Molecular Design Principles for Tailoring the Partitioning of CO₂-Soluble Surfactants in CO₂/Water Systems

The molecular design of CO2-soluble surfactants is critical for improving CO2 mobility control in enhanced oil recovery and geological carbon storage. However, current design strategies remain largely empirical due to the lack of predictive structure–property relationships. The CO2 surfactant's performance is governed by its gas–liquid partition coefficient (k) between CO2 and water, which dictates its transport efficiency by supercritical CO2 and in situ foaming behavior. Here, we establish quantitative molecular design principles that enable the predictive tuning of k. A homologous series of nine nonionic surfactants (DFQ-series) with systematically varied architectures is evaluated over a broad range of temperatures (35–65 °C) and pressures (10–26 MPa) using a PVT system. This study proposes a four-level design strategy to optimize the CO2–water partition coefficient. Key structural features─molecular size, segmental composition, and hydrophobic architecture─jointly govern the k value. First, minimizing the total number of propylene oxide (PO) and ethylene oxide (EO) units is essential. A compact surfactant (DFQ-4, 10 units) tends to yield a higher k value. In contrast, an oversized analog (DFQ-3, 22 units) shows poor partitioning. Second, increasing the PO/EO ratio enhances CO2-philicity when the total number of PO and EO groups is fixed. At 35 °C and 26 MPa, DFQ-2 (9PO/9EO) yields k = 1.41, while DFQ-5 (5PO/13EO) reaches only k = 1.07. Third, shortening the linear alkyl tail significantly improves partitioning. Reducing the chain length from C12 to C6 raises k from 0.38 (DFQ-8) to 1.46 (DFQ-6) at 35 °C and 26 MPa. A C18 tail results in complete CO2 insolubility. Finally, introducing tail branching further boosts CO2 solubility. A multibranched surfactant (DFQ-7) reaches a higher k value than its linear counterpart DFQ-6. The proposed design principles enable targeted development of CO2-soluble surfactants for effective carbon capture, utilization, and storage (CCUS) deployment.