Forces in Solution

Dipole-dipole forces, in the absence of H bonding, aeeount for the solubility of polar organic molecules, such as ethanal (aeetaldehyde, CH3CHO), in polar, nonaqueous solvents like chloroform (CHCI3). 

Ion-induced dipole forces are one of two types of charge-induced dipole forces, which rely on the polarizability of the components. They result when an ion s charge distorts the electron cloud of a nearby nonpolar molecule. This type of force plays an essential biological role that initiates the binding of the Fe ion in hemoglobin and an O2 molecule in the bloodstream. Because an ion increases the magnitude of any nearby dipole, ion-induced dipole forces also contribute to the solubility of salts in less polar solvents, such as LiCl in ethanol. 

Dispersion forces contribute to the solubility of all solutes in all solvents, but they are the principal type of intermolecular force in solutions of nonpolar substances, such as petroleum and gasoline. 

As you ll see in Chapter 15, these intermolecular forces are also responsible for keeping cellular macromolecules in their biologically active shapes. 

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Octane and cyclohexane are another liquid pair whose intermolecular interactions are alike. Both have low polarities, so these molecules in the pure liquids are held together by the dispersion forces caused by their polarizable electron clouds. Dispersion forces in solutions of octane and cyclohexane are about the same as in the pure liquids. Again, these two liquids are miscible. 

The qualitative discussion of solubility has focussed so far on the attractive forces in solute-solvent interactions. However, where water is concerned, it is also important to consider the forces of repulsion due to the so-called hydrophobic interactions that may arise in certain cases (Franks, 1975). These hydrophobic interactions may be explained in terms of thermodynamic concepts. 

The program of calculating the BO-level potentials from Schroedinger level cannot often be carried through with the accuracy required for the intermolecular forces in solution theory. (9.) Fortunately a great deal can be learned through the study of BO-level models in which the N-body potential is pairwise additive (as in Eq. (3)) and in which the pair potentials have very simple forms. (2, 3, 6) Thus for the hard sphere fluid we have, with a=sphere diameter,... 

This volume of Topics in Current Chemistry on supramolecular chirality aims to acquaint the researcher with the principles and applications of noncovalent chiral assemblies or aggregates. The first chapter illustrates the reader the state of the art on the construction of synthetic chiral supramolecular assemblies held together by means of weak intermolecular noncovalent forces in solution except metal-ligand coordination. The next chapter deals with dynamic heli-... 

The organic chemist usually works with compounds which possess labile covalent bonds and are relatively involatUe, thereby often rendering the gas-phase unsuitable as a reaction medium. Of the thousands of reactions known to occur in solution only few have been studied in the gas-phase, even though a description of reaction mechanisms is much simpler for the gas-phase. The frequent necessity of carrying out reactions in the presence of a more or less inert solvent results in two main obstacles The reaction depends on a larger number of parameters than in the gas-phase. Consequently, the experimental results can often be only quahtatively interpreted because the state of aggregation in the liquid phase has so far been insufficiently studied. On the other hand, the fact that the interaction forces in solution are much stronger and more varied than in the gas-phase, permits to affect the properties and reactivities of the solute in manifold modes. 

We shall now discuss briefly the dependence of the heat of mixing on the intermolecular forces in solutions. 

It was mentioned in the Introduction that solution takes place when the selfattraction forces in solute and solvent molecules are of the same order of... 

The internal pressure is due to the cohesional forces (see Section 3.4.3) between the molecules that contribute to the internal energy, 17, and is equal to zero for ideal gases. The approximate equality of internal pressure values in liquids is a good criterion for the behavior of ideal liquid solutions obeying Raoult s law. In ideal liquid solutions, the molecules of the components are under similar forces in solution as in the pure liquids, and the approximate equality of internal pressures, especially for non-polar components, is the reason for their ideal behavior. On the other hand, the repulsive thermal pressure represents the tendency of a fluid to expand. The (3P/3T)V parameter is called an isochore and can be measured directly, or it is more often computed as the ratio of the coefficient of thermal expansion, a = 1/V(3V/3T)P = (31nV/3P)p, to the coefficient of compressibility, p = - l/VOV/3P)T = — (31n V/3P)T, so that (3P/3T)V = -alp. 

When an ion and a nearby polar moleeule (dipole) attraet eaeh other, an ion-dipole force results. The most important example takes plaee when an ionic compound dissolves in water. The ions become separated because the attractions between the ions and the oppositely charged poles of the H2O molecules overcome the attractions between the ions themselves. Ion-dipole forces in solutions and their associated energy are discussed fully in Chapter 13. 

Figure 1 3.1 The major types of intermolecular forces in solutions. Forces are listed in decreasing order of strength (with values in kJ/mol), and an example of each is shown with space-filling models.

The very large dipole moment of polymers results in strong intermolecular forces in solution. Atactic and isotatic polymers have different dipole moments. The dipole moment of the atactic poly(vinyl isobutyl ether) is 10% lower than that of the isotactic form, showing that the isotactic polymer adopts a more ordered structure with group dipoles tending to align parallel to each other. 

Tulpar, A., Subramaniam, V., and Ducker, W. A. 2001. Decay lengths in double-layer forces in solutions of partly associated ions, Langmuir 17,8451-8454. 

I/ Precipitation The solubility of Ln ions can be altered either by changes in pH (destabilizes the ion according to electrostatic forces in solution) or by changes in polarity (alters the miscibility). The polarity of the solvent affects the solubility of an Ln ion, eventually causing it to be immiscible. Coupled with centrifugation, the heavier of the ions can be forced out of solutions and separated using the appropriate centrifugation protocol. 

Models of polymer dynamics are also partitioned by their assumptions as to the dominant forces in solution, these assumptions being totally independent of the assumed concentration dependence. In some models, excluded-volume forces (topological constrmnts) dominate, while hydrodynamic interactions dress the monomer diffusion coefficient. In other models, hydrodjmamic interactions dominate, while chcun-crossing constriunts cure secondary. Experimentally, Dg c) is directly accessible, but the intermolecular forces Ccm at best only be inferred from numerical coefficients D, a, and so forth. 

In Fig. VI.6 we show the adhesion number of quartz particles on a glass surface as a function of the concentration of electrolytes with different cation valences [12]. The adhesion number of glass particles on a glass surface is shown in Fig. VI.7 as a function of the applied detaching force, in solutions of KCl (curves 1 and 2), CaCU (curves T and 2 ), and AICI3 (curves l" and 2") with concentrations of 0.01 and 0.001 mol/liter. It will be noted that, on the one hand, the adhesion drops off for all of the electrolytes with decreasing electrolyte concentration on the other hand, the lower the solution concentration, the more rapidly the adhesion drops off [77]. Also, for solutions with concentrations from 0.01 to 0.001 M, the adhesion increases with increasing valence of the cation. 

In these series, the adhesive force in solutions of a given concentration will increase for each successive electrolyte. The lyotropic series coincides with a series of electrolytes ranked in order of decreasing thickness of the residual layer of liquid between plane-parallel disks [96]. This fact confirms the validity of the theoretical premises (see Section 26). 

In Fig. VI.8 we show the adhesive force (or more properly, sin a, where a is the slope angle at which all of the particles are removed) as a function of the electrophoretic velocity for univalent (curve 1) and bivalent (curve 2) cations [4]. The first curve lies above the second curve, i.e., for the same f potential, the adhesion in solutions of univalent cations is greater than in solutions of bivalent cations this may be explained by the different thicknesses of the liquid boundary layer. With a zero potential, the adhesive force in solutions of univalent and bivalent cations should be the same. In this case, with specific forces of molecular interaction, the disjoining pressure of the thin layer of liquid is also identical. 

This enables us to clarify our minds as to how to exploit interaction forces in solution and showed that since, in the nonpolar stationary phases such as long chain paraffin hydrocarbons, where London dispersion forces were operative, that the molecular weight (rather than BP) was the major factor controlhng separation, with molecular configuration as the secondary, though stiU important, effect. In solvents such as alcohols and polyethers, hydrogen bonding with donor or acceptor solutes occurred in addition to London dispersion interactions. ... 

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