How ions affect the swelling characteristics of polyurethane-based microcapsules?

Abstract

Microcapsule-based polymer flooding is an innovative and efficient enhanced oil recovery (EOR) method, known for its controlled release and enhanced stability. Despite advances in the design and synthesis of microcapsule materials, the impact of reservoir conditions, particularly saline water, on the swelling and rupture of microcapsule membranes remains largely unexplored. In this study, we investigate how various types and concentrations of ions affect the interaction between the microcapsule membrane and the aqueous phase. To model the atomistic interactions between saline solutions and polyurethane (PU)-based polymers, we first optimized atomic charges via quantum chemistry and then performed classical all-atom molecular dynamics simulations to study the interactions between various PU membranes and saline water. The results reveal that ions have condition-dependent effects on PU membrane swelling, depending on the molecular structure of PUs. In unmodified PU membranes, Na⁺ and Ca²⁺ suppress swelling by lowering the effective water activity, with Ca²⁺ exhibiting a stronger suppressing effect due to its higher hydration capacity. In contrast, anionically modified PU membranes exhibit strengthened PU-ion interactions that allow ions to penetrate the membrane and promote water diffusion. These findings offer valuable insights for designing microcapsules with finely tuned controlled-swelling properties.