Forces acting between molecules

The excess free energy is divided into terms representing the contributions due to repulsive and attractive nonassociative forces acting between molecules, as well as into a contribution arising from association [38,39]... 

The following four forces acting between molecules contribute to the lattice energy of a crystal consisting of molecules ... 

The structure of living cells depends very much on the covalent bonds within individual molecules and on covalent crosslinks that sometimes form between molecules. However, weaker forces acting between molecules and between different parts of the same molecule are responsible for many of the most important properties of biochemical substances. These are described as van der Waals forces, electrostatic forces, hydrogen bonds, and hydrophobic interactions. In the discussion that follows the thermodynamic quantities AH, AS, and AG will be used. If necessary, please see Chapter 6 for definitions and a brief review. 

The forces acting between molecules 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, introduced in Section A. Water, for instance, can exist in any of these three phases, as ice, liquid, and vapor. 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 diamond and another is graphite. A condensed phase means simply a solid or liquid phase. 

Strong intramolecular forces (covalent bonds) hold the atoms in molecules together. Relatively weak intermolecular forces act between molecules. 

Figure 3.4 Schematic representation of forces acting between molecules, and between molecules and solid wall (adapted from Kerkhof [22]).

Cohesive forces Act between molecules in the adhesive layer. Sufficient cohesive strength depends on the observance of the determined curing time and temperature as well as on a homogeneous mixture of the adhesive components. 

First of all, we must be aware of the fact that all physical forces acting between molecules are essentially electrostatic (Coulomb forces between charges and dipoles induced dipole forces) or quantum mechanical (dispersion, and repulsive forces) in origin apart from a minor contribution due to mass attraction gravitational forces. Gravitational forces... 

Intermolecular forces act between molecules or between molecules and ions. Generally, these attractive forces are much weaker than bonding forces. 

Forces acting between molecules are, by definition, noncovalent. A full discussion of intermolecular forces is well beyond the scope of this book, but some of the more significant features are important for understand-... 

I. Chemical nature of the medium and the solid, i.e. the nature of forces acting between molecules (atoms) of both phases and in particular at the interface. 

Finally, it should be pointed out that all of the equations of state that are proposed for gases are based on the two fundamental ideas first suggested by van der Waals (1) molecules have size, and (2) forces act between molecules. The more modern equations include the effects of the dependence of the intermolecular forces on the distance of separation of the molecules. 

Besides forces acting among atoms on one molecule intramolecular forces), there are also intermolecular forces acting between molecules. Such forces depend on distances between molecular centers and, in nonspherical molecules, on the relative orientations of the molecules. When molecules are widely separated, as in a gas, intermolecular forces are small see Figure 1.3. If we squeeze the gas, it may condense to form a liquid evidently, when molecules are pushed moderately close together they attract one another. But if we squeeze on the condensate, the liquid resists strongly when molecules are dose together they repel one another. This behavior is typical. 

In many mixtures having LCSTs, relatively strong attractive forces act between molecules of different species as well as between molecules of the same species often such forces are caused by hydrogen bonding. At low temperatures T attractions between unlike molecules dominate and prevent a liquid-liquid split. But the strength... 

Dispersion forces also known as the London forces act between molecules and atoms that are even neutral. The dispersion forces are long-range forces. Their range of action varies from 0.2 to 10 nm. 

At temperatures near the critical temperature of a hquid, the cohesive forces acting between molecules in the liquid become very small and the surface tension approaches zero. That is, since the vapor cannot be condensed at the critical temperature, there will be no surface tension. A number of empirical equations that attempt to predict the temperature coefficient of surface tension have been proposed one of the most useful is that of Ramsey and Shields ... 

An important point in the use of surface energy components is the realization that for non-polar liquids, such as alkanes, only dispersion forces act between molecules, so... 

To maintain his doctrine Kekule also had to devise an artificial division of compounds into atomic compounds with the atoms held together by normal afSnities and molecular compounds composed of atomic ones combined by odd forces acting between molecules rather than between atoms. For example, Kekule classified sal-ammoniac (NH4CI) and phosphorus pentachloride (PCI5) as molecular compounds and assigned them the formulas NH3,HC1 and PCl3,Cl2 to preserve the trivalence of nitrogen and phosphorus. 

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