1591
Langmuir 1990,6, 1591-1595
Reaction Kinetics in Microemulsion Medium. 1. Inversion of Cane Sugar in Quaternary System of Microemulsion Containing Water/Triton X-lOO/l-Butanol/(Cholesteryl Benzoate + n-Heptane) M. L. Das, P. K. Bhattacharya, and S. P. Moulir Department of Chemistry, Jadavpur University, Calcutta- 700032, India Received August 29, 1989. I n Final Form: December 27, 1989 The H30+ion catalyzed inversion of cane sugar was studied in water/poly(oxyethylene) isooctylphenyl ether/ 1-butanol/(cholesteryl benzoate + n-heptane) microemulsion medium at a constant surfactant/ cosurfactant ratio and at different oil/water ratios. Increasing the O/W ratio increased the rate, 0.570 (w/v) dextran and 2% (w/v) brine moderately retarded it, whereas urea retarded it markedly. In waterdioxane medium, increasing the proportion of dioxane increased the rate of inversion. The results are analyzed in light of the polarity of the medium as well as the effective concentration of the catalyst (H30+) in the microemulsion. The temperature effect on the rate of inversion revealed that with increased proportion of oil in microemulsion and dioxane in water the transition state complex ends up with a lower activation energy; the variation is systematic in the former but not in the latter.
Introduction Surfactant, cosurfactant, oil, and water can mix together t o form micro emulsion^^-^ which are isotropic a n d a thermodynamically stable liquid media. These microemulsions are used widely in the pharmaceutical field, in industry, and for enhanced oil r e c ~ v e r y They . ~ ~ ~have the prospect to be used as liquid membranes7-10 and reaction media,ll-15 but these aspects have been little explored. Microemulsions have some similarities to micelles which are known to influence18 reaction kinetics. The kinetics of reactions in microemulsion media are expected to exhibit distinct features: the microdroplets of either oil or water in the solution may provide surfaces to catalyze a reaction through convenient and preferential adsorption. Very recently, we have studied17J8the phase behaviors and physical properties of microemulsions using biologically compatible and biologically occurring compounds with a view to employ them as model biomembranes and reaction media. The present paper deals with the use of such a system composed of water/Triton X-100/ 1-butanol/(nheptane + cholesteryl benzoate) as a medium of a hydrolytic reaction: the acid-catalyzed inversion of cane (1)Hoar, T.P.; Schulman, J. H. Nature (London) 1943,152,102. (2)Shinoda, K.; Lindman, B. Langmuir 1987,3,135. (3)Microemulsions, Theory and Practice; Prince, L. M., Ed.;Academic Press: New York, 1977. (4)Winsor, P. A. Trans. Faraday SOC.1948,44,376. (5)Healy, R. N.; Reed, R. L.; Carpenter, C. W. SPEJ. SOC.Pet. Eng. J . 1975,15,87. (6)Gogarty, W.B. 81st National Meeting of AIChE, Kansas City, MO, 1976. (7) Xenakis, A. J. Colloid Interface Sci. 1987,117,442. (8)Small, D.M.; Bourges, M. Mol. Cryst. 1964,I , 541. (9)Fontell, K. froc. 5th Intl. Cong. Surf., Active Substances 1969,2, 1033. (10)Tondre, C.; Xenakis, A. Faraday Discuss. Chem. SOC.1984,77, 771. (11)Letts, K.;Mackay, R. A. Inorg. Chem. 1975,14, 2993. (12)Letts, K.; Mackay, R. A. Inorg. Chem. 1975,14,2990. (13)Tondre, C.; Xenakis, A. J. Phys. Chem. 1989,93,846. (14)Mishra, B.K.; Valaulikar, B. S.;Kunjappu, J. T.;Monohar, C. J . Colloid Interface Sci. 1989,127,373. (15)Fletcher, P. D.I.; Robinson, B. H.; Barrera, B.; Oakenfull, D. G. In Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1982. (16)Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (17)Das, M. L.; Bhattacharya, P. K.; Moulik, S.P. Ind. J. Biochem. Biophys. 1989,26,24. (18)Kunieda, H. J. Colloid Interface Sci. 1987,116 (l),224.
0743-7463/90/ 2406-1591$02.50/0
sugar. A comparative study of the inversion in waterdioxane medium has also been made for the understanding of the function of the microemulsion with respect to its polar-nonpolar characteristics.
Materials and Methods The materials (n-heptane, 1-butanol, and Triton X-100 ((CH&CH CH2(CH3)~CCH&H&~H40)9.~0H, having MW 680 and a cmc of 20 mmol dm-3at 300 K)) used for microemulsion formation were the same as described earlier." The sucrose, hydrochloric acid, urea, dextran, and sodium chloride were of Excelar grade BDH, England. Doubly distilled conductivity water of specific conductance 2-3 ps cm-l at 303 K was used to form the aqueous medium. All measurements were taken in a temperature-controlled room of fluctuations 0.2 K. In most of the rate studies, the hydrochloric acid concentration was kept at 0.69 mol dm-3, which was the optimum concentration. Below this concentration, it was difficult to dissolve sucrose in the microemulsion, and above it, the rate of inversion became too fast to be followed conveniently. The oil/water ratio was varied between 0.06 and 1.25; beyond 1.25, there appeared again the problem of sucrose solubility. Rate studies were also carried out in microemulsions and water both containing brine (2% w/v) and dextran (0.5% w/v) as well as three concentrations (0.1, 1,and 10 mmol dm-7 of Triton X-100. In the media of brine and dextran, however,the HCl concentration had to be kept at 0.85 mol dm-3. Below this acid concentration at oil/water (O/W) = 0.23, the microemulsion was unstable. Rate studies were also carried out both in microemulsion- and water-containing urea. The microemulsions consisted of water (or brine or aqueous dextran solution), oil (cholesteryl benzoate (1.83% w/v) in nheptane), surfactant (TX-loo), and cosurfactant (1-butanol). The preparation of the microemulsion has been described e1~ewhere.l~ The surfactant/cosurfactant ratio (S/CS) was maintained at 1:l. A Hilger polarimeter was used for the inversion study. In the actual experiment, the prepared microemulsion and all other components were kept in the temperaturecontrolled room well in advance of experiment for temperature equilibration. In a clean and dry stoppered conical flask (100 mL), a weighed amount of solid sucrose was taken, and to it were added requisite volumes of the microemulsion and stock HC1 of known concentration. Stirring gave a clear homogeneous solution. The final volume of the reaction mixture was 25 mL. The solution was immediately transferred into the polarimeter tube (2-dm length), and the reaction rate was followed by measuring the optical rotation at regular intervals of time. The reading at infinity was obtained after keeping the
*
0 1990 American Chemical Society
1592 Langmuir, Vol. 6, No. 10, 1990
Das et al.
s+cs
A
Time ( m i n )
/
Woter
" 20
60
LO
wt
80
\01
I
Vi,
Figure 1. Phase diagram of TX-lOO/l-butanol/(n-heptane+ CB)/0.69 mol dm-3 HC1 at S/CS = 1 at 307 K.
Figure 2. log ( a a- a - ) / ( a t- am)vs t plots for the inversion (300 K) in different environments at [H+]= 0.85 mol dm-3: ( 0 )water; ( 8 )0 . 5 5 dextran; ( 0 )2.0% brine; (0) 2.0% brine in microemulsion; (A)0.5 50 dextran in microemulsion; (0)microemulsion.
mixture overnight. Until otherwise stated, in all the cases the initial concentration of sucrose was 3 % (w/v) and that of HCl 0.69 mol dm-3. Besides sucrose, all other components were optically inactive.
Results and Discussion Figure 1shows the pseudoternary phase diagram of the microemulsion system at a 1:l S/CS ratio in 0.69 mol dm-3 HC1. The compositions of the solutions used for the kinetic study are indicated by full circles in the one-phase region, the O / W compositions being 0.06, 0.21, 0.80, and 1.25. Their selection was based on their ability to dissolve enough cane sugar to follow the changes in the optical activity during inversion. The rate equation of the H+ ion catalyzed inversion of the sugar (S) is
.61
12
-
5 E
0 Y
--dS - k'[H+][S] = k[S]
dt where t h e first-order rate constant k a t a constant temperature includes a particular concentration of the H+ ion. At different acidities, k should have different values. The rate constant was evaluated from the slope of the plot of log (a, - a m ) / ( a-t am)vs time, where aB,at,and am refer to angles of optical rotation of the reaction mixture a t the start, at time t, and a t the end of the inversion process, respectively. Figure 2 illustrates the kinetic plots for the observed pseudo-first-order inversion of sucrose at 300 K in various media. The rate (as well as the rate constant) increases in the order brine ( 2 % ) > dextran (0.5%) > water. The order is reversed in the microemulsion media. The rate, however, increased with increased (O/W) ratio a t S/CS = 1 (Figure 3). The effect of urea on the rate of inversion is presented in Figure 4; t h e amide has a prominent rate-decreasing effect. The surfactant TX100 (an ingredient of the microemulsion),at its critical micelle concentration (cmc) and above, exhibited a minor rateincreasing effect. The effect of dioxane on the rate of inversion was also studied; the rate decreased with increased proportion of dioxane (graphical presentation is not shown). In Table I, typical kinetic results at 300 K are presented. Role of Polarity of the Medium. The rate-increasing properties of the O/W microemulsion and aqueous dioxane media may not be simple and straightforward. Although both are isotropic combinations of polar and nonpolar media, the microstructures are entirely different. The
Figure 3. Rate constant for the inversion as a function of the oil/water ratio at different temperatures at [H+]= 0.69 mol dm-3: (0) 293 K; ( 8 )300 K; (A)307 K. important common property is low polarity (the dielectric constant t) of both media; that of the microemulsion is unknown, whereas precise values of the dielectric constant, e of aqueous dioxane mixtures are available in 1iterat~re.l~ The study in the water-dioxane medium may help one to understand the influence of the polarity of the medium on the rate of inversion; analogically the knowledge can be extended to explain the polarity effect in a microemulsion. The influences of the increased oil in the microemulsion and increased dioxane in water are parallel: both enhance the rate. An increased dielectric constant has an adverse effect on the rate of reaction. There is, however, an exception. In 30% (v/v) dioxane at 293 K, a comparative retardation of the rate has been observed. Beyond this composition at 293 K, and a t all other compositions a t 293, 300, and 307 K, the rate constants decrease with increased polarity of the medium. An effect similar to this has been reported by Kundu et a1.20 Ion(19) Timmermans, J. The Physico-Chemical Constants of Binary Systems in Condensed Solutions; Interscience Publishers: New York, 1960, Vol. 4. (20) Mandal, U.; Das, K.; Kundu, K. K. Can. J . Chem. 1986,64,1638.
Langmuir, Vol. 6, No. 10, 1990 1593
Reaction Kinetics in Microemulsion Medium
I
/
IVo
-2 5r 0 01
I
o
I
0 02
I
I
0 03
l/€ Timelmini
Figure 4. log (a, - n m ) / ( a-t am)vs t plots for the inversion (300 K) at different concentrations of urea solution. Curves 1-5 denote 1.74, 0.9, 0.47, and 0.19 mol dmT3urea, respectively, in microemulsion. Table I. Typical Kinetic Results of Cane Sugar Inversion at 300 K in Different Environments
system
water
dioxane-water ( o;8 v/v) 9.7 19.5 29.0 38.8 48.4 58.0 microemulsiona O / W (v/v) 0.06 0.21 0.80 1.25 microemulsion (O/W = 0.21) with (mol dm-3 urea) 0.2 0.47 0.92 1.74 water water with 0.5''< (w/v) dextran 2.0"; (w/v) brine 0.10 mmol dm-3 TX-100 1 mmol dm-3 TX-100 10 mmol dm-3 TX-100 1.74 mol dm-3 urea microemulsion (O/W = 0.16) with 0.5('( (w/v) dextran 2% (w/v) brine
dielectric constant
[H301+, equiv 102k, AG*,kJ dm-3 min-l mol-'
77
0.69 0.69
1.15
84.9
69.5 61.0 53.0 44.0 35.5 27.5
0.69 0.69 0.69 0.69 0.69 0.69
1.12 1.2 1.53 1.56 2.12 2.98
84.8 84.8 84.2 84.1 83.4 82.5
33.3 27.8 23.2 11.4
0.69 0.69 0.69
3.00 3.51 7.24
82.5 82.1 80.3
0.69 0.69 0.69 0.69 0.85
1.28 1.00 0.59 0.21 4.30
84.6 85.3 86.6 89.1 81.4
0.85 0.85 0.69 0.69 0.69 0.69
1.55 1.73 1.28 1.42 1.46 0.35
83.9 83.6 85.5 85.3 85.2 87.9
0.85 0.85
3.67 3.05
82.0 82.5
Dielectric constant values derived from the plot at 300 K shown in Figure 5.
dipole type reactions (acid hydrolysis of esters and inversion of sucrose are examples of this kind) are guided by the dielectric constant of the medium in accordance with the relation21 In k = In ko - (p2' - pA2 - pB2)/Ct (2) where k and ko are the rate constants in mixed solvent and (21)Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper and Row: New York, p 203.
Figure 5. log K vs t-l plots at different temperatures in aquodioxane medium. 0, A, and 0 refer to 293, 300, and 307 K,
respectively.
water, respectively; pz, PA, and p~ are the dipole moments of the activated complex, reactant A, and reactant B, respectively; C is a constant containing several usual terms; and t is the dielectric constant of the reaction medium. The plot of log k vs t-l should yield a negative slope which is normally observed for ester hydrolysis. The inversion in water-dioxane medium has yielded a positive slope (Figure 5). Analogically, a microemulsion medium with increased O / W ratio (i.e., with decreased dielectric constant) ought to yield a positive slope, which has been achieved in practice. This is possible if the dipole moment of the activated complex is lessz1than that of the sucrose (the dipole moment of the other interacting species, i.e., of H+ is zero). I t is intriguing to theoretically justify the proposition. Experimental determination of the dielectric constant of the microemulsion system is necessary for quantitative agreement between the rate constant and t according to the above proposition. Assuming that the rate-influencing effect is entirely due to the changed polarity of the medium, the values for the microemulsion solutions at different O / W ratios can be evaluated from the linear plots observed in the water-dioxane medium (Figure 5). These values a t 300 K for the O/W ratios 0.06,0.21,0.80, and 1.25 are 33.3,27.8,23.2, and 11.4, respectively (Table I). About a 20-fold increase in oil over water in microemulsion causes a 3-fold change in both t and k. On the other hand, about a 6-fold increase in dioxane in the water-dioxane medium also alters t and k by nearly 3-fold. The difference between the performance of the water-dioxane medium and the microemulsion medium is apparent. Effect of Local Concentration of Catalyst. The pseudo-first-order inversion kinetics must be independent of sucrose concentration b u t must depend on t h e concentration of the catalyst (H+ ion). In the discussion made above, the dielectric constant refers to bulk values. In the location of the reaction, it is the local or microscopic dielectric constant that influences the process. In the water-dioxane medium, because of complete miscibility, molecules of water and dioxane exist side by side without phasing out. But in the microemulsion, the water and oil are not in the same phase, and there is a sharp difference of the dielectric constant, the water phase having t only insignificantly less than pure water owing to the presence of HC1, sugar, dissolved head groups of TX-100, and 1butanol (approximately 7 5% v/v or less). Therefore, the rate-influencing property of the microemulsion does not
1594 Langmuir, Vol. 6, No. 10, 1990
Das et al.
depend on the polarity of the medium. The vital fact to our comprehension is the changed concentration of the catalyst (H+ ion) in the aqueous phase of the microemulsion. Though the inversion reaction is performed in microemulsion systems, the H30+ ions are expected to be present entirely in the aqueous part and not in the oil. With the increase in the O/W ratio, a decrease in the volume fractionofwater occurswithanenhancementofH~0+concentration in the aqueous phase. Consequently, more of the activated complex is formed with concomitant increase in the rate of reaction. Assuming that the water phase contains all the H30+ ions produced by HC1 in microemulsion, the effective concentration of H30+ has been estimated. There is also the possibility of water binding through hydration of the head groups of the surfactant TX100, and if this occurs, more free water will be lost from the aqueous phase with a further rise in the H30+ ion concentration. The effective H30+ ion concentration in the aqueous phase has also been calculated by taking the hydration of the ethylene oxide residue in TX-100 into account (hydration number taken to be 8 mol/mo1).22 On the basis of the modified acid concentrations, the rate constants have been calculated considering the linearity of the observed rate constant with [H30+]. Those with hydration and no hydration of TX-100 are presented in Figure 6A along with observed values. It is apparent that the experimentally obtained values of the rate constants are more in agreement with those calculated without hydration of the head groups of TX-100. This suggests that the hydration does not affect the catalytic concentration of H30+ ion. The fact that hydration of TX-100 hardly affects the rate of inversion gets support from the very minor effect of TX-100 micelles on the rate. The above procedure of the treatment of modified acid concentration when applied to the waterdioxane medium (considering both H30+ ions and glucose molecules remain entirely in association with the water molecules in the mixed solvent) produces curves presented in Figure 6B. Here also, the observed rate constants are fairly close to the calculated ones (the results deviate a t a higher percentage of dioxane). These results support that the effective concentration of H30f gets modified in the mixed media to influence the rate and hence the rate constant. Thermodynamics of Inversion. In the light of the transition-state theory, the inversion follows the course kl
S + H30+==SH30+
-. k
products
(3)
k-i
where SH3O+ is the activated complex that decomposes into products and kl and k-1 are the rate constants for the formation and the decomposition of the transition-state complex, respectively. The free energy of activation
AG = -RT In k , / k - , is related to the rate constant as
k = (RT/hN)e-AG'JRT where the terms have their usual significance. T h e energetics of the inversion process have been evaluated from the temperature effect on the rate constants by applying the Eyring equation. The maximum uncer(22) Moulik, S. P.; Mandal, A. B.; Biswas, A. M.; Roy, S. J. Phys. Chem. 1980, 34, 856. (23) Dasgupta, P. K.; Bhattacharya, P. K.; Moulik, S. P. Ind. J. Chem. 1984,23A, 192. (24) Dasgupta, P. K.; Moulik, S. P. J. Phys. Chem. 1987, 91,5826.
J
9k
,----"I rL
293K
r
I
I
I
05
15
10
o/w
IV/"l
I'
et
i d2
6t
2 2
I
,
20
,
LO Vol % D i o x a n e
, 60
Figure 6. (A) Rate constants of inversion as a function of oil/ water ratio in microemulsion at different temperatures at [H+] = 0.69 mol dm-3. In each set, curve 1 represents experimental results and curves 2 and 3 represent results on the basis of [H+] in the aqueous part of the solution without hydration of TX100 and with hydration of TX-100, respectively. (B) Rate constants of inversion as a function of vol 76 of dioxane in aquo= 0.69 mol dioxane medium at different temperatures at [H+] dm-3. In each set, curve 1 represents experimental results and curve 2 represents results on the basis of [H+] in the aqueous part of the solution. tainties of the derived energetic parameters AE* (energy of activation), AG* (free energy of activation), and AS' (entropy of activation) are k2.796, hl.096, and k0.7 74, respectively. The dependence of the activation parameters on the O / W ratio is presented in Figure 7. The entropy of activation varies between +60 J mol-' and -80 J mol-' for a O/W ratio ranging between 0 and 1.25. Hydrolysis of ester in aqueous medium normally offers a negative AS* (=-40J mol-'); the inversion of cane sugar provides a positive AS*. The activated complex favorably dissociates into products in the latter system and not in the former. A t O/W > 0.2, A S * is negative; the magnitude increases at higher ratios. Entrapped sugar molecules and H@+ ions in the dispersed water enclosures in the microemulsion help
Langmuir, Vol. 6, No. 10, 1990 1595
Reaction Kinetics in Microemulsion Medium
Figure 7.
Ib
310 40 V o l % Dioxane 210
5b
$0
I
I
10
20
I
1
40 Vol % Dioxane
30
, 50
1, 60
Figure 8. Thermodynamic parameters for the inversion as a function of vol % of dioxane at 300 K. catalyst,23.24 resulting in formation of a lesser amount of to form a favorable transition-state complex with lower the activated complex SH30+; consequently, it decreases activation energy which passes over to the product side. the observed rate of the reaction. Such a decrease has been In pure water, the complex transforms to a lesser extent also reported by Kundu et a1.20for the inversion and Mouto the products, and the corresponding rate of inversion lik et al.23for the acid-catalyzed hydrolysis of esters. The is smaller with larger M S .A systematic variation in the moderate rate-decreasing phenomenon witnessed for 2 76 energetic parameters has been observed with O / W brine and 0.5% dextran in microemulsion is due to the composition; the variation is nonsystematic in waterdecreased activity of H30+ ion by the additives. The TXdioxane medium (Figure 8). The curvature of the plot in 100 has a negligible rate-increasing effect; the formation Figure 8 a t 10% and 30% dioxane medium is thermoand the nature of the polar sugar-H30+ transition complex dynamically more complex than that in the microemulare hardly affected by TX-100 micelles. sion medium. The microaqueous pool in the microemulsion is different than normal water which is manifested Acknowledgment. The work was financed by a grant in the energetics of the inversion process. from t h e Department of Science a n d Technology, Influence of Other Additives. The additives used Government of India. have been urea, 2 7;) brine, 0.5 % dextran, and TX-100 of concentrations 0.1, 1, and 10 mmol dm-3. The significant Registry No. TX-100, 9002-93-1; CB, 604-32-0; 1-butanol, rate-decelerating effect of urea in both aqueous and mi71-36-3; n-heptane, 142-82-5; dextran, 9004-54-0; brine, 7647croemulsion media is by virtue of its influence on the 14-5; urea, 57-13-6; dioxane, 123-91-1; cane sugar, 57-50-1; hydrochloric acid, 7647-01-0. activity of the catalyst H+ ion. The amide deactivates the
