Condensed phases intermolecular forces

The large amplitude motion that sometimes takes place within the molecule, should above all be studied in the gas phase. In a condensed phase intermolecular forces may... 

The physical forces described above aptly account for most molecular interactions in the gas phase. We now direct our discussion toward the condensed phases. Sohds and liquids form when the net attractive intermolecular forces are stronger than the thermal energy in the system and, consequently, hold the molecules together. While the force of attraction can sometimes be attributed to the electrostatic and van der Waals interactions described above, chemical forces also frequently play a role in condensed phases. Chemical forces are based on the nature of covalent electrons, the concept of the chemical bond, and the formation of new chemical species. The main difference between chemical and physical forces is that chemical forces saturate whereas physical forces do not, since chemical interactions are specific to the electronic wavefunc-tions of the chemical species involved. Indeed, a complete quantitative description of chemical interactions involves solution of the Schrodinger equation to describe the overlap of the molecular orbitals involved. We will consider chemical interactions only qualitatively. The goal of this discussion is to realize that there may be other important forces that govern the behavior of solids and liquids and to get a flavor of what these forces might be. 

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

Intermolecular forces are responsible for the existence of several different phases of matter. A phase is a form of matter that is uniform throughout in both chemical composition and physical state. The phases of matter include the three common physical states, solid, liquid, and gas (or vapor), introduced in Section A. Many substances have more than one solid phase, with different arrangements of their atoms or molecules. For instance, carbon has several solid phases one is the hard, brilliantly transparent diamond we value and treasure and another is the soft, slippery, black graphite we use in common pencil lead. A condensed phase means simply a solid or liquid phase. The temperature at which a gas condenses to a liquid or a solid depends on the strength of the attractive forces between its molecules. 

A 7/vap always is 3.10 kJ/mol greater than A fi vap At 298 K (25 °C, room temperature) A /Tvap always is 2.48 kJ/mol greater than A ivap The difference between A vap and A i7vap arises because, in addition to overcoming intermolecular forces in the condensed phase (A E), the escaping vapor must do work, w = A(P V ) — RT as it expands against the constant external pressure of the atmosphere. 

Gases and condensed phases look very different at the molecular level. Molecules of F2 or CI2 move freely throughout their gaseous volume, traveling many molecular diameters before colliding with one another or with the walls of their container. Because much of the volume of a gas is empty space, samples of gaseous F2 and CI2 readily expand or contract in response to changes in pressure. This freedom of motion exists because the intermolecular forces between these molecules are small. 

A substance exists in a condensed phase when its molecules have too little average kinetic energy to overcome intermolecular forces of attraction. 

A gas condenses to a liquid if it is cooled sufficiently. Condensation occurs when the average kinetic energy of motion of molecules falls below the value needed for the molecules to move about independently. Thus, the molecules in a liquid are confined to a specific volume by intermolecular forces of attraction. Although they cannot readily escape, liquid molecules remain free to move about within the liquid phase, hi this behavior, liquid molecules behave like the molecules of a gas. The large-scale consequences of the molecular-level properties are apparent. Like gases, liquids are fluid, so they flow easily from place to place. Unlike gases, however, liquids are compact, so they cannot expand or contract significantly. 

Phase changes are characteristic of all substances. The normal phases displayed by the halogens appear in Section II-L where we also show that a gas liquefies or a liquid freezes at low enough temperatures. Vapor pressure, which results from molecules escaping from a condensed phase into the gas phase, is one of the liquid properties described in Section II-I. Phase changes depends on temperature, pressure, and the magnitudes of intermolecular forces. 

In this section we will briefly review some of the main different approaches that have been used up to now in order to evaluate the potential energy of each configuration in a Monte Carlo run. As we have already stated, the force fields that describe intra- and intermolecular interactions are at the heart of such statistical calculations because the free energy differences that we want to evaluate are directly dependent on the changes of those interactions. In fact, the important advances of the last ten years in the power of computer techniques for chemical reactions in the condensed phase, that we have mentioned in the Introduction, have been due, to a great extent, to the continual evolution in force fields, with added complexity and improved performance. 

Intermolecular forces can affect phase changes. Strong intermolecular forces require more kinetic energy to convert a liquid into a gas. Stronger intermolecular forces, make it easier to condense a gas into a liquid. 

Our interest is in the connection between the intermolecular forces that cause condensation and/or gas phase molecular clustering and thermodynamics. To set the stage consider the following simple model ... 

The previous chapter dealt with chemical bonding and the forces present between the atoms in molecules. Forces between atoms within a molecule are termed intramolecular forces and are responsible for chemical bonding. The interaction of valence electrons between atoms creates intramolecular forces, and this interaction dictates the chemical behavior of substances. Forces also exist between the molecules themselves, and these are collectively referred to as intermolecular forces. Intermolecular forces are mainly responsible for the physical characteristics of substances. One of the most obvious physical characteristics related to intermolecular force is the phase or physical state of matter. Solid, liquid, and gas are the three common states of matter. In addition to these three, two other states of matter exist—plasma and Bose-Einstein condensate. 

Throughout this chapter we have dealt with surface tension from a phenomenological point of view almost exclusively. From fundamental perspective, however, descriptions from a molecular perspective are often more illuminating than descriptions of phenomena alone. In condensed phases, in which interactions involve many molecules, rigorous derivations based on the cumulative behavior of individual molecules are extremely difficult. We shall not attempt to review any of the efforts directed along these lines for surface tension. Instead, we consider the various types of intermolecular forces that exist and interpret 7 for any interface as the summation of contributions arising from the various types of interactions that operate in the materials forming the interface. 

The very fact that the vapor phase of many substances can condense to form a liquid is a consequence of the existence of attractive van der Waals forces between atoms or molecules. An attractive intermolecular force is not needed for a gas to condense into a solid solidification can occur purely as a result of excluded-volume interactions among the molecules at sufficiently large densities. The pressure in a fluid, the cohesion between materials, and the existence of surface energy or surface tension all result, partially or wholly, from van der Waals forces. 

The temperature at which a gas condenses depends on the pressure and the strength of the attractive forces between its molecules. Intermolecular forces pull molecules together and, provided the temperature is low enough, produce a condensed phase. In the gas phase, the properties of the substance are dominated by the nearly free motion of the molecules, so they are nearly independent of the identity of the gas. In condensed phases, however, the molecules are very close to one another all the time and intermolecular forces are of dominating importance. 

With the exception of gas/gas mixtures, such as air, the different kinds of solutions listed in Table 11.1 involve condensed phases, either liquid or solid. Thus, all the intermolecular forces described in Chapter 10 to explain the properties of pure... 

During condensation, gaseous particles slow down and are overcome by the intermolecular forces at work in liquids. During sublimation and deposition, temperature and pressure conditions are so extreme that the liquid phase simply gets skipped. 

E. Wasserman (The DuPont Company, U.S.A.). When you talk about the possible phases of something like C60 (is it a gas, a liquid or a low density liquid of high compressibility), we really have to compare it with another phase which may be accessible under the same temperature and pressure conditions. In many such cases, some of the features of C60 are due to intermolecular interactions, in some of the more condensed phases, rather than to individual molecular properties that you were concentrating on. For example, the very strong tenacity of one C60 molecule to bond to another, as well as to incorporate solvent molecules in the interstitial spaces, depends critically on how well they seem to fit together, as well as to the intrinsic forces that may be found in smaller molecules. We find that if you have small degrees of substitution of C6, for example, alkyl groups, the volatility increases dramatically. 

Since chemical species in condensed phases interact strongly, solids and liquids are more complicated than gases, where intermolecular forces are usually negligible. The interactions of atoms, molecules, and ions are electrostatic in origin, but there are several types of interaction with quite different energies ... 

Other flexible molecular models of nitromethane were developed by Politzer et al. [131,132]. In these, parameters for classical force fields that describe intramolecular and intermolecular motion are adjusted at intervals during a condensed phase molecular dynamics simulation until experimental properties are reproduced. In their first study, these authors used quantum-mechanically calculated force constants for an isolated nitromethane molecule for the intramolecular interaction terms. Coulombic interactions were treated using partial charges centered on the nuclei of the atoms, and determined from fitting to the quantum mechanical electrostatic potential surrounding the molecule. After an equilibration trajectory in which the final temperature had been scaled to the desired value (300 K), a cluster of nine molecules was selected for a density function calculation from which... 

In this chapter, the basic types of chemical bonds existing in condensed phases are discussed. These interactions include ionic bonds, metallic bonds, covalent bonding (band theory), and intermolecular forces. In Chapter 10, the structures of some inorganic crystalline materials will be presented. 

The solid and liquid phase is known as a condensed phase. The condensed phase results due to intermolecular forces. An approximate relationship between the strength of intermolecular forces and the boiling points is provided by Trouton s Law. According to it the value of AS°yap is around 90 JK-1 mol-1... 

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