Hydrophobic partitioning

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

Now the sum in the numerator indicates that both ion exchange and hydrophobic partitioning with a co-ion (here Na+) are important. The sum in the denominator shows that at this dissolved DS concentration the KDSi is still on the linear portion of the isotherm. 

First, we calculate the fraction in particulate form, (1 -fw), at sorption equilibrium by assuming that hydrophobic partitioning is the dominant sorption mechanism. From Eq. 23-4 and using the values given in Table 24.3 we get ... 

Among many methods proposed for hydrophobic partition, only the method of Shanbhag and Axelsson (1975) is described here. The principle of this method is phase separation by partitioning the hydrophobic upper phase (palmitate is used as it is the most abundant saturated fatty acid in food fat) and the hydrophilic lower phase of proteins. A protein will be separated more into the top layer when it is hydrophobic. The method used by Keshavarz and Nakai (1979), based on Shanbhag and Axelsson (1975), is described in this protocol. 

The most popular method for measuring the polarity of a solute entails determination of the distribution constant between water and a water-immiscible solvent, e.g., octanol. However, because there is difficulty in dissolving proteins in the solvent, a two-phase aqueous system was developed (Shanbhag and Axelson, 1975). Albertson (1986) reported the construction of various aqueous phase systems for partitioning proteins, other macromolecules, and even cells. Recently, simpler aqueous biphase systems were selected for hydrophobic partitioning of proteins (Hachem et al., 1996). However, because of restrictions similar to those for HIC, as discussed above, it may be premature to replace the method used in Basic Protocol 5. The definition of hydrophobicity is based on the polarity of chemical compounds, which is closely related to the distribution between solvents of different polarities. This theory is similar to the elution mechanism of phase distribution chromatography as well as phase partition. However, complexity in the partition system and procedure hampers the broad use of the phase partition approaches. 

Most fluorescence probe methods can accommodate more than ten samples per day. However, other methods such as detergent binding, hydrophobic interaction chromatography, and hydrophobic partitioning can accommodate only one or two protein samples a day. 

Hachem, F., Andrews, B.A., and Asenjo, J.A. 1996. Hydrophobic partitioning of proteins in aqueous two-phase systems. Enzyme Microb. Technol. 19 507-517. 

In practice, the above equation is valid only if the organic carbon content is the primary sorbent, for organic pollutants with molecular weights less than 400, and when the organic pollutant does not have special-function groups that could promote ion exchange or complexation. In other words, when hydrophobic partition is the sole adsorption mechanism, Koc is strongly correlated with the hydrophobic partition coefficient (Koa,). The octanol/ water partition coefficient is defined as follows ... 

Sorption to surfaces can have important effects on the rates of contaminant transformation, but these effects may be very different, depending on how the mechanism of sorption (i.e., hydrophobic partitioning, donor-acceptor interactions, or ligand exchange) relates to the mechanism of contaminant transformation (i.e., reaction in solution, reaction at surface sites, etc.). In general, however, the contributions of each compartment can be treated as additive as long as the kinetics of adsorption/desorption are fast, relative to contaminant transformation (168). Just as with the effect of pH (Section 4.2.3), each term is simply the product of the reactant concentrations in the compartment and the corresponding rate constant. 

Organic ions are known to sorb to soils and sediments via cation exchange. This sorption decreases as pH increases above the pKa. Thus, sorption should be important for a compound with a pKa below or near the pH range of natural waters (i.e., 5-7). The processes responsible for sorption are not limited to cation exchange, however, and the uncharged portion of the molecule can sorb via hydrophobic partitioning. 

In considering an organic-rich slurry wall additive, the governing sorption mechanism is often considered to be hydrophobic partitioning and the effects of solution chemistry (pH, ionic strength, etc.) are considered secondary. The specification ofthe appropriate sorption model requires several choices ... 

Tanaka, A., Nakamura, K., Nakanishi, I. and Fujiwara, H. (1994). A Novel and Useful Descriptor for Hydrophobicity, Partition Coefficient Micellar-Water, and Its Application to a QSAR Study of Antiplatelet Agents. J.Med.Chem., 37,4563-4566. 

It appears that absorption of RDX in soils is not solely the result of hydrophobic partitioning of RDX to the organic carbon phase of the soils. 

Since OPs are hydrophobic compounds, the lipid composition of an organism governs their hydrophobic partitioning (Wallace, 1992). The protective biood-brain barrier (BBB) is permeable to these pesticides. Likewise, the high lipid content of the brain results in a disproportionate amount of accumulation in the brain and partially explains... 

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