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Spectral studies of methyl violet in aqueous solutions of different surfactants in supermicellar concentration region
Spectrochimica Acta Part A 55 (1999) 1737 – 1742
Spectral studies of methyl violet in aqueous solutions of different surfactants in supermicellar concentration region M. Sarkar *, S. Poddar Department of Chemistry, Uni6ersity of Kalyani, Kalyani 741 235, India Accepted 27 November 1998
Abstract The absorption spectra of methyl violet, a cationic dye, were investigated in aqueous solutions containing anionic, non-ionic and cationic surfactants above their critical micelle concentration (cmc). The dye forms 1:1 electron donor acceptor or charge transfer complexes with all of the non-ionic surfactants. The dye acts as the electron acceptor and the surfactants as the electron donors. The length of the alkyl hydrocarbon chain of the non-ionic surfactants influences the stability of the complex. The greater the chain length the more stable the complex will be. There is an electrostatic interaction of the dye with the anionic surfactant as exhibited in its electronic spectra while no perturbation in the spectra was observed for the cationic surfactant. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Methyl violet; Supermicellar concentration; Electrostatic interaction; Charge transfer interaction
1. Introduction The surfactants are drawing the attention of analytical chemists in recent years, due to their uses in analytical methods which provide an increase in selectivity and sensitivity [1]. Surfactants increase the solubility of organic compounds in water [2] and also catalyse some reactions, modifying the micro-environment in which the reactants are produced [3]. The nature and mechanism of interactions of surfactant with chemical systems are still not clearly understood [4]. Electrostatic interactions and/or hydrophobic interactions may take place [5]. The dye – surfactant * Corresponding author. Tel.: +91-33-582 8750.
interactions are also interesting due to their complex nature [6–13]. Molecular complexes having specific and characteristic physico–chemical features may be formed. The dye Phenosafranine in normal micelle [14] and Methylene blue in reverse micelle [15] are reported to form 1:1 molecular complexes with both the non-ionic and the anionic surfactant, while Congo red is reported to form 1:2 dye–surfactant complexes with CTAB and TX-100 [16]. The metachromatic dye Acridine orange has been observed to undergo complicated interactions with normal and reverse micelle [17,18]. The acid base behavior of indicator dyes are significantly influenced by reverse micellar solution [19–21]. Micellar media is being considered as simple biological mimetic system
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 3 4 4 - 8
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M. Sarkar, S. Poddar / Spectrochimica Acta Part A 55 (1999) 1737–1742
[22]. In continuation to our work on Methyl violet with surfactants [23] in submicellar concentration and to understand the nature of localisation and interaction of the dye with surfactants, the present communication deals with methyl violet–surfactants interaction in supercritical micellar concentration. The dye forms 1:1 charge transfer type complexes with all non-ionic surfactants while with anionic surfactant the complex formation is mainly due to electrostatic attraction. Cationic surfactant shows no perturbation of the electronic spectra of the dye.
2. Experimental The surfactants Sodium dodecylsulphate (SDS), Cetyltrimethyl ammoniumbromide (CTAB), Polyoxyethylene sorbitan-monolaurate (Tw-20), Polyoxyethylene sorbitan-monopalmitate (Tw-40), Polyoxyethylene sorbitan-monostearate (Tw-60), Polyoxyethylene sorbitan-monooleate (Tw-80), Octylphenylpolyoxyethylene ether (Triton X-100 or TX-100) were used of either BDH or Sigma products. Methyl violet (E. Merck) was recrystallised twice before use. The spectrophotometric measurements were carried out in a Perkin Elmer UV 200 spectrophotometer with a matched pair of silica cuvettes of 1 cm optical path length placed in a thermostated cell holder.
ing intensity as the SDS concentration increases (Fig. 1). A sharp isobestic at 450 nm indicates a 1:1 complex formation between methyl violet and SDS. Spectral investigations with non-ionic surfactants above their cmc indicate some different features. An enhancement in the absorbance with spectral investigations with non-ionic surfactants above their cmc indicate some different features. An enhancement in the absorbance with bathochromic spectral shift was observed (Figs. 2–6). The spectrum of the dye due to the interaction with surfactant micelle passes through two isobestic points, supporting the formation of a 1:1 dye–micellar complex above the cmc. Dye–surfactant complex formation can be assumed to follow the equilibrium Kc
D+ M X DM where D, M, DM and Kc represent the dye, micelle, dye–micelle complex and the complexation constant, respectively. For a 1:1 complex, the equilibrium constant Kc and molar extinction co-efficient oc can be determined using the Scott equation [24] in the following modified form [D][S]l [S] 1 = + d−d0 oc − o0 Kc(oc − o0)
(1)
3. Results and discussion The visible absorption spectrum of methyl violet (2.5×10 − 5 mol dm − 3) in aqueous solution exhibits an absorption maximum at 590 nm with an extinction coefficient (o) of 32108 dm3 mol − 1 cm − 1 and a shoulder at 540 nm with (o) 27 200 dm3 mol − 1 cm − 1 at 298 K. In the presence of increasing SDS concentration below its critical micelle concentration (cmc), the absorption spectra of methyl violet yield dimer and trimer of the dye [23]. In the presence of SDS above its cmc, a spectacular change occurs. The intensity of the band at 590 nm gradually diminishes with the appearance of a new band at 420 nm with increas-
Fig. 1. Visible absorption spectra of methyl violet and SDS at 298 K [methyl violet] =2.5 ×10 − 5 mol dm − 3 at [SDS]×101 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
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Fig. 4. Visible absorbance spectra of methyl violet and Tw-60 at 298 K [methyl violet] = 2.5 × 10 − 5 mol dm − 3 at [Tw-60] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0. Fig. 2. Visible absorption spectra of methyl violet and TX-100 at 298 K [methyl violet]= 2.5× 10 − 5 mol dm − 3 at [TX100] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
where [D] and [S] are the initial molar concentrations of methyl violet and the surfactant, respectively, l is the optical path length of the solution, d and d0 are the absorbances of methyl violet at the absorption maximum of the complex with and without surfactant, respectively and oc and o0 are the respective molar extinction coefficient of the complex and methyl violet. However, it is assumed that Eq. (1) is valid when [S]\ \ [D] and the complex absorbs at a wavelength where the surfactant is completely transparent. The plot of [D][S]l/(d −d0) against
Fig. 3. Visible absorption spectra of methyl violet and Tw-80 at 298 K [methyl violet] = 2.5 ×10 − 5 mol dm − 3 at [Tw-80] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
[S] was found to be linear in all cases confirming 1:1 complex formation. The extent of methyl violet–surfactant interaction in the aqueous medium Kc and oc was calculated from the slope and intercept. The Bensi Hildebrand equation [25] in the following modified form [D]l 1 1 = + d− d0 o−o0 Kc[S](oc − o0)
(2)
has also been used to calculate Kc and oc for the methyl violet–surfactant complexes; from the linear plots of [D]l/(d −d0) against 1/[S]. The almost identical values were obtained for Kc and oc from the Scott equation which confirmed the reliability of the experimental results. The data obtained from the spectral observations indicate that the different surfactants yield dye–micelle complexes with different affinity which follow the order Tw80\ Tw - 60\ Tw - 40\Tw - 20\ TX - 100\SDS (Table 1). The cationic dye should form a stable complex with anionic surfactants considering only the electrostatic factor. This is further established by the fact that there is also no perturbation in the spectrum of the dye with CTAB. However, experimental results show that methyl violet forms stronger complexes with all non-ionic surfactants compared to SDS, indicating that the nature of interactions in both cases are different. This is comparable with the interaction of phenozinium dyes and methyl orange [26] with a micelle of various charge type. It can be assumed
M. Sarkar, S. Poddar / Spectrochimica Acta Part A 55 (1999) 1737–1742
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Table 1 Spectrophotometric properties of methyl violet–surfactant interactions at 298 Ka Surfactant
pH
Kc×10−2 (dm3 mol−1)
lmax(oc)
Band shift Dn (cm−1)
cmc×104 (mol dm−3)
Tw-80 Tw-60 Tw-40 Tw-20 TX-100 SDS
3.42 3.28 4.02 4.74 5.80 6.08
4.46 3.79 2.90 1.90 1.50 0.52
618 (35602) 616.5 (36025) 616 (38075) 615.5 (38346) 602 (37865) 420 (18412)
764 728 715 702 337 686
0.10 0.21 0.23 0.50 2.50 5.00
a
Error limit of oc is 9 1% cmc and is taken from [3,27,28].
Fig. 5. Visible absorption spectra of methyl violet and Tw-40 at 298 K [methyl violet] = 2.5× 10 − 5 mol dm − 3 at [Tw-40] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
that non-ionic micelles being composed of polyethylene oxide residues are more favorable for the location of a dye cation. The polar oxygen centres interact in a concerted manner through the orientation of the head groups of the surfactant molecules to offer a relatively large electron density for stronger complex formation. The molecular interactions between methyl violet and non-ionic surfactants in an aqueous medium are considered to be charge transfer interactions which are comparable with starch – iodine complex formation [27,28]. The glucose units of the polymeric starch in the above complex causes induction through the polarity effect of oxygen centres with a specific conformational orientation. The increasing alkylhydrocarbon chain length of non-ionic surfactants enhancing the electron donor capacity of the molecule, due to an induc-
Fig. 6. Visible absorption spectra of methyl violet and Tw-20 at 298 K [methyl violet] =2.5 ×10 − 5 mol dm − 3 at [Tw-20] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
tive effect, also supports the above trend of complex formation. The order of Kc (binding constant) may represent the trend of the effect produced by the surfactant micelles. It is difficult to examine the role of the hydrophobic effect on the tendency of formation of the molecular complex. Tweens possess identical head groups [– (CH2 –CH2 –O)2 H] but different non-polar tails. It is observed that the larger the non-polar tail in the Tweens, the stronger the complex formation, indicating a direct correlation of complexation with hydrophobicity. The log Kc versus carbon number (n) profile for the Tweens when fitted to the linear equation (Eq. (3)) give the intercept value corresponding to log Kc at zero carbon number of the hydrophobic tail of the surfactants, indicating the complexing magnitude of the polyoxyethylene head group alone (20 ethyleneoxide
M. Sarkar, S. Poddar / Spectrochimica Acta Part A 55 (1999) 1737–1742
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Table 2 Thermodynamic parameters for methyl violet–surfactant interactionsa Surfactant
Tw-80 Tw-60 Tw-40 Tw-20 TX-100 SDS
Kc×10−2 (dm3 mol−1) 298 K
303 K
308 K
4.46 3.79 2.90 1.90 1.50 0.52
3.62 3.08 2.38 1.62 1.28 0.45
2.98 2.53 1.97 1.38 1.11 0.40
−DG° (kJ mol−1)
−DH° (kJ mol−1)
−DS° (Jmol−1 deg−1)
15.12 14.71 14.05 13.00 12.41 9.79
32.08 30.89 29.21 24.05 22.98 20.11
56.91 54.29 50.87 37.08 35.46 34.63
Error limit in Kc is 95%. The correlation coefficients are 0.9944, 0.9942, 0.9972, 0.9996, 0.9932, and 0.9932 for Tw-80, Tw-60, Tw-40, Tw-20, TX-100 and SDS, respectively. a
residues) of the Tweens. The linear equation takes the following form log Kc = 1.70+ 0.056 n
(3)
The presence of an aryl group in TX-100 results in an opposite effect compared to the Tweens. With negatively charged micelles of SDS, the cationic dye will be held in the Stern layer due to the Coulombic interaction, resulting in the dye being repelled by the positively charged micelles of CTAB. The results in Table 1 show that Kc values vary directly with the band shift (Dn) and are indirectly proportional to cmc [29,30]. The plot of log Kc against log cmc (Eq. (1)) can be fitted to the following equation log Kc = 1.15− 0.287 log cmc
(4)
where the slope and intercept take values 0.287 and 1.15, respectively. The Kc value calculated for methyl violet with TX-100 in the present case are quite close to that reported earlier [31]. The thermodynamic quantities of the complexes were obtained (Table 2) from Kc values at three different temperatures using the following relations DG° −RT ln Kc DH° = − R(T2T1/T2 −T1) ln K2/K1 DS°=(DH°− DG°)/T The exothermic process exhibits a negative entropy and the release of heat leads to a stable or organised state.
References [1] E. Blanco Gorzalez, J.I. Garcia Alonso, M.E. Diaz Garcia, A.Sanz. Medel, Quim. Anal. 5 (1986) 428. [2] L. Hinze, Use of surfactants and micellar systems in Anal Chem, in: K.L. Mittaled (Ed.), Solution Chem of surfactants, vol. 1, Plenum Press, NY, 1979. [3] J.H Fendler, E. Fendler, Catalysis in micellar and macromolecular systems, Academic Press, NY, 1975. [4] A. Sonz Medal, M.M. Fernandez Perez, M. Guardai Cirugeda, J.L. dela Carrion Dominguez, Anal. Chem. 58 (1986) 2161. [5] G.S. Hartley, Trans. Faraday Soc. 30 (1986) 541. [6] M.E. Diaz Garcia, A. Sanz Medal, Talanta 33 (1986) 255. [7] R.C. Kapoor, J. Ind. Chem. Soc. 63 (1986) 541. [8] J. Havel, I. Buresova-Jancarova, V. Kuban, Collect. Czech. Chem. Commun. 48 (1983) 1290. [9] V. Kuban, J. Hedvabny, I. Jancarova, M. Vrchlabsky, Collect. Czech. Chem. Commun. 54 (1989) 622. [10] K. Shinoda et al., Colloidal Surfactants, Academic Press, NY, 1963, p. 155. [11] S.C. Bhattacharya, H. Das, S.P. Moulik, J. Photochem. Photobiol, A: Chem 74 (1993) 239. [12] P. Pal, et al., J. Photochem. Photobiol, A: Chem. 98 (1996) 65. [13] B.B. Bhowmik, D.C. Mukherjee, J. Surf. Sci. Technol. 10 (1996) 97. [14] K.K. Rohatgi Mukherjee, R. Chaudhuri, B.B. Bhoumik, J. Colloid Interface Sci. 106 (1985) 45. [15] D. Pramanik, D. Mukherjee, J. Colloid Interface Sci. 157 (1993) 131. [16] M. Hayashi, Bull. Chem. Soc., Jpn 34 (1961) 119. [17] S.P. Moulik, S. Ghosh, A.R. Das, Colloid Polym. Sci. 257 (1979) 645. [18] O. Ortona, V. Vitagliano, B.H. Robinson, J. Colloid Interface Sci. 125 (1988) 271. [19] R.C. Vieiva, O.A.El. Seound, J. Colloid Interface Sci. 141 (1991) 295.
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[20] C. Oldfield, B.H. Robinson, R.B. Freedman, J. Chem. Soc. Faraday Trans. 86 (1990) 833. [21] R. Guo, X. Zhu, J.Surf. Sci Technol. 7 (1991) 41. [22] J.H. Fendler, Membrane mimetic chemistry, Wily Interscience, NY, 1982. [23] M. Sarkar, S. Poddar, communicated. [24] R.L. Scott, Trav. Chim. 75 (1956) 787. [25] H.A. Benesi, J.A. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2073.
.
[26] [27] [28] [29] [30]
R.K. Dutta, S.N. Bhat, Colloids Surf., A 106 (1996) 127. R.E. Rundle, R.R. Baldwin, JACS 65 (1943) 554. R.E. Rundle, D. French, JACS 65 (1943) 1707. P. Stibles, B. Lindman, J. Colloid Interface 99 (1984) 290. M.J. Schick, Non-ionic Surfactants, Marcel Dekker, NY, 1967, p. 481. [31] B.B. Bhowmik, R. Chaudhury, K.K. Rohatgi-Mukherjee, Ind. J. Chem. 26A (1987) 95.
.
Spectral studies of methyl violet in aqueous solutions of different surfactants in supermicellar concentration region M. Sarkar *, S. Poddar Department of Chemistry, Uni6ersity of Kalyani, Kalyani 741 235, India Accepted 27 November 1998
Abstract The absorption spectra of methyl violet, a cationic dye, were investigated in aqueous solutions containing anionic, non-ionic and cationic surfactants above their critical micelle concentration (cmc). The dye forms 1:1 electron donor acceptor or charge transfer complexes with all of the non-ionic surfactants. The dye acts as the electron acceptor and the surfactants as the electron donors. The length of the alkyl hydrocarbon chain of the non-ionic surfactants influences the stability of the complex. The greater the chain length the more stable the complex will be. There is an electrostatic interaction of the dye with the anionic surfactant as exhibited in its electronic spectra while no perturbation in the spectra was observed for the cationic surfactant. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Methyl violet; Supermicellar concentration; Electrostatic interaction; Charge transfer interaction
1. Introduction The surfactants are drawing the attention of analytical chemists in recent years, due to their uses in analytical methods which provide an increase in selectivity and sensitivity [1]. Surfactants increase the solubility of organic compounds in water [2] and also catalyse some reactions, modifying the micro-environment in which the reactants are produced [3]. The nature and mechanism of interactions of surfactant with chemical systems are still not clearly understood [4]. Electrostatic interactions and/or hydrophobic interactions may take place [5]. The dye – surfactant * Corresponding author. Tel.: +91-33-582 8750.
interactions are also interesting due to their complex nature [6–13]. Molecular complexes having specific and characteristic physico–chemical features may be formed. The dye Phenosafranine in normal micelle [14] and Methylene blue in reverse micelle [15] are reported to form 1:1 molecular complexes with both the non-ionic and the anionic surfactant, while Congo red is reported to form 1:2 dye–surfactant complexes with CTAB and TX-100 [16]. The metachromatic dye Acridine orange has been observed to undergo complicated interactions with normal and reverse micelle [17,18]. The acid base behavior of indicator dyes are significantly influenced by reverse micellar solution [19–21]. Micellar media is being considered as simple biological mimetic system
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 3 4 4 - 8
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M. Sarkar, S. Poddar / Spectrochimica Acta Part A 55 (1999) 1737–1742
[22]. In continuation to our work on Methyl violet with surfactants [23] in submicellar concentration and to understand the nature of localisation and interaction of the dye with surfactants, the present communication deals with methyl violet–surfactants interaction in supercritical micellar concentration. The dye forms 1:1 charge transfer type complexes with all non-ionic surfactants while with anionic surfactant the complex formation is mainly due to electrostatic attraction. Cationic surfactant shows no perturbation of the electronic spectra of the dye.
2. Experimental The surfactants Sodium dodecylsulphate (SDS), Cetyltrimethyl ammoniumbromide (CTAB), Polyoxyethylene sorbitan-monolaurate (Tw-20), Polyoxyethylene sorbitan-monopalmitate (Tw-40), Polyoxyethylene sorbitan-monostearate (Tw-60), Polyoxyethylene sorbitan-monooleate (Tw-80), Octylphenylpolyoxyethylene ether (Triton X-100 or TX-100) were used of either BDH or Sigma products. Methyl violet (E. Merck) was recrystallised twice before use. The spectrophotometric measurements were carried out in a Perkin Elmer UV 200 spectrophotometer with a matched pair of silica cuvettes of 1 cm optical path length placed in a thermostated cell holder.
ing intensity as the SDS concentration increases (Fig. 1). A sharp isobestic at 450 nm indicates a 1:1 complex formation between methyl violet and SDS. Spectral investigations with non-ionic surfactants above their cmc indicate some different features. An enhancement in the absorbance with spectral investigations with non-ionic surfactants above their cmc indicate some different features. An enhancement in the absorbance with bathochromic spectral shift was observed (Figs. 2–6). The spectrum of the dye due to the interaction with surfactant micelle passes through two isobestic points, supporting the formation of a 1:1 dye–micellar complex above the cmc. Dye–surfactant complex formation can be assumed to follow the equilibrium Kc
D+ M X DM where D, M, DM and Kc represent the dye, micelle, dye–micelle complex and the complexation constant, respectively. For a 1:1 complex, the equilibrium constant Kc and molar extinction co-efficient oc can be determined using the Scott equation [24] in the following modified form [D][S]l [S] 1 = + d−d0 oc − o0 Kc(oc − o0)
(1)
3. Results and discussion The visible absorption spectrum of methyl violet (2.5×10 − 5 mol dm − 3) in aqueous solution exhibits an absorption maximum at 590 nm with an extinction coefficient (o) of 32108 dm3 mol − 1 cm − 1 and a shoulder at 540 nm with (o) 27 200 dm3 mol − 1 cm − 1 at 298 K. In the presence of increasing SDS concentration below its critical micelle concentration (cmc), the absorption spectra of methyl violet yield dimer and trimer of the dye [23]. In the presence of SDS above its cmc, a spectacular change occurs. The intensity of the band at 590 nm gradually diminishes with the appearance of a new band at 420 nm with increas-
Fig. 1. Visible absorption spectra of methyl violet and SDS at 298 K [methyl violet] =2.5 ×10 − 5 mol dm − 3 at [SDS]×101 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
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Fig. 4. Visible absorbance spectra of methyl violet and Tw-60 at 298 K [methyl violet] = 2.5 × 10 − 5 mol dm − 3 at [Tw-60] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0. Fig. 2. Visible absorption spectra of methyl violet and TX-100 at 298 K [methyl violet]= 2.5× 10 − 5 mol dm − 3 at [TX100] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
where [D] and [S] are the initial molar concentrations of methyl violet and the surfactant, respectively, l is the optical path length of the solution, d and d0 are the absorbances of methyl violet at the absorption maximum of the complex with and without surfactant, respectively and oc and o0 are the respective molar extinction coefficient of the complex and methyl violet. However, it is assumed that Eq. (1) is valid when [S]\ \ [D] and the complex absorbs at a wavelength where the surfactant is completely transparent. The plot of [D][S]l/(d −d0) against
Fig. 3. Visible absorption spectra of methyl violet and Tw-80 at 298 K [methyl violet] = 2.5 ×10 − 5 mol dm − 3 at [Tw-80] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
[S] was found to be linear in all cases confirming 1:1 complex formation. The extent of methyl violet–surfactant interaction in the aqueous medium Kc and oc was calculated from the slope and intercept. The Bensi Hildebrand equation [25] in the following modified form [D]l 1 1 = + d− d0 o−o0 Kc[S](oc − o0)
(2)
has also been used to calculate Kc and oc for the methyl violet–surfactant complexes; from the linear plots of [D]l/(d −d0) against 1/[S]. The almost identical values were obtained for Kc and oc from the Scott equation which confirmed the reliability of the experimental results. The data obtained from the spectral observations indicate that the different surfactants yield dye–micelle complexes with different affinity which follow the order Tw80\ Tw - 60\ Tw - 40\Tw - 20\ TX - 100\SDS (Table 1). The cationic dye should form a stable complex with anionic surfactants considering only the electrostatic factor. This is further established by the fact that there is also no perturbation in the spectrum of the dye with CTAB. However, experimental results show that methyl violet forms stronger complexes with all non-ionic surfactants compared to SDS, indicating that the nature of interactions in both cases are different. This is comparable with the interaction of phenozinium dyes and methyl orange [26] with a micelle of various charge type. It can be assumed
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Table 1 Spectrophotometric properties of methyl violet–surfactant interactions at 298 Ka Surfactant
pH
Kc×10−2 (dm3 mol−1)
lmax(oc)
Band shift Dn (cm−1)
cmc×104 (mol dm−3)
Tw-80 Tw-60 Tw-40 Tw-20 TX-100 SDS
3.42 3.28 4.02 4.74 5.80 6.08
4.46 3.79 2.90 1.90 1.50 0.52
618 (35602) 616.5 (36025) 616 (38075) 615.5 (38346) 602 (37865) 420 (18412)
764 728 715 702 337 686
0.10 0.21 0.23 0.50 2.50 5.00
a
Error limit of oc is 9 1% cmc and is taken from [3,27,28].
Fig. 5. Visible absorption spectra of methyl violet and Tw-40 at 298 K [methyl violet] = 2.5× 10 − 5 mol dm − 3 at [Tw-40] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
that non-ionic micelles being composed of polyethylene oxide residues are more favorable for the location of a dye cation. The polar oxygen centres interact in a concerted manner through the orientation of the head groups of the surfactant molecules to offer a relatively large electron density for stronger complex formation. The molecular interactions between methyl violet and non-ionic surfactants in an aqueous medium are considered to be charge transfer interactions which are comparable with starch – iodine complex formation [27,28]. The glucose units of the polymeric starch in the above complex causes induction through the polarity effect of oxygen centres with a specific conformational orientation. The increasing alkylhydrocarbon chain length of non-ionic surfactants enhancing the electron donor capacity of the molecule, due to an induc-
Fig. 6. Visible absorption spectra of methyl violet and Tw-20 at 298 K [methyl violet] =2.5 ×10 − 5 mol dm − 3 at [Tw-20] × 102 mol dm − 3: (1) 0.0; (2) 3.0; and (3) 5.0.
tive effect, also supports the above trend of complex formation. The order of Kc (binding constant) may represent the trend of the effect produced by the surfactant micelles. It is difficult to examine the role of the hydrophobic effect on the tendency of formation of the molecular complex. Tweens possess identical head groups [– (CH2 –CH2 –O)2 H] but different non-polar tails. It is observed that the larger the non-polar tail in the Tweens, the stronger the complex formation, indicating a direct correlation of complexation with hydrophobicity. The log Kc versus carbon number (n) profile for the Tweens when fitted to the linear equation (Eq. (3)) give the intercept value corresponding to log Kc at zero carbon number of the hydrophobic tail of the surfactants, indicating the complexing magnitude of the polyoxyethylene head group alone (20 ethyleneoxide
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Table 2 Thermodynamic parameters for methyl violet–surfactant interactionsa Surfactant
Tw-80 Tw-60 Tw-40 Tw-20 TX-100 SDS
Kc×10−2 (dm3 mol−1) 298 K
303 K
308 K
4.46 3.79 2.90 1.90 1.50 0.52
3.62 3.08 2.38 1.62 1.28 0.45
2.98 2.53 1.97 1.38 1.11 0.40
−DG° (kJ mol−1)
−DH° (kJ mol−1)
−DS° (Jmol−1 deg−1)
15.12 14.71 14.05 13.00 12.41 9.79
32.08 30.89 29.21 24.05 22.98 20.11
56.91 54.29 50.87 37.08 35.46 34.63
Error limit in Kc is 95%. The correlation coefficients are 0.9944, 0.9942, 0.9972, 0.9996, 0.9932, and 0.9932 for Tw-80, Tw-60, Tw-40, Tw-20, TX-100 and SDS, respectively. a
residues) of the Tweens. The linear equation takes the following form log Kc = 1.70+ 0.056 n
(3)
The presence of an aryl group in TX-100 results in an opposite effect compared to the Tweens. With negatively charged micelles of SDS, the cationic dye will be held in the Stern layer due to the Coulombic interaction, resulting in the dye being repelled by the positively charged micelles of CTAB. The results in Table 1 show that Kc values vary directly with the band shift (Dn) and are indirectly proportional to cmc [29,30]. The plot of log Kc against log cmc (Eq. (1)) can be fitted to the following equation log Kc = 1.15− 0.287 log cmc
(4)
where the slope and intercept take values 0.287 and 1.15, respectively. The Kc value calculated for methyl violet with TX-100 in the present case are quite close to that reported earlier [31]. The thermodynamic quantities of the complexes were obtained (Table 2) from Kc values at three different temperatures using the following relations DG° −RT ln Kc DH° = − R(T2T1/T2 −T1) ln K2/K1 DS°=(DH°− DG°)/T The exothermic process exhibits a negative entropy and the release of heat leads to a stable or organised state.
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