Partition phenomena

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The extremely large number of partitioning phenomena greatly exaggerates the differences in the relative solubilities of the solutes and results in the effective separation of the components. Those solutes that show greater solubility in the stationary phase will move less rapidly up the paper than those solutes that show a greater solubility in the mobile phase. 

Ethoxylated nonionic surfactants approximately obey a hnear mixing rule expressed as Eq. 1 when the characteristic property is the averaged number of ethylene oxide groups per molecules (EON) [35]. The goodness of the fit depends on the partitioning phenomena, which will be discussed later, in Sect. 4. 

In the absence of electron donor-acceptor interactions, the London dispersive energy is the dominant contributor to the overall attractions of many molecules to their surroundings. Hence, understanding this type of intermolecular interaction and its dependency on chemical structure allows us to establish a baseline for chemical attractions. If molecules exhibit stronger attractions than expected from these interactions, then this implies the importance of other intermolecular forces. To see the superposition of these additional interactions and their effect on various partitioning phenomena below, we have to examine the role of dispersive forces in more detail,... 

Figure 3.9—Adsorption and partition phenomena. Unlike absorption, adsorption is a surface phenomenon (reproduced by permission of M. Lagues, L Actualite Chimique 1990, (1) p. 17).

An important step in the understanding of partitioning phenomena is the ability to link between a drug molecule s pKa and its pH-dependent solubility. The major tool for this estimation has been, for the past 80 years, the Henderson-Hasselbalch equation [133]. The ionized fraction of a drug can be calculated for any pH value if its pKa is known [134], as follows ... 

In order to be able to treat such partition phenomena with COSMO-RS, we have developed a slightly more empirical extension to the COSMO-RS theory, which is called the V-moment approach , [see C14,C16,C21 for more details]. The basic idea of this approach results from the finding that the er-potential /is(a) for almost all solvents can be well represented as a linear combination of about six er-functions. These are the simple polynomials with exponent i = 0,1,2, and 3,... 

Leppard, G.G., DeVitre, R.R., Perret, D. and Buffle, J. (1989) Colloidal iron-oxhydroxy-phosphate the sizing and morphology of an amorphous species in relation to partitioning phenomena. Sci. Total Environ., 87/88, 345-354. 

The amount transferred depends on the type and strength of the interactions between / and the source and receptor phases. The key interaction types are the dipole-dipole and hydrogen bonding. Surface or interface adsorption may also play a key role in certain partitioning phenomena however, only bulk interactions (i.e., dipole - dipole and hydrogen bonding) are considered in the present discussion. 

Hydrophobieity parameters for organic substituents have been developed by Hansch et al. using partitioning phenomena [296], and by Menger et al. using kinetic measurements (hydrolysis of long-chain esters) [297] see Section 7.2. Further results connected with the presence of hydrophobic interactions in solutions are discussed in Sections 2.5 and 5.4.8. 

In order to avoid possible associations of the solute in the organic phase, partition coefficients should be measured at low concentrations or extrapolated to infinite dilution of the solute. They are dimensionless measures of the relative affinity of a molecule with respect to the two phases and depend on absorption, transport, and partitioning phenomena. Compounds for which F > 1 or logP > 0 are lipophilic, and compounds for which F < 1 or logF < 0 are hydrophilic. In particular, lipophilicity depends on solute bulk, polar and hydrogen-bonding effects. 

Electrokinetic chromatography (see Section 23.6) is ahybrid of HPLC and CE. For this technique the capillaries are packed with a stationary phase and the separation is based on partition phenomena. The mobile phase acts as in CE it consists of a buffer solution and moves thanks to the applied electrical field. 

The kinetic features of POase in solid-phase assays may be masked by diffusion or partitioning phenomena (Section 9.2.2). Most probably, an incorrect H2O2 concentration is the most important single factor in interassay variations and enzyme inactivation. 

Equations [18] to [22] represent a homolytic pathway for the oxidation of phenolic compounds, in which a collection of reactive free radical intermediates are produced. Several reactions in parallel eventually consume the phenoxy radical ArO , possibly involving readsorption and additional oxidation by the mineral surface. Dimers, trimers, etc., produced by oxidative coupling are less soluble and more likely to adsorb to the mineral surface. Partitioning phenomena are therefore important in determining the nature and final distribution of reaction products. 

Partition coefficients are dimensionless measures of the relative affinity of a molecule with respect to the two phases and depend on absorption, transport, and partitioning phenomena. 

Selection of the octanol-water system is often justified in part beeause, like biological membrane components, oetanol is flexible and contains a polar head and a nonpolar tail. Hence, the tendency of a drug molecule to leave the aqueous phase and partition into oc-tanol is viewed as a measure of how efficiently a drug will partition into and diffuse across biological barriers such as the intestinal membrane. Although the octanol-water partition coefficient is, by far, most commonly used, other solvent systems such as cyclohexane-water and chloroform-water systems offer additional insight into partitioning phenomena. 

Bums DC, Ellis DA, Li H et al (2008) Experimental pKa determination for perfluorooctanoic acid (PFOA) and the potential impact of pKa concentration dependence on laboratory-measured partitioning phenomena and environmental modeling. Environ Sci Technol 42 9283-9288... 

CHAPTER 16 Sorption, lipophilicity and partitioning phenomena of ionic liquids in... 

Sorption, Lipophilicity and Partitioning Phenomena of Ionic Liquids in Environmental Systems... 

---