Dynamics, Aggregation, and Interfacial Properties of the Partially Hydrolyzed Polyacrylamide Polymer for Enhanced Oil Recovery Applications: Insights from Molecular Dynamics Simulations

We hereby employ molecular dynamics (MD) simulations (∼6 μs in total) to investigate the chain dynamics, aggregation, and interfacial properties of the partially hydrolyzed polyacrylamide (HPAM) polymer. HPAM is widely used in chemical enhanced oil recovery (cEOR) applications. The conformational changes and aggregation properties are examined in different conditions simulating cEOR activities. Also, we examined the degree of polymerization (20-, 50-, and 100 -mers) effect on the polymer chain dynamics and aggregation. MD simulations showed that HPAM has a high conformational diversity ranging from coiled to compact conformations. The former is abundantly found in fresh water. In brine solutions, HPAM is found to be very sensitive to ions and adopts a more compact conformation. HPAM-ion interactions drive the conformational thermodynamic equilibrium between the compact and coiled conformations toward the compact conformation. Furthermore, ion interactions are largely impcating its aggregation. HPAM has a high propensity to form large-size aggregates in brine solution. An interesting ionic effect has been observed; Ca²⁺ ions showed a high affinity toward HAPM compared to Mg²⁺ and Na⁺ ions. The electrostatic forces and ionic dehydration free energy penalty are the two main factors that determine the HPAM ionic affinity. Short oligomers are noted to overestimate the tendency of the polymer to have compact conformations and underestimate its aggregation capacity in brine solutions. Simulations of oil-water systems show that HPAM has a spectator role on the interfacial tension in the absence of surfactants.