Catalytic effect of CTAB reverse micelles on the reduction of toluidine blue by ascorbic acid

Transcription

1 Indian Journal of Chemistry Vol. 5A, February 013, pp. 15 Catalytic effect of CTAB reverse micelles on the reduction of toluidine blue by ascorbic acid M Padma a, P Shyamala a, *, A Satyanarayana a, P V Subba Rao b & Asha Mathew a a Department of Physical and Nuclear Chemistry & Chemical ceanography, School of Chemistry, Andhra University, Visahapatnam, , India shyamalapulipaa@rediffmail.com b Department of Chemistry, Gayatri Vidya Parishad, Visahapatnam, , India Received 6 June 01; revised and accepted 15 January 013 The inetics of the reduction of toluidine blue by ascorbic acid has been studied in the presence of CTAB/CHCl 3 /hexane/water reverse micelles. In the absence of added acid, the reaction obeys simple first order inetics with respect to both the reactants. The second order rate constant in the presence of reverse micelles, w (0.07 M 1 s 1 ) was found to be about sixty times greater than that in aqueous medium, o ( M 1 s 1 ) under identical experimental conditions. The significant increase is attributed to the concentration effect in the reverse micelles, and, lower micropolarity due to the reverse micelles, which facilitates the reaction between oppositely charged ions. The effect of variation of W ( [H ]/[CTAB]) at constant [CTAB] and variation of [CTAB] at fixed W has been studied. The inetics at W < 6 is explained on the basis of the special properties of the entrapped water, while the inetics at W > 10 is explained on the basis of ionic strength. Keywords: Kinetics, Toluidine blue, Ascorbic acid, Water pools, CTAB, Micelles, Reverse micelles Reverse micelles and microemulsions are the subject of increasing interest as reaction rates, reaction pathways and equilibrium positions are greatly altered in these systems as compared to conventional solvents 13. This is particularly true in the case of enzyme catalysed reactions where water droplets provide a unique microenvironment for proteins and serve as powerful models for biological systems 4. Reverse micelles, which are nanosized aqueous droplets encapsulated by surfactant molecules existing at various compositions, are used widely in the synthesis of many types of nanoparticles 5. When surfactants aggregate in nonpolar solvents, their polar or charged groups are located in the interior or core of the aggregate while their hydrocarbon tails extend into the bul solvent. Thus, water is readily solubilised in the polar core, forming a water pool and this solubilised water exhibits peculiar properties characterised by a parameter W ( [H ]/[Surfactant]) 6. When the reactants are in the water pool they are concentrated and act as a nanoreactor leading to a concentration effect in the micellar pseudophase. At low water contents where W 5, the water is structured due to interactions with the polar head groups and counter ions. When W > 1 unstructured free water appears in the water pools. The properties of the structured water are different from those of free water and these properties alter the reactions rates. For example, the solubilising capacities of the two pseudophases are different; the structured water has comparatively lower dielectric constant, lower mobility and a high ionic strength due to the counter ion in the case of ionic surfactants. Systematic studies on the inetics of electron transfer reactions in the presence of CTAB/CHCl 3 /hexane/water reverse micelles are limited. In our previous wor, we have studied the aquation of tris, bipyridyl iron(ii) 7, base hydrolysis of tris1,10 phenanthroline iron(ii) 8 and oxidation of iodide by V(V) 9 in the presence of CTAB reverse micelles. The inetics of toluidine blueascorbic acid reaction in the presence of CTAB and SDS aqueous micelles has been studied 10,11. In order to investigate further the influence of these unique properties of the water pools on inetics of electron transfer reactions, we have taen up a inetic study of the reduction of toluidine blue by ascorbic acid in the presence of CTAB reverse micelles. Experimental All the reagents used were of analytical reagent grade and solutions were prepared in doubly distilled water. Toluidine blue () (3amino7dimethyl amino methyl phenazathionium chloride) and ascorbic acid (H A) were purchased from Aldrich and were used as received. Stoc solutions of toluidine blue M and ascorbic acid 0.6 M were prepared by dissolving the appropriate amounts of these compounds in doubly distilled water. Cetyl trimethyl ammonium bromide (CTAB) received from Merc (India) was used without further purification. Hexane and chloroform were distilled before use.

2 INDIAN J CHEM, SEC A, FEBRUARY 013 A 0.1 M solution of CTAB was prepared by dissolving appropriate amount of CTAB in a chloroformhexane (3: v/v) mixture. A typical experiment was carried out as follows: Small quantities of aqueous solutions of toluidine blue ( L) were injected into L of CTAB solution using a micro pipette. Ascorbic acid ( L) was then added to initiate the reaction. The mixtures were shaen sufficiently to obtain a transparent and apparently homogenous solution that can be regarded as a reverse micellar system. In all the experiments, the molar ratio of [Water] to [CTAB], i.e., W was varied in the range (corresponding to L L of water). Although the reverse micellar systems are heterogeneous on the molecular scale, they are macroscopically homogeneous and isotropic. Since they are optically transparent, a change in the concentration of the reactant/product can be followed spectrophotometrically. The reduction inetics of toluidine blue ( ) with ascorbic acid ( ) was carried out under pseudofirst order conditions, i.e., [ ] >> [ ], by taing the initial concentration of toluidine blue as M and that of ascorbic acid in the range M. The reactions were monitored by measuring the decrease in absorbance of at 65 nm using a Shimadzu UV1800 spectrophotometer. The temperature was maintained at 30.0±0.1 K in the reaction cell of the spectrophotometer with a thermostatic bath. The observed first order rate constants were calculated by a linear least squares method from the relationship log (A t A ) versus time. The inetic data are the averages from triplicate runs with reproducibility less than ±5%. Results and discussion In the studies on reactions in reverse micelles involving watersoluble reactants, the reactivity in the aqueous droplets of the reverse micelle is usually compared with that in bul aqueous medium. In the case of first order reactions the measured rate constants may be compared directly in both the systems. However, in the case of bimolecular processes, the calculation of the second order constant involves consideration of the concentration of the species in the reverse micelle. Also, the question arises as to whether the overall concentration or the concentration in the entrapped water, i.e., the effective concentration, should be taen into account. In the present reaction, the reduction of toluidine blue by ascorbic acid has been investigated in neutral medium. In the neutral medium, the negatively charged species of ascorbic acid, i.e.,, is the dominant species 10,11,1 (pk a1 4.17, pk a 11.57). Since both the reactants, i.e., and, are ionic, they cannot exist in the oil phase and are constrained to the solubilised water only. Therefore, their effective concentrations are considered for calculation of. For the calculation of second order rate constant, the effective concentration of a reactant in the water pool is calculated by dividing the overall concentration by the volume fraction (f) of solubilised water 13. The volume fractions lie in the range ( ) 10 3 L for L of 0.1 M CTAB, and therefore, there is a fold increase in concentration. For example, if the concentration of CTAB is 0.1 M, W 4.61, f (0.083/10) and [ ] overall M then [ ] effective {[ ] overall /f } { /0.0083} M. Herein, the subscript o is used to represent overall concentrations and subscript e to represent effective concentration. The reaction between and in CTAB/CHCl 3 /hexane/water mixtures obeys first order inetics with respect to as observed by the linear plots of log (absorbance) versus time, under the condition of {[ ] o >>[ ] o }. The pseudofirst order rate constant is directly proportional to [ ], indicating first order inetics with respect to the [ ] (Table 1). The reaction is shown in Scheme 1. The second order rate constant,, has been determined over a wide range of W ( ) at constant [CTAB] and also at varying [CTAB] (Table ). At constant [CTAB], increases with W but in the range of W , the increase in is much less compared to the increase in in the range In the lower W range, the confined water is utilised to solvate the ionic surfactant head groups and counter ions, and therefore, the reaction can be considered to tae place at the interface. The water is highly structured in this low W range and Table 1 Effect of varying [ ] on the observed rate constants ( ). {[ ] M; [CTAB] M; T 30.0 K} 10 3 [ ] o 10 [ ] e W 5.61 W (s 1 ) 10 3 [ ] e 10 3 (s 1 )

3 NTES 3 CH3 N N N S Cl H NH N S NH is associated with special properties lie lower micro polarity. The reaction, being a cationanion reaction, is facilitated by the lower micropolarity of the structured water at the interface. With increase in W, the micropolarity increases, resulting in decrease in rate. At the same time, with increasing W, the ionic strength decreases, and therefore, should increase since lower ionic strength favours a cation anion reaction. In the case of ionic surfactants, the value of W regulates the concentration of polar heads of the surfactant (CTA ) and its counter ions (Br ) in the aqueous water pool, and therefore, W gives the value of ionic strength 14. For example, at W 5.61 and [Br ] e µ and [Br ] o /f 0.1/ M. For a change in W from to 5.0, the ionic strength ( [Br ] e ) changes from 63.8 M to. M. The combined effect of these two factors results in a small overall increase in indicating a slight predominance of ionic strength effect over polarity effect. At higher W ( ), the increase in is much more pronounced. In this range, the special properties of water become extinct and it is no longer micropolar. Thus, the polarity effect on is not significant. The acceleration of the reaction is due to decrease in ionic strength with increase in W. Under these conditions, the reaction can be considered to have two components, one at the interface and other in the water pool. With increase in W, the latter component becomes increasingly important. At high W (W > 10) the entrapped water behaves lie bul water and assuming that only ionic strength effect is present, the effect of W has been H H CH H Toluidine blue Ascorbate anion ( ) HCl Leuco toluidine blue Scheme 1 H H CH H Dehydro ascorbic acid Table bserved first order ( ) and second order rate constants ( ) for the reduction of toluidine blue by ascorbic acid. {[ ] M; [ ] M; T 30.0 K} [CTAB] 0 assessed in terms of ionic strength relating the rate constant (Eq. 1), 0 # (1) where, and # are the activity coefficients of, and the transition state and 0 is the rate constant at zero ionic strength. According to Guggenheim equation 15, the activity coefficient i of an ion is related to ionic strength (µ) (Eq. ), log i [Br ] e iµ AZ 1 µ 10 3 f (vol. fraction) B j i, j C W j 10 3 (s 1 ) 10 3 (M 1 s 1 ) ()

4 4 INDIAN J CHEM, SEC A, FEBRUARY 013 where A is equal to 0.5, Z i is the charge on the ions and the summation extends over all ions of concentration C j. In the present case, the relevant specific interactions are between positively charged micellar surface and and and Br, represented respectively by the specific interaction terms B M,, B,Br. The specific interactions between ions whose concentrations are very small as compared to the concentration of Br have been neglected. Using Eqs (1) and (), it can be shown that log log 1 µ B,Br B M, [Br ] e [M] e (3) and [M] e is equal to [CTAB] e /N where N is the aggregation number and [Br ] e is equal to [CTAB] e since the source of Br is only CTAB. log log B {[Br ] e / } N 1 µ M, (4) B [Br ],Br e log log b[br ] e (5) 1 µ With increase in CTAB concentrations at constant W, the second order rate constant increases. This study has been carried out in a detailed manner at low W (5.61) where the entrapped water is highly structured and is associated with lower micro polarity compared to bul water. It has been reported that the Berezin pseudophase model is applicable to reactions taing place in reverse micellar media 16,17 and according to this model, the second order rate constant is given by Eq. (6), m P P (1 K m pk (1 K CV W C)(1 K (1CV) C) C w(1 CV ) C)(1 K C) (6) (7) where m is the rate constant of the reaction at the interface, w is the rate constant of the entrapped water, C is the concentration of CTAB, V is the molar volume of surfactant and P, P are the partition coefficients of ascorbic acid and toluidine blue respectively. K, K are the binding constants of toluidine blue, ascorbic acid and K P V and K P V. Neglecting CV in comparison with 1 and if 1 (K )C and (K )C, Eq. (7) becomes Eq. (8). where b (B / N ) (B ) (B ) Br M,, #,Br The ionic strength of the water pool at W 5.61 has been calculated to be 9.91 M and at W 5.0, it is. M. It is nown that at high ionic strengths, variation of the second term in Eq. () with µ can be neglected in comparison with the third term 14. If this is true, a plot of log versus [Br ] e should be linear. Interestingly, the Guggenheim plots are linear and parallel to each other for data at different [CTAB] (Fig. 1). The specific interaction effect is included in the value of b in Eq. (5), which is equal to the slope of the plot between log versus [Br ] e.the value of b includes the interaction effect of both the interface and water pool components and was found to be 0.15, 0.11, 0.14 for 0.1, 0. and 0.3 M [CTAB] respectively. Fig. 1 Effect of ionic strength (µ) ( effective bromide ion concentration [Br ] e ) on reduction of toluidine blue by ascorbic acid, [1, 0.1 M CTAB;, 0. M CTAB; 3, 0.3 M CTAB].

5 NTES 5 o (0.30 M 1 s 1 ). This shows that the properties of the free, unstructured water at W 5.0 are very similar to that of the bul water in the aqueous medium. The present study shows that the reduction of toluidine blue by ascorbic acid is around 60 times faster in the CTAB/CHCl 3 /hexane/water reverse micellar medium compared to conventional aqueous medium. The significant increase is attributed to the concentration effect produced in the reverse micelles and lower micropolarity of the reverse micelles, which facilitates the reaction between oppositely charged ions. Fig. Effect of [CTAB] at varying water:ctab ratio on the reduction of toluidine blue by ascorbic acid. [1, 0.1 M CTAB;, 0. M CTAB]. mp K C w (1 CV) (8) In the present case, a plot of versus C is linear with a positive intercept (Fig. ) supporting the assumption, 1>> (K )C and (K )C. The slope corresponds to m P and intercept gives the rate constant ( w ) for the reaction in the entrapped water. The value of w (0.07 M 1 s 1 ) is about 60 times greater than the rate constant of the reaction in aqueous medium in the absence of surfactant o ( M 1 s 1 ) and bromide ion. The value of o should have been much smaller at this ionic strength i.e., µ 9.91 M (at W 5.61), but this high ionic strength cannot be achieved in aqueous medium to determine the value of o under identical conditions. This large increase of rate in the reverse micelles illustrates the special properties of entrapped water, i.e., micropolarity at low W. The Berezin pseudophase model has been applied to the data corresponding to higher W also (W 5.0) and the plot of versus [CTAB] o is linear with a positive intercept. The value of w (0.185 M 1 s 1 ) is very close to the rate constant of the reaction in aqueous medium at the same ionic strength in the absence of surfactant, Acnowledgement MP and PS than University Grants Commission, New Delhi, India for financial assistance under the major research project: F No. 370/009(SR). References 1 Diamond J M & Wright E M, Ann Rev Physiol, 31 (1969) 581. Fletcher P D I, Donohue J A & Kevan L, J Phys Chem, 95 (1991) Bardez E, Monnier E & Valuer B, J Colloid Interf Sci, 11 (1986) Mariano Correa N & Juana J, J Mol Liq, 7 (1997) Reverse Micelles edited by Luisi P L & Straube B E, (Plenum Press, New Yor) Wong M, Thomas J K & Nowa T, J Am Chem Soc, 99 (1977) Shyamala P, Subba Rao P V & Ramarishna K, Indian J Chem, 39A (000) Venateswarlu G, Shyamala P, Subba Rao P V & Ramarishna K, Indian J Chem, 41A (00) Shyamala P & Subba Rao P V, Kinet and Catal, 51 (010) Arian B & Tuncay M, Dyes Pigm, 1 (005) Arian B & Tuncay M, Colloids Surf, 73 (006) 0. 1 Taqui Khan M M & Matell A E, J Am Chem Soc, 89 (1967) Reverse Micelles Vol. 73 edited by Fletcher P D I, Howe A M, Robinson B H, Steytler D C, Luisi P L & Straub B (1984). 14 Munoz E, GomezHerrera C, Garciani M, Moya M L & Sanchez F, J Chem Soc Faraday Trans, 87(1) (1991) Guggenheim E A, Phil Mag, 19 (1935) Silber J J, Biasutti A, Abuin E & Lissi E, Adv In Colloid and Interface Sci, 8 (1999) Fletcher P D I & Robinson B H, J Chem Soc, Faraday Trans 1, 80 (1984) 417.