Carbon dioxide/water, water/carbon dioxide emulsions and double emulsions stabilized with a nonionic biocompatible surfactant Enza Torino a,b , Ernesto Reverchon b , Keith P. Johnston a, * a Department of Chemical Engineering, The University of Texas at Austin, 1 University Station C0400, Austin, TX 78712-0231, USA b Chemical and Food Engineering Department, University of Salerno, Via Ponte Don Melillo, 1, 84084 Fisciano (SA), Italy article info Article history: Received 7 January 2010 Accepted 10 April 2010 Available online 24 April 2010 Keywords: Supercritical fluids Emulsions Tween 80 Nonionic surfactant CO 2 -in-water Water-in-CO 2 Double emulsions abstract Whereas microemulsions and emulsions of water and carbon dioxide have been reported for various surfactants with fluorocarbon surfactants, relatively few studies have been successful in forming these emulsions with hydrocarbon surfactants. Stable CO 2 /water and water/CO 2 emulsions with droplets smal- ler than 1 lm were formed at high shear with the nonionic surfactant polysorbate 80 (Tween 80). In order to understand the emulsion phase behavior at high shear, low shear phase behavior experiments were performed at the same temperature and pressure. For pressures up to 250 bar and temperatures of 25–60 °C, C/W emulsions were formed for water concentrations as low as 10%, as the surfactant is highly hydrophilic with limited CO 2 -philicity. However, with added NaCl, the surfactant partitioned away from water toward CO 2 , and W/C emulsions were formed with droplet sizes from a few 100 nm to a few lm in diameter, which were stable for at least 24 h. In addition C/W/C double emulsions are reported for the first time, as well as W/C/W/C triple emulsions. The ability to form emulsions with environmentally benign solvents, CO 2 and water, and biocompatible surfactants is desirable for green reaction and sepa- ration processes, as well as production of materials including pharmaceutical particles and composites. Published by Elsevier Inc. 1. Introduction Water and carbon dioxide (CO 2 ) are the two most abundant and environmentally benign solvents on earth. Liquid or supercritical CO 2 (Tc = 31.1 °C, Pc = 73.8 bar) exhibits solvent properties that are tunable with pressure, and it is nontoxic and nonflammable. Dense CO 2 is nonpolar and has weak van der Waals forces. Disper- sions of water-in-CO 2 and CO 2 -in-water on the nanometer (micro- emulsions) or micrometer (emulsions) scale offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules. The use of C/W or W/C in pharmaceutical applications has great potential and is a relatively unexplored area of research. Applications can include proteins, ions, heavy metals, particle formation, organometallic catalysis and synthesis of polymer colloids, and inorganic nanoparticle pho- toresist drying, emulsion templating in polymerization, and drug delivery. Microemulsions are optically transparent, thermodynam- ically stable, isotropic mixtures of water, CO 2 , and surfactant, and typically consist of disperse phase droplets from 2 to 50 nm in diameter [1,2]. In contrast to microemulsions, (macro) [3–8] emul- sions contain relatively large droplets (>0.1 lm) that are opaque and, although no longer thermodynamically stable, may be kineti- cally stable for long periods. Furthermore, emulsions may be formed with higher interfacial tensions between water and oil (or CO 2 ) than in the case of microemulsions and, thus, with lower values of surfactant adsorption at the interface. Therefore, emulsions may be formed for a wider variety of surfactants than microemulsions, and with lower surfactant concentrations. This becomes especially important when dealing with expensive fluori- nated surfactants often used in emulsions containing CO 2 [9]. Emulsions are inherently unstable because of the large interfa- cial free energy. This thermodynamic instability is manifested in the various mechanisms of emulsion destabilization: aggregation, coalescence, sedimentation, and creaming. Aggregation is the re- sult of attractive forces between droplets and can be prevented by supplying a sufficiently strong steric or electrostatic repulsive force. For water-in-CO 2 (W/C) emulsions, electrostatic repulsion is often negligible, although it has stabilized charged emulsion droplets when the counterions were prevented from ion pairing by micelles in CO 2 [10]. As is well-known for water-in-oil emulsions and microemul- sions, the phase-inversion point, more specifically, the phase-inver- sion temperature (PIT), corresponds to a minimum in interfacial tension and zero net curvature. Under these conditions, the surfac- tant has equal affinity for the oil and the water phases (a so-called balanced system). At the balanced state, the interfacial tension (c) and thus gradients in c are small. Consequently Marangoni stabiliza- tion is weak and emulsions tend to be very unstable. Changing any formulation variable away from the phase-inversion point causes 0021-9797/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.jcis.2010.04.027 * Corresponding author. Fax: +1 512 471 7060. E-mail address: kpj@che.utexas.edu (K.P. Johnston). Journal of Colloid and Interface Science 348 (2010) 469–478 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis