Pseudophase Approach to Reactivity in Microemulsions
J. Phys. Chem., Vol. 100, No. 26, 1996 10987
systems.10 Those of the alkyl nitrites, K3 and K4, show BEN
to be more hydrophobic than EEN, as in micellar systems.13,23
Anomalous BehaVior with W < 12. The reactions with
NMBA and MOR were slower than predicted when carried out
in media with W < 12; similarly anomalous behavior has
previously been reported for the nitrosation of these amines by
MNTS in microemulsions with low W.10 We attribute these
anomalies to the scarcity of water molecules in such media, in
which a large proportion of the water is engaged in solvating
AOT head groups. Numerous studies have shown that the
dielectric constant of the interface decreases with decreasing
W when W is less than 10-15,24 and electron solvation
experiments,25 1H-NMR studies,26 and studies of solvolysis in
microemulsions27 suggest that other kinetically relevant proper-
ties of water (solvating power, nucleophilicity, electrophilicity,
etc.) also undergo significant alteration in microemulsions with
low W. This implies that for media with W less than a certain
threshold, the kinetic constant ki depends on the amount of
available water at the interphase and is therefore a function of
the composition of the microemulsion.
Percolation. The conditions used in some of the experiments
carried out in this work were such that electric percolation may
have taken place, i.e., interdroplet mass transport made possible
by the fleeting formation of interdroplet channels during
droplet-droplet collisions.28 Percolation is facilitated by the
presence of amines.29 This phenomenon is not taken into
account by the pseudophase model. However, our results for
the reactions studied in this work are valid regardless of the
occurrence of percolation, because these reactions are chemically
controlled and have half-lives very much longer than those of
the interdroplet mass transport phenomena.30 To check this,
we carried out the reaction between PIP and EEN both in the
absence and in the presence of 1,3-dimethylurea (DMU), which
drastically lowers the percolation threshold of AOT/iC8/water
microemulsions;29 under conditions in which percolation occurs
in the presence of DMU but not in its absence, the slope k2,app
of a plot of kobs against [PIP] was effectively the same in the
absence of DMU (3.01 × 10-3 M-1 s-1) as in its presence (2.97
× 10-3 M-1 s-1). Nor did percolation alter the mechanism of
the reaction, to judge from the solvent isotope effects of 1.76
and 2.0 observed in experiments with heavy water in respec-
tively the presence and absence of DMU. Such kinetic and
mechanistic independence of percolation has previously been
reported for the hydrolysis of 4-nitrophenyl chloroformate in
microemulsions.27
SCHEME 5
interphase due to the polarity of the interface, like the polarity
of the interface of normal micelles,31 is less than that of bulk
water.
The General Model. The pseudophase models used in this
work are all particular cases of the general model shown in
Scheme 5. This general model does not seem to be readily
verifiable by means of its full application to a single reaction,
since most reactions exhibit the same sort of limitations as have
led to the use of reduced models in the present work:
insolubility of one or more reagents in one of the pseudophases
and great disparity (by several orders of magnitude) between
the reaction rates in different paseudophases. It is nevertheless
useful to have at hand the expression of kobs implied by the
general model:
kwW
koilZ
ki +
+
[amine]T
[AOT]
K1K3 K2K4
kobs
)
(14)
W
Z
W
Z
1 +
+
1 +
+
(
)(
)
K1 K2
K3 K4
Appropriate simplification of this one expression yields the
expression for kobs for any reduced model, including all those
used in this work.
Acknowledgment. Financial support from Xunta de Galicia
(Project XUGA 20906B93) and from the Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica of Spain (Project PB93-0524)
is gratefully acknowledged. J.C.M. thanks Xunta de Galicia
for a Research Training Grant.
References and Notes
Comparison with Reaction Rates in Water. For comparison
of reactivities in the AOT interface with the corresponding
reactivities in bulk water, the ki of Table 3 (defined in terms of
mole per mole concentrations and expressed in s-1) must be
(1) (a) ReVerse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum
Press: New York, 1984. (b) Structure and ReactiVity in reVerse micelles;
Pile´ni, M. P., Eds.; Elsevier: Amsterdam, 1989. (c) Zulauf, M.; Eicke, H.
F. J. Phys. Chem. 1979, 83, 480.
(2) Fendler, J. H. Chem. ReV. 1987, 87, 877.
converted to conventional reaction rates expressed in M-1 s-1
.
(3) (a) Mukerjee, P.; Ray, A. J. Phys. Chem. 1966, 70, 2144. (b)
Mukerjee, P.; Cardinal, J. R.; Desai, N. R. Micellization, Solubilization and
Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977. (c)
Funasaki, J. J. Phys. Chem. 1979, 83, 1998. (d) Ferna´ndez, M. S.; Fromherz,
P. J. Phys. Chem. 1979, 83, 1755.
This requires knowledge of the molar volume of AOT under
the conditions obtaining at the interface in our microemulsions,
which is far from easy to ascertain with confidence since it
implies precise definition and knowledge of the volume of the
interface. In the interests of comparability with our results on
nitrosation by MNTS in microemulsions,10 in this work we have
adopted the same criterion as in our earlier paper: we assume
the molar volume of AOT in the interface, Vh, to be given by its
density, and we accordingly define a “conventional” bimolecular
(4) Ruasse, M. F.; Blagoeva, Y.; Gray, P. Book of Abstrats ESOR IV-
MMBP; The Royal Society of Chemistry Perkin Division: Newcastle upon
Tyne, 1993.
(5) (a) Wong, M.; Thomas, J. K.; Gra¨zel, M. J. Am. Chem. Soc. 1976,
98, 2391. (b) Bakale, G.; Beck, G.; Thomas, J. K. J. Phys. Chem. 1981,
85, 1062. (c) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (d) Keh, E.;
Valeur, B. J. Colloid Interface Sci. 1981, 79, 465.
(6) (a) Thomas, J. K. Chem. ReV. 1980, 80, 281. (b) Marcel, W.
Proteins: Struct. Funct. Genet. 1986, 1, 4.
i
rate constant, k2 for the reaction at the interface by
(7) Lissi, E. A.; Engel, D. Langmuir 1992, 8, 452.
i
(8) (a) Khmelnitsky, Y. L.; Neverova, I. N.; Polyakov, V. I.; Grinberg,
V. Y.; Levashov, A. V.; Martinek, K. Eur. J. Biochem. 1990, 190, 155. (b)
Verhaert, R. M. D.; Hilhorst, R. Recl. TraV. Chm. Pays-Bas 1991, 110,
236.
(9) (a) Menger, F. M.; Donohue, J. A.; Williams, R. F. J. Am. Chem.
Soc. 1973, 95, 286. (b) Blandamer, J. M.; Burges, J.; Clark, B. J. Chem.
k2 ) kiVh
(13)
where under our conditions Vh ) 0.37 M-1
Comparison of k2 and corresponding k2 in Table 3 shows
that the reactions studied proceed much more slowly in the
.
i