Langmuir 1987,3, 377-382 Another argument against the crown ether effect can be made from the observation that there is no significant difference between the binding for systems containing an ethoxylated nonionic surfactant and systems containing a nonethoxylated nonionic surfactant. In addition, the binding of cationic counterions to anionic/nonionic micelles is essentially the same as the binding of anions to cationic/nonionic micelles. The insensitivity of counterion binding to all these factors suggests that there are no specific interactions between the counterions and the polyether chain of the ethoxylated nonionics. It is important to stress that this discussion of the crown ether formation is only speculative because binding is certainly not a direct method of quantifying this effect. There is no evidence in these results to support the idea that two types of micelles, one rich in ionic surfactant and the other rich in nonionic, coexist in solution for any of the systems studied. If a micellar phase rich in ionic surfactant were present, the fractional counterion binding would be expected to be essentially constant as the overall composition varied. The lengths of the alkyl and polyethoxylate groups of the nonionics used in this investigation are most likely not in the range in which the two separate micellar phases exist.15J6
Conclusions The localized adsorption model can be used to accurately predict the fractional counterion binding on mixed ionic/nonionic micelles. The model works well for surfactants of markedly different structure, including both ethoxylated
377
and nonethoxylated nonionics. The minimal effect of surfactant structure indicates that electrostatics is the dominant force in determining counterion binding.
Acknowledgment. Financial support for this work was provided by the Mobil Research and Development Corp., the Shell Development Co., DOE Contract 1985BC10845.000, the OU Energy Resources Institute, and the Oklahoma Mining and Minerals Resources Research Institute. We thank the Proctor and Gamble Co. for donation of the DDPO surfactant. Kevin Stellner, Steven Hendon, Terry Davis, and Ronda Huffines helped obtain the data presented here. Nomenclature a micellar surface area per charged hydrophilic group in micelle, m2/molecule a for a single-component ionic micelle, m2/molecule a1 micellar surface area per hydrophilic group for a aN single-componentnonionic micelle, m2 molecule cmc critical micelle concentration, kmol/m constant from localized adsorption model of counKB terion binding, m3/kmol R gas constant, 1.987 kcal/(kmol K) T temperature, K regular solution theory interaction parameter, WR kcal/ kmol electrical Stern layer potential, V YO Registry No. NP(EO),, 9016-45-9; DPC, 104-74-5; CPC, 123-03-5;SDS, 151-21-3;OBS,28675-11-8;DE(EO)B, 3055-96-7; DDPO, 871-95-4; DDAO, 1643-20-5; DMS, 3079-28-5.
B
Spectroscopic Studies of Dye Solubilizates in Micellelike Complexes of Surfactant with Polyelectrolyte Katumitu Hayakawa,* Junko Ohta, Tamaki Maeda, and Iwao Satake Department of Chemistry, Faculty of Science, Kagoshima University, Korimoto-1, Kagoshima, Japan 890
Jan C. T. Kwak Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received May 2, 1986. I n Final Form: December 1, 1986 Absorption and fluorescence spectra of the cationic dyes rhodamine 6G (R6G),proflavin (PF),and acridine orange (AO) were measured in the presence of anionic polyelectrolytes, i.e., sodium salts of dextran sulfate (DxS) and poly(viny1sulfate) (PVS),with or without the cationic surfactant dodecyltrimethylammonium bromide (DTAB). The dimer bands of R6G and PF appear at 498 and 435 nm in aqueous solutions with excess DxS or PVS, respectively. Addition of DTAB induces a red shift of the bands indicative of dissociation of the dimer into the monomer. The monomer band maximum appears at a longer wavelength than that of aqueous dye. A similar red shift of the monomer band of the dyes is observed in micellar solutions of DTAB and sodium dodecyl sulfate. These results indicate that PF and R6G dissolve into DTA+-polyanion complexes in the monomeric form. The increase in fluorescence intensity of both dyes induced by DTAB addition also points at the solubilization of monomeric dye in the polymer-surfactant complex. DTAB addition to AO-DxS solution induces only a minor change in the absorption and fluorescence spectra, indicative of strong cooperative binding of A 0 with DxS.
Introduction dialysis,lPH solubility,7 NMR? potentiometric titration,&'l and neutron scattering.12 Our studies of the binding of The interaction between polyelectrolytes and oppositely charged surfadants produces a special type of organization. The formation of soluble polymer-surfactant complexes (1) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. can be deduced from the marked changes observed in (2) Goddard,E. D.; Hannan, R. B. J.Colloid Interface Sei., 1976,55, many properties of the solution by the use of a variety of 73. techniques such as surface tension,lS2dye s o l ~ b i l i z a t i o n , ~ ~ ~ (3) Murata, M.; Arai, H. J. Colloid Interface Sei. 1973, 44, 475. 0743-7463/S7/2403-0377$01.50/0 0 1987 American Chemical Society
378 Langmuir, Vol. 3, No. 3, 1987
Hayakawa
ionic surfactants by polyions of opposite charge using This paper concerns the absorption and fluorescence surfactant-selective electrodes have revealed a strongly spectra of the cationic dyes rhodamine 6G M G ) , proflavin (PF, 3,6-diaminoacridine), and acridine orange (AO, 3,6cooperative character of surfactant binding by polyanions, bis(dimethylamino)acridine),in the presence of the comwhich is shown in a steep rise in the binding isotherms at plexes formed by the addition of dodecyltrimethyla concentration far below the critical micelle concentration ammonium ion (DTA+) to solutions of the sodium salts (cmc) of ~urfactant.'~-'~The theoretical treatment of of dextran sulfate (DxS) and poly(viny1 sulfate) (PVS). cooperative binding is based on the effect of interactions between bound ligands in the polymer d ~ m a i n . ~ J " ~ ~This study will show that the dyes PF and R6G tend to dissolve in a monomeric form in the complexes, but A 0 Thermodynamic behavior of the polyion-surfactant comkeepts its aggregated form in the complexes. plexes is found to be similar to micelle f~rmation.'~J"'* Photochemical techiques have played an important role in the examination of the nature of organized systems of various types.23-27 For example, luminescence techniques have been employed to determine micellar parameters such as aggregation number, cmc, and partition of solute between the micellar and aqueous pseudo phase^.^"^^ On the other hand, organized systems provide new environments where photoreactions frequently proceed differently from the case observed in homogeneous s o l ~ t i o n . ~ ~ , ~ ~ R6G However, aggregates formed by the interaction of surfactants with polyelectrolytes have been the subject of only limited study. In particular, photochemical techniques have received very little attention in the case of polyelectrolyte-surfactant systems.33 H
(4)Shirahama, K. Colloid Polym. Sci. 1974,252,978. (5)Sen, M.; Mitra, S. P.; Chattoraj, D. K. Colloids Surf. 1981,2,259. (6) Chatterjee, R.;Mitra, S. P.; Chattoraj, D.K. Indian J. Biochem. Biophys. 1979,16, 22. (7)Ohbu, K.; Hiraishi, 0.; Kashiwa, J. J . Am. Oil Chem. SOC.1982, 59, 108. (8) Birch, B. J.; Clarke, D. E.; Lee, R. S.; Oakes, J. Anal. Chim. Acta 1974,70,417. (9)Satake, I.; Yang, J. T. Biopolymers 1976,15,2263. (10)Gilanyi, T.; Wolfram, E. Colloids Surf. 1981,3,181. (11)Shirahama, K.; Tashiro, M. Bull. Chem. Soc. Jpn. 1984,57,377. (12)Cabane, B. J. Phys. Chem. 1977,81,17. (13)Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982,86,3866,1983, 87,506. (14)Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Biophys. Chem. 1983,17,175. (15)Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983,16, 1642. (16)Malovikova, A.; Hayakawa, K.; Kwak, J. C. T . J. Phys. Chem. 1984,88,1930. (17)Santerre, J. P.;Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1986, 13, 35. (18)Satake, I.; Hayakawa, K.; Komaki, M.; Maeda, T. Bull. Chem. SOC.Jpn. 1984,57,2995. (19)Hill, A. V. J . Physiol. (London),1910,40, 180. (20)Schwarz, G. Eur. J. Biochem. 1970,12,442. (21)McGhee, J. D.;Hippel, P. H. v. J.Mol. Bid. 1974,86, 469. (22)Miyazawa, S.Biopolymers 1983,22,2253. (23)Leung, P.S.;Goddard, E. D.;Han, C.; Glinka, C. J. CoEEoids Surf. 1985,13, 63. (24)Ananthapadmanabhan, K. P.;Leung, P. S.; Goddard, E. D. Colloids Surf. 1985,13,63. (25)Thomas, J. K. Chem. Reu. 1980,83,203. (26)Turro, N.J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980,19,675. (27)Fendler, J. H.Acc. Chem. Res. 1980,13,7. (28)Lianos, P.;Long, J.; Zana, R. J. Colloid Interface Sci. 1983,91, 276. (29)Turro, N.J.; Yekta, A. J. Am. Chem. Soc. 1978,100, 5951. (30)Encinas, M. V.;Lissi, E. A. Chem. Phys. Lett. 1982,91,55. (31)Sato, H.;Kawasaki, M.; Kasatani, K. J . Phys. Chem. 1983,87, 3759. (32)Casal, H. L.; deMayo, P.; Miranda, J. F.; Scaiano, J. C. J . Am. Chem. SOC.1983,105,5155. (33)Abuin, E. B.;Scaiano, J. C. J. Am. Chem. SOC.1984,106, 6274.
PF -
H
A0 -
Experimental Section R6G (Aldrich, 99%), PF hydrochloride (Aldrich, 95%), and A 0 hydrochloride (Nakarai Chemicals, fluorescence analytical
grade) were used without further purification. Absorption spectra of dilute aqueous solutions (0-2 X lob M) were in good agreement with those reported in the l i t e r a t ~ r e . ~The ~ - ~concentrations ~ of the stock solutions were determined from the linear part of the absorbance-concentration plot at the concentrations below 2 X 10" M, where the dye monomers predominate. The molar absorption coefficient, e, is 81OOO M-'cm-' for R6G:' 41 OOO M-' cm-' for PF,38and 53 000 M-' cm-' for A0.36 Freshly prepared dye stock solutions were used within 2 days. Each dye concenM. tration in test solutions was kept constant at (1.5-2.0) X The sodium salt of DxS (Nakarai Chemicals, GR)and the potassium salt of PVS (Wako Chemicals, colloid titration reagent grade) were dialyzed against 1 M NaCl, 0.1 M NaC1, and then water until no chloride ion was detected in the outside solution. In the case of NaDxS, the concentration of the resulting stock solution was determined by converting part of the stock solution into acid form by using a Dowex 50W ion exchanger, and the equivalent concentration was determined by titration with standardizedNaOH solution. This procedure is necessary because DxS hydrolyses in acid solution. The PVS stock solution was completely changed into acid form. Part of the acid solution was titrated with 0.01 M NaOH. The equivalent amount of NaOH was added to the remainder of the PVS acid stock solution and (34) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972,76,762. (35)Schwarz, G.;Klose, S.; Balthasar, W.Eur. J. Biochem. 1970,12, 454. (36)Blake, A.; Peacocke, A. R. Biopolymers 1966,4, 1091. (37) Haugen, G.R.; Melhuish, W. H. Trans. Faraday Soc. 1964,60, 386.
Langmuir, Vol. 3, No. 3, 1987 379
Dye Solubilizates in Micellelike Complexes
I
400
500
600
500
X/nm
Figure 1. Absorption spectra of R6G in aqueous (a), DxS (b), and DxS-DTAB mixed solutions (c,d). [DxS] = 2.0 X lo4 equiv dm-3 and [DTAB] = 1.0X lo-, (c) to 12.0 X lo4 (d) M. then the volume of the solution was adjusted to obtain a NaPVS stock solution of known concentration. The equivalent concenequiv dm-3 tration of the polyelectrolytes was fixed at 2 X in all test solutions. Surfactants, dodecyltrimethylammonium bromide (DTAB, Tokyo Kasei, GR)and sodium dodecyl sulfate (SDS, Nakarai Chemicals, protein research reagent grade), were purified by repeated recrystallization from acetone and ethanol solution, respectively. All solutions were prepared from double-distilled water, the second distillation being from alkaline KMnO, solution in an all-glass distillation apparatus. A Hitachi 228 spectrophotometer and a Hitachi 650-10s spectrofluorimeter were used. The sample solution contained in 1-cmnonfluorescence rectangular quartz cell was excited with 420 nm in A 0 and R6G solutions and with 350 nm in PF solutions with a slit width of 2 nm. The absorbance at the excitation wavelength was independent of the DTAB concentration in R6G and A 0 solutions. The absorbance at the excitation wavelength was less than 0.05 in R6G solutions and 0.18in A 0 solutions. In the case of PF, there is no isosbestic point and the excitation wavelength was selected at the wavelength of lowest absorbance (0.08) and lowest difference in absorbance at various DTAB concentrations. Attenuation corrections in the excitation light may result in minor changes in fluorescence intensity at these low absorbances. Excitation spectra were strongly deformed from the absorption spectra because of reabsorption at the absorption band maximum (absorbancesat the maximum are 0.5 for A 0 and PF and 1.6 for R6G)and internal screening due to dye aggregates in polyion-DTAB systems. Since the maximum wavelength in the emission band is well separated from the absorption band in PF and AO, the emission spectrum may show the real maximum wavelength. The emission spectrum of R6G,however, gives only an apparent maximum, because of the large overlap of the emission band with the absorption band and large absorbance at the wavelength of maximum emission. For this reason, excitation spectra were not used in these systems, and fluorescence intensities at these high dye concentrations cannot be discussed quantitatively. We do use fluorescence intensity as a qualitative measure of nonfluorescence aggregate formation. No attenuation correction was made in the fluorescence measurements. All spectra were measured at a room temperature. The molar absorption coefficients were calculated based on the total dye concentration. Results and Discussion Absorption and Fluorescence Spectra of R6G i n Polyion-DTAB Solutions. Self-associating molecules such as surfactants and hydrophobic dyes form dimers, higher order aggregates, and micelles in relatively concentrated aqueous solutions. These solutes also tend to associate in the polyelectrolyte domain a t concentrations far below the concentration of their self-association.l"l*~s
I
I
Excitation 420 nm
Wnm
600
Figure 2. Fluorescence spectra of R6G. The symbols a', b', c', and d' correspond to a, b, c, and d in Figure 1, respectively. Spectrum a in Figure 1 is of the aqueous solution a t 2 lod M R6G. R6G in aqueous solution has an absorption band a t 527 nm for the monomer and a t 498 nm for the dimer.34p37Spectrum a, therefore, is assigned to the R6G monomer. The weak shoulder around 500 nm is ascribed to an original shoulder of the monomer band34and/or a dimer content of ca. ,590.~~ Addition of DxS a t a concentration of 2 X equiv dm-3 induces the formation of dimer, as shown by spectrum b in Figure 1, with a maximum a t 498 nm. This phenomenon is caused by the aggregation of R6G bound to DxS even for 8 large excess of polyion (polyion/dye ratio of Two bands of monomeric and dimeric R6G appear a t 535 and 502 nm, respectively, in spectrum c in Figure 1, obtained after addition of a small amount of DTAB (concentration of 1 X lo4 M) to the 2 X equiv dm-3 DxS solution. Upon further addition of DTAB, the dimer band disappears and a remarkable enhancement with a small red shift is observed for the monomer band now at 539 nm. The molar absorption coefficient E is 81000 M-l cm-'. This peak position is at longer wavelength than that of the aqueous monomer band (539 vs. 527 nm, compared with a in Figure 1). In micellar SDS solution the absorption maximum of monomer band is found a t 535 nm (E 91 000 M-l cm-I), in DTAJ3 micellar solution it is at 537 nm (E 89OOO M-' cm-I). These findings show that R6G bound to DxS dissolves in the monomeric form into the hydrophobic region of the surfactant aggregates formed when DTA+ binds cooperatively to DxS. Figure 2 shows the fluorescence spectra of R6G in aqueous (a'), DxS (b'), and DxS-DTAI3 (c', d') solutions. The strong fluorescence intensity in aqueous solution (a') decreases drastically in DxS solution (b'). The wavelength of maximum emission a t 557 nm is the same in both solutions. This decrease is due to R6G aggregation in the polyion domain through hydrophobic interactions; the R6G dimer is nonfluorescent. Spectrum b', therefore, may be ascribed either to R6G monomers remaining in bulk solution or to bound but isolated dye monomers. The addition of DTAB causes a red shift of the fluorescence maximum emission wavelength to 572 nm and an intensity enhancement of the band (c', d' in Figure 2). This intensity X
(38) Vitagliano,V. In Aggregation Processes in Solution; Wyn-Jones, E., Gormally, J., Eds.; Elsevier: New York, 1983; p 271. (39) This value is estimated from the dissociation constant of dimer, 5.9 x 10-4." (40) Michaelis, L.; Granick, G. J. Am. Chem. SOC.1945, 67, 1212.
380 Langmuir, Vol. 3, No. 3, 1987
Hayakawa
Table I. Characteristics of Absorption and Fluorescence Spectra in Various Solutions rhodamine 6G proflavin acridine orange absorption fluorescence absorption fluorescence absorption fluorescence solutions aqueous polyion DxS PVS
Xmax,
nm
emax,
Amam
xmax,
a
emax,
Amax,
nm
Illo
1
492
53000
530
1
Cmw
Amax,
nm
Ill,,
1
445
41000
512
Iflo
558
Aman
527 498 499
40000 46000
557 558
0.09 0.17
435 435
26000 27000
512 512
0.08 0.15
455
25000
532
0.01
539 537
81000 83000
572 571
0.28 0.42
470 464
53000 40000
514 511
0.36 0.32
466
28000
535
0.16
537 533
89000 91000
569 562
0.91 1.10
446 454
43000 54000
512 504
0.64 1.00
497" 498
57O0Oa 84000
533" 530
0.65' 2.3
micellar DTAB SDS
M-l cm-'
M-' cm-I
nm
complex DxS-DTAB PVS-DTAB
nm
nm
M-l cm-' 81000
In 0.08 M DTAB solution.
recovery is much less than the absorbance recovery in the absorption band at 539 nm. The ratio of the fluorescence intensity a t the maximum emission wavelength in DxSDTAB mixed solution (I)to that in aqueous solution (lo) is 0.28 at 11 X 10" M DTAB, while the absorbance ratio, DIDo, is 0.96a t the same concentration. The red shift in the wavelength of the fluorescence maximum is also found in micellar solutions. For instance, the peak is found a t 569 nm with I l l o = 0.91 in a DTAB micellar solution and a t 562 nm with I / I o = 1.10 in a SDS micellar solution. Notice that in micellar solution the intensity a t the band maximum is comparable to that of the aqueous solution of R6G alone. The weak intensity of R6G fluorescence in DxS-DTAB mixed solutions may be caused by (a) a considerable dimer content and/or (b) a decrease in quantum yield of R6G due to the change in the microenvironment through the interaction of monomeric dye molecules with the DxS-DTA+ complexes or with the other dye monomers bound by DxS. Self-absorption and/or internal screening may affect the intensity, but because the absorbances are similar this factor should be about the same in aqueous, micellar, and polyion-DTAB solutions. The absorption spectrum of R6G at the maximum concentration of DTAB examined has only a small shoulder around 510 nm (d in Figure l),suggesting only a small dimer content. "he weak fluorescence intensity, therefore, is not due to dimer formation, but to a decrease in quantum yield of R6G in the DxS-DTA" complexes. The spectroscopic behavior of R6G in PVS-DTAB mixed solutions is very similar to that in DxS-DTAB mixed solutions. PVS addition to an aqueous dye solution induces the blue shift and absorbance reduction in the absorption band and a remarkable reduction in fluorescence intensity, indicative of the dimer formation. The absorption and fluorescence maxima are given in Table I. Figure 3A shows the dependence of the molar absorption coefficient ( e ) and relative fluorescence intensity ( I / l o )at the spectrum maximum in both systems, where solid symbols correspond to the dimer band. The molar absorption coefficient is always larger in PVS-DTAB than in DxS-DTAB. The shoulder on the monomer band around 530 nm is very clear in the PVS solution (em 35000 M-' cm-l) but hardly noticeable in the DxS solution (ehao 26 000 M-' cm-l), as shown by (b) in Figure 1. This fact suggests there is less aggregation of R6G bound to PVS and thus the binding of R6G by PVS is expected to have lower c o o p e r a t i ~ i t y .At ~ ~high ~ ~ ~DTAB concentration the molar absorption coefficient converges to that of aqueous solution for both the PVS and the DxS systems. In the PVS system, the fluorescence intensity of R6G reaches a maximum at a relatively low DTAB concentration (7 X lo4 M) system, whereas it is still increasing even at the highest
A. R 6 G
0
B. PF
IO
C. A 0
10
0
0
10
IO4[ DTAB I / M
Figure 3. Dependence of molar absorption coefficient (c) and relative fluorescence intensity (Z/Zo) at the spectral maximum on DTAB concentration. Squares corresponded to c in aqueous solution. Circles and triangles are obtained in DxS-DTAB and PVS-DTAB mixed solutions, respectively. Open marks indicate the values at the monomer band peak and solid marks indicate the values at the dimer band peak.
X/nm
Figure 4. Absorption (solid lines) and fluorescence (broken lines) spectra of PF in aqueous (a and a'), DxS (b and b'), and DxS-
DTAB mixed solutions (c, d, c', and d'). Absorbance and fluorescence intensity are represented by molar absorbance (e, left axis) and relative fluorescence intensity (right axis). concentration of DTAB in the DxS system. This fact suggests that the R6G dimer dissociates more easily in PVS-DTAB complexes through solubilization of the dye in surfactant aggregates on the polyion. Absorption and Fluorescence Spectra of PF and A 0 in DTAB-Polyion Mixed Solutions. The binding
Langmuir, Vol. 3, No. 3, 1987 381
Dye Solubilizates in Micellelike Complexes I
1
I
PF and
R6G
I DTAB
A0
400
500
I DTAB
600 X/nm
Figure 5. Absorption (solid linea) and fluorescence (broken lines) spectra of A 0 in aqueous (a and a’), DxS (b and b’), and DxSDTAB mixed solutions (c, d, c’, and d’).
of aminoacridines by polymers frequently leads to a shift in the wavelength of maximum absorption of the bound molecule^.^^ Figure 4 shows this shift for PF in the presence of DxS. The absorption maximum at 445 nm (a) of PF in aqueous solution shifts to 435 nm (b) accompanied by an absorbance reduction in the presence of 2 X equiv dm-3 DxS. This blue shift is attributed to the formation of dimers38 or a stacked form of the dye on the polyion chain.% DTAB addition once again induces a red shift of this bond maximum and the appearance of a new band maximum a t 470 nm. The maximum absorbance of the 470-nm band increases with increasing DTAB concentration and eventually exceeds that of aqueous PF solution a t high DTAB concentrations (d). The fluorescence maximum of PF appears at the same position (512 nm) in both aqueous and DxS solutions (a’ and b’ in Figure 4). A very minor shift in the maximum emission wavelength (512-514 nm) is observed upon DTAB addition (d’ in Figure 4). The intensity recovery is fairly low (Illo = 0.36 a t the highest concentration of DTAB). The dependence of absorbance and fluorescence intensity at the band maximum on DTAB concentration is given in Figure 3B for both the DxS- and PVS-DTAB systems. In DxS-DTAB the PF absorbance for DTAB concentrations above 4 X lo4 M is larger than in aqueous solution, while in PVS-DTAB the absorbance converges to the aqueous solution value. The PF fluorescence intensity tends to saturate at I l l o = 0.32 in PVS-DTAB solutions, whereas in DxS-DTAB it tends to increase even a t 12 X lo4 M DTAB. In a DTAB micellar solution (i.e., in the absence of polyion) the absorption maximum is found a t 446 nm, with e 43 OOO. In SDS micellar solution it is at 454 nm, with e = MOO0 M-’ cm-’. The maxima of the fluorescence spectra are a t 512 nm with a relative intensity I/Ioof 0.64 for the DTAB micellar solution and a t 504 nm with I/Ioof 1.00 for the SDS micellar solution. These resulta indicate there is a considerable difference between micelles of SDS and DTAB with respect to the solubilization of PF, as might be expected taking into account electrostatic effects. The comparison of the absorption and fluorescence spectra of PF obtained in polyion-DTAB solutions with those obtained in micellar solutions indicates that the polymer-surfactant complexes solubilize P F in monomeric form and diminish the quantum yield of PF. (41) Vitagliano, V.;Costantino, L.;Zagari, A. J. Phys. Chem. 1973, 77, 204.
Dye
gr S u r f a c t a n t
Figure 6. Schematic diagram of dye solubilization in polyionsurfactant complexes.
When two amino groups in P F are substituted by dimethylamino groups (AO), the behavior induced by DTAB addition is greatly different from the case of PF. The spectrum of 1.4 X lo4 M A 0 (a in Figure 5) in an aqueous solution consists of a band a t 492 nm and a band a t 465 nm which belong to the monomer and dimer of AO, respectively.42 DxS addition induces a band at 460 nm (b in Figure 5) attributed to the A 0 aggregate^.^^ The tendency of A 0 to aggregate is strong even in an aqueous solution. For example, the dimerization constants are 11000 for A042but only 500 for PF.35This tendency to aggregate is enhanced in the presence of a polyanion. As already discussed in R6G and PF systems, upon electrostatic binding to polyanions the dye molecules come closer to each other, thus occupying adjacent ionic sites they have a tendency to stack, and aggregation formation increase ~ . DTAB ~ ~ *addition ~ ~ induces a red shift in the absorbance maximum to 462 nm and an absorbance enhancement a t 500 nm (b-d in Figure 5), but the effect is weak. The fluorescence of A 0 almost disappears upon addition of DxS even at a polyion/dye ratio of 140 (a’, b’ in Figure 5). This strong quenching of the green fluorescence again indicates the strong tendency of A 0 to stack in the polyion domain.43 Subsequent DTAB addition causes an enhancement of the fluorescence intensity with a maximum 5-nm red shift at the highest concentration of DTAB examined (d’ in Figure 5). However, the effect is the weakest among the three dyes, as shown in Figure 3. These findings indicate the strong cooperative binding of A 0 by DxS and the relatively weak solubilizing effect of DxS-DTAB complexes. For cooperative binding, the degree of binding ,8 is given by eq 1,as derived from a theoretical treatment based on
B = Y2[1 + (s - l)/[(s - 1 ) 2
+ 4s/u]’/2]
s = Ku[surfactantIf
(1) (2)
the nearest-neighbor interaction model between surfactants bound by linear polymers with an infinite chain length?20 where u is the cooperativity parameter, which is a measure of the increased tendency for a surfactant ion (42)Schwarz, G.; Balthasar, W.Eur. J . Biochem. 1970,12,461. (43) Muller, G.; Fenyo, J . C . J . Polym. Sci. Polym. Chem. Ed. 1978, 16, 17.
Langmuir 1987, 3, 382-381
382
to bind to a site adjacent to a site already occupied by a surfactant; K is the binding constant between the surfactant and an isolated polyion binding site; and [surfactantIf is the equilibrium concentration of surfactant. s is equivalent to the parameter s in the Zimm-Bragg theory for the cooperative helix-coil transition of biopolymers.44 The average cluster size m in surfactant aggregates in a polyion domain can be derived as f01lows:~
m = 2P(u - 1)/[[4P(1 - P)(u - 1) +
l]l/’-
11 (3)
The cooperativity parameter u was estimated to be 650 for DTA+ binding by DxS polyion in the presence of NaC1.13 This value predicts the following values of m as a function of @ ( m (P)): 9 (0.1); 13 (0.2); 17 (0.3); 22 (0.4);(26) 0.5; (32) 0.6; 42 (0.7). This trial estimation of m shows the presence of relatively large clusters of bound DTA+ ions even at a small degree of binding in DxS-DTAB solutions. Since the cooperativity parameter is smaller in solutions without NaCl than in solutions with NaC1,l4J5the above values of m may be overestimated. A clear indication of surfactant clustering in polyion-surfactant complexes was deduced from the existence of a distinct solubilization area for an oil-soluble dye, orange OT, and from the lowering of the microenvironment polarity of the fluorescence probe (44) Zimm, B. H.; Bragg, J. K. J . Chem. Phys. 1959, 30, 526.
pyrene aldehyde, in mixtures of a cationic cellulose ether (Polymer JR) and SDS a t a conceqtration of SDS 1 order of magnitude lower than its cmc.23,24 The solubilization of dye by the polyion-surfactant complex is due to these surfactant clusters. The surfactant clusters are formed around the dimers or aggregates of bound dye in a polyion domain. Bound dyes such as R6G and PF are dissolved in monomeric form in these surfactant clusters. The dimerization constant of A 0 (11000 M-’)42is much larger in aqueous solution than that of R6G (1700 M-l)%and PF (500 M-’),35and A 0 itself binds highly cooperatively to DxS. In this case, the solubilizing power of the surfactant clusters is not sufficient to effect the dissociation of A 0 aggregates bound by polyions. Figure 6 shows a schematic diagram of the solubilization of polyion-bound dyes by surfactant clusters in the polyion domain. The aggregates of R6G and PF are easily dissociated into the monomeric form by surfactant clusters, but the A 0 aggregates remain in the aggregated form even in the surfactant clusters.
Acknowledgment. We are grateful to Professor Masako Sat0 for her help in the measurement of fluorescence spectra. Registry No. R6G, 989-38-8;PF, 92-62-6;AO, 65-61-2;DxS (sodium salt), 9011-18-1;PVS (sodium salt), 26701-97-3;DTAB, 1119-94-4; SDS, 151-21-3.
Surfactant Aggregation in the Presence of Polymers E. Ruckenstein,*+ G. Huber, and H. Hoffmann Institut fur Physikalische Chemie der Universitat Bayreuth, 0-8580 Bayreuth, West Germany Received August 26, 1986. I n Final Form: December 1, 1986 A model for surfactant aggregation in the presence of macromolecules that involves the adsorption of micellar aggregates in the “free space” of the coiled macromolecules is suggested. The model accounts for the interactions between macromolecules and surfactant molecules via the changes the former cause in the interfacial free energies between the hydrocarbon core of bound aggregates and the microenvironment (the water located in the “free space” of the macromolecular coil) and between the polar headgroups of the bound aggregates and the microenvironment. If the headgroup is sufficiently small, the overall surface free energy is decreased and thus aggregation is stimulated. Under such conditions, micelles which are bound to macromolecules form at a critical concentration which is smaller than the cmc for the formation of the free micelles. In contrast, if the headgroup is sufficiently large, the overall interfacial free energy between micelles and the microenvironment is increased and no bound micelles will form. This model is able to explain the variety of experimental data which are available, particularly why surfactants with large headgroups do not stimulate the formation of bound micelles while those with sufficiently small headgroups do stimulate their formation. Calculations have been carried out for sodium dodecyl sulfate, Triton X-100 and dodecyltrimethylammonium chloride in the presence of poly(ethy1ene oxide).
2. Small-angle neutron-scattering experiments11J2 have Introduction shown that (1) the surfactant molecules bind to the The effect of macromolecules on surfactant aggregation in aqueous solutions was investigated over the last decade because of its relevance to various biological, pharmaceu(1)Robb, I. D. Anionic Surfactants-Physical Chemistry of Surfactical, mineral processing, and oil recovery appli~ations.l-~ tant Action; Lucassen-Reinders, E. H., Ed.; Marcel Dekker: New York, 1981; p 109. Robb’ has provided a detailed review. The main experi(2) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 13, 531. mental results can be summarized as follows: (3) Steinhardt, J.; Reynolds, J: A. Multiple Equilibria in Proteins; 1. Binding of some anionic surfactants to nonionic Academic: New York, 1969. (4) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. polymers occurs above a critical surfactant concentration (5) Saito, S.J . Colloid Interface Sci. 1967, 24, 227. which is lower than the critical micelle concentration of (6) Fishman, M. L.; Eirich, F. R. J . Phys. Chem. 1971, 75, 3135. the surfactant in the polymer-free aqueous s o l ~ t i o n . ~ - ~ ~ (7) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. *Present address: State University of New York at Buffalo, Buffalo, New York, 14260. Humboldt Award Winner.
0743-7463/87/2403-0382$01.50/0
(8) Smith, M. L.; Muller, N. J. Colloid Interface Sci. 1975, 52, 507. (9) Gilanyi, T.; Wolfram, E. In Proceedings of the International Conference on Colloid Surface Science; Wolfram, E., Ed.: Elsevier: Amsterdam, 1975; Vol. 1, p 633. (10) Gilanyi, T.; Wolfram, E. Colloid Surf. 1981, 3, 181.
0 1987 American Chemical Society
