, the driving forces for phase behavior, modeling approaches, and terminologies used to describe them differ markedly, reflecting historical examples and applications that underlie the development and understanding of phase diagrams in these fields. Here, for example, we link the concept of theta and non-theta solvent, in colloid phase diagrams, to upper critical end points arising in polymer + solvent binary mixtures in simple fluids by integrating concepts from both fields. We show that the phase behavior of silica nanoparticles (7 nm diameter) + polystyrene (237 kg/mol) + cyclohexane is qualitatively similar to the phase behavior of chemically separated Athabasca pentane asphaltenes and physically separated Athabasca retentate (comprising 43.1 wt % pentane asphaltenes) + atactic polystyrene (400 kg/mol) + toluene. All three mixtures exhibit two-phase regions, where one phase is enriched with polymer and the other phase is enriched with nanoparticles. The phase boundaries are reversible and include critical points, underscoring the overlap in particulate states in both phases. The experimental methods, phase boundaries, and fluid–fluid critical points are presented and discussed. X-ray transmission was found to be more robust than acoustic transmission for the identification of two-phase to one-phase boundaries and critical points for these mixtures. The outcomes of this work add to our understanding of the phase behavior of solvent + non-adsorbing polymer + nanoparticle mixtures for cases where dispersive energies are weak. More specifically, they improve our understanding of asphaltene and asphaltene-rich fluid behaviors in reservoirs and production, transport, and refining processes. We broaden the conceptual understanding of asphaltene behavior and underscore the importance of a colloidal approach for modeling asphaltene stability.