greatly upsets the interpretation of results on diffusivity, since it is not well
known what factors cause crack formation; it greatly depends on the
composition of the matrix.8.4.2 Chemical Reactions
Examples of the dependence of the rate of chemical reactions, including an
enzyme-catalyzed one, are in Figure 8.10. The reader should realize that
relative rates are given, i.e., relative to the maximum rate shown in the
graph. The latter rate may be much slower than the maximum possible rate
(as in c and e). Moreover, the absolute rates of the various reactions
considered may differ by orders of magnitude.
Most authors plot relative reaction rates against water activity, as in
Figure 8.10, but this is not always practical, and Figure 8.11, below, gives
rates against mass fraction of waterw. These figures show that the relations
can vary widely. Important factors affecting reaction rates are- Water activity. If water is a reactant, the rate will decrease with
decreasingaw. For instance, ifaw¼0.5, the rate would be slower by a factor
of 2 than ataw?1, if nothing else changes. In practice, the rate may be
smaller by an order of magnitude or more; consequently, other factors must
FIGURE8.9 Effective diffusion coefficient (Deff) of molecules in systems of various
water contents. (a) Diffusivity of solutes of various molar mass as a function of mass
fraction water (w). (b) Diffusivity of water at some temperatures as a function ofw.
(c) Diffusivity of water in two systems as a function of water activity. Very
approximate, only to illustrate trends.