Apolar solute

This effect is explained by a structuring of the solvent surrounding the apolar solute. Table 2 shows a comparison of the thermodynamical excess quantities for mixing the pure solvent with the pure solute to an infinitely diluted solution for hydrophobic and non-hydrophobic solutes, according to Chan et al. 42). 

However, microemulsions have a significant advantage as reaction media over micelles and vesicles in one key respect. They are excellent solubilizing media and will tolerate high concentrations of apolar solutes they are much superior to micelles and vesicles in this respect (Mackay, 1981). 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

AntimonyCni), surface formation, 30 113 Antimony-oxygen system, 30 101 Apolar solute, 32 432... 

Fio. 21. Ehwtropic strengtha of water-methanol on three different columns with polar and apolar solutes. Tlie data were taken on a phenyl phase 0 -Bondapak. fiuty acids) with (a) polar and (b) apolar sohite. on Psitisfl OOS-2 with (c) polar and (d) apolar eluites. and on pyrocaibon witt (e) polar and (0 apolar solutes. Reprinted with permission fiom Colin and Ouiochon W). 

Solvent (Polarity) Hexane apolar) Solute Benzene Diethylether monopolar, monopolar, H-acceptor) H-acceptor) Ethanol bipolar)... 

So far, we have considered rather small-sized organic molecules. Larger molecules such as the PAHs or the PCBs exhibit large positive excess enthalpies (Table 5.3). Apparently, with increasing apolar solute size, water is not able to maintain a maximum of hydrogen bonds among the water molecules involved. Hence, for these types of compounds the excess enthalpy term may become dominant (Table 5.3). 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

The action of salts as inhibitors is somewhat different than that of alcohols or glycols. The salt ionizes in solution and interacts with the dipoles of the water molecules with a much stronger Coulombic bond than either the hydrogen bond or the van der Waals forces that cause clustering around the apolar solute molecule. The stronger bonds of water with salt ions inhibit hydrate formation water is attracted to ions more than water is attracted to the hydrate structure. 

Contradictory to the radius dependence of apolar solutes the solubility of the rare gases in water increases with the radius87 . But this may be an effect of the increasing dispersion forces with the molecular weight of the rare gases. 

Gadelle et al. (1995) investigated the solubilization of various aromatic solutes irbfftRSS-b-PEO (ABA)/PPO-bPEO-bPPO (BAB) triblock copolymers. According to the experimental results, they indicated two different solubilization processes. To understand better the mechanism for solubilization in the polymeric surfactant solutions, it was postulated that (1) the addition of apolar solutes promotes micellization of the polymeric surfactant molecules, (2) the central core of the polymeric micelles contains some water molecules, and (3) solubilization is initially a replacement process in which water molecules are displaced from the micellar core bythesolubilizate. Adetailed discussion of the solubilization process can be found in the next section and the pharmaceutical application section of this chapter. 

The low solubility of hydrocarbons and other mainly apolar substances in water has been ascribed phenomenologically to the hydrophobic interaction. The hydro-phobic free energy can be defined4 as the difference between the standard chemical potentials of an apolar solute at infinite dilution in a hydrocarbon solvent juhc and in water... 

For a more generalized approach to this solubility problem, the question of reproducibility of experimentally known properties of water seems to be in order. Previous computational studies have suggested that the insertion of compounds in aqueous solution is vastly dependent upon the ability of the water model to reproduce structural properties at the desired conditions. Even so, this comparative type of analysis would only be effective in ideal solutes or apolar solutes in the solution. If the solute was to deviate from this spherical shape, the method would need to be modified accordingly. One may even suggest that the CO2 may be treated as more of a single LJ sphere with a series of charges that would reproduce the quadrapole moment, and therefore, coordinate the water in an appropriate solvation, although further study of the aqueous C02 system would be needed to confirm this. 

Kinetics of Reactions Involving Apolar Solutes in Water. . 256... 

Calculation of A//e -quantities from the dependence of AG on temperature is less reliable than direct calorimetric measurements (Franks and Reid, 1973 Frank, 1973 Reid et al., 1969). However, disagreement between published A//-functions for apolar solutes in aqueous solutions may also stem from practical problems associated with low solubilities (Gill et al., 1975). Calorimetric data have the advantage that, as theory shows, the standard partial molar enthalpy H3 for a solute in solution is equal to the partial molar enthalpy in the infinitely dilute solution, i.e. x3 - 0. A similar identity between X3 and X3 (x3 - 0) occurs for the volumes and heat capacities but not for the chemical potentials and entropies. The design of a flow system for the measurement of the heat capacity of solutions (Picker et al., 1971) has provided valuable information on aqueous solutions. 

The actual organization of water molecules around an apolar solute and the mechanism of structure enhancement are not clear cut. Nemethy and Scheraga (1962b) suggested that a solute molecule stabilizes water molecules having four hydrogen bonds by means of dispersion interactions between the solute and water. Regions of... 

The most conceptually attractive model for these solutions is to consider that the organization of water resembles that in the clathrate hydrates (p. 225), the structure being based on pentagonal dodecahedra of hydrogen bonded water molecules (Glew and Moelwyn-Hughes, 1953). This model receives some support from the observation that there is an optimum molecular radius of 4-5 x 10 8 cm for solubility of apolar solutes in water (Franks and Reid, 1973). 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

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