Front Matter

dissolution in the bulk solvent (Jenta et al., 1997a; Karlsson et al., 1998). ‘Hard’,

opaque gels (8–14 % gelatin) are less active, presumably due to diffusional limita-

tions caused by the additional gelatin, but are much more stable than soft gels (Jenta

et al., 1997a). As thewovalue of the gel increased from 40 to 200, the gel underwent a

soft-to-hard transition. The gels must be stored in closed containers to reduce the

evaporation of solvent and water, which renders the gels ineffective (Uemasu

and Hinze, 1994).

The gels were successfully applied to catalyze a variety of reactions, particularly

enantioselective esterification of racemic alcohols and acids, even on a preparative

scale and at
208 C (Rees et al., 1991; 1993; 1995b; Uemasu and Hinze 1994).

Surprisingly, even polyols were successful substrates. The polyols were adsorbed

by the gels, rendering them as swollen; but, as esterification proceeded, the swelling

decreased due to the consumption of polyol (Rees et al., 1993). The only substrates

not tolerated were short-chain (C 1 –C 3 ) and branched fatty acyl substrates (Rees et al.,

1991), which induced a phase transition of the gel to a liquid phase (Rees et al.,

1993). In addition, the presence of substrates (fatty acids and alcohols) during

gel preparation prevented gel formation (Jenta et al., 1997b). Substrates that accu-

mulated in the gel phase were readily extracted away by solvent (Rees et al., 1991).

An attempt to employ the gels to catalyze acidolysis (racemic 2-octanolþvinyl

acetate) yielded the hydrolysis side reaction, producing acetic acid to a significant

extent (Karlsson et al., 1998). Several different lipases were active in the gel-phase,

with the exception ofC. rugosalipase (Nascimento et al., 1992; Rees et al.; 1995b;

Karlsson et al., 1998). [However, one group reported the successful resolution of

polyphenolics byC. rugosalipase encapsulated in AOT–gelatin gel (Parmar et

al., 1996).]

Importantly, the leakage of AOT, water, and gelatin from the gels is minimal,

simplifying downstream separations (Rees et al., 1993). The gels demonstrated ex-

cellent stability, allowing them to be reused over a period of months (Rees et al.,

1991; Nascimento et al., 1992; Backlund et al., 1996). The majority of the activity

loss that occurred was due to the accumulation of water, an ester synthesis product, in

the gels (Jenta et al., 1997a; Rees and Robinson, 1995). Moreover, the gel activity

decreased withwobecause of the soft-to-hard gel transition (Jenta et al., 1997a).

However, the activity loss was reversed when water was extracted from the gels

by low-womicroemulsion solution (Jenta et al., 1997a).

Surprisingly, the gels exhibited minimal diffusional limitations. Moreover, the

turnover number and activation energy for esterification, determined by applying

a Ping Pong Bi-Bi model, was very similar to the value achieved in a w/o-ME solu-

tion (Jenta et al., 1997b). However, many of the transformations took several days to

complete, due to a limit in lipase loading. Moreover, a concentration of lipase over

200 lg/liquified the gel (Jenta et al., 1997a).

The solvent type employed in the gel formation and in the reaction medium only

slightly affected the performance on the gels (Nascimento et al., 1992; Jenta et al.,

1997a). [The solvent type contained in the gel did not necessarily have to be the same

as that in the bulk reaction medium. Of interest, solvent molecules transported be-

tween the solid and bulk liquid phases (Karlsson et al., 1998).] In addition, the gels

successfully esterified a liquid mixture of substrates in the absence of solvent.

3.5 Microemulsion gels 61