et al., 1990). The inability to hydrolyze triglycerides was attributed to the presence of
the PEG palisade layer at the interface, which is known to repel proteins (Holmberg
and O ̈sterberg, 1990; Stark et al., 1990). In addition, a side reaction occurred: the free
hydroxyl on the free end of PEG acted as an acyl acceptor for free fatty acid (Holm-
berg and O ̈sterberg, 1987; Stark et al., 1990). Likewise, when PEG-sorbitan fatty
acid esters were employed as surfactant to host lipolysis, they were more readily
hydrolyzed than the triglyceride substrate (O ̈sterberg et al., 1985; Yamada et al.,
1993). In subsequent work, Holmberg and co-workers screened several surfactants
for their ability to hydrolyze palm oil (Skagerlind and Holmberg, 1994). Their work
demonstrated that the two-tailed, or branched, PEG-fatty acid ether, octa(ethylene
glycol) mono(2-butyloctyl)ether, was better suited for encapsulating lipase than sin-
gle-tail nonionic surfactants (Skagerlind and Holmberg, 1994). In agreement, Wien-
cek and co-workers have demonstrated that the best surfactant structure for w/o-ME
formation consists of two tail groups of nonequal length, with an HLB value of about
10 (Vasudevan et al., 1995).
An additional novel approach was to employ Winsor III microemulsion systems,
which consist of a middle bicontinuous microemulsion phase that contains the sur-
factant and enzyme, and excess aqueous and apolar (isooctane) phases. Holmberg
and co-workers developed an oxyethylene ether phosphonate surfactant that readily
formed Winsor III systems over a 30 8 C temperature range (Sonesson and Holmberg,
3.2 Lipases encapsulated in water-in-oil microemulsions 55
Table 2.Microemulsion systems employed for hosting lipase-catalyzed reactions
Microemulsion system Reference
Anionic surfactants
Aerosol-OT (sodium bis[2-ethylhexyl] sulfosuccinate) Hayes and Gulari, 1990
Cationic surfactants
Cetyl trimethylammonium bromide (CTAB)/1-Pentanol Valis et al., 1992
Didodecyldimethylammonium bromide Skagerlind and Holmberg, 1994
Nonionic surfactants
Poly(ethylene glycol) fatty alcohol ethers) Bello et al., 1987
Holmberg and O ̈sterberg, 1987
Tween 80 (polyoxyethylene sorbitan fatty acid esters) O ̈sterberg et al., 1985
Kermasha et al., 1993
Octa(ethylene glycol) mono(2-butyloctyl) ether Skagerlind and Holmberg, 1994
Nonidet P-40 (octylphenoxy polyethoxyethanol) Yang and Russell, 1994
surfactants derived from natural sources
Phosphatidylcholine (lecithin)/Alkanol Morita et al., 1984
Schmidli and Luisi, 1990
C 10 -C 12 acid/C 10 -C 12 FA soap Oh et al., 1996
Skagerlind et al., 1997
Mixed surfactant systems
AOT/Polyoxyethylene sorbitan fatty acid ester (Tween 85) Yamada et al., 1993
Hossain et al., 1999
AOT/Taurodeoxycholate Kuboi et al., 1992
AOT/Lecithin Nagayama et al., 1998
Winsor III microemulsion systems
Branched-tail oxyethylene phosphonate surfactant Sonesson and Holmberg, 1991