Dissolution Mechanism of Nonionic Polyether Surfactants in Supercritical CO2

Abstract

Polyoxyethylene ether is an efficient CO₂-philic compound with significant potential applications in CO₂ flooding, which requires CO₂-philic surfactants with high solubility in supercritical CO₂ (scCO₂). This study elucidates the molecular and atomic-level dissolution mechanism of the fatty alcohol polyoxyethylene ether (AEO) in scCO₂ by combining phase-behavior experiments and molecular dynamics simulations. The solubility of AEO in scCO₂ is measured using a semiconductor laser. AEO containing three polyoxyethylene (EO) groups exhibited the highest solubility in scCO₂.The solubility of AEO in scCO₂ decreased with an increase in the number of EO groups. Molecular dynamics simulations reveal that the interaction force between the AEO and CO₂ is predominantly determined by van der Waals forces, which account for 70% of the total interaction force, while the interactions between the alkyl chain, EO group, and CO₂ in AEO are primarily driven by Lewis acid–base (LA-LB) interactions and dispersion forces. The strength of these interactions is a key determinant in the dissolution of AEO–CO₂, while the structure of AEO facilitates its dissolution in CO₂. The number of CO₂ binding sites in AEO molecules increases with the free volume fraction and diffusion coefficient, allowing an optimal conformation with superior CO₂ affinity owing to greater contact with CO₂ molecules. Ethanol interacts with CO₂, thereby strengthening the LA-LB interactions and dispersion forces between AEO and CO₂ and thus significantly enhancing the solubility of AEO in CO₂.