Micelles, volatilization

Another method is based on the evaporation of a w/o microemulsion carrying a water-soluble solubilizate inside the micellar core [221,222], The contemporaneous evaporation of the volatile components (water and organic solvent) leads to an increase in the concentration of micelles and of the solubilizate in the micellar core. Above a threshold value of the solubilizate concentration, it starts to crystallize in confined space. Nanoparticle coalescence could be hindered by surfactant adsorption and nanoparticle dispersion within the surfactant matrix. 

For CE—MS, volatile buffers are common. To use MEKC systems combined with mass detection, volatile micelles have been tested.When using atmosferic pressure photo-inonization (APPI), non-volatile BGE constituents do not deteriorate the mass signal to... 

Besides CZE and NACE, micellar electrokinetic chromatography (MEKC) is also widely used, and ionic micelles are used as a pseudo-stationary phase. MEKC can therefore separate both ionic and neutral species (see Chapter 2). Hyphenating MEKC with ESI/MS is problematic due to the non-volatility of micelles, which contaminate the ionization source and the MS detector, resulting in increased baseline noise and reduced sensitivity. However, MEKC—ESI/MS was applied by Mol et al. for identifying drug impurities in galantamine samples. Despite the presence of non-volatile SDS, all impurities were detected with submicrogram per milliliter sensitivity and could be further characterized by MS/MS. 

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

Freshwater mammals such as heaver may leave odors on the surface of their ponds and olfactorily sample the water or layer of air immediately above it. Lipids on water may form micelles, small blobs of molecules (from Latin mica, a grain, crumb, morsel) that enhance evaporation into the air layer by increased chemical potential. Some seahirds hunt hy odor (e.g. Hutchison and Wenzel, 1980 Nevitt, 1999). They may respond to prey volatiles (from krill, squid, or fish) that rise to the water surface and evaporate into the air. The air-water equilibrium for dilute solutions can be expressed by using partition coefficients, relative volatility, or Henry s law (Thibodeaux, 1979). 

The oil-in-water emulsion method consists Lrst of preparing an aqueous solution of the copolymer. To this a solution of the drug in a water-insoluble volatile solvent (e.g., chloroform is added to form an oil-in-water emulsion) (Jones and Leroux, 1999). The micelle-drug complex forms as the solvent evaporates. The main advantage of the dialysis procedure over this method is that potentially toxic solvents can be avoided. Both dialysis and oil-in-water emulsion methods were compared for the incorporation of DOX in PECb-PBLA micelles (Kwon et al., 1997). The emulsiLcation method was more efLcient with a DOX loading of 12% (w/w) (Kwon et al., 1997) compared with 8% (w/w) for the dialysis technique (Kwon etal., 1995). 

Since one of the issues raised in this paper is whether the objects seen in the TEM images are a proper representation of the structures present in solution, we will describe briefly sample preparation strategies. For solutions of micelles in hexane, a very volatile solvent, samples for TEM studies could be obtained by aspirating a dilute solution directly onto a carbon-coated copper grid. Most of the solvent likely evaporated as the sample was deposited on the substrate. Alternatively, the TEM substrate could be dipped briefly into a dilute solution of the micelles and allowed to dry. This method also worked for less volatile solvents like decane. For decane, we could also place a small drop (a few pi) of solution on the grid and then touch the edge of the droplet with a Kimwipe to remove excess solvent. For several samples these methods were compared, and we observed the same morphology. 

Reverse micelle solutions of known concentration were prepared in 2 mL constant volume 2.5 inch o.d. by 5/8 inch i.d. stainless steel cells fitted with 1" diameter x 3/8 thick sapphire windows. The path length was 1 cm. The cell was equipped with cartridge heaters and thermostated to 0.1 C by means of a platinum resistance thermometer and a temperature controller. A 60 mL syringe pump pressurized the system, and pressure was controlled to 0.2 bar. The surfactant and pyridine-N-oxide were introduced into the static cell as solutions in volatile solvents, so that concentrations were known to 1 %. The solvent was removed by volatilization and water, if needed, was added with a syringe. Experiments were always performed in the order of increasing pressure so that the overall molarities of surfactant, water, and pyridine-N-oxide were constant. 

For many metals and semi-metals and even for an element such as cadmium, it has recently been described that volatilization can be obtained by vesicle mediation. Indeed, surfactants are able to organize reactants at a molecular level, by which chemical generation of volatile species is enhanced. It was shown, by Sanz-Medel et al. [169], that by adding micelles or vesicles to cadmium solutions it is possible to generate volatile CdH2 with a high efficiency. This volatile compound can even be transported to a measurement cell where a cold vapor of cadmium can be measured. 

The activity coefficient is defined as the relative volatility of the solute in the micelle, compared to the volatility of the solute in an ideal solution at the same mole fraction, i.e., pjp% where p, is the partial pressnre above the micellar solntion and p° the partial pressure above the pure solute. 

The degradation of poly-a-methylstyrene is unaffected by the presence of polystyrene, but depolymerization of the latter polymer can be brought about at temperatures below 300°C by heating in the presence of poly-a-methylstyrene [320], The rate of polystyrene volatilization then varies as an inverse function of the molecular weight of poly-a-methylstyrene. The system is heterogeneous, consisting of micelles of poly-a-methylstyrene embedded in a polystyrene matrix. It has been suggested that the poly-a-methylstyrene chain unzips completely to a monomer radical which diffuses into the polystyrene matrix and attacks a polystyrene molecule. 

PHN, ACE, PYR, CHY, B[a]P, and benzo[e]pyrene were separated in a 50 mM borate buffer (pH 9) containing a mixture of 20 mM neutral methyl-(3-cyclodextrin (M(3CD) and 25 mM anionic sulfobutylether-(3-cyclodextrin (SB(3CD) at 30 kV and 30°C. " B[a]P and benzo[e]pyrene were successfully resolved with the other compounds in under 11 min in a 50-cm effective length of capillary without micelles in the mobile phase. The system was also less sensitive to temperature and separation potential. LIE detection with excitation at 325 nm at 2.5 mW from a He/Cd laser coupled to an optical fiber allowed for detection limits in the sub ppb range. The method described above was applied to the analysis of contaminated soil that had been extracted by supercritical CO2 for 20 min at 120°C and collected in methanol/DCM. ° Of the 16 EPA PAH mixtures, eleven compounds were detectable in the low ppb range. Ten of the eleven detectable compounds were measured in the soil extract. When compared to RP-HPLC, CE values were slightly lower but only six compounds were detected by HPLC-FLD. No direct relationship between PAH molecular size, polarity, or volatility with migration order was observed and B[b]F/B[k]F isomers were readily separated. 

Imidazolium ILs easily form micro-emulsions using different surfactants such as long-chain alcohols and the properties of the new micelle-ionic liquid soluhons can be explored in inverse gas chromatography processes [124]. Moreover, ILs have been used as run buffer additives in capillary electrophoresis [125] and as ultra-low-volatility liquid matrixes for matrix-assisted laser desorption/ionizahon mass spectrometry [126]. 

Altered physical-chemical and solvatochromatic properties (e.g., solubility, volatility, critical micelle concentration [CMC])... 

The first and most widely studied area of chemical-chemical interactions is on the surface of the skin. The types of phenomena that could occur are governed by the laws of solution chemistry and include factors such as altered solubility, precipitation, supersaturation, solvation, or volatility as well as physical-chemical effects such as altm ed surface tension from the presence of surfactants, changed solution viscosity, and micelle formation (Idson et al., 1983 Williams and Barry, 1998 Barry, 2001 Moser etal., 2001). For some of these so-called solvatochromatic effects, chemicals act independmit of one another. However, for many the presence of other component chemicals may modulate the effect seen. 

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