Water-miscible substances

Miscible A term used to specify the degree that two substances will mix with one another. Completely miscible substances such as water and ethanol will mix uniformly no matter the proportions. 

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

A potent irreversible inhibitor (abbreviated DFP) of many serine proteinases and serine esterases (especially acetylcholinesterase). This substance is EXTREMELY POISONOUS, but the vapor state can be minimized by using dry, water-miscible solvents such as 2-propanoL. Aqueous solutions become inactivated by hydrolysis, but solutions made with dry 2-propanol are stable at -20°C for many months. 

Often the enzyme stability can be improved by using a suitable water-immiscible solvent instead of a water-miscible one. Two-phase systems are obtained with the enzyme and other hydrophilic substances present in the aqueous phase while hydrophobic substrates and products mainly partition to the organic phase (Figure 9.1). Water immiscible solvents often used for enzymatic reactions are hydrocarbons, ethers and esters further details on solvents are found in the section 9.5 Selection of solvents , below. In order for the bioconversion to occur, the substrates must be transferred to the enzyme in the aqueous phase after the reaction hydrophobic... 

For most purposes, it is convenient to classify suppository bases according to their physical characteristics into two main categories and a third miscellaneous group (i) fatty or oleaginous bases (ii) water-soluble or water-miscible bases, and (iii) miscellaneous bases, general combinations of lipophilic and hydrophilic substances (43). Appendix V presents a survey of commercial pharmaceutical suppositories and the respective suppository bases. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

As indicated by equation 35, the vapor pressure—solubility ratio (eq. 34), only provides accurate results when (/) reliable vapor pressure and aqueous solubility data are employed, (2) the quantity x is small enough to be neglected, and (3) Henry s law applies for the dissolved substance in water up to the saturation limit. If water miscibility in the organic is expected to be significant, equation 35 is much preferred, assuming miscibility data are available (42). This correction can amount to several percent even when the aqueous solubility is low. For example, water solubility in /2-octanol is about 5% whereas the -octanol solubility in water is in the parts per thousand range. 

The use of organic solvents is an alternative to enhance availability of hydrophobic substances, either substrates or products of the reaction, as well as to reduce side reactions. The first studies of peroxidase catalysis in water-miscible and water-immiscible organic solvents were initiated by Dordick and Klibanov using commercial HRP, one of the most often investigated enzymes [60, 61]. Several studies have been focused on the effect of solvents in peroxidase structure and function... 

Hydrocarbons will dissolve in water BECAUSE substances that have the same polarity are miscible and can dissolve each other. 

Organic solvents have long been used for extraction and sequential extraction, which is fractionation of a sort (Flaig et al., 1975 Schnitzer, 1978). While the direct use of organic solvents in fractionation has not been widespread, nonetheless, the technique has received some attention. For instance, the separation of hymatomelanic acid from precipitated humic acid is obtained by extraction with ethanol (Oden, 1919). Ethanol has been used to bring about fractional precipitation by addition to alkaline solutions of humic acid (Kyuma, 1964 Kumada and Kawamura, 1968). There is no reason why other water-miscible solvents such as acetone and methanol should not be used in this way. Solvents that are highly immiscible with water (e.g., hexane and benzene) do not appear to remove any substantial fraction of humic substances. These are perhaps best used to remove nonhumic substances (such as fats and waxes) prior to extraction. However, recent work by Allen and MacCarthy (personal communication) has shown that more polar water-immiscible solvents, such as methyl isobutyl ketone and diethyl ether, can be used successfully to purify and fractionate humic substances. 

The extraction solvent should be chosen such that the analyte is extracted with as little co-extraction of interfering substances as possible. Furthermore, choose a solvent that is compatible with the solid-phase extraction method that is being used. For example, samples that are high in fat are best homogenized with nonpolar solvents, such as hexane. The use of hexane as an extractant dictates normal-phase sorbents with silica, alumina, Florisil, or a CN sorbent. Samples with high water content are best homogenized with acids, bases, or polar solvents, such as methanol, acetonitrile, or acetone. The use of methanol or one of the water-miscible solvents suggests the use of reversed phase (C-18) and dilution of the extract with water or buffer. Thus, the choice of the extraction solvent will dictate the type of sorbent that will be used in SPE. 

Some liquid substances are able to mix completely with water while others will mix only partially or not at all. If a substance mixes freely with water, only one liquid layer will be seen in the container. If a substance does not mix freely with water, two separate liquid layers will be observed in the container. With the water miscibility test, we observe this ability or lack of ability to mix. If an unknown mixes freely with water, we can conclude with certainty that it is not the same liquid as any liquid studied in Part A that does not mix with water. While at the same time we can conclude that it may be one of those that does mix with water. Thus, this test helps to narrow down the possibilities of what an unknown s identity might be. 

Immunoassay techniques rely upon synthetic antibodies that have been developed to form a complex with petroleum substances. The antibodies in the test kit are immobilised on the walls of a special cell or membrane. Water samples can be added directly, whereas soils are solvent extracted into a suitable water miscible solvent and added to the cell. A known amount of enzyme with an affinity for the antibody is added. After equilibrium is established, the cell is washed to remove any unreacted material. Colour development reagents which react with the enzyme are added. A solution that stops colour development is also added at a specific time, and the optical density is then measured. Samples showing high optical density (colour intensity) contain low concentrations of analytes. Concentration is inversely proportional to optical density. Kits are generally available for, among others, TPH, BTEX and PAH. A correction factor supplied by the manufacturer is used to calculate TPH and this is subject to variation depending on the product type. These tests do not provide information on product type and have limitations dependent upon soil type and homogeneity. Also, field extraction techniques are not as efficient as laboratory-based extraction techniques. 

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