Enthalpic component

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,... 

It can be anticipated that the computation of A//soi and AAsoi is more delicate than the prediction of AGsoi, which benefits from the enthalpy-entropy compensation. Accordingly, the suitability of the QM-SCRF models to predict the enthalpic and entropic components of the free energy of solvation is a challenging issue, which could serve to refine current solvation continuum models. This contribution reports the results obtained in the framework of the MST solvation model [15] to estimate the enthalpy (and entropy) of hydration for a set of neutral compounds. To this end, we will first describe the formalism used to determine the MST solvation free energy and its enthalpic component. Then, solvation free energies and enthalpies for a series of typical neutral solutes will be presented and analyzed in light of the available experimental data. Finally, collected data will be used to discuss the differential trends of the solvation in water. 

Figure 3 Overview of the receptor-ligand binding process. All species involved are solvated by water (symbolized by gray spheres). The binding free energy difference between the bound and unbound state is a sum of enthalpic components (breaking and formation of hydrogen bonds, formation of specific hydrophobic contacts), and entropic components (release of water from hydrophobic surfaces to solvent, loss of conformational mobility of receptor and ligand).

Guillet approach we did not separate )f12 into entropic and enthalpic components. Instead, a more general term, 0 (23) was introduced to represent a combination of interaction terms. By introducing the contact energy per unit volume, B (23), a simpler form of the expression is obtained ... 

Calderone, C. T, Wilhams, D. H. An enthalpic component in coop-erativity the relationship between enthalpy, entropy, and noncovalent structure in weak associations. J. Am. Chem. Soc. 2001, 123, 6262-6267. 

The results on the values of the cis/trans equilibrium of simple amides in different solvents confirm observations made with Gly-Pro and acetyl-Gly-Pro-OMe [53,54], In all these cases the free energy difference for the isomers originates primarily from enthalpic differences. According to AH0 values obtained in different solvents, it was concluded that the enthalpic component that differentiates the cis and trans isomer is similar in protic and aprotic solvents. Thus, the entropic term was found to favor the cis isomer in both aqueous buffers and aprotic organic solvents. 

Figure 2. Calculated and experimental values of the Flory-Huggins parameter x and its entropic and enthalpic components, x and x, for the system poly(vinyl acetate)-benzene (Scigliano (34)). The solid lines are computed and the points are experimental by Makajima et al. (50).

Few reports of the temperature dependence of 4Fe-4S cluster potentials have appeared in the literature. With the exception of a small number of c-type cytochromes, the enthalpic component is typically found to be negative for the available data on heme proteins and 4Fe-4S proteins, reflecting favorable bonding changes on reduction. For HiPIPs, the A// values typically lie within 4 to 5 kcal moP of each other (Table VII), which is consistent with their similar values and the importance of the enthalpy term in defining the free en-... 

The enthalpic component of the interaction parameter calculation according to the Eqs. (56) and (57) for PESF in the indicated solvents (Fig. 28) shows opposite tendencies of change for PESF solutions in chloroform and tetrachloroethane. Besides, the dependence Xf /orJ solutions in chloroform has additive character and for solutions in tetrachloroethane— nonadditive one (Fig. 28). And at last, the values and calculated ac-... 

FIGURE 28 The dependences of enthalpic component of Floiy-Huggins interaction parameter on formal blocks contents for PESF solutions in tetrachloroethane (1) and chloroform (2). 

The first attempt to compile all the factors contributing to the stability of a microemulsion is due to Ruckenstein and Chi [15], who summarized calculations of enthalpic components (van der Waals attractive potential, electrical double layer repulsive potential, and the interfacial stretching and bending free energy) and entropic contributions from the location of droplets. These calculations as well as those following [16] were useful because they revealed the importance of extremely low interfacial tension. 

The enthalpic component of polymer-solvent interaction parameter Xh can be related to the solubihty parameters via... 

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