Forces from Bulk Properties

Intermolecular forces are important because they dictate many of the macroscopic properties of the bulk matmal, for example the behavior of the gas and liquid phases, the crystalline structure, the elastic modulus, the ultimate strength, the heat of sublimation, and the chemical reactivity. Of course, the intermolecular forces are also responsible ultimately for the adhesion between bodies. Table 5.2 illustrates some of the properties which have been predicted from a knowledge of intermolecular forces. But the properties of gases are easiest to understand. 

In 1873, van der Waals was the first person to show how intermolecular forces must be introduced to explain the properties of gases. Gases exist because each atom has so much kinetic energy that the adhesion between the atoms is relatively insufficient to cause the atoms to stick together. Thus the compression of a volume V of atoms by a pressure P is dominated by the kinetic energy of the atoms as shown in Fig. 5.5. The kinetic energy of each atom is 3kT/2 where k is Boltzmann s constant 1.38 x 10 IK. and Tis the absolute temperature. 

As the gas is compressed at constant temperature, the energy of the atoms remains constant and so the perfect gas law, PV= constant, is obeyed for a perfect gas. 

Properties which can be predicted from fundamental knowledge of intermolecular forces 

Surface science Surface structure, adhesion of materials, phase structures, crystal structures, catalytic processes, clusters 

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The development of various techniques has led to important advances. The possibility to measure intermolecular and intercolloidal forces directly represents a qualitative change from the indirect way such forces had been inferred in the past from aggregation kinetics or from bulk properties such as the compressibility (deduced from small angle scattering) or phase behavior. Both static (i.e., equilibrium) and dynamic (e.g., viscous) forces can now be directly measured, providing information not only on the fundamental interactions in liquids but also on the structure... 

But probably the most serious barrier has been the paralysis that overtakes the inexperienced mind when it is faced with an explosion. This prevents many from recognizing an explosion as the orderly process it is. Like any orderly process, an explosive shock can be investigated, its effects recorded, understood, and used. The rapidity and violence of an explosion do not vitiate Newton s laws, nor those of thermodynamics, chemistry, or quantum mechanics. They do, however, force matter into new states quite different from those we customarily deal with. These provide stringent tests for some of our favorite assumptions about matter s bulk properties. 

A useful property of liquids is their ability to dissolve gases, other liquids and solids. The solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution s properties can differ significantly from the individual constituents. Solvents are covalent compounds in which molecules are much closer together than in a gas and the intermolecular forces are therefore relatively strong. When the molecules of a covalent solute are physically and chemically similar to those of a liquid solvent the intermolecular forces of each are the same and the solute and solvent will usually mix readily with each other. The quantity of solute in solvent is often expressed as a concentration, e.g. in grams/litre. 

The surface tension 7 is a measure of the work required to create unit area of surface from molecules in the bulk it is expressed in ergs per square centimeter or dynes per centimeter. The surface tension is a bulk property, not a molecular property. There appears to be some trend of y with other measures of polarity, but a lower limit of y is reached with very nonpolar liquids this limit (evidently about 15 dyn/cm) reflects the ever-present dispersion force between the molecules of liquid. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

The properties of any material are dependent on the particles present and the forces operating on the particles. Since these forces are different at the frontier than the forces in the bulk, the properties of the frontier region, the interphase region, will differ from the bulk properties. Thus, the uniform properties of the electrolyte are perturbed in the interphase region by the presence of another phase. 

Spectroscopic techniques have been applied most successfully to the study of individual atoms and molecules in the traditional spectroscopies. The same techniques can also be applied to investigate intermolecular interactions. Obviously, if the individual molecules of the gas are infrared inactive, induced spectra may be studied most readily, without interference from allowed spectra. While conventional spectroscopy generally emphasizes the measurement of frequency and energy levels, collision-induced spectroscopy aims mainly for the measurement of intensity and line shape to provide information on intermolecular interactions (multipole moments, range of exchange forces), intermolecular dynamics (time correlation functions), and optical bulk properties. 

The virial equation of state is especially important since its coefficients represent the nonideality resulting from interactions between two molecules. The second coefficient represents interactions between two molecules.Thus,a link is formed between the bulk properties of the gas (P,V,T) and the individual forces between molecules. 

As revealed by structure studies, the environment of surface atoms differs markedly from atoms in the bulk. They have fewer nearest neighbors and consequently different bonding configurations. As a result, net forces beyond those found in the bulk are exerted on the surface atoms. The existence of these forces is the basis of the thermodynamic properties of a surface. These properties can be defined separately from bulk thermod5mamic parameters in the following way. The energy specific to a surface, E, is related to the total energy of the... 

Professor Wakeham is interested in the relationship between the bulk thermophysical properties of fluids and the intermolecular forces between the molecules that comprise them. Thus, at one extreme, he is involved in the determination of intermolecular forces from measurements of macroscopic properties and the development and application of the statistical mechanics and kinetic theory that interrelate them. He is also actively involved in the measurement of the thermophysical properties of fluids under a very wide variety of thermodynamic states. The same thermophysical properties find application in the process industries within the design of a plant. A part of Professor Wakeham s activities are therefore concerned with the representation and extension of a body of accurate information on thermophysical properties in a fashion that allows their use with software packages for process simulation. 

Interpreting bulk properties qualitatively on the basis of microscopic properties requires only consideration of the long-range attractive forces and short-range repulsive forces between molecules it is not necessary to take into account the details of molecular shapes. We have already shown one kind of potential that describes these intermolecular forces, the Lennard-Jones 6-12 potential used in Section 9.7 to obtain corrections to the ideal gas law. In Section 10.2, we discuss a variety of intermolecular forces, most of which are derived from electrostatic (Coulomb) interactions, but which are expressed as a hierarchy of approximations to exact electrostatic calculations for these complex systems. 

Since dynamic friction of polymers has a large contribution from internal viscoelastic dissipation, plots of the friction force, determined, e.g., by AFM, vs./D are qualitatively similar to plots of tan<5 vs. fa. Hence, by measuring dynamic friction forces under well-controlled conditions, the dynamics of a given polymer can be directly probed at the free surface of a sample specimen. Thereby surface vs. bulk properties can be probed as well as confinement effects. 

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