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 CO 2 , a series of multi-ester headgroup surfactants were designed and synthesized by leveraging the CO 2 -philic properties of ester groups. The molecular structures were characterized using Fourier transform infrared (FTIR) spectroscopy and 1 H NMR. A custom-designed laser-based apparatus was developed to quantify surfactant solubility and systematically investigate phase behavior in CO 2 . 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 CO 2 is below 12 MPa. This value is substantially lower than the typical minimum miscibility pressure (MMP) between crude oil and CO 2 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-CO 2 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 CO 2 binding sites, enhancing CO 2 -philicity via Lewis acid–base 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 CO 2 contact efficiency during dissolution. Given that hydrocarbon surfactants primarily interact with CO 2 through Lewis acid–Lewis base (LA–LB) and dispersion forces, the design focus is on optimizing the density and arrangement of CO 2 -philic segments. This modulates entropy changes to enhance favorable enthalpic interactions. This study elucidates the dissolution mechanisms of multi-ester headgroup surfactants in supercritical CO 2 (scCO 2 ) and identifies the molecular factors controlling their CO 2 -philicity. These insights provide a theoretical basis for designing green, fluorine-free, cost-effective CO 2 -philic surfactants, thereby enhancing oil recovery efficiency in CO 2 -EOR applications.