Polarization, solute molecule

Fig. 6. Free energies of hydration calculated, for a series of polar and non-polar solute molecules by extrapolating using (3) from a 1.6 ns trajectory of a softcore cavity in water plotted against values obtained using Thermodynamic Integration. The solid line indicates an ideal one-to-one correspondence. The broken line is a line of best fit through the calculated points.

The Poisson equation assumes that the solvent is completely homogeneous. However, a solvent can have a significant amount of charge separation. An example of a heterogeneous solution would be a polar solute molecule surrounded by water with NaCl in solution. The positive sodium and negative... 

The strength of the interaction between the adsorbent and the solute molecules increases as the polarity of the solute increases. Thus we can increase retention of our solute molecules (X) by decreasing the polarity of the mobile phase (S), which will shift the equilibrium above to the right. Polar solute molecules are strongly held on unmodified silica and tail badly, so the method is useful only for solutes having low or medium polarity. 

It is important to keep in mind that the ability of dipoles to associate is influenced by their environment. Many studies on association of dipoles have been carried out in solutions containing the polar molecules. If the molecules of the solvent are polar or can have polarity induced in them (see Section 6.3), association of the solute molecules will be hindered. The solvent molecules will surround the polar solute molecules, which will inhibit their interaction with other solute molecules. The solute... 

Figure 6. A simple model for the aggregation of supercritical fluid molecules around a polar solute molecule.

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

Interactions between a solute and a solvent may be broadly divided into three types specific interactions, reaction field and Stark effects, and London-van-der-Waals or dispersion interactions. Specific interactions involve such phenomena as ion pair formation, hydrogen bonding and ir-complexing. Reaction field effects involve the polarization of the surrounding nonpolar solvent by a polar solute molecule resulting in a solvent electric field at the solute molecule. Stark effects involve the polarization of a non-polar solute by polar solvent molecules Dispersion interactions, generally the weakest of the three types, involves nonpolar solutes and nonpolar solvents via snap-shot dipole interactions, etc. For our purposes it is necessary to develop both the qualitative and semiquantita-tive forms in which these kinds of interactions are encountered in studies of solvent effects on coupling constants. 

Non-polar Solutes in Polar Solvents the (Solvent Stark EffecP. At first sight a non-polar solute molecule cannot polarize the surrounding solvent since it develops no electric field. However, the solvent fluctuates around the non-polar solute, so that there is a small instantaneous electric field which acts on the solute to produce a fluctuating induced dipole which leads to... 

Fig. 3.1. Schematic illustration of the situation of a polar solute molecule (water) in hexane (left) and in water (right).

The Kamlet-Taft u polarity/polarizability scale is based on a linear solvation energy relationship between the n it transition energy of the solute and the solvent polarity ( 1). The Onsager reaction field theory (11) is applicable to this type of relationship for nonpolar solvents, and successful correlations have previously been demonstrated using conventional liquid solvents ( 7 ). The Onsager theory attempts to describe the interactions between a polar solute molecule and the polarizable solvent in the cybotatic region. The theory predicts that the stabilization of the solute should be proportional to the polarizability of the solvent, which can be estimated from the index of refraction. Since carbon dioxide is a nonpolar fluid it would be expected that a linear relationship... 

Solvent Effects. The values oi Pim obtained from dilute solution measurements differ somewhat from values obtained from the pure solute in the form of a gas. This effect is due to solvent-solute interactions in which polar solute molecules induce a local polarization in the nonpolar solvent. As a result p. as determined in solution is often smaller than p for the same substance in the form of a gas, although it may be larger in other cases. Usually the two values agree within about 10 percent. A discussion of solvent effects is given in Ref. 4. 

A revised picture is needed to describe the structure of films composed of coadsorbed polar solute molecules and nonpolar solvent molecules. The close-packed model of Reis implies that the coadsorbed solvent should be relatively tightly bound. That this is apparently not the case is suggested by the ease with which the n-octadecane was removed from all the films. The data show that n-octadecane is more firmly attached to the surfaces on which stearic acid is adsorbed than to surfaces on which no acid is adsorbed. This means that the coadsorbed n-octadecane found is not present in the film as relatively large aggregates of randomly oriented solvent molecules on the surface. These aggregates, if they exist on the surface initially, should have been removed by the 5-second rinse. This is based on the fact that solvent was removed from the silver mirrors which had been immersed in pure n-octadecane containing no stearic acid. 

In solutions containing solutes consisting of polar molecules, the solvent strongly affects the association of the dipoles. In general, if the solvent has low polarity and/or dielectric constant, the dipoles will be more strongly associated. If the solvent is also polar, it is likely that the solvation of each polar solute molecule will be strong enough that solute molecules will be... 

The observed hydrophobic effect is to be understood at two levels, as follows. First, we need to understand the hydrophobic interaction between one isolated non-polar solute molecule and the surrounding water moleeules. Seeond, we need to understand the interaction between two non-polar solute molecules, as a function of distance mediated by intervening water moleeules. Both are important and need to be understood together. There are also independent attributes that need to be studied separately. 

While insertion of a non-polar solute into water is entropically unfavorable (at room temperature) and can now be understood nearly quantitatively, the effective interaction between a pair of non-polar solute molecules is more difficult to understand. This effective interaction is termed the potential of mean force, often just called... 

The calculation of pair hydrophobicity is a bit more complex. This is defined by the work done to bring two hydrated non-polar solute molecules from infinite separation R —> oo) to a separation R. When R is closer than the length of the spatial correlations that exist in pure water, the solvents begin to reorganize further. For each pair separation, R, the excluded volume Av(7 2.) depends on R. 

Consider a dilute solution of a polar solute molecule M in a polar solvent S for example, a solution of CHjBr in water. The water molecules near the CBr side of a solute molecule will tend to be oriented with their positively charged hydrogen atoms... 

The device of replacing Vo(t) or its transform by a reflection pulse, or its transform, from a reference sample of known permittivity can be very useful when one is interested in small relaxation effects produced, for example, by adding impurities, ions, or polar solute molecules in low concentration to a solvent medium. Using measurements of solution and solvent reflections Rs(t) and Rx(t) for permittivities nd s, one can obviously combine the two admittance equations for any of the methods to eliminate Vq and obtain x or x " s 0 terms of s od the transforms rx and r. The important quantity for good precision in x " improved precision from digital processing of separate records of Rs(t) and Rx(t). Methods of this kind have been described in some detail by Cole, Mashimo, and Winsor,15 and should be useful for a variety of systems. 

Wax in oil The interactions formed between non-polar solvent and non-polar solute molecules are similar in kind and strength. Mechanical agitation of the solid by collisions from solvent molecules is sufficient to allow a solution to form. 

If the net interaction between solvent and solute is small, their molecules will remain far apart since the van der Waals radii of molecules are relatively large. Their MOs will not therefore overlap significantly, so the corresponding terms P, in equation (5.80) will be small. The main second-order effect therefore corresponds to a polarization of solvent and/or solute by the electric fields due to the other. Thus if the solvent is nonpolar and the solute is polar, the solute will remain unperturbed (since the solvent molecules have no net fields), but the solvent will be polarized by the fields of the polar solute molecules. The polarization is such as to lead to an attraction between solute... 

If water is not the solvent, the interaction between solvent molecules and solute ions or polar solute molecules decreases substantially and can disappear completely. This means that, while ionic compounds and polar molecular solutes may partially dissolve in nonaqueous polar solvents (such as methyl alcohol), their solubility is not as high as in water. This is illustrated in Figure 10.7. 

The main solute-column interactions can be classified as dispersion forces and dipole-dipole interactions. The dispersion forces are present in any solute-solvent system, a hydrocarbon solute interacting with a nonpolar paraffin being often shown as an example. The polar solute molecules have permanent dipoles that can interact with those of the polar phases on occasions, the dipole moments can also be induced in certain solute molecules in the presence of highly polar column materials. Dipole-dipole interactions are clearly evident in the separations of alcohols, esters, amines, aldehydes, and so on, on the poly glycol, polyamide, polyester, or cyanoalkylsilicone stationary phases. 

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