Solvophobic effects

While the previous receptors are typically used in organic solvents, except for the cyclodextrins, there are special cases of cyclophane receptors supphed with peripheral charges (ammonium units) (107—12) or ionizable groups (carboxylate functions) (113,114) (Fig. 17) to allow substrate recognition, as in nature, in an aqueous medium, profiting from the solvophobic effects of water (115). 

In section 8.2 we described the solvophobic effect, which theory leads us to expect is related to the solvent surface tension. Abraham et al. have developed a different measure of solvophobicity by relating the transfer free energy W)... 

Solvents, UV cut-olf values, 70 Solvents, miscibility, 75 Solvophobic effect, 201,203 Solvophobic inleHlclidHk, IS2, 20i Solvophobic ion chromatography, 242 Solvophobic theory, 141,148,152,155, 158, 202, 203, 226, 228, 246 8omatostedn, 263,290 Sorbents, polymeric, 127 Sorption isottom, 159 Soiption kineties, efbet on column effi-cieney in RPC, 227 Speed of aepantion, optimization [Pg.172]

Scheme 10.8 Guest encapsulation by softball hosts is driven by entropic (solvophobic) effects.

It is general considered that the driving force for the self-assembly of amphiphilic molecules is a solvophobic effect, more specific in an aqueous environment, this is referred to as the hydrophobic effect. The type of aggregate morphology formed can be predicted... 

Two main theories, the so-called solvophobic and partitioning theories, have been developed to explain the separation mechanism on chemically bonded, non-polar phases, as illustrated in Figure 2.4. In the solvophobic theory the stationary phase is thought to behave more like a solid than a liquid, and retention is considered to be related primarily to hydrophobic interactions between the solutes and the mobile phase14-16 (solvophobic effects). Because of the solvophobic effects, the solute binds to the surface of the stationary phase, thereby reducing the surface area of analyte exposed to the mobile phase. Adsorption increases as the surface tension of the mobile phase increases.17 Hence, solutes are retained more as a result... 

For a more detailed discussion of these questions, see references [76, 77, 176, 343-347] and references cited therein. More recent results [346, 347] have shown that the classical view (a) seems to be basically correct. The essential condition for sol-vophobicity is that solvent/solvent interactions are much stronger than solute/solvent interactions. However, the solvophobic effect is not necessarily always an entropie phenomenon it can be enthalpic or entropie depending on the temperature and the geometrical size of the solute molecules [346]. 

The exact mechanism(s) of solute retention in reversed-phase high-performance liquid chromatography (RPLC) is not presently well understood. The lack of a clear understanding of the mechanics of solute retention has led to a myriad of proposals, including the following partition (K21, L6, S16) adsorption (C9, CIO, H3, H15, H16, K13, L3, T2, U2) dispersive interaction (K2) solubility in the mobile phase (L7) solvophobic effects (H26, K6, M5) combined solvophobic and silanophilic interaction (B9, M12, Nl) and a mechanism based upon compulsary absorption (B5). 

The principle of solvophobicity as presented by Horvath et al. is based upon the tendency of the mobile phase to minimize the site of the cavity occupied by the solute molecules in the hydroorganic mobile phase. This can be viewed as a reversible association of the solute molecules with the hydrocarbonaceous stationary phase. The magnitude of the solvophobic effect for a given solute is due largely to the following four properties of the hydrooi anic solvent system (H23) ... 

In addition to this classical polarity effect, the solvent can have a more subtle influence, hi the case of monomer 16 (Fig. 20) hydrogen bonding in chloroform leads to the formation of a usual flexible supramolecular polymer. However, in dodecane the dimerization of the ureidotriazine is reinforced by a solvophobic stacking of the aromatic parts, which yields a columnar architecture [93]. A similar solvophobic effect has been demonstrated with a UPy-based monomer [94]. Another possible side effect of the nature of the solvent is the occurrence of specific host-guest interactions between the HBSP and the solvent [42,95,96]. 

In such a conformation, which features an anti arrangement of the two alkaloids, the quinoline and the phthalazine rings form a U-shaped cleft, well suited to host an incoming alkene through attractive noncovalent interactions. Steric crowding at two opposite regions of the complex is then expected to be responsible for the 7t-face discrimination. In this scenario, solvophobic effects may also be partially responsible for the stacking. In fact, ten-butanol is reported to behave much better than the less polar toluene. 

Vesicles (Latin vesicula, bladder) are sealed, extremely thin (< 10 nm), often spherical membraneswhich enclose aqueous or other solvent volumes of approximately 10 -10 nm. Aggregation numbers are in the order of 10 -10. The monomers are held together by the same solvophobic effects which produce micelles, and solvation forces together with membrane undulations prevent crystallization. The state of the vesicle membrane is therefore also essentially of a fluid character. Supramolecular ordering within vesicle membranes is negligible. 

Spherical micelles and vesicles are formed from amphiphiles and bolaamphi-philes by the solvophobic effect and are protected against crystallization by head group repulsion. What happens if the head groups carry secondary amide groups which have an inborn and irresistible drive to form linear hydrogen bond chains in polar and apolar environments (see section 5.4) Chains will be formed, of course. The usual result will be vesicular tubules, as in the case of amphiphiles with a low cmc (typically < 10 M) and thinner micellar rods in the case of amphiphiles with a relatively high cmc (typically 10 -10" M). 

Bond angle constraint aided by solvophobic effect Fig. 15.1. Examples of polymers with conformational restriction due to steric and bond angle constraints. 

However, there are systems with gross deviations from these predictions. Liquid-liquid immiscibility was observed with some of these salts even in aqueous solutions [37, 71]. In such cases, the ionic forces are not expected to drive the phase separation. Rather, solvophobic effects of salts with large ions in solvents of high cohesive energy density may be responsible for these transitions [37],... 

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