Apolar contributions water

Fig. 1. Analysis of the apolar contribution to the dissolution thermodynamics of cyclic dipeptides into water. Each thermodynamic quantity is plotted against the number of apolar hydrogens (aH) (i.e., hydrogens bonded to carbon) (a) AC , (b) AH°, (c) AS0, and (d) AG°. Lines are the linear regression fit of the data. As described in the text, the slope gives the hydrophobic contribution. Data are from Murphy and Gill (1990).

Because the apolar contribution to AH° of dissolution into water at 25°C is negative for the solids rather than near zero as seen in the... 

The apolar contribution to AS0, ASap, is better characterized than AHap. The value of Tt has been shown to be a universal temperature for all processes involving the transfer of an apolar surface into water and has a value of 112°C (Murphy et al., 1990). At this temperature the AS0 of transfer, ASf, represents the mixing entropy of the process. The universal value of Tt was determined using mole fraction concentration units, so that the liquid transfer ASf takes on a value of zero. The value of Tt remains the same using the local standard state of Ben-Naim (i.e., molar concentration units) (Ben-Naim, 1978), but the value of Ais increased by R ln(55.5), where R is the gas constant and 55.5 is the molarity of water. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

Another important result that was obtained recently concerns the evaluation of the contribution to the reorganization energy arising from the polarization of the medium, protein and solvent from a microscopic model including the residual charges and induced dipoles of the protein as well as bound water molecules, a value of about 0.2 eV was calculated for different eleetron transfer processes [97], This weak value results from the apolar character of the medium, and is compatible with the kinetic data which indicate that reorganization energies are small in the reaction center (Sect. 3.2.2)... 

Rank the four compounds (I-FV) indicated below in the order of increasing tendency to distribute from (a) air into hexadecane (mimicking an apolar environment), (b) air to olive oil, and (c) air to water. Use the ah j3 and Vn values given in Table 4.3 and calculated by the method given in Box 5.1. Assume, that the four compounds have about the same nDi value. Do not perform unnecessary calculations. Comment on your choices. Finally, check your result (c) by applying the bond contributions given in Table 6.4. 

Contributions of Peptide and Apolar Hydrogen Groups to the Thermodynamics of Dissolution into Water ... 

The endothermic maxima observed for apolar solutes and salts in water-rich mixtures must also contribute towards the minima in AH for alkaline ester hydrolysis in these mixtures. As before, the tendency for the rate constant to decrease is determined by the behaviour of 5m AS. Plots of AH against AS are complicated but in mixtures for which x2 < xf the data points generally fall on a straight line. Of course, there are new problems in this class of reactions. For example, the possibility arises that the rate constant is a function of quantities describing the equilibrium between, say, RO- and OH". However, the patterns which emerge indicate that this may not usually be an important consideration in water-rich mixtures. One exception may be the alkaline hydrolysis of ethyl acetate and methyl acetate (Tommila and Maltamo, 1955) in methyl alcohol + water mixtures for which AH increases gradually as x2 increases. 

The chemical nature of a solid determines its adsorptive and wetting properties. Now, the energy of immersion mainly depends on the surface chemistry but also, to some extent, on the nature of the bulk solid. For example, the interaction between water and silica has contributions from the bulk Si02 together with contributions from the silanol groups of the interface. Polar molecules are very sensitive to the local surface chemistry, whereas nonpolar molecules are more sensitive to the bulk composition. Interactions between a bulk Hquid and a bulk solid through an interface are often described in terms of Hamaker constant [16]. Immersion calorimetry in apolar liquids was proposed to estimate the Hamaker constant [17]. The sensitivity of immersion calorimetry to the surface polarity has justified its use for characterising the surface sites. 

Surface waters are displaced on formation of the protein-ligand complex and thus provide a favorable entropic contribution to the free energy of complex formation. In particular, displacement of the water found interacting on apolar surfaces makes a large contribution to the AG and provides the driving force for many interactions (the hydrophobic effect). 

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