Reversed micelles are dynamic entities, which can exchange their constituents
including water, surfactant, or other contents (Luisi and Magid, 1986). The exchange
process may take place between two micelles or between each micelle and the bulk
organic solvent. Upon collision of two micelles their coalescence occurs through a
transient dimer, that permits a rapid exchange of material (Luisi, 1985).
The solutes such as enzymes, co-surfactants, substrates or products, may induce the
structural change of the micelles, namely the size or water content affecting both the
micelles and the microencapsulated biomolecules (Luisi and Magid, 1986).
SurfactantsSurface active agents, commonly known as surfactants, are amphiphilic molecules
containing non-polar and polar parts capable of interacting with interfaces. One of the
most widely used parameters to evaluate surfactant activity is the critical micelle
concentration (CMC). The CMC is the minimum surfactant concentration required to
reach the lowest surface tension. Above the CMC, the surfactant molecules readily
associate to form supramolecular structures, such as micelles, vesicles, bilayers or others.
Surfactants can be classified according to their ionic character in: anionic, cationic,
ampholytic or zwitterionic and non-ionic. The former group includes bis-(2-ethylhexyl)
sulfosuccinate usually known as AOT as well as calcium dodecyl sulfate (CDS) and
sodium dodecylsulphate (SDS). Cetyl trimethyl ammonium bromide or chloride (CTAB
or CTAC), didodecyl dimethyl ammonium bromide (DDAB), tetradecyl trimethyl
ammonium bromide (TTAB) and trioctyl methyl ammonium chloride (TOMAC) are
examples of cationic surfactants. Ampholytic surfactants include lecithin or
phosphatidylcholine dérivátes with origin in egg yolk or soy bean. Finally, non-ionic
surfactants include polyoxyethylene alcohols, esters and ethers.
The type of surfactant used to form the reversed micelles can largely influence enzyme
activity (Patel et al., 1996a). Rees et al. (1995a) compared the activity of five microbial
lipases and verified that lactonisation activity was higher for the systems based on
anionic surfactants (AOT), than for those based on cationic surfactants (CTAB). Lipase
stability was higher in CTAB, but reached high levels in AOT reversed micelles with
reduced water content (Rees et al., 1995a).
A comparative study was carried out by Valis et al. (1992), in reversed micelles of
anionic, cationic and nonionic surfactants, using Rhizopus delemar lipase. The
assembling conditions of pH, Wo and temperature to reach the maximum enzyme
activity, differ significantly in each reversed micellar system.
Cationic surfactants often need the presence of a co-surfactant to form reverse
micelles. The same happens with most of the zwitterionic surfactants. Non-ionic and
anionic surfactants yield reversed micellar systems in defined concentration regions.
The tailoring of surfactants to fulfill biocatalytic needs has been gaining interest. A
new class of anionic surfactants (e.g. dioleyl phosphoric acid or DOLPA) was
synthesised with long alkyl chains included in the hydrophobic moiety ensuring a high
encapsulation ratio of proteins, such as hemoglobin, that could not be extracted with
AOT (Goto et al., 1997).
Concerns about the toxicity of surfactants led some authors to study alternatives and to
suggest nontoxic microemulsions. Kahlweit et al. (1997) proposed the use of unsaturated
Multiphase bioreactor design 192