Surfactant in a Polyol-CO 2 Mixture: Insights from a Classical Density Functional Theory Study

Silicone-polyether (SPE) surfactants, made of a polydimethyl-siloxane (PDMS) backbone and polyether branches, are commonly used as additives in the production of polymeric foams with improved properties. A key step in the production of polymeric foams is the nucleation of gas bubbles in the polymer matrix upon supersaturation of dissolved gas. However, the role of SPE surfactants in the nucleation of gas bubbles is not well understood. In this study, we use classical density functional theory to investigate the effect of an SPE surfactant on the nucleation of CO bubbles in a polyol foam formulation. We find that the addition of an SPE surfactant leads to a ∼3-fold decrease in the polyol-CO interfacial tension at the surfactant's critical micelle concentration. Additionally, the surfactant is found to reduce the free energy barrier and affect the minimum free energy pathway (MFEP) associated with CO bubble nucleation. In the absence of a surfactant, a CO -rich bubble nucleates from a homogeneous CO -supersaturated polyol solution by following an MFEP characterized by a single nucleation barrier. Adding a surfactant results in a two-step nucleation process with reduced free energy barriers. The first barrier corresponds to the formation of a spherical aggregate with a liquid-like CO core. This spherical aggregate then grows into a CO -rich bubble (spherical aggregate with a vapor-like CO core) of a critical size representing the second barrier. We hypothesize that the stronger affinity of CO for PDMS (than polyether) stabilizes the spherical aggregate with the liquid-like CO core, leading to a lower free energy barrier for CO bubble nucleation. Stabilization of such an aggregate during the early stages of the nucleation may lead to foams with more, smaller bubbles, which can improve their microstrustural features and insulating abilities.