Solvent-exclusion

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

E6.7 Bronsted measured the solubility of solid orthorhombic and monoclinic sulfur at 298.15 K in various solvents (benzene, diethyl ether and ethanol). He found that the solubility of monoclinic sulfur is always 1.28 times that of orthorhombic sulfur. That is, the concentration of sulfur in the solution in equilibrium with monoclinic sulfur is always 1.28 times the concentration of sulfur in the solution in equilibrium with orthorhombic sulfur. Sulfur exists in these solvents exclusively in the form of Sg molecules. 

Liquid-Liquid extraction is a versatile and dependable separation technique wherein an aqueous solution is usually brought into contact with another organic solvent, exclusively immiscible with the former, so as to affect a legitimate and actual transfer of either one or more solutes into the latter. The normal-feasible separations which can thus be achieved are found to be rather easy, fast, convenient and effective resonably. Invariably such separations may be performed by shaking the two liquids in a separatory funnel for a few minutes and may be extended either to large quantities of pharmaceutical substances or trace levels. 

Oligomers substituted with a methyl group at the ortho position (19-27) display more stable folded states in solution than 10-18 because of a more solvophobic backbone and increased solvent exclusion in the helical cavity [23, 25]. In trying to determine the solid-state properties of the methyl-substituted... 

In addition to maximizing contact area, inclusion also leads to more or less complete solvent exclusion from the receptor site, thus minimizing the number of solvent molecules to be displaced by the substrate on binding. 

Fig. 3. Preferential protein hydration in the presence of precipitating agents used in crystallization experiments. When high concentrations of salts are used as precipitants, a precipitant-poor layer forms near the protein (P) surface due to a higher affinity of the protein for water than for the precipitant. Other precipitants (e.g., polyethylene glycol polymers) induce formation of a similar precipitant-depleted region near the protein by solvent exclusion effects. In either case formation of the precipitant-depleted layer is energetically unfavorable. Consequently, the overall effect of precipitants is to promote molecular associations that decrease the total protein surface area exposed to solvent. After Timasheff and Arakawa (1988).

Fig. 4.15 Solvent-exclusion contribution to image contrast is demonstrated with scanning force data acquired on a co-block-polyethylene glycolpoly amide (Nylon-12) surface (PEBAX 1,074), prepared by melting/resolidification. (a) Topographic image in water, methoxy tip termination (15 x 15 pm2) (b) friction LFM image in water, methoxy tip termination (15 x 15 pm2) (c) topographic image in water, amide tip termination (7.5 x 7.5 pm2), (d) friction image in water, amide tip termination (7.5 x 7.5 pm2). (Reprinted with permission from [32]. Copyright 1996. American Chemical Society.)...

Using a glass pipette, cast the film in about five successive layers allowing time for solvent exclusion between each layer. Do not to touch the cast polymer with the tip of the pipette as this introduces bubbles. The film is now left in a desiccator containing silica gel (to absorb moisture) and paraffin wax (to absorb dichloromethane) for around 1 h such that the majority of the solvent is excluded. 

The method of solvent exclusion has been used to explore issues of accessibility on a somewhat larger scale. An approach pioneered by Stone and Scallan and Stone et relied on static measurement using a series of oligomeric sugars and dextrans of increasing size to establish the distribution of pore sizes in different preparations of a variety of native celluloses. 

Each curve represents the kinetics of the reaction occurring exclusively in the contact area of an individual AFM tip modified with a SAM of NHS - CIO. The observed trends are fully consistent with solvent exclusion effects [144]. Increasing conversion leads to a progressively more solvated carboxylate surface in the case of the hydrolysis, while for the aminolysis an increasingly less solvated, hydrophobic surface is obtained. 

Waltho )P, Williams DH. Aspects of molecular recognition Solvent exclusion aiul dimerization of the antibiotic ristocetin when bound to a model bacterial cell-wall precursor. J Am Chem Soc 1989 111 2475-2480. 

In both RPC and HIC, immediately prior to the interaction between the nonpolar ligates and the protein or peptide moleculc(s), the nonpolar ligates and some of the nonpolar regions accessible at the surface of the biosolutes will still have struetured water molecules associated with their nonpolar surfaces [205]. When these two surfaces are brought into close contact, solvent exclusion events... 

---