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Conductometric studies of interaction between anionic dyes and

cetylpyridinium bromide in wateralcohol mixed solvents

A.A. Rafati , S. Azizian, M. Chahardoli

Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

Received 16 May 2006; accepted 20 March 2007

Available online 13 May 2007

Abstract

The interactions between two anionic dyes and a cationic surfactant were studied by conductometric technique. The conductance of aqueous

solutions of methyl orange (MO) and methyl red (MR) was measured in the presence of a cationic surfactant, cetylpyridinium bromide (CPB) at

different temperatures in waterethanol, waterpropanol and waterbutanol mixed solvents, containing different concentration of alcohols. The

equilibrium constants and other thermodynamic functions for the process of dyesurfactant ion pair formation were calculated on the basis of a

theoretical model. The results showed that the presence of alcohol, as well as increasing the length of the alcohol chain, decreases the tendency for

ion pair formation. The results have shown that an increase in temperature lowers the tendency for ion pair formation as the equilibrium constants

decrease with increasing temperature. According to the results, long range as well as short range interactions are responsible for the formation of

the ion pair. The importance of long range electrical forces is basically to bring the dye anion and the surfactant cation close enough to enable the

action of short range interactions whose contribution represents the major part of the standard free energy change for the formation of the anionic

dyecationic surfactant ion pair. By using the association constant (K1) for the first step of the association [D+ S+ (DS)], the standard free

energy change, standard enthalpy change, and standard entropy change of the association were calculated at low surfactant concentrations.

2007 Elsevier B.V. All rights reserved.

Keywords: Dye; Surfactant; Interaction; Ion pairing; Mixed solvent; Conductometry

1. Introduction

The interaction between dyes and surfactants is subject

of some investigations [16]. Although a lot of research work

has already been done into dyesurfactant interactions the

studies in this area are still important and interesting for the

theory and technology of dyeing. Electrostatic interactions andsteric factors are both important in the binding process of dye

surfactant.

The ion pair formation in an aqueous solution is known to be

effected by organic additives. Recently, increasing attention is

being devoted to the study of the effect of neutral molecules into

ion pair formation in aqueous solution. Some of the most

studied solubilizates are alcohols, because of the important role

they have in dying. It is generally accepted that the alcohol

binds to the dye and surfactant molecules, leading to some

principal effects: (a) reducing tendency for ion pair formation;

(b) the dielectric constant of media decreases with increasing

concentration of alcohols; (c) the molecular order of the inter-

face region of the ion pair changes [7

9].Investigation of dye interaction with surfactant aqueous

solutions can give useful information about the mechanisms

according to which surfactants operate as leveling agents, ther-

modynamics and kinetics of dyeing process and finishing of

textile material.

Experimental methods mostly used were spectroscopy [10

15], potentiometry [1619] and tensiometry [20,21]. Recently it

was reported that the formation of dyesurfactant ion pair can

be investigated by the conductometric method [4,9].

In continuation of our research project on interaction of

surfactant with other molecules [2227], here we report the

Journal of Molecular Liquids 137 (2008) 80 87

www.elsevier.com/locate/molliq

Corresponding author. Fax: +98 811 825 7407.

E-mail addresses: [email protected], [email protected](A.A. Rafati).

0167-7322/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2007.03.013
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.molliq.2007.03.013http://dx.doi.org/10.1016/j.molliq.2007.03.013mailto:[email protected]:[email protected]

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interaction of methyl orange (structure I) and methyl red (struc-

ture II), two anionic dyes, with CPB (structure III), cationic

surfactant, by coductometric technique. The equilibrium con-

stants of ion pairing between dyes and surfactant were cal-

culated by a method which was described in [4] and using

experimental conductometric data.

Analysis of the results shows how both environmentalcondition and temperature affect the ion pairing.

2. Experimental

The anionic azo dye, methyl orange (MO) and methyl red

(MR) were obtained from Merck and used without furtherpurification. Cetylpyridinium bromide (CPB) was obtained

from Aldrich Chem. Co. and used as received. All solutions

were prepared in double distilled water.

The conductance was measured by using Methrohm

Conductometer (Model 712) and the conductivity cell was

calibrated with KCl solution in the appropriate concentration

range. The cell constant was 0.8 cm1. The measuring cell was

immersed in a thermostat bath, keeping the temperature con-

stant within 0.1 C.

3. Results and discussion

Fig. 1a, b and c show plot of the specific conductance of pure

solution of CPB, MR and MO against concentration and at

different temperatures, respectively. These plots show how the

specific conductivity of pure electrolyte solutions is varied by a

range of concentrations. Base on Kohlrausch's law, we have the

following relations:

K-SBr k-S k-Br 1

K-DNa k-D k-Na 2

The values were determined experimentally by measuring

the specific conductivities of the surfactant and the dye and

converting them to equivalent conductance by the following

equation:

K 103j

C3

Then values can be determined by plot of versusffiffiffiffi

Cp

and

extrapolation of this plot to infinite dilution. The values are

Fig. 1. Plot of the specific conductance of pure solution of (a) CPB, (b) MR and

(c) MO versus concentration at different temperatures.

81A.A. Rafati et al. / Journal of Molecular Liquids 137 (2008) 8087

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Table 1

Equivalent conductances at infinite dilution in water and wateralcohol mixtures for individual species and correspondence ion pairs [units: (1 cm2 mol1)]

%Alcohol t (C) CPB NaBr MO MR (CPBMO) (CPBMR)

Water 15 80.57 102.90 78.58 79.36 56.25 57.03

20 91.55 118.00 80.54 88.20 54.09 61.75

25 100.10 128.20 92.70 94.09 64.60 65.99

30 111.64 145.40 101.57 103.62 67.81 69.865% Ethanol 15 74.65 93.08 72.88 73.94 54.45 55.51

20 84.82 106.23 80.98 82.01 59.57 60.60

25 94.76 118.9 87.16 88.32 63.02 64.18

30 105.96 131.7 96.04 97.02 70.30 71.28

10% Ethanol 15 57.42 78.65 63.97 65.00 42.74 43.77

20 69.71 89.25 69.78 71.03 50.24 51.49

25 82.01 99.83 75.57 76.72 57.75 58.90

30 90.78 113.63 81.37 82.46 58.52 59.61

15% Ethanol 15 49.03 62.52 54.97 56.05 41.48 42.56

20 56.22 75.27 61.52 62.72 42.47 43.67

25 63.41 88.45 67.94 69.11 42.90 44.07

30 74.92 101.15 75.74 76.82 49.51 50.59

20% Ethanol 15 40.89 54.19 46.07 47.13 32.77 33.38

20 49.64 64.29 53.67 54.86 39.02 40.21

25 58.43 73.86 61.37 62.43 45.94 47.0030 65.08 87.16 68.12 69.06 46.04 46.98

25% Ethanol 15 30.42 44.86 37.07 38.09 22.63 23.65

20 39.62 57.1 44.11 45.24 26.63 27.76

25 47.01 68.78 51.73 53.00 29.96 31.23

30 56.29 79.76 59.37 60.49 35.90 37.02

5% Propanol 15 73.24 90.93 71.20 72.34 53.51 73.24

20 82.97 104.21 79.65 80.80 58.41 64.38

25 93.52 116.41 85.74 86.13 62.85 70.11

30 104.36 129.00 94.13 95.62 69.49 74.08

10% Propanol 15 56.33 76.21 62.24 63.50 42.36 56.39

20 68.45 87.48 68.95 70.22 49.92 55.74

25 80.89 97.92 74.39 75.01 57.36 63.63

30 89.78 111.93 80.43 81.09 58.28 66.87

15% Propanol 15 47.88 61.10 53.92 54.87 40.70 47.88

20 54.68 73.14 60.67 61.21 42.21 48.4525 61.99 86.61 66.89 67.53 42.27 50.06

30 73.21 99.05 74.46 75.63 48.62 54.13

20% Propanol 15 39.71 52.13 45.01 45.00 32.59 39.71

20 48.56 62.98 52.41 53.07 37.99 41.43

25 57.65 72.29 60.12 61.00 45.48 47.74

30 63.99 85.17 66.97 67.71 45.79 52.70

25% Propanol 15 29.66 43.64 36.24 36.95 22.26 29.66

20 38.82 55.60 43.05 43.48 26.27 32.13

25 46.09 66.66 50.19 51.47 29.62 33.97

30 55.17 77.85 58.32 59.04 35.64 39.98

0.5% Butanol 15 78.02 99.50 78.32 58.44

20 88.10 110.31 86.73 64.52

25 99.15 119.52 96.37 76.00

30 108.13 128.32 104.69 84.50

1% Butanol 15 76.88 99.00 76.56 56.44 20 86.66 108.48 82.76 58.70

25 91.89 116.39 91.30 66.80

30 102.12 126.37 99.99 75.74

1.5% Butanol 15 74.23 97.43 75.10 51.90

20 84.65 107.11 84.04 61.58

25 94.33 116.69 93.33 70.97

30 104.00 126.75 102.09 79.73

2% Butanol 15 72.64 95.99 74.02 50.67

20 82.21 106.27 84.04 61.58

25 94.33 116.69 93.33 70.97

30 104.03 126.03 101.45 79.73

82 A.A. Rafati et al. / Journal of Molecular Liquids 137 (2008) 8087

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shown in Table 1. By applying i values from literature for Br

and Na

+

, one can write:

k-S k-D K-SBr K-DNa k-Na k-Br 4

where (SBr) and (DNa) are the equivalent conductance of

surfactant and dye at infinite dilution, respectively and S+, D,

Br and Na+ are the equivalent conductance of the ions S+, D,

Br and Na+ at infinite dilution, respectively.The specific con-

ductivity, , of dye solutions can easily be calculated in terms of

the molar ionic conductivities of ions, i. Electrical conductivity

before ion pair formation is written as:

j kD Df kC CfDf Cf Ct

5

where [D

_]f and [C

+]f are the concentration of free dye and its

counterion, respectively and D and C+ are corresponding to

equivalent ionic conductivities. The complete dissociation of

ionic dye is assumed in the absence of surfactant. The slope(S1) of

molar conductivity becomes:

S1 j=Ct kC kD 6

With addition of surfactant to the solution ion pair formation is

occurred, and an abrupt change in concentration dependence of

specific conductivity was observed. Fig. 2 shows a typical plot ofthe specific conductivity of the MO solution as a function of the

CPB concentration at 15 C, 20 C, 25 C and 35 C. The change

in the electrical conductance of aqueous ionic dye solution with

addition of surfactant solution is due to the ion pair formation

between surfactant ion and opposite charge dye ion.

In the presence of surfactant ion, for a 1:1 ion pair formation

one should be able to define the equilibrium for this reaction:

S DXS D K S DSfDf7

where [S+]f and [SD] are the concentration of free surfactant

and ion pair, respectively. The mass conservation law equations

for above equilibrium could be written for the total surfactant

and dye concentrations as:

St Sf S D 8

Dt Df S D 9where [S+]t and [D

]t are total surfactant and dye ion con-

centrations, respectively. In this case, the observed specific

conductance could be expressed as:

jobs jS jD 10where obs, S and D are observed, surfactant and dye specific

conductance, respectively. If ion pair is considered as a non-

conducting species, and if there was no interaction between the

surfactant and dye, we can explain Eq. (10) based on equivalent

ionic conductivity as follows:

103jobs kS St kC Ct kD Dt kC Ct 11

where [C]t and C are the total concentration and ionic molar

conductivity of surfactant counter ion, respectively. The ion pair

formation caused a decrease in the concentration of free ions,

hence:

103jobs kSSt S D kC Ct kDDt S D kC Ct 12

After deduction of Eq. (11) from Eq. (12) we have:

103Dj S DkD kS 13

where is the difference between the theoretical and measured

specific conductances at a given surfactant concentration. Now,

Fig. 2. Plot of the specific conductance of MO versus [CPB] in water at different

temperatures. The concentration of MO is equal to 105 mol dm3.

Fig. 3. A typical plot ofG versus temperature for ion pairing of MR with

CPB in different percentages of propanol. The concentration of MR is equal

to 10

5

mol dm

3

. Obtained data for other systems are tabulated in Tables 2and 3.


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Kohlrausch's law for infinite dilute solution of an electrolyte can

be used for the above system, hence Eq. (13) can be written as:

103DjcS Dk-D k-S S DK-SD 14where (SD) is the equivalent conductance of the surfactant

dye ion pair at infinite dilution. The

(S

D) values weredetermined experimentally by measuring the specific conduc-

tivities of the surfactants and the dyes. Obtained data are

tabulated in Table 1. By using Eq. (14), we can estimate ion pair

concentration at each concentration of dye and surfactant.

Table 2

Equilibrium constants and thermodynamic parameters for MOCPB ion pair

formation in different media

Solvent t

(C)

K

(dm3 mol1)

G

(kJ mol1)

H

kJ mol1)

S

(J mol1)

Water 15 3.96 105 30.878 33.834 10.26

20 3.11 105

30.825

10.2625 2.46 105 30.773 10.28

30 1.97 105 30.726 10.25

5% Ethanol 15 3.92 105 30.854 41.972 38.58

20 2.87 105 30.629 38.69

25 2.18 105 30.470 38.58

30 1.64 105 30.264 38.62

10% Ethanol 15 3. 20 105 30.368 50.595 70.20

20 2.31 105 30.100 69.91

25 1.65 105 29.780 69.81

30 1.12 105 29.303 70.24

15% Ethanol 15 3. 05 105 30.253 104.233 256.74

20 1.13 105 28.358 258.83

25 5.44 104 27.029 258.94

30 3.58 104 26.428 256.65

20% Ethanol 15 7. 54 104

26.905 238.828 735.4620 1.34 104 23.161 735.69

25 2.32 103 19.209 736.61

30 5.65 102 15.971 735.14

25% Ethanol 15 1. 32 104 22.730 339.037 1097.71

20 1.29 103 17.457 1096.98

25 1.60 102 12.580 1094.94

30 1.10 101 6.044 1098.44

5% Propanol 15 3. 12 105 30.307 44.008 47.55

20 2.45 105 30.244 46.95

25 1.79 105 29.982 47.04

30 1.26 105 29.600 47.53

10% Propanol 15 2.99105 30.205 52.751 78.25

20 2.14 105 29.914 77.90

25 1.54 105 29.609 77.62

30 9.93 104

28.999 78.3515% Propanol 15 2.88105 30.115 120.578 313.94

20 9.96 104 28.050 315.63

25 4.66 104 26.646 315.05

30 2.34 104 25.356 314.11

20% Propanol 15 6.33104 26.486 243.459 752.99

20 1.01 104 22.472 753.83

25 2.11 103 18.974 752.93

30 4.00 102 15.101 753.28

25% Propanol 15 9.94103 22.051 396.395 1299.13

20 1.00 103 16.836 1294.76

25 5.00 101 9.697 1296.99

30 3.00 2.769 1298.45

0.5% Butanol 15 3.97105 30.884 34.022 10.89

20 3.10 105 30.817 10.93

25 2.55 105 30.859 10.6130 1.94 105 30.687 11.00

1% Butanol 15 3.70 105 30.716 34.726 13.92

20 2.90 105 30.655 13.89

25 2.38 105 30.688 13.54

30 1.78 105 30.470 14.04

1.5% Butanol 15 3.61105 30.657 38.253 26.36

20 2.84 105 30.604 26.09

25 2.21 105 30.504 25.99

30 1.63 105 30.248 26.40

2% Butanol 15 3.44 105 30.541 41.860 39.28

20 2.71 105 30.490 38.79

25 2.02 105 30.281 38.84

30 1.45 105 29.954 39.28

Table 3

Equilibrium constants and thermodynamic parameters for MRCPB ion pair

formation in different media

Solvent t

(C)

K

(dm3 mol1)

G

(kJ mol1)

H

(kJ mol1)

S

(J mol1)

Water 15 4.18 105 31.008 31.510 1.74

20 3.34 105 30.999 1.74

25 2.69 105 31.991 1.74

30 2.18 105 30.981 1.74

5% Ethanol 15 3.96 105 30.878 40.225 32.44

20 2.97 105 30.713 32.45

25 2.35 105 30.656 32.09

30 1.70 105 30.354 32.56

10% Ethanol 15 3.25 105 30.405 49.850 67.48

20 2.41 105 30.204 67.02

25 1.75 105 29.926 66.83

30 1.15 105 29.369 67.56

15% Ethanol 15 3.10 105 30.292 104.091 256.49

20 1.36 105

28.809 257.1725 5.82 104 27.197 258.27

30 3.78 104 26.565 256.09

20% Ethanol 15 7.58 104 26.917 238.271 734.26

20 1.46 104 23.370 733.84

25 2.34 103 19.230 735.42

30 5.87 102 16.068 733.72

25% Ethanol 15 1.53 104 23.084 338.363 1095.25

20 1.04 103 16.932 1097.55

25 1.61 102 12.596 1093.69

30 1.20 101 6.263 1096.54

5% Propanol 15 3.30 105 30.442 39.585 31.87

20 2.63 105 30.417 31.41

25 1.94 105 30.181 31.68

30 1.47 105 29.988 31.79

10% Propanol 15 3.13105 30.315 50.715 70.9720 2.30 105 30.090 70.53

25 1.69 105 29.839 70.19

30 1.08 105 29.211 71.79

15% Propanol 15 2.99105 30.205 116.147 298.66

20 1.01 105 28.084 300.80

25 4.76 104 26.698 300.40

30 2.68 104 25.698 298.75

20% Propanol 15 6.48104 26.542 240.075 741.91

20 1.08 104 22.635 742.59

25 2.17 103 19.044 742.18

30 4.46 102 15.375 742.04

25% Propanol 15 9.95103 22.053 394.516 1293.87

20 1.01 103 16.860 1289.52

25 5.60 101 9.978 1290.97

30 3.00 2.769 1293.46


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Thermodynamic parameters can be extract by calculating of

equilibrium constant for ion pair formation. Eq. (7) can be

written in the form:

K S DSt S DDt S D15

where [S]t and [D]t are the total concentration of surfactant and

dye in the solution, respectively. Values of K were determined

by application of Eq. (15), and used to determine the values of

the Gibbs free energy of ion pair formation, G:

DG- RT lnK 16

The enthalpy of ion pair formation was obtained from the

temperature dependence of the equilibrium constant (K) using

the van't Hoff relation:

d

lnK

=d

1=T

DH-=R

17

and finally the S values could be obtained from the following

equation:

DG- DH- TDS- 18Fig. 3 shows a typical plot ofS versus temperature in different

percentage of propanol for MRCPB system. It is obvious that

the tendency of ion pair formation is decreased with increasing

of temperature as well as concentration of alcohol. Calculated

thermodynamic parameters are listed in Table 2. The negative

values ofH could be attributed to the attraction between the

cationic head of surfactant and anionic dye species.

3.1. Effect of alcohols on ion pair formation

In Fig. 4, the molar conductance of the MR versus added

concentration of CPB in waterethanol mixtures was pre-

sented typically at 15 C. As mentioned earlier, there is an

appreciable decrease in the tendency of the dye for ion pair

formation with an increase in hydrophobicity of the solvent.

Also, Fig. 5 shows a typical variation of dielectric constant of

medium with variation of concentration of alcohol in water.

The estimated dielectric constants are listed in Table 4. As has

been shown, the permittivity of medium was decreased with

increasing percentage of ethanol. Similar trends were observed

with other alcohols media also. The decrease in ion pair

formation constant indicates a decrease in the polarity or the

dielectric constant of the medium surrounding the ionic head

group. Thus, for example, in wateralcohol mixtures of dif-

ferent dielectric constants or in the presence of solvents of

different polarity, similar changes in ion pair formationconstant were also observed. It is seen that as the dielectric

constant of the matrix in the wateralcohol mixture decreases

or the medium is changed from high to low polar solvents, the

dyesurfactant interaction decreases with a progressive

decreasing in tendency for ion pair formation. The change in

G with the medium polarity and also the dielectric constant

Fig. 4. A typical plot of molar conductivity of MR versus surfactant

concentration in different percentage of ethanol at 15 C. The concentrationof MR is equal to 105 mol dm3.

Fig. 5. A typical plot of dielectric constant vs. percentage of ethanol in water

(%v/v) at different temperatures. Data for other solvents are listed in Table 4.

Table 4

Dielectric constant for WaterAlcohol mixtures at different temperatures [28]

Solvent Temperature

15 20 25 30Water 81.63 80.37 78.34 76.73

5% EtOH 78.82 77.51 75.58 73.96

10% EtOH 75.87 74.60 72.72 71.14

15% EtOH 72.74 71.52 69.70 68.18

20% EtOH 69.92 68.66 66.94 65.46

25% EtOH 66.68 65.53 63.83 62.41

5% PrOH 78.32 77.04 75.09 73.48

10% PrOH 74.80 73.52 71.68 70.12

15% PrOH 71.02 69.83 68.03 66.54

20% PrOH 67.71 66.54 64.80 63.34

25% PrOH 63.72 62.62 60.97 59.60

0.5% BuOH 81.57 80.29 78.28 76.66

1% BuOH 81.49 80.22 78.20 76.58

1.5% BuOH 81.41 80.14 78.12 76.51

2% BuOH 81.33 80.06 78.05 76.43


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gives a descending curve which approaches to a constant value

at 80 (see Fig. 6).

The values ofG, H and S for the reaction of ion pair

formation between MO and CPB were plotted versus percentage

of ethanol in Fig. 7, typically. As have been shown, tendency for

ion pair formation is decreased with increasing concentration of

alcohol. In addition, the H values are negative representing

exothermic reaction and become progressively more negative

with increasing concentration of alcohol. The negative values of

S indicate that the process is enthalpy driven. By considering

obtained thermodynamic parameters in Tables 2 and 3, we can

observe that all enthalpy and entropy changes related to ion pair

formation are negative, so the driving force for ion pairing isenthalpy and system becomes more unfavorable due to decrease

in entropy, i.e. S b0.

Obtained thermodynamic data for other media (water

alcohol) are tabulated in Tables 2 and 3.

A comparison of the behaviour of the two anionic dyes with

similar hydrophobic portion structure showed a similar very

closed values for thermodynamic parameters and indicates thathydrophobic structure of dye had a significant influence on

formation of ion pairs with the particular surfactant used. As an

example for this similarity, Fig. 8 shows the values of H

against percentage of ethanol.

4. Conclusion

The conductometric technique allows us to estimate value

for ion pair formation constants between dyes and ionic

surfactants in aqueous and mixed solvents. The main drawback

to conductometric investigations of dyesurfactant ion pair

formation is that for a numerical description, namely the

calculation of equilibrium constants, a suitable theoreticalmodel is required and this can include certain simplifications.

The equilibrium constants have been calculated for two anionic

dyes with a cationic surfactant, and the tendency of ion pair

formation as a function of polarity or dielectric constant of

medium has been presented. The calculated thermodynamic

parameters show a non-linear dependency between tendency of

ion pair formation and percentage of alcohols. The values of

thermodynamic parameters indicate that ion pair formation is an

enthalpy driven process for interaction between ionic surfactant

and ionic dyes. In addition, at higher temperatures, a higher

concentration of the surfactant CPB was required to initiate the

process of ion pair formation.According to the results, long range as well as short range

interactions are responsible for the formation of the ion pair. The

importance of long range electrical forces is basically to bring

the dye anion and the surfactant cation close enough to enable

the action of short range interactions whose contribution

represents the major part of the standard free energy change

for the formation of the anionic dyecationic surfactant ion pair.

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Fig. 8. Comparison between H for ion-paring of CPB with () MO and ()

MR in waterethanol mixture.Fig. 6. Plot ofG for ion pair formation between MOCPB and MRCPB vs.

dielectric constant of media at various temperatures.


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