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Microemulsions as reaction media for the synthesis of sodium decyl sulfonate 1: Role of microemulsion composition
COLLOIDS AND ELSEVIER
Colloids and Surfaces A: Physicochemicaland Engineering Aspects97 (1995), 169-179
SURFACES
A
Microemulsions as reaction media for the synthesis of sodium decyl sulfonate 1: Role of microemulsion composition Seong-Geun Oh *, Jerzy Kizling, Krister Holmberg Institute for Surface Chemistry, P.O. Box 5607, Stockholm S-114 86, Sweden Received 7 November 1994; accepted 11 December 1994
Abstract
Reaction kinetics for the formation of sodium decyl sulfonate from alkyl halide and sodium sulfite in microemulsions based on nonionic surfactant were investigated and compared with those in oil-water two-phase systems. Reactions were run at room temperature at various oil-water ratios. Whereas at room temperature almost no reaction occurred in the two-phase systems, all microemulsion-based reactions proceeded at a fair rate. No clear relationship between microemulsion structure and reaction rate could be seen. An equation describing reaction kinetics in the microemulsion system has been derived based on the pseudophase model. Keywords: Microemulsion; Nucleophilic substitution; Phase diagram; Sodium decyl sulfonate; Solubilization
1. Introduction
One c o m m o n problem in preparative organic chemistry is the attainment of proper phase contact between nonpolar organic compounds (oil soluble) and inorganic salts (water soluble). Hydrolysis of esters with caustic soda, oxidative cleavage of olefins with permanganate periodate, addition of hydrogen sulfite to aldehydes and to terminal olefins, and preparation of alkyl sulfonates by treatment of alkyl chloride by sulfite or by addition of hydrogen sulfite to ~-olefin oxides are examples of organic reactions having compatibility problems between reactants [ 1 ]. Several methods exist to overcome poor phase contact in organic synthesis. Use of a solvent or a solvent combination capable of dissolving both the
* Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(94)03064-2
organic compound and the inorganic salt is one way. Polar, aprotic solvents such as D M F , D M S O or acetonitrile can be used for this purpose, but m a n y of these are unsuitable for large scale work owing to toxicity and/or difficulties in their removal by low vacuum evaporation. Alternatively, the reaction may be carried out in a mixture of two immiscible solvents. The contact area between the phases can be increased by agitation, and the reaction may be facilitated by use of quaternary a m m o n i u m compounds or crown ethers to improve the transfer of reagents across interfaces. Recently, microemulsions have been introduced as reaction media to overcome reactant solubility problems [2,3]. Being microheterogeneous mixtures of oil, water and surfactant, they are excellent solvents for nonpolar organic compounds as well as inorganic salts. Representative examples of reactions performed in microemulsions include alkylation of 2-alkylindan-l,3-diones with
170
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. A,spects 97 (1995) 169-179
benzyl bromide [2], oxidation of Fe(CN)64- by 8 2 O8z [4], detoxification of mustard (C1CH/CH2SCH2CHzC1) [5], and synthesis of metalloporphyrin from porphyrin and metal salts [6]. Microemulsions have also been employed as solvents for enzymic reactions to enhance the stability of biocatalysts and increase reaction rate [7,8]. Besides the very large interfacial area between the phases in a microemulsion, the droplet structure may catalyze or inhibit a certain chemical reaction by compartmentalization and by concentration of reactants and products [9,10]. Also, the presence of an oil-water interface may induce orientation of reactants at the interface, which in turn may affect the regioselectivity of the reactions. For example, the conventional nitration of phenol in aqueous media results in an approximate 2:1 ratio of para to ortho isomer. When this reaction is performed in an AOT-based microemulsion, a selectivity for ortho-nitration in the order of 80% is obtained. This preference for nitration in the ortho position is believed to be due to the accumulation of phenol at the oil-water interface, with the phenolic hydroxyl group oriented towards the water phase [ 11 ]. With a few exceptions [ 12,13], in all prior works related to organic reactions in microemulsions, ionic surfactants such as AOT (sodium bis(2-ethylhexyl) sulfosuccinate) have been used. From a practical point of view, nonionic surfactants are of more interest since after the completion of reaction they can be relatively easily removed from the product by either heating or cooling [ 14]. Easy removal and re-use of the surfactant are prerequisites for the large scale use of microemulsions as vehicles for chemical reactions. In this paper, reaction kinetics for the synthesis of sodium decyl sulfonate from 1-bromodecane (oil-soluble) and sodium sulfite (water-soluble) in nonionic microemulsions have been investigated. CaoHzlBr + NazSO 3 --*CloHzlSO3Na + NaBr
This reaction is a typical second-order nucleophilic substitution (SN2) reaction, with the reaction rate in a homogeneous system being proportional to the concentration of both components. In a microheterogeneous system, the reaction rate will also
depend on the transfer of reactants across the interface. Reaction rates were measured in microemulsions and liquid crystalline phases of various compositions, and compared with those obtained in oil-water two-phase systems.
2. Experimental 2.1. Materials Experiments were carried out using the following materials. Dodecane (> 99%, Sigma), sodium sulfite (Na2SO3, 98%, Merck), nonionic surfactant C12E 5 (pentaethylene glycol monododecyl ether) from Nikko Chemicals, Tokyo, Japan, 1-bromodecane (>97%, Fluka) and sodium carbonate (>98%, Kebo, Stockholm). All reagents were used without further purification. Water was purified by decalcination, prefiltration and reverse osmosis followed by passing through a modified Milli Q unit. 2.2. Phase diagram A ternary phase diagram was constructed using dodecane, aqueous 1.0 wt.% sodium sulfite solution and C12E5 as components. The lipophilic reagent, 1-bromodecane, was not added to the oil phase, since it would initiate a SN2 reaction leading to the formation of a surface-active alkyl sulfonate product. Phases obtained by titration of various oil-surfactant combinations with the aqueous solution were identified using a cross-polarizer. The influence of temperature on phase behavior was determined by slow (1 °C min -1) heating or cooling in a water bath. 2.3. Solubilization capacity of water into microemulsions The capacity of a surfactant-dodecane solution (weight ratio C~2E5 to dodecane 15:85) to solubilize water was determined as a function of temperature. Heating was performed in a water bath at a rate of 1°C fnin -~. The solubilization limit was measured by visual observation of the onset of opacity of the solutions. From several runs it was
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
171
found that the temperature limits were reproducible within I°C.
the inorganic salts, was removed. The oil phase was analyzed for 1-bromodecane.
2.4. NMR measurements
2.7. Analysis of oil phase by UVspectroscopy
Self-diffusion coefficients of the individual components in one microemulsion were measured by the pulsed field gradient spin echo method [15]. The experiments were performed on a Jeol FX-100 spectrometer operating at v0 = 100 MHz for protons. The length of the gradient pulse was 1-18 ms and the time separation of the gradient pulse was 140 ms. The structure of a liquid crystalline phase was analyzed by determination of quadruple splitting using 2H NMR. The spectrum was recorded at 25°C on a Bruker MSL-200 spectrometer, operating at Vo= 30.72 MHz. A quadruple echo sequence with a ~/2 pulse length of 10.3 las was used.
The total amount of unreacted 1-bromodecane in the oil phase was determined by measuring the UV absorbance at 207nm using a Lambda 6/PECSS spectrophotometer (Perkin Elmer). The reaction kinetics were monitored by measuring the rate of depletion of 1-bromodecane in the oil phase during the reaction. (Direct determination of 1-bromodecane concentration in microemulsion was not possible, since both unreacted sulfite and the reaction product, decyl sulfonate, exhibit UV absorbance in the region of interest.)
2.5. Sample preparation for reactions
3.1. Phase behavior
Dodecane containing 1-bromodecane was used as the oil phase. The concentration of 1-bromodecane in dodecane was kept at 1.75 wt.% in order to give the same molar concentration of lipophilic reagent in the oil phase as hydrophilic reagent in the aqueous phase (1.0 wt.% sodium sulfite in water). By this measure, the molar ratio of the two reactants is equivalent to the weight ratio of the initial oil and water phases regardless of the total composition of the system. Reactions were run at various compositions along lines representing molar ratios of 1-bromodecane (in the oil phase) to sodium sulfite (in the aqueous phase) of 9:1, 8:2, 6.5:3.5 and 5"5. The reaction temperature was kept constant at 2 5 + 0 . 5 ° C and all the samples were vigorously agitated by a magnetic stirrer during reaction.
Fig. 1 shows the phase diagram of the system dodecane-C12Es Na2SO3 solution at 25cC. The general shape of the diagram is similar to that of tetradecane-ClzEs-water published by other authors [16]. As can be seen from the phase diagram, the capacity to solubilize water (the extent of the L2 region) is rather limited and relatively insensitive to the surfactant to oil ratio. Fig. 2 shows the phase behavior at different temperatures for the formulation corresponding to point A (25 wt.% C12E5, 58% dodecane and 17% aqueous solution) of Fig. 1. The sample was turbid below 19°C, isotropic up to 26°C, liquid crystal up to 43°C, again isotropic up to 57°C and finally turbid above this temperature. These temperature-induced phase conversions are of interest since they are indicative of the temperature at which the maximum solubilization of water (and, consequently, maximum incorporation of water soluble reagent) can take place. Fig. 3 shows the contour of water solubilization as a function of temperature. For practical reasons, the reaction temperature used in this work is well below the maximal solubilizing temperature of the system, which was found to be 48 ° C. The microstructure of composition A in Fig. 1
2.6. Phase separation of samples after reaction Samples (0.5 g) were taken after reaction and diluted with dodecane (3.0 g). Addition of aqueous Na2COa (4.0 wt.%, 2.0 g) gave a turbid solution. After centrifugation at 3000 r.p.m, for 10 min, the mixture had separated. The water phase which contained the product, decyl sulfonate, as well as
3. Results and discussion
172
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169 179 C12E5
,\ LC gB I
/
I I
\ I
\\ \\ \
5
H20
6.5 : 3.5
8 :2
9:1
C12
Fig. 1. Phase diagram of the system dodecane-ClzEs-l% Na2SO a solution at 25~'C. The dark spots represent the compositions at which the reactions have been carried out, and points A and B are the compositions which have been structure determined by NMR.
100 oC
2 phases
57 °C i sotropic 43 o c liquid crystal 26 °C isotropic 19 °C 2 phases 0 °C
Fig. 2. Temperature-induced phase conversion for the system dodecane-C12Es-l% NazSO 3 solution at the composition represented by point A in Fig. 1.
was determined by self-diffusion NMR. The 1H NMR spectrum consisted of several resonances: 6(HDO)=4.76p.p.m., 6(Es)=3.Sp.p.m., ~(C12, methylene groups)= 1.2 p.p.m., and 6(C12, methyl groups)=0.96 p.p.m. Obviously, the alkyl groups, including the terminal methyls, of the surfactant and the oil overlap and cannot be separated. However, the oxyethylene groups of the surfactant give rise to a separate signal, allowing a simple evaluation of the intensity decay data. The same applies to water. To obtain the self-diffusion coefficient of dodecane, it was necessary to consider the doubly exponential behavior in the evaluation procedure. Table 1 shows the values of self-diffusion coefficients for each component of microemulsion A from Fig. 1. Since the D-value for water represents an average of "free" and surfactant-bound water, and since the self-diffusion of the latter is relatively slow, the method underestimates the D-value of free water. The discrepancy is not large unless very high surfactant concentrations are used, and cot-
S.-G. Oh et al. 'Colloids Surfaees A. Physicochem. Eng. Aspects 97 (1995) 169 179
173
70
6(
\ \ \ \ \ \ N O
N \\
50Isotropic
/
/
/
\
\
"%.
40
30
[ 10
20 % H20
I 3O
Fig. 3. Water solubilization capacity as a function of temperature of a C~2Es-dodecane mixture of weight ratio 15:85. The m a x i m u m solubilization capacity was found to be at 48°C.
Table 1 Self-diffusion coefficients, D, for the individual components of microemulsion A in Fig. 1 at 25~C. Self-diffusion of neat water, Do ( H D O h was 1920 x 10 12 m 2 s-1 and that of neat C12H26 was 596× 10 12m 2 s 1. Component
HDO
C12E5
C12Hz~
D x 1012[m2s 1}
368 0.19
26.6
271 0.45
D/D o
rection is therefore not normally necessary. In this case, with more than 20 wt.% surfactant, deviation from the true value for free water is too large to be neglected. Jonstr6mer et al. [17] have studied the self-diffusion behavior of the micellar phase of C12E 5 and C12E8 solutions over wide concentration ranges. It was found that the D/Do-value of D 2 0 (Do being the value of neat O 2 0 at the same temperature) decreases almost linearly with surfactant concentration. The D/Do-value at 20 wt.%
174
S.-G. Oh et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
surfactant was approximately 2/3 of that obtained at very low surfactant concentration. The selfdiffusion data were analyzed according to a two-site discrete exchange model. The water is described as being in either one of the two sites, bound or free, respectively, where the sites have different diffusivities. The bound water hydrates the surfactant headgroups, while the free water is unpertubed by the surfactant film and possesses bulk water properties. The results obtained may not be directly applicable to this work, since in a microemulsion the surfactant molecules align at the oil-water interface instead of self-assembling as in micelles. However, it is reasonable to consider the experimentally obtained D/Do-value as being too low to represent the state of the free water in the system, Even without proper quantification of the selfdiffusion values, it is obvious from the NMR measurements that the microemulsion of composition A has a typical bicontinuous structure. A long, narrow channel separating a liquid crystalline phase and a two-phase region, and extending from the L1 region almost into the L2 region has been observed previously in a similar system [-16]. The structure of composition B in the liquid crystalline region of the ternary phase diagram (see Fig. 1) was determined by 2H NMR. The spectrum obtained showed a Pake pattern with a typical quadruple splitting (peak-to-peak separation) indicative of a liquid crystalline structure. The recorded splitting was 1600 Hz. Since several other studies have shown the splitting for hexagonal phases to be below 1000 Hz and for lamellar phases to be above 1000 Hz, the NMR data is a clear indication of a lamellar liquid crystalline structure. 3.2. Reaction kinetics
To describe the reaction kinetics in a micellar system it is useful to divide the reaction medium into three pseudophases: (a) the micellar core, (b) the interface consisting of a palisade layer of surfactant molecules and (c) bulk water [18]. Extension of the model to a microemulsion gives three pseudophases: (a) the volume of the oil-rich domain, Vo, (b) the interface, V~, and (c) the waterrich domain, Vw [-2]. The equilibrium distribution
of reactants between the pseudophases depends on their solubilities in each phase. During the course of the reaction, the distribution of reactants in the pseudophases is diffusion-controlled and considered to be fast due to vigorous agitation of the samples. If the reaction A + B ~ P occurs in all three domains, it can be described by the following reaction scheme: k~
Aw + B w ~ k~ T~ A~ + Bs ~ ks k's T~ Ao + Bo, ko ' k;
Pw T,~ Ps T Po
(1)
The indices refer to the domains in which the molecules or ions are dissolved. The overall amount of molecules or ions distributed over three pseudophases is given by (for A) NA=N'~+N°A+N~=C~Vw+C°AVo+C~V~
(2)
where CAi is the concentration of A in domain i. For a reaction occurring simultaneously in all three domains, the rate is given by dNa dt - VwkwC'~C~ + Kk~C~, + VokoC°~C~ t
w
-- V w k w C p -
t
S
VsksC a
--
!
0
VokoCe
(3)
In the following discussion, related to the reaction carried out in this study, A, B and P represent CloHE1Br, SO32- and CloH21Br, SO3, respectively. By neglecting the concentration of A in water and of B in oil, and by regarding the reaction to be irreversible (which is, in practice, the case for an SN2 reaction of this type), Eq. (3) can be simplified to dNA dt
S V~k~C A CBs
= V~kdPAC°A)(PBC~)
(4)
where the partition coefficients for the reactants between interface and bulk are defined as PA=CA/CA,S 0 Pa=CB/CB'S W
S.-G. Oh et al./Colloids Surfaces A." Physicochem. Eng. Aspects 97 (1995) 169-179
Eqs. (4), (7) and (8) give
For an equimolar reaction Vo(C ° ' -
C °) - K ( C ~ - C~,)
= Vw(C~'-
C~) -
V,(C~ - C~,)
dEVo+ VsPA)C~] dt = VsksPAPBC°A
(5)
Vo(C°' - C°) _ V~PA(Co" _ C°)
×[C~'
= Vw{C~'-- C ~ ) - V s P B ( C ~ ' - C~)
(6)
Therefore Cw = C w'
175
V ° - V~PA(C~' - C~) Vw- V~P,
(7)
where C°' and Cw' In the following discussion, related to the reaction carried are the initial concentration of A and B in the oil and water domains, respectively. The total number of moles of A in the oil and interfacial domains (NA) is given by N A = VoC°A A- VsCSA = VoC°A -4- VsPAC°A
= (Vo + V~PA)C°A
(8)
V~_v~(CAV°--V~PA o'
_
C~,)]
Eq. (9) shows that the concentration of 1-bromodecane in the oil phase (C°) decays as a function of time, and that the rate of decay depends on many parameters such as the volume of each domain (Vo, Vw, Vs), the partition coefficient of reaction between bulk and interface (PA, PB), the reaction rate constant at the interface (ks) and the initial concentration of reactants (C°', CW'). Owing to experimental difficulties, no attempt has been made in this work to determine the values of PA and PB. Figs. 4-7 show the decay of 1-bromodecane with time for reaction media of different compositions.
1,8
water-oil mixture v
0 o9 ca. _, m
1,7
' ~
~
1,6
=
-_liquid crystal
_.
L2 microemulsion
0 c "o t-t~ o•
1,5
maximum conversion
isotropic channel
1,4
o
E 0 cn
1,3
e
"~
1,2
tO •"
1,1
,m
E o to
o
1,0
,
0
I
2
,
{9)
l
4
,
I
6
,
8
Reaction T i m e (h)
Fig. 4. D e c a y of 1 - b r o m o d e c a n e with time for r e a c t i o n systems h a v i n g an oil to w a t e r ratio of 9 : 1.
S.-G. Oh et al./Colloids SurJi~ces A: Physicochem. Eng. Aspects 97 (1995) 169 179
176
1,8
v {9 tO-
water-oil mixture
1,7
~
liquid crystal
1,6
0 .C:
1,5
tO (D "0 0
E ~
1,4
1,3
maximum conversion
!
'*"O tO
1,2
~
1,1
E O rO
O
,
1,0 0
I
i
2
I
,
4
I
6
,
8
Reaction Time (h) Fig. 5. Decay of 1-bromodecane with time for reaction systems having an oil to water ratio of 8 : 2.
Reactions were run at four different compositions (indicating four different surfactant concentrations) along lines representing oil to water weight ratios as well as initial molar ratios of 1-bromodecane to sodium sulfite, of 9 : 1, 8 : 2, 6.5 : 3.5 and 5 : 5, as shown in Fig. 1. Regardless of the reactant ratio, the extent of reaction was negligible when no surfactant was present. This is a good illustration of the problem of phase contact in a reaction between a nonpolar organic compound and an inorganic salt. The two microemulsion domains, denoted L2 and isotropic channel, proved to be good reaction media, giving relatively fast initial reaction rates. The formulations having the largest excess of 1-bromodecane to sodium sulfite (9:1 and 8:2) gave the most complete reactions (Figs. 4 and 5). The reactions run at smaller (6.5:3.5) or no (5:5) 1-bromodecane excess resulted in lower reaction yields (Figs. 6 and 7). In these reactions the decay of 1-bromodecane levelled offlong before the maximum conversion, representing total conversion of sulfite into product. The reason for the
cessation of the reaction will be discussed below. This study does not show any definite trend as to which type of microemulsion, bicontinuous (as in the isotropic channel) or w/o (as in the L2 phase), is more suitable as a reaction medium. The liquid crystalline phase, with a composition intermediate between the two microemulsion regions, gave a considerably more sluggish reaction than the microemulsions. This is noteworthy since the liquid crystalline phase, like the microemulsions, contains a very large oil water interfacial area. The low reaction rate may be related to the high rigidity of the lamellar liquid crystalline structure, which may hinder the diffusion of reactants across the interface. Various types of liquid crystals have been investigated as media for organic [-19,20] and bioorganic [21,22] reactions, but no previous comparison between reaction rates in microemulsions and liquid crystals based on the same components appears to have been carried out. From the concentration vs. time curves of Figs. 4 7 it is not possible to determine the reaction
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169 179 ~
1,8
177
water-oil mixture :
1,7
~ liquid crystal
131,6 ¢"--
1,5
(D "0 0
1,4
E ~ m
isotropic channel L2 microemulsion
1,3
!
O eO
1,2
~
1,1
E O eO
O
1,0
, 0
I 2
,
I 4
,
I 6
, 8
Reaction Time (h) Fig. 6. Decay of l-bromodecane with time for reaction systems having an oil to water ratio of 6.5 : 3.5. M a x i m u m conversion of 1-bromodecane corresponds to 0.8 wt.%.
order. The decay curves of 1-bromodecane in the microemulsion-based reactions have the general appearance of second-order reaction kinetics, but the shape of the time dependence of the reactant during a first-order reaction is similar. (With the same initial rate, the change in concentration at the later stage of reaction is more rapid for a firstorder than for a second-order reaction.) In order to verify that the reactions are true second order and not pseudo-first order, one would need to plot the reciprocal of 1-bromodecane concentration against time to see if a straight line, indicative of a second-order reaction, is obtained. To do this with acceptable accuracy many more experimental points are needed. Ideally, one should continuously monitor the reaction. Unfortunately, for this system no simple monitoring process was available. The cessation of reaction before all the 1-bromodecane has been consumed may be seen as an interracial phenomenon. The product formed, decyl sulfonate, is surface active and accumulates
at the interfaces. The nature of the surfactant palisade layer gradually changes from a purely nonionic monolayer to a mixed assembly of nonionic and anionic species. A charged interface is known to repel ions of the same charge from that interface [23]. It is reasonable to believe that in the reaction studied here the gradual increase of negative charge at the oil water interface leads to a reduced ability of sulfite ions to diffuse into the interfacial region where reaction with the lipophilic 1-bromodecane takes place. The influence of surfactant charge on rate and yield of reaction is presently being studied in our laboratory. An attempt was made to enhance the reaction rate by addition of small amounts of potassium iodide to the reaction mixture. Iodide ions, being both strong nucleophiles and good leaving groups, are known to accelerate many SN2 reactions [24]. Unfortunately, in this reaction iodide did not function as a catalyst but rather as a reducing agent. Sulfite ions rapidly oxidized iodide to iodine, as
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
178
o~ 1,8
water-oil mixture
.¢.-,
8
II.
1,7 t./} A: Q"
~
liquid crystal
1,6
.B
0
~_
~
_
L2microemulsion
.I;::: 1,5 E O
1,4
0
isotropic channel
E
£ 1,a m i
0
1,2
E 0
~
1,1
e0
1,0
0
i
0
I
=
I
2
t
4
I
6
u
8
Reaction Time (h) Fig. 7. Decay of 1-bromodecane with time for reaction systems having an oil to water ratio of 5:5. Maximum conversion of l-bromodecane corresponds to 0.0 wt.%.
demonstrated by an immediate discoloration of the solution. In conclusion, the rates of synthesis of sodium decyl sulfonate from 1-bromodecane and sodium sulfite were high in microemulsions based on nonionic surfactant, more sluggish in liquid crystalline systems obtained from the same surfactant, and very low in surfactant-free oil-water mixtures. The reaction went to completion at high oil to water ratio compositions in which there is a large excess of organic reactant. The reaction yield diminished as the oil to water ratio (as well as the 1-bromodecane to sodium sulfite ratio) approached unity. The cessation of the reaction is likely to be due to a build-up of negative charge at the oilwater interface by the surface-active reaction product, decyl sulfonate. This work does not give any decisive information as to what type of microemulsion structure - - bicontinuous or droplet - is optimal as a reaction medium.
Acknowledgment The authors are grateful to Dr. Erik S6derlind from the Royal Institute of Technology for the N M R measurements, to Dr. Bj6rn Lindman and Dr. Ulf Olsson for comments on the interpretation of the self-diffusion data, and to Dr. Mikael Jansson, our previous colleague, for helpful discussions. SGO wishes to express his sincere thanks for financial support from the Wennergren Foundation in Sweden.
References [1] R. Schom~tcker, Nachr. Chem. Tech. Lab., 40 (1992) 1344. [2] R. Schom~.cker, K. Stickdorn and W. Knoche, J. Chem. Soc., Faraday Trans., 87 (1991) 847. 1-3] K. Holmberg, Adv. Colloid Interface Sci., 51 (1994) 137. 1-4] P. Lopez, A. Rodriguez, C. Gomez-Herrera, F. Sanchez
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
[5]
[6] [7] I-8]
1-9] [10] [11] [12] [13] [14]
and M.A. Moya, J. Chem. Soc., Faraday Trans., 88 (1992) 2701. A.R. Elrington, Rapid Deactivation of Mustard in Microemulsion Technology, Ph.D. Thesis, Emory University, Atlanta, GA, 1990. R.A. Mackay, Adv. Colloid Interface Sci., 15 (1981) 131. K. Holmberg and E. Osterberg, Prog. Colloid Polym. Sci., 82 (1990) 181. Y.L. Khmelnitski, A.V. Kabanov, N.L. Klyachko, A.V. Levashov and K. Martinek, in M.P. Pileni (Ed.), Structure and Reactivity in Reversed Micelles, Elsevier, Amsterdam, 1989. M.L. Moya, C. Izquierdo and J. Casado, J. Phys. Chem., 95 ( 1991 ) 6001. B.K. Mishra, B.S. Valandikar, J.J. Knujappu and C. Manohar, J. Colloid Interface Sci., 127 (1989) 373. A.S. Chhatre, R.A. Joshi and B.D. Kulkarni, J. Colloid Interface Sci., 158 (1993) 183. F.M. Menger and A.R. Elrington, J. Am. Chem. Soc., 113 (1991) 9621. F.M. Menger, J.U. Rhee and H.K. Rhee, J. Org. Chem., 40 (1975) 3803. K.M. Larsson, P. Adlercreutz, B. Mattiasson and U. Olsson, Biotechnol. Bioeng., 36 (1990) 135.
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[15] P. Stilbs, Prog. Nucl. Magn. Resonance Spectrosc., 19 (1987) 1. [16] H. Kunieda and K. Shinoda, J. Dispersion Sci. Technol., 3 (1982) 233. [17] M. Jonstr6mer, B. J6nsson and B. Lindman, J. Phys. Chem., 95 (1991) 3293. El8] W. Knoche, V.R. Hanke and E. Dutkiewicz, J. Chem. Soc., Faraday Trans., 83 (1987) 2847. [19] V. Ramesh and M.M. Labes, Mol. Cryst. Liq. Cryst., 144 (1987) 257. [20] V. Ramesh and M.M. Labes, J. Am. Chem. Soc., 110 (1988) 738. [21] N.L. Klyachko, A.V. Levashov, A.V. Pshezhetsky, N.G. Bogdanova, I.V. Berezin and K. Martinek, Eur. J. Biochem., 161 (1986) 149. [221 P. Miethe, R. Gruber and H. Voss, Biotechnol. Lett., 11 (1989) 449. [23] R. Johannsson, M. Almgren and R. Schm~icker, Langmuir, 9 (1993) 1269. [24] K. Rorig, J.D. Johnston, R.W. Hamilton and T.J. Telinski, Org. Synth., Coll. Vol. 4 (1963) 576.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects97 (1995), 169-179
SURFACES
A
Microemulsions as reaction media for the synthesis of sodium decyl sulfonate 1: Role of microemulsion composition Seong-Geun Oh *, Jerzy Kizling, Krister Holmberg Institute for Surface Chemistry, P.O. Box 5607, Stockholm S-114 86, Sweden Received 7 November 1994; accepted 11 December 1994
Abstract
Reaction kinetics for the formation of sodium decyl sulfonate from alkyl halide and sodium sulfite in microemulsions based on nonionic surfactant were investigated and compared with those in oil-water two-phase systems. Reactions were run at room temperature at various oil-water ratios. Whereas at room temperature almost no reaction occurred in the two-phase systems, all microemulsion-based reactions proceeded at a fair rate. No clear relationship between microemulsion structure and reaction rate could be seen. An equation describing reaction kinetics in the microemulsion system has been derived based on the pseudophase model. Keywords: Microemulsion; Nucleophilic substitution; Phase diagram; Sodium decyl sulfonate; Solubilization
1. Introduction
One c o m m o n problem in preparative organic chemistry is the attainment of proper phase contact between nonpolar organic compounds (oil soluble) and inorganic salts (water soluble). Hydrolysis of esters with caustic soda, oxidative cleavage of olefins with permanganate periodate, addition of hydrogen sulfite to aldehydes and to terminal olefins, and preparation of alkyl sulfonates by treatment of alkyl chloride by sulfite or by addition of hydrogen sulfite to ~-olefin oxides are examples of organic reactions having compatibility problems between reactants [ 1 ]. Several methods exist to overcome poor phase contact in organic synthesis. Use of a solvent or a solvent combination capable of dissolving both the
* Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(94)03064-2
organic compound and the inorganic salt is one way. Polar, aprotic solvents such as D M F , D M S O or acetonitrile can be used for this purpose, but m a n y of these are unsuitable for large scale work owing to toxicity and/or difficulties in their removal by low vacuum evaporation. Alternatively, the reaction may be carried out in a mixture of two immiscible solvents. The contact area between the phases can be increased by agitation, and the reaction may be facilitated by use of quaternary a m m o n i u m compounds or crown ethers to improve the transfer of reagents across interfaces. Recently, microemulsions have been introduced as reaction media to overcome reactant solubility problems [2,3]. Being microheterogeneous mixtures of oil, water and surfactant, they are excellent solvents for nonpolar organic compounds as well as inorganic salts. Representative examples of reactions performed in microemulsions include alkylation of 2-alkylindan-l,3-diones with
170
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. A,spects 97 (1995) 169-179
benzyl bromide [2], oxidation of Fe(CN)64- by 8 2 O8z [4], detoxification of mustard (C1CH/CH2SCH2CHzC1) [5], and synthesis of metalloporphyrin from porphyrin and metal salts [6]. Microemulsions have also been employed as solvents for enzymic reactions to enhance the stability of biocatalysts and increase reaction rate [7,8]. Besides the very large interfacial area between the phases in a microemulsion, the droplet structure may catalyze or inhibit a certain chemical reaction by compartmentalization and by concentration of reactants and products [9,10]. Also, the presence of an oil-water interface may induce orientation of reactants at the interface, which in turn may affect the regioselectivity of the reactions. For example, the conventional nitration of phenol in aqueous media results in an approximate 2:1 ratio of para to ortho isomer. When this reaction is performed in an AOT-based microemulsion, a selectivity for ortho-nitration in the order of 80% is obtained. This preference for nitration in the ortho position is believed to be due to the accumulation of phenol at the oil-water interface, with the phenolic hydroxyl group oriented towards the water phase [ 11 ]. With a few exceptions [ 12,13], in all prior works related to organic reactions in microemulsions, ionic surfactants such as AOT (sodium bis(2-ethylhexyl) sulfosuccinate) have been used. From a practical point of view, nonionic surfactants are of more interest since after the completion of reaction they can be relatively easily removed from the product by either heating or cooling [ 14]. Easy removal and re-use of the surfactant are prerequisites for the large scale use of microemulsions as vehicles for chemical reactions. In this paper, reaction kinetics for the synthesis of sodium decyl sulfonate from 1-bromodecane (oil-soluble) and sodium sulfite (water-soluble) in nonionic microemulsions have been investigated. CaoHzlBr + NazSO 3 --*CloHzlSO3Na + NaBr
This reaction is a typical second-order nucleophilic substitution (SN2) reaction, with the reaction rate in a homogeneous system being proportional to the concentration of both components. In a microheterogeneous system, the reaction rate will also
depend on the transfer of reactants across the interface. Reaction rates were measured in microemulsions and liquid crystalline phases of various compositions, and compared with those obtained in oil-water two-phase systems.
2. Experimental 2.1. Materials Experiments were carried out using the following materials. Dodecane (> 99%, Sigma), sodium sulfite (Na2SO3, 98%, Merck), nonionic surfactant C12E 5 (pentaethylene glycol monododecyl ether) from Nikko Chemicals, Tokyo, Japan, 1-bromodecane (>97%, Fluka) and sodium carbonate (>98%, Kebo, Stockholm). All reagents were used without further purification. Water was purified by decalcination, prefiltration and reverse osmosis followed by passing through a modified Milli Q unit. 2.2. Phase diagram A ternary phase diagram was constructed using dodecane, aqueous 1.0 wt.% sodium sulfite solution and C12E5 as components. The lipophilic reagent, 1-bromodecane, was not added to the oil phase, since it would initiate a SN2 reaction leading to the formation of a surface-active alkyl sulfonate product. Phases obtained by titration of various oil-surfactant combinations with the aqueous solution were identified using a cross-polarizer. The influence of temperature on phase behavior was determined by slow (1 °C min -1) heating or cooling in a water bath. 2.3. Solubilization capacity of water into microemulsions The capacity of a surfactant-dodecane solution (weight ratio C~2E5 to dodecane 15:85) to solubilize water was determined as a function of temperature. Heating was performed in a water bath at a rate of 1°C fnin -~. The solubilization limit was measured by visual observation of the onset of opacity of the solutions. From several runs it was
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
171
found that the temperature limits were reproducible within I°C.
the inorganic salts, was removed. The oil phase was analyzed for 1-bromodecane.
2.4. NMR measurements
2.7. Analysis of oil phase by UVspectroscopy
Self-diffusion coefficients of the individual components in one microemulsion were measured by the pulsed field gradient spin echo method [15]. The experiments were performed on a Jeol FX-100 spectrometer operating at v0 = 100 MHz for protons. The length of the gradient pulse was 1-18 ms and the time separation of the gradient pulse was 140 ms. The structure of a liquid crystalline phase was analyzed by determination of quadruple splitting using 2H NMR. The spectrum was recorded at 25°C on a Bruker MSL-200 spectrometer, operating at Vo= 30.72 MHz. A quadruple echo sequence with a ~/2 pulse length of 10.3 las was used.
The total amount of unreacted 1-bromodecane in the oil phase was determined by measuring the UV absorbance at 207nm using a Lambda 6/PECSS spectrophotometer (Perkin Elmer). The reaction kinetics were monitored by measuring the rate of depletion of 1-bromodecane in the oil phase during the reaction. (Direct determination of 1-bromodecane concentration in microemulsion was not possible, since both unreacted sulfite and the reaction product, decyl sulfonate, exhibit UV absorbance in the region of interest.)
2.5. Sample preparation for reactions
3.1. Phase behavior
Dodecane containing 1-bromodecane was used as the oil phase. The concentration of 1-bromodecane in dodecane was kept at 1.75 wt.% in order to give the same molar concentration of lipophilic reagent in the oil phase as hydrophilic reagent in the aqueous phase (1.0 wt.% sodium sulfite in water). By this measure, the molar ratio of the two reactants is equivalent to the weight ratio of the initial oil and water phases regardless of the total composition of the system. Reactions were run at various compositions along lines representing molar ratios of 1-bromodecane (in the oil phase) to sodium sulfite (in the aqueous phase) of 9:1, 8:2, 6.5:3.5 and 5"5. The reaction temperature was kept constant at 2 5 + 0 . 5 ° C and all the samples were vigorously agitated by a magnetic stirrer during reaction.
Fig. 1 shows the phase diagram of the system dodecane-C12Es Na2SO3 solution at 25cC. The general shape of the diagram is similar to that of tetradecane-ClzEs-water published by other authors [16]. As can be seen from the phase diagram, the capacity to solubilize water (the extent of the L2 region) is rather limited and relatively insensitive to the surfactant to oil ratio. Fig. 2 shows the phase behavior at different temperatures for the formulation corresponding to point A (25 wt.% C12E5, 58% dodecane and 17% aqueous solution) of Fig. 1. The sample was turbid below 19°C, isotropic up to 26°C, liquid crystal up to 43°C, again isotropic up to 57°C and finally turbid above this temperature. These temperature-induced phase conversions are of interest since they are indicative of the temperature at which the maximum solubilization of water (and, consequently, maximum incorporation of water soluble reagent) can take place. Fig. 3 shows the contour of water solubilization as a function of temperature. For practical reasons, the reaction temperature used in this work is well below the maximal solubilizing temperature of the system, which was found to be 48 ° C. The microstructure of composition A in Fig. 1
2.6. Phase separation of samples after reaction Samples (0.5 g) were taken after reaction and diluted with dodecane (3.0 g). Addition of aqueous Na2COa (4.0 wt.%, 2.0 g) gave a turbid solution. After centrifugation at 3000 r.p.m, for 10 min, the mixture had separated. The water phase which contained the product, decyl sulfonate, as well as
3. Results and discussion
172
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169 179 C12E5
,\ LC gB I
/
I I
\ I
\\ \\ \
5
H20
6.5 : 3.5
8 :2
9:1
C12
Fig. 1. Phase diagram of the system dodecane-ClzEs-l% Na2SO a solution at 25~'C. The dark spots represent the compositions at which the reactions have been carried out, and points A and B are the compositions which have been structure determined by NMR.
100 oC
2 phases
57 °C i sotropic 43 o c liquid crystal 26 °C isotropic 19 °C 2 phases 0 °C
Fig. 2. Temperature-induced phase conversion for the system dodecane-C12Es-l% NazSO 3 solution at the composition represented by point A in Fig. 1.
was determined by self-diffusion NMR. The 1H NMR spectrum consisted of several resonances: 6(HDO)=4.76p.p.m., 6(Es)=3.Sp.p.m., ~(C12, methylene groups)= 1.2 p.p.m., and 6(C12, methyl groups)=0.96 p.p.m. Obviously, the alkyl groups, including the terminal methyls, of the surfactant and the oil overlap and cannot be separated. However, the oxyethylene groups of the surfactant give rise to a separate signal, allowing a simple evaluation of the intensity decay data. The same applies to water. To obtain the self-diffusion coefficient of dodecane, it was necessary to consider the doubly exponential behavior in the evaluation procedure. Table 1 shows the values of self-diffusion coefficients for each component of microemulsion A from Fig. 1. Since the D-value for water represents an average of "free" and surfactant-bound water, and since the self-diffusion of the latter is relatively slow, the method underestimates the D-value of free water. The discrepancy is not large unless very high surfactant concentrations are used, and cot-
S.-G. Oh et al. 'Colloids Surfaees A. Physicochem. Eng. Aspects 97 (1995) 169 179
173
70
6(
\ \ \ \ \ \ N O
N \\
50Isotropic
/
/
/
\
\
"%.
40
30
[ 10
20 % H20
I 3O
Fig. 3. Water solubilization capacity as a function of temperature of a C~2Es-dodecane mixture of weight ratio 15:85. The m a x i m u m solubilization capacity was found to be at 48°C.
Table 1 Self-diffusion coefficients, D, for the individual components of microemulsion A in Fig. 1 at 25~C. Self-diffusion of neat water, Do ( H D O h was 1920 x 10 12 m 2 s-1 and that of neat C12H26 was 596× 10 12m 2 s 1. Component
HDO
C12E5
C12Hz~
D x 1012[m2s 1}
368 0.19
26.6
271 0.45
D/D o
rection is therefore not normally necessary. In this case, with more than 20 wt.% surfactant, deviation from the true value for free water is too large to be neglected. Jonstr6mer et al. [17] have studied the self-diffusion behavior of the micellar phase of C12E 5 and C12E8 solutions over wide concentration ranges. It was found that the D/Do-value of D 2 0 (Do being the value of neat O 2 0 at the same temperature) decreases almost linearly with surfactant concentration. The D/Do-value at 20 wt.%
174
S.-G. Oh et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
surfactant was approximately 2/3 of that obtained at very low surfactant concentration. The selfdiffusion data were analyzed according to a two-site discrete exchange model. The water is described as being in either one of the two sites, bound or free, respectively, where the sites have different diffusivities. The bound water hydrates the surfactant headgroups, while the free water is unpertubed by the surfactant film and possesses bulk water properties. The results obtained may not be directly applicable to this work, since in a microemulsion the surfactant molecules align at the oil-water interface instead of self-assembling as in micelles. However, it is reasonable to consider the experimentally obtained D/Do-value as being too low to represent the state of the free water in the system, Even without proper quantification of the selfdiffusion values, it is obvious from the NMR measurements that the microemulsion of composition A has a typical bicontinuous structure. A long, narrow channel separating a liquid crystalline phase and a two-phase region, and extending from the L1 region almost into the L2 region has been observed previously in a similar system [-16]. The structure of composition B in the liquid crystalline region of the ternary phase diagram (see Fig. 1) was determined by 2H NMR. The spectrum obtained showed a Pake pattern with a typical quadruple splitting (peak-to-peak separation) indicative of a liquid crystalline structure. The recorded splitting was 1600 Hz. Since several other studies have shown the splitting for hexagonal phases to be below 1000 Hz and for lamellar phases to be above 1000 Hz, the NMR data is a clear indication of a lamellar liquid crystalline structure. 3.2. Reaction kinetics
To describe the reaction kinetics in a micellar system it is useful to divide the reaction medium into three pseudophases: (a) the micellar core, (b) the interface consisting of a palisade layer of surfactant molecules and (c) bulk water [18]. Extension of the model to a microemulsion gives three pseudophases: (a) the volume of the oil-rich domain, Vo, (b) the interface, V~, and (c) the waterrich domain, Vw [-2]. The equilibrium distribution
of reactants between the pseudophases depends on their solubilities in each phase. During the course of the reaction, the distribution of reactants in the pseudophases is diffusion-controlled and considered to be fast due to vigorous agitation of the samples. If the reaction A + B ~ P occurs in all three domains, it can be described by the following reaction scheme: k~
Aw + B w ~ k~ T~ A~ + Bs ~ ks k's T~ Ao + Bo, ko ' k;
Pw T,~ Ps T Po
(1)
The indices refer to the domains in which the molecules or ions are dissolved. The overall amount of molecules or ions distributed over three pseudophases is given by (for A) NA=N'~+N°A+N~=C~Vw+C°AVo+C~V~
(2)
where CAi is the concentration of A in domain i. For a reaction occurring simultaneously in all three domains, the rate is given by dNa dt - VwkwC'~C~ + Kk~C~, + VokoC°~C~ t
w
-- V w k w C p -
t
S
VsksC a
--
!
0
VokoCe
(3)
In the following discussion, related to the reaction carried out in this study, A, B and P represent CloHE1Br, SO32- and CloH21Br, SO3, respectively. By neglecting the concentration of A in water and of B in oil, and by regarding the reaction to be irreversible (which is, in practice, the case for an SN2 reaction of this type), Eq. (3) can be simplified to dNA dt
S V~k~C A CBs
= V~kdPAC°A)(PBC~)
(4)
where the partition coefficients for the reactants between interface and bulk are defined as PA=CA/CA,S 0 Pa=CB/CB'S W
S.-G. Oh et al./Colloids Surfaces A." Physicochem. Eng. Aspects 97 (1995) 169-179
Eqs. (4), (7) and (8) give
For an equimolar reaction Vo(C ° ' -
C °) - K ( C ~ - C~,)
= Vw(C~'-
C~) -
V,(C~ - C~,)
dEVo+ VsPA)C~] dt = VsksPAPBC°A
(5)
Vo(C°' - C°) _ V~PA(Co" _ C°)
×[C~'
= Vw{C~'-- C ~ ) - V s P B ( C ~ ' - C~)
(6)
Therefore Cw = C w'
175
V ° - V~PA(C~' - C~) Vw- V~P,
(7)
where C°' and Cw' In the following discussion, related to the reaction carried are the initial concentration of A and B in the oil and water domains, respectively. The total number of moles of A in the oil and interfacial domains (NA) is given by N A = VoC°A A- VsCSA = VoC°A -4- VsPAC°A
= (Vo + V~PA)C°A
(8)
V~_v~(CAV°--V~PA o'
_
C~,)]
Eq. (9) shows that the concentration of 1-bromodecane in the oil phase (C°) decays as a function of time, and that the rate of decay depends on many parameters such as the volume of each domain (Vo, Vw, Vs), the partition coefficient of reaction between bulk and interface (PA, PB), the reaction rate constant at the interface (ks) and the initial concentration of reactants (C°', CW'). Owing to experimental difficulties, no attempt has been made in this work to determine the values of PA and PB. Figs. 4-7 show the decay of 1-bromodecane with time for reaction media of different compositions.
1,8
water-oil mixture v
0 o9 ca. _, m
1,7
' ~
~
1,6
=
-_liquid crystal
_.
L2 microemulsion
0 c "o t-t~ o•
1,5
maximum conversion
isotropic channel
1,4
o
E 0 cn
1,3
e
"~
1,2
tO •"
1,1
,m
E o to
o
1,0
,
0
I
2
,
{9)
l
4
,
I
6
,
8
Reaction T i m e (h)
Fig. 4. D e c a y of 1 - b r o m o d e c a n e with time for r e a c t i o n systems h a v i n g an oil to w a t e r ratio of 9 : 1.
S.-G. Oh et al./Colloids SurJi~ces A: Physicochem. Eng. Aspects 97 (1995) 169 179
176
1,8
v {9 tO-
water-oil mixture
1,7
~
liquid crystal
1,6
0 .C:
1,5
tO (D "0 0
E ~
1,4
1,3
maximum conversion
!
'*"O tO
1,2
~
1,1
E O rO
O
,
1,0 0
I
i
2
I
,
4
I
6
,
8
Reaction Time (h) Fig. 5. Decay of 1-bromodecane with time for reaction systems having an oil to water ratio of 8 : 2.
Reactions were run at four different compositions (indicating four different surfactant concentrations) along lines representing oil to water weight ratios as well as initial molar ratios of 1-bromodecane to sodium sulfite, of 9 : 1, 8 : 2, 6.5 : 3.5 and 5 : 5, as shown in Fig. 1. Regardless of the reactant ratio, the extent of reaction was negligible when no surfactant was present. This is a good illustration of the problem of phase contact in a reaction between a nonpolar organic compound and an inorganic salt. The two microemulsion domains, denoted L2 and isotropic channel, proved to be good reaction media, giving relatively fast initial reaction rates. The formulations having the largest excess of 1-bromodecane to sodium sulfite (9:1 and 8:2) gave the most complete reactions (Figs. 4 and 5). The reactions run at smaller (6.5:3.5) or no (5:5) 1-bromodecane excess resulted in lower reaction yields (Figs. 6 and 7). In these reactions the decay of 1-bromodecane levelled offlong before the maximum conversion, representing total conversion of sulfite into product. The reason for the
cessation of the reaction will be discussed below. This study does not show any definite trend as to which type of microemulsion, bicontinuous (as in the isotropic channel) or w/o (as in the L2 phase), is more suitable as a reaction medium. The liquid crystalline phase, with a composition intermediate between the two microemulsion regions, gave a considerably more sluggish reaction than the microemulsions. This is noteworthy since the liquid crystalline phase, like the microemulsions, contains a very large oil water interfacial area. The low reaction rate may be related to the high rigidity of the lamellar liquid crystalline structure, which may hinder the diffusion of reactants across the interface. Various types of liquid crystals have been investigated as media for organic [-19,20] and bioorganic [21,22] reactions, but no previous comparison between reaction rates in microemulsions and liquid crystals based on the same components appears to have been carried out. From the concentration vs. time curves of Figs. 4 7 it is not possible to determine the reaction
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169 179 ~
1,8
177
water-oil mixture :
1,7
~ liquid crystal
131,6 ¢"--
1,5
(D "0 0
1,4
E ~ m
isotropic channel L2 microemulsion
1,3
!
O eO
1,2
~
1,1
E O eO
O
1,0
, 0
I 2
,
I 4
,
I 6
, 8
Reaction Time (h) Fig. 6. Decay of l-bromodecane with time for reaction systems having an oil to water ratio of 6.5 : 3.5. M a x i m u m conversion of 1-bromodecane corresponds to 0.8 wt.%.
order. The decay curves of 1-bromodecane in the microemulsion-based reactions have the general appearance of second-order reaction kinetics, but the shape of the time dependence of the reactant during a first-order reaction is similar. (With the same initial rate, the change in concentration at the later stage of reaction is more rapid for a firstorder than for a second-order reaction.) In order to verify that the reactions are true second order and not pseudo-first order, one would need to plot the reciprocal of 1-bromodecane concentration against time to see if a straight line, indicative of a second-order reaction, is obtained. To do this with acceptable accuracy many more experimental points are needed. Ideally, one should continuously monitor the reaction. Unfortunately, for this system no simple monitoring process was available. The cessation of reaction before all the 1-bromodecane has been consumed may be seen as an interracial phenomenon. The product formed, decyl sulfonate, is surface active and accumulates
at the interfaces. The nature of the surfactant palisade layer gradually changes from a purely nonionic monolayer to a mixed assembly of nonionic and anionic species. A charged interface is known to repel ions of the same charge from that interface [23]. It is reasonable to believe that in the reaction studied here the gradual increase of negative charge at the oil water interface leads to a reduced ability of sulfite ions to diffuse into the interfacial region where reaction with the lipophilic 1-bromodecane takes place. The influence of surfactant charge on rate and yield of reaction is presently being studied in our laboratory. An attempt was made to enhance the reaction rate by addition of small amounts of potassium iodide to the reaction mixture. Iodide ions, being both strong nucleophiles and good leaving groups, are known to accelerate many SN2 reactions [24]. Unfortunately, in this reaction iodide did not function as a catalyst but rather as a reducing agent. Sulfite ions rapidly oxidized iodide to iodine, as
S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
178
o~ 1,8
water-oil mixture
.¢.-,
8
II.
1,7 t./} A: Q"
~
liquid crystal
1,6
.B
0
~_
~
_
L2microemulsion
.I;::: 1,5 E O
1,4
0
isotropic channel
E
£ 1,a m i
0
1,2
E 0
~
1,1
e0
1,0
0
i
0
I
=
I
2
t
4
I
6
u
8
Reaction Time (h) Fig. 7. Decay of 1-bromodecane with time for reaction systems having an oil to water ratio of 5:5. Maximum conversion of l-bromodecane corresponds to 0.0 wt.%.
demonstrated by an immediate discoloration of the solution. In conclusion, the rates of synthesis of sodium decyl sulfonate from 1-bromodecane and sodium sulfite were high in microemulsions based on nonionic surfactant, more sluggish in liquid crystalline systems obtained from the same surfactant, and very low in surfactant-free oil-water mixtures. The reaction went to completion at high oil to water ratio compositions in which there is a large excess of organic reactant. The reaction yield diminished as the oil to water ratio (as well as the 1-bromodecane to sodium sulfite ratio) approached unity. The cessation of the reaction is likely to be due to a build-up of negative charge at the oilwater interface by the surface-active reaction product, decyl sulfonate. This work does not give any decisive information as to what type of microemulsion structure - - bicontinuous or droplet - is optimal as a reaction medium.
Acknowledgment The authors are grateful to Dr. Erik S6derlind from the Royal Institute of Technology for the N M R measurements, to Dr. Bj6rn Lindman and Dr. Ulf Olsson for comments on the interpretation of the self-diffusion data, and to Dr. Mikael Jansson, our previous colleague, for helpful discussions. SGO wishes to express his sincere thanks for financial support from the Wennergren Foundation in Sweden.
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S.-G. Oh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 97 (1995) 169-179
[5]
[6] [7] I-8]
1-9] [10] [11] [12] [13] [14]
and M.A. Moya, J. Chem. Soc., Faraday Trans., 88 (1992) 2701. A.R. Elrington, Rapid Deactivation of Mustard in Microemulsion Technology, Ph.D. Thesis, Emory University, Atlanta, GA, 1990. R.A. Mackay, Adv. Colloid Interface Sci., 15 (1981) 131. K. Holmberg and E. Osterberg, Prog. Colloid Polym. Sci., 82 (1990) 181. Y.L. Khmelnitski, A.V. Kabanov, N.L. Klyachko, A.V. Levashov and K. Martinek, in M.P. Pileni (Ed.), Structure and Reactivity in Reversed Micelles, Elsevier, Amsterdam, 1989. M.L. Moya, C. Izquierdo and J. Casado, J. Phys. Chem., 95 ( 1991 ) 6001. B.K. Mishra, B.S. Valandikar, J.J. Knujappu and C. Manohar, J. Colloid Interface Sci., 127 (1989) 373. A.S. Chhatre, R.A. Joshi and B.D. Kulkarni, J. Colloid Interface Sci., 158 (1993) 183. F.M. Menger and A.R. Elrington, J. Am. Chem. Soc., 113 (1991) 9621. F.M. Menger, J.U. Rhee and H.K. Rhee, J. Org. Chem., 40 (1975) 3803. K.M. Larsson, P. Adlercreutz, B. Mattiasson and U. Olsson, Biotechnol. Bioeng., 36 (1990) 135.
179
[15] P. Stilbs, Prog. Nucl. Magn. Resonance Spectrosc., 19 (1987) 1. [16] H. Kunieda and K. Shinoda, J. Dispersion Sci. Technol., 3 (1982) 233. [17] M. Jonstr6mer, B. J6nsson and B. Lindman, J. Phys. Chem., 95 (1991) 3293. El8] W. Knoche, V.R. Hanke and E. Dutkiewicz, J. Chem. Soc., Faraday Trans., 83 (1987) 2847. [19] V. Ramesh and M.M. Labes, Mol. Cryst. Liq. Cryst., 144 (1987) 257. [20] V. Ramesh and M.M. Labes, J. Am. Chem. Soc., 110 (1988) 738. [21] N.L. Klyachko, A.V. Levashov, A.V. Pshezhetsky, N.G. Bogdanova, I.V. Berezin and K. Martinek, Eur. J. Biochem., 161 (1986) 149. [221 P. Miethe, R. Gruber and H. Voss, Biotechnol. Lett., 11 (1989) 449. [23] R. Johannsson, M. Almgren and R. Schm~icker, Langmuir, 9 (1993) 1269. [24] K. Rorig, J.D. Johnston, R.W. Hamilton and T.J. Telinski, Org. Synth., Coll. Vol. 4 (1963) 576.