1

Catalysis in reverse micellar media: Kinetics of fading of rosaniline

hydrochloride by periodate in AOT/heptane and CTAB/CHCl3/hexane

systems.

M.Padmaa, P.Shyamala*a, K.Suneethab, K.Sundara Raoa

a.Department of Physical and Nuclear Chemistry &Chemical Oceanography,

School of Chemistry, Andhra University, Visakhapatnam-530003.

b.Department of Chemistry, Govt. Arts college (Autonomous), Rajahmundry,

A.P.

ABSTRACT

The kinetics of fading of rosaniline hydrochloride (RA+) by periodate (IO4- ) was studied in

two different reverse micellar systems AOT/heptane and CTAB/CHCl3/hexane. The reaction

between RA+ and IO4- obeys first order kinetics with respect to RA+ and IO4

- in both the

reverse micellar media. In the AOT system, the pseudo first order rate constant was found to

be fifteen times greater than in aqueous medium, while in the CTAB system the rate increases

by three hundred times. The significant increase in reverse micelles is attributed to the unique

properties of the water pools. The study was carried at different surfactant concentrations and

different W = ([H2O]/[Surfactant]) values and Berezin pseudo phase model was used to

explain the effect of surfactant. Decreasing trend in the activation parameters in the reverse

micelles compared to aqueous medium also indicates the acceleration in reverse micelles.

Key Words: AOT, CTAB, reverse micelles, water pool, rosaniline hydrochloride.

*E-mail: shyamalapulipaka@rediffmail.com

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INTRODUCTION

When surfactants aggregate in nonpolar solvents forming reverse micelles, their polar

or charged groups are located in the interior or core of the aggregate while their hydrocarbon

tails extend into the bulk solvent. Water is readily solubilised in the polar core which exhibits

peculiar properties characterised by a parameter W=([H2O]/[Surfactant]).1 At low water

contents W≤5, the water is structured due to interactions with the polar head groups and

counter ions. When W>12 unstructured ‘free’ water appears and a water pool is formed. The

properties of the structured water are different from free water and these properties alter

reactions rates. For example, the solubilising capacities of the two pseudo phases are

different; the structured water has comparatively low dielectric constant, low mobility and a

high ionic strength (due to the counter ions in the case of ionic surfactants). The pH

experienced by a species is also dependent on W and type of surfactant used, hence pKa of

charged species changes.

In our previous work, we have studied the aquation of tris-2, 2′ bipyridyl iron (II),

base hydrolysis of tris-1,10′ phenanthroline iron (II) and oxidation of iodide by V(V) in the

presence of CTAB reverse micelles. 2-4 We have extended our work to the study of fading of

rosaniline hydrochloride (RA+) by IO4 - in AOT/heptane and CTAB/CHCl3/hexane along with

the determination of activation parameters. The results are reported.

EXPERIMENTAL

Materials:

Stock solutions of rosaniline hydrochloride (BDH), sodium periodate (Merck India)

were prepared in doubly distilled water. Chloroform, hexane and heptane were distilled

before use. CTAB (Cetyl tri methyl ammonium bromide) purchased from (Merck India) and

AOT (Aerosol-T) from Sigma-Aldrich were used without further purification. A 0.1mol/l

solution was prepared by dissolving an appropriate amount of CTAB in chloroform-hexane

(3:2 v/v) mixture and AOT in heptane.

Kinetic measurement:

The kinetic study of fading of RA+ by IO4- was carried out using a shimadzu uv-1800

double beam spectrophotometer by measuring the decrease in absorbance of RA+ at a wave

length of 555nm. The reaction was carried out at a temperature of 29oC under pseudo first

order condition, [IO4- ]>> [RA+]. The concentration of RA+ was 7.9×10-6 mol/l and IO4

- was

varied from 0.47×10-3 to 5.8×10-3mol/l. In all cases good linear plots were obtained for log

3

(absorbance) versus time (Fig.1). The kinetic data are the averages from triplicate runs with

reproducibility less than ±5%. Activation parameters were obtained from the linear fit of log

(absorbance) versus time.

Stoichiometry:

The stoichiometry of the reaction was determined in both the reverse micellar

systems, AOT/heptane and CTAB/chloroform/hexane by spectrophotometry using mole ratio

method, keeping the concentration of the rosaniline hydrochloride constant at 1×10-5mol/l and

varying the concentration of IO4- from 0.5×10-5 to 50×10-5mol/l. The absorbance of the

reaction mixture was measured at λmax=555, T=29oC after the reaction has gone to completion

over a period of 48 hours (the absorbance corresponding to unreacted rosaniline). The mole

ratio corresponding to the intersection point from the plot of absorbance vs mole ratio

([RA+]/[IO4- ]) indicates that one mole of the dye was consumed by one mole of periodate in

both the reverse micellar systems. Similar results were reported by O.A. Babatunde et al.5

Product analysis:

The product analysis was carried out by mixing equimolar amount of dye and IO4- .

After the completion of the reaction a colourless solution indicating destruction of the

quinonoid group was obtained and uv visible spectrum of the product showed no absorption

peak at λmax=555nm. The other product, IO3- (Fig.2), was confirmed qualitatively through the

starch-indicator test for liberated I2 when KI was added to the reaction mixture at the end of

the reaction. 6

Preparation of the reverse micellar system and initiation of the reaction:

A typical experiment was carried out as follows: Aqueous solution of rosaniline

hydrochloride (0.015×0-3 L) was injected into 10 ml of 0.1 mol/ surfactant solution using a

micro pipette. Periodate (0.045×10-3L) was then added to initiate the reaction. These mixtures

were shaken sufficiently to obtain a transparent and apparently homogenous solution that can

be regarded as a reverse micellar system. In the case of CTAB system, the molar ratio of

[Water] to [CTAB] i.e. W was varied in the range 1.33-16.0 and in the AOT system from 5.0

to 12.0.

Calculation of the effective Concentrations in reverse micelles:

In the study of reactions in reverse micelles involving water-soluble reactants, the

reactivity in the aqueous droplets of the reverse micelle is usually compared with that in bulk

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aqueous medium. In the case of first order reactions the measured rate constants may be

compared directly in the two types of systems. However, in the case of bimolecular processes

taking place in the water pools or at the interface of the reverse micelles, the calculation of

second order rate constant involves the consideration of effective concentration of the

reactants. The effective concentration of a reactant in the water pool is calculated by dividing

the overall concentration by the volume fraction of solubilised water, f (= volume of aqueous

phase/ total volume of the solution). 7 The volume fractions lie in the range 6.00 to 28.8×10-3

for 0.1mol/l CTAB. Hence there is a 166.63 to 34.70 fold increase in concentrations. For

example, if the concentration of CTAB is 0.1 mol/l and W=6.00 then f =(0.108/10). If [IO4-]

overall =1.87×10–3 then [IO4- ]effective=[IO4

- ] overall /f}={1.87×10–3/0.0108}= 0.173mol/l. From now

onwards, the subscript ‘o’ is used to represent overall concentration and subscript ‘e’ to

represent effective concentrations.

RESULTS AND DISCUSSION

Fading of RA+ by IO4- in AOT/ heptane and CTAB/chloroform/hexane:

The reaction between RA+ and IO4- in both the reverse micellar media obey first order

kinetics with respect to RA+ as observed by the linear plots between log(absorbance) versus

time under the conditions {[RA+] << [IO4- ]}. The pseudo first order rate constant is directly

proportional to [IO4- ] indicating first order kinetics with respect to IO4

- (Table I, III). Based on

these results and stoichiometry determined by mole ratio method the mechanism is given

below.

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Effect of variation of W and AOT in AOT/heptane:

The reaction is markedly accelerated in the presence of both AOT and CTAB reverse

micelles. The significance increase in the presence of reverse micelles is due to the unique

properties of the solubilised water of reverse micelles like low micro polarity and high ionic

strength. The pseudo first order rate constant ko and second order rate constant k2 have been

determined over a W range 5-12 at constant [AOT] and has been repeated at different

concentrations of AOT (Table II). At constant [AOT], ko decreases with W and also decreases

with increase in concentration AOT. This is because of two reasons (i) at low W, most of the

water present is bound and at W>4, ″free″ water appears.8 RA+ being hydrophobic and by

electrostatic interactions gets attached to the surfactant chains irrespective of the size of the

water pool. The IO4- is present in the aqueous water pool because it is repelled by LS- group

of AOT. For small W, the probability of IO4- being nearer to RA+ is more since the movement

of IO4- is restricted. As W increases, IO4

- is present in the central aqueous water pool and less

vulnerable to attack by the RA+ which is left at the interface. (ii) At low W, dielectric constant

is less which favours a cation-anion reaction like the present one. Since the reaction obeys

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first order kinetics with respect to [IO4- ], the second order rate constant is calculated as k2=

ko/[IO4- ]e as given in table II. It can be seen that k2 is nearly constant for all values of W for all

concentrations of AOT. The constancy is explained as follows: As W increases ionic strength

decreases and dielectric constant increases. These two have opposing effects on the rate of

the cation-anion reaction and their effects cancel each other. Therefore only concentration

effect exists and it is the only factor responsible for acceleration.

Effect of W and CTAB in CTAB/CHCl3/hexane/Water:

The rate of the reaction increases around three hundred times more than in the

aqueous medium while the rate increases only by fifteen times in the presence of AOT

reverse micelles. Larger increase of rate in CTAB micelles is attributed to the existence of

electrostatic interactions between IO4- and CTA+ in addition to the special properties. Table IV

shows that at constant [CTAB], both ko and k2 increase with W. At low W, the reaction being

a cation-anion reaction is facilitated by lower micropolarity of structured water. At the same

time, with increase in W, ionic strength decreases which favours a cation-anion reaction (in

the case of ionic surfactants, the value of W regulates the concentration of CTA+ polar heads

of the surfactant and its counter ions, Br- in the aqueous water pool and therefore W gives the

value of ionic strength, µ).9 At higher W(>6)micro polarity becomes closer to water and its

effect is not significant. The acceleration of the reaction is only due to decrease in ionic

strength with increase in W. Therefore the effect of W has been assessed in terms of ionic

strength relating the rate constant k2

k 2=k2o

γRA+ γ

IO4−

γ¿.. .. . ..( I )

Where IO4,-RA+ and # are the activity coefficients of IO4

- , RA+ and the transition state and

k2o is the rate constant at zero ionic strength. According to Guggenheim equation, the activity

coefficient i of an ion is related to ionic strength (µ)

−log γi=AZi

2 μ1 /2

1+μ1/2 −∑j

B i , j C j. . .. .( II )

Where A is equal to 0.5, Zi is the charge on the ions and the summation for specific

interaction term (Bi,j) extending over all ions of concentration Cj. In the present case, the

relevant specific interactions are between positively charged micellar surface, M and IO4- &

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RA+ and Br– represented respectively by the specific interaction terms BM, IO4-, BRA+, Br-. Using

equations (I) and (II), it can be shown that

log k 2=log k2o− μ1/2

1+μ1/2+b[ Br− ]e .. . ..( III )

(BM, IO4–/N) + (BRA+, Br-)

A plot of log k2 versus [Br-]e{=µ} was found to be linear at different concentrations of

CTAB(Fig.3) and the specific interaction effect included in the value of ‘b’ represents the

slope of plot between log k2 and [Br–]e. The Value of ‘b’ was found to be-0.071, -0.074, and -

0.070 for 0.1, 0.2 and 0.3 mol/l CTAB respectively. Since there is effect of W on rate even at

W=16 the value of ‘b’ includes the specific interaction effect of both the interface and water

pool components.

At constant W, with increase in CTAB concentration, the second order rate constant

increases. This study has been carried out in a detailed manner at W (=3.33, 10.0,16.0). It has

been reported that the Berezin pseudo phase model is applicable to reactions taking place in

reverse micellar media 10, 11 and according to this model, the second order rate constant k2 is

given by

k 2=km P

RA+ KIO4

=C+kw (1−CV )

(1+K IO4−

C )(1+K RA+ C ).. .. .( IV )

In the above equation, km is the rate constant of the reaction at the micellar interface, kw is the

rate constant of water pool, C is the concentration of CTAB, V is the molar volume of CTAB.

PRA+ is the partition coefficient of rosaniline. KRA

+, KIO4– are the binding constants of rosaniline

and periodate KRA+=PRA

+V, and KIO4–=PIO4

–V. Neglecting CV in comparison to 1 and taking 1

≥ (KRA+).C (since IO4

- is strongly bound to the micellar surface while RA+ is weakly repelled).

Also since there is an increase in rate with [CTAB], km>kw. The above equation now becomes

k 2=km P

RA+ KIO4

−C

(1+K IO4−

C ). .. .. (V )

1k2

= 1k m P

RA+ KIO4

−C+ 1

km PRA+

. . .. .(VI )

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If above equation holds good, the plot of 1/k2 vs 1/C should be linear and such a plot was

obtained showing the validity of Berezin pseudophase model to reverse micelles (Fig.4). The

values of KIO4- at different values of W, calculated from the slope and intercept using eq.VI are

given below

W (dm3mol-1)

KIO4

−

3.33 10.02

10.0 3.10

16.0 1.98

The binding constant KIO4- of periodate with micellar interface decreases with increasing W.

This can be explained as follows. At low W values the negative periodate is bound to the

cationic interface by electrostatic forces. As the size of water pools increase the water soluble

periodate goes more into the bulk water decreasing the binding constant.

Activation parameters:

The temperature effect on the structural properties of a reverse micelle has been

studied by Lang et al. They reported a 25-35% decrease in the radius of the droplet for a

temperature increase of 35oC.12 Thus change in temperature alters the structure of a reverse

micelle which influences the kinetics of reactions. In the present study the temperature

variation was 24-34oC, hence very little structural changes can be expected. Good linear plots

were obtained for Arhenius equation and the activation parameters derived from Arhenius

and Eyring equation are shown in table V. The activation energies are much lower in CTAB,

followed by AOT reverse micelles compared to aqueous medium. This is also a proof for the

accelerating effects in the reverse micelles. The enthalpy, entropy and free energy of

activation also show the same trend.

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CONCLUSION:

The reaction is markedly accelerated in the presence of both AOT and CTAB reverse

micelles. The significanct increase in the presence of reverse micelles is attributed to the

unique properties of the solubilised water of reverse micelles like low micro polarity and high

ionic strength, while the rate constant was found to be fifteen times greater in AOT compared

to aqueous medium, the rate increases by three hundred times in CTAB reverse micelles.

Large increase of rate in CTAB micelles is attributed to the existence of electrostatic

interactions between IO4- and CTA+ in addition to the special properties. The effect of

variation of W and [surfactant] on rate was explained in terms of concentration effect in AOT

reverse micelles and in terms of ionic strengths effect in CTAB.

ACKNOWLEDGEMENTS

Two of us (PS&MP) gratefully acknowledge the financial support work by University Grants

Commission, New Delhi, India under the major research project grant F.No.37-20/2009(SR).

REFERENCES

1. Wong M., Thomas J.K., Nowak T. J. American. Chem. Soc. 99, (1977), 4730.

2. Shyamala P., SubbaRao P.V., Ramakrishna K Indian. J. Chem. 39A, (2000), 643.

10

3. Venkateswarlu G., Shyamala P., SubbaRao P.V., Ramakrishna K. Indian. J. Chem.

41A., (2002), 1410.

4. Shyamala P., SubbaRao P.V. Kinetics and Catalysis. 51, (2010), 207.

5. Babatunde O.A.. World journal of chemistry. 4(1), (2009), 39.

6. Jeffer yG.H. ,Basset tJ., Mendham J., Denny R.C. Vogel’s textbook of quantitative

chemical analysis. 5thedition, England: Longman. (1992), 391.

7. Fletcher P.D.I., Howe A.M., Robinson B.H., Steytler D.C., Luis iP.L., Straub B.E.

Reverse Micelles.eds, plenum press, Newyork. (1984). 73.

8. Ruasse M.F., Blagoeva I.B., Ciri R., Garcio-Rio L., Marques A., Mejuto J., Monnier,

E. Pure & Appl. Chem. 68, 9, (1997), 1923.

9. Munoz E., Gomez-Herrera C., Garciani M., Moya M.L., Sanchez F. J. Chem. Soc

Faraday Trans1. 87, (1991), 129.

10. Silber J.J., Biasutti A., Abunin E., Lissi E. // Adv in Colloid and Interface Sci. 1999.

V. 82. P.189.

11. Fletcher P.D.I, Robinson B.H. J. Chem. Soc Faraday Trans1. 80, (1984), 2417.

12. Lang J., Jada A.., Malliaris A. J. Phys. Chem. 92, (1988), 1946.

TABLE I. Effect of varying periodate [IO4- ] on observed

first order (ko) at W = 6.0 and W = 10.0: [RA+] = 7.9×10-6

mol/l; [AOT] = 0.1 mol /l; T = 29oC.W = 6.0 W = 10.0

103×[IO4]o

(mol/l)102×[IO4]e

(mol/l)105×ko

(s-1 )102

×[IO4-]e

(mol/l )

105×ko

( s-1 )

0.470 4.40 0.390 2.60 0.2500.940 8.70 0.620 5.20 0.4601.87 17.3 1.43 10.4 0.7702.80 25.9 1.60 15.5 0.9603.74 34.6 2.00 20.7 1.69

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TABLE II. Effect of AOT and W on observed first order (ko) and second order rate constants (k2): [RA+] = 7.9×10-6

mol /l; [IO4

−

] = 1.87×10-3 mol/l; T = 29oC.[AOT](mol/l)

[Na+]e

(mol/l)103 ×f(volume fraction)

W 105×ko

(s-1)104 ×k2

( mol-1 l s1)

0.05 6.945.554.63

7.209.0010.8

8.0010.012.0

2.201.811.53

8.468.578.80

0.1 11.19.266.945.554.63

9.0010.814.418.021.6

5.006.008.0010.012.0

1.601.430.9600.7700.630

7.708.097.447.207.24

0.2 11.19.266.945.554.63

18.021.628.836.043.2

5.006.008.0010.012.0

0.7700.6500.5200.4100.320

7.407.477.287.697.39

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TABLE III. Effect of varying periodate [IO4

−

] on observed first order (ko) at W = 6.0 and W = 10.0. RA+] = 7.9×10-6

mol/l; [CTAB] = 0.1mol/l; T=29oC.

W = 5.0 W =12.0

103×[IO4

−

]o

(mol/l)102×[

IO4−

]e

(mol/l)

105×ko

(s-1 ) 102×[IO4

−

]e

(mol/l)

105×ko

( s-1 )

0.620 6.90 8.10 3.44 10.11.20 13.3 16.0 6.60 18.91.87 20.7 24.1 10.4 33.02.50 27.7 31.0 13.8 37.33.30 36.6 42.0 18.3 49.04.20 --- --- 23.3 57.05.80 --- --- 32.2 85.0

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TABLE IV. Effect of CTAB and W on Observed first order (ko)and second order rate constants (k2) [RA+] = 7.9×10-6

mol/l; [IO4−

] = 1.87×10-3 mol/l; T = 29oC[CTAB](mol/l)

[Br-]e

(mol/l)103×f(volume fraction)

W 105×ko

(s-1)104×k2

(mol-1 ls-1)

0.05

6.945.564.633.973.47

7.209.0010.812.614.4

8.0010.012.014.016.0

27.631.040.042.058.0

10.614.923.128.438.8

0.1

16.711.19.266.945.564.633.973.47

6.009.0010.814.418.021.625.228.8

3.335.006.008.0010.012.014.016.0

17.724.025.026.528.833.038.239.5

5.8011.614.520.427.937.951.660.8

0.2

33.316.711.19.266.945.564.633.973.47

6.0012.018.021.628.836.043.250.457.6

1.663.335.006.008.0010.012.014.016.0

8.7315.420.022.023.124.226.228.831.5

2.789.9019.225.336.046.561.078.0100

0.3

50.033.316.711.19.266.945.564.633.973.47

6.008.9018.027.032.443.254.064.875.686.4

1.111.663.335.006.008.0010.012.014.016.0

2.024.4114.618.820.522.623.725.126.427.6

0.6402.5014.227.235.351.466.686.691.0145

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Table V. Activation parameters for the reaction between RA+ and IO4- ,[RA+] = 7.9×10-6

mol/l; [IO4- ] = 1.87×10-3mol/l.

Ea/KJ mol-1 ΔH#/KJ mol-1 ΔS#/J mol-1K-1 ΔG#/KJ mol-1

Water 116.6 114.04 76.20 94.49

CTAB(0.1mol/l)W= 5.0

W=10.0

72.07

56.25

73.54

50.45

-55.91

-116.01

90.42

85.45

AOT(0.1mol/l)W= 6.0

W= 10.0

76.15

91.29

75.69

85.42

-48.44

-23.35

89.51

92.56

15

0 20 40 60 80 100 120 140 1600

0.1

0.2

0.3

0.4

0.5

0.6

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Time (min)

Abso

rban

ce

2+lo

g(ab

sorb

ance

)

Fig.1 Plot of absorbance versus time. The log(absorbance) curve (the right y axis) is superimposed.

260 270 280 290 300 310 320 330 340 3500

0.1

0.2

0.3

0.4

0.5

0.6

Wave length (nm)

Abso

rban

ce

Fig.2 Spectrum of IO3-

● Absorbance■2+log(absorbance)

16

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.01.0

1.5

2.0

2.5

3.0

3.5

[Br -]e={µ} mol/l

5+lo

gk2

mol

-l l s

-l

Fig.3 Effect of ionic strength on fading of rosaniline hydrochloride by periodate at different concentrations of 0.1,0.2,0.3mol/l CTAB.

0 5 10 15 20 250

100

200

300

400

500

600

700

800

1/C mol-1 l

1/k2

mol

-1 l

s-1

● W=3.33■ W=10.0▲W=16.0

Fig.4 A plot of 1/k2 versus 1/C at various W values.

● 0.1mol/l CTAB■ 0.2mol/l CTAB♦ 0.3mol/l CTAB

17