Solvent intermolecular forces acting

Like other measures of pressure, c has units of MPa. In theory, a liquid will break all solvent-solvent interactions on vaporization, and so c is a measure of the sum of all the attractive intermolecular forces acting in that liquid. Hydrogen-bonding and dipolar solvents therefore have high c values. Water has a large value for c, and fluorocarbons very low values (Table 1.5). 

The thermal motion of molecules of a given substance in a solvent medium causes dispersion and migration. If dispersion takes place by intermolecular forces acting within a gas, fluid, or solid, molecular diffusion takes place. In a turbulent medium, the migration of matter within it is defined as turbulent diffusion or eddy diffusion. Diffusional flux J is the product of linear concentration gradient dCldX multiphed by a proportionality factor generally defined as diffusion coefficient (D) (see section 4.11) ... 

At best, this approach provides a quantitative index to solvent polarity, from which absolute or relative values of rate or equilibrium constants for many reactions, as well as absorption maxima in various solvents, can be derived. Since they reflect the complete picture of all the intermolecular forces acting in solution, these empirical parameters constitute a more comprehensive measure of the polarity of a solvent than any other single physical constant. In applying these solvent polarity parameters, however, it is tacitly assumed that the contribution of intermolecular forces in the interaction between the solvent and the standard substrate is the same as in the interaction between the solvent and the substrate of interest. This is obviously true only for closely related solvent-sensitive processes. Therefore, an empirical solvent scale based on a particular reference process is not expected to be universal and useful for all kinds of reactions and absorptions. Any comparison of the effect of solvent on a process of interest with a solvent polarity parameter is, in fact, a comparison with a reference process. 

Chemical transformations can be performed in a gas, liquid, or solid phase, but, with good reasons, the majority of such reactions is carried out in the liquid phase in solution. At the macroscopic level, a liquid is the ideal medium to transport heat to and from exo- and endothermic reactions. From the molecular-microscopic point of view, solvents break the crystal lattice of solid reactants, dissolve gaseous or liquid reactants, and they may exert a considerable influence over reaction rates and the positions of chemical equilibria. Because of nonspecific and specific intermolecular forces acting between the ions or molecules of dissolved reactants, activated complexes as well as produets and solvent molecules (leading to differential solvation of all solutes), the rates, equilibria, and the selectivity of chemical reactions can be strongly influenced by the solvent. Other than the fact that the liquid medium should dissolve the reactants and should be easily separated from the reaction products afterwards, the solvent can have a decisive influence on the outcome (i.e., yield and product distribution) of the chemical reaction under study. Therefore, whenever a chemist wishes to perform a certain chemical reaction, she/he has to take into account not only suitable reaction partners and their concentrations, the proper reaction vessel, the appropriate reaction temperature, and, if necessary, the selection of flic right reaction catalyst but also, if the planned reaction is to be successful, flic selection of an appropriate solvent or solvent mixture. 

Gasoline is a solution composed mainly of nonpolar hydrocarbons and is also an excellent solvent for fats, oils, and greases. The major intermolecular forces acting between the nonpolar molecules are weak London forces. 

A key to both methods is the force field that is used,65 or more precisely, the inter- and possibly intramolecular potentials, from which can be obtained the forces acting upon the particles and the total energy of the system. An elementary level is to take only solute-solvent intermolecular interactions into account. These are typically viewed as being electrostatic and dispersion/exchange-repulsion (sometimes denoted van der Waals) they are represented by Coulombic and (frequently) Lennard-Jones expressions ... 

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. 

The desired high diffusivity and low viscosity results from the lack of strong intermolecular forces between solvent molecules. After all, if there were strong intermolecular forces, the solvent would condense to form a liquid. Without such strong interactions, the solute molecules are less impeded as they diffuse through the fluid. Despite weak intermolecular forces, when an external force is applied to push the molecules close together, they collectively act as a solvent. 

Both the degree of order in liquid crystals and the average orientation of guest molecules in liquid crystals are closely related to the anisotropy of the intermolecular forces. The measurements of the solute or the solvent order are therefore most important in order to test theoretical models of the forces acting between non-spherical molecules. The use of nematic phases as model systems for the investigation of anisotropic intermole.cular interaction potentials is another important scientific application of liquid crystals. 

Interfacial tension is defined as the surface free energy for the interface between two immiscible liquids. As with surface tension, it results from an imbalance in intermolecular forces across the interface. It has the same units as surface tension, conventionally mN m Surfactants are very effective at reducing the interfacial tension between water and organic solvents, and this is one of the mechanisms by which they act as detergents (see Section 4.7). The difference between the surface tensions of the two liquids (Ya,Yp) and the interfacial tension between them (y g) defines the work of adhesion (Fig. 4.8a) ... 

When dissolved in water, compounds of this type assemble around nonpolar substances to form spheres called micelles (Figure 1.54). The nonpolar substance is located at the center of the micelle, where it interacts with the nonpolar ends of the soap molecules via intermolecular London dispersion forces. The surface of the micelle is comprised of polar groups, which interact with the polar solvent. That is, the micelle acts as a unit that is solvated by the polar solvent. In this way, soap molecules can solvate nonpolar substances, such as grease, in polar solvents, such as water. 

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