Reactions in microemulsion media: Schiff bases with targeting/anchoring module as kinetic sensors to map the polarity pocket of a microemulsion droplet

Article abstract of DOI:10.1002/kin.1042

The hydrolysis of some tailor-made Schiff bases having flexible spacers between aldimine groups and alkoxy groups at ortho (o) or para (p) position in the benzene ring has been investigated in microemulsion media. The kinetic data of acid-catalyzed hydrolysis in anionic (sodium lauryl sulphate: NaLS) and cationic (cetyltrimethyl ammoniumbromide: CTAB) microemulsion media have been explained considering the localization of the Schiff bases at various probable pockets of the microemulsion droplet. The results are in conformity to the solubilization studies of the reported Schiff bases in microemulsions (Dash et al., Spectrochim Acta 1996, 52A, 349). The change in reactivity due to change in the spacer length and position of the alkoxy group in the Schiff bases has been rationalized on the basis of localization sites of the reaction center at different polarity pockets of the reaction media.

Full text of DOI:10.1002/kin.1042

Reactions in Microemulsion
Media: Schiff Bases with
Targeting/Anchoring
Module as Kinetic Sensors
to Map the Polarity Pocket
of a Microemulsion Droplet
B. K. MISHRA, NAMITA PATEL, P. K. DASH, G. B. BEHERA
Center of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur University, Jyoti Vihar—
7
68 019
Received 30 August 1997; accepted 28 March 2001
ABSTRACT: The hydrolysis of some tailor-made Schiff bases having flexible spacers between
aldimine groups and alkoxy groups at ortho (o) or para (p) position in the benzene ring has been
investigated in microemulsion media. The kinetic data of acid-catalyzed hydrolysis in anionic
(sodium lauryl sulphate: NaLS) and cationic (cetyltrimethyl ammoniumbromide: CTAB) mi-
croemulsion media have been explained considering the localization of the Schiff bases at
various probable pockets of the microemulsion droplet. The results are in conformity to the
solubilization studies of the reported Schiff bases in microemulsions (Dash et al., Spectrochim
Acta 1996, 52A, 349). The change in reactivity due to change in the spacer length and position
of the alkoxy group in the Schiff bases has been rationalized on the basis of localization sites
of the reaction center at different polarity pockets of the reaction media. ᭧ 2001 John Wiley &
Sons, Inc. Int J Chem Kinet 33: 458–464, 2001
INTRODUCTION
biological systems. In particular, the use of micro-
emulsion water droplets as a novel environment for
enzyme-catalyzed reactions has attracted much inter-
est [2]. Microemulsions contain divided domains of
oil and water topologically ordered by surfactants at
the internal interfaces and sometimes provide a strong
barricade to separate the reactants from each other [3].
They find extensive use in selective syntheses [4,5],
oxidation [6,7], hydrolysis [8–10], and photochemical
[11–16] reactions, etc.
Microemulsion is an optically isotropic, transparent,
and thermodynamically stable medium formed from
water, oil, surfactants, and cosurfactants [1]. Owing to
the varied constituents and compositions, it can solu-
bilize a wide variety of compounds simultaneously
and offer the possibility of reagent compartmentali-
zation and of providing an interface for chemical re-
actions similar to the lipid–water interface found in
Schiff bases act as intermediates in many biological
processes. Some examples of such processes are re-
actions involving pyridoxal enzyme [17], aldol con-
Correspondence to: B. K. Mishra (bijaym@hotmail.com)
2001 John Wiley & Sons, Inc.
᭧

REACTIONS IN MICROEMULSION MEDIA
459
densations [18], decarboxylation [19], transamination
20], and visual processes [21]. In an earlier work [22],
ionic surfactant (sodium lauryl sulphate: NaLS)—iso-
butanol–hexane–water (ABH) were used.
[
we have studied the solubilization of the reported
Schiff bases to map the polarity pockets of a micro-
emulsion droplet.
EXPERIMENTAL
Materials
The Schiff bases (1) reported in this article have a
hydrophobic alkoxy group attached to the aromatic
ring and also have a spacer with varied hydrophobicity
in between the aldimine groups. The former has the
capability of targeting the molecule to anchor at the
interface, and the spacer group may tend to pull the
anchoring polar site from the interface to a different
extent into the microemulsion droplet. For the pur-
pose, microemulsion droplets formulated from cat-
ionic surfactant (cetyltrimethyl ammoniumbromide:
CTAB)—iso-butanol–hexane–water (CBH) and an-
The synthesis of Schiff bases (1, m-p/o-n: m is the
number of carbon atom in the alkyloxy chain attached
to the ortho (o) or para (p) position in the benzene
nucleus with respect to aldimine group, and n is the
number of methylene groups in the spacer) and puri-
fication of surfactants and solvents were reported ear-
lier [22]. The 16-p/o-n Schiff bases (1, m ϭ 16, n ϭ
0, 2, 4, 6 and cetyloxy group in para or ortho position)
have been mostly used for various studies.
(
p/o)CH (CH )
O9
(p/o)CH (CH )
O9
3
2 m ϭ 1
3
2 m ϭ 1
CH"N9(CH ) 9N"CH
2
n
1
Phase Diagram
Phase diagrams of the pseudoternary system [water–
surfactant–cosurfactant)–hexane] were constructed
by the method reported earlier (Fig. 1) [10]. The dif-
ferent microenvironments in the phase diagrams were
identified by the conductivity method [10].
Vis.160A spectrophotometer equipped with a water-
jacketed cell compartment at 27ЊC. For the kinetic
measurements, the microemulsions were prepared by
(
3
mixing NaLS/CTAB (0.6 g), isobutanol (1.8 cm ), and
3
different amount of hexane and water in a 25-cm stop-
pered conical flask and were kept for 30 min to attain
equilibrium. Sulfuric acid of varying strength was in-
troduced into the microemulsion to maintain the re-
quired acid concentration in the reaction medium.
Kinetic Measurement
Ϫ
4
Then Schiff base (SB) in hexane solution ([SB] Ϸ 10
The kinetic measurements were carried out spectro-
photometrically using a Shimadzu model UV-
M) was added to the microemulsion, and the whole
solution was transferred to a quartz cell. The change
in optical density (D ) with time (t) at the respective
t
analytical wavelength (for ortho series Schiff bases,
2
77 nm, and for para series, 320 nm) was noted. The
optical density at infinite time (D ) was measured after
ϱ
keeping the reaction mixture for 48 h. The first-order
rate constants were calculated from the plot of log(D –
t
D ) against time. The kinetic plots yielded a good
ϱ
straight line up to three half-lives (r ϭ 0.999), and
duplicate runs reproduced the pseudo-first-order rate
constant within an error of Ϯ5%. The values of the
first-order rate constants under various conditions are
given in Tables I–IV.
RESULTS AND DISCUSSION
Schiff bases undergo acid-catalyzed hydrolysis to
form the corresponding aldehyde and amine (Scheme
I). The reported Schiff bases are insoluble in water;
Figure 1 Phase diagram of NaLS–iso-butanol–hexane–
water system (A) o/w region, (B) bicontinuous region, (C)
w/o region [22].

4
60
MISHRA ET AL.
Table I First-Order Rate Constants of the Hydrolysis of 16-p/o-2 in ABH w/o Microemulsion at 27ЊC with Variation
of Substrate Concentration; [Acid] ϭ 0.412 N
4
3
Ϫ
1
4
3
Ϫ1
Sl. No.
[16-p-2] ϫ 10 M
k ϫ 10 s (R)
[16-o-2] ϫ 10 M
k ϫ 10 s (R)
1
2
3
4
5
6
7
0.142
0.212
0.354
0.425
0.495
0.566
0.637
4.76 (0.9996)
4.61 (0.999)
4.48 (0.9992)
4.69 (0.999)
4.61 (0.999)
4.61 (0.9991)
4.45 (0.9996)
0.235
0.705
1.176
1.411
1.646
1.881
—
8.44 (0.999)
8.39 (0.9997)
8.23 (0.9997)
8.39 (0.9990)
8.44 (0.9999)
8.23 (0.9993)
—
therefore, the acid hydrolysis has been carried out in
microemulsion media so as to provide the appropriate
environment to the Schiff bases and the acid.
croemulsions is noticed at 30 s of the reaction. There-
fore, these compounds are fully protonated and the
hydrolysis of the protonated form is slow. With de-
creasing chain length of the spacer, the ␭max for the
Schiff base appears in both the microemulsions even
at the beginning of the reaction. In acid, the Schiff
bases (SB) remain in equilibrium with their protonated
1
(p/o)CH (CH )
O9
3
2 m ϭ 1
CHO
ϩ
ϩ
forms (SB ϩ H L SBH ).
ϩ NH (CH )nNH
2
2
2
The appearance of the inflection point in all these
curves indicates a smooth changeover of the reactants
to the products. This is achieved by a one-step mech-
anism or through the formation of an intermediate get-
ting converted to the products in a fast process.
Because the Schiff bases and products absorb
approximately at the same wavelength, the decrease in
Scheme I
Spectral Characteristics
The protonated Schiff bases are found to absorb at a
higher wavelength than their corresponding Schiff ba-
ses. It is noticed that the optical density value at the
ϩ
[SBH ] and not the increase in [product] has been used
ϩ
␭max for protonated Schiff bases (SBH ) decreases and
for evaluation of rate constants.
that at the ␭max value in the region of absorption of
Schiff base increases with time (Figs. 2 and 3). The
wavelengths of maximum absorption for the Schiff ba-
ses and the products are almost in the same region and
are not easily distinguishable. The peak heights at the
absorption maxima of the Schiff bases go on increas-
ing with time, indicating the formation of the prod-
uct(s) due to hydrolysis of the protonated Schiff base.
No absorption for the Schiff bases (16-p-6 and 16-o-
Kinetic Results
The acid concentration is always much higher than the
concentration of Schiff bases and, therefore, first-order
plots have been made with success (r ϭ 0.999). The
first-order rate constants are found to remain constant
with change in concentration of the Schiff bases. Table
I gives the values of the rate constants with the vari-
ation in the concentration of 16-p/o-2. To see if the
6
) or their product(s) in both the CBH and ABH mi-
Table II First-Order Rate Constants of the Hydrolysis of m-p-2 in w/o and o/w ABH Microemulsions at 27ЊC with
Variation of Alkyl Chain
w/o ME [Acid] ϭ 0.326 N
o/w ME [Acid] ϭ 0.088 N
ϩ
4
3
Ϫ
1
4
3
Ϫ1
Sl. No.
Alkyl Group
␭max of (SBH )
[m-p-2] ϫ 10 M
k ϫ 10 s (R)
[m-p-2] ϫ 10 M
k ϫ 10 s (R)
1
2
3
4
5
CH3
321
323
323
325
327
0.507
0.542
0.497
0.568
0.334
4.18 (0.999)
2.78 (0.998)
2.84 (0.997)
2.61 (0.999)
2.79 (0.998)
0.608
0.488
0.396
0.682
0.425
7.48 (0.999)
4.91 (0.999)
4.60 (0.999)
4.91 (0.999)
4.72 (0.999)
CH H
8
17
C H
25
12
C H
16
33
C H
18
37

REACTIONS IN MICROEMULSION MEDIA
461
Table III Rate Constant of the Hydrolysis of 16-p/o-2 in o/w ME at 27ЊC with Variation of Surfactant (NaLS)
Concentration; [Acid] ϭ 0.089 N
16-p-2
16-o-2
4
3
Ϫ
1
4
3
Ϫ1
Point in Phase Diagram
[NaLS] M
[16-p-2] ϫ 10 M
k ϫ 10 s (R)
[16-o-2] ϫ 10 M
k ϫ 10 s (R)
A (50% E)
0.531
0.417
0.313
0.71
0.71
0.71
8.06 (1.000)
9.02 (1.000)
8.06 (1.000)
1.14
1.14
1.14
17.12 (0.997)
15.98 (0.999)
16.21 (0.998)
1
A (40% E)
2
A (30% E)
3
hydrophobicity of the alkyl group in the substrate
changes the site of the location of the Schiff bases and
thereby affects the rate constants, the first-order rate
constants are determined with various alkoxy groups
in the Schiff bases (m-p-2). The values are found to
remain unaltered (Table II) except when m ϭ 1 (me-
thoxy-substituted Schiff base, 1-p/o-n). The rate con-
stant for 1-p-2 is slightly higher than all other Schiff
bases. It is therefore clear that 1-p-2 is well exposed
to the water surface, whereas the increasing hydro-
phobicity of the alkoxy group assists in anchoring the
aldimine group in the decreasing polar medium in the
micellar droplet. Because the boundary is not sharp
but blurred, there is water penetration in the micellar
medium. The rate constants remain the same for the
Schiff bases with m ϭ 8–18, which indicates a similar
environment for the chromophoric group of the sub-
strate. The similarity in behavior of these Schiff bases
with varying alkoxy chain (octyloxy to octadecyloxy)
has been noticed by Behera et al. [14] while carrying
out the quenching behavior of the fluorescence of 1-
alkoxy naphthalene with varying R in w/o and o/w
ABH microemulsions. Therefore, these alkoxy groups
act merely as targeting groups. However, the rate-con-
stant values are dependent on the spacer chain. These
values change in the order 2 ϾϾ 4 Ͼ 6. The ortho series
compounds are also found to react much faster than
the corresponding para series compounds. The 16-p/
o-2 compounds are able to trap a proton due to the
topology of the protophilic groups.
In order to see the effect of surfactant concentra-
tion, the first-order rate constants have been evaluated
at various [NaLS]. The rate constants are found to re-
main unaltered with changing [NaLS] (Table III).
It is observed that at any specific concentration of
ϩ
acid, the optical density of SBH is higher in ABH
microemulsion than in CBH microemulsion, clearly
indicating the greater extent of protonation of Schiff
bases in the former. The ABH microemulsions have a
negative charge at the interface and, therefore, a
greater proton concentration, resulting in decrease in
the pH is expected at the interface. Bissell et al. [2]
have observed that the effective proton density near
the surface of several anionic micelles changes over
2–3 orders of magnitude compared to the bulk. In
CBH microemulsions, proton depletion is natural.
Effect of Variation of Water in ME with
؉
[
H ] Constant
The first-order rate constants have been evaluated with
ϩ
varying [H O] and constant [H ] and are presented in
2
ϩ
Table IV Rate Constants with Variation of Water in ABH Microemulsion for the Hydrolysis of 16-p/o-n Keeping [H ]
Constant at 27ЊC
4
Ϫ1
k ϫ 10 s at Various Mole Fraction of Water in ABH Microemulsion
Schiff Base
0.12
0.16
0.20
0.26
0.34
0.36
0.40
0.44
0.50
0.56
0.60
16-p-2
16-p-4
16-p-6
16-o-0
16-o-2
16-o-4
16-o-6
26.14
0.72
0.65
25.2
61.32
1.92
1.25
29.91
0.99
0.74
23.3
66.50
2.32
1.54
31.62
1.25
0.62
20.21
71.25
2.72
41.31
1.48
0.98
15.25
78.05
3.25
42.43
1.71
1.05
14.26
80.65
3.43
42.77
1.66
1.17
14.07
87.50
3.46
42.91
1.74
1.25
14.41
90.32
3.42
43.12
1.95
1.30
15.99
90.50
3.56
45.53
1.85
1.39
19.72
93.40
3.95
51.50
3.00
1.47
23.98
115.2
5.92
59.62
3.75
1.63
26.83
120.6
6.75
1.76
2.02
2.64
2.73
3.10
3.33
3.43
3.51
3.62

4
62
MISHRA ET AL.
Figure 3 Successive scans during the hydrolysis of 16-o-
n (n ϭ 0, 2, 4, and 6) in the presence of sulfuric acid in
ABH w/o microemulsion at 27ЊC; direction of arrow is with
time.
Figure 2 Successive scans during the hydrolysis of 16-p-
n (n ϭ 2, 3, 4, and 6) in the presence of sulfuric acid in
ABH w/o microemulsion at 27ЊC; direction of arrow is with
time.
Table IV. The plots of k vs. nH2O consist of three
regions (Fig. 4). The rate constants initially increase
with increasing nH2O for all compounds except for 16-
o-0 (where it decreases), then levels off with a sub-
sequent sharp rise (which is not observed for 16-p-6
and 16-o-6). This behavior reveals the existence of
three zones of microemulsions, that is, w/o ME,
bicontinuous zone, and o/w ME, which are inter-
convertible by addition or removal of water
(Fig. 1).
ϩH
2
O
2
ϩH O
w/o Microemulsion IR Ϫ H JO Bicontinuous IRJO o/w Microemulsion
2
Ϫ H
2

REACTIONS IN MICROEMULSION MEDIA
463
Ͻ 16-p/o-2 at all concentrations of water. This may
be attributed to the localization site of the Schiff bases.
With increasing length of spacer, the hydrophobicity
of Schiff bases increases, and, hence, the exposure of
the substrate to the water core becomes less. The so-
lubilization study of the Schiff bases also substantiates
this proposition [22]. Kevan et al. [23] have carried
out photoinduced electron transfer (PET) from (alkox-
yphenyl) biphenyl porphyrins (CnOPTPP) to the in-
terface water of anionic aerosol dioctyl AOT and
CTAB/alcohol-reversed micelle at 77 K. The photol-
ysis is found to depend on three factors, namely, (1)
Cn alkyl chain length, (2) the interface charge, and (3)
the kind of cosurfactant alcohol. Because of increasing
hydrophobic interaction between the surfactant alkyl
chain length and the alkyl chain of CnOPTPP, the dis-
tance between the porphyrin moiety and interface wa-
ter (D O) is supposed to increase. The decrease in the
2
photo-yield confirms this proposition. In the present
study, the change of alkyl chain length is not found to
influence the rate constant.
CONCLUSION
The model of the microemulsion presented earlier [22]
explains satisfactorily the behavior of Schiff bases to-
wards acid-catalyzed hydrolysis. The Schiff bases be-
have as sensors to study the following:
4
Ϫ1
Figure 4 Plots of k ϫ 10 s vs. mole fraction of water
for 16-o-n and 16-p-n compounds (a: 16-o-0; b: 16-p-n; and
c: 16-o-n).
1. The presence of a targeting hydrophobic chain
is responsible for anchoring the molecule at the
interface.
2
. A hydrophobic spacer pulls the reaction site
away from the interface.
3
. The extent of the hydrophilic unit protruding
into the water is determined by the hydropho-
bicity of the spacer and not of the targeting
chain.
The polarity and the nucleophilicity of water present
at the interface changes with the nature of the zone.
The sharp increase in the rate constants in the wa-
ter-rich region is attributed to a greater exposure of the
reaction site to the nucleophilic water. The 16-o-0
Schiff base does not show a bathochromic shift in acid
medium. The compound also does not undergo hy-
drolysis in the absence of acid. The nature of the plot
of k vs. nH2O for the compound indicates that the hy-
drophilic moiety is in the bulk water pool, in which it
undergoes protonation slowly followed by fast hy-
drolysis of the protonated form. The initial decrease
in the rate constant with increasing nH2O in the w/o ME
may be due to dilution of acid, whereas the steep in-
crease in the rate constant in o/w ME may be ascribed
to increasing availability of nucleophilic water for at-
tack at the protonated aldimine linkage.
One of the authors (B.K.M.) thanks the Science and Tech-
nology Department, Government of Orissa, Bhubaneswar
for financial assistance.
BIBLIOGRAPHY
1
2
3
. Menger, F. M. Angew Chem, Int Ed Engl 1991, 30,
086.
1
. Bissel, R. A.; Bryan, A. J.; de Silva, P.; McCoy, C. P.
J Chem Soc, Chem Commun 1994, 405.
. Panda, M.; Behera, P. K.; Mishra, B. K.; Behera, G. B.
Adv Colloid Interface Sci 1999, 1.
It is observed from Table IV that the reactivity of
the Schiff bases is of the order 16-p/o-6 Ͻ 16-p/o-4
4. Luthi, P.; Luisi, P. L. J Am Chem Soc 1984, 106, 7285.

4
64
MISHRA ET AL.
5
6
7
8
9
. Ranganathan, D.; Singh, G. P.; Ranganathan, S. J Am
Chem Soc 1989, 111, 1144.
. Miyake, Y.; Owari, T.; Matsuura, K.; Teramoto, M. J
Chem Soc, Faraday Trans 1993, 89, 1993.
. Chauhan, S. M. S.; Gulati, A.; Sahay, A.; Nizar, P. N.
H. J Mol Catal, Chem 1996, 105, 159.
. Lavallee, D. K.; Huggins, E.; Lee, S. In ACS Sympo-
sium Series, Part I, 1982; p 196.
. Mackay, R. A.; Longo, F. R.; Kneir, R. L.; Durst, H. D.
J Phys Chem 1987, 91, 861.
15. Panda, M.; Behera, P. K.; Mishra, B. K.; Behera, G. B.
J Photochem Photobiol 1998, 113, 73.
16. Panda, M.; Behera, P. K.; Mishra B. K.; Behera, G. B.
Indian J Chem 1998, 37A, 201–205.
17. Benecky, M. J.; Copeland, R. A.; Rava, R. P.; Feldhaus,
R.; Scott, R. D.; Metzler, C. M.; Metzler, D. E.; Spiro,
G. T. J Biol Chem 1985, 260, 11671.
18. Speck, J. C., Jr.; Rowley, P. T.; Horecker, B. L. J Am
Chem Soc 1963, 85, 1012.
19. Chaturvedi, R. K.; Cordes, E. H. J Am Chem Soc 1967,
89, 4631.
20. Abbott, E. H.; Martell, A. E. J Am Chem Soc 1971, 93,
5852.
21. Dupius, P.; Corcoran , T. C.; El Sayed, M. A. R. Proc
Natl Acad Sci, USA 1985, 82, 3662.
1
0. Senapati, S.; Dash, P. K.; Mishra, B. K.; Behera, G. B.
995, 34A, 278.
1. Mackay, R. A. Adv Colloid Interface Sci 1981, 15, 149.
2. Panda, M.; Behera, P. K.; Mishra, B. K.; Behera, G. B.
Indian J Chem 1995, 34A, 11.
1
1
1
1
1
3. Panda, M.; Behera, P. K.; Mishra, B. K.; Behera, G. B.
J Photochem Photobiol, Chem 1995, 90, 69.
4. Panda, M.; Mishra, A.; Behera, P. K.; Mishra, B. K.;
Behera, G. B. J Lumin 1996, 69(2), 510.
22. Dash, P. K.; Mishra, B. K.; Behera, G. B. Spectrochim-
ica Acta 1996, 52A, 349.
23. Kevan, L.; Liang, K.; McManus, H. J. D.; Kang, S. Y.
J Phys Chem 1994, 98, 1044.