Interfaces in supercritical fluid

Colloids and Polymers at Interfaces in Supercritical Fluids Theory and Simulation... 

Simulations of chains grafted to interfaces in vacuum and in liquid solvents have been reviewed[73]. The repulsive force of interaction between surfaces coated with grafted athermal chains (good solvent conditions) has been calculated with lattice MC[74] and continuum molecular dynamics[75] methods. The first simulations of interfaces in supercritical fluids considered the adsorption of pure solvent (no chains) in a flat-wall pore[76]. Near the solvent critical temperature (7 ) a maximum in adsorbed amount was observed at densities slightly below the solvent critical density (pc) The maximum in adsorbed amount was attributed to local density enhancement of solvent in the pore. 

Figure 8.22 Schematic diagram of the Suprex MPS/225 integrated aupercritical fluid extractor, cryogenically focused interface and supercritical fluid chromatogra d>. The bold lines represent the direction of fluid flow in the load and inject positions.

Dispersions in Supercritical Fluids. The ability to design surfactants for the interface between water (or organics) and supercritical fluids... 

Johnston, K.P. (2000) Block copolymers as stabilizers in supercritical fluids. Curr. Opin. Colloid Interface Sci., 5(5-6), 351-6. 

Major applications of SFE-SFC are somewhat limited at the moment to the analysis of lipids and pesticides from foods and similar matrices and different types of additives used in the production of polymers [79,146,188-194]. The approaches used cover a wide range of sophistication and automation from comprehensive commercial systems to simple laboratory constructed devices based on the solventless injector [172,174,175,188]. Samples usually consist of solid matrices or liquids supported on an inert carrier matrix. Aqueous solutions are often analyzed after solid-phase extraction (SPE-SFE-SFC) to minimize problems with frozen water in the interface [178,190]. The small number of contemporary applications of SFE-SFC reflects a lack of confidence in supercritical fluid chromatography as a separation technique and competition for... 

At the liquid-SC CO2 inter ce, a constant, and sufficient high concentration of extractant Cyanex 302 is assumed. Concentration effects of generated metal-complexes are assumed to be negligible, due to diffusion coefficient of solutes in supercritical fluids of about 10" m /s (17), which is approximately 2 orders of magnitude fester than in the aqueous phase. A continuous flow of solvent during extraction even reduces surfece effects, due to both continuous supply of exbactant and continuous removal of metal-extractant complex. Thus, mass transfer is a limiting factor at the liquid-SC CO2 interface, as studied previously by Tai et al. (18). Furdier research is required to study a possible impact on the overall extraction of SFE from humid MSWI fly ash. 

THEORY AND SIMULATION OF COLLOID AND INTERFACE SCIENCE IN SUPERCRITICAL FLUIDS... 

In this paper we review principles relevant to colloids in supercritical fluids colloids in liquids are discussed elsewhere [24]. Thermodynamically unstable emulsions and latexes in CO2 require some form of stabilization to maintain particle dispersion and prevent flocculation. Flocculation may be caused by interparticle van der Waals dispersion forces (Hamaker forces). In many of the applications mentioned above, flocculation of the dispersed phase is prevented via steric stabilization with surfactants, in many cases polymeric surfactants. When stabilized particles collide, polymers attached to the surface impart a repulsive force, due to the entropy lost when the polymer tails overlap. The solvent in the interface between the particles also affects the sign and range of the interaction force, and the effect of solvent is particularly important for highly compressible supercritical solvents. Since the dielectric constant of supercritical CO2 and alkanes is low, electrostatic stabilization is not feasible [24] and is not discussed here. For lyophobic emulsion and latex particles (-1 xm), the repulsive... 

The simulations give information on the local solvent environment and structure of polymers at interfaces not currently available from experiments in supercritical fluids, as in Figure 5b. For example local solvent density enhancement observed in simulations and experiments of pure supercritical solvents near interfaces,[76] also occurs near Tc and pc in the simulations for grafted chain-solvent systems. The enhancement of solvent density persists out to large distances (6-7 G in Figure 5b) from the chains because of the intermolecular forces in the confined geometry and since the correlation length of the fluid increases near the critical point. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

T. L. Chester and J. D. Pinkston, Pressure-regulating fluid interface and phase behavior considerations in the coupling of packed-column supercritical fluid chromatography with low-pressure detectors , ]. Chromatogr. 807 265-273 (1998). 

Figure 12.23 SFC-SFC analysis, involving a rotaiy valve interface, of a standard coal tar sample (SRM 1597). Two fractions were collected from the first SFC separation (a) and then analyzed simultaneously in the second SFC system (h) cuts a and h are taken between 20.2 and 21.2 min, and 38.7 and 40.2 min, respectively. Peak identification is as follows 1, tii-phenylene 2, chrysene 3, henzo[g/ i]perylene 4, antliracene. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al, Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switcliing interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

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