Solvophobic mechanism

Solvent-induced effects on phase equilibria have also been described by models based on solvent-averaged Friedman-Gumey potentials using the HNC approximation [81]. The difficulty in extracting phase transition lines from HNC calculations has been noted earlier, but only the HNC seems to be flexible enough to account for specific interactions (e.g., present in solvophobic mechanisms). 

The retention behavior of nucleic acid constituents in the presence of increasing concentrations of an organic modifier is typical of that expected based upon solvophobic mechanisms with increasing concentrations of organic modifier, the capacity for solute retention is decreased. As expected, the use of a stronger eluent under reversed-phase conditions (e.g., acetonitrile vis-a-vis methanol) results in a subsequent decrease in the capacity ratios of all compounds in the chromatographic analysis. 

From the viewpoint of molecular interactions, the number of fundamentally distinct chromatographic stationary phases is very limited.17 One mechanism for adsorption to the stationary phase is solvophobic, or mobilestationary phase transfer free energy effects, in which the adsorption of an analyte to the stationary phase liberates bound solvent. There is often an accompanying enthalpic component to such binding through dispersion interactions. Another mechanism for adsorption is that of specific interactions,... 

The mechanism of reversed-phase chromatography arises from the tendency of water molecules in the aqueous-organic mobile phase to self-associate by hydrogen bonding. This ordering is perturbed by the presence of nonpolar solute molecules. As a result, solute molecules tend to be excluded from the mobile phase and are bound by the hydrophobic stationary phase. This solvophobic... 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

If the retention enthalpies of the two sites differ, curvature may be observed in the plots. Moreover, if the enthalpies are opposite in sign, a minimum will occur in the van t Hoff plot at a temperature where the ratio of the retention foctors for the two mechanisms equals the absolute value of the reciprocal of the ratio of the corresponding enthalpies. Most frequently, however, less dramatic curvature would be expected. Such behavior may be anticipated in the RPC of amines with- arge nonpolar moieties which could be retained by silmiophilic interactions with surface silanols and by solvophobic interactions with nonpolar ligates of a reversed phase with low surface coverage. Recently an lihalysis of this behavior has been reported 93). 

The retention mechanism is not yet fully understood. The solvophobic theory does not account for any interaction in the stationary phase, which plays a passive role. The partition mechanism as described by Dill and Dorsey (27) is generally accepted. The most relevant feature is the linear plot of In k versus carbon number in a homologous series (Fig. 10), which is similar to what is observed in isothermal gas-liquid chromatography. Retention is governed mainly by hydro-... 

LC Tan, PW Carr. Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. II. Partitioning vs. adsorption mechanism in monomeric alkyl bonded phase supports. J Chromat A 775 1-12, 1997. 

Another subtle case, where specific interactions may obscure the effects of Coulombic criticality, is ethylammonium nitrate (EtNH3N03) +l-octanol (Tcs315K) [85], In contrast to all other known examples, the critical point is located in the salt-rich regime at a critical mole fraction of Xc = 0.77. Electrical conductance data indicate strong ion pairing, presumably caused by a hydrogen bond between the cation and anion which stabilizes the pairs in excess to what is expected from the Coulombic interactions [85]. This warns that, beyond the Coulombic/solvophobic dichotomy widely discussed in the literature, additional mechanisms may affect the phase separation [5]. 

As viewed from the Coulombic versus solvophobic dichotomy, the mean-field-like behavior is quite unexpected. Thermodynamic properties of the salt-free systems clearly point toward a hydrophobic mechanism for phase... 

Third, in real ionic solutions, solvophobic and Coulombic interactions may define different length scales. This case is, of course, not covered by the RPM and similar continuum models. Anisimov et al. [322] have argued that such a mechanism may be responsible for the observed shift of the crossover temperature closer to Tc found in solutions of a picrate in a homologous series of alcohols. 

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... 

Why does protein concentration exert such strong stabilizing influences on protein structure and, thereby, on enzymatic activity And why can a protein such as BSA affect the stability of an unrelated enzymatic protein like MDH To explain the effects shown in figure 6.21, another type of stabilizing influence must be introduced, one that differs from the mechanisms involving solvophobic (hydro-phobic and osmophobic) effects. The primary phenomenon involved in accounting for stabilization of macromolecular structure by other macromolecules is termed the excluded volume effect, which is one consequence of the molecu-... 

Ben-Naim (1972b, c) has examined hydrophobic association using statistical mechanical theories of the liquid state, e.g. the Percus-Yevick equations. He has also examined quantitative aspects of solvophobic interactions between solutes using solubility data for ethane and methane. The changes in thermodynamic parameters can be calculated when two methane molecules approach to a separation of, 1-533 x 10-8 cm, the C—C distance in ethane, and the solvophobic quantities 8SI/i, s 2 and 8SiS2 can be calculated. In water (solvophobic = hydrophobic) 5si/i is more negative than in other solvents and decreases as the temperature rises both 8s iH%... 

Influence of Micellar and Solvophobic Interactions on Reaction Rates and Mechanisms... 

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). 

From these investigations, it is apparent that no single retention mechanism is operative in RPLC rather solute selectivity is based upon mixedmode mechanisms. At present it appears that solvophobicity (hydropho-bicity) is the primary mechanism for solute retention [Horvath et al. (B9, H23, H25, M12, M15, Nl) and Kargerer al. (K6, M5)]. 

Recently, Horvath and co-workers (B9, N1) introduced the concept of a dual binding mechanism to explain the atypical behavior of some solutes under reversed-phase conditions. In addition to solvophobic forces, it is possible for solutes to interact with the free surface silanols of the silica-based hydrocarbonaceous packing material. The term silanophilic interaction has been introduced to denote a reversible binding mechanism between solute molecules and silanol groups. 

This dual binding mechanism satisfactorily explains the anomalous retention of many solutes when viewed in purely solvophobic terms and will no doubt lead to the explanation of other solutes. 

Zakaria and Brown (Zl) have found that, whereas nucleoside and base retention mechanisms can be adequately explained in terms of solvophobic considerations, nucleotide retention behavior can best be explained in terms of a mixed-mode mechanism. In an acidic mobile phase, it has been observed that ribonucleotides elute in order of increasing negative charge. This elution pattern is atypical for the reversed-phase... 

This proposed mechanism for protein separations is supported by the recent theoretical studies of Horvath ef al. (29) and Horvath and Melander (28). In these studies, the hydrophobic effect in aqueous-organic systems (termed the solvophobic theory) was used to predict the retention of peptides on a nonpolar column. These authors found that the dominant interactions were between the mobile and stationary phases and between the mobile phase and the sample molecules. The driving force in both interactions was the shielding of a nonpolar region of either the column or sample molecule from the polar aqueous phase. 

Probably the most widely studied is the solvophobic theory [12] based on the assumption of the existence of a single partitioning retention mechanism and using essentially equation (10-1) for the calculation of the analyte retention. Carr and co-workers adapted the solvophobic theory [12,13] and LSER theory [11, 14-17] to elucidate the retention of solutes in a reversed-phase HPLC system on nonpolar stationary phases. 

In spite of widespread applications, the exact mechanism of retention in reversed-phase chromatography is still controversial. Various theoretical models of retention for RPC were suggested, such as the model using the Hildebrand solubility parameter theory [32,51-53], or the model supported by the concept of molecular connectivity [54], models based on the solvophobic theory [55,56) or on the molecular statistical theory [57j. Unfortunately, sophisticated models introduce a number of physicochemical constants, which are often not known or are difficult and time-consuming to determine, so that such models are not very suitable for rapid prediction of retention data. 

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