Ground Rules for Colloidal Stability

Because the lower limit of the colloidal range is just larger than the size of some molecules and solvated species it is difficult to determine exactly where the distinction between surface and bulk ends and a molecularly dispersed system begins. For macromolecular systems, of course, the molecular size is such that even a molecular dispersion or solution easily falls into the size range of colloids. For that reason, primarily, such systems are referred to as lyophilic colloids, even though the properties of such systems are governed for the most part by phenomena distinct from the classic surface interactions considered in lyophobic colloids. It is no trivial matter, therefore, to decide just where surface effects end and the characteristics of the individual free, solvated units begin. 

Keeping in mind what we mean by colloid, the next step is to define what is meant by a stable colloid, or stability in general. Basic thermodynamics tells us that any system, left to its own devices, will spontaneously tend to alter its condition (chemical and/or physical) in an effort to attain the condition of minimum total free energy. It says nothing about how fast such a transformation will occur—that is the province of kinetics—nor necessarily whether the system will stop along the way in the form of some metastable configuration. We only know that given a viable mechanism, it will occur. 

The same general scheme is used to describe the situation in a chemical or physicochemical system in the form of a reaction coordinate. In that case, the height of the Up above the level of the green corresponds to the activation energy for the system which must be overcome in order for a given reaction or transformation to occur. If elements of a physicochemical system have sufficient energy (kinetic, electronic, vibrational, etc.) to overcome the barrier, reaction wiU occur. If not, the system will remain in the metastable state. In golf, of course, there is only one ball involved (we assume ) while chemical systems may involve on the order of 10 (for typical model colloidal sys- 

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