Organic Liquid-Water Partitioning

Effect of Temperature and Salt on Organic Solvent-Water Partitioning 

3 Comparison of Different Organic Solvent-Water Systems 

LFERs Relating Partition Constants in Different Solvent-Water Systems Model for Description of Organic Solvent-Water Partitioning Illustrative Example 7.1 Evaluating the Factors that Govern the Organic Solvent-Water Partitioning of a Compound 

One-Parameter LFERs for Estimation of Octanol-Water Partition Constants Polyparameter LFERs for Estimation of the Octanol-Water Partition Constant 

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Log P values have been studied in approximately 100 organic liquid-water systems. Since it is virtually impossible to determine log P in a realistic biological medium, the octanol-water system has been widely adopted as a model of the lipid phase (Leo et al. 1971). Whilst there has been much debate about the suitability of this system (see, e.g., Dearden and Bres-nen 1988), it is the most widely used in pharmaceutical studies. Octanol and water are immiscible, but some water does dissolve in octanol in a hydrated state. This hydrated state contains 16 octanol aggregates, with the hydroxyl head groups surrounded by trapped aqueous solution. Lipophilic (unionized) species dissolve in the aliphatic regions of the octanol, whilst ionized species (see below) are drawn to the polar regions (Franks et al. 1993). The partitioning of solutes in different solvent systems has been reported by El-Tayar et al. (1991). 

The resulting estimated half-life is inversely proportional to vapor pressure the greater the vapor pressure, the greater the extent of volatilization. Conversely, the rate of volatilization will be reduced if the solvent readily dissolves into water or is adsorbed by the soil. Organic carbon-water partition coefficients were compiled for each solvent (see 17.1.6.2.), and vapor pressure data (not shown) were collected from Howard. The resulting half-life estimates (Table 17.1.6) indicated that volatilization would be a major pathway if the liquid solvents were spilled on soil all of the half-life estimates were less than one hour. Thomas cautioned, however, that soil moisture, soil type, temperature, and wind conditions were not incorporated in the simple Dow Model. 

As mentioned before, POP transport in the environment depends on their physicochemical properties [40-54], and these include saturated vapor pressure, solubility, Henry s law constant, octanol-water, octanol-air, and organic carbon-water partition coefficients. The saturated vapor pressure characterizes the capability of a substance to be transferred to the gaseous state. Eollowing the study of Wania and Mackay [40], the efficiency of POP condensation with subcooled liquid pressure (p°L) at 25°C above 1 Pa is very low. POPs with a vapor pressure between 1 and 10" Pa are condensed at a temperature of about -30°C and their deposition may be expected mostly in the polar latitudes. POPs with a vapor pressure of subcooled liquid from 10" to 10" Pa are condensed at a temperature above 0°C and they may reach to the middle latitudes. EinaUy, POPs of low volatility with a vapor pressure of subcooled liquid below 10" Pa are practically not vaporized and these substances may be transported and deposited as fine aerosols or coarse particles [39]. Using the vapor pressure of the subcooled liquid it is possible to characterize the partitioning of a POP between the gas phase and the solid phase of the atmospheric aerosol. The POPs having a lower vapor pressure are better bound with... 

The other way of removing plasticizers from water is their adsorption at liquid-solid interface (i.e., soil or sediments) [11]. This process depends on hydrophobicity of the substance, i.e. is proportional to organic carbon-water partition coefficient Kqq - plasticizers with Kqq value greater than 1000 dm /kg are regarded as having relatively significant affinity for sediments and son. It means that it is of greater importance in the case of 2-ethylhexyl (hydrophobic alkyl chain) than in the case of dimethyl phthalate (see Table 3). 

Fig. 2. Graphs showing influence of organic liquid/water ratios on percent of total acetic, propionic, and butyric acids dissolved in water coexisting with oils solid curves) or octanol dashed curves). Curves are calculated from partition coefficients given by Leo et al. (1971)...

The liquid junction potential from the organic side may be negligible, owing to the use of a nitrobenzene-water partition system containing tetraethylammonium picrate as the salt bridge. The mobilities of both ions in nitrobenzene are similar, and they have similar Gibbs energies of... 

Hansch et al. (1968) established the linear free-energy relationship between aqueous and octanol-water partition of organic liquid. Others, such as Tulp and Hutzinger (1978), Yalkowsky et al. (1979), Mackay et al. (1980), Banerjee et al. (1980), Chiou et al. (1982), Bowman and Sans (1983), Miller et al. (1985), Andren et al. (1987) and Doucette and Andren (1988b) have all presented similar but modified relationships. 

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