Organic liquid/water distribution coefficients

Some general trends for organic liquid/water distribution coefficients, exist... 

It is always advisable to measure the distribution coefficient for the specific liquid/ liquid extraction before designing a full treatment system however, for a preliminary trade-off study between different treatment alternatives, this may not be practical. In addition, the selection of the extraction solvent would be facilitated if we could narrow the choices to a subset of organic solvents. There are a number of organic liquid/water distribution coefficients, available in the literature however, by far the system... 

Environmental Factors Affecting Organic Liquid/Water Distribution Coefficients... 

Therefore, monopolar solute values vary significantly between different organic liquid/water systems in a predicable way. As shown above, the complementary H-donor functionality of chloroform increases the distribution coefficient of acetone (an H-acceptor) by a factor of 10. Therefore, chloroform would be the better solvent for the extraction of acetone from water. 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

It is known that the RPLC retention parameters are often strongly correlated to the analyte s distribution coefficient in organic solvent/ water. Generally, the relationship between liquid/liquid (LL) distribution and RPLC retention are of the form of the dimensionless Collander-type equations, e.g., see Eq. (15.21)... 

Continuous extractions of liquids. When the organic compounds is more soluble in water than in the organic solvent (i.e. the distribution coefficient between the organic solvent and water is small), very large quantities of organic solvent must be employed to obtain even a moderately efficient extraction. This may be avoided by the use of an apparatus for continuous extraction where only relatively small volumes of solvent are required. 

Where H is Henry s Law constant for a VOC in water, atm/unit concentration in the aqueous phase P is the total pressure, atm and y is the VOC mole fraction in the gas phase. The product P y is the partial pressure of the VOC above the liquid phase. Henry s Law constants are specific to both the solute and the solvent. The higher the value of H q, the easier it is to remove the VOCs from the aqueous phase. In the nonvolatile solvent, the Henry s Law constant of the VOC (H jj = H q/D) is lowered relative to water due to the greater affinity of the organic solvent for the VOC as measured by the large distribution coefficient. Substituting equation 2 into equation 3 yields ... 

To compute a point on the liquid-liquid saturation curve at the specified T and P, we choose an overall composition and apply the Rachford-Rice algorithm. For example, consider the overall composition having z = 0.3436 and Z2 = 0.3092. We set the convergence tolerance to e = 10 (or 10 near the critical point). Then we guess the fraction of material in the water-rich phase, R = 0.4, and guess the component distribution coefficients = 7, C2 = 2, and C3 = 0.5. Here we use C = jxf, where a indicates the water-rich phase and (1 indicates the organic-rich phase. 

One may first consider the relation between log K and log S (aqueous solubility) since S is the major parameter that influences the partitioning of slightly soluble organic compounds. Aqueous solubility may also be thought of as a special form of partition coefficient in which the compound distributes between an ideal solvent (itself) and water. Hansch et aL (44) were the first to correlate octanol-water partition coefficients with aqueous solubilities for various types of low-molecular-weight organic liquids. Their results are shown in Table II. The regression equation between log and log S for a total of 156 compounds from various classes is... 

When a product is very soluble in water, it is often difficult to extract using the techniques described in Sections 12.4 12.7 because of an unfavorable distribution coefficient. In this case, you need to extract the aqueous solution numerous times with fresh batches of an immiscible organic solvent to remove the desired product from water. A less labor-intensive technique involves the use of a continuous liquid-liquid extraction apparatus. One type of extractor, used with solvents that are less dense than water, is shown in Figure 12.15. Diethyl ether is usually the solvent of choice. 

The values of the distribution coefficient k,i of a volatile liquid i between two essentially completely immiscible liquid phases, an organic liquid (/ = 1) and water (j = 2), are provided in Table 3.P.1 for two molar concentrations, C,i, of liquid i in organic liquid phase y = 1 at 25 °C. 

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