Interaction of phenazinium dyes and methyl orange with micelles of various charge types

COLLOIDS

AND Colloids and Surfaces A: Physicochemicaland EngineeringAspects 106 (1996) 127-134

ELS EVI E R

SURFACES

A

Interaction of phenazinium dyes and methyl orange with micelles of various charge types Robin K. Dutta 1, Subray N. Bhat * Department of Chemistry, North Eastern Hill University, Shillong-793 003, Meghalaya, India Received 6 January 1995; accepted 7 August 1995

Abstract

Interactions of phenazinium dyes, phenosafranine (PSF), safranine O (SFO), safranine T (SFT) and methyl orange (MO) with micelles of surfactants having head groups of various charge types have been investigated spectroscopically. The interactions are found to be electrostatic as well as hydrophobic. No interaction was observed in similarly charged dye-surfactant systems. The strength of the interaction of the dyes with nonionic surfactant Triton X100 increases in the order PSF < SFT < SFO < MO.

Keywords: Dye surfactant interaction; Methyl orange; Phenazinium dyes; Triton X100

1. Introduction Micelles have been considered as simple chemic a l m o d e l s f o r b i o m e m b r a n e s [11. Dye-surfactant interaction pertains to the interactions between large organic molecules and biomembranes, etc. [23. In addition, the studies of binary dye-surfactant systems are also pertinent to other scientific fields including pharmaceutical science, analytical chemistry, photography, luminescence and lasers [3]. Dyes exhibit significant spectral changes in micellar media [ 3 - 6 ] . It has been suggested that owing to electrostatic interactions, the ionic dyes can form molecular complexes with oppositely charged micelles [7,8]. Bunton and co-workers [9,10] have shown that aromatic compounds with sulphonic acid groups are incorporated into the 1 Present address: Department of Chemistry, Darrang College, Tezpur-784 001, Assam, India. *Corresponding author,

0927-7757/96/$15.00© 1996ElsevierScienceB.V. All rights reserved SSDI 0927-7757(95)03374-2

Stern layer of cationic micelles in a sandwich arrangement and suggested that a van der Waals interaction between adjacent surfactant chains and the dye organic moiety (hydrophobic forces), causes alteration of the chromophore microenvironment. Many researchers have reported the absence of any interaction in similarly charged dye-micelle systems owing to electrostatic repulsion [7,8]. However, the presence of a strong hydrophobic effect has been known to overcome such an electrostatic repulsion in hydrophobic dye-micelle systems [11-14]. Dill et al. [15] are of the opinion that dye molecules are placed in a hydrophobic micellar environment when they are exposed to water. When surfactants are added to the dye solutions, the spectral band of the dyes shifts. This has been ascribed to the deaggregation of dyes upon incorporation into micelles [6,16-18], joint effect of deaggregation and the change in microenvironment [ 19,20], localization of the chromophore within the hydrophobic micellar interior [21], electrostatic interaction [22],

128

R.K. Dutta, S.N. Bhat/Colloids SurJaces A: Physicochem. Eng. Aspects 106 (1996) 12~134

and charge-transfer (electron donor-acceptor) interactions [8,22 24]. Diaz Garcia and SanzMedel [5] in a review article pointed out that it is not possible to pinpoint which force is responsible for dye-surfactant interactions. Therefore, it was thought worthwhile to carry out a systematic investigation on the dye-micelle interaction of various charge type combinations in order to have a better understanding of the nature of the interaction and the cause of the spectral changes. The dyes chosen for such studies were phenazinium (cationic) dyes, phenosafranine (PSF), safranine O (SFO), safranine T (SFT) and a sulphonated azo (anionic) dye, methyl orange (MO). Surfactants of all three charge types, anionic (sodium dodecyl sulphate (SDS)), nonionic (Triton X100 (TX100)) and cationic (N-hexadecylpyridinium chloride (CPC)) were used.

tained within _+0.1 K which was attained after keeping the solitions in the thermostated cells for about 15 min.

3. Results and discussion 3.1. Phenazinium dyes

The variation in the visible absorption spectra of 2.22 x 10 5 mol dm -3 solution of PSF, on addition of TX100 of various concentrations are shown in Fig. 1. No detectable changes in the spectra of the dye (2max=520 nm and £ma,,=3600 m 2 tool -1) were observed when the concentrations of the surfactant were below the CMC (the CMC of TX100 in water is 3.5 x 10 .4 mol dm -3 [27]). However, when the concentration of TX100 was above the CMC, the spectral character of the dye changed. With an increase in the concentration of TX100, the spectral absorbances of the 520nm band shifted slightly towards longer wavelengths with a gradual increase in the absorbance. An absorption maximum of the red shifted band was at 535nm (Emax=4990 m 2 mo1-1 T a b l e l ) was observed at higher concentrations ofthesurfactant. There was an isosbestic point at 518 n m (6"max = 3600 m 2 mol-1) and the presence of the isosbestic point indicates that an equilibrium exists between the free dye and its interaction product with the micelles. The spectral changes of SFT and SFO in the micellar medium of TX100 were similar to those observed in the case of PSF (Table 1). PSF, in the visible region, has an absorption

2. Experimental The sources and purification of PSF, SFT, SFO and SDS have been mentioned elsewhere [25]. The sources and the purification of MO and (CPC) have been reported earlier [26] TX100 (p-tCsH17C6H4o(CHzO),H, n~9.5), was from Sigma and was used as supplied. The electronic absorption spectra were recorded on a Hitachi 330 UV-VIS-NIR spectrophotometer using a matched pair of 1 cm path-length cells placed in a thermostated cell holder. Solutions were prepared in triply distilled water. The temperature of the experimental solutions were main-

ClPSF

N~ SFO

SFT

/o

"e2NN=N--L O-N MO

Structure of the dyes used.

o

CI-

R.K. Dutta, S.N. Bhat/Colloids SurJaces A." Physicochem. Eng. Aspects 106 (1996) 12~134

9 l 0.8 -

1

,j, 1

0.6 i

9 0.z,[1~

~

.£3

,~ 0.2 ,j

0.0.

I 400

I 500 (nm)

600 ,

Fig. 1. Spectra of PSF (2.22 x 10 -5 mol dm 3) in the presence of TXI00 of various concentrations at 298.15 K. [TX100] x 10 3 mol dm-3: (1)=0.0, (2)=2.57, (3)=5.13, (4)=10.3, (5)=15.4, (6)=23.1, (7)=30.8, (8)=41.1 and (9)= 51.3.

band with a maximum at 520 nm (emax=3600 m 2 mol-1 ). On increasing the concentration of SDS above 6 x 10 -3 mol dm -3 (this may be the CMC of SDS in presence of the dye [25], the CMC of SDS in pure water is 8.0 x 10 -3 mol dm -3 [28]), the absorption spectra of PSF give rise to a new band at 531 nm [25]. The band at 531 nm has been attributed to the dye-SDS micelle interaction, We did not observe any detectable change in the spectra of the phenazinium dyes on the addition of CPC (a cationic surfactant), which can be easily understood from electrostatic repulsion between the dye and the surfactant having similar charges [22]. 3.2. Methyl orange

MO, an anionic dye, interacts with TX100 micelles. The variations in spectral absorbances of

129

(2.54 x 10 5 mol dm 3) MO solution on addition of TX100 is shown in Fig. 2. In aqueous medium MO has an absorption maximum at 463 nm (e= 2520 m 2 mol 1) [26]. On the addition of small amount of TX100 (below CMC) to MO solutions no detectable changes in the spectra of MO were noticed. However, when the concentrations of TX100 were increased above the CMC, the absorbance at the 463 nm band started decreasing and at the same time the absorbance at lower wavelength region started increasing. At sufficiently high concentration of TX100, (about 5.6 x 10 3 mol dm 3)' a new abs°rpti°n band' with abs°rpti°n maximum at 430 nm (~ma×=2640 m 2 mol 1 was observed. An isosbestic point was observed at 444 nm (eiso= 2400 m 2 mo1-1) and the presence of the isosbestic point indicates the presence of an equilibrium between the free dye and the micelle bound dye. The variations in the absorption maximum of MO with the variation in concentration of CPC, a cationic surfactant, are shown in Fig. 3(a) (b). On the addition of CPC, a new absorption band, with absorption maximum at 369 n m w a s observed w i t h gradual d e c r e a s e in t h e intensity o f t h e 463 n m band. When the concentration of the surfactant was increased above 4.0 x 10 - 4 mol d m -3, the intensity of the 369 nm band started decreasing with a corresponding appearance of another absorption band with an absorption maximum at 435 nm (£max=2600 m 2 mol 1). The 369 nm band of MO at concentrations of CPC far below its CMC (9 x 10 -4 mol dm 3 in pure water [27]) can be ascribed to the dye-surfactant ion pairs [26]. The 435 nm band can be attributed to the interaction of MO with the CPC micelles where the micelles probably start forming at around 4.0 x 10 4 mol dm -3 in presence of the dye. The CMC of CPC in the presence of the dye is probably decreased to about 4.0 x 10 -1 mol dm 3. The gradual disappearance of the 369 nm band and corresponding developement of the 435 nm band can be attributed to conversion of the ion pair into a charge-transfer complex between dye and micelle. There was no change in the spectral character of MO on addition of the anionic surfactant, SDS. This is due to the electrostatic repulsion between

R.K. Dutta, S.N. Bhat/Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 127 134

130

Table 1 Spectral properties of the dyes in water and in micellar solutions of SDS, TX100 and CPC at 298.15( _+0.1) K Dye

Aqueous solution

Surfactant

)tisoa (rim)

eisob (m2 mol 1)

518 518c

3600

/~maxa

Emax b

,~maxa

Emax b

(nm)

(m 2 mol- 1)

(nm)

(m 2 mol - 1)

CPC SDS TX 100 CPC SDS TX100 CPC SDS TXI00

531 531 ~ 535 537.5¢ 520 531 535 520 531 535 520 463 430

4550 4870 4990 5270c 3600 3600 4080 2900 4600 5280 3700 2500 2640

518 518

2900

444

2400

CPC

435

2600

-

SDS PSF

Micellar solution

520 520c

3600 3612c

SFT

520

2900

SFO

520

3700

MO

463 463a

2520 462 ~

TX100

3700

a Error limit = _ 1 nm. b Error limit = _+ 50 m z mol 1. c Ref. [22]. d Ref. [31]. e Ref. [32].

0.6 -

7

~

,' ]

0.4

1 /

0.2

0.0

I ~00 x(nm)

I 500 >

60o

Fig. 2. Spectra of MO (2.54 × 10 -s mol dm 3) in the presence of TX100 of various concentrations at 298.15 K. [TX100] x 10 -3 mol dm 3: (1)=0.00, (2)=0.84, (3)=1.40, (4) = 2.80, (5)= 5.60, (6)= 14.0 and (7)= 28.0.

the dyes a n d the micelles o f s i m i l a r charges, p a r t i c ularly, w h e n t h e r e is n o s t r o n g h y d r o p h o b i c effect. T h e i n t e r a c t i o n s b e t w e e n c a t i o n i c dyes a n d a n i o n i c micelles o f S D S are e x p e c t e d to be s t r o n g e r t h a n t h o s e b e t w e e n c a t i o n i c dyes a n d n o n i o n i c micelles of TX100. T h e s h i f t / p e r t u r b a t i o n o f the '~max of the dye is n o r m a l l y c o n s i d e r e d to be an i n d i c a t i o n of the s t r e n g t h of the i n t e r a c t i o n . H o w e v e r , in the p r e s e n t case, it has b e e n n o t i c e d t h a t t h e shift in the a b s o r p t i o n b a n d of P S F is less in S D S micelles ( f r o m 520 to 531 nm) c o m p a r e d to t h a t in the T X 1 0 0 micelles ( f r o m 520 to 535 nm). T h e s a m e t r e n d has b e e n o b s e r v e d in the o t h e r t w o dyes, S F O a n d S F T ( T a b l e 1). It m a y be m e n t i o n e d h e r e t h a t R o h a t g i - M u k h e r j e e et al. [ 2 2 ] also h a d r e p o r t e d such b e h a v i o u r . W e feel t h a t for c o m p a r i n g the s t r e n g t h s of the i n t e r a c t i o n b e t w e e n the t w o systems, the " n a t u r e " of i n t e r a c t i o n s h o u l d be same. I n the p r e s e n t case, the n a t u r e of the i n t e r a c t i o n in P S F - S D S a n d M O - C P C systems are e l e c t r o s t a t i c / i o n i c w h e r e a s t h e y are of c h a r g e t r a n s f e r t y p e in P S F - T X 1 0 0 a n d M O T X I 0 0 systems. It m a y be n o t e d t h a t l a r g e r shifts in the a b s o r p t i o n b a n d s of the dye m a y n o t necessarily i m p l y a s t r o n g e r i n t e r a c t i o n b e t w e e n the d y e a n d the micelles, as the p o s i t i o n of an e l e c t r o n i c a b s o r p -

R.K. Dutta, S. ~ Bhat/Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 127-134

o.61--

0,4l-- I/x|

/

I/I/

X \

/

~

\\~

I

0.6

I

0.4

0.2

~-

0.2

..Q <

<

0.0 ~ 400

(a)

131

Mnm)

~ 500

0.0 600

>

400

(b)

Mnm)

500

600

>

Fig. 3. Variation in the spectra of M O (2.85 x 10 -5 mol dm 3) on the addition of CPC at 298.15 K. (a) [CPC] x 10 ,5 mol dm 3: (1)=0.0, (2)=0.8, (3)=1.2, (4)=1.6, (5)=2.0, (6)=2.4, (7)=2.8, (8)=3.2, (9)=4.0, (10)=6.0 and (11)=20.0. (b) [-CPC] x 10 4 mol dm 3: (1)=0.0, (2)=4.0, (3)=6.0, (4)=6.5, (5)=7.2, (6)=7.5, (7)=8.0, (8)= 10.0, (9)=50.0 and (10)= 100.0.

tion band is largely affected by factors such as the dielectric constant of the microenvironment of the dye, hydrogen bonding of the chromophore with the solvent, etc. Owing to the electrostatic attraction, the ionic dyes are likely to be bound to the surface of micelles of oppositely charged surfactants [-15]. However, in the case of nonionic micelle~s, the dyes can penetrate into the micelles further from the surface of the micelles and can reside in, a more hydrophobic (lower dielectric) microenvironment than in the case of ionic micelles of oppositely charged surfactant and this may be the reason for the greater effect of the nonionic micelles on the spectra of MO than that of the cationic micelles. PSF, owing to electrostatic attraction, is likely to be located on the surface of the anionic micelles, whereas, it is likely to be located towards the core in the nonionic micelles. Thus, the nonionic micelles may perturb the dye chromophore to a greater extent (compared to perturbation caused

by the anionic micelles) and this this may cause the absorption maxima of the phenazinium dyes in TX100 micelles to shift to a larger extent compared to those in the SDS micelles.

3.3. Determination of equilibrium constant The equilibrium of for the interaction of the dyes with the surfactants can be represented as [-22] D q-S m .

Kc " D.S m

(1)

Kc = [-D.Sm]/ED] [-Sm]

(2)

or

where, [D.Sm] and [D] stand for the concentration of bound and flee dye, respectively, and [Sin] is the concentration of the micellized surfactant, ( [ S ] - CMC) [29]. The equilibrium constants for the interaction of

R.K. Dutta, S.N. Bhat/Colloids Surjaces A: Physicochem. Eng. Aspects 106 (1996) 127 134

132

dyes with TX100 have been determined by using the well known Ketelaar equation 1-22,30]: , ED]o 1 1 =---+ (3)

d-do

ec-eo

(eo-eo)K¢[Sm]

where I-D]o is the total concentration of the dye, do and d are the absorbances in the absence and presence of the micelles, eo and ec are the molar extinction coefficients of the dye in the free and micelle-bound states respectively, and [Sm]>>[D]o. The absorbances at the values of)~max of the micelle-bound dyes were used for the plots of [Do]/(d - do) vs. [Sm] 1. The plots were found to be quite linear (Fig. 4). The Kc values were calculated from the slopes and the intercepts of such plots. The Kc values for the interaction of

T 0.0031

-1

PSF, SFO, SFT and MO with TX100 micelles at three temperatures are given in Table 2. It can be seen from Table2 that the phenazinium dyes strongly interact with TXI00 micelles and the strength of the interaction increases in the order PSF < SFT < SFO. The relatively stronger interaction of SFT and SFO compared to that of PSF can be attributed to the presence of two methyl groups in SFT and SFO. The presence of two methyl groups increases the hydrophobicity of SFO and SFT Compared to that of PSF. The slightly higher K c values for SFO compared to those for SFT may be due to the differences in the positions of the methyl and amino groups in the two dyes. From the present limited data, we are tempted to infer that the hydrophobicity of the

r "~

0.0032

0.0033

!

I

t2

5-0

T

?E ,o "5 E

4"8

o X

8

T "

o I

r-~ ao

4"6 6

4

1

0-0

I

50 t00 [Sm] -t dm3mol-I

150

>

Fig. 4. Plots of [Doll(d-do) vs. [ S m ] for PSF-TX100 system at 298.15K, (C)); 308.15 K, ( × ) and 318.15K, ( ~ ) [ P S F ] o = 2.22 × 10 5 mol dm 3. ( 0 ) van't Hoff plot.

R.I(2 Dutta. S.N. Bhat/Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 12~134

Table 2 Equilibrium constant, K~, and molar extinction coefficients, Ec for the interaction of the dyes with micellar TX100 at different

temperatures

Dye

PSF

SFT

Temperature" (K)

Ecu

Kc

(m a mo1-1)

(dma mol ')

298.15

4990

155( _+ 10)

308.15 318.15 298.15 308.15

5270¢ 4800 4710 4080 4010

308.15 318.15 298.15 308.15 318.15

5260 5200 2640 2610 2580

318.15 298.15

SFO

MO

3860 5280

188.10c 128(_+ 10) 108(_+10) 329(_+ 15) 281(_+15)

228( + 15) 403( _+ 20) 327(_+20)

257( _+2o) 524(_+20)

420(_+20) 338(_+20)

Error limit = _+0.1 K. b Error limit = _+ 50 m 2 mol 1.

CRef. [22]. dye plays an important role in the interaction between the dye and the micelles, It can be seen from Tables 2 and 3 that the interaction of MO with TX100 is stronger than the interaction of phenazinium dyes with TX100. MO, being a relatively long molecule, may bind to the micelles more strongly than the phenazinium dyes, by incorporating into the Stern layer of the micelle in a sandwich arrangement [9,10]. Our attempts to determine the equilibrium constants for the interaction of the oppositely charged dyes with the surfactants, i.e. for the phenazinium Table 3 Thermodynamic parameters for the interactions of the dyes with micellar TX100 at 298.15( _+ 0.1) K Dye

-(AG°) a (kJ mol -I)

-(AH°) b (kJ mo1-1)

-(AS°) c (J mol 1 K - l )

PSF

12.5 13.05d 14.4 14.8 15.5

14.3

5.9

16.6 17.8 17.3

8.3 9.6 6.0

SFT SFO MO

" Error limit = _+0.2 kJ mol 1 b Error limit = _+ 0.4 kJ mol 1. c Error limit = _+ 1 J m o l i K - ~. d Ref. [22].

133

dyes-SDS micelles and M O - C P C micelles were not successful owing to the complete incorporation of the dyes into the micelles (i.e. saturation is reached) within a very narrow range of concentration of surfactants leading to very high errors in the K c values. Moreover, no isosbestic points were observed for these systems above the CMC. The presence of other types of interactions (such as ion pair formation) of the dyes with oppositely charged surfactants in the submicellar concentration ranges of the surfactants also renders the determination of the equilibrium constant difficult

[-25,26].

The thermodynamic parameters for the dye-TX100 adducts were calculated using the van't Hoff equation and the results are summarised in Table 3. It can be seen from Table 3 that the interactions of TX100 micelles with all the dyes studied are exothermic and the enthalpies of the interactions are small. The small, and negative entropy changes indicate that the mixed aqueous dye-micelle systems are slightly more ordered than the separated systems. So, it appears that the stability of the dye-micelle systems arises owing to the overall free energy changes rather than the contribution of the entropies of the interactions.

Acknowledgement Financial assistance from the U G C under Special Assistance Programme is gratefully acknowledged.

References

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R.K. Dutta, S.N. Bhat/Colloids SurJaces A: Physicochem. Eng. Aspects 106 (1996) 127-134

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