Solution formation intermolecular forces

Complex coacervation in aqueous solution may be considered as a special case of network formation. Intermolecular forces such as Coulomb, van der Waals, hydrophobic, hydrogen bond, and dipol-charge transfer between polymers themselves, or polymers and low... 

Now, let s look at a polymeric system. To begin with, the motion in polymer chains is hindered. The massive size of the polymer itself and the intermolecular forces within the chains create an inflexible system, especially when compared to the aqueous systems with which we are most familiar. Secondly, the entropy of mixing is not actually as great as that seen in typical solution formation. Polymers are inherently highly entropic, so the benefit of mixing them together is modest. Therefore, any two polymers that form a miscible blend depend primarily... 

Other methods of film formation discussed in this book depend on allowing a melt or a solution of the material to be deposited to spread on the substrate and subsequently to solidify. An ordered structure can sometimes be imposed on such a film by the application of an electric or magnetic field if the film is in a mesophase (otherwise known as a liquid crystal) before solidification. However, any such method presupposes that the melt or solution used will spread evenly over the substrate. It is thus important to understand a little about the conditions which allow a liquid to spread on a solid surface. This topic depends on the nature of intermolecular forces, a subject which is of general relevance to the formation of organic films and which is discussed in the following section. 

In many solutions strong interactions may occur between like molecules to form polymeric species, or between unlike molecules to form new compounds or complexes. Such new species are formed in solution or are present in the pure substance and usually cannot be separated from the solution. Basically, thermodynamics is not concerned with detailed knowledge of the species present in a system indeed, it is sufficient as well as necessary to define the state of a system in terms of the mole numbers of the components and the two other required variables. We can make use of the expressions for the chemical potentials in terms of the components. In so doing all deviations from ideal behavior, whether the deviations are caused by the formation of new species or by the intermolecular forces operating between the molecules, are included in the excess chemical potentials. However, additional information concerning the formation of new species and the equilibrium constants involved may be obtained on the basis of certain assumptions when the experimental data are treated in terms of species. The fact that the data may be explained thermodynamically in terms of species is no proof of their existence. Extra-thermodynamic studies are required for the proof. 

FIGURE 7.4 Of the 16 chemistry topics examined (1-16) on the final exam, overall the POGIL students had more correct responses to the same topics than their L-I counterparts. Some topics did not appear on all the POGIL exams. Asterisks indicate topics that were asked every semester and compared to the L-I group. The topics included a solution problem (1), Lewis structures (2), chiral center identification (3), salt dissociation (4), neutralization (5), acid-base equilibrium (6), radioactive half-life (7), isomerism (8), ionic compounds (9), biological condensation/hydrolysis (10), intermolecular forces (11), functional group identification (12), salt formation (13), biomolecule identification (14), LeChatelier s principle (15), and physical/chemical property (16). 

We have already encountered the effects of intermolecular forces in our discussion of precipitates and solubility. Here the intermolecular attractions between water molecules are instrumental in the ion-cage formation that allows some salts to go into aqueous solution. The glycerin molecule shares some similarities with water, but the individual glycerin molecules are still strongly attracted to each other and admit water to their ranks only when there is sufficient provocation. In this demonstration, the provocation occurs in the form of stirring, but no amount of stirring will force the canola oil into the glycerin solution until soap is added. 

Polar compounds will dissolve in polar solvents because the latter will solvate the compound and thereby overcome the electrostatic forces which hold the crystal together. It is for this reason that polar compounds will not dissolve in nonpolar solvents. Similarly, nonpolar compounds -will not dissolve in polar solvents because the relatively strong intermolecular forces in the liquid would be decreased if a solution were formed. This makes the formation of a solution energetically unfavorable. 

When a solid or liquid dissolves, the structural units—ions or molecules— become separated from each other, and the spaces in between become occupied by solvent molecules. In dissolution, as in melting and boiling, energy must be supplied to overcome the interionic or intermolecular forces. Where does the necessary energy come from The energy required to break the bonds between solute particles is supplied by the formation of bonds between the solute particles and the solvent molecules the old attractive forces are replaced by new ones. 

This heat of mixing, calculated on the basis of the model of ideal associated solutions, must be interpreted as a heat of reaction resulting from the formation or dissociation of complexes. Besides this contribution we must also expect to find in the heat of mixing a contribution from less intense intermolecular forces, of the type mentioned at the end of the previous paragraph, which do not result in complex formation. In the simplest case these interactions contribute a term to M and g of the form... 

Description of these phenomena requires quantitative specifications of the amount of solute in the solution, or the composition of the solution. Solutions are formed by mixing two or more pure substances whose molecules interact directly in the mixed state. Molecules experience new intermolecular forces in moving from pure solute or solvent into the mixed state. The magnitude of these changes influences both the ease of formation and the stability of a solution. 

Although the solute and solvent can be any combination of solid, liquid, and gas phases, liquid water is indisputably the best known and most important solvent. Consequently, we emphasize aqueous solutions in this chapter, but you should always remember that dissolution also occurs in many other solvents. We describe formation of aqueous solutions by considering the intermolecular forces between the solute and water molecules. Because these forces can be quite different for molecular solutes and ionic solutes, we discuss these two cases separately. 

Describe the formation of a solution in molecular terms by comparing intermolecular forces in the pure phases and in the solution (Section 11.2). 

The fact that the volume change associated with mixing the components is zero gives another important property of an ideal solution. On the other hand, a volume change does accompany the formation of most solutions. One example was analyzed above in section 1.4. This change is another reflection of the fact that the energy due to the intermolecular forces between the components changes with solution composition. 

The intermolecular forces play an important role in determining the solubility of a solute dissolved in a solvent. The old rule of thumb like dissolves like usually provides a good qualitative means to predict solubility. The energetics of solutions can be summarized as follows. Keep in mind that there will usually be an entropy-driving force favoring the formation of solution. The solute-solute, solvent-solvent, and solute-solvent interactions must be considered in qualitative estimation of the enthalpy effect, i.e., the enthalpy of solution can be expressed as A//go[ (jojj — A/fjQj jg.jQj jg... 

Systems Containing More Than Two Components. As in binary systems, the behavior of systems containing more than two components can be understood on the basis of intermolecular forces and solubility parameters. Water and tetrachloromethane have widely differing solubility and hydrogen bond parameters, and are therefore immiscible. Added acetone dissolves partly in the aqueous phase due to hydrogen bond formation, and partly in the tetrachloromethane phase due to dispersion and induction forces. Twice as much acetone dissolves in the aqueous phase as in tetrachloromethane. On increasing the acetone concentration a homogeneous solution is obtained. The added solvent thus acts as a solubilizer for the two immiscible solvents. 

In general, the attractive intermolecular forces between solvent and solute particles must be comparable or greater than solute-solute interactions for significant solubility to occur. Explain this statement in terms of the overall energetics of solution formation. 

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