In certain surfactant solutions, such as aqueous cetyltrimethylammonium bromide (CTAB) in 0.1 M KBr, the amphiphiles are found to assemble reversibly into long, flexible worm-like micelles. Above a few per cent surfactant, these form an entangled viscoelastic fluid, reminiscent of a polymer solution. The authors review some recent experimental and theoretical progress concerning the equilibrium statistics and dynamics of these systems, which differ from normal polymer solutions in that the chains can break and reform reversibly.

http://iopscience.iop.org/article/10.1088/0953-8984/2/33/001/pdf

Statics and dynamics of worm-like surfactant micelles

A novel concept for performing stoichiometric and catalytic chemical transformations has been developed that is based on the limited miscibility of partially or fully fluorinated compounds with nonfluorinated compounds. A fluorous biphase system (FBS) consists of a fluorous phase containing a dissolved reagent or catalyst and another phase, which could be any common organic or nonorganic solvent with limited or no solubility in the fluorous phase. The fluorous phase is defined as the fluorocarbon (mostly perfluorinated alkanes, ethers, and tertiary amines)-rich phase of a biphase system. An FBS compatible reagent or catalyst contains enough fluorous moieties that it will be soluble only or preferentially in the fluorous phase. The most effective fluorous moieties are linear or branched perfluoroalkyl chains with high carbon number; they may also contain heteroatoms. The chemical transformation may occur either in the fluorous phase or at the interface of the two phases. The application of FBS has been demonstrated for the extraction of rhodium from toluene and for the hydroformylation of olefins. The ability to separate a catalyst or a reagent from the products completely at mild conditions could lead to industrial application of homogeneous catalysts or reagents and to the development of more environmentally benign processes.

https://science.sciencemag.org/content/sci/266/5182/72.full.pdf

Facile Catalyst Separation Without Water: Fluorous Biphase Hydroformylation of Olefins

The permeability of poly(dimethylsiloxane) [PDMS] to H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8, and solubility of these penetrants were determined as a function of pressure at 35 °C Permeability coefficients of perfluorinated penetrants (CF4, C2F6, and C3F8) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH4, C2H6, and C3H8), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon-based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C3F8/PDMS segment pair requires 460 J/mol more energy than the formation of a C3H8/PDMS pair A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons © 2000 John Wiley & Sons, Inc J Polym Sci B: Polym Phys 38: 415–434, 2000

Gas sorption, diffusion, and permeation in poly(dimethylsiloxane)

Room-temperature ionic liquids (RTILs) are nonvolatile, tunable solvents that have generated significant interest across a wide variety of engineering applications. The use of RTILs as media for CO2 separations appears especially promising, with imidazolium-based salts at the center of this research effort. The solubilities of gases, particularly CO2, N2, and CH4, have been studied in a number of RTILs. Process temperature and the chemical structures of the cation and anion have significant impacts on gas solubility and gas pair selectivity. Models based on regular solution theory and group contributions are useful to predict and explain CO2 solubility and selectivity in imidazolium-based RTILs. In addition to their role as a physical solvent, RTILs might also be used in supported ionic liquid membranes (SILMs) as a highly permeable and selective transport medium. Performance data for SILMs indicates that they exhibit large permeabilities as well as CO2/N2 selectivities that outperform many polymer membra...

/pdf/guide-to-co2-separations-in-imidazolium-based-room-1amwcp9iiq.pdf

Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids

Fluorous solvents have limited miscibility with conventional organic solvents Combined with the fact that compounds functionalized with perfluorinated groups often dissolve preferentially in fluorous solvents this can be used to extract fluorous components from reaction mixtures This review discusses the application and potential of fluorous phase separation techniques for the recovery of soluble metal catalysts

Fluorous phase separation techniques in catalysis

Existing data on 64 fluorocarbon solutions (vapor pressures, solubilities, heats of mixing and volume changes) are summarized, and the free energies deduced are compared in a consistent way with the predictions of Hildebrand's solubility parameter theory. Many solutions, notably fluorocarbon-- hydrocarbon mixtures, show very much larger positive deviations from ideal behavior than can be accounted for by the difference of thermodynamic'' solubility parameters. However, other solutions (e.g., C7F16 with CCl4 and with I2) are entirely normal. Variou s explanations of this anomalous behavior are reviewed and criticized: (1) the interpenetration of hydrocarbon molecules, (2) an empirical shift in 6 for hydrocarbons, (3) volume changes, (4) corresponding states theories, (5) polarity of C-- F bonds, (6) differences in ionization potential, and (7) non-central force fields. Of these, only the last two appear promising and they do not account for the behavior of all of the anomalous systems. Additional experimental and theoretical studies are needed; some directions for future research are suggested.

The Anomalous Behavior of Fluorocarbon Solutions

Phase Equilibria in Solutions of Liquid Sulfur. I. Theory

The f.c.c. structure of a-Nz, a-CO, COz, and KzO is a consequence of the quadrupolequadrupole interactions between the molecules. The classical electrostatic energies resulting from fixed dipoles and quadrupoles in these crystals have been calculated by summation over the first 449 shells on an electronic computer. Depending upon the somewhat uncertain values of the quadrupole moments, the quadrupole-quadrupole contribution ranges from 10 to 40% of the total lattice energy. The additional energy arising froni the dipoledipole interactions is very small, -4.5 and -10.7 cal. mole-l for a-CO and NzO, respectively. 'The Curie temperatures for dipole orientation in these substances are thus around 5 andl llOK., a t which temperatures the rate of the ordering process will be almost infinitesimally slow. Consequently, the disorientation entropy of R In 2 for these substances is always present above these temperatures and will in practice persist to the lowest temperatures.

Energies of Disorientation in Solid Carbon Monoxide and Nitrous Oxide1

Liquid-liquid and liquid-solid phase equilibria in the system methane--perfluoromethane have been studied and the phase diagram constructed. The critical solution temperature is 94.5°K, at a critical mole fraction of CF4 of approximately 0.43; these values are consistent with the free energies of mixing previously reported by Thorp and Scott. The eutectic temperature is 84.5°K at x = 0.88. The volume change on mixing CH4 and CF4 has been measured dilatometrically at 107°K. At x = 1/2 the volume change is 0.88 cm3 mole or 2.1%. This is smaller than the values observed for fluorocarbon- hydrocarbon mixtures at room temperature (ca 5 cm3 per mole); when allowance is made for the smaller excess free energy and the small coefficient of thermal expansion of CF4, the whole series fits together consistently.

Robert L. Scott

Papers

The Anomalous Behavior of Fluorocarbon Solutions

Phase Equilibria in Solutions of Liquid Sulfur. I. Theory

Energies of Disorientation in Solid Carbon Monoxide and Nitrous Oxide1

Fluorocarbon Solutions at Low Temperatures. III. Phase Equilibria and Volume Changes in the CH4-CF4System

Heats of Mixing of Nonelectrolyte Solutions. III. Solutions of the Five Hexane Isomers with Hexadecane1