Hydrophobic solute

Additives, whether hydrophobic solutes, other surfactants or polymers, tend to nucleate micelles at concentrations lower than in the absence of additive. Due to this nucleating effect of polymers on micellization there is often a measurable erne, usually called a critical aggregation concentration or cac, below the regular erne observed in the absence of added polymer. This cac is usually independent of polymer concentration. The size of these aggregates is usually smaller than that of free micelles, and this size tends to be small even in the presence of added salt (conditions where free micelles tend to grow in size). 

FIGURE 2.5 Formation of a clathrate structure by water molecules surrouudiug a hydrophobic 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). 

Figure 4.1. (a) A cellular automata model of hydrophilic solutes in water, (b) A cellular automata model of hydrophobic solutes in water... 

The third application reveals the effects of wall and solute hydropathic states on the accumulation of solute molecules at the wall. The studies show that hydrophobic solutes accumulate in significant concentration at walls with hydrophobic character. 

In thermodynamic terms, solutes can be divided into two classes. For hydrophobic solutes in dilute solution in water, the partial Gibbs free energy of solution is positive. This is because water molecules that surround a less polar molecule in solution are more restricted in... 

SW Kim, JR Cardinal, S Wisniewski, GM Zenter. Solute permeation through hydrogel membranes, hydrophilic vs. hydrophobic solutes. ACS Symp Ser 127 347-359, 1980. 

Our first example of an application of the potential distribution formula [Eq. (5)] is in the calculation of the excess chemical potential of a simple hydrophobic solute dissolved in water (Hummer et al., 1998a, 2000), which we consider to be a hard-core particle that excludes all water molecules from its molecular volume. We note that these purely... 

Thus, for a hydrophobic solute is determined by quantifying the probability po of successfully inserting a hard-core solute of the same size and shape into equilibrium configurations of water, as illustrated in Figure 4. A virtue of this approach is that the thermodynamics of hydrophobic hydration characterized by is determined from the properties of pure water alone. The solute enters only through its molecular size and shape (see Fig. 4). 

Various models have been proposed to describe the interaction between hydrophobic solutes and water [19,20,41-45]. In a recent publication, Moel-... 

Phase separation of the saturated solution from the excess solid solute is a critical process. If a filter is employed, it must be inert to the solvent, it must not release plasticizers, and its pore size must be small enough to retain the smallest particles of the solid solute. Furthermore, steps must be taken to monitor, minimize, and preferably avoid losses of the dissolved solute by adsorption onto the filter material [27-30] and/or onto the vessels, pipettes, and syringes. Typically, the first small volume of filtrate is discarded until the surfaces of the filter and/or vessels are saturated with the adsorbed solute, to ensure that the filtrate analyzed has not suffered significant adsorption losses. Adsorption can be a serious problem for hydrophobic solutes, for which filtration would not be recommended. 

The flow column method has useful advantages. Manipulation of the system prior to analysis is minimized, and so problems such as adsorption or evaporation that may arise from separation of the saturated solution and the undissolved solute are reduced. The method is rapid and precise [22,33,34], and it is valuable for sparingly soluble systems, such as hydrophobic solutes in water. 

Molecules that possess both hydrophilic and hydrophobic structures may associate in aqueous media to form dynamic aggregates, commonly known as micelles. The properties of micellar structures have been discussed in great detail [66-69], but thejr main pharmaceutical application lies in their ability to provide enhanced solubility to compounds lacking sufficient aqueous solubility [70], The ability of a micelle to solubilize compounds of limited aqueous solubility can be understood from consideration of the schematic drawing of Fig. 10a. Above the critical micelle concentration, these molecules orient themselves with the polar ends in interfacing with the aqueous solution and the nonpolar ends at the interior. A hydrophobic core is formed at the interior of the micelle, and hydrophobic solute molecules enter and occupy this region. 

The models of Fromherz and Dill allow the solubilization of hydrophobic solutes near the surface of the micelle and explain how these solutes (in addition to part of the hydrocarbon chains of the surfactants) can be in contact with the water when they are associated with the micelle. However, the debate concerning the structure of the clusters is not yet finished and research on the subject continues. 

Most of the applications of the micelles in aqueous medium are based on the association or solubilization of solutes. The interactions between both can be electrostatic, hydrophobic, or, more normally, a combination of both effects [23, 24], It was thought initially that hydrophobic solutes dissolve in the core of the micelle in the same way in which they would do so in an organic solvent, but... 

Such hydrophobic H-bonding naturally leads to an appreciable reduction in volume, and is therefore increasingly favored at higher pressures. Similarly, in the spirit of LeChatelier s principle, one may expect that the presence of a hydrophobic solute promotes formation of cage structures, i.e., tends to shift the cluster... 

N. Nishi, S. Takahashi, M. Matsumoto, A. Tanaka, K. Muraya, T. Takamuku, and T. Yamaguchi, Hydrogen bonding cluster formation and hydrophobic solute association in aqueous solution of ethanol. J. Phys. Chem. 99, 462 468 (1995). 

In summary the hydrophobic effect can be viewed as a result of the attraction not of the solute to the surface or the water, but of the solvent (water) for itself, which restricts the entry of the hydrophobic solute into the aqueous phase. 

The effect that hydrophobic organic substances can become sorbed to organic coatings can be used to make sorbents by coating inorganic colloids such as ferri-hydrite with surfactants. Such surfactant-coated surfaces contain films of hemicelles (see 4.5) which are able to remove hydrophobic solutes, such as toluene or chloro-hydrocarbons, from solution (Holsen, et al., 1991). 

Influence of Organic Cosolvents. Rao et al. (49) have recently presented a solvophobic model for estimating the sorption of a hydrophobic solute from a mixed solvent. This model is based on the work of Yalkowsky et al. (27), who developed an empirical relationship between the solubility in a mixed solvent system, Sm, and that in pure water given by... 

Since adsorption at a mineral surface is a replacement process, we would expect mineral surfaces with weak affinity for water to have the strongest affinity for hydrophobic solutes. Infrared spectroscopy shows that siloxane surfaces on clays with little isomorphic substitution form weaker hydrogen bonds than water forms with itself (64), which corresponds to one of the definitions of a hydrophobic surface offered by Texter et al. (65) Therefore,... 

The earlier rather complicated evidence for clathrate structures enforced by hydrophobic pairs (from EPR lineshape phenomena for paramagnetic hydrophobic solutes, (23)) and for two states of the hydrophobic bond (from thermodynamic excess functions (2k,25)) is provided with a detailed background by these important theoretical developments. 

Organic modifiers have been frequently employed in CE to increase the solubility of hydrophobic solutes in the aqueous buffer system. Unfortunately, many organic modifiers are UV absorbent and cannot be used without considerable loss of sensitivity of detection. A contactless conductivity detection system has been developed which extends the application range of UV-absorbing solvents [ 119]. As both natural pigments and synthetic dyes absorb in the visible part of the spectra, the application of UV-absorbing organic modifiers in their CE analysis does not cause detection problems. 

Mitragotri S (2002) A theoretical analysis of permeation of small hydrophobic solutes across the stratum corneum based on scaled particle theory. J Pharm Sci 91 744-752. 

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