Multiphase Bioreactor Design

highlights the attenuated effect of solvents in microencapsulated biocatalysts.
Nevertheless, studies performed by Han and Rhee (1986) indicated large differences in
the initial velocity of hydrolysis depending on the solvent. Isooctane produced the highest
velocity of reaction followed by cyclohexane and octane (Han and Rhee, 1986).
Another aspect much less explored is the effect of the solvent on the reaction rate. The
physical properties of the solvent are important in determining the substrate
concentrations in the enzyme microenvironment. The development of models of substrate
distribution explain how substrate concentrations can vary and the capacity of the organic
solvent to solubilize the substrate(s) may be important to the process.
The droplet size and interdroplet interaction are also affected by the solvent and
increase with the increment of the chain length of the oil, at a constant W 0 (Hou et al.,
1988). Alkanes of small chain length are able to penetrate into the surfactant layer more
effectively, by reducing the solubilisation of water (Hayes and Gulari, 1995).

Temperature

The influence of temperature on biocatalysis in reversed micelles is regulated by the
same rules as in other aqueous or non-conventional media; here the activity is improved
by raising the temperature up to a value dependent on the stability of the catalyst in the
operating conditions. Nevertheless, the importance of temperature in the definition of the
phase diagrams should be kept in mind, namely to keep the L 2 phase correspondent to the
reversed micelles.
High temperature and high droplet concentration lead to percolation, leading to
changes in the electrical conductivity and eventually to the formation of bicontinuous
structures (Almgren et al., 1993). The electrical conductivity studies carried out by
Suarez and Lang (1995) support this statement, as they verified that an increase in
temperature strengthens the conductivity value above W 0 10 (again when free water starts
to exist in reversed micelles). The percolative behaviour appears to be consequence of the
increase in attractive interdroplet interactions.
Zulauf and Eicke (1979) presented a study of the Stokes radii (rh) as a function of
temperature and W 0 that corroborates the importance of temperature to the physical
properties of reversed micellar systems.

Kinetic Evaluation

The kinetics of reactions catalysed by microencapsulated enzymes in reversed micelles
usually obeys the classical Michaelis-Menten model. However, the kinetic constants
determined for enzymes in these conditions differ significantly from those observed with
the same enzyme in aqueous solution. Since the microencapsulated enzyme is subject to
alterations in conformation, and micellar systems may have associated partition
limitations, the enzyme kinetics in these systems is always inherent or even apparent
depending on the role of diffusion (see chapter 5).
The consequences of the values determined reflect a general pattern. The most
remarkable observation of kinetics in reversed micelles is the increment of Km by 100 to
1000-fold and a simultaneous decrease in kcat by a factor of 2–5 times (Gupte et al.,
1995b; Fletcher et al., 1984).

Multiphase bioreactor design 202