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

Abstract

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.