Investigation of interactions between some anionic dyes and cationic surfactants by conductometric method

Fluid Phase Equilibria 251 (2007) 1–7

Investigation of interactions between some anionic dyes and cationic surfactants by conductometric method Sibel Tunc¸ ∗ , Osman Duman Akdeniz University, Faculty of Arts and Sciences, Department of Chemistry, 07058 Antalya, Turkey Received 24 July 2006; received in revised form 27 October 2006; accepted 27 October 2006 Available online 2 November 2006

Abstract The interactions of cationic surfactants with anionic dyes were studied by conductometric method. Benzyltrimethylammonium chloride (BTMACl), benzyltriethylammonium chloride (BTEACl) and benzyltributylammonium chloride (BTBACl) were used as cationic surfactants and indigo carmine (IC) and amaranth (Amr) were chosen as anionic dyes. The specific conductance of dye–surfactant mixtures was measured at 25, 35 and 45 ◦ C. A decrease in measured specific conductance values of dye–surfactant mixture was caused by the formation of non-conducting or less-conducting dye–surfactant complex. The equilibrium constants, K1 , the standard free energy changes, G◦1 , the standard enthalpy changes, H1◦ and the standard entropy changes, S1◦ for the first association step of dye–surfactant complex formation were calculated by a theoretical model. The results showed that the equilibrium constants and the negative standard free energy change values for all systems decreased as temperature increased. Also these values decreased for all systems studied with increasing alkyl chains of surfactants due to the steric effect. When the equilibrium constant values, K1 , for the first association step of IC–surfactant and Amr–surfactant systems with the same surfactant were compared, the values of K1 for IC–surfactant system were higher than that of Amr–surfactant system. © 2006 Elsevier B.V. All rights reserved. Keywords: Anionic dyes; Cationic surfactants; Interactions; Conductivity; Equilibrium constant

1. Introduction The interactions between dye and surfactant molecules are important in various dyeing processes such as textile dyeing, photography and printing ink as well as in chemical researches such as biochemistry, analytical chemistry and photosensitization [1]. Knowledge of these interactions helps to understand the chemical equilibria, mechanisms and kinetics of surfactantssensitized colour and fluorescence reactions [2]. Surfactants are widely used as auxiliaries in textile finishing processes [3]. Their interactions with dyes play a very significant role in achieving level of dyeing [2]. Various parameters (the charge and the alkyl tail length of the surfactants and the type and the position of the substituents in the aromatic ring of the dye molecules) can effect the interactions between surfactant and dye molecules.

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0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2006.10.020

Surfactants contain both hydrophobic and hydrophilic part in their structure. Solution properties of dye solutions such as electrical conductivity and absorption spectrum are changed by the addition of small amounts of surfactant with varying the environment of the solution. The changes as a function of surfactant concentration in the measured quantities indicate a significant variation in the nature of the solution [4]. When the charge of the surfactant is opposite of that of dye, the attractive forces between the dye and surfactant molecules lead to dye–surfactant complex formation in the solution [5]. Various techniques such as spectrophotometry [6–9], potentiometry [5,10], voltammetry [11] and conductometry [4,12] were used to understand the interactions between dyes and surfactants. These techniques have some advantages and disadvantages. Surfactant-selective membrane electrodes were used for the potentiometric methods. So, surfactant-selective electrodes should be prepared to study dye–surfactant interactions. Moreover, these electrodes work only in a specific concentration and pH ranges. These are disadvantages of potentiometric methods. Spectrophotometry and voltammetry

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are more expensive equipments. Conductometer is a cheap equipment and conductometric method is also easy to investigate the interactions between molecules. For these reasons, conductometric method was preferred to use in this study. There are many studies reported in the literature about the interactions between dyes and surfactants. The interactions of a cationic dye, Safranin-O, with various surfactants having the same hydrophobic lauryl group but different polar groups, such as sodium dodecylsulfate, sodium dodecylsulfonate, laurylsulfobetaine and dodecyltrimethylammonium-bromide were ˇ studied by G¨okt¨urk and Tunc¸ay [2]. Forte-Tavˇcer and Span [6] reported the interactions between anionic dye (acid orange 7) and two cationic surfactants (N-cetylpyridinium chloride and cetyltrimethylammonium bromide) in aqueous solutions using the method of continuous variations. They found that the molar binding ratio of both surfactant/dye associates was 1:1 and also the calculated equilibrium constants were high (about 106 ). Yang [1] investigated the interaction of dye and surfactant by their spectroscopic and surface properties and it was found from the static and dynamic surface properties that there was a strong interaction in mixture of cationic surfactant and aminoindophenol dye. Recently, Khan and Sarwar [4] studied the interactions between N-dodecylpyridinium chloride, cationic surfactant and eosin yellowish, anionic dye, by the spectrophotometric and conductometric methods at different temperatures. Their results have shown that low temperature favours the tendency for ion pair formation between surfactant and dye. The purpose of this study was to investigate the interactions of cationic surfactants; benzyltrimethylammonium chloride (BTMACl), benzyltriethylammonium chloride (BTEACl) and benzyltributylammonium chloride (BTBACl) with anionic dyes; indigo carmine (IC) and amaranth (Amr) by conductometric method. It was also aimed to determine the effect of alkyl chain length of surfactants, of the types of anionic dye and of temperature on to the interaction between dye and surfactant.

2. Experimental 2.1. Materials Anionic dyes, indigo carmine and amaranth, were purchased from Merck and Sigma, respectively. The cationic surfactants, benzyltrimethylammonium chloride, benzyltriethylammonium chloride and benzyltributylammonium chloride, were obtained from Aldrich. The structures of the dyes and surfactants used in this study are shown in Fig. 1. Deionized water was used in the preparation of solutions and in the measurements. 2.2. Method The conductivity measurements were carried out with a CMD 757 conductivity meter equipped with a black platinum electrode. The temperature control of solutions was achieved with Heto DT Hetotherm type thermostat circulating water through the jacketed glass cell in which the solution was continuously stirred. The specific conductivity of deionized water was measured before the each series of measurement at each temperature. Then the specific conductivity of an exact volume and known concentration of dyes solution (5 × 10−5 mol dm−3 ) was measured. Binary mixtures of dye/surfactant were prepared by keeping the dye concentration constant but by increasing the surfactant concentration. Then the specific conductivity of each solution was measured. The specific conductivity of surfactant alone was also measured at the concentration that is exactly the same as in the binary mixtures. Measurements were made at 25, 35 and 45 ◦ C. The temperature of solutions was kept within the range of ±0.1 ◦ C. 3. Results and discussion IC with two sulfonate groups and Amr with three sulfonate groups are reacted with the studied cationic surfactants in various ratios (in 1:2 and 1:3 ratios for IC and Amr, respectively).

Fig. 1. Structures of dyes and surfactants.

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Fig. 2. Specific conductivity of IC–BTMACl mixture in aqueous solution as a function of the BTMACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

Fig. 4. Specific conductivity of IC–BTBACl mixture in aqueous solution as a function of the BTBACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

The specific conductances of IC–BTMACl, BTEACl and BTBACl mixtures at the different temperatures were shown in Figs. 2–4, respectively. If there were no interaction between IC and surfactants in the solution, the experimentally measured conductance of the mixed solution should be the sum of the conductivities of the individual IC ions and surfactant ions in the solution [13]. Figs. 2–4 show that the measured conductances of the IC–surfactant mixtures are lower than the sum of the specific conductivities of the individual IC and of the individual surfactant molecule. The decrease in the measured values can be explained by the formation of a non-conducting or a lessconducting specie in the solution. The increase in temperature from 25 to 45 ◦ C caused a decrease in the formation of nonconducting or less-conducting species for all studied systems. At a given surfactant concentration for each temperature, the deviation from the theoretical values increased in the order of IC–BTMACl > IC–BTEACl > IC–BTBACl. This order may be

explained with the structure of the surfactant molecules. Each surfactant molecule has a positive nitrogen centre containing three alkyl groups and a benzyl group connected to the nitrogen centre as seen from Fig. 1. The increase in the chain length of the alkyl groups connected to nitrogen centre may cause a steric effect by preventing the approach of the surfactant molecules to IC molecules. For this reason, as the chain length of alkyl groups in the surfactant molecules increases, the formation of non-conducting or less-conducting complex between the IC and surfactant molecules decreases as understand from decreasing in deviation (Figs. 2–4). A similar result was reported by Forteˇ Tavˇcer and Span [6]. They studied the interactions between a dye, acid orange 7 and two surfactants, N-cetylpyridinium chloride and cetyltrimethylammonium bromide. They explained the weaker interactions as the result of steric hindrance, arising from the tetrahedral structure of the quaternary ammonium ion. The specific conductivity of another dye, Amr, with BTMACl, BTEACl and BTBACl mixtures in aqueous solution as a function of the surfactant concentration were shown in Figs. 5–7, respectively. It is seen from these figures that there are also a deviation of the measured conductance values from the sum of the specific conductivities of the individual Amr and of the individual surfactant molecules. However for each dye–surfactant system, this deviation is lower for the Amr–surfactant systems than that of IC–surfactant systems. The chain length of alkyl groups in surfactant molecules was also effective for the formation of non-conductive or less-conductive complexes in Amr–surfactant systems as in IC–surfactant systems. The equilibrium constants were calculated by using a theoretical model based on the deviation from linear behaviour. This model is based on the comparison between the measured conductivity of the dye–surfactant mixture and a theoretical straight line that represents the sum of the specific conductivities of the dye and the surfactant [14]. If a non-conducting complex formation (DSn ) occurs between dye (Dn− ) and surfactant (nS+ ) molecules, the equi-

Fig. 3. Specific conductivity of IC–BTEACl mixture in aqueous solution as a function of the BTEACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

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Fig. 5. Specific conductivity of Amr–BTMACl mixture in aqueous solution as a function of the BTMACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

librium reaction for this case can be written as: Dn− + nS+  DSn

(1)

Fig. 7. Specific conductivity of Amr–BTBACl mixture in aqueous solution as a function of the BTBACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

vidual surfactant molecule. The formation of a non-conducting dye–surfactant complex causes a decrease in the concentration of free ions:

If there was no interaction between dye and surfactant molecules in the solution, the measured conductance should be the sum of conductance of each species in the solution:

103 κ = (CD − CDSn )λDn− + CD · nλNa+

103 κ = CD · nλNa+ + CD · λDn− + nCS · λS+ + nCS · λCl−

where CDSn is the concentration of the non-conducting dye–surfactant complexes. By subtracting Eq. (3) from Eq. (2), Eq. (4) is obtained:

(2) where CD and CS are the molar concentrations of the dye and surfactant, respectively and λNa+ , λDn− , λS+ and λCl− are the equivalent conductances of the ions Na+ , Dn− , S+ and Cl− . It can be seen from Figs. 2–7 that the measured conductance of the dye–surfactant mixtures was lower than the sum of the specific conductivities of the individual dye and of the indi-

+ (CS − CDSn )nλS+ + nCS · λCl−

103 κ = CDSn (λDn− + nλS+ )

(3)

(4)

where κ is the difference between the theoretical and measured conductance at a given surfactant concentration. For dilute solutions: (λ◦ Dn− + nλ◦ S+ ) ≈ Λ◦DSn

(5)

and Eq. (4) can be written as 103 κ ≈ CDSn · Λ◦DSn

(6)

where Λ◦DSn is the equivalent conductance of the dye–surfactant complex at infinite dilution. The molar conductivity of dyes and surfactants at infinite dilution, Λ◦ , were determined according to Kohlrausch equation: ΛC = Λ◦ − b · C1/2

Fig. 6. Specific conductivity of Amr–BTEACl mixture in aqueous solution as a function of the BTEACl concentration at 25 ◦ C (), 35 ◦ C (䊉) and 45 ◦ C (). Solid lines show the sum of the conductivities of individual species in the solution, dashed lines show the measured conductivity of dye–surfactant mixtures.

(7)

where ΛC is molar conductivity of the solution at concentration C and b is a constant. Λ◦ value of a salt can be calculated from the intercept of a graph by plotting ΛC versus C1/2 [15]. Typical ΛC versus C1/2 plots for calculating the Λ◦ values of Amr as a dye and BTMACl as a surfactant were shown in Figs. 8 and 9, respectively. Calculated Λ◦ values for dyes and surfactants studied were given in Table 1. Λ◦DSn values determined from Λ◦ values of dyes, surfactants and NaCl were given in Table 1.

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where N is the total stepwise association constant. This value is 2 and 3 for the IC–surfactant and Amr–surfactant systems, respectively. The equilibrium constants, K, and the first stepwise association constants, K1 , calculated from Eqs. (8)–(10) for IC–surfactant complex formations and for Amr–surfactant complex formations were given in Tables 2 and 3, respectively. The standard free energy changes, G◦ , the standard enthalpy changes, H◦ and the standard entropy changes, S◦ , for the reaction of complex formation can be calculated using Eqs. (11)–(13), respectively. G◦ = −RT lnK   ∂(G◦ /T ) H ◦ = ∂(1/T ) P

Fig. 8. ΛC vs. C1/2 graph for the determination of molar conductivity of amaranth at infinite dilution (at 25 ◦ C (䊉), 35 ◦ C () and 45 ◦ C ()).

(11) (12)

H ◦ − G◦ (13) T The values of equilibrium constants, K1 , standard free energy changes, G◦1 , standard enthalpy changes, H1◦ and standard entropy changes, S1◦ for the first association step of IC–surfactant and Amr–surfactant complexes were given in Tables 2 and 3, respectively. The increasing of temperature caused a decrease in the K and the negative G◦1 values for all the systems studied as seen from Tables 2 and 3. It is clear that as the temperature increases, the tendency to form dye–surfactant complex decreases. The similar trends were reported in the literature [4,6]. For each dye–surfactant systems, the K and G◦1 values decreased with increasing the length of alkyl chains of surfactants due to the steric effect. A large decrease was observed for the K values of IC–BTBACl system. When the K values for IC–surfactant systems were compared with the Amr–surfactant systems, it was seen from Tables 2 and 3 that the K values of Amr–surfactant systems were higher than the IC–surfactant systems. This result may be explained with the structures of dye molecules. Amr dye molecule has three binding sides; on the other hand, IC dye molecule has two binding sides (Fig. 1). This property of Amr favours the tendency for complex formation with surfactant molecules. Since the K values were higher in the case of the surfactant with shorter aliphatic chain, electrostatic forces are mainly responsible for the dye–surfactant complex formation. The negative values of H1◦ indicate that complex formation processes of dye–surfactant systems are exothermic. IC–surfactant and Amr–surfactant complex formations were accompanied by the negative values of S1◦ (Tables 2 and 3). The negative values of S1◦ for dye–surfactant complex formations indicated that the binding of a surfactant to a dye caused S ◦ =

Fig. 9. ΛC vs. C1/2 graph for the determination of molar conductivity of BTMACl at infinite dilution (at 25 ◦ C (䊉), 35 ◦ C () and 45 ◦ C ()).

The equilibrium constant for the reaction (Eq. (1)) of complex formation is given by Eq. (8): K=

CDSn (CD − CDSn )(CS − nCDSn )n

(8)

The relationship between the equilibrium constant and stepwise association constant is given by Eq. (9) [16]: K = K1 K2 K3 . . . Ki

(9)

where Ki is the association constant of the ith step. The relationship between stepwise association constants as statistically is defined by Eq. (10) [16]: Ki (N − i + 1)(i + 1) = Ki+1 (N − i)(i)

(10)

Table 1 Equivalent conductances at infinite dilution in water for dyes (IC and Amr), surfactants (BTMACl, BTEACl and BTBACl) and complexes (DS2 and DS3 ) T (◦ C)

25 35 45

Equivalent conductance (S cm2 mol−1 ) Λ◦ICNa2

Λ◦AmrNa3

Λ◦BTMACI

Λ◦BTEACI

Λ◦BTBACI

Λ◦IC(BTMA)

Λ◦IC(BTEA)

Λ◦IC(BTBA)

Λ◦Amr(BTMA)

Λ◦Amr(BTEA)

Λ◦Amr(BTMA)

199.15 238.68 276.02

329.02 399.69 477.63

107.24 131.11 154.50

102.36 124.04 147.56

95.091 115.33 136.27

160.63 193.40 219.72

150.87 179.26 205.84

136.33 161.84 183.26

271.24 331.77 393.10

256.60 310.56 372.36

234.79 284.43 338.49

2

2

2

3

3

3

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Table 2 The values of K, K1 , G◦1 , H1◦ and S1◦ for IC–BTMACl, BTEACl and BTBACl complex formations System

T (◦ C)

K (dm3 mol−1 )2

K1 (dm3 mol−1 )

G◦1 (kJ mol−1 )

H1◦ (kJ mol−1 )

S1◦ (J mol−1 K−1 )

IC–BTMACl

25 35 45

5.77 × 109 2.09 × 109 0.77 × 109

15.2 × 104 9.14 × 104 5.55 × 104

−29.6 −29.3 −28.9

−40.0

−35.0 −34.8 −35.0

IC–BTEACl

25 35 45

1.97 × 109 0.76 × 109 0.28 × 109

8.88 × 104 5.51 × 104 3.35 × 104

−28.2 −28.0 −27.6

−37.1

−29.9 −29.6 −29.9

IC–BTBACl

25 35 45

1.76 × 108 0.55 × 108 0.24 × 108

2.65 × 104 1.48 × 104 0.98 × 104

−25.2 −24.6 −24.3

−38.7

−45.2 −45.7 −45.2

Table 3 The values of K, K1 , G◦1 , H1◦ and S1◦ for Amr–BTMACl, BTEACl and BTBACl complex formations System

T (◦ C)

K (dm3 mol−1 )3

K1 (dm3 mol−1 )

G◦1 (kJ mol−1 )

H1◦ (kJ mol−1 )

S1◦ (J mol−1 K−1 )

Amr–BTMACl

25 35 45

15.1 × 1011 3.14 × 1011 0.70 × 1011

4.17 × 104 2.47 × 104 1.50 × 104

−26.4 −25.9 −25.4

−41.3

−50.0 −50.0 −50.0

Amr–BTEACl

25 35 45

5.45 × 1011 1.69 × 1011 0.17 × 1011

2.97 × 104 2.01 × 104 0.93 × 104

−25.5 −25.4 −24.2

−44.7

−64.4 −62.6 −64.4

Amr–BTBACl

25 35 45

1.08 × 1011 0.45 × 1011 0.05 × 1011

1.73 × 104 1.29 × 104 0.62 × 104

−24.2 −24.2 −23.1

−40.4

−54.4 −52.6 −54.4

an increase in the order of the system [17]. Taking the values of thermodynamic parameters into consideration, it can be said that the complex formations of dye–surfactant systems are enthalpy-driven process. The interactions between an anionic dye, C.I. Reactive Orange 16, and the cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB), were studied using a conductometric method by Akbas¸ and Kartal [13]. They found that CTAB molecules have a longer aliphatic chain than DTAB molecules and CTAB has greater interaction with dye in solution. In another study, Braˇcko ˇ and Span [14] determined that the equilibrium constants of acid orange 7 with dodecylpyridinium chloride was lower than that of the same dye with hexadecylpyridinium chloride which has longer aliphatic chain. These results are contrary to our results. In their case, mainly the interactions between the hydrophobic parts of dye and surfactants are important for the formation of ion pair formation. On the other hand, Akbas¸ and Kartal [13] ˇ and Braˇcko and Span [14] indicated that K and negative G◦ values for all the systems studied decreased with the increasing of temperature as the tendency obtained in this work. 4. Conclusions Conductometric method was applied successfully for the investigation of dye–surfactant systems. Amr and IC formed a non-conducting or a less-conducting species with BTMACl, BTEACl and BTBACl surfactants. A theoretical method was used for determining the degree of interaction between dye and

surfactant molecules. The equilibrium constants for the process of dye–surfactant complex formation were calculated by this method. Standard free energy change values confirmed the spontaneity of the complex formation process. Increase of temperature lead to a decrease in the equilibrium constant values for all systems studied. The increase of the alkyl chain length of surfactants lowered the tendency to form non-conducting complex formation. The equilibrium constant values, K1 , for the first association step of dye–surfactant systems were higher for IC–surfactant system than Amr–surfactant system. It may be concluded from these results that electrostatic forces played important roles for the dye–surfactant complex formation. Acknowledgement The authors would like to thank to the Management Unit of Scientific Research Projects of Akdeniz University for the support of this work. References [1] [2] [3] [4] [5] [6] [7] [8]

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