Formation of Water-in-Carbon Dioxide Microemulsions with a Cationic Surfactant: A Small-Angle Neutron Scattering Study (original) (raw)
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Super-Efficient Surfactant for Stabilizing Water-in-Carbon Dioxide Microemulsions
Langmuir, 2011
The fluorinated double-tailed glutarate anionic surfactant, sodium 1,5-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)2), was found to stabilize water-in-supercritical CO2 microemulsions with high water-to-surfactant molar ratios (W0). Studies were carried out here to obtain detailed information on the phase stability and nanostructure of the microemulsions by using a high-pressure UV−vis dye probe and small-angle neutron scattering (SANS) measurements. The UV−vis spectra, with methyl orange as a reporter dye, indicated a maximum attainable W0 of 60 at 45 and 75 °C, and SANS profiles indicated regular droplet swelling with a linear relationship between the water core nanodroplet radius and W0. This represents the highest water solubilization reported to date for any water-in-CO2 microemulsion. Further analysis of the SANS data indicated critical packing parameters for 8FG(EO)2 at the microemulsion interface >1.34, representing approximately 1.1 times the value for common aerosol-OT in water-in-heptane microemulsions under equivalent conditions.
2005
During the last two decades colloid and interface science in the field of supercritical fluid technology has brought enormous potentials in the utilization of supercritical carbon dioxide as an environmentally benign solvent. Liquid or supercritical CO2 exhibits solvent properties that are tunable with pressure, and is essentially nontoxic and nonflammable. Emulsions and microemulsions of water and CO2, whether in the form of water-in-CO 2 (w/c) or CO2-in-water (c/w), offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules. For the first time, formation of thermodynamically stable c/w microemulsions was characterized by dynamic light scattering (DLS) technique. High-pressure carbon dioxide swells potassium carboxylate perfluoropolyether (PFPE-K) cylindrical micelles in water, elongating the micelles significantly from 20 up to 80 nm. As the micelles swell to form microemulsions, the solubility of pyrene increa...
Langmuir, 2002
Formation and stability of water-in-carbon dioxide microemulsions are described with eight related fluorinated analogues of the anionic surfactant Aerosol-OT. The aim was to identify a structure-performance relationship in CO2 with surfactants of high surface chemical purity that can also be synthesized from readily available reagents. The most effective CO2-philes from this group were sodium bis(1H,1Hperfluoropentyl)-2-sulfosuccinate (di-CF4), sodium bis(1H,1H,2H,2H-perfluorohexyl)-2-sulfosuccinate (di-CF4H), and sodium bis(1H,1H-perfluoroheptyl)-2-sulfosuccinate (di-CF6). All three of these compounds stabilized microemulsions at CO2 bottle pressure (57 bar) at 15°C, with a w value ([water]/[surf]) of 10. A close correlation is demonstrated between limiting aqueous phase surface tension of a given surfactant at its critical micelle concentration, γcmc, and its performance in water-CO2 microemulsions, as measured by the phase transition pressure Ptrans. This finding has important implications for the rational design of CO2-philic surfactants.
The relationship between the tail architecture and performance of hybrid AOT analogue surfactants hasbeen investigated. Three hybrid surfactants were synthesised using hydrocarbon CO2-philic tails with dif-ferent levels of chain branching. The performance of each surfactant was investigated via high-pressurephase behaviour, UV-visible spectroscopy, and air–water (a/w) surface tension measurements. Notably,the incorporation of hydrocarbon CO2-philic tails with a high degree of branching has been found tosignificantly boost CO2-philicity, allowing the surfactant to stabilise water-in-CO2(w/c) microemul-sions at low cloud pressures, Ptrans. The newly synthesised hybrid CF2/SIS1 (sodium (4H, 4H, 5H, 5H,5H-pentafluoropenyl-5,7,7-trimethyl-2-(1,3,3-trimethyl-buthyl)-octyl)-2-sulfosuccinate) is a CO2-philicsurfactant that contains the lowest amount of fluorine (15.01 wt%) and exhibits the highest efficiencyof any di-chain surfactant to date. High-pressure phase behaviour studies provided a maximum water-to-surfactant molar ratio (wmax) of wmax= 39, which is usually only observed from surfactants with longfluorocarbon chains. The present results are beneficial for expanding the pool of economical, effective,and efficient surfactants available for CO2-based technology.
Shape Transitions in Supercritical CO2 Microemulsions Induced by Hydrotropes
Langmuir : the ACS journal of surfaces and colloids, 2013
The ability to induce morphological transitions in water-in-oil (w/o) and water-in-CO 2 (w/c) microemulsions stabilized by a trichain anionic surfactant 1,4-bis-(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2sulfonate (TC14) with simple hydrotrope additives has been investigated. High-pressure small-angle neutron scattering (SANS) has revealed the addition of a small mole fraction of hydrotrope can yield a significant elongation in the microemulsion water droplets. For w/o systems, the degree of droplet growth was shown to be dependent on the water content, the hydrotrope mole fraction, and chemical structure, whereas for w/c microemulsions a similar, but less significant, effect was seen. The expected CO 2 viscosity increase from such systems has been calculated and compared to related literature using fluorocarbon chain surfactants. This represents the first report of hydrotrope-induced morphology changes in w/c microemulsions and is a significant step forward toward the formation of hydrocarbon worm-like micellar assemblies in this industrially relevant solvent.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
We describe perfluoropolyether (PFPE) surfactants which are capable of stabilising the water/CO 2 interface and present FTIR spectroscopic evidence for the formation of water in supercritical carbon dioxide microemulsions. A wide variety of single chain surfactants of differing chain lengths but similar structure has been screened and the effect of the surfactant chain length on the water uptake was studied. The ammonium carboxylate of the PFPE surfactant Krytox FSL TM with an average molecular weight of 2500 g mol (1 was demonstrated to be the surfactant capable of dissolving the most water out of all the tested surfactants and hence to have the optimum chain length.
The Journal of …, 2010
Measurements of interfacial tensions for 2-ethyl-hexanol-(propylene oxide)∼ 4.5-(ethylene oxide)∼ 8 (2EH-PO 4.5-EO 8) at the planar water-CO 2 interface and the surfactant distribution coefficient are utilized to explain microemulsion and macroemulsion phase behavior from 24 to 60 • C and 6.9 to 27.6 MPa. A CO 2 captive bubble technique has been developed to measure the interfacial tension at a known surfactant concentration in the aqueous phase, with rapid equilibration at the water-CO 2 interface. The surface pressure (o −) decreases modestly with density at constant temperature as CO 2 solvates the surfactant tails more effectively, but changes little with temperature at constant density. The area per surfactant at the CO 2-water interface determined from the Gibbs adsorption equation decreases from 250 A 2 /molecule at 24 • C and 6.9 MPa, to 200 A 2 /molecule at 27.6 MPa. It was approximately twofold larger than that at the water-air interface, given the much smaller o driving force for surfactant adsorption. For systems with added NaCl, decreases with salinity at low CO 2 densities as the surfactant partitions from water towards the W-C interface. At high densities, salt drives the surfactant from the W-C interface to CO 2 and raises. Compared with most hydrocarbon surfactants, this dual tail surfactant is unusually CO 2philic in that it partitions primarily into the CO 2 phase versus the water phase at CO 2 densities above 0.8 g/ml, and produces values below 1 mN/m. With this small , a middle phase microemulsion and a C/W microemulsion were formed at low temperatures and high CO 2 densities, whereas macroemulsions were formed at other conditions.
Soft fluctuating surfactant membranes in supercritical CO2-microemulsions
Physical Chemistry Chemical Physics, 2011
The bending rigidity of surfactant membranes in novel bicontinuous CO 2-microemulsions of the type H 2 O/NaCl-scCO 2-Zonyl FSH/Zonyl FSN 100 was determined using both high pressure small angle neutron scattering and neutron-spin echo spectroscopy. As an important result it was found, that the stiffness of the membrane increases solely by an increase of the pressure. The waste of chemical solvents from chemical processing and related industries represent a huge environmental concern. During the last decades supercritical fluids have attracted much attention as potential replacements for these conventional organic solvents. Supercritical CO 2 (T c = 31.1 1C, p c = 72.8 bar) is seen as the most promising candidate because it is cheap, abundant, incombustible, non-toxic, bio-and foodcompatible. Unfortunately scCO 2 is generally a very poor solvent, in particular for polar and/or high molecular weight solutes. Thus, in applications where both polar and nonpolar components needed to be dissolved, emulsions and microemulsions were used to overcome this severe limitation. 1 Furthermore, scCO 2 microemulsions are also of great interest from a theoretical point of view because, contrary to classical ''state of the art'' microemulsions, the properties of scCO 2 microemulsions can be strongly influenced just by varying the pressure, i.e. the solvent properties of CO 2. So far studies on these novel microemulsion systems concentrate on the phase behavior and the microstructure of CO 2-rich microemulsions. 1-3 Recently, we were able to formulate balanced supercritical CO 2-microemulsions containing equal volumes of water and CO 2 using technical grade n-alkyl-polyglycolether-and perfluoroalkyl-polyglycolether-surfactants. 4,5 The phase behavior of such a microemulsion system H 2 O/NaCl-scCO 2-Zonyl FSH/Zonyl FSN 100 is shown in Fig. 1 for pressures of p = 160 (top), 220 (middle) and 300 bar (bottom) as a function of the overall surfactant mass fraction g and the temperature T. 5 Thereby the mass fraction a of CO 2 in the mixture of water and CO 2 was adjusted to a = 0.40 and the surfactant mixture consists of equal amounts of Zonyl FSH and Zonyl FSN 100 (d = 0.50). NaCl (1 wt% in the mixture of water and NaCl; e = 0.01) was added to screen possible electrostatic interactions induced by ionic impurities.