The kinetics of slow coagulation of GR-S Latex (Pt.I). : The kinetics of the cis-trans isomerization of azobenzene in solution (Pt.II).

The phenomenon of coagulation of colloidal systems is analogous in many respects to a chemical reaction, proceeding at a definite rate dependent on temperature and concentration, and being markedly sensitive to the presence of foreign substances. It is, however, mainly a diffusion-controlled process. The consequence of this is that no equilibrium state can be set up to describe the reacting system. Hence the problem cannot be treated on the basis of either the collision theory of chemical reactions or the theory of the transition state, since both of these depend on an equilibrium configuration of the reactants, activated intermediates and products in the reaction. In coagulation the rate is governed by the viscosity of the medium, which determines the rate at which the particles diffuse to each other, since the ensuing collision of the particles generally results in coalescence. This process disturbs the equilibrium greatly and it is only when the probability of coalescence of these collisions is reduced that an equilibrium state can be postulated. Therefore an all-inclusive theory of coagulation cannot be deduced by the standard kinetic methods. However, the application of other methods leads to rate equations similar to those used for chemical reactions. In any colloidal system the sum of the free energies of the separate components is less than that of the system which is formed from them [...