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Characterization of Mixed Micelles of Sodium Dodecyl Sulfate and Tetraoxyethylene Dodecyl Ether in Aqueous Solution Hueder Paulo Moise´s de Oliveira and Marcelo Henrique Gehlen* Instituto de Quı´mica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, Sa˜ o Carlos, Brazil Received October 1, 2001. In Final Form: February 5, 2002 The mixed micelles of sodium dodecyl sulfate (SDS) and tetraoxyethylene dodecyl ether (C12E4 or Brij 30) were studied by surface tension, ζ potential, and fluorescence spectroscopy measurements. In the range of 0-0.5 mole fraction (x) of added Brij 30 to a 30 mM SDS solution, the mixed micellar aggregation number (Nagg) changed from 65 to 171, and the ζ potential of the micelles varied from -60 to -15 mV. The change in size was acompanhied by a 2-fold increase in the internal micellar viscosity, indicating that addition of the nonionic surfactant with similar alkyl chain enhances the surfactant packing in the mixed aggregate. The reduction of the electrical surface potential causes a proportional decrease of the binding constant of the cationic dye acridine orange. The change in temperature from 15 to 45 °C of a solution with x ) 0.5 results in a decrease in the Nagg from 175 to 128, but the SDS to Brij 30 monomers ratio remains constant. This fact is typical of ionic micelles, opposite to the usual behavior of pure nonionic micellar aggregates with an oxyethylene surfactant headgroup.
Introduction Micelles are dynamic nanoscopic aggregates of surfactant molecules having a specific capability to solubilize a wide variety of organic molecules with different polarities and hydrophobicities.1,2 They are host structures used in several types of organic reactions, catalysis, polymerization, and stabilization and transport of dyes and drugs.3-5 In these applications, the size and the stability of the micelles are important features that determine and explain their role in a variety of chemical process. The tailoring of micelle properties may be achieved by adding salts, organic solutes such as alcohols,6 or a second type of surfactant forming the so-called mixed micellar system.7-17 Mixed micelles of ionic with nonionic surfactants have been a topic of several investigations owing to their extended colloidal stability when compared with pure * Corresponding author. E-mail:
[email protected]. (1) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708-11714. (2) Rodrigues, M. A.; Alonso, E. O.; Chang, Y. W.; Farah, J. P. S.; Quina, F. H. Langmuir 1999, 15, 6770-6774. (3) Bunton, C. A.; Nome, F. J.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357-364. (4) Colombie, D.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 2000, 33, 7283-7291. (5) Lasic, D. D. Nature 1992, 355, 279-280. (6) Romani, A. P.; Gehlen, M. H.; Lima, G. A. R.; Quina, F. H. J. Colloid Interface Sci. 2001, 240, 335-339. (7) Baglioni, P.; Dei, L.; Minten, E. R.; Kevan, L. J. Am. Chem. Soc. 1993, 115, 4286-4290. (8) Feitosa, E.; Brown, W. Langmuir 1998, 14, 4460-4465. (9) Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 38553858. (10) Huang, H.; Verrall, R. E.; Skalski, B. Langmuir 1997, 13, 48214828. (11) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 7166-7182. (12) Misselyn-Bauduin, A. M.; Thibaut, A.; Grandjean, J.; Broze, G.; Je´roˆme, R. Langmuir 2000, 16, 4430-4435. (13) Goloub, T. P.; Pugh, R. J.; Zhmud, B. V. J. Colloid Interface Sci. 2000, 229, 72-81. (14) Sierra, M. L.; Svensson, M. Langmuir 1999, 15, 2301-2306. (15) Ruiz, C. C.; Aguiar, J. Langmuir 2000, 16, 7946-7953. (16) Thomas, H. G.; Lomakin, A.; Blankschtein, D.; Benedek, G. B. Langmuir 1997, 13, 209-218. (17) Douglas, C. B.; Kaler, E. W. Langmuir 1994, 10, 1075-1083.
nonionic micellar systems.7-17 On the other hand, an ionic surfactant that forms usually small globular aggregates at a low surfactant concentration may have size increase upon addition of a nonionic surfactant. The larger size and the thermodynamic stability of the mixed micelles would enhance the incorporation capability of solutes into the micellar phase, which is an important issue in many of the applications of micellar solutions. In this work, the mixed micelles formed by sodium dodecyl sulfate (SDS) and tetraoxyethylene dodecyl ether (C12E4 or Brij 30) were studied by using stationary and time-resolved fluorescence spectroscopy, surface tension, and ζ potential measurements. The size (or aggregation number) of the mixed micelles, polydispersity, micellar surface polarity, microviscosity, electrical potential, and the cationic dye binding as a function of surfactant composition are reported and discussed. Furthermore, the rate of solute intermicellar diffusion obtained from dynamic quenching experiments is correlated with the size of the mixed micelles formed in different surfactant compositions and temperatures. Materials and Methods The dye acridine orange (Merk), pyrene, and perylene (Aldrich) were recrystallized from methanol. N-Dodecylpiridinium chloride was recrystallized from acetone. Sodium dodecyl sulfate (Sigma, 99%) was used after recrystalization from acetone and treated by solvent extraction to remove traces of long-chain N-alkyl alcohol. Tetraoxyethylene dodecyl ether (Brij 30, Sigma) was used as purchased. The surfactants and dye concentrated stock solutions were prepared using Milli-Q pure water. Stock solutions of the aromatic probes were prepared in methanol or in cyclohexane. Working solutions were prepared by adding, via a microseringe, the appropriate amounts of concentrated stock solution of the fluorophores and quencher to the SDS/Brij 30 solution, followed by stirring and a stabilization period before spectral measurements. For measurements above cmc, the total concentration of SDS was fixed at 30 mM while the concentration of Brij 30 was change from 0 to 30 mM. Absorption measurements were performed on a Hitachi U-2000 spectrophotometer, and corrected steady-state fluorescence spectra were recorded on a CD-900 Edinburgh spectrofluorimeter. For measurements, samples in a 1 × 1 cm quartz cuvette were
10.1021/la011501g CCC: $22.00 © 2002 American Chemical Society Published on Web 04/12/2002
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thermostated by circulating fluid through a jacketed cuvette holder, in air-equillibrated condition. Surface tensions of the mixed surfactant aqueous solutions were determined using a platinum ring tensiometer (KSV model Sigma 70) with samples thermostated at 298 K. Stock solutions of the surfactant mixture at fixed 120 mM SDS concentration and Brij 30 from 0 to 120 mM were added sequentially in small portions to a proper water volume for surface tension measurements. The reported cmc of the mixed micelles is given in terms of the SDS concentration. Electrophoretic light scattering measurements of the mixed micelles in aqueous solution were made using the ZetaPlus (Brookhaven Instruments Corp.) equipment, with samples thermostated at 298 K. The ζ potential was calculated from the measured mobility (µ) using the expression µ ) 2ζf1(κa)/3η, where and η are the dielectric constant and viscosity of the solvent, respectively. The values of and η for water at 298.15 K, which are 78.54 and 0.89 cP, respectively, were assumed in the calculation of ζ. f1(κa) is the Henry correction factor18 which takes into account the product of the Debye-Hu¨ckel parameter (κ) and the radius of the particle (a). The f1 factor is in the range of 1-1.1 when dilute micellar solution in the absence of added salt is considered. Fluorescence decays of pyrene were measured by the singlephoton counting technique using a CD-900 Edinburgh spectrometer operating with a hydrogen-filled nanosecond flash lamp at 40 kHz pulse frequency. Time-resolved anisotropy measurements were performed with the same spectrometer equipped with Glan-Thompson polarizers, but the ligth pulse was provided by frequency doubling the 200 fs laser pulse of Mira 900 Ti: Sapphire laser pumped by a Verdi 5W Coherent. The laser pulse frequency was reduced by a Conoptic pulse picker system. The respective fluorescence decays were analyzed by reconvolution procedure with the micellar quenching and spherical rotor models of the Edinburgh Instruments Level 2 software. Aggregation numbers of the mixed micelles were obtained from the fitting of the fluorescence decay at different concentrations of added quencher using the Infelta-Tachiya equation,19-21
f(t) ) f(0) exp[-k0t + 〈n〉(exp[-kqt] - 1)]
(1)
where f(0) is the fluorescence intensity at time t ) 0, k0 the decay constant of the excited probe in absence of added quenchers, 〈n〉 the average number of quenchers per micelle, and kq the firstorder intramicellar quenching rate constant. The aggregation numbers are calculated from
Nagg )
〈n〉Sm Qm
Figure 1. Plot of the cmc values of SDS + Brij 30 in aqueous solution at 298 K as a function of surfactant composition obtained using different methods: (1) surface tension; (9) dye fluorescence.
(2)
Sm and Qm are the surfactant and quencher micellized concentrations. The kinetics model behind eq 1 assumes that excited probe and quencher stay in the same micelle during the entire fluorescence decay; i.e., the final slope of the decay in a log plot is independent of quencher concentration. The system here investigated followed this condition. The rotational relaxation time τr was obtained by global fitting of the crossed and parallel polarization time-resolved data using respectively
I⊥(t) ) exp[-t/τ0](1 - r0 exp[-t/τr])
(3)
I|(t) ) exp[-t/τ0](1 + 2r0 exp[-t/τr])
(4)
where r0 is the initial value of anisotropy and τ0 the fluorescence lifetime of the probe. τ0 was obtained in a separate experiment with fluorescence decay measured at the magic angle polarizer configuration and single-exponential fit. (18) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: New York, 1981; p 71. (19) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190-195. (20) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289-292. (21) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199221.
Figure 2. ζ potential (9) and the binding constant of acridine orange (2) to SDS + Brij 30 mixed micelles at 298 K.
Results and Discussion (i) Characterization of the Mixed Micelles. The addition of the nonionic surfactant Brij 30 decreased the cmc of the mixed surfactant solution when compared with the cmc of pure SDS. The values of cmc as a function of composition, obtained from the two different methods, are reported in Figure 1. There is a good agreement between the values obtained from surface tension and fluorimetric measurements. The cmc of the SDS is 8.0 mM, and for Brij 30 the cmc at 298 K is 0.064 mM.22 The cmc values found are within this range, in accordance with the regular solution theory for mixed micelles.23 Mixed micelles of anionic-nonionic surfactants usually are nonideal solutions, and the aggregation process has a degree of synergism.11 This synergism would result in part from the charge screening and reduction of the repulsion between sulfate headgroups caused by intercalating the nonionic surfactant between SDS monomers. This effect is reflected in the change of the ζ potential of the mixed micelles with composition. The ζ potential of the mixed micelles as a function of the molar fraction of Brij 30 is shown in Figure 2. It changes from -58 mV to about -15 mV as the mole fraction of Brij 30 varies from 0 to 0.5. At larger concentration of Brij 30, (22) Rosen, M. J.; Cohen, A. W.; Dahanyake, M.; Hua, X. J. Phys. Chem. 1982, 86, 541-545. (23) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 337.
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Table 1. Rotational Relaxation Times of Perylene (τr) in SDS/Brij 30 Mixed Micelles as a Function of the Mole Fraction of Brij 30a
Table 2. Aggregation Number,a Micellar Radius, and Intramicellar Quenching Rate Constant of SDS/Brij 30 Mixed Micelles
X
τF, ns
τr, ns
χ2
x
Nagg(SDS)
Nagg(SDS + Brij 30)
Rm, Å
10-7kq, s-1
0 0.05 0.10 0.20 0.30 0.40 0.50
5.87 ( 0.02 5.92 ( 0.02 5.95 ( 0.02 5.84 ( 0.02 5.84 ( 0.02 5.86 ( 0.02 5.84 ( 0.02
0.54 ( 0.07 0.65 ( 0.09 0.67 ( 0.07 0.76 ( 0.08 0.92 ( 0.12 1.03 ( 0.12 0.92 ( 0.11
1.006 1.016 1.085 1.052 1.001 1.023 1.028
0 0.05 0.10 0.15 0.20 0.30 0.40 0.50
65 77 84 85 88 84 83 85
82 94 101 110 121 140 171
18 19 20 21 22 23 25 27
4.84 ( 0.15 3.85 ( 0.12 3.12 ( 0.04 2.66 ( 0.09 2.35 ( 0.09 1.80 ( 0.06 1.63 ( 0.07 1.32 ( 0.06
a τ and χ2 are the fluorescence lifetime and the chi-square of the F fitting, respectively. The intrinsic anisotropy (r0) measured is 0.12 ( 0.01.
micellar solution will be less stable because the ζ potential will be reduced to about -10 mV and eventually micellar agregation would occur. The local polarity of the mixed micelle can be reported by the I1/I3 ratio of vibronic band intensity of pyrene fluorescence,24 and the change in micelle viscosity can be investigated from the average orientational relaxation time (τr) of a fluorescent probe like perylene.25-27 The I1/I3 ratio decreased gradually upon addition of Brij 30, indicating a formation of a less polar environment sensed by pyrene probe in the mixed micelle. A linear equation I1/I3 ) 1.16-0.19x is obtained from the fitting of the ratio of intensity as a function of mole fraction of Brij 30. This trend has been previously observed in SDS/Brij 30 mixed micellar system;28 however, it contrasts with the behavior of I1/I3 ratio found in mixtures of SDS with Triton X100, a surfactant with a much larger nonionic poly(ethylene oxide) headgroup.29 This difference could be ascribed to the effect of increase of the micellar size upon addition of nonionic surfactant in the SDS/Brij 30, resulting in a change in location of the pyrene probe in the aggregate, probably with pyrene located more toward the core of the mixed micelle. The anisotropy decays of the perylene probe in the micellar solutions were single-exponential, indicating that the fluorophore reorients probably as a prolate rotor. This behavior has been also observed in n-alkanes solvents from n-pentane to n-hexadecane.30 In a nanosecond time window of the fluorescence decay of perylene, the effect of the whole micelle assembly rotation, which is a slow process, can be neglected. The orientational relaxation time of perylene depends on the composition of the mixed micelle solution (the results obtained are listed in Table 1). τr doubles as the mole fraction of Brij 30 varies from 0 to 0.5, indicating an increase of the viscous friction on the reorientational diffusion motion of the probe. This effect may be ascribed to an enhancement of surfactant chain packing in the mixed micelles. The ordering of the micelle interface and the hydration capacity of the ethylene oxide headgroups may reduce the degree of water penetration in the alkyl layer, in accordance with the reduction observed in the micropolarity sensed by the pyrene probe. The packing is favored by the size matching between the two types of surfactant tails and proper location of the SDS sulfate headgroup in the middle of the ethylene oxide (24) Kalyanasundaram, K. Langmuir 1988, 4, 942-945. (25) McCarrol, M.; Toerne, K.; von Wandruszka, R. Langmuir 1998, 14, 2965-2969. (26) McCarrol, M. E.; Joly, A. G.; Wang, Z.; Friedrich, D. M.; von Wandruszka, R. J. Colloid Interface Sci. 1999, 218, 260-264. (27) Ruiz, C. C. J. Colloid Interface Sci. 2000, 221, 262-267. (28) Iglesias, I.; Montenegro, L. Phys. Chem. Chem. Phys. 1999, 1, 4865-4874. (29) Ruiz, C. C.; Aguiar, J. Mol. Phys. 1999, 97, 1095-1103. (30) Jiang, Y.; Blanchard, G. J. J. Phys. Chem. 1994, 98, 6436-6440.
a The standard deviations of the aggregation numbers are in the range of 4-10%.
branch. If one assumes a molecular volume of perylene30,31 of about 225 Å3 in the Debye-Stokes-Einstein equation, the micelle viscosity will have values of 10 ( 1 cP for pure SDS and 20 ( 2 cP at 0.5 mol fraction. These results are in agreement with a progressive uptake of the nonionic surfactant and micellar grow upon addition of Brij 30 (see the aggregation numbers in Table 2), and the microviscosity found is within the range observed in other micellar environments.32 (ii) Cationic Dye Association. Cationic dyes bind strongly to anionic micelles. In the premicellar region, dye-surfactant aggregates with dye pairing are formed.33 When a fluorescent dye is used, the first addition of an anionic surfactant quenchs its fluorescence because the dimers formed have no emission in most of the cases. At the cmc, micelles are formed, the dye is redistributed in monomeric form, and the fluorescence intensity is recovered. The change in fluorescence intensity has been used as a spectroscopic method to estimate the cmc of surfactants. Its onset is always less than the true cmc of pure surfactant system due to the nucleation/self-association of surfactants onto the premicellar clusters, but the inflection point of the fluorescence intensity agains surfactant concentration gives one good estimative of the cmc. Above the cmc, the change in fluorescence intensity may be correlated with the dye association to the micellar pseudophase following a similar method described by Almgren et al.,34
Is 1 )1+ ∆I K[m]
(5)
where Is is the saturation value of fluorescence intensity at high surfactant concentration and ∆I is the difference in fluorescence intensity at concentrations above and at the cmc. K is the dye-micelle association constant, and [m] stands for micelle concentration. The values of K of acridine orange in mixed micelles of SDS/Brij 30 were estimated using fluorescence intensity measurements, and the values found are plotted in Figure 2. The binding of a cationic dye to an anionic micelle depends on the balance of electrostatic and hydrophobic contributions to the Gibbs energy of the process. The decrease of the electrostatic potential of the mixed micelle, as seem from the change in ζ, seems to be the main factor of the lowering of K with addition of Brij 30. However, the hydration of the headgroup of Brij 30 (oxyethelene units) increases the (31) Edward, J. T. J. Chem. Educ. 1970, 47, 261-270. (32) Shinitzky, M.; Dianoux, A. C.; Gitler, C.; Weber, G. Biochemistry 1971, 10, 2106-2113. (33) Neumann, M. G.; Gehlen, M. H. J. Colloid Interface Sci. 1990, 135, 209-217. (34) Almgren, M.; Grieser, F.; Thomas, K. J. J. Am. Chem. Soc. 1979, 101, 279-291.
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Figure 3. Aggregation numbers (Nagg) of mixed micelles of SDS + Brij 30 in aqueous solution at 298 K as a function of surfactant composition obtained from fluorescence quenching: (2) Nagg (SDS + Brij 30); (9) Nagg (SDS).
uptake of water molecules in the micellar surface35 and, therefore, will also decrease the hydrophobic contribution related to the association of a dye molecule to the surface of the mixed micelle. Considering minor this last effect, the change in K can be related to a corresponding change in the electrical potential of the micelle site where the dye is solubilized. In this way, the decrease of K in the presence of 0.5 mol fraction of Brij 30 of a factor of about 5 when compared to the value of K in the absence of Brij 30 would correspond to a reduction of about 45 mV in the surface electrical potential at 298 K. Note that this result is in agreement with the change observed in the ζ potential, as seen in Figure 2. (iii) Aggregation Numbers and the Average Reaction Time. The mean aggregation numbers of SDS/Brij 30 mixed micelles were determined by the time-resolved fluorescence quenching (TRFQ) method using up to six different concentrations of added quencher. Using eq 2, and the concentration of micellized surfactants, Sm ) [SDS] + [Brij 30] - cmc, the aggregation number can be expressed as a sum of the two surfactants. Considering that the cmc of SDS is much larger than the cmc of Brij 30, the partial aggregation number of SDS monomers in the mixed micelles is calculated by assuming the micellized SDS as Sm(SDS) ) [SDS] - cmc. The results obtained are given in Table 2, and the plots of aggregation numbers as a function of bulk mole fraction of Brij 30 are shown in Figure 3. Upon addition of Brij 30, the mixed micelle aggregation number increases progressively, but the number of SDS monomers/micelle achieves a constant value above the mole fraction 0.1. The fluorescence quenching parameters, like Nagg from TRFQ method, are correlated with the type of size distribution of the micelles.36 The weighted average aggregation number, Nw, is related to the quencher-average aggregation number, Nagg, and quencher concentration by37
Nagg ) Nw -
β σ2 θ + θ2 - ... 2 6
(6)
where σ2 is the variance and β the third cumulant of the micelle size distribution. θ is the ratio of quencher to (35) Garamus, V. M. Chem. Phys. Lett. 1998, 290, 251-254. (36) Almgren, M.; Lo¨froth, J. E. J. Chem. Phys. 1982, 76, 27342743. (37) Warr, G. G.; Grieser, F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813-1828.
Figure 4. Plot of the inverse of the intramicellar quenching rate constant (kq) as a function of the parameter φ, according to eqs 7 and 8.
micellized surfactant. For most purposes and working with low θ, the linear fitting is appropriate, from which a Gaussian shape of the size distribution can be obtained. The mixed micelles investigated grew with addition of Brij 30, and their size distribution became broad. The ratio between standard deviation (σ) and the weighted average aggregation number can be used as a polydispersity index showing trends in size distribution. In the range of surfactant composition investigated, the value found for this ratio is practically constant (about 0.22 ( 0.05). The result obtained here is close to the standard polydispersity index of SDS micelles.38 The intramicellar quenching rate constant of pyrene fluorescence by dodecylpyridinium was determined from the decay analysis using eq 1. The values of kq as a function of surfactant composition are listed in Table 2. kq decreases as the size or aggregation number of the mixed micelle increases, a typical behavior found in diffusion-controlled reaction in a confined space like micelles.39 When 1/kq is considered as the mean reaction time and probe and quencher species diffuse over the surface of the micelle of radius Rm, the following relation holds:39
kq-1 ) φ ) Rm2
(
φ D
(7)
)
2Rm 2 -1 ln 2 d 1 - (d/2Rm)
(8)
D is the mutual diffusion coefficient of probe and quencher, and d is the sum of molecular radii of probe and quencher. The volume of a globular micelle, and therefore Rm, were calculated by considering the average number of SDS and Brij 30 molecules/micelle and their respective molecular volumes. The molecular volumes of SDS anion and of Brij 30, obtained by quantum chemical calculations using the Gaussian 98 with STO-3G basis set, were 316 and 547 Å3, respectively. The plot of 1/kq according to the model given by eq 7, with values of Rm from Table 2 and assuming d ) 5 Å, is given in Figure 4. The mutual diffusion coefficient of probe and quencher is therefore the inverse of the slope of the curve in Figure 4. The graphical results show that D decreases by approximately a factor 2 with the addition (38) Attwood, D.; Florence, A. T. Surfactant Systems. Their Chemistry, Pharmacy and Biology; Chapmann and Hall: London, 1984; p 794. (39) Tachiya, M. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; John Wiley & Sons: New York, 1987; p 602.
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Table 3. Micellar Quenching Parameters in SDS/Brij 30 at x ) 0.50 as a Function of Temperaturea temp, °C
10-7kq, s-1
Nagg(SDS)
Nagg(SDS + Brij 30)
Rm, Å
15 25 35 45
0.60 ( 0.04 1.32 ( 0.06 2.01 ( 0.40 3.38 ( 0.77
85 85 66 63
173 171 130 128
27 27 24 24
a The standard deviations of the aggregation numbers are in the range of 4-10%.
of Brij 30, in this case corresponding to a 2-fold increase in micellar surface viscosity. Note that the same effect was previously observed with the rotational times of the perylene probe in the mixed micelles. Nevertheless, D in these micelles is in the range of (4-8) × 10-6 cm2 s-1. The results obtained here are based on the assumption that the mixed micelles retain their globular or spherical geometry upon addition of the nonionic surfactant. This is an approximation and large mixed micelles will appear as nonspherical aggregates, but as long as the radii of prolate ellipsoidal micelle are not very different, the model applied will describe in a good approximation the intramicellar quenching kinetics. In a last part, the mixed micellar solution at mole fraction 0.5 of Brij 30 was investigated at different temperatures. The aim was to see whether these mixed micelles have the behavior of nonionic micelles or the behavior of ionic micelles with respect to temperature, which for the former the aggregation number increases while for the later it decreases when T is increased. The loss of hydration water with temperature leads to a micellar grown in micelles with ethylene oxide headgroups.35 Aggregation numbers and quenching rate constant at different temperatures are given in Table 3. The total aggregation number (SDS + Brij 30) decreases as the temperature increases, which is the typical behavior of ionic micelles in solution. However, the ratio between Brij 30 to SDS monomers/micelle stays constant (in a factor 2) with temperature. The increase of kq with temperature may be ascribed to an increase of the mutual diffusion coefficient of probe and quencher D in addition to the effect of reduction of the micelle volume. The effect of the
variation in micelle volume (or Rm) in kq(T) is removed by multiplying the experimental value of the quenching rate constant by its corresponding φ value (see eq 8). Thus, the slope of the Arrhenius plot of kq(T)φ gives the activation barrier of the mutual diffusion of the probe and quencher in the micellar environment. To compare results, the quenching rate constant of pyrene fluorescence by dodecylpyridinium chloride was also measured in pure ethylene glycol (η ) 18 cP at 298 K). From the Arrhenius plot, the activation energies of the quenching in the mixed micelle with x ) 0.5 and in ethylene glycol were determined as 8.5 ( 1.2 and 7.1 ( 0.5 kcal/mol, respectively. Thus, the translation diffusion of pyrene and dodecylpyridinium chloride has similar activation barriers in these media of comparable viscosities. Conclusions Sodium dodecyl sulfate (SDS) and tetraoxyethylene dodecyl ether (C12E4 or Brij 30) form a mixed micellar aqueous solution with properties that depend on the surfactant composition. In the range of 0-0.5 mol fraction of added Brij 30 to a 30 mM SDS solution, the mixed micellar aggregation number (Nagg) changes from 65 to 171 and the ζ potential of the aggregates varies from -60 to -15 mV. The change in size is acompanhied by a 2-fold increase in the internal micellar viscosity (or microviscosity), as indicated by the rotational and translational intramicellar difusion of the perylene and pyrene fluorescent probes, respectively. In this way, the addition of the nonionic surfactant enhances the surfactants packing in the mixed aggregate. The reduction of the electrical surface potential results in a decrease by a factor 5 of the binding constant of the cationic dye acridine orange. The mixed micelles at higher Brij 30 concentration still keep the behavior of an ionic micellar aggregate since the increase in temperature from 15 to 45 °C decreases the Nagg value from 175 to 128. Acknowledgment. Financial supports by the FAPESP, through Project 98/14481-9, and the CNPq are gratefully acknowledged. H.P.M.d.O. thanks the FAPESP for a graduate fellowship. LA011501G
