Hydrophobic cosolvent

On the other hand, more hydrophobic cosolvents usually decrease the stabihty of the threedimensional form of the protein the less hydrophihc the solvent the greater the effect on (temperature at which half the protein is unfolded). 

The second approach that has been rather popular with mixed aqueous solvents is to assume that the mixture is more or less structured than that of pure water. There is much evidence to show that the particular hydrogen-bonded structure of water influences many of the properties of electrolytes in water (15). If nonelectrolytes can modify the structure of water (15), they can have an indirect effect on the properties of electrolytes. This explanation has been particularly successful in the case of U + W mixtures (1,2). Such a simple approach is not as successful with hydrophobic cosolvents. For example, AHe°(W — W + TBA) are positive for both alkali halides (16) and tetraalkylammonium bro-... 

Alcohol and acids as enhancers (methanol, ethanol, decyl alcohol, dodecyl alcohol, capric acid, lauric acid, myristic acid) that have been mixed with hydrophobic cosolvents such as n-hexane, n-dodecane, and n-hexadecane... 

Aqueous dispersions are alternatives to solutions of Hquid and soHd resins. They are usuaUy offered in 50% soHds and may contain thickeners and cosolvents as stabilizers and to promote coalescence. Both heat-reactive (resole) and nonheat-reactive (novolak) systems exist that contain unsubstituted or substituted phenols or mixtures. A related technology produces large, stable particles that can be isolated as discrete particles (44). In aqueous dispersion, the resin stmcture is designed to produce a hydrophobic polymer, which is stabilized in water by an interfacial agent. 

With increasing alcohol concentration non-bulk electrostatic contributions become relevant. Because these non-bulk electrostatic contributions depend on the concentration of the cosolvent as well on the size of the alkyl-group, one can conclude that there is a relation to the smaller free energy necessary for exposing hydrophobic surfaces to the medium. It yields ... 

Further, for studying the role of pH and salt concentrations on bulk-electrostatic and non-bulk electrostatic contributions the same approach was made to experiments on the influence of the alcohols mentioned above on the oxygen affinity at various KC1 concentrations and pH-values 144,146). The results obtained indicate that at a low alcohol concentration the bulk-electrostatic contributions are dominant and that with increasing size of the alkyl group, alcohol and KC1 concentration, the nonbulk electrostatic, hydrophobic contributions increase. Recent results of kinetic measurements of 02 release show that cosolvents such as alcohols and formamide influence mainly the allosteric parameter L, i.e. -the equilibrium between T and R conformation and that the separation of the alcohol effects into bulk-electrostatic and hydrophobic (non-bulk electrostatic) contributions is justified. 

Hydrolysis of substrates is performed in water, buffered aqueous solutions or biphasic mixtures of water and an organic solvent. Hydrolases tolerate low levels of polar organic solvents such as DMSO, DMF, and acetone in aqueous media. These cosolvents help to dissolve hydrophobic substrates. Although most hydrolases require soluble substrates, lipases display weak activity on soluble compounds in aqueous solutions. Their activity markedly increases when the substrate reaches the critical micellar concentration where it forms a second phase. This interfacial activation at the lipid-water interface has been explained by the presence of a... 

An intriguing influence of a cosolvent immiscible with water on the enantioselec-tivity of the enzyme-catalyzed hydrolysis was observed. It was proven that enzyme enantioselectivity is directly correlated with the cosolvent hydrophobicity. In the best example, for ethyl ether as cosolvent, the reaction proceeded with E = 55, and the target compound was obtained in 33% yield with 92.7% ee. This finding may be of great practical importance, particularly in industrial processes [24], since it will enable better optimization of enzyme-catalyzed processes. It is clear that, in future, immobilized enzymes, as heterogeneous catalysts, wiU be widely used in most industrial transformations, especially in the preparation of pharmaceuticals [25]. 

For carbon-carbon bond-formation purposes, S 2 nucleophilic substitutions are frequently used. Simple S 2 nucleophilic substitution reactions are generally slower in aqueous conditions than in aprotic organic solvents. This has been attributed to the solvation of nucleophiles in water. As previously mentioned in Section 5.2, Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions (Scheme 6.1). In alkylations of phenoxide ions by benzylic chlorides, S/y2 substitutions can occur both at the phenoxide oxygen and at the ortho and para positions of the ring. In fact, carbon alkylation occurs in water but not in nonpolar organic solvents and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para). The effects of phenol substituents and of cosolvents on the rates of the competing alkylation processes... 

Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions of phenoxide ions by benzylic chlorides carbon alkylation occurs in water but not in nonpolar organic solvents, and it is observed only when the phenoxide has at least one methyl substituent (ortho, meta, or para). This has been discussed in Chapter 6 (Section 6.4.2). 

Density functional theory study of aqueous-phase rate acceleration and endo/exo selectivity of the butadiene and acrolein Diels-Alder reaction72 shows that approximately 50% of the rate acceleration and endo/exo selectivity is attributed to hydrogen bonding and the remainder to bulk-phase effects, including enforced hydrophobic interactions and cosolvent effects. This appears to be supported by the experimental results of Engberts where a pseudothermodynamic analysis of the rate acceleration in water relative to 1-propanol and 1-propanol-water mixtures indicates that hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process are the two main causes of the rate enhancement in water.13... 

Smith, P. E., Computer simulation of cosolvent effects on hydrophobic hydration, J. Phys. Chem. B 1999,103, 525-534... 

Nzengung VA, Voudrias EA, Nkedi-Kizza P, Wampler JM, Weaver CE (1996) Organic cosolvent effects on sorption equilibrium of hydrophobic chemicals by organoclays. Environ Sci Technol 30 89-96... 

Nkedi-Kizza, P, Rao, P.S.C., Hornsby, A.G. (1985) Influence of organic cosolvent on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol. 19, 975-979. 

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... 

Fig. 3 Concept of the ion-association method for fabricating ion-based organic dye nanoparticles in pure aqueous media. The approach is based on ion-pair formation between the ionic dye (for example, cationic dye) and the hydrophobic counterion that is soluble in water [for example, tetraphenylborate (TPB) or its derivative anion], which gives rise to a hydrophobic phase in water. For preparation, organic cosolvent is unnecessary. The size of the dye nanoparticles can be controlled by adjusting the interionic interaction between the dye cation and the associative hydrophobic counteranion...

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

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