Multiphase Bioreactor Design

of increasing the salt concentration is the salting-out of surfactant from the micelles to the
organic phase due to the partial hydrophobic character of surfactant. Moreover, it also
favors the undissociated form of the surfactant by the commom ion effect (Wang et al.,
1994).

Additives and protecting compounds

The mechanisms of stabilisation in reversed micelles or in other systems are normally
associated with hydrogen bonding, dipole-dipole interactions and dispersion interactions
(Luisi and Magid, 1986). It should be emphasized that the micellar properties previously
referred to, such as low dielectric constant and higher microviscosity, can intensify
hydrogen bonding in the protein thereby modifying its conformation.
A variety of additives are capable of improving these processes. The efficiency of an
additive to preserve enzymatic activity depends on its chemical character and on the
enzyme structure. Among the additives tested, alcohols, sugars and polyols have
dominated prospective studies. This is mainly due to the fact that these compounds
interact with the enzymes, strengthening hydrogen bonds and decreasing the water
contact with protein unfolding, thereby preventing protein unfolding.
Some additives are used to improve the capacities of the micellar systems, namely
their ability to form inverted micelles, as is the case with co-surfactants. Their effects on
microencapsulated enzymes are variable, as some may be detrimental to activity/stability
while others enhance enzymatic performance and the retention of activity. The addition
of co-surfactants may increase or decrease the droplet size of microemulsions, depending
on the chemical structure of the co-surfactant added.
The effects of increasing or decreasing the co-surfactant chain length are not
straightforward. The most common situation when long chain co-surfactants are applied,
is the hindrance of attractive interactions and the decrease of interfacial thickness,
although, these co-surfactants may also increase the interfacial thickness when
enhancement of interactions between the micellar interface and the bulk organic solvent
occurs (Hayes and Gulari, 1995).
Freeman et al. (1998) reported an enhancement of chymotrypsin enzymatic activity in
AOT reversed micelles by the addition of a bile salt cosurfactant, sodium taurocholate
(NaTC), which modifies the interfacial properties increasing the water uptake, hence
modifying the catalytic microenvironment. Additionally, this bile salt improves the
aggregates organisation and may structurally alter the enzymes.
The addition of alcohols to micellar solutions of surfactants affects micellar properties
such as CMC, ionisation degree, micellar molecular weight and micellar dynamics (Yiv
et al., 1981). Short chain alcohols usually increase the attractive interactions diminishing
the degree of interpénétration of droplets whereas the interfacial fluidity is enhanced. The
same happens with the addition of Arlacel-20 (non-ionic surfactant) to the ternary system
AOT/dodecane/water (Hou et al., 1988).
Hayes and Marchio (1998) studied the expulsion of water and protein from reversed
micelles upon addition of different alcohols as co-surfactants. The addition of co-
surfactant released (α-chymotrypsin, pepsin, bovine serum albumin and catalase from
reversed micelles and the release was enhanced with co-surfactant chain length (from 1-
butanol to 1-dodecanol). The water was also co-expelled except on the addition of

Multiphase bioreactor design 200