Estimation of the Hydrophobicity

Hydrophobicity is consideied one of the most important physicochemical properties in the design process of a new bioactive substance. It can be related to biological activities such as absorptivity, transportation, bioaccumulation, effectivity, and toxicity, and it is taken as a parameter able to reflect interactions of the drugs with biomembranes [7,8]. 

A typical microemulsion in MEEKC consists of sodium dodecyl sulfate (SDS) as surfactant that also implements charges to the microdroplets, 1-butanol as cosurfactant, octane, or hexane as oil and an aqueous buffer at pH 7.50, in order to mimic physiological acid-base conditions and also to establish EOF in the capillary. 

Apart from the most commonly used microanulsion, other systans are used to estimate the hydrophobicity of several analytes based on PC as biosurfactant and sodium cholate or sodium deoxycholate as cobiosurfactant, isopropyl myristate as oil and an aqueous phosphate buffer at pH 7.50 (Table 19.1). 

3 Retention Factor and 1-Octanol-Water Partition Coeeeicient 

The partition coefBcients, P, of the analytes in EKC systems can be calculated from their k valnes by nsing reference compounds whose values are known. The refer- 

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Dilling, J., and Kaiser, K. (2002). Estimation of the hydrophobic fraction of dissolved organic matter in water samples using UV photometry. Water Res. 36,5037-5044. 

Since the estimation of the hydrophobicity of a solute requires the determination of the difference between the chemical potentials of the solute in both phases, it is clear that the partition coefficient value should be measured for the solute molecules being in the same form in both phases, i.e., for the nonprotonated or the ionized monomeric species. The basic methods of corrections for the degree of association... 

The effect of the pH and ionic composition of the aqueous phase of the solvent system on the estimates of the hydrophobic character of a number of solutes have been shown by Wang and Lien 60). It has been particularly shown that even the partition coefficients of nonionic solutes depend upon the type of buffer used as an aqueous phase in the w-octanol-buffer system. It seems possible to explain the effects of the ionic composition of the aqueous phase on the partitioning of solutes in the above biphasic system 60) by the influence of the ions on the state and/or structure of water in the phase of the system. 

Hence, the estimates of the hydrophobic character of solutes can be used only for a relative rating of thf solutes of the same or very similar chemical nature. Additionally, the experimental methods considered in this section of the review cannot, as has been repeatedly noted above, be used in the studies of biological solutes, the intact properties of which may be altered by an organic solvent. 

Khayet, M., Matsuura, T. and Mengual, J.l. 2005a. Porous hydrophobic/hydrophilic composite membranes Estimation of the hydrophobic layer thickness, 266 68-79. 

ATo is the Pq/ distribution constant of A", the anionic form of AH. Eq. 9 clearly shows that the D value measured by CCC for an ionizable compound is not Po/w It is highly dependent on the pH and solutepK.. It was demonstrated that three measurements at three pH values around the pKa value allows the determination of the molecular Pq/w value of the solute as well as the Pq/w value of its corresponding anion. Even though the Pq/w values of the ions were always small as expected for such hydrophilic entities, they were not nil. CCC may be the only experimental method allowing estimation of the hydrophobicity of ions. ... 

FIGURE 5.5 (a) The hydroxy amino acids serine and threonine are slowly destroyed during the course of protein hydrolysis for amino acid composition analysis. Extrapolation of the data back to time zero allows an accurate estimation of the amonnt of these amino acids originally present in the protein sample, (b) Peptide bonds involving hydrophobic amino acid residues snch as valine and isolencine resist hydrolysis by HCl. With time, these amino acids are released and their free concentrations approach a limiting value that can be approximated with reliability. 

A recent breakthrough in molecular theory of hydrophobic effects was achieved by modeling the distribution of occupancy probabilities, the pn depicted in Figure 4, rather than applying a more difficult, direct theory of po for cavity statistics for liquid water (Pohorille and Pratt, 1990). This information theory (IT) approach (Hummer et al., 1996) focuses on the set of probabilities pn of finding n water centers inside the observation volume, with po being just one of the probabilities. Accurate estimates of the pn, and po in particular, are obtained using experimentally available information as constraints on the pn. The moments of the fluctuations in the number of water centers within the observation volume provide such constraints. 

Bun ton et al., 1981b). Estimation of the extent of micellar binding becomes a non-problem if the organic iori is very hydrophobic, because then it is completely micellar bound under essentially all conditions (Martinek et al., 1977). Perhaps for this reason, there are many examples of good fits between experimental rate constant-surfactant profiles and those calculated using (5), (6) or equivalent expressions. 

Generally, it is clear how Kd can be predicted for organic hydrophobic pollutants which obey a linear isotherm relationship. First, the organic carbon partition coefficient (i. e.,K0C) is predicted based on either solubility or the octanol-water partition coefficient (K0Vf). Then based on an estimate of the organic carbon fraction in the fine and coarse sediments/soils, Kd can be estimated from Eqs. (a and b) (Table 1). 

Several models have been suggested for the estimation of the distribution ratios of nonionic solutes between water and (practically) immiscible organic solvents. One model takes 1-octanol to represent, in general, lipophilic ( fat-liking ) media, which hydrophobic ( water-fearing ) solutes would prefer over water. Such media may be oils, biological lipid membranes, and, somewhat less suitably, hydrocarbon solvents. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

The same authors [98] have presented a method for the estimation of the number of electroactive disulfide bonds in proteins adsorbed on mercury. The developed approach was based on the assumption that electroactive disulfides are located in more hydrophobic regions of the protein molecule. 

Aqueous solubility is a direct measure of the hydrophobicity of a substance. Therefore, perhaps the most practical way of estimating the intrinsic water solubility (logS) for structurally diverse organic substances is through the use of the Yalkowsky equation [47], which uses regression-derived correlation with logPQ w and melting point (MP) for solids ... 

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