J. of Supercritical Fluids 91 (2014) 77–83 Contents lists available at ScienceDirect The Journal of Supercritical Fluids j our na l ho me page: www.elsevier.com/locate/supflu New method for the determination of surfactant solubility and partitioning between CO 2 and brine Guangwei Ren a,1 , Aaron W. Sanders b , Quoc P. Nguyen a,∗ a Department of Petroleum and Geosystems Engineering, University of Texas, Austin, TX 78712, United States b The Dow Chemical Company, Freeport, TX 77541, United States a r t i c l e i n f o Article history: Received 29 January 2014 Received in revised form 16 April 2014 Accepted 19 April 2014 Available online 2 May 2014 Keywords: Supercritical CO2 Non-ionic surfactant Solubility in CO2 Partition coefficient a b s t r a c t A novel method is presented for measuring solubility in supercritical CO 2 (scCO 2 ), 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, surfac- tant from a known amount of CO 2 solution was transferred into an aqueous solution and the surfactant concentration of the aqueous solution was measured directly by HPLC (high-performance liquid chro- matography). In this work, the partitioning of a series of 2-ethylhexanol (2-EH) alkoxylate surfactants among an aqueous phase (water or brine) and scCO 2 as a function of electrolyte concentration, tem- perature, 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 par- titioning into CO 2 . An understanding of surfactant partitioning between brine and scCO 2 is particularly important in the design of CO 2 foam processes, particularly for surfactant stabilized foam in subsurface systems, where it can affect surfactant transport and foam propagation. In general, the solubility in scCO 2 increased with pressure and decreased with temperature. The partitioning of the surfactants between CO 2 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 scCO 2 and the aqueous phase increased with decreasing EO content. Published by Elsevier B.V. 1. Introduction In recent years, the use of supercritical fluids (ScFs) has offered the opportunity to replace conventional toxic organic solvents in a variety of applications due to their unique properties, such as adjustable solvent power, enhanced mass transfer character- istics, and low surface tension. CO 2 is nontoxic, inexpensive, volatile, nonflammable, readily available in large quantities, and also has an easily accessible critical temperature and pressure (31.1 ◦ C and 7.38 MPa). Due to a very low dielectric constant with weak intermolecular forces, low polarizability per volume and correspondingly weak Van der Waals forces [1], CO 2 is a poor solvent for hydrophilic molecules and polar compounds. In the early 1990s, Consani and Smith [2] tested the solubility of more than 130 commercially available surfactants in scCO 2 at 50 ◦ C and 10–50 MPa. Practically all of those conventional alkyl-functional ∗ Corresponding author. Tel.: +1 5124711204; fax: +1 5124719605. E-mail address: quoc p nguyen@mail.utexas.edu (Q.P. Nguyen). 1 Now at Total E&P R&T USA, Houston, TX 77002, United States. ionic amphiphiles were found to be insoluble or only slightly solu- ble at moderate pressure. To increase the solubility of CO 2 , it was necessary to include fluo- rinated or silicone based hydrophobes to lower the intermolecular interactions of the surfactants. Although these early CO 2 soluble surfactants were used successfully in supercritical fluid research, some inevitable drawbacks have impeded their use in commer- cial applications. Expensive fluorous and silicone-based surfactants have environmental and biological persistence issues. Surfactants with perfluoroalkylpolyether tails also have toxicity concerns [3]. Therefore, efforts have been made to obtain low toxicity and less expensive CO 2 -soluble hydrocarbon based surfactants, including: hydrocarbon polymers [4,5], oxygenated hydrocarbon ionic surfac- tants [6], and nonionic surfactants [7–10]. There are several oxygenated hydrocarbon groups that exhibit more favorable thermodynamic interactions with CO 2 . For example, low molecular weight PPO (poly-(propylene oxide)) (<2000 g/mol) is somewhat CO 2 -soluble at moderate temperatures [11], and higher molecular weight PPO (>2000 g/mol) is also solu- ble in CO 2 at elevated temperatures [12]; therefore, PPO has been used as a CO 2 -philic segment in diblock and triblock nonionic http://dx.doi.org/10.1016/j.supflu.2014.04.010 0896-8446/Published by Elsevier B.V.