Indian Journal of ChemistryVol. 22A. October 1983. pp. 887-889

Notes

Aggregation of Neutral Redt: Part IIt

N V RAO· & K L NARAYANADepartment of Engineering Chemistry, Andhra University,

Waltair 530003

Received 5 April 1983; rerised and accepted 20 June 1983

The effects of temperature, co-solvents and added salts on theaggregation of Neutral Red have been studied spectrophoto-metrically, based on the observed deviation from Beer's law. Thedimerization constant has also been calculated.

We have recently reported the effects of time andconcentration on the aggregation of Neutral Red!. Inthis note, we present the results of our investigationson the effects of temperature, co-solvents and addedsalts on the aggregation of Neutral Red, an azine dyewith Colour Index No. 50040.

A Chroma sample of the dye was used ill theinvestigation. The solvents and salts employed were ofreagent grade quality. The dye was purified and testedfor purity as already reported.". A Shimadzu doublebeam spectrophotometer (model UV -140.02) was usedfor spectral studies and the average aggregationnumber (AAN) was calculated from the observeddeviation from Beer's law, as reported earlier".

Solutions of different concentrations of the dye wereprepared in deionised water and stored for 24 hr beforetaking measurements in order to ensure aggregation.To avoid the adsorption of the dye on the walls of thespectrophotometer cells, dilute solutions of the dyewere stored in the cells for considerable time.

Effect of temperature on aggregationThe cells and dye solutions were placed in a

thermostat (accuracy: ± 1°C) for about 5 to 10 min toensure equilibration and then the absorbance wasmeasured. The results obtained for AAN at differenttemperatures are presented in Table 1.

Effect of salts and co-solventsSolutions of the dye, taken in a series of 25 ml

volumetric flasks, were treated with the requisiteamounts of the various salt solutions and solvents suchthat the concentration of the salt was 1M and theprecentage of the solvent (v/v) was 10. Theabsorbancesof these solutions were then measured at 25cC and30"C, and the results for AAN are given in Table 2.

tPresented at the 52nd Annual Session of the National Academy ofSciences, India, Bhavnagar, 31 Oct.-2 Nov. 1982.~See ref.. I for Part I.

Table I-Variation of Average Aggragation Number(AAN) with Temp. at [Neutral Red] = 1.558 x 10 -4 M

Temp. Threshold AAN(DC) cone. x 105 M

30 8.7 935 9.2 840 lOA 745 11.0 650 11.3 660 12.6 5

The "threshold concentration", i.e., the con-centration above which only the aggregation of the dyeoccurs in the absence of any salt is 1.2 x 10 -4 M and itvaries with temperature and the salt and solvent added(Tables 1 and 2).

Calculation of the dimerization constant of the dyeMoulik et at". have proposed an equation for

calculating the dimerization constant of AcridineOrange. We have used this equation to calculate thedimerization constant of Neutral Red as follows.From our earlier studies", we have observed thatNeutral Red can exist in solution as a dimer, trimer,and tetramer, etc., depending upon various factors,especially the concentration. We have, therefore,carried out experiments to find out the concentrationof the dye at which it exists in the dimeric form. Havingestablished this concentration, we have calculated thedimerization constant and studied the effects of addedsalts and co-solvents on the dimerization constant; theresults are presented in Table 3.

The dimerization equilibrium may be representedas,

where M is the monomer, D is the dimer and Kd is thedimerization constant, given by Eq. I,

... (1)

The dimerization constant was calculated using Eq. 2suggested by Moulik et al. 2,

... (2)

where Em = molar absorptivity of the monomer, C,= total concentration of the dye, and A = absorbance.The results of these studies are incorporated in Table 3.

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INDIAN J. CHEM., VOL. 22A, OCTOBER 1983

Table 2-EfTect of Added Salts and Co-solvents on Average Aggregation Number at [Neutral Red] = 10-4 M

Saltt Threshold Om X 10-4 AAN Solvent] Threshold Om X 10-4 AAN(1M) cone. X 105M (lit (10% v/v) cone. X 105M (lit

mol " +IMKCI mol "em -I) cm "")

Nil 12.0 2.025 I Nil 6.0 1.8 9KCI 63 1.82 7 Glycerol 8.2 2.13 6NaCI 6.0 1.88 9 Isopropanol 8.5 1.55 6Na2S04 4.0 1.65 II Ethanol 8.3 2.15 6KBr 6.2 1.36 13 Methanol 8.0 2.08 5K OJ 6.2 1.98 13 Dioxane 9.0 2.35 4

~NaBr 5.8 1.53 15 Acetone 8.9 2.35 4

tTemp. = 30°C; [Temp. = 25°C

Table 3 -Effect of Added Salts and Co-solvents on theDimerization Constant of Neutral Red.

[Salt], (M) ,q X ro-J Solvent (%)(v/v)

KCl,0.250.51.0

KNO),0.250.51.0

Na2SQ4,0.250.51.0

0.4560.7561.3240.4111.162.2670.2060.5591.146

Dioxane,510IS

Methanol,51015

Ethanol,510IS

Glycerol,51015

1.40.2550.1521.6120.6920.1521.670.750.471.611.320.864

tat [Neutral Red] = 6.21 x 10-5 Mtat [Neutral Red] = 1.0 x 10 -4 M

Thermodynamic parametersThe thermodynamic parameters, !J.H, !J.S and !J.G,

for the dimerization process were calculated using Eqs3-S,

-!J.G = RT In Kd (3)!J.S = (!J.H - !J.G), T (4)

In K = _ !J.H + !J.S (S)d RT R

A plot of In Kd versus liT was linear the slope ofwhich (-!J.Hj2.303 R) was found to be 3 X 103. Hence,!J.H = -2.303 x 1.98 x 3 x 103 = -13.68kcalmol-l.

The values of!J.G and!J.S were calculated using the Eqs3 and 4, and these data are incorporated in Table 4.

The low values for !J.H indicate weakly bondedspecies (hydrogen bonds) (Coates:') and negativevalues for !J.S suggest more order in the dimerizationprocess (Vinogradov and Linnell").

The results in Tables 1 and 2 show that the AAN(i) decreases with increasing temperature,

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Table 4- Thermodynamic Parameters of Dimerization at[Neutral Red] = 1.212 x 10 -4 M

x, X 10-2 /:'G /:,H(kcal mol -I) (kcal mol -I)

-4.089- 3.932-3.725-3.625

Temp.CC)30354045

-13.68

/:,S

(e.u.)- 31.65- 31.65- 31.80- 31.62

9.26.324.053.16

(ii) decreases in the presence of co-solvents, and(iii) increases in the presence of salts, whereas theresults in Table 3 indicate that the dimerizationconstant (i) decreases with increasing temperature andincreasing percentage of co-solvent, and (ii) increaseswith increase in salt concentration.

There is shift in the I·max towards shorterwavelengths (lO-IS nm) in the presence of salts,whereas in the presence of co-solvents, the effect isopposite. In this connection, it may be mentioned thatPalchevskii and Polyanskaya ' also have observed asimilar behaviour with Neutral Red.

The observed values of AAN indicate that at [dye]= 1.212 x 10-4 M the order of AAN at 30°C is NaBr> KN03 = KBr > Na2S04 > NaCI > KCI. The ne-gative environment of the anions surrounding thecationic dye will melt the so-called "icebergs -,6 andhelp the hydrophobic association. Cationic speciesmay have little or opposite effect due to electrostaticrepulsion. The order of AAN in the presence of co-solvents, at 2ScC and at [dye] = 1.212 x 10-4 M isisopropanol = glycerol = ethanol> methanol> dioxane = acetone, the values being less than thosein the absence of the solvent.

The dimerization constant, Kd, decreases signi-ficantly with increasing temperature. This can beexplained on the basis of the disruption of hydrogenbond. The aggregation of Neutral Red is believed to bedue to hydrogen bonding I, and since increasingtemperature puts stress on the monomer-dimerequilibrium, the equilibrium has to shift to the

monomer side so as to relieve the stress (LeChatlier'sprinciple).

We are grateful to the CSIR, New Delhi, forfinancial assistance.

References1 Rao N V & Narayana K L, Indian J Chern, 21A (1982) 995.

NOTES

2 Moulik S p, Ghosh S & Das A R,lndian J Chern, 14A(1976) 306.

3 Coates E, J Soc Dyers Col, 85 (1969) 355.

4 Vinogradov S N & Linnell R H, Hydrogen bonding (VanNostrand-Reinhold Company, New York) 1971, 120.

5 Palchevskii V V & Polyanskaya LA, Optika i spektroskopiya, 20(1966) 415.

6 Frank H S & Evans M W, J chern Phys, 13 (1945) 507.

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