The Importance of Long-Range Physical Forces

Bond Type Strength (kJ moL ) Bond Type Strength (kJ moL ) 

While physical interactions lack the qualifications to be considered classic chemical bonds, it is through such interactions that all but the most elemental properties of most materials in our world are determined. They also exhibit themselves as the fundamental factors involved in all colloidal systems, all biological assemblies, and all natural phenomena not involving chemical interactions. That covers quite a lot of territory Because of their importance, physical interactions will be covered in somewhat more theoretical detail below than most topics to be presented in the following chapters. 

We have seen from the discussion in Chapter 2 that as two uncharged surfaces in a vacuum are separated to an effectively infinite distance, their free energies increase to some maximum value characteristic of the system involved. This means that the net force acting between the two surfaces must be attractive. That will always be the case for pure substances in vacuum, but in reality, many situations exist in which a maximum in the free energy-separation distance curve is encountered. 

Many important systems and processes, especially biological assemblies such as cell walls and protein secondary and tertiary structures form as a result of physical, inter- and intramolecular interactions. Such assemblies and 

Under certain circumstances it is possible to utilize physical interactions to maintain surfaces at some minimum distances of separation as a result of an energy maximum in the interaction energy. The practical result of such long-range energy maxima is that, properly utilized, they can prevent or at least retard the natural tendency of surfaces to approach and join spontaneously, thereby reducing interfacial area. This effect is especially important in colloids, foams, emulsions, and similar systems. 

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THE IMPORTANCE OF LONG-RANGE PHYSICAL FORCES 41 TABLE 4.1. Characteristic Strengths of Covalent Bonds... 

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