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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: rafati_aa@yahoo.com, aa_rafati@basu.ac.ir(A.A. Rafati).
0167-7322/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2007.03.013
mailto:rafati_aa@yahoo.commailto:aa_rafati@basu.ac.irhttp://dx.doi.org/10.1016/j.molliq.2007.03.013http://dx.doi.org/10.1016/j.molliq.2007.03.013mailto:aa_rafati@basu.ac.irmailto:rafati_aa@yahoo.com -
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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.
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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
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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 ()
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dielectric constant of media at various temperatures.
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