Water relative volatility effects

Although SPME was applied initially for the analysis of relatively volatile environmental pollutants in waters, rapid developments have enabled SPME to be successfully applied for the analysis of pesticides in water, wine and more complex food samples such as honey, fruit juice and pears, vegetables and strawberries. With food samples, most analysts recognize the need for some sample pretreatment in order to minimize matrix effects. The matrix can affect the SPME efficiency, resulting in a reduced recovery of pesticides. The most common method is simply to dilute the sample or sample extract with water. Simpltcio and Boas comminuted pears in water prior to the determination of pesticides. Volante et al. extracted over 100 pesticides... 

In the chemical processing industry, extraction is used when distillation is impractical or too costly. Extraction may be more practical than distillation when the relative volatilities of two components are close. In other cases, the components to be separated may be heat sensitive like antibiotics or relatively nonvolatile like mineral salts. When unfortunate azeotropes form, distillation may be ineffective. Several examples of cost-effective liquid-liquid extraction processes include the recovery of acetic acid from water using ethyl ether or ethyl acetate and the recovery of phenolics from water with butyl acetate. 

Figure 13.23. Examples of vapor-liquid equilibria in presence of solvents, (a) Mixture of-octane and toluene in the presence of phenol, (b) Mixtures of chloroform and acetone in the presence of methylisobutylketone. The mole fraction of solvent is indicated, (c) Mixture of ethanol and water (a) without additive (b) with 10gCaCl2 in 100 mL of mix. (d) Mixture of acetone and methanol (a) in 2.3Af CaCl2 ip) salt-free, (e) Effect of solvent concentration on the activity coefficients and relative volatility of an equimolal mixture of acetone and water (Carlson and Stewart, in Weissbergers Technique of Organic Chemistry IV, Distillation, 1965). (f) Relative volatilities in the presence of acetonitrile. Compositions of hydrocarbons in liquid phase on solvent-free basis (1) 0.76 isopentane + 0.24 isoprene (2) 0.24 iC5 + 0.76 IP (3) 0.5 iC5 + 0.5 2-methylbutene-2 (4) 0.25-0.76 2MB2 + 0.75-0.24 IP [Ogorodnikov et al., Zh. Prikl. Kh. 34, 1096-1102 (1961)].

Figure 3, taken from the data of Dobroserdov (2), gives another example of the substantial effect that a salt, even at reasonably moderate concentration, can have in certain systems. The key components are again ethanol and water, but here the salt is calcium chloride, present at a constant concentration of 10 grams/100 ml of alcohol-water solution. The azeotrope has been completely eliminated, and relative volatility increased substantially. 

Of course, liquid-hquid extraction also may be a useful option when the components of interest simply cannot be separated by using distillation methods. An example is the use of hquid-liquid extraction employing a steam-strippable solvent to remove nonstrippable, low-volatility contaminants from wastewater [Robbins, Chem. Eng. Prog., 76(10), pp. 58-61 (1980)]. The same process scheme often provides a cost-effective alternative to direct distillation or stripping of volatile impurities when the relative volatility of the impurity with respect to water is less than about 10 [Robbins, U.S. Patent 4,236,973 (1980) Hwang, Keller, and Olson, Ind. Eng. Chem. Res., 31, pp. 1753—1759 (1992) and Frank et al., Ind. Eng. Chem. Res., 46(11), pp. 3774-3786 (2007)]. 

Direct measurement of y would confirm whether or not the solution is infinitely dilute at saturation. Lobien and Prausnitz (23) have attempted to measure this effect in a few systems by comparing the solubility limit with measurements of y from differential ebulliometry. The systems they studied all had solubilities of a few percent, and for these systems they found significant deviations from yi = 1/xi. It would be useful to have measurements for more dilute solubilities, but in this case the limiting activity coefficient becomes very large, and ebulliometry is inapplicable for high relative volatilities. Perhaps such data could be taken by ebulliometry for systems where the solute is much less volatile than water, or by chromatographic methods. 

Dynamic headspace isolation is a relatively mild method and can be carried out in a relatively short time. With thermal desorption it can be used for a wide range of compound volatilities. However, thermal desorption can also cause molecular clmges to sensitive molecules. Solvent desorption is milder and applicable to large traps but has the same drawback as the other methods which use solvent in that very volatile compounds are lost or obscured. The use of closed loop stripping and binding of water with excess Na2S04 markedly increases the effectiveness of high flow dynamic headspace isolation for very water soluble volatiles. 

Water had a relatively weak effect on the sensor array. This is helpful if we are trying to detect substances that are much more reactive than water. In fact, it might be possible to configure the sensor array to study flavor notes in aqueous substances such as coffee or tea. If, on the other hand, humidity measurement is desired, then the presence of more volatile constituents in the system may mask any response to humidity. 

Carbohydrates can have a measurable influence on the release and perception of flavors. Carbohydrates change the volatility of compounds relative to water, but the effect depends on the interaction between the particular volatile molecule and the particular carbohydrate. As a general rule, carbohydrates, especially polysaccharides, decrease the volatility of compounds relative to water by a small to moderate amount, as a result of molecular interactions. However, some carbohydrates, especially the monosaccharides and disaccharides, exhibit a salting-out effect, causing an increase in volatility relative to water (Godshall, 1997). 

While the effect maybe so powerful that the relative volatility of the two solvents can be reversed (e.g. it is possible using highly polar water as the entrainer to make ethanol more volatile than methanol) this is seldom done in industrial practice because, unlike the choice of azeotropic entrainers which must lie... 

To illustrate the latter limitation, suppose we take the same ternary mixture considered in Section 10.1. However, now the feed composition is 45 mol% DME, 50mol% MeOH, and 5 mol% water. Can a vapor sidestream column be effectively used The normal boiling point of MeOH is 337.7 K. The normal boiling point of water is 373.2 K. This is a much smaller difference than is the case for DME and MeOH (248.4 K vs. 337.7 K). The result is a relative volatility between MeOH and water of about 1.73. 

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