Solute nonpolar surface area

As the logarithm of 1-octanol-water partition coefficient (log P) describes the hydrophobicity of molecules and the retention of solutes in RP-HPLC depends on the hydrophobicity, a strong correlation can be expected between the log V value and the retention of solutes in RP-HPLC. Besides log P, a considerable number of physicochemical parameters have been tested for their capacity to predict retention in RP-HPLC. Thus, Snyder s polarity index, fraction of positively and negatively charged surface area, molecular bulkiness, nonpolar surface area, electron donor and acceptor capacity, various ster-ical parameters, and the energy of highest occupied molecular orbit have all been included in QSRR calculations. 

When a nonpolar molecule is surrounded by water, stronger than normal water-water interactions are formed around the solute molecule to compensate for the weaker interactions between solute and water. This results in an increasingly ordered arrangement of water molecules around the solute and thus a negative entropy of dissolution. The decrease in entropy is roughly proportional to the nonpolar surface area of the molecule. The association of two such nonpolar molecules in water reduces the total nonpolar surface area exposed to the solvent, thus reducing the amount of structured water, and therefore providing a favourable entropy of association. 

Clearly eq. [5.5.24] possesses the functional flexibility to describe the data. (In some systems a 1 -step (2-state) equation is adequate. To transform eq. [5.5.24] to a 1-step version, set K2 = 0 and let / = Yj - Yi ) The next step is to examine the parameter values for their possible physical significance. It seems plausible that K, and Kj should be larger than tmity, but not very large, on the basis that the solutes are organic and so are the cosolvents, but the cosolvents are water-miscible so they are in some degree water-like. In fact, we find that nearly all Ki and Kj values fall between 1 and 15. Likewise the gA values seem, in the main, to be physically reasonable. Earlier estimates of g (reviewed in ref. ) put it in the range of 0.35-0.5. A itself can be estimated as the solvent-accessible surface area of the solute, and many of the gA values found were consistent with such estimates, though some were considerably smaller than expected. Since gA arises in the theory as a hydrophobicity parameter, it seemed possible that A in the equation represents only the nonpolar surface area of the... 

In solubility studies of some substituted biphenyls, it was found (see 5.5.3.1) fliat gA evaluated via eq. [5.5.23] was linearly correlated wifli die nonpolar surface area of the solutes rather than with dieir total surface area the correlation equation was gA = 0.37 A poiar- K was concluded diat die A in the parameter gA is the nonpolar surface area of die solute. This conclusion, however, was based on die assumption diat g is fixed. But the correlation equation can also be written gA = 0.37 F p ,3Aoiai, where F p<,b,= A,K.npoiar/A, is the fraction of solute surface area that is nonpolar. Suppose it is admiUed that g may depend upon the solute (more particularly, it may depend upon the solute s polarity) then the correlation is consistent with the identities A = A fai and g = 0.37... 

Attempts have been made to distinguish between these theories on the basis of the AH° and values anticipated for the two theories, but it may be illusory to think of them as independent alternatives. The eavity model has been criticized on the basis that it eannot account for certain observations such as the denaturing effect of urea, but it must be noted that the cavity theory includes not only the cavity term AAy, but also a term (or terms) for the interaction of the solutes and the solvent. A more eogent objeetion might be to the extension of the macroseopic concepts of surface area and tension to the molecular scale. A demonstration of the validity of the cavity concept has been made with silanized glass beads, which aggregate in polar solvents and disperse in nonpolar solvents. 

There are surprisingly few studies of the retention mechanism for open tubular columns but the theory presented for packed columns should be equally applicable. For normal film thicknesses open tubular columns have a large surface area/volume ratio and the contribution of interfacial adsorption to retention should be significant for those solutes that exhibit adsorption tendencies. Interfacial adsorption has been shown to affect the reproducibility of retention for columns prepared with nonpolar phases of different film thicknesses [322-324]. The poor reproducibility of retention index values for columns prepared from polar phases was demonstrated to be c(ue to interfacial... 

AGg (X) can be removed by assuming that it is equivalent to the polar contribution to the free energy of solution of solute X in a nonpolar hydrocarbon solvent, such as squalane. A second reason for using a reference hydrocarbon solvent is to correct, at least partially, for the fact that the hardcore van der Haals volume is a poor estimate of the size of the cavity and its accessible surface for solvent interactions for aromatic and cyclic solutes. The solvent accessible surface area would logically be the preferred parameter for the cavity term but is very difficult to calculate while the van der Haals volume is readily accessible. With the above approximations the solvent interaction term for... 

For three nonpolar solvents (cyclohexane, hexadecane and benzene), solvation is promoted by the solute having a large total surface area and large fl 47... 

Given the interest in extended carbon systems in recent years, it seemed useful to study the solubility of C60 (fullerene) in various organic liquids.54 55 It was now for the solvents that the molecular surface properties were computed. The resulting Eq. (14) shows that, for this large nonpolar solute, solubility is enhanced by solvent molecule surface area and by the latter having somewhat... 

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