Solubility and dissolution mechanism of novel multi-ester headgroup surfactants in supercritical CO₂

To address the limited solubility and applicability of conventional hydrocarbon surfactants in supercritical CO₂, a series of multi-ester headgroup surfactants were designed and synthesized by leveraging the CO₂-philic properties of ester groups. The molecular structures were characterized using Fourier transform infrared (FT-IR) spectroscopy and ¹H NMR. A custom-designed laser-based apparatus was developed to quantify surfactant solubility and systematically investigate phase behavior in CO₂. Molecular dynamics (MD) simulations were employed to elucidate structure–solubility relationships across multiple scales, including solubility parameters, interaction energies, radial distribution functions (RDFs), and free volume fractions. Results indicate that, at 323.15 K, the cloud-point pressure of a 1 wt% multi-ester headgroup surfactant in CO₂ is below 12 MPa. This value is substantially lower than the typical minimum miscibility pressure (MMP) between crude oil and CO₂ in Chinese reservoirs. The number and arrangement of ester groups synergistically affect solubility, with linear configurations outperforming cyclic ones and an optimal ester count (n = 5) maximizing solubility. Surfactant–CO₂ interactions are primarily governed by van der Waals and electrostatic forces, with van der Waals contributions exceeding 70%. RDF analysis reveals that carbonyl oxygen (O1) and terminal methyl carbon (C1) in the ester group are the primary CO₂ binding sites, enhancing CO₂-philicity via Lewis acid–base (LA–LB) and dispersion interactions. Linear ester side chains exhibit enhanced flexibility and conformational adaptability. The free volume fraction initially increases and then decreases with increasing ester group count, with the optimal configuration (MEG-L5) showing maximal chain extension and CO₂ contact efficiency during dissolution. Given that hydrocarbon surfactants primarily interact with CO₂ through LA–LB and dispersion forces, the design focus is on optimizing the density and arrangement of CO₂-philic segments. This modulates entropy changes to enhance favorable enthalpic interactions. This study elucidates the dissolution mechanisms of multi-ester headgroup surfactants in supercritical CO₂ (scCO₂) and identifies the molecular factors controlling their CO₂-philicity. These insights provide a theoretical basis for designing green, fluorine-free, cost-effective CO₂-philic surfactants for enhanced oil recovery.