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Articles Nucleophilic Substitution Sulfonation in Microemulsions and Emulsions Maen M. Husein, Martin E. Weber, and Juan H. Vera* Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada H3A 2B2 Received November 17, 1999. In Final Form: July 13, 2000 Microemulsions and emulsions can be used to carry out reactions in which the reactants are not soluble in the same phase. In this work, the nucleophilic substitution reaction between an organic soluble compound, decyl bromide, and a water-soluble salt, Na2SO3, to produce sodium decyl sulfonate was carried out in batches in o/w microemulsions and emulsions formed with the two-tailed cationic surfactant dioctyldimethylammonium chloride, R2(Me)2N+Cl-. The surfactant counterion Cl-, substituted for bromide to give decyl chloride, an intermediate that further reacted to give the final product. The effects of stirring and preconditioning, surfactant concentration, the concentration of decyl bromide, and the initial mole ratio of the reactants on the conversion to sodium decyl sulfonate and decyl chloride were determined. Although phase separation occurred and emulsions formed in most of the samples, stirring and preconditioning did not have a significant effect on the conversion. An optimum surfactant concentration was found at which the maximum conversion to the final product was obtained. Increasing the concentration of decyl bromide increased the molar rate of sodium decyl sulfonate formation, but the conversion dropped. Increasing the mole ratio of Na2SO3 to decyl bromide increased conversion to the final product and reduced conversion to the intermediate. A pseudophase ion-exchange model with the interfacial volume as a fitted parameter, varying only with the surfactant concentration, represented well the experimental results.
Introduction Reactions between oil-soluble and water-soluble compounds can be carried out by solubilizing the reactants in two separate phases. Phase-transfer catalysts, such as crown ethers or quaternary ammonium salts, can be used to move the reacting water-soluble compound into the organic phase.1 This technique has two drawbacks: the presence of an organic solvent, which may be an environmental hazard;2 and the fact that the formation of the product involves a sequence of many steps, that is, complexation reaction in the aqueous phase, mass transfer of the complex to the organic phase, formation of the final product in the organic phase, complexation reaction in the organic phase, and finally, mass transfer of the complex to the aqueous phase. Slow kinetics for any of the reactions or mass transfer steps can limit the rate of product formation.2 Another way to carry out such reactions is to use polar aprotic solvents, such as dimethylformamide or acetonitrile,3 or a mixture of solvents, such as acetone and water,4 which can solubilize both reactants in one phase. This technique eliminates the mass transfer between phases, and thus, the rate of product formation is governed only by the reaction kinetics. However, the difficulty of separation of polar aprotic solvents, the need for an organic solvent, and the limits on the amounts of reactants that can be solubilized limit the large-scale application of this technique. * To whom correspondence should be addressed. (1) Naik, S.; Doraiswamy, L. AIChE J. 1998, 44, 612. (2) Siswanto, C.; Battal, T.; Schuss, O.; Rathman, J. Langmuir 1997, 13, 6047. (3) Oh, S.-G.; Kizling, J.; Holmberg, K. Colloids Surf., A 1995, 97, 169. (4) Bunton, C.; Halevi, E. J. Chem. Soc. 1952, 4, 4541.
Microemulsions are thermodynamically stable dispersions of oil and water. The thermodynamic stability comes from the fact that the droplets are very small, around 10 nm in diameter,5 and the interfacial tension is low because amphiphile molecules are present at the interface. Microemulsions are suitable media to carry out reactions involving oil-soluble and water-soluble compounds due to their ability to solubilize these compounds and provide a large contact surface between them. A major problem that restricts the industrial application of microemulsions is their low reactant loading.2,6 Increasing the reactant concentrations affects the type of microemulsion or results in emulsion formation, thus changing the reaction rate.2,3,6-9 Selectivity and separation of the product are also of concern. When cationic surfactants were employed to carry out nucleophilic substitution reactions, side products formed as a result of the reaction between the surfactant counterion and the organic substrate.2 Another disadvantage of cationic surfactants is that many of them are toxic. Cationic surfactants, however, are the best choice for nucleophilic substitution reactions because they attract the reacting nucleophile to the interface by electrostatic forces.2,3,6-15 (5) Langevin, D. J. Am. Chem. Soc. 1988, 21, 255. (6) Battal, T.; Siswanto, C.; Rathman, J. Langmuir 1997, 13, 6053. (7) Gutfelt, S.; Kizling, J.; Holmberg, K. Colloid Surf., A 1997, 128, 265. (8) Holmberg, K.; Oh, S.-G.; Kizling, J. Prog. Colloid Polym. Sci. 1996, 100, 281. (9) Oh, S.-G.; Kizling, J.; Holmberg, K. Colloids Surf., A 1995, 104, 217. (10) Holmberg, K. Adv. Colloid Interface Sci. 1994, 51, 137. (11) Al-Lohedan, H.; Bunton, C.; Moffatt, J. J. Phys. Chem. 1983, 87, 332. (12) Bunton, C.; Gan, L.-H.; Hamed, F.; Moffatt, J. J. Phys. Chem. 1983, 87, 336.
10.1021/la991512y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
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The pseudophase separation, PS, model in combination with the pseudophase ion exchange, PIE, model has been used to describe reactions between ionic reactants and organic substrates taking place in micelles.11,12,14-17 According to this model, the rate enhancement or inhibition occurs because micelles concentrate or repel the reactants from the interfacial region. The ion-exchange constant governs the concentration of the ions at the interface. The relative concentrations of the ions at the interface are controlled by their specific interactions, which generally follow the Hofmeister series; large, weakly hydrated, polarizable anions displace strongly hydrated anions.15,18,19 Although the surface of the micelles is charged due to the dissociation of the ionic surfactant, Coulombic interactions were neglected. In some of the previous work, the degree of counterion association, β, which is the fraction of the surfactant molecules at the interface that are not dissociated, was assumed to be constant, independent of the type of ions present at the interface.13-15 The volume of the interfacial region was taken as directly proportional to the number of surfactant molecules forming the micelles. This limited the application of the PIE model to reactant concentrations that are well below the surfactant concentration, and thus do not affect the micellar system properties. The main objective of this work was to carry out the second-order nucleophilic substitution reaction, SN2, between decyl bromide and sodium sulfite to form sodium decyl sulfonate at high reactant loading, in o/w microemulsions and emulsions, based on the two-tailed cationic surfactant dioctyldimethylammonium chloride, R2(Me)2N+Cl-, with decyl bromide forming the oil core of the microemulsion and the emulsion. Gutfelt et al.,7 Holmberg et al.,8 and Oh et al.10,11 carried out the same reaction in w/o microemulsions based on the nonionic surfactant penta(ethyl glycol)monododecyl ether, with decyl bromide solubilized in dodecane forming the oil phase. The rate of product formation was governed by the type of microemulsion which, in turn, was affected by the reactant and the surfactant concentrations. The highest rate was obtained at relatively small concentrations of the reactants. Gutfelt et al.7 and Holmberg et al.8 studied the effect of adding a small amount of cationic surfactant to the microemulsion. An increase or a decrease in the rate, depending on the surfactant counterion, occurred. The possibility of side-product formation upon reaction between the counterion of the cationic surfactant and the decyl bromide was not considered, however. Gutfelt et al.7 compared the microemulsion route and the phasetransfer route and found that phase-transfer catalysis was much less efficient due to formation of a strong ion pair between the decyl sulfonate product and the phasetransfer catalyst. An additional objective was to develop a model to describe the reaction under high reactant and surfactant concentrations. In previous studies,13-15 simplifying as(13) Martinek, K.; Yatsimirski, A.; Levashov, A.; Berezin, I. In Micellization, Solubilization and Microemulsions; Mittal, K., Ed.; Plenum Press: New York, 1977; p 489. (14) Romsted, L. In Micellization, Solubilization and Microemulsions; Mittal, K., Ed.; Plenum Press: New York, 1977; p 509. (15) Bunton, C.; Nome, F.; Quina, F.; Romsted, L. Acc. Chem. Res. 1991, 24, 357. (16) Athanassakis, V.; Bunton, C.; Mckenzie, D. J. Phys. Chem. 1986, 90, 5858. (17) Mackay, R. J. Phys. Chem. 1982, 86, 4756. (18) Thompson, R.; Allenmark, S. J. Colloid Interface Sci. 1992, 148, 241. (19) Bartet, D.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84, 272.
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sumptions were made that restricted these earlier models to low concentrations. Experimental Methods Reaction Procedure. The experiments were carried out by adding a volume of Bardac (Lonza Corp., Fair Lawn, NJ) containing 80 wt % dioctyldimethylammonium chloride (R2(Me)2N+Cl-), 10 wt % ethanol, and 10 wt % water to a 50-mL volumetric flask. Sodium sulfite (Sigma, St. Louis, MO) was then dissolved into the Bardac aqueous solution. At time zero, decyl bromide (Aldrich, Milwaukee, WI) was added, and the volume was made up to 50 mL with distilled water. Less than two minutes were required to add the chemicals unless otherwise noted. The reaction mixture was then transferred to a 100-mL vial and placed in a water bath at 25.0 ( 0.5 °C. In all runs, the vials were stirred unless otherwise noted. Stirring was achieved with a cylindrical magnetic bar (1.3 cm × 0.8 cm) spinning at 300 rpm. Analysis. Runs with or without stirring were mixed for one minute before sampling. A sample of 1.0 mL from the reaction mixture was diluted by addition of methanol (Anachemia, Montreal, PQ) containing dodecanol (A&C, Montreal, PQ) at a concentration of 14.9 mM as an internal standard. The samples with concentrations of decyl bromide of 57 and 189 mM were diluted with 5.0 and 19.0 mL of methanol solution, respectively. After dilution, the concentration of decyl bromide in the initial reaction mixture was 9.5 mM in both cases. Upon addition of methanol, sodium sulfite and sodium decyl sulfonate precipitated. The precipitation of these components was confirmed in separate tests involving the addition of methanol to aqueous solutions of R2(Me)2N+Cl- containing sodium sulfite or sodium decyl sulfonate (Sigma, St. Louis, MO). A 10-µL volume was taken from the methanol mixture and injected into the gas chromatograph (GC). A Hewlett-Packard 5980 A GC having a 183 × 0.32 cm stainless steel Tenax TA 60/80 mesh packed column interfaced with an HP 3392 integrator was used with helium as the carrier gas (oven temperature, 225 °C; injector and FID temperature, 275 °C). The peak areas were related to the concentrations of decyl bromide and decyl chloride through a calibration solution. The calibration solution consisted of decyl bromide and decyl chloride (Aldrich, Milwaukee, WI), each at 57 mM, in methanol. The calibration solution was diluted with 5.0 mL of methanol solution to make a final concentration of decyl bromide of 9.5 mM. No precipitation occurred during the analysis of the calibration solution. Determination of the cmc of Bardac. The cmc of Bardac, containing 80 wt % dioctyldimethylammonium chloride (R2(Me)2N+Cl-), 10 wt % ethanol, and 10 wt % water, was determined by measuring the surface tension of 14 samples of aqueous solutions of Bardac at concentrations of R2(Me)2N+Cl- between 0.25 and 25 mM using a Fisher 215 Autotensiomat. A plot of the surface tension versus the logarithm of the R2(Me)2N+Clconcentration was used to determine the cmc.20 Solubility of Decyl Bromide in Distilled Water. A mixture of decyl bromide and water was left in an Orbit Environ Shaker at 350 rpm and 25 ( 0.5 °C for 24 h. Shaking was stopped for another 24 h to allow for the two phases to separate. A sample from the aqueous phase was prepared, as outlined in the analysis section, for analysis by GC. Oxidation of SO32- to SO42-. The possibility of SO42formation upon oxidation of SO23 in the reaction mixture was followed using an ion chromatograph (IC). A Dionex DX 100 IC containing a 4-mm AS/2A column with an eluent solution of 10.5 mM Na2CO3 and 0.5 mM NaHCO3 was used.
Results and Discussion The percent conversions to decyl chloride and sodium decyl sulfonate were calculated from the disappearance of decyl bromide and the appearance/disappearance of decyl chloride according to (20) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: New York, 1998, pp 57, 63.
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%C10H21Cl )
[C10H21Cl] [C10H21Br]STD
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× 100
(1)
%C10H21SO3Na ) [C10H21Br]STD - [C10H21Br] - [C10H21Cl] [C10H21Br]STD
× 100 (2)
where, [i] is the total concentration of species i (mM) in the sample and the superscript STD refers to the concentration in the calibration solution. Decyl bromide was always the limiting reactant. Three replicates were prepared for some of the runs. The 95% confidence intervals are shown on some of the figures. Observations on Phase Separation. Cloudiness appeared, indicating phase separation and emulsion formation, at time zero in all the samples, except the ones at a R2(Me)2N+Cl- concentration of 200 mM and a decyl bromide concentration of 57 mM. Under these conditions, cloudiness appeared at different times, depending on the initial mole ratio of sodium sulfite to decyl bromide. For mole ratios of nine, six, and three, cloudiness appeared after 20 min, 1 h, and 3 h, respectively. This suggests that sodium decyl sulfonate causes phase separation if its concentration exceeds a certain limit that depends on the concentrations of the other components of the reaction mixture. In experiments with no stirring, two bulk phases separated: an upper R2(Me)2N+Cl- rich phase containing most of the decyl bromide and the product and a cloudy lower R2(Me)2N+Cl- poor phase containing most of the sodium sulfite, some of the decyl bromide, and the sodium decyl sulfonate product. The volume of the upper phase decreased with time and reached a constant value at about 4 h. This final volume was larger for higher concentrations of R2(Me)2N+Cl- and decyl bromide and lower concentrations of sodium sulfite. Experiments to investigate phase separation by titrating decyl bromide into a solution of 200 mM R2(Me)2N+Cl- showed that concentrations of more than 60 mM decyl bromide would cause phase separation. Phase separation also occurred in 200 mM R2(Me)2N+Clsolution containing 57 mM decyl bromide when the concentration of sodium decyl sulfonate was 3.5 mM or more. Increasing the sodium sulfite concentration to more than 600 mM in the 200 mM R2(Me)2N+Cl- and 57 mM decyl bromide solution also caused phase separation. To summarize, a relatively high concentration of any component of the reaction mixture resulted in phase separation and emulsion formation. Effect of Stirring and Preconditioning. To investigate the roles of mass transfer of the reactants to the interface and equilibrium between the ions there, the effects of stirring and preconditioning were studied. The concentrations of the components added to the reaction mixture were [R2(Me)2N+Cl-]a ) 200 mM, [C10H21Br]a ) 57 mM, and initial mole ratio Na2SO3/C10H21Br ) 6.0. Three replicates were provided for the unstirred experiment. The results of two experiments, one stirred and the other unstirred, are presented in Figure 1. The differences in the conversion to C10H21SO3Na and C10H21Cl between the stirred and unstirred runs fall within the 95% confidence interval shown in the figure. Mass transfer of the reactants occurs sufficiently rapidly that the reaction rate is controlled by the reaction kinetics. This conclusion is supported by other studies.13-15 The preconditioning experiment was also performed under the above conditions. In this experiment, R2(Me)2-
Figure 1. Effect of stirring on the conversion to sodium decyl sulfonate, solid symbols, and decyl chloride, open symbols: [R2(Me)2N+Cl-]a ) 200 mM, [C10H21Br]a ) 57 mM, mole ratio Na2SO3/C10H21Br ) 6.0; stirring ([,]), no stirring (b,O).
Figure 2. Effect of preconditioning on the conversion to sodium decyl sulfonate, solid symbols, and decyl chloride, open symbols: [R2(Me)2N+Cl-]a ) 200 mM, [C10H21Br]a ) 57 mM, mole ratio Na2SO3/C10H21Br ) 6.0; preconditioning (2,4), no preconditioning ([,]).
N+Cl- and Na2SO3 were mixed in distilled water for 24 h prior to adding the decyl bromide to allow for the replacement of the surfactant counterion. The run with no preconditioning was performed following the procedure outlined in the Experimental Methods section, where R2(Me)2N+Cl-, Na2SO3, and C10H21Br were added at almost the same time. The results, plotted in Figure 2, are almost identical and show that ion-exchange equilibrium between reacting anion and the surfactant counterions, Cl- and Br-, at the interface is attained rapidly. Effect of R2(Me)2N+Cl- Concentration. Figure 3 shows the time variation of the conversion to the intermediate and the final products, as calculated from eqs 1 and 2, for R2(Me)2N+Cl- concentrations from 50 to 800 mM. The initial mole ratio of Na2SO3/C10H21Br was 9.0, and the initial concentration of C10H21Br was 57 mM. The conversion to sodium decyl sulfonate after about 9 h of reaction is plotted versus the initial concentration of R2(Me)2N+Cl- in Figure 4. Without surfactant, [R2(Me)2N+Cl-]a ) 0, the reaction was slow because the solubility of C10H21Br in water is low, 0.17 ( 0.04 mM. This indicates that the reaction takes place mainly at the interface between the phases of the microemulsion or the emulsion. The conversion to C10H21SO3Na and C10H21Cl increased
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Figure 4. The conversion to sodium decyl sulfonate versus [R2(Me)2N+Cl-]a after ∼9.0 h run. [C10H21Br]a ) 57 mM, mole ratio Na2SO3/C10H21Br ) 9.0. Points are from experiment; curve is calculated with the model.
Figure 3. Effect of [R2(Me)2N+Cl-]a on the conversion to sodium decyl sulfonate, solid symbols, and decyl chloride, open symbols: [C10H21Br]a ) 57 mM, mole ratio Na2SO3/C10H21Br ) 9.0. a) [R2(Me)2N+Cl-]a ) 50 mM. (b) [R2(Me)2N+Cl-]a ) 200 mM. (c) [R2(Me)2N+Cl-]a ) 800 mM. Points are from experiments; curves are calculated with the model. The maximum conversion as predicted by the model is based on %C10H21Bre.
with increasing R2(Me)2N+Cl- concentration, with the conversion to the final product reaching a maximum at a R2(Me)2N+Cl- concentration between 200 and 400 mM. This behavior is typical for second-order reactions carried out in micelles,13-15 and it can be explained as follows. As the R2(Me)2N+Cl- concentration increased, more reac+ were bound to the tants, C10H21Br, SO23 , and Na interface, and thus, the conversion to the final product increased. The conversion to C10H21Cl also increased
because there was more Cl- at the interface. Since the intermediate, C10H21Cl, reacted more slowly than the original reactant, C10H21Br, the conversion to the final product, C10H21SO3Na, decreased. Another factor could be the dilution caused by the increase in the volume of the interfacial region with increasing R2(Me)2N+Cl- concentration.14 Effect of Decyl Bromide Concentration. The objective of this experiment was to test the effect of loading the system with the reactants. The C10H21Br concentration was increased from 57 to 189 mM at a constant R2(Me)2N+Cl- concentration of 400 mM and a constant initial mole ratio Na2SO3/C10H21Br of 9.0. The results are shown in Figure 5. The molar rate of formation of the final product increased upon increasing the amounts of reactants added due to the increase in the reactants bound to the interface. However, the conversion to sodium decyl sulfonate dropped. This drop in the conversion suggests that not all of the C10H21Br was available for the reaction at the interface. Since the solubility of C10H21Br in water is low, the excess amount remains in the oil core of the microemulsion or emulsion. This amount will not react at the interface because the product, C10H21SO3Na, is a surfactant that remains at the interface, thus blocking further C10H21Br from approaching it and reacting. It can also be that the increase in the interfacial concentration of the product, which is an anionic surfactant, repels the anionic reacting nucleophile from approaching the interface.2 The increase in the molar rate of sodium decyl sulfonate formation can also be due to its participation in the formation of mixed micelles with the cationic surfactant R2(Me)2N+Cl-. The increase in the mixed micelle concentration within the emulsion system increases the partition of decyl bromide from the emulsified droplets into the mixed micelles where it reacts. However, at some concentration of C10H21SO3Na, the retarding effect of the anionic surfactant outweighs the effect of the increasing concentration of mixed micelles and results in trapping in the emulsified droplets some of the C10H21Br that will not react. Effect of Mole Ratio of Sodium Sulfite to Decyl Bromide. The effect of the initial mole ratio Na2SO3/ C10H21Br on the conversion to sodium decyl sulfonate and decyl chloride is shown in Figure 6. The concentration of R2(Me)2N+Cl- was 200 mM and that of C10H21Br was 57 mM. The conversion to C10H21SO3Na increased and the conversion to C10H21Cl decreased with increasing mole
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Figure 5. Effect of [C10H21Br]a on the conversion to sodium decyl sulfonate, solid symbols, and decyl chloride, open symbols: [R2(Me)2N+Cl-]a ) 400 mM, mole ratio Na2SO3/C10H21Br ) 9.0. (a) [C10H21Br]a ) 57 mM. (b) [C10H21Br]a ) 189 mM. Points are from experiments; curves are calculated with the model. The maximum conversion as predicted by the model is based on %C10H21Bre.
ratio, with the mole ratio having less significance at high values. The increase in the conversion to C10H21SO3Na and the decrease in the conversion to C10H21Cl suggest an-ion exchange step between reacting anion and Cl-. Increasing the bulk concentration of Na2SO3 increased the interfacial concentration of SO23 at the expense of the interfacial concentration of Cl-. The conversion to C10H21SO3Na was not directly proportional to the initial mole ratio, since the reaction takes place at the interface and and Na+ are the interfacial concentrations of SO23 governed by ion exchange. Despite the fact that phase separation occurred at different times depending on the mole ratio, the general trend of the conversion was not affected. This indicates that mass transfer of reactants was not limited by phase separation. Proposed Reaction Mechanism. It was found that less than 1.0% of the Na2SO3 added oxidized to Na2SO4 after 6 h reaction time. Thus, the oxidation reaction was neglected. On the basis of the results obtained from the experiments, it is proposed that sodium decyl sulfonate is formed in the following steps. (1) The surfactant counterion, Cl-, reacts with some of the decyl bromide at the interface to form decyl chloride, and some bromide replaces chloride as the surfactant counterion:
Figure 6. Effect of the mole ratio of Na2SO3/C10H21Br on the conversion to sodium decyl sulfonate, solid symbols, and decyl chloride, open symbols: [R2(Me)2N+Cl-]a ) 200 mM, [C10H21Br]a ) 57 mM. (a) mole ratio Na2SO3/C10H21Br ) 3.0. (b) mole ratio Na2SO3/C10H21Br ) 6.0. (c) mole ratio Na2SO3/C10H21Br ) 9.0. Points are from experiments; curves are calculated with the model. The maximum conversion as predicted by the model is based on %C10H21Bre.
R2(Me)2N+Cl- + C10H21Br f C10H21Cl + R2(Me)2N+Br- [R1] (2) Sulfite anion, the predominant anion in the bulk aqueous phase at the measured pH of about 10, exchanges with the surfactant counterions forming the anionic complex, R2(Me)2N+SO23 . This complex interacts immediately with Na+ from the bulk aqueous phase to give + R2(Me)2N+SO23 Na . For simplicity, this reaction se-
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Figure 7. Apparent second-order reaction rate constants for the sulfonation of decyl bromide, k1′, and decyl chloride, k2′, versus mole ratio Na2SO3/C10H21Br. + quence is written as an ion exchange of SO23 Na from the bulk aqueous phase with the surfactant counterion:
K
R2(Me)2N+Cl-/Br- + SO32-Na+ 798 R2(Me)2N+SO32-Na+ + Cl-/Br- [R2] + reacts with (3) The surfactant counterion SO23 Na decyl bromide and decyl chloride to form the sodium decyl sulfonate, according to reactions [R3] and [R4]:
k1
R2(Me)2N+SO32-Na+ + C10H21Br 98 C10H21SO3Na + R2(Me)2N+Br- [R3] k2
R2(Me)2N+SO32-Na+ + C10H21Cl 98 C10H21SO3Na + R2(Me)2N+Cl- [R4] Modeling As noted in the Experimental Section, there is an increase in the conversion to the final product and a decrease in the conversion to the intermediate as the mole ratio of sodium sulfite to decyl bromide increases. This effect, which is clearly seen in Figures 6a to 6c, suggests that ion exchange plays an important role in this system. To represent such behavior with a second-order reaction model based on the total concentrations without ion exchange, it would be necessary to use apparent reaction rate “constants” that depend on the mole ratio of the reactants. Preliminary calculations using such apparent rate constants were made for the runs shown in Figures 6a, 6b, and 6c. Figure 7 shows the apparent rate constants for the sulfonation of decyl bromide (k1′) and decyl chloride (k2′), which produced data fits comparable to those in Figures 6a to 6c. The values of k1′ and k2′ decreased by a factor of about two over the range of mole ratios studied. In the pseudophase separation model, the microemulsion consists of oil and aqueous pseudophases separated by an interfacial region, the Stern layer. The Stern layer extends from the oil core to the shear surface containing the surfactant headgroups and the counterions associated
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with the surfactants.3,14,15,21 The reaction rate in the pseudophases is dictated by the reactant concentrations in each phase, which are determined by partition coefficients that are specific for each reactant.22 Romsted14 used partition coefficients to calculate the concentration of the oil-soluble reactant in the oil and the aqueous pseudophases and the pseudophase ion-exchange, PIE, model to describe the distribution of ions between the phases. Pereira et al.21 and Minero et al.23 employed a three-pseudophase model consisting of water, interface, and oil to account for reactions carried out in microemulsions. Partition coefficients were used to calculate the reactant concentrations in each phase. It is believed that such partition coefficients are irrelevant to this work, since the solubility of decyl bromide in water is low, SO23 and Na+ are insoluble in the oil phase, and there was almost no reaction in the absence of R2(Me)2N+Cl-. In view of the work of Menger et al.,24 it is unlikely that the cationic surfactant will increase the solubility of the reacting anion in the oil phase by acting as a phase-transfer catalyst. Hence, in this work, the interfacial region is assumed to be the main reaction site. Since phase separation occurred in most of the samples, the value of the counterion association, β, is set equal to unity and thus, the concentration of an ion at the interface is dictated by the specific interactions following the Hofmeister series.15 All of the surfactant is assumed to be at the interface because the cmc, 2 mM, is only about 5% of the lowest R2(Me)2N+Clconcentration. The remaining assumptions in the model are given below for each step in the proposed mechanism. (1) Reaction [R1] takes place instantaneously at t ) 0. This assumption is based solely on the experimental findings as shown in Figures 1 to 3. The largest concentration of the decyl chloride intermediate was always measured shortly after the reaction started. Earlier work by Al-Lohedan et al.11 showed that nucleophilic substitution reactions involving the cationic surfactant counterions occurred readily due to the compartmentalization of the reactants in the micellar phase. The moles of decyl chloride formed at t ) 0, nCo 10H21Cl, are a function of the total concentrations of dioctyldimethylammonium bromide, [R2(Me)2N+Cl-]a, decyl bromide, [C10H21Br]a, and sodium sulfite, [Na2SO3]a, added to the reaction mixture. (2) A single ion-exchange constant, K, for reaction [R2] describes the exchange of Cl- and Br-. Although according to the Hofmeister series, Cl- exchanges more readily than Br-,15,18,19 this simplifying assumption reduces the number of adjustable parameters. The ion-exchange reaction is + instantaneous, and the mass transfer of SO23 and Na to the interface is not the limiting step. The concentration + at the interface of R2(Me)2N+ SO23 Na is governed by K, + - a a [R2(Me)2N Cl ] , and [Na2SO3] as follows:
K)
[R2(Me)2N+SO32-Na+] × [Cl-/Br-]
) [R2(Me)2N+Cl-/Br-] × [SO32-Na+] nR2(Me)2N+SO32-Na+ × nCl-/BrnR2(Me)2N+Cl-/Br- × nSO32-Na+
(3)
where [R2(Me)2N+i-] is the interfacial concentration of the surfactant counterion (mM), [i-] is the concentration of this ion in the bulk aqueous phase (mM), nR2(Me)2N+i- is (21) Da Rocha Pereira, R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990, 94, 356. (22) Berezin, I.; Martinek, K.; Yatsimirski, A. Russ. Chem. Rev. 1973, 42, 787. (23) Minero, C.; Pramauro E.; Pelizzetti, E. Langmuir 1988, 4, 101. (24) Menger, F.; Rhee, J.; Rhee, H. J. Org. Chem. 1975, 40, 3803.
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the amount of the surfactant counterion at the interface (mmol), and ni- is the amount of this ion in the bulk aqueous phase (mmol) where i- stands for Cl-, Br- or + SO23 Na . Combining eq 3 with the mass balance when only the reacting salt is added gives25
y2 K) a (n Na2SO3 - y)(nRa 2(Me)2N+Cl- - y)
(4)
a a where y ) nR2(Me)2N+SO2+, and nNa SO and nR (Me) N+Cl2 3 2 2 3 Na are the amounts of Na2SO3 and R2(Me)2N+Cl- added to the reaction mixture (mmol). (3) Reactions [R3] and [R4] are the rate determining steps. The conversion to sodium decyl sulfonate and decyl chloride was calculated as follows. If the surfactant concentration is insufficient to bind all of the organic reactant to the interface, the moles of decyl bromide trapped in the oil core at t ) 0, nCe 10H21Br, varies with [R2(Me)2N+Cl-]a and [C10H21Br]a. The moles of decyl bromide available for reaction at t ) 0 can then be calculated from eq 5 below.
noC10H21Br ) nCa 10H21Br - nCo 10H21Cl - nCe 10H21Br
(5)
where nCa 10H21Br is the amount of C10H21Br added to the reaction mixture (mmol). Since the reactions take place only at the interface, the second-order rate equations for reactions [R3] and [R4] can be written as
(
)(
)
nR2(Me)2N+SO2nCi 10H21Br + dx1 3 Na ) k1 dt Vi Vi and
(
n dx2 ) k2 dt
)(
+ R2(Me)2N+SO23 Na
i
V
)
nCi 10H21Cl V
i
(6)
(
)(
)
and
(
)(
)
nR2(Me)2N+SO2nCi 10H21Cl + dX2 3 Na ) k2 dt Vi Vt
X1 )
(8)
(9)
where X1 and X2 are the total concentrations of C10H21SO3Na (mM). When the initial mole ratio of sodium sulfite to decyl bromide was 9.0, the pseudo first-order approximation was used. The value of nR2(Me)2N+SO2+ was taken to be 3 Na constant at the value obtained from eq 4. Equations 8 and (25) Husein, M. Nucleophilic Sulfonation in Microemulsions and Emulsions. Ph.D. Thesis, McGill University, Montreal, September, 2000.
{
nCo 10H21Br t
V
(
k1
(
k2
1 - exp t ×
Vi
)}
× nR2(Me)2N+SO2+ 3 Na
(10)
and
X2 )
{
nCo 10H21Cl t
V
1 - exp t ×
Vi
)}
× nR2(Me)2N+SO2+ 3 Na
(11)
Equations 4, 10, and 11 were used to calculate X1 and X2 with K, k1, and k2 as fitted parameters. The value of Vi was also a fitted parameter, which varied only with [R2(Me)2N+Cl-]a. In previous studies, the volume of the interfacial region was taken as directly proportional to the number of surfactant molecules forming the micelles, and the reactant and surfactant concentrations were kept low to avoid changing the nature of the microemulsion.13-15 It should be noted, however, that the PIE model does not explicitly account for the shape, the size, and the aggregation number of micelles, and thus, since it is not sensitive to these variables,15 it should be applicable to reactions in emulsions. The quantity nCe 10H21Br from eq 5 was taken as a fitted parameter, a function of [R2(Me)2N+Cl-]a and [C10H21Br]a. Finally, nCo 10H21Cl was taken as a fitted parameter that varies with [R2(Me)2N+Cl-]a, [C10H21Br]a, and [Na2SO3]a. Microsoft Excel Solver was used to obtain the fitted parameters by minimizing the average squared deviation between the experiments and the model. The percent conversion to decyl chloride and sodium decyl sulfonate was calculated from
%C10H21Cl )
(7)
where x1 and x2 are the interfacial concentrations of C10H21SO3Na (mM) produced from reactions [R3] and [R4], respectively, and nCi 10H21Br and nCi 10H21Cl are the amounts of decyl bromide and decyl chloride at the interface at any time (mmol). Since the experimental measurements were based on the total volume, Vt, eqs 6 and 7 are multiplied by Vi/Vt, assuming that the volumes are constant throughout the reaction period, to obtain
nR2(Me)2N+SO2nCi 10H21Br + dX1 3 Na ) k1 dt Vi Vt
9 were then integrated to give
nCo 10H21Cl - X2Vt nCa 10H21Br
× 100
(12)
× 100
(13)
and
%C10H21SO3Na )
(X1 + X2)Vt nCa 10H21Br
When the mole ratio of sodium sulfite to decyl bromide was less than nine, nR2(Me)2N+NaSO3- was calculated from eq 4, with nNa2SO3, obtained as a function of time from a - (X1 + X2)Vt nNa2SO3 ) nNa 2SO3
(14)
a in eq 4. Equations 4, 8, 9, and was substituted for nNa 2SO3 14 were solved simultaneously via a fourth-order RungeKutta method using Matlab. No optimization of parameters was done. The values of the fitted parameters obtained from the pseudo first-order results were used except for the values of nCo 10H21Cl, which were adjusted. Equations 12 and 13 were used to calculate the percentage conversions.
Performance of the Model The following values were obtained from the pseudo first-order fit for all the runs at the initial mole ratio of reactants of 9.0: K ) 1.0 × 10-5, k1 ) 2.0 × 10-2 (mM h)-1, and k2 ) 9.0 × 10-3 (mM h)-1. Decyl bromide reacts faster
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Husein et al.
Table 1. Values of the Volume of the Interfacial Region, the Percentage Decyl Chloride Formed at t ) 0, and the Percentage Decyl Bromide Trapped for Different [R2(Me)2N+Cl-]a:[C10H21Br]a ) 57 mM, mole ratio Na2SO3/ C10H21Br ) 9.0 [R2(Me)2N+Cl-]a (mM)
Vi × 103 (L)
%C10H21Clo
%C10H21Bre
50 100 200 400 800
2.6 3.7 4.6 6.9 11.8
6.9 11.0 12.2 26.6 43.1
28.6 4.6 1.7 0 0
Table 2. Values of the Percentage Decyl Chloride Formed at t ) 0 and the Percentage Decyl Bromide Trapped for Different [C10H21Br]a:[R2(Me)2N+Cl-]a ) 400 mM; mole ratio Na2SO3/C10H21Br ) 9.0 [C10H21Br]a (mM)
%C10H21Clo
%C10H21Bre
57 189
27.9 7.6
0 30.2
than decyl chloride (k1 > k2), a finding consistent with the literature.26 Effect of R2(Me)2N+Cl- Concentration. The model fit for experiments on the effect of the R2(Me)2N+Clconcentration is compared to the experimental data in Figures 3 and 4. In addition to the ion-exchange and reaction constants, the volumes of the interfacial region listed in Table 1 and %C10H21Clo and %C10H21Bre calculated from eqs 15 and 16 below and listed in Table 1 were used to obtain the model curves: o
%C10H21Cl )
e
%C10H21Br )
nCo 10H21Cl nCa 10H21Br nCe 10H21Br nCa 10H21Br
× 100
(15)
× 100
(16)
The maximum conversion predicted by the model for the different concentrations of R2(Me)2N+Cl- is based on the value of %C10H21Bre and is shown in Figure 3. The volume of the interfacial region increased roughly linearly with surfactant concentration above 50 mM. The model fitted well the conversion to the final and the intermediate products for the different concentrations. The model accounted for the maximum conversion (at about 9 h) observed between R2(Me)2N+Cl- concentrations of 200 and 400 mM. As discussed earlier, this optimum is caused by two factors: the dilution effect and the increase in the conversion to the C10H21Cl intermediate as the R2(Me)2N+Cl- concentration increases. Table 1 shows that a concentration of 400 mM R2(Me)2N+Cl- was enough to bind all of the decyl bromide to the microemulsion interface, whereas a concentration of 200 mM was not. Apparently, the optimum occurred at the R2(Me)2N+Cl- concentration, enough to bind all the organic substrate to the interface. Effect of Decyl Bromide Concentration. The model fit for the effect of C10H21Br concentration is shown in Figure 5. Values of K, k1, k2, and Vi at [R2(Me)2N+Cl-]a ) 400 mM were used in the model calculations. Table 2 shows the values of the parameters calculated from eqs 15 and 16. There is good agreement between the model fit and the experimental results. The maximum conversion for dif(26) Bruice, P. Organic Chemistry, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1998; pp 363-369.
Table 3. Values of the Percentage Decyl Chloride Formed at t ) 0 for Different Mole ratios Na2SO3/ C10H21Br: [R2(Me)2N+Cl-]a ) 200 mM; [C10H21Br]a ) 57 mM mole ratio Na2SO3/C10H21Br
%C10H21Clo
3.0 6.0 9.0
26.5 18.1 12.2
ferent [C10H21Br]a predicted by the model was compared to experimental measurements after a 24 h run. The model and experimental values differed by less than 5%. This difference is less than the 95% confidence interval for the measured value. Effect of the Mole Ratio of Sodium Sulfite to Decyl Bromide. The model fit for this experiment was obtained using values of K, k1, k2, and Vi and %C10H21Bre at [R2(Me)2N+Cl-]a ) 200 mM from Table 1. The data from this experiment were not included in the optimization of the parameters except for the one result at a mole ratio of 9.0. The only adjustable parameter in this fit was %C10H21Clo. The model curves in Figure 6, including the one at a mole ratio of 9.0, were obtained by solving eqs 4, 8, 9, and 14 simultaneously. The results at a mole ratio of 9.0 should be compared with the fit obtained using the pseudo first-order approximation in Figure 3b. The difference between the pseudo first-order fit and the one in Figure 6c is less than 0.5%. Table 3 shows that the values for %C10H21Clo decrease as the mole ratio increases. There is more NaSO3 at the interface replacing the Cl . The model fit agreed well with the experimental results for the different mole ratios. The difference between the experimental and the model results usually fall within the 95% confidence interval for the measured conversions. Conclusions Microemulsions and emulsions based on the two-tailed cationic surfactant dioctyldimethylammonium chloride, R2(Me)2N+Cl-, were used to prepare sodium decyl sulfonate with high reactant loading. Concentrations of about 20 times and a rate of reaction of about 8 times previous studies were obtained. The cationic surfactant concentrated the reacting anion, SO23 , at the interface and minimized the effect of phase separation and interphase mass transfer. The equilibrium between the ions exchanging at the interface was attained rapidly. The product of the reaction between the surfactant counterion, Cl-, and the decyl bromide reacted further to form the final product. At a surfactant concentration between 200 and 400 mM, the highest conversion to sodium decyl sulfonate was obtained. This concentration was sufficient to bind all of the organic substrate to the interface. Increasing the R2(Me)2N+Cl- concentration above the optimum value resulted in a decrease in the conversion due to having more of the decyl chloride intermediate, which reacted more slowly, and due to the dilution effect at the interface. Increasing the concentration of C10H21Br at constant R2(Me)2N+Cl- concentration and constant initial mole ratio of Na2SO3/C10H21Br increased the average rate of formation of C10H21SO3Na since more reactants were bound to the interface and/or due to increasing the concentration of the mixed micelles. A drop in the conversion occurred, however, due to the entrapment of some C10H21Br in the oil core and/or due to the repulsion between reacting nucleophile and the product anionic surfactant.
Nucleophilic Substitution Sulfonation
Increasing the mole ratio of Na2SO3 to C10H21Br increased the conversion to sodium decyl sulfonate and reduced the conversion to decyl chloride. Increasing the bulk concentration of the salt, at constant R2(Me)2N+Clconcentration, increased the interfacial concentration of NaSO3 by exchanging more Cl at the interface. A single-pseudophase model in combination with the pseudophase ion-exchange, PIE, model with three new assumptions was used to describe the sulfonation of decyl halides in microemulsion/emulsion systems. The three new assumptions employed in the current model are (1) the volume of the interfacial region varies only with the amount of surfactant, but it is not directly proportional to the surfactant concentration; (2) decyl bromide may be trapped within the oil core and not participate in the reaction if there is insufficient surfactant to bind all of the organic substrate to the interface; and (3) a single ion-
Langmuir, Vol. 16, No. 24, 2000 9167
exchange constant accounts for the exchange between the reacting anion and the surfactant counterions. The first assumption was needed to handle high surfactant and reactant concentrations. The second assumption accounted for the retarding effect of an anionic surfactant, such as C10H21SO3Na, on nucleophilic substitution reactions. The third assumption reduced the number of fitted parameters. The model described well the experimental results over the wide range of concentrations employed for microemulsion and emulsion systems. Acknowledgment. We gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada and from McGill University through the Max Stern Fellowship awarded to M.M.H. LA991512Y
