Physical Chemistry: Effect Of Cetylammonium Bromide Micelles On Decolouration Of Rosaniline Hydrochloride By Periodate

Abstract

Rosaniline hydrochloride (RH) is a triaminotriphenyl methane dye, which is widely used as biological stain, mordant, printing, in cotton tannings, and dyeing in textile industry. The reaction obeys first order kinetics with respect to RH and IO4-. The reaction is around hundred times faster in the presence of CTAB compared to aqueous medium under identical conditions. This is due to lower dielectric constant in micellar medium in which the transition state in more stabilized. The rate of the reaction increases with increase in concentration of CTAB up to 0.08 mol dm-3 and thereafter shows a limiting behavior with increase in micellar concentration. The monotonic increase indicates a unimolecular pathway. From Menger and Portnoy’s pre equilibrium kinetic model binding constant was found to be 21.84 mol-1dm3. Where we calculated theoretical binding constant KM = p = 39.2, where p is partition co-efficient and is molar volume which is obtained good agreement with of the same order and in good agreement with KM experimentally.

In order to account for the catalytic behavior of CTAB, theoretical calculations of single point energy and energy gap were done using GAUSSIAN 09 W program employing density functional theory (DFT) with Becke’s Three – parameters hybrid functional in combination of Lee-Yang-Parr functional implemented with LANL2DZ basis sets using conductor like polarizable continuum model (CPCM). (DFT/ B3LYP/ LANL2DZ/ CPCM (solvent = water)). Based on the structures of RH, KIO4 and CTAB, binding energies (BEs) were calculated for the dimer complexes RH — KIO4 and RH — KIO4 — CTAB. BEs of the RH — KIO4 and RH—KIO4 — CTAB systems have been found to be -60.4 kcal/mol and -70.4 kcal/mol, respectively. The high binding energy (RH—KIO4 — CTAB, -70.4 kcal/mol) indicates the catalytic behavior. The HLEG (HOMO-LUMO Energy Gap) measures the stability of the system. A large HLEG implies high stability (less reactive) and slight gap implies low stability (highly reactive). In this work, RH – KIO4 — CTAB system, the gap is reduced around 1.28 times than the RH – KIO4 system which clearly indicates that the CTAB surfactant plays a vital role in this reaction. It shows that catalytic behavior of CTAB by theoretical approach.

INTRODUCTION

Rosaniline hydrochloride (RH) which is a triaminotriphenyl methane dye, is widely used as biological stain, mordant, printing, in cotton tannings, and dyeing in textile industry1-6. These dyes enter the environment in natural waters. It is also an important constituent of Schiff’s reagent as an indicator for the detection of aldehydes and ketones7-9. The intermediates of these dyes are also encountered in several organic and enzymatic reactions. Studies on the chemical reactions of these dyes are therefore important. Since rates of reactions change in the presence of micelles the oxidation reactions of the dye in the presence of micelles will be helpful in exploiting their use in industries.

Babatunde et al10-12 have studied the reduction of rosaniline hydrochloride extensively in tri oxo carbonate (IV), hydroxide ion and nitrite ions. In these reactions, a stoichiometry of 1:1 was found for rosaniline hydrochloride and the reductant. In the reaction of rosaniline hydrochloride with chlorite a stoichiometry of 1:2 was found, the order of the reactions with respect to both the reactants was found to be two13. The rate of reaction increases with increase in ionic strength13. In the reduction of rosaniline hydrochloride by hydroxide ion11, the rate of reaction decreases with increase in ionic strength. The oxidation of malachite green, brilliant green and crystal violet14, other members of the tri amino tri phenyl methane dyes by peroxydisulphate was studied by Joshi et al in the presence of different micellar media. London Singh et al15 have studied the effect of anionic and non-ionic micelles on nucleophilic addition reaction of rosaniline hydrochloride (RH) with hydroxide ion. Mishra et al16 were reported that rate enhancement (2.25 fold) of hydroxylation of rosaniline hydrochloride by NaOH in the presence of CTAB micelles. Padma et al17 studied fading of rosaniline hydrochloride by periodate (IO4-) in presence of reverse micellar systems. Since rates of reactions are altered in the presence of micelles, kinetic studies of rosaniline hydrochloride by periodate in the presence of CTAB micelles have been carried out and the results are presented in this paper.

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RESULTS AND DISCUSSIONS

Experimental Analysis

The kinetic investigation of the decolouration of RH by IO4- has been carried out in the presence of CTAB micelles at constant ionic strength (μ) of 0.5 mol dm-3 under the experimental conditions [RH] > [RH]. Plots of log (At) versus time (where At is the absorbance at time t) were found to be perfectly linear for at least 95% of the reaction. The kinetic data are the averages from triplicate runs with a reproducibility ± 3%. Critical micellar concentration (CMC) of CTAB determined by surface tension measurements under the present experimental conditions was found to be 2.62×10-3 mol dm-3. The theoretical calculations of single point energy and energy gap were done using GAUSSIAN 09 W program22 employing density functional theory (DFT) with Becke’s Three – parameters hybrid functional in combination of Lee-Yang-Parr functional23, 24 implemented with LANL2DZ basis sets using conductor like polarizable continuum model (CPCM). In this method water consider as solvent (DFT/ B3LYP/ LANL2DZ/ CPCM (solvent = water)).

Conclusions

The reaction between rosaniline hydrochloride and periodate is around hundred times faster in CTAB micelles compared to aqueous medium under identical conditions. Since CTAB is an ionic surfactant, in addition to micellar medium effect and proximity effect of reactants, ionic strength of ionic micellar surface and electrostatic interactions between reactants and ionic head groups have effect on rates of reactions.

The effect of CTAB on the rate of the reaction confirms to a unimolecular pathway as shown by limiting behavior in a rate–surfactant profile. The value of KM obtained from the plot of 1/k' vs. 1/CM was found to be 21.84 mol-1 dm3 and is of the same order as the calculated value i.e., 39.2. This reasonably large binding constant is indicative of a strong interaction between reactants and CTAB micelles. BEs of the RH — KIO4 and RH—KIO4 — CTAB systems have been found to be -60.4 kcal/mol and -70.4 kcal/mol, respectively. The above BE value fairly suggest that the substrates – surfactant complex RH — KIO4— CTAB is more stable than the dimer complex RH — KIO4. The high binding energy facilitates the rate of reaction. The HLEGs of the RH – KIO4 and RH – KIO4 – CTAB are 1.22 eV and 0.95 eV respectively which indicate that the RH – KIO4 — CTAB is chemically more reactive than the RH – KIO4 system.

The HLEG measures the stability of the system. A large HLEG implies high stability (less reactive) and slight gap implies low stability (highly reactive). In this work, RH – KIO4 — CTAB system, the gap is reduced around 1.28 times than the RH – KIO4 system which clearly indicates that the CTAB surfactant plays a vital role in this reaction.

REFERENCES

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