Polarity Dependent Positional Shift of Probe in a Micellar Environment

3114

Langmuir 1996, 12, 3114-3121

Polarity Dependent Positional Shift of Probe in a Micellar Environment Tarasankar Pal* and Nikhil R. Jana Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Received August 11, 1995. In Final Form: April 11, 1996X Spectroscopic properties and acid-base equilibrium of hydroxyanthraquinone dyes have been studied in nonionic, cationic, and anionic micellar media as well as in other solvents, and it has been shown that the relative position of dye in micelle depends on the polarity of dye and nature of micelle. The shift of pKa value in nonionic micelle, pKia, compared to that in water, pKw a , has been explained considering the shift of the relative position of the dye during deprotonation, associated with medium effect. Surface potentials, ψ, for charged micelles at different bulk ionic strengths are calculated for different dyes, and it is noted that the influence of the bulk ionic strength on acid-base equilibrium of charged micelle bound dye depends on the relative position of the dye in the micelles. A simplified form of the pseudophase ion exchange model has been found suitable for quantitative interpretation of the change in apparent pKa values of the dyes with the change in the bulk ionic strength. This model has been utilized to calculate m i the intrinsic micellar pKa, pKm a , for different dyes. Comparison of these pKa values with pKa gives an idea about the average polarity of location of the dyes in micelles.

Knowledge regarding the property of micelle-solubilized dye is of importance in understanding the chemical equilibria, mechanisms, and kinetics of micelle-sensitized color and fluorescence reactions.1-10 It has also been used to measure the critical micelle concentration (cmc) of amphiphiles11-14 and surface property of micelles.15-17 Dyes are useful probes for the measurement of micellar surface properties due to their high molar extinction coefficients which permits their use at a very low concentration, to minimize perturbation of the micellar surfaces. Micellar medium leads to the shift of acid-base equilibrium of dyes which was first observed by Hartley,18 and since then a wide variety of dyes7,9,19-22 were studied in micellar media to examine the acid-base properties of dyes. The apparent pKa shifts arise primarily due to a combination of electrostatic and microenvironmental X

Abstract published in Advance ACS Abstracts, June 1, 1996.

(1) Mittal, K. L.; Lindman, B. Surfactants in Solution; Plenum Press: New York, 1984. (2) Mittal, K. L. Micellization, Solubilization and Microemulsions; Plenum Press: New York, 1977. (3) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley and Sons: New York, 1978. (4) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (5) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1978, 74, 10. (6) Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 169. (7) Diaz-Garcia, M. E.; Sanz-Medel, A. Talanta 1986, 33, 255. (8) Cline Love, L. J.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1133A. (9) Hinze, W. L. Use of Surfactant and Micellar Systems in Analytical Chemistry, Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1. (10) Hinze, W. L.; Singh, H. N.; Baba, Y.; Harvey, N. G. Trends Anal. Chem. 1984, 3, 193. (11) Corrin, M. L.; Harkins, W. D. J. Am. Chem. Soc. 1974, 96, 679. (12) Ledbetter, J. W.; Bowen, J. R. Anal. Chem. 1969, 41, 1345. (13) Rosenthal, K. S.; Koussale, F. Anal. Chem. 1983, 55, 1115. (14) Mukerjee, P.; Mysels, K. J. J. Am. Chem. Soc. 1955, 77, 2937. (15) Ferna´ndez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (16) Mukerjee, P.; Banerjee, K. J. Phys. Chem. 1964, 68, 3567. (17) Fendler, J. H. Membrane Mimetic Chemistry; Wiley Interscience: New York, 1982. (18) Hartley, G. S. Trans. Faraday Soc. 1934, 30, 444. (19) Minch, M. J.; Giaccio, M.; Wolff, R. J. Am. Chem. Soc. 1975, 97, 3766. (20) McIntire, G. L. Crit. Rev. Anal. Chem. 1990, 21, 257. (21) Pramauro, E.; Saini, G.; Pelizzetti, E. Anal. Chim. Acta 1984, 166, 233. (22) Rosendorfova´, J.; C ˇ erma´kova´, L. Talanta 1980, 27, 705.

S0743-7463(95)00680-9 CCC: $12.00

effects of the micelles.15,16,23-26 Addition of salt to an aqueous micellar solution changes the surface properties of the micelles, which includes changes in polarity, pH, and potential, and thus also affects the indicator equilibrium.15,16,26,27 In previous studies it has been assumed that the relative position of the probe in micelle remains unchanged. But during micelle-sensitized chemical reactions, if the probe changes its polarity, the relative position of it may also be changed. This would further influence the properties of the probe. To investigate this effect we have chosen seven hydroxyanthraquinone (HAQ) dyes with one or more hydroxyl groups in different positions and studied their solubility, UV-visible and fluorescence spectral properties, and acid-base equilibria in different micellar media and also in different solvents. Both R- and β-HAQ have been used because of their difference in polarity. Dyes containing more than one hydroxyl group are used to compare the influence of micellar media on their successive pKa values (Figure 1). Many HAQ derivatives are biochemically active28 natural products29 and many of them are widely used as dyes30 as well as analytical reagents.31 But there is little information available about the nature of interaction32 of anthraquinone dyes with micelles, although some of them have been used as improved spectrophotometric reagent in the presence of micelles.33-35 These results would enable one to choose dye molecules as improved analytical reagent (23) Romsted, L. S. J. Phys. Chem. 1985, 89, 5107. (24) Burton, C. A.; Romsted, L. S.; Sepulveda, L. J. Phys. Chem. 1980, 84, 2611. (25) Pesavento, M. J. Chem. Soc., Faraday Trans. 1992, 88, 2035. (26) Romsted, L. S.; Zanette, D. J. Phys. Chem. 1988, 92, 4690. (27) Romsted, L. S. J. Phys. Chem. 1985, 89, 5113. (28) (a) Anke, H.; Kolthoum, I.; Laatsch, H. Arch. Microbiol. 1980, 126 (3), 231. (b) Fuzellier, M. C.; Mortier, F.; Lectard, P. Ann. Pharm. Fr. 1982, 40 (4), 357; Chem. Abstr. 1983, 98, 86112W. (c) Logrange, E. C. R. Soc. Biol. 1946, 140, 1186. Chem. Abstr. 1948, 42, 1694a. (29) Thomson, R. H. Naturally Occurring Quinones; Academic Press: London, 1971. (30) Okawara, M.; Kitao, T.; Hirashima, T.; Matsuoka, M. Organic Colorants, A Handbook of Data of Selected dyes for Electro-optical Applications; Elsevier: Amsterdom, 1988. (31) Diaz, A. N. Talanta 1991, 38, 571. (32) Malik, W. U.; Verma, S. P. J. Phys. Chem. 1966, 70, 26. (33) Shao-pu, L. Fenxi Hsi Hua Hsueh. 1977, 5, 366. (34) Truong Son, N.; Ruzicka, E.; Lasovsky, J. Collect. Czech. Chem Commun. 1979, 44, 3264.

© 1996 American Chemical Society

Dyes in Micellar Environments

Langmuir, Vol. 12, No. 13, 1996 3115

Figure 1. Structure of anthraquinone dyes: (I) R1 ) OH; R2, R3, R4, R5, R6, and R7 ) H; (II) R1 and R2 ) OH; R3, R4, R5, R6, and R7 ) H; (III) R1 and R4 ) OH; R2, R3, R5, R6, and R7 ) h; (IV) R1 and R7 ) OH; R2 R3, R4, R5, and R6 ) H; (V) R1, R3, and R7 ) OH; R6 ) CH3; R2, R4, and R5 ) H; (VI) R1, R2, R5, and R7 ) OH; R3, R4, and R6 ) H; (VII) R1 and R2 ) OH; R3 ) SO3Na; R4, R5, R6, and R7 ) H.

in micellar media, to measure the cmc of surfactants and membrane surface potential.36 The knowledge may be helpful to understand the biochemical properties and anticancer activities37 of these anthraquinone compounds.

bound A and B, and [Hw] represents bulk hydrogen ion concentration. If we denote the dissociation constant of an indicator in water by Kw a , the dissociation constant of nonionic micelle-bound acid and base forms of indicator and proton in water near the interface as Kia, and the dissociation constant of charged micelle-bound acid and base forms of indicator and proton in the bulk water phase as Kmw a , and in all cases the dye concentrations are very low so that concentration and activity of dye are same, then according to Ferna´ndez and Fromherz

∆pKia ) pKia - pKw a )

1 (µm - µw B) 2.3RT B 1 (µm - µw A ) (2) 2.3RT A

and

Theory Thermodynamic Model. Acid-base equilibria of micelle bound indicators have been studied by several authors to understand the ionic composition of the micellar surface.3,16,19,24 Apparent shift of the pKa value in the micellar medium, compared to pure aqueous solution, is due to the change of the local interfacial proton activity at the surface of the charged micelle as compared to that in the bulk water, which was originally suggested by Hartley and Roe.38 However, the equilibrium of an indicator, bound to a micellar surface, may be affected not only by an electrostatic potential but also by a local environment. Ferna´ndez and Fromherz15 provided a quantitative estimate of the micelle-induced pKa shifts of two coumarin dyes, bound to both cationic as well as anionic micelles by taking into account two factors, viz., the shift due to polarity of the interface and the surface pH. In our study, we have used the same model15 to understand the acid-base equilibria of hydroxyanthraquinones in micelles. Due to the presence of a large hydrophobic group with the coumarin dye,15 the relative positional shift of dye in micelle was insignificant, but in our case there are possibilities of a significant shift due to the absence of a large hydrophobic group. Now as the micellar pseudophase possesses both the polarity and pH gradient so the positional shift of dye in micelle may lead to further shift of acid-base equilibrium. The acid-base equilibrium may be represented as

A-n ) B-n-1 + H+ where n ) 0, 1, 2, etc. For an indicator (in its acidic or basic form), which is completely bound in the micelle, during titration one measures the proton activity in the bulk aqueous phase and the ratio of acidic and basic forms of the dye in micelle. The apparent pKa in eq 1 will be the bulk pH, for which the indicator in the micelle is 50% dissociated.

pKAa ) -log

[Bm][Hw] [Am]

(1)

Here [Am] and [Bm] represent the concentrations of micelle (35) Truong Son, N.; Ruzicka, E. Collect. Czech, Chem. Commun. 1980, 45, 703. (36) Gennis, R. B. Biomembrane, Molecular Structure and Function; Springer-Verlag: New York, 1989. (37) Nicolaou, K. C.; Dai, W. M.; Tsay, S. C.; Estevez, V. A.; Wrasidlo, W. Science 1992, 256, 1172. (38) Hartley, G. S.; Roe, J. W. Trans. Faraday Soc. 1940, 36, 101.

i ) pKmw - pKw ∆pKmw a a a ) ∆pKa -

F ψ 2.3RT

(3)

where µ, R, T, and F have their usual meaning. Equation 3 was obtained considering the same ∆pKia value for nonionic and charged micelles, where ψ is the potential operative at the plane where the dissociable group of the dye is located. Pseudophase Ion Exchange Model. The pseudophase ion exchange (PIE) model provides a quantitative interpretation of the change in the apparent acid-base equilibrium of organic dyes in charged micellar media. The assumption of the PIE model and the derivation of the relevant equations have been published many times.23,26 Here we have summarized the derivations for indicator equilibrium in cationic and anionic micellar media. In this model the ionization of the indicator is assumed to occur in the micellar pseudophase and deprotonation within the micellar pseudophase is defined by an intrinsic acidity constant

Km a )

msBmsH msA

(4)

The superscript s indicates reaction in the Stern layer. The concentration of each species is defined as the ratio of moles of bound species per mole of micellized surfactant in order to avoid to specify the exact reaction volume in the micellar media. So

msB )

[Bm] [C] - cmc [Am]

msA )

[C] - cmc

msH )

[C] - cmc

and

[Hm]

where [C] is the concentration of surfactant. We have assumed that under the experimental conditions, all forms of the dyes are completely micelle bound, and so

[Atotal] ) [Am] + [Bm]

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Pal and Jana

The proton concentration at the anionic micelle surface will be in equilibrium with Na+ counterion, i.e., Hm + Naw ) Hw + Nam.26 Similarly for the cationic micelle OHm + Clw ) OHw + Clm, where counterion Cl- is in equilibrium with OH- ion on the micelle surface.23 Now for the anionic micelle, the distribution of H+ ion can be expressed as ion exchange constant

KNa H )

[Hw]msNa [Naw]msH

(5)

(subscript w denotes the species in water) and the fraction of the micelle surface covered by the two counterions H+ and Na+ is given by

msH

+

msNa

)β≈

msNa

(6)

s (considering mNa . msH). The proton concentration at the anionic micellar surface obtained from eqs 5 and 6 is

msH )

β[Hw] Na KH [Naw]

(7)

Putting an msH value of eq 7 in eq 4, on simplification we get A pKm a ) pKa + log

KNa H [Naw] β

(8)

Similarly, for the cationic micellar medium, the distribution of OH- ion can be expressed as

KCl OH

[OHw]msCl )

[Clw]msOH

(9)

(10)

s ). (assuming msCl . mOH Now combining eqs 4, 9, and 10 we get

A pKm a ) pKa + log

β KCl OH[Clw]

[A-n] ) [B

]

-n-1

D - Dmin Dmax - D

where Dmax and Dmin are the absorbances for A-n and B-n-1, respectively, for a particular wavelength. To confirm the maximum possible complete binding of the dye in all its forms with the micelle and to confirm minimum perturbation of the micellar surface, we have always used [micelle]/[dye] ∼ 2. For this we have used the cmc values and aggregation numbers from previous studies.8,20 Beer’s law was obeyed for all the dyes (in neutral or anionic forms) in different organic solvents, water, and also in different micellar media, and hence we concluded that the possible dye aggregation was insignificant in the working concentration range of the dyes. In water, the solubility of the neutral dyes was low at low pH but it increased at high pH due to deprotonation. For these cases the dyes were solubilized at high pH in water and then the pH was lowered successively but quickly and electronic spectra or absorbance were noted for low pH, before their precipitation. The relative standard deviations for λmax were (1.0 nm and values for pKa were (0.05 wherever not mentioned.

Results

and the fraction of the micellar surface covered by the two counterions is given by

msOH + msCl ) β ≈ msCl

The final concentration of H3PO4 used for the buffer system was kept moderately high (0.1 mol dm-3) in most cases to overcome the influence of micellar media on the solution pH of the buffer. However, when measurements were made at low ionic strengths, the buffer concentration was reduced. Spectral Measurements. All fluorescence measurements were done with a luminescence spectrometer (Perkin-Elmer LS 50B). All absorption spectra were measured with a Shimadzu UV160 digital spectrophotometer with a quartz cell having 1 cm path length. The concentration ratio of two consecutive forms of the dye were calculated from measured absorbance value (D) using the equation

(11)

Equations 8 and 11 are used as working equations to calculate intrinsic micellar pKa values for charged micelles. Experimental Section The dyes were recrystallized from methanol and their concentrations were in the range between 10-4 and 10-5 mol dm-3. Surfactants used were cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and poly(oxyethylene) isooctylphenyl ether (Triton X-100), and all were purchased from Aldrich Chemical Co. Sodium chloride, sodium hydroxide, phosphoric acid, and diethylamine used were of analytical reagent grade. Deionized double distilled water was used throughout. Method. pH Measurement. All pH measurements were carried out using a digital (ECIL, Hyderabad, Govt. of India Enterprise) pH meter, calibrated using different buffer solutions of known pH ranging from 4.0 to 11.0. Buffer Solutions. Distilled diethylamine was used as the buffer system in DMSO and the H3PO4-NaOH system was used as buffer for all measurements in the micellar media, water, 50% (v/v) methanol-water, and methanol.

Solubilities in Different Media. All the dyes except alizarin red S were insoluble in water but soluble in nonionic, cationic, and anionic surfactant solution above their cmc. Below the cmc only the cationic surfactants showed turbidity with all dyes due to the interaction between CTAB monomer and dye to form a mixed micelle or dye-surfactant salt.14 In the case of nonionic or anionic surfactants, no such effect was observed. The solubility of the dyes was also studied in different solvent systems (ethanol, ether, acetone, benzene, and cyclohexane) to get a qualitative idea about the relative position of the dye in the micelles.39,40 The dyes I, III, and IV were soluble in cyclohexane and benzene, but the remaining dyes were soluble in ethanol, ether, and acetone. Spectral Measurements. We have studied the nature of UV-visible and fluorescence spectra of these dyes in water, 50% (v/v) methanol-water, methanol, DMSO, and nonionic, cationic, and anionic micellar media. The deprotonation equilibrium always gave a bathochromic shift of λmax values of the dyes and the magnitude of the shift was solvent dependent. The shift of λmax in different media is a result of two factors: (i) a shift due to deprotonation and (ii) a shift due to solvent-dye interactions. We have studied the UV-visible absorption spectrum and deprotonation equilibrium over a wide range of pH (2.0-13.0) in different media and thus isolated the λmax shift due to solvent-dye interaction. In Table 1, the λmax values corresponding to minimum energy transitions are given for neutral and anionic forms of each dye in different media. (39) Mukerjee, P.; Jeong-Soo, Ko J. Phys. Chem. 1992, 96, 6090. (40) Aamodt, M.; Landgren, M.; Jo¨nsson, B. J. Phys. Chem. 1992, 96, 945.

Dyes in Micellar Environments

Langmuir, Vol. 12, No. 13, 1996 3117

Table 1. Absorption λmax Values (nm) in Different Media at 0.3 mol dm-3 Ionic Strengtha dye I III IIII2III IIIIII2IV IVV VV2-/3VI VIVI2-/3VIIVII2VII3-

water

50% (v/v) methanol water

methanol

DMSO

neutral micelle

anionic micelle

cationic micelle

405 (480, 560) 485 410 460 555 470 (535) 548 557, 594 430 (520) 500 445 (-) 496 520 492 (550) 584 584 423 520 555, 593

404 (480, 560) 486 410 465 560 470 (535) 545 560, 594 430 (520) 506 441 (520) 493 522 483 (550) 550 584 429 520 556, 595

400 (480, 560) 490 414 462 566, 607 475 (536) 546 557, 594 428 (520) 510 444 (520) 494 525

403 (560) 527 422 487, 555

405 (560) 485 417 465 555 480 (535) 560, 593 560, 593 430 (520) 500 445 (520) 510 520 490 (550) 575 575 429 520 555, 596

405 (480, 560) 485 418 465 560 472 (535) 550 559, 594 430 (520) 500 445 (520) 500 520 487 (550) 570 570 423 520 555, 595

407 (560) 500 418 476 550 482 (535) 556, 590 572, 610 430 (520) 515 445 (520) 511 525 492 (550) 592 592 433 548 572, 615

(550) 551 588 428 535 556, 597

478 (540) 588 572 430 (420) 568 443 (520) 544 544 492 (550) 574 435 552

a [micelle]/[dye] ) 2 for all three micellar media. Aggregation numbers8,20 used were 143, 62, and 78 for neutral, anionic, and cationic micellar media, respectively. Values within parentheses are emission λmax values.

Table 2. Apparent pKa Values in Different Media at 0.3 mol dm-3 Ionic Strength and at 25 °Ca apparent pKa shift due to potential effect

medium anionic cationic ∆pKmw ∆pKmw water neutral ∆pKia a a + w i mwmw+ i w mww (pKa ) micelle (pKa) micelle (pKa ) micelle (pKa ) (pKa - pKa ) (pKa - pKa ) (pKmw - pKw a a) -

dye

pKa

I II

pKa pKa1 pKa2

III

pKa1 pKa2 pKa1 pKa1 pKa2 pKa1 pKa2

IV V VI

VII pKa1 pKa2 a

10.60 6.50 12.10 (0.10 10.90

10.40 6.70 12.30 (0.10 10.10

10.70 7.40 12.00 (0.10 11.30

9.60 7.30 10.40 6.60 9.70 (0.10 5.35 10.50 (0.10

9.40 6.90 11.70 6.35 10.85 (0.10 5.20 11.95 (0.10

9.60 8.05 10.50 7.60 11.15 (0.10 5.45 10.50 (0.10

8.70 5.70 12.20 (0.10 8.10 12.75 7.45 5.20 10.65 4.90 10.10 4.70 11.50 (0.10

+

anionic micelle

cationic micelle

-0.20 +0.20 +0.20

+0.10 +3.90 -0.10

-1.90 -0.80 +0.10

-0.30 -0.70 +0.30

+1.70 +1.00 +0.10

-0.80

+0.40

-2.80

-1.20

+2.00

-0.20 -0.40 +1.30 -0.25 +1.15

0.00 +0.75 +0.10 +1.00 +1.45

-2.15 -2.10 +0.25 -1.70 +0.40

-0.20 -1.15 +1.20 -1.25 -0.30

+1.95 +1.70 +1.05 +1.45 +0.75

-0.15 +1.45

+0.10 0.00

-0.65 +1.00

-0.25 +1.45

+0.50 +0.45

[micelle]/[dye] ) 2 for all three micellar media.

It can be noted from Table 1 that the shifts in λmax values in different media compared to water were bathochromic in most cases and the maximum shifts were observed in DMSO and in cationic micelles. The observed bathochromic shift in DMSO and cationic micellar systems increases with the increase in anionic character of the dye by successive acid-base dissociation. Shifts in methanol, 50% (v/v) methanol-water, and nonionic and anionic micellar media were either zero or small. The fluorescence spectra of the dyes were studied in the above solvents as well as in three different micellar media. All the dyes showed fluorescence in neutral form (Table 1) but do not have any fluorescence in anionic form. Fluorescence spectra were taken for each dye with the same excitation wavelength for different solvent systems. The emission λmax of each dye remained almost same in all media, but the emission intensity was different in different media. In most cases the emission intensity is maximum in methanol and minimum in water, and for other systems it was in between. However, the fluorescence intensity in three different micellar media followed

the order SDS > CTAB > TX100 in all cases excepting for VI where it is TX100 >SDS > CTAB. Acid-Base Equilibrium. Nonionic Micellar Media. Apparent pKa values in nonionic micellar medium (pKia) and their relative shift (∆pKia) compared to pKw a values are given in Table 2. ∆pKia values for the first i ) were small and negative in proton dissociations (∆pKa1 i most cases while ∆pKa values for second proton disi ) were large and positive in most cases. sociations (∆pKa2 Charged Micellar Media. Apparent pKa values were lower in magnitude in cationic micellar medium but higher in anionic micellar medium and we have measured the + ) and anionic relative shifts for cationic (∆pKmw a (∆pKmw ) micellar media compared to corresponding a mw+ pKw values (Table 2). The ∆pK values for the first a a mw+ proton dissociations (∆pKa1 ) were negative while + ∆pKmw values for second proton dissociations a mw+ (∆pKa2 ) were positive. However, ∆pKmw values were a either zero or positive for all cases.

3118 Langmuir, Vol. 12, No. 13, 1996

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For charged micelles, ∆pKmw values are combinations a of ∆pKia shift and the shift due to the potential effect, and the potential component has been calculated using eq 3 (Table 2). For the first proton dissociation these were large and positive for cationic micelles and comparatively low but negative for anionic micelles. For the second proton dissociation these were small positive for cationic and large positive for anionic micelles. Ionic Strength Dependent Spectral Profile and Acid-Base Equilibria. Electronic spectra and pKa values for all the dyes in water and different micellar media were studied as a function of the ionic strength. Dyes in neutral or anionic forms have low solubility in water for high ionic strength and so studies were restricted with 0.0-1.0 mol dm-3 ionic strength in most cases but where solubility of the dyes were reasonable, higher ionic strengths were used. SDS is precipitated at high NaCl concentration (added to maintain ionic strength) and so for anionic micellar medium we used up to 1.0 mol dm-3 NaCl. However, for nonionic and cationic micellar media, we used 0.0-5.0 mol dm-3 NaCl. The λmax values of only the anionic forms of the dyes have been observed to be red-shifted by 4-5 nm in the case of a cationic micelle for a change in ionic strength from 0.0 to 5.0 mol dm-3 (exception, VII- and V3- have hypsochromic shift by 4 nm). However, the molar absorptivity remained unaltered in all cases. Addition of NaCl to cationic or anionic micellar solution does not affect the emission λmax but increases the emission intensity of the dyes, and the effect is more pronounced in CTAB than in SDS. The apparent pKa values decreased in water and anionic micellar media but increased in cationic micellar medium i with the increase in ionic strength. However, the pKa1 values did not change with ionic strength in cases of I, III, IV, and V, but in other cases values decreased significantly with increase in ionic strength. i As both the pKw a and pKa values were ionic strength i dependent, the ∆pKa value was also dependent on ionic strength. However, the change of ∆pKia value with ionic strength was either zero or very small for all cases except i changed from positive to negative for VII- where ∆pKa1 when ionic strength was increased (Figure 2). For calculation of ψ, we have used ∆pKia values at the respective ionic strength. For charged micelles, the electrical surface potentials (ψ) at different ionic strength were calculated for all dyes using eq 3 and plotted against bulk ionic strength (Figure 3). The values of ψ were positive for all the dyes in cationic micelles, and they are found to decrease exponentially with increase in the bulk ionic strength (Figure 3a). For anionic micelles, a negative ψ was obtained for first proton dissociation and both positive and negative ψ values were obtained for second proton dissociation, and the values increased with the increase in ionic strength in all cases. The mean activity coefficient (ν()41 was used in the activity calculations. Calculations for PIE Model. For anionic micelles we have used the optimum values26 of β ) 0.75 and m + KNa H ) 1.0 for the calculation of pKa . The activity of Na ion (aNa) has been calculated from aNa ) [Natotal]ν(, where [Natotal] ) [Nabuffer] + [Naadded] + (1 - β)[C], representing the contribution of Na+ ion from buffer, added NaCl, and dissociated SDS.26 For cationic micelle, we have used the Cl optimum values of β ) 0.75 and KOH ) 4.5, established earlier23 and the aCl has been calculated according to aCl

The solubility of the dyes with β-OH groups increases in ethanol, ether, and acetone due to intermolecular hydrogen bonding with the solvent. The dyes containing R-OH groups are less polar and are soluble in nonpolar solvents like benzene and cyclohexane because of the formation of intramolecular hydrogen bonding with quinone carbonyl. So it is expected that, the R-isomers are more deeply “buried” in the micelle in comparison to β-isomers. Hydroxyanthraquinones are weakly polar donor-acceptor chromogens,42 where the hydroxyl group behaves as a donor and the anthraquinone skeleton behaves as a complex acceptor. The electron-donating capability of the hydroxyl group is enhanced greatly by deprotonation at higher pH, which causes a bathochromic shift of the λmax values. The excited states of these compounds generally show a high degree of charge transfer from donor to acceptor and thus have a large dipole moment. Polar solvents will stabilize the excited state more than the ground state and, thus, show a bathochromic shift of the first absorption band when the solvent polarity is increased. Again the hydrogen bonding between the anthraquinone carbonyl group and protic solvent may lead

(41) Meites, L., Ed. Handbook of Analytical Chemistry, McGrawHill: New York, 1963; Table 1-7.

(42) Griffiths, J. Colour and Constitution of Organic Molecules; Academic Press: London, 1976.

i Figure 2. pKw a or pKa values against bulk ionic strength; b, pKia; and O, pKw . For other dyes pKia values do not change with a the ionic strength and so are not given. The superscript represents first or second proton dissociation of dyes. [micelle]/ [dye] ) 2.

) [Cl]ν( where [Cl] ) [Cladded]. The value of ν( is obtained from literature.41 The graphical plot of pKAa vs log[aNa] or log[aCl-] showed a straight line for most cases. Linear regression analysis was done for all cases to determine the pKm a values, and for most cases the correlation coefficients were between 0.94 and 1.0. The pKm a is a unitless number and it has been converted to micellar acidity constant, Kva, dividing by the molar volume of the Stern layer,23,26 i.e., Kva ≈ v Km a /0.14. Table 3 represents the results of pKa values for charged micelles. Discussion

Dyes in Micellar Environments

Langmuir, Vol. 12, No. 13, 1996 3119 Table 3. Intrinsic Micellar pKa Values (pKm a ) of the Hydroxyanthraquinone Dyes + pKva v0 pKa pKva

Figure 3. (a) ψ values against bulk ionic strength for cationic micellar medium. Superscript represents first or second proton dissociation of dyes. [micelle]/[dye] ) 2. (b) ψ values against bulk aNa+ for anionic micellar medium. Superscript represents first or second proton dissociation of dyes. [micelle]/[dye] ) 2.

to a hypsochromic shift of the absorption band. Thus water, 50% (v/v) methanol-water, and methanol, all being protic polar solvents, lead to insignificant shift of λmax values, but DMSO being a nonprotic polar solvent leads to a large bathochromic shift. The previous study showed that the effective hydrogen bonding capacity of water, if present in the micellar Stern layer is low.19 So the bathochromic shift in micellar media compared to water

I

II

III

IV

V

VI

VII

7.7 10.4 -

4.9 6.9 6.2

7.2 10.1 -

6.5 9.3 -

4.3, 9.7 6.9, -, -

4.0 6.45 6.0

3.2 5.9 4.3

indicated that the dyes were in the micellar Stern layer rather than in the bulk water or in the micellar hydrocarbon core. Cationic micellar medium exhibited more bathochromic shifts for dye anions, sometimes comparable to DMSO, indicating that the dyes were situated in a more polar region. This is due to strong electrostatic attraction between the cationic surfactant head groups and the dye anions which keeps the dye anions in the Stern layer. The comparatively low bathochromic shift for neutral dyes in all types of micelles was due to deeper penetration of the dyes into the micelles. However, the low bathochromic shift for anionic dyes in nonionic or anionic micelle is very difficult to explain. An insignificant shift does not generally imply negligible interaction between the dye and the micelle but may be a combination of several factors. VI and VI- showed hypsochromic shift while moving from water to nonionic or anionic micellar media. This dye has four -OH groups (two -OH on each terminal benzene ring), so both the ends of the quinone skeleton are considerably polar and hence the dye would be more closer to the bulk water phase, where availability of water is high. This caused a hypsochromic shift due to increased hydrogen bonding with water. However, for cationic micelles, usual bathochromic shifts were observed due to predominate electrostatic attraction between the surfactant head groups and the dye anion. For V, hypsochromic shifts were not observed, though three -OH groups are present in the two terminal benzene rings of the anthraquinone skeleton, because of the presence of one hydrophobic -CH3 group with one of the rings. The decrease in emission intensity from methanol to water for neutral dyes is due to the quenching effect of water. Increase in emission intensity in micellar media compared to water is due to the extraction of dyes in micelle. High emission intensity in SDS micelle is due to deeper penetration of dye in the micelle. Weak intensity in TX100 means a significant amount of water can penetrate at the solubilization site of dye. Comparatively low emission intensity in CTAB compared to SDS is due to the presence of Br- quencher. Successive addition of NaCl replaces the Br- quencher from cationic micelle surface and thus the emission intensity gradually increases and reaches a maximum value and further addition of NaCl does not increase the emission intensity. This maximum intensity is comparable to SDS micelle indicating that the dyes are deeply penetrated in CTAB also. Being more polar, the VI is less penetrated in charged micelles and so the intensities are quite less, even less than TX100. At high ionic strength cationic and anionic micelles change their size and shape43 and the saturation of micellar surface charge is atttained, which causes more effective incorporation of the dye in micelle.44 This may be a possible cause for a small bathochromic shift of λmax values or increase in emission intensity at high ionic strength. A significant contribution to ∆pKia is the energy required to transfer the conjugate acid and conjugate base forms of the dyes from water to the nonaqueous medium of the nonionic micelle. A negative ∆pKia means a rightward shift of equilibrium and a positive ∆pKia means (43) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 744. (44) Miyashita, Y.; Hayano, S. J. Colloid Interface Sci. 1982, 86, 344.

3120 Langmuir, Vol. 12, No. 13, 1996

a leftward shift of equilibrium. As the dielectric constant of the nonionic micelle surface is low in comparison to water, a positive ∆pKia was expected for these types of i value acid-base reactions. A small negative ∆pKa1 indicated that the difference in free energy of neutral and monoanionic forms of the dyes is less in nonionic micelles than in water. This is opposite to the expectation and was mainly due to the relative shift of the position of dyes in the micelles for their better stability. The anionic form of the dye is more polar and may be located nearer to the i exterior part of the micelle. A high positive ∆pKa2 indicates that the difference in free energy between monoand dianionic forms is greater in nonionic micelles than in water. This result showed that dyes did not shift their positions for mono- to dianionic forms in micelles and thus showed the usual behavior. The shift was restricted for the mono- to dianionic forms of dyes, because a greater displacement toward the bulk water phase from the stern layer means complete loss of the hydrophobic interaction. Apparent pKa shifts in charged micelles are the combination of pKia component and potential component. Though the pKia components may be identical for both the charged micelles, the potential components are opposite in sense for two oppositely charged micelles. This is the mwmw+ reason for positive ∆pKa1 and negative ∆pKa1 values in most cases. Separation of potential component using eq 3 showed a comparatively large component for the potential contribution. For a constant ionic strength, the potential component should be equal for all the dyes in either cationic or anionic micellar medium. In most of the cases, this fact has been corroborated from the very small variation in the potential component values which were -1.15 to -1.25 for anionic micellar media and +1.45 to +2.00 for cationic micellar media for the first proton dissociation. Deviation from these average values was mostly dependent on three factors, viz., (i) validity of our assumption that ∆pKia is the same for nonionic and charged micelles, (ii) whether the dye is situated at the surface stern layer or not, and (iii) whether the dye is bound exclusively in the micelle. The ∆pKia value is related to the average polarity of location of conjugate acid and conjugate base forms of dye in the micelles, and this may be different for different micelles. This is a possible reason for an abnormal positive potential component of pKa in anionic micellar media. The potential at the surface of the micelle is highest, and on either side it will be low.36 If the dye is located at the interface, the incoming H+ or OH- will be affected by highest surface potential, but if the dye is on the either side of the interface, the effective potential for H+ or OHwill be less. Low potential effects for I and IV may be explained as just inside the surface Stern layer, but a similar low value for VII- could be explained considering the position of the dye just outside the Stern layer. Exclusive binding of di- or trianionic dyes is possible in cationic micelles but will not be feasible for anionic micellar medium due to large electrostatic repulsion between the anionic dye and anionic head groups of the micelles. This was one of the reasons for abnormal results. Being less polar; R-isomers (I, III, and IV) are more deeply buried into micelle than β-isomer and their pKia values are ionic strength independent, as if their conjugate acid-base forms are isolated from bulk water. Ionic strength dependence of pKia values for β-isomers indicated that hydrophobic -OH groups of the dyes were displayed in bulk water. Presence of hydrophobic -CH3

Pal and Jana

groups in V leads to more effective incorporation of this i β-isomer into micelle, and so pKa1 is independent of ionic strength. Increase in ionic strength decreases the surface potential of the charged micelle and so also reduces the electrostatic potential part of the apparent pKa value in charged micelles. This should lead to a decrease in apparent pKa value in anionic micelles and an increase in apparent pKa value in cationic micelles, with increase in the bulk ionic strength, and such an effect was well observed for the first proton dissociations. The change of ψ with ionic strength is high at low ionic strength but low at high ionic strength. This is observed for all the dyes in a cationic micelle, and all the curves were of similar types (Figure 3a). The R-isomers of the dyes have higher ψ values compared to β in all bulk ionic strength, indicating that the average positions of the conjugate acid-base forms for these isomers are more closer toward the Stern layer of cationic micelle. For β-isomers this is toward bulk water from the Stern layer. For anionic micelle the curves were linear or almost linear and the slopes were different for different dyes. For first proton dissociation, the slopes were zero or small for all cases except VI (Figure 3b). If the dye is firmly attached at the inner side of the Stern layer, its acid-base equilibrium should have little effect on ψ and the value of ψ should be low. This happened in the case of I and IV. The slope was high for VI as it lies toward the bulk water phase of the Stern layer. The abnormal positive values of ψ (Figure 3b) for the second proton dissociation (also first proton dissociation for VII-) were due to deviations from the above assumptions. For a particular ionic strength, the difference in ψ value for all the dyes in either cationic or anionic micelles can be correlated with the relative position of dyes only if the above assumptions are well obeyed. According to the PIE model, the shift of pKa value in micellar media compared to pKw a is caused primarily due to the transfer of dye (in both acidic as well as basic form) and H+/OH- ion from large volume of water into the much smaller volume of micellar pseudophase. Addition of NaCl in SDS micellar solution results in the displacement of H+ ions from the micelle surface by Na+ ion and thus decreases the apparent pKa of micelle bound dye. Similarly, the addition of NaCl in CTAB micellar solution results in the displacement of OH- ions from micelle surface by Cl- ions and thus increases the apparent pKa of micelle bound dye. Dye molecules which are located on the surface of the micelle followed the PIE model closely. However, exceptions have been observed which are reflected in nonlinear relationship in the plot of pKAa vs log[aNa] for I, III, IV, and + V. For other dyes the pKva and pKva , the so-called intrinsic pKa values, have been isolated as a counterpart of apparent pKa values using the PIE model. So these intrinsic pKa values represent the nature of the microenvironment of the micellar media where the dye is located. If the average polarity of locations of dyes in all three + 0 micelles is same, then pKva , pKva , and pKva should be almost the same. But in actual case they are different. 0 In most cases pKva is highest followed by pKva and v+ pKa has the lowest value. This signifies that the average polarity of location of the conjugate acid-base form of dyes is lowest in nonionic micelle, higher in the anionic micelle, and highest in the cationic micelle. In anionic micelle, the dye penetrated more deeply in micelles compared to cationic micelle and this leads to minimization of the polarity effect, which is consistent with the experimental findings.

Dyes in Micellar Environments

Conclusion

pKia

shift of the probe molecules in micellar media The is generally explained by medium effects. We have shown that during deprotonation, when the probe changes its polarity, it may change its relative position in the micelle for better stability, and this has to be considered for the pKia shift. Again, the positional shift of the probe is restricted when it reaches the micellar surface coming from the interior of the micelle. Whether the probe will reflect the true surface potential of the charged micelle or not will depend on the location of the probe in the micelle. A probe can only reflect the true surface potential if it is located at the surface of the micelle. If the probe is located on either side of the micellar surface, it will detect a lower potential value. This important fact should be considered in the use of dye

Langmuir, Vol. 12, No. 13, 1996 3121

molecules as probes for the measurement of membrane surface potential. These observations led to develop methods for quantification of metal ions in micellar media with definite improvement over the existing method.45 The observed shifts of λmax values (Table 1) would be helpful for choosing dye molecules for cmc measurements. Intrinsic micellar pKa values can be calculated using the PIE model for dyes which are located at the micellar Stern layer, representing the average polarity of location of conjugate acid-base form of dye molecules in micelle. Acknowledgment. The work was supported by the Council of Scientific and Industrial Research, India. LA950680X (45) Pal, T.; Jana, N. R. Talanta 1994, 41, 1291.