New method for the determination of surfactant solubility and partitioning between CO2 and brine

A novel method is presented for measuring solubility in supercritical CO₂ (scCO₂), which can be used in conjunction with traditional cloud point measurements to obtain information directly on the soluble portion of a given sample and, ultimately, a much more informative data set. In this method, surfactant from a known amount of CO₂ solution was transferred into an aqueous solution and the surfactant concentration of the aqueous solution was measured directly by HPLC (high-performance liquid chromatography). In this work, the partitioning of a series of 2-ethylhexanol (2-EH) alkoxylate surfactants among an aqueous phase (water or brine) and scCO₂ as a function of electrolyte concentration, temperature, and pressure was also investigated. Surfactant partition coefficient was determined based on the reduction of HPLC measured surfactant concentration in the aqueous phase due to surfactant partitioning into CO₂. An understanding of surfactant partitioning between brine and scCO₂ is particularly important in the design of CO₂ foam processes, particularly for surfactant stabilized foam in subsurface systems, where it can affect surfactant transport and foam propagation. In general, the solubility in scCO₂ increased with pressure and decreased with temperature. The partitioning of the surfactants between CO₂ and water phases was almost proportional to pressure, and decreased as temperature increased, where the latter held more sensitivity. The partition coefficient was very sensitive to surfactant formula. For the 2-EH-PO5-EO_x series, the partition coefficient between scCO₂ and the aqueous phase increased with decreasing EO content.