W 0 versus aw
Hydration level and W 0The hydration degree of the reversed micelles is commonly given by the W 0 parameter,
which represents the molar ratio of water to surfactant concentrations. The optimum W 0
value for catalysis depends on several aspects. A common feature is that normally the
optimum Wo corresponds to a micellar size comparable to that of the protein. Martinek et
al. (1989) found a direct relation between the effective radii of entrapped enzyme and the
corresponding size of the micellar core (regulated by the W 0 parameter). The optimal W 0
also depends on the enzyme concentration and the higher the concentration the higher the
W 0 needed to attain maximal activity (Han et al., 1990; Patel et al., 1996a).
The study of the physical properties of water in the aqueous pool proved that they are
quite different from the bulk water solutions, which reflects the enzymatic reactivity. As
an example, at low W 0 , water has a freezing temperature below zero, allowing
cryoenzymology applications (Luisi et al., 1990; Rees et al., 1995b). The micellar
solutions containing AOT were examined as a function of temperature by Douzou et al.
(1979) to determine the range over which they remain homogeneous and it was verified
that AOT microemulsions are feasible at temperatures as low as −45 °C and the presence
of enzymes and electrolytes does not markedly affect the solubility of water.
Goto et al. (1995a) studied the solubilisation of water in AOT reversed micelles
classifying the water as immobilised, hydration and free water. Below W 0 2, only
immobilised water exists whereas in the range 4–10 the water hydrates the AOT polar
heads and finally the presence of free water is verified at W 0 values above 10 (Figure
7.1).
Multiphase bioreactor design 196