Acceleration of hydrolysis of acetylsalicylic acid in AOT/supercritical ethane reverse micelles

Article abstract of DOI:10.1246/cl.2002.178

In situ UV-vis spectroscopy has been applied to study the hydrolysis of acetylsalicylic acid, as a model reaction, in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/supercritical ethane reverse micelles in the presence of imidazole as a catalyst. A 55-fold increase in the rate constant was observed in AOT/supercritical ethane micelles compared to the reaction carried out in aqueous buffer medium. The promotion of hydrolysis in the micelles as compared to aqueous solution has originated from compartmentalization of water-soluble reactant and the catalyst in the aqueous core of micelles.

Full text of DOI:10.1246/cl.2002.178

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78
Chemistry Letters 2002
Acceleration of Hydrolysis of Acetylsalicylic Acid in AOT/Supercritical Ethane Reverse Micelles
ꢀ
Zameer Shervani and Yutaka Ikushima
Supercritical Fluids Research Center, National Institute of Advanced Industrial Science and Technology,
4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551
(Received May 10, 2001; CL-010979)
In situ UV-vis spectroscopy has been applied to study the
hydrolysis of acetylsalicylic acid, as a model reaction, in sodium
bis(2-ethylhexyl)sulfosuccinate (AOT)/supercritical ethane re-
verse micelles in the presence of imidazole as a catalyst. A 55-fold
increase in the rate constant was observed in AOT/supercritical
ethane micelles compared to the reaction carried out in aqueous
buffer medium. The promotion of hydrolysis in the micelles as
compared to aqueous solution has originated from compartmen-
talization of water-soluble reactant and the catalyst in the aqueous
core of micelles.
The concentration of AOT, acetyl salicylic acid, and imidazole
ꢄ4
ꢄ2
was taken as 0.1, 2:5 ꢃ 10 and 2:5 ꢃ 10 M, respectively. For
rate constant determination, the time-course of the reaction was
monitored at 298.6 nm, the band of the product salicylic acid, with
a Jasco V-570 spectrometer. The conversion rate (%) was
determined by recording the UV absorption spectrum of the
product at various reaction time. 100 mM phosphate buffer was
used to adjust the water content (W0) of micelles and also as an
aqueous reaction medium. W0 is the molar ratio of water-to-AOT.
In all the measurements temperature was kept constant at 310 K.
The pressure of AOT/SC ethane micelles was varied in the range
of 27to 44 MPa, while the reaction in aqueous buffer were carried
out at ambient pressure.
The pseudo first order rate constants for the hydrolysis of
acetylsalicylic acid under various conditions are shown in Figure
1. The introduction of AOT/SC ethane micelles in the presence of
imidazole catalyst has enhanced greatly the reaction rate as
compared to the reaction carried out in aqueous buffer solution.
The addition of imidazole in the aqueous solution has no influence
on the reaction rate whereas the incorporation of imidazole in
micelles has increased the reaction rate remarkably. Figure 1 also
shows that increasing water content (W0) of micelles containing
imidazole decelerates the reaction. In a separate experiment of
conversion rate (%) of acetylsalicylic acid to salicylic acid vs the
reaction time in presence of imidazole, it was found that in the
initial 33 min of reaction time 100% conversion was obtained in
micelles of W0 ¼ 1 at 40.2 MPa pressure of ethane, while in
aqueous solution it took 61 longer hrs to complete the reaction.
The above findings could be explained on the basis of
compartmentalization of water-soluble imidazole catalyst and
acetylsalicylic acid within the aqueous core of micelles rather
existence of a diluted solution of these solutes in aqueous buffer
medium. Because of the increase in concentration of the water-
soluble catalyst and substrate in the compartmentalized aqueous
Reverse micelles are formed by adding a small quantity of
water to a surfactant solution usually in hydrocarbon (oil)
1
solvents. Thermodynamically stable and optically transparent
reverse micelles solution is composed of water droplets
surrounded by a layer of surfactant dispersed in continuous oil
2
phase. Because of the presence of different polarity domains,
reverse micelles are known as universal solvents which can
3
solubilize hydrophilic, hydrophobic and amphiphilic solutes.
Therefore, they have potential as reaction media for both organic
4
5
6
and inorganic synthesis, and are also used in extraction and
7
separation processes. Depending upon the nature of reactant, the
reaction can occur inside the aqueous core of micelles, at the
interface or in the continuous oil phase. For water soluble
reactants the core of micelle can act as a nanoreactor and the size
of core can be adjusted by changing the water content of micelles
which can have profound effect on the kinetics of reactions. In
1
987, the existence of reverse micelles in supercritical (SC)
8
ethane and propane was observed since then much attention has
been focussed on these high pressure micelles as supercritical
continuous phase that has liquid-like density and gas-like
diffusivity and viscosity.9 In this article, we find out if
supercritical fluid micelles have the potential as the medium for
chemical reactions. To this end, we have selected a model
reaction, the hydrolysis of acetylsalicylic acid carried out in AOT/
SC ethane micelles in presence of catalyst imidazole and
compared with the reaction carried out in aqueous buffer solution.
The contribution of water content of micelles and pressure effect
of ethane have also been reported.
The reaction was conducted in a high pressure UV cell
consisted of a cylindrical stainless steel block fitted with a pair of
sapphire windows and had an internal volume of 2.2 ml. The
ꢂ
temperature of the reaction was controlled within ꢁ0:2 C with a
temperature controller attached to the cell. Sample preparation
was done by loading AOT, acetylsalicylic acid, imidazole, and
water into the high pressure cell and after closing the cell micelles
solution was obtained by introducing pre-cooled ethane to a
desired pressure inside the cell. The reaction mixture was stirred
continuously during the measurement by a Teflon-coated bar
driven by an outside magnet attached to the high pressure UV cell.
Figure 1. Hydrolysis rate constant of acetylsa-
licylic acid in AOT/SC ethane micelles (P ¼
40:2 MPa) and in aqueous buffer at 310 K.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
179
domain of micelles, the enhancement of the reaction rate and
shorter reaction time were noticed. Increasing amount of water in
micelles decreases the concentration of reactants in the core of
micelles, which results in the lowering of the values of rate
constant. The effect of concentration on the reaction kinetics was
also studied in micelles and aqueous buffer solution; in both the
systems the rate constant increased with increasing the concen-
tration of catalyst and substrate proportionately (Figure 2) which
further proves that the compartmentalization of the water-soluble
reactants in the aqueous domain of micelles is indeed the reason
for acceleration of the reaction in micelles. A comparison of the
rate constants was made in AOT/SC ethane (40.2 MPa) and AOT/
isooctane micelles at 310 K for W0 ¼ 1. The rate constant was
found 1.3 times higher in AOT/SC ethane micelles attributed to
the higher diffusivities of the reactants in SC ethane phase. The
hydrolysis of acetylsalicylic acid using the imidazole catalysis is a
generally acid and base catalyzed reaction with the generation of
hydroxide ions followed by the transfer of proton from protonated
imidazole in the transition state.10 The effect of pressure on the
hydrolysis of acetylsalicylic acid in AOT/SC ethane micelles has
also been investigated (Figure 3). Increasing the pressure of
ethane has no influence on the kinetics of the reaction, which is
contrary to the pressure dependence observed in normal AOT/
octane micelles for enzyme catalysed hydrolysis reactions
1
1
attributed to the stability of the enzyme in structurally ordered
micelles at higher pressures of octane. In contrast to the large
sized enzyme molecules which are in contact to the micellar
interface most efficiently, the samller sized imidazole molecule
located inside the core of micelles, where it does not experience
any change thus rendering nopressure dependence on the reaction
rate at high pressures of the system. Pressure dependence has also
1
2;13
been observed for the reactions carried out in carbon dioxide
in the near-critical region. The maximum reaction yield obtained
in the near-critical region has been attributed to the increased
solubility and diffusivity of reactants in this region. In the present
reaction, the supercritical state of ethane continuous phase of
micelles has been maintained throughout the reaction. Also, the
system containing catalyst imidazole and acetylsalicylic acid
existed in a single homogeneous solution, as confirmed by
looking across the windows of high pressure UV cell, at all the
pressures therefore, the solubility and diffusivity of the solutes
have not contributed to the kinetics of the reaction and thus no
pressure dependence of the reaction was noticed.
In this work, we demonstrated that in supercritical fluid
micelles a greater enhancement of catalytic activity was noticed
as compared to aqueous buffer medium. The enhancement of the
reaction rate in micelles and the influence of water content have
been confirmed to be attributed to compartmentalization of the
water-soluble imidazole and acetylsalicylic acid in aqueous core
of micelles. No influence of increasing pressure of supercritical
ethane was observed on the kinetics of the reaction, and reaction
can be carried out efficiently in lower pressure region of ethane.
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