Methanol, properties viscosity

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Fio. 16. Dependence of solvent properties pertinent to RPC on composition of water-methanol mixture at 25 C. SuifisiM tension y data were obtained from Hmmemums (/i ) the viscosity and dielectric constant data c were taken from Carr and Riddick (135) and Akeriof (136), respectively. Reprinted from Horvdth and Melander (/29). J. Chroma-togr. Sci., with permission from Reston Publications. 

As its name suggests, supercritical fluid extraction (SEE) relies on the solubilizing properties of supercritical fluids. The lower viscosities and higher diffusion rates of supercritical fluids, when compared with those of liquids, make them ideal for the extraction of diffusion-controlled matrices, such as plant tissues. Advantages of the method are lower solvent consumption, controllable selectivity, and less thermal or chemical degradation than methods such as Soxhlet extraction. Numerous applications in the extraction of natural products have been reported, with supercritical carbon dioxide being the most widely used extraction solvent. However, to allow for the extraction of polar compounds such as flavonoids, polar solvents (like methanol) have to be added as modifiers. There is consequently a substantial reduction in selectivity. This explains why there are relatively few applications to polyphenols in the literature. Even with pressures of up to 689 bar and 20% modifier (usually methanol) in the extraction fluid, yields of polyphenolic compounds remain low, as shown for marigold Calendula officinalis, Asteraceae) and chamomile Matricaria recutita, Asteraceae). " ... 

There remains little more for the operator to decide. Sometimes, alternative but similar solvent mixtures that have a lower viscosity or higher solute diffusivity could be selected. For example, a n-hexane/methanol mixture might be chosen as an alternative to the more viscous n-heptane/isopropyl alcohol mixture as it has similar elution properties. However, it will be shown later, that if a fully optimized column is employed the viscosity of the mobile phase does not seem to effect the column performance as it is taken into account in the optimization procedure. The operator would, under some circumstances, be free to choose less toxic or less costly solvents for example, in reverse phase chromatography the operator could select methanol/ water solvent mixtures as opposed to acetonitrile/water mixture on the basis of lower cost or less... 

A serious problem results from the fact that methanol is a toxic fluid (extreme permissible concentration of methanol in the air is 5 mg/m3) and handle it is possible only in specialised laboratories. For the pipeline laboratory research it is useful to find such substitute of methanol, which is absolutely harmless and its physical properties (density and viscosity) strictly correspond to methanol. 

The volumetric gas-liquid mass transfer coefficient, khaL, largely depends on power per unit volume, gas velocity (for a gassed system), and the physical properties of the fluids. For high-viscosity fluids, kLaL is a strong function of liquid viscosity, and for low-viscosity fluids (fi < 50 mPa s), kLaL depends on the coalescence nature of the bubbles. In the aeration of low-viscosity, pure liquids such as water, methanol, or acetone, a stable bubble diameter of 3-5 mm results, irrespective of the type of the gas distributor. This state is reached immediately after the tiny primary bubbles leave the area of high shear forces. The generation of fine primary gas bubbles in pure liquids is therefore uneconomical. 

Most of the common alcohols, up to about 11 or 12 carbon atoms, are liquids at room Physical Properties temperature. Methanol and ethanol are free-flowing volatile liquids with characteris-Of AI CO hols c frui y °dors. The higher alcohols (the butanols through the decanols) are somewhat viscous, and some of the highly branched isomers are solids at room temperature. These higher alcohols have heavier but still fruity odors. Propan-1 -ol and propan-2-ol fall in the middle, with a barely noticeable viscosity and a characteristic odor often associated with a physician s office. Table 10-2 lists the physical properties of some common alcohols. 

The PAA-PVP system in methanol and the PMAA-PVP system in DMF display the typical properties of polycomplexes, namely low values of intrinsic viscosity independent of the matrix molecular weight (for PVP with molecular weight of 50 MO and 560000, the intrinsic viscosity [ /], of PA A-PVP complexes in methanol is equal to 0.07 dl/g), the absence both of the concentration dependence of the reduced viscosity, and of the polyelectrolyte anomaly. 

Water has interesting and unusual thermal properties, which have only recently been significantly exploited by chromatographers [56-58], As the temperature is increased, thermal motion weakens the hydrogen-bonding so that the polarity of water is reduced (Figure 18-8). At 200°C, water has a polarity similar to that of methanol in addition, the viscosity also drops markedly with temperature and the diffusion rate increases. Flowever, the vapor pressure remains low and by 250°C has only reached 30 bar, well within the normal... 

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