Effect of Electrolytes on the Pyrene Solubilization Capacity of Dodecyl

Figure 1 Pyrene solubilization by SDS micelles as measured by UV absorbance at several NaCl concentrations: triangles, 0 mM NaCl; squares, 20 mM NaCl;...
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Langmuir 2000, 16, 10037-10043

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Effect of Electrolytes on the Pyrene Solubilization Capacity of Dodecyl Sulfate Micelles Joon-Hyung Kim,†,‡,§ Michael M. Domach,† and Robert D. Tilton*,†,‡ Department of Chemical Engineering, and Colloids, Polymers and Surfaces Program, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received April 12, 2000. In Final Form: August 17, 2000 Micellar aggregation numbers for ionic surfactants are sensitive to both the ionic strength and the type of counterion present. The current study considers the effects of electrolyte conditions on solubilization of pyrene, a polycyclic aromatic hydrocarbon, in micellar dodecyl sulfate surfactant solutions. We use ultraviolet-visible spectrophometry to measure the pyrene solubilizing powers, and we use an excimer fluorescence technique both to measure the micellar aggregation numbers in the presence of pyrene solubilizates and to measure the pyrene solubilization capacities. The aggregation number increases with increasing concentration of background electrolyte. When considering different types of counterions (NH4+, Na+, and Li+), the aggregation number increases with increasing counterion binding affinity. We estimate the latter by electrical conductivity measurements. In all cases of varying electrolyte conditions examined, the solubilization capacity increases siginficantly with increasing aggregation numbers, and the solubilizing power is therefore only weakly dependent on the electrolyte conditions.

Introduction Micelle-enhanced solubilization of nonpolar compounds is one of the more significant applications of surfactants. It provides the basis for detergency, micellar catalysis and extraction, and microemulsion polymerization, for example. It is important in enhanced oil recovery, and it may accelerate Ostwald ripening in suspensions of sparingly soluble colloids. To describe solubilization phenomena in general, Ikeda and Maruyama1 defined the (macroscopic) solubilizing power as the number of molecules solubilized per molecule of micellized surfactant and the (microscopic) solubilization capacity as the average number of molecules solubilized at saturation in a single micelle. The solubilizing power has been studied frequently. On the other hand, there have been relatively few studies of solubilization capacity, probably for lack of suitable experimental techniques. Solubilization capacity data shed light on the molecular mechanisms of micellar solubilization. It is most common to take the approach of Kolthoff and Stricks,2 that is, to calculate solubilization capacity by multiplying the solubilizing power by the micellar aggregation number that was measured in the absence of the solubilizate. If the aggregation number were significantly different in the presence of solubilizate, then the Kolthoff and Stricks calculation would need to be refined. Alternatively, Kuriyama3 and Nakagawa et al.4 calculated the number of solubilizates per micelle from the micelle molecular weight as deduced from light-scattering measurements. In doing so, they assumed that micelles * To whom correspondence should be addressed. E-mail: tilton@ andrew.cmu.edu. † Department of Chemical Engineering. ‡ Colloids, Polymers and Surfaces Program. § Current address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025. (1) Ikeda, S.; Maruyama, Y. J. Colloid Interface Sci. 1994, 166, 1. (2) Kolthoff, I. M.; Stricks, W. J. Phys. Colloid Chem. 1949, 53, 424. (3) Kuriyama, K. Kolloid-Z. 1962, 180, 55. (4) Nakagawa, T.; Kuriyama, K.; Inoue, H. J. Colloid Sci. 1960, 15, 268.

had fixed compositions at different surfactant concentrations provided that the solubilizate/surfactant ratio was fixed. The effects of ionic strength on the critical micelle concentration (cmc) and on the solubilizing power have been studied for many systems (see McBain and Hutchinson5 and Rosen6 and references therein). The simple solubilization capacity calculation of Kolthoff and Stricks has been widely adopted. Still, more direct measurements of solubilization capacity and its dependence on electrolyte are not readily available. Because the type and concentration of background electrolyte are such obvious and important variables for solubilization in ionic surfactant solutions, more direct studies of the details of the solubilization process are warranted. Recently we developed an excimer fluorescence technique to more directly measure the solubilization capacities of micelles and of polymer-surfactant complexes.7,8 Here, we use this technique to measure the effects that electrolyte concentration and the type of counterion have on the solubilization capacity of dodecyl sulfate micelles for pyrene, a polycyclic aromatic hydrocarbon. We use ultraviolet/visible spectrophotometry to measure electrolyte effects on solubilizing power. We find that the solubilization capacity of dodecyl sulfate micelles increases significantly when the aggregation number increases, regardless of whether the latter is changed by changing the ionic strength of the solution or by changing the counterion binding affinity. Theory Pyrene, our model solubilizate, has well-known environment-sensitive photophysical characteristics. The pyrene fluorescence emission spectrum has several peaks. (5) McBain, M. E. L.; Hutchinson, E. Solubilization and related phenomena; Academic Press: New York, 1955. (6) Rosen, M. J. Surfactants and interfacial phenomena; Wiley: New York, 1978. (7) Kim, J.-H.; Domach, M. M.; Tilton, R. D. Colloids Surf., A 1999, 150, 53. (8) Kim, J.-H.; Domach, M. M.; Tilton, R. D. J. Phys. Chem. B 1999, 103, 10582.

10.1021/la0005560 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000

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Three prominent monomer emission peaks occur at 370 nm (denoted by I1 or Imon), 380-381 nm (I3), and 390-391 nm (I5). When the local concentration of pyrene is high, an excited pyrene molecule may bind to a ground-state pyrene molecule and form an excited dimer (excimer). The broad excimer fluorescence emission peak is observed at 470 nm (I e). The ratio of two monomer peaks I1/I3 is sensitive to the microenvironmental polarity around the pyrene molecule,9 and the excimer to monomer fluorescence intensity ratio, I e/Imon, for solubilized pyrene is closely related to its distribution among micelles. Since excimer fluorescence requires dimerization during an excited-state lifetime, a minimum of two pyrenes per micelle is required for solubilized pyrene to produce excimer emission.10 The solubilizate distribution among micelles is frequently described by the Poisson distribution model. The statistical approach is mandated by the low occupancy numbers of the micelles.7,8,11-17 The Poisson model assumes that solubilizates are randomly distributed among the micelles. Hunter18 has analyzed solubilization statistics when, instead of a random distribution, there is a finite limit on the number of solubilizates that can be contained within a micelle. In that case, he showed that the binomial distribution is more appropriate. Nevertheless, we and others have shown that the Poisson distribution is an excellent approximation when the average number of solubilizates per micelle is small (less than approximately five), a condition that is satisfied in the current work.7,10,11,13 Here we use the Poisson distribution, wherein the fraction of micelles containing j solubilized molecules is

[Mj] [M]

)

nje-n j!

[Py]m [M]

)

[Py]mNagg [Surf]total - cmc

∞

∞

Ime ) Kmφme

where [Py]m, Nagg, and [Surf]total are the analytical concentration (i.e., the concentration based on the total solution volume) of pyrene that resides in the micellar pseudophase, the aggregation number of the micelles, and the total analytical surfactant concentration, respectively. Equation 2 assumes that the surfactant monomer activity remains constant above the cmc, whereas it is well recognized that the monomer activity in fact decreases above the cmc for ionic surfactants.19-22 We will show below (9) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 9, 2039. (10) Infelta, P. P. Chem. Phys. Lett. 1979, 61, 88. (11) Almgren, M.; Leofroth, J. E.; van Stam, J. J. Phys. Chem. 1986, 90, 4431. (12) Bales, B. L.; Stenland, C. J. Phys. Chem. 1993, 97, 3418. (13) Barzykin, A. V.; Tachiya, M. J. Phys. Chem. 1994, 98, 2677. (14) Gehlen, M. H.; Van der Auweraer, M.; Reekmans, S.; Neumann, M. G.; De Schryver, F. C. J. Phys. Chem. 1991, 95, 5684. (15) Infelta, P. P.; Graetzel, M. J. Chem. Phys. 1979, 70, 179. (16) Barzykin, A. V.; Tachiya, M. Heterogen. Chem. Rev. 1996, 3, 105. (17) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (18) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152. (19) Lindman, B.; Puyal, M. C.; Kamenka, N.; Rymde´n, R.; Stilbs, P. J. Phys. Chem. 1984, 88, 5048. (20) Kale, K. M.; Cussler, E. L.; Evans, E. F. J. Phys. Chem. 1980, 84, 593.

j

(

j(j - 1)

∑ j)2 R + j - 1

)

)

[Mj] [M]

[Mj] [M]

(3)

(4)

where Km is a proportionality factor involving the absorptivity as well as instrumental factors, φmmon and φme are the monomer and excimer quantum yields inside the micelle, respectively, and R is a kinetic factor arising from the fluorescence mechanism. The subscript “m” emphasizes that these refer to pyrene in micelles. If the surfactant concentration is low, (i.e., very near the cmc), a significant fraction of the fluorophores resides in the aqueous pseudophase and contributes mainly to the monomer emission intensity. However, in this study we focus our analysis on surfactant concentrations that are sufficiently far above the cmc that both the excimer and the monomer fluorescence intensities arising from the aqueous pseudophase are negligible compared to those from the micellar pseudophase. This is well supported by the very large partition coefficients previously demonstrated for pyrene partitioning between micelles and the aqueous pseudophase.7 Thus, the ratio of monomer and excimer fluorescence intensities can be expressed here as

φme

(2)

(

∑ j)1 R + j - 1

Immon ) KmφmmonR

(1)

j are the total micelle concentration, where [M], [Mj], and n the concentration of those micelles that contain j solubilized molecules, and the average number of solubilized molecules per micelle, respectively. The last is simply given by

n)

that this assumption has no significant effect on our conclusions. According to Infelta and Gra¨tzel,15 the fluorescence intensities due to pyrene monomers and excimers solubilized in surfactant aggregates are, respectively

∞

(

Ie

)

Imon

φm

∞

(

j

∑ j)1 R + j - 1

R

)

j(j - 1) [Mj]

∑ mon j)2 R + j - 1

)

[M]

[Mj]

(5)

[M]

Because of its relation to the Poisson distribution (eq j . By fixing the 1), the I e/Imon ratio must depend on n analytical pyrene concentration and incrementally increasing the surfactant concentration, i.e., manipulating n j , we determine the number of micelles in solution, as well as the respective aggregation numbers, from the I e/ Imon ratios measured under conditions where virtually 100% of the pyrene in the system resides within micelles. (See Results and Discussion.) As we described in a previous paper,8 the model is dominated by the quantitative dependence of n j on the surfactant concentration and is quite insensitive to the values of R and Ommon/Ome. Experimental Section Materials. We purchased sodium dodecyl sulfate (SDS) from Fluka Chemical Corp. (>99% pure), and further purified it by washing with hot acetone (Fisher, HPLC grade) more than three times and then drying under vacuum at room temperature. Surface tension measurements of the purified SDS solutions showed no dip near the cmc (8.3 mM in deionized water).23 SDS solutions were prepared from the purified powder immediately (21) Cutler, S. G.; Meares, P.; Hall, D. G. J. Chem. Soc., Faraday Trans. 1 1978, 74 1758. (22) Gunnarsson, G.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1980, 84, 3114. (23) Kim, J.-H. Pyrene Solubilization in Micelles and PolymerSurfactant Complexes. Ph.D. Dissertation, Carnegie Mellon University, 1999.

Effect of Electrolytes on Pyrene Solubilization prior to each experiment. Comparing purified and unpurified SDS, we observed nevertheless that pyrene solubilization is not affected by the presence of trace dodecanol that is commonly found in commercial dodecyl sulfate surfactant preparations. Thus, we purchased lithium dodecyl sulfate (LDS) from Sigma (>99% pure) and used it as received. We purchased pyrene from Aldrich Chemical Co. (optical grade, >99% pure) and methanol (ACS grade) from Fisher, using both as received. We purchased lithium chloride and ammonium chloride from Aldrich and sodium chloride from Fisher, using each as received. All water was purified by reverse osmosis followed by treatment to 18 MΩ cm resistivity with a Milli-Q Plus system from Millipore Corporation. Methods. All experiments were conducted at 25 °C. Five milliliter glass centrifuge tubes with snap caps were used for sample preparation. Pyrene solutions of 5.0 µM analytical concentration (i.e., approximately seven times the aqueous solubility limit for pyrene) were made by mixing 10.0 mM pyrene methanol solutions into SDS solutions having the desired electrolyte concentrations. The solutions were mixed by a vortex mixer for about 10 s, then were allowed to equilibrate for at least 8 h after adding pyrene, and were finally centrifuged at 3000 rpm for 1 h. Centrifugation removed the suspended excess pyrene microcrystals from the low surfactant concentration samples. We used a Perkin-Elmer LS-5B fluorescence spectrometer to measure fluorescence spectra after the centrifugation, using 240 nm excitation and recording the emission spectrum between 270 and 750 nm. The excitation and emission slit widths were 5 and 3 nm, respectively. We used 10 mm path length quartz cuvettes and corrected for inner filter effects as described previously.7 For the UV-vis measurements of solubilizing power, we added an excess of powdered pyrene to SDS solutions that contained the desired electrolyte concentrations. These were sonicated for more than 8 h and then centrifuged at 3000 rpm for 1 h to sediment excess pyrene before analysis. Supernatants were taken and diluted appropriately into 50 mM SDS solutions in order to assay the total solubilized pyrene concentration via the absorbance at 336 nm under conditions where Beer’s law holds. (50 mM is more concentrated than any of the SDS solutions being studied; therefore, it was sufficient to ensure that all solubilized pyrene remained solubilized during the UV-vis measurement.) Absorbance was measured using a Hewlett-Packard 8451 diode array spectrophotometer. The molar absorptivity of pyrene solubilized in micellar dodecyl sulfate solutions is 2.06 × 10-5 M/cm. The degree of micelle ionization was estimated by electrical conductance using an Orion model 162 conductivity meter. First, conductivities of 50 mL samples of 100 mM NH4Cl, NaCl, or LiCl solutions in surfactant-free water were measured, and then the conductivity increment was measured while adding 1 mL aliquots of 15 mM SDS or LDS solutions that contained 100 mM NH4Cl, NaCl, or LiCl.

Results and Discussion Pyrene Solubilization in SDS Solutions of Varying Ionic Strength. It is well-known that increasing the ionic strength increases the micelle aggregation number and decreases the cmc for ionic surfactants. Figure 1 shows the effect of NaCl concentration on pyrene solubility in SDS solutions. Pyrene solubility was very low until the surfactant concentration reached the cmc, above which it increased linearly as expected. The slopes of the lines after the cmc equal the solubilizing powers. The cmc and pyrene solubilizing power values for SDS in NaCl solutions are summarized in Table 1, where it may be seen that although NaCl decreases the cmc, it has only a small effect on the pyrene solubilizing power. The solubilizing power increases by approximately 16% as the NaCl concentration is increased from 0 to 100 mM. As part of a recent study of solubilization in polymer/ surfactant complexes,8 we exploited the tendency of the SDS aggregation number to increase with increasing ionic strength in order to test the excimer fluorescence model. The data are reproduced in Figure 2. We varied the total

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Figure 1. Pyrene solubilization by SDS micelles as measured by UV absorbance at several NaCl concentrations: triangles, 0 mM NaCl; squares, 20 mM NaCl; circles, 100 mM NaCl. For each data set, the cmc is plainly evident and the solubilizing power is the slope of the line after the cmc. Table 1. NaCl Concentration Effect on Solubilization SDS micelles NaCl (mM)

cmca (mM)

0 8.33 ( 0.05 (8.1) 20 3.95 ( 0.03 (3.82)

Nagg

solubilizing power (Py/surfactant)

78 ( 1 (6.8 ( 0.1) × 10-3 86 ( 1 (7.4 ( 0.1) ×

10-3

100 1.35 ( 0.04 (1.39) 105 ( 6 (7.9 ( 0.1) × 10-3

solubilization capacityb (Py/micelle) 0.68 ( 0.01 (0.53 ( 0.01) 0.74 ( 0.01 (0.64 ( 0.01) 1.4 ( 0.09 (0.83 ( 0.05)

a Cmc values from Philips and Mysels24 in parentheses for comparison. b Values from solubilizing power × Nagg in parentheses.

surfactant concentration in three sets of experiments conducted at either 0, 20, or 100 mM NaCl concentration, while maintaining a constant analytical pyrene concentration. Thus we examined the sensitivity of the I e/Imon ratio to changes in both Nagg and n j . This provides a test of the method and allows us to check a key assumption in the analysis, namely, that a significant change in micelle aggregation number has no significant effect on the photophysical parameters R and φmmon/φme. Each series of measurements in Figure 2 follows a similar trend, namely, a sharp increase in I e/Imon beginning at the cmc, passage through a maximum somewhat above the cmc, and an asymptotic approach at high SDS concentrations to a level that is slightly below the level for zero SDS concentration. This last effect probably occurs because at high SDS concentrations, intramicellar excimer formation is extremely unlikely since very few micelles contain two pyrene molecules. Then micelle-encapsulated pyrene molecules are hindered from forming excimer pairs with each other, whereas at zero micelle concentration freely dissolved pyrene molecules can more readily form excimers. Note that the sharp increase in I e/Imon from its low, pre-cmc value marks the first appearance of micelles, or of any premicellar clusters that are capable of solubilizing pyrene, should those occur. (The latter are, in fact, unlikely to occur in SDS solutions.)21 The peak in I e/Imon results from the passage of n j through a maximum after which all the pyrene in the system is solubilized in micelles. Further increases in SDS concentration simply decrease n j ; i.e., the finite population of pyrene molecules is diluted over an increasing number of micelles. Thus, the SDS concentration corresponding to the peak in I e/Imon represents the highest concentration at which micelles are saturated with pyrene. We only analyze data after the peak position, as this ensures that

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Figure 2. (a) Pyrene excimer-to-monomer intensity ratio dependence on total SDS concentration for several NaCl concentrations: triangles, 0 mM NaCl; squares, 20 mM NaCl; circles, 100 mM NaCl. Arrows denote the cmc for each NaCl concentration. (b) Pyrene excimer-to-monomer intensity ratio data from (a), replotted against the SDS micelle concentration (same symbols). The scale at the top of the figure shows the average number of solubilized pyrene molecules per micelle. The analytical pyrene concentration is 5 µM in all solutions at and beyond the peaks.

all pyrene is solubilized in micelles. This is supported by the UV-vis pyrene solubilization data shown in Figure 1. Recall that the total analytical pyrene concentration for the fluorescence studies was 5.0 µM. The solubilization data show that SDS solutions are capable of solubilizing 5 µM pyrene after the SDS concentration is approximately 0.6-0.7 mM above the cmc. The SDS concentrations at which the peaks occur in Figure 2 satisfy this condition. Therefore, given the large micellar partition coefficients for pyrene, it is safe to assume that effectively all the pyrene in the system resides within micelles after the I e/Imon peaks. The same is true of the peak positions corresponding to different counterion types, to be discussed below. The parameters R, φmmon/φme, and the micelle aggregation numbers are obtained by least-squares regression analysis of these data sets (data from the peak and above) according to the model described by eq 5. Only the fluorescence data after the peaks are used in the analysis, since that condition ensures that approximately 100% of the pyrene originally added is solubilized in micelles and pyrene residing in the aqueous pseudophase is negligible. The calculated SDS micellization parameters are listed in Table 1. The best fit values for the parameters, R ) 0.8 and φmmon/φme ) 1.9, were used in all regressions for all

Kim et al.

solutions in this study. Aggregation numbers for pyrenebearing SDS micelles obtained by excimer fluorescence spectroscopy are as follows: 78 ( 1 in 0 mM NaCl, 86 ( 1 in 20 mM NaCl, and 105 ( 6 in 100 mM NaCl. These are comparable to aggregation numbers for SDS micelles determined by other means. Philips and Mysels24 used light scattering to determine the following weight average aggregation numbers: 80, 94, and 112 for 0, 20, and 100 mM NaCl. Cabane25 reported an aggregation number of 76 from neutron scattering in salt-free water, while Almgren and Lo¨froth26 reported the following aggregation numbers obtained via fluorescence quenching with trace probes: 63 in salt-free water and 104 in 300 mM NaCl. Since the scattering methods detect a weight average aggregation number, our excimer fluorescence results would be expected to more closely resemble the fluorescence-quenching results than the scattering results. A possible discrepancy is that the excimer technique averages the aggregation number over the entire surfactant concentration range examined. Although we consider a small range of concentrations, there is a weak dependence of aggregation number on surfactant concentration. Quina et al.27 showed that the SDS aggregation number scales with the 1/4 power of the free counterion concentration. In 100 mM NaCl, the Na+ contributed by the NaCl swamps the Na+ contribution from the SDS, and the scaling relationship predicts that the aggregation number should only change by approximately 0.7% over the SDS concentration range that we considered. The effect is most significant in the absence of added salt, in which case the scaling relationship would indicate an 8% increase in aggregation number over the concentration range considered. The averaging of aggregation numbers therefore may explain some of the discrepancy with the quenching result in salt-free water, but not at the higher NaCl concentrations. Last, we consider the extent to which the decrease in monomeric surfactant activity above the cmc might impact this calculation. This occurs because the degree of micellar counterion binding is less than unity, so the concentration of free counterion tends to increase as surfactant is added to solution above the cmc, and in order to preserve constant ionic activity, the surfactant ion activity must decrease. This effect is minimal for the SDS concentration range we consider. In the absence of added salt, surfactant specific electrode results of Cutler et al.21 indicate that the monomeric dodecyl sulfate concentration decreases by just 2% from the cmc to approximately 16 mM total SDS concentration, justifying the constant monomer concentration approximation in eq 2. Thus, we conclude that the somewhat higher aggregation numbers that we report may be caused by the presence of large amounts of pyrene in the system. The same I e/Imon data is plotted against the total micelle concentration [M] in Figure 2b to illustrate consistency between the model and the data. The total micelle concentration was calculated from the total surfactant concentration and the cmc using the regressed aggregation number. The data beyond the peak position all collapse onto a single curve that is well-matched by the Poisson distribution model. When the data is plotted in this way, equal values of [M] for any ionic strength correspond to equal values of n j . Thus we see that when we vary the (24) Phillips, J. N.; Mysels, K. J. J. Phys. Chem. 1955, 59, 325. (25) Cabane, B. J. Phys. 1985, 46, 2161. (26) Almgren, M., Lo¨froth, J. E. J. Colloid Interface Sci. 1981, 81, 486. (27) Quina, F. H.; Nassar, P. M.; Bonilha, J. B. S.; Bales, B. L. J. Phys. Chem. 1995, 99, 17028.

Effect of Electrolytes on Pyrene Solubilization

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Table 2. Counterion Effect on Solubilization in 100 mM Salt Solutions salt and surfactant NH4Cl + SDS NaCl + SDS LiCl + SDS LiCl + LDS

cmc (mM)a 1.12 ( 0.04 ∼1.3 1.1 ( 0.1 1.42 ( 0.03 ∼1.6 1.4 ( 0.1 1.67 ( 0.03 ∼1.7 1.4 ( 0.1 1.67 ( 0.03 ∼2.1 1.6 ( 0.1

micelle degree of ionization

solubilizing power (Py/surfactant)

solubilization capacityb (Py/micelle)

Naggc

0.34 ( 0.02

(7.8 ( 0.2) ×

10-3

1.9 ( 0.3 (1.1 ( 0.1)

135 ( 7.2

0.46 ( 0.04

(7.9 ( 0.1) × 10-3

1.4 ( 0.1 (0.8 ( 0.2)

105 ( 5.6

0.76 ( 0.05

(7.1 ( 0.1) × 10-3

0.9 ( 0.1 (0.6 ( 0.2)

80 ( 4.3

0.63 ( 0.07

(7.1 ( 0.1) × 10-3

0.9 ( 0.3 (0.6 ( 0.2)

80 ( 4.3

a Values from UV absorbance (top), conductivity (middle), and fluorescence I /I (bottom). bValues in parentheses are from mass balance 1 3 calculation of solubilization capacity ) solubilizing power × Nagg. c Micelle aggregation number determined by excimer fluorescence method.

ionic strength, I e/Imon depends only on n j via the statistical distribution model. The only significant effect of Nagg is that it fixes [M] for a particular SDS concentration. This observation that excimer emission does not depend intrinsically on the aggregation number itself is important, because it allows us to safely treat the parameters R and φmmon/φme as constants in all of our experiments. In other words, the statistics of solubilizate distribution control the excimer fluorescence spectroscopy in dodecyl sulfate micelle solutions. From the value of n j at the I e/Imon peaks (analytical pyrene concentration divided by micelle concentrations at the peaks), we determined solubilization capacities of 0.68 pyrene per micelle at zero NaCl, 0.74 pyrene per micelle at 20 mM NaCl, and 1.40 pyrenes per micelle at 100 mM NaCl. Note that the peak heights are larger for larger NaCl concentrations, where micelles have greater aggregation numbers. This is a direct consequence of the increase in solubilization capacity, as micelles containing more pyrene molecules on average allow more excimer formation. Note that the Poisson model used here cannot predict a peak, since it places no mathematical limit on solubilization capacity. The solubilization capacity values obtained directly from the excimer fluorescence measurements are summarized in Table 1 along with values calculated as the product of solubilizing power and Nagg. Although of comparable magnitude, the solubilization capacities determined directly from excimer fluorescence alone are consistently greater than those calculated as the product of the solubilizing power (measured by UV-vis spectrophotometry) and the aggregation number (measured via excimer fluorescence). Nevertheless, both calculation methods indicate the same relative change in solubilization capacity values when the NaCl concentration is changed. Measuring the solubilization capacity entirely from the peak position in the excimer fluorescence data alone requires an accurate determination of the SDS concentration that corresponds to the peak in a plot such as Figure 2b, but the difficulty is that the peak position becomes less sensitive to the value of the solubilization capacity as the solubilization capacity increases (steeper slope in I e/Imon at lower SDS concentration). Effect of Different Counterions on Pyrene Solubilization. Figure 3 shows the effect that the type of counterion (NH4+, Na+, or Li+) has on pyrene solubility in dodecyl sulfate solutions. We used only SDS and LDS as the surfactants, because those are commercially available, while ammonium dodecyl sulfate is not. In the case of NH4+, we simply added NH4Cl to a solution of SDS, providing a mixed counterion situation. We also compared LDS in LiCl with SDS in LiCl. In each case the added salt

Figure 3. Pyrene solubilization by dodecyl sulfate micelles: filled circles, SDS + 100 mM NH4Cl; filled squares, SDS + 100 mM NaCl; filled triangles, SDS + 100 mM LiCl; unfilled triangles, LDS + 100 mM LiCl.

concentration was 100 mM. This was at least 20 times the surfactant concentration. Therefore, the cation introduced by the added salt always swamped the cation introduced by the surfactant. As a result the 100 mM LiCl + SDS system and the 100 mM LiCl +LDS system showed no difference in either the absorbance (Figure 3) or the fluorescence measurements discussed below (Figures 4 and 5). From Figure 3, it is readily seen that the cmc increases in the order cmc(NH4+) < cmc(Na+) < cmc(Li+). The cmc values (as determined by UV-vis spectrophotometry as well as fluorescence spectroscopy and electrical conductivity) are shown in Table 2. That table also shows estimates of the degree of ionization of micellized surfactants in 100 mM solutions of NH4Cl, NaCl, or LiCl. The degree of ionization, R, is simply the fraction of surfactant headgroups in micelles that are not neutralized by bound counterions, and it is readily estimated by comparing the slopes above and below the cmc on an electrical conductivity versus surfactant concentration plot. From Table 2 we see that the counterion binding affinity (as indicated by the degree of counterion binding, 1 - R) decreases in the order NH4+ > Na+ > Li+; i.e., NH4+ has the highest binding affinity of the three. The solubilizing powers determined from the data in Figure 3 are summarized in Table 2. While the solubilizing powers of dodecyl sulfate micelles in 100 mM NaCl or 100 mM NH4Cl solutions are similar within the experimental error, the solubilizing power of dodecyl sulfate micelles in 100 mM LiCl solutions is approximately 10% smaller (regardless of whether we dissolve LDS in LiCl solutions

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do not significantly affect our excimer fluorescence analysis for these systems. The fluorescence behaviors of the two “LDS” systems (SDS + LiCl and LDS + LiCl) were indistinguishable, confirming that the background salt cation dominated the cation that came in with the surfactant under these conditions. The excimer behavior in different background salt solutions is shown in Figure 4a. The aggregation numbers determined by data regression according to the model (forcing R and φmmon/φme to be equal in all cases) are tabulated in Table 2, as are the solubilization capacities determined from the I e/Imon peak position. As was the case with SDS in the presence of varying NaCl concentrations, the excimer fluorescence data for DS surfactants in 100 mM solutions of various 1:1 electrolytes again collapse onto a single curve when plotted against the total micelle concentration (Figure 4b). The curve is again well matched by the pyrene distribution model. For the sake of comparison, Bales et al.28 showed that the LDS aggregation number scales with the total counterion concentration as

Nagg ) κ2([Li+]aq)γ

Figure 4. (a) Pyrene excimer-to-monomer intensity ratio plotted against total surfactant concentration for the following: filled circles, SDS in 100 mM NH4Cl; filled squares, SDS in 100 mM NaCl; filled triangles, SDS in 100 mM LiCl; unfilled triangles, LDS in 100 mM LiCl. (b) Pyrene excimer-to-monomer intensity ratio data from (a), replotted against the micelle concentration (same symbols).

Figure 5. Pyrene I1/I3 ratios in the presence of different 100 mM electrolyte solutions: filled circles, SDS + 100 mM NH4Cl; filled squares, SDS + 100 mM NaCl; filled triangles, SDS + 100 mM LiCl; unfilled triangles, LDS + 100 mM LiCl.

or SDS in LiCl solutions). Although the difference is small, it is beyond the experimental error. This is similar to the 16% difference between the solubilizing power of SDS in 100 mM NaCl solution versus SDS in salt-free water. Fluorescence results for these systems are plotted in Figures 4 and 5. Note that because of the swamping concentrations of added salt, the concentration dependence of the micellar aggregation number and the decrease in the monomeric surfactant ion concentration above the cmc

(6)

Using their values for K2 ) 112 and γ ) 0.18, their relationship would indicate an aggregation number of 74 for the LDS concentration range that we considered in the presence of 100 mM LiCl, whereas we found Nagg ) 80. As before, the small discrepancy may indicate a slightly larger aggregation number for pyrene-loaded micelles. The results show that both the solubilization capacity and the aggregation number decrease in the order NH4+ > Na+ > Li+, consistent with the order of counterion binding affinity to the micelle. Approximately, the solubilization capacity is proportional to the aggregation number. As was the case with SDS in NaCl solutions of varying concentration, there was a consistent discrepancy between the two methods of calculating solubilization capacity. Values calculated from the excimer peak position alone were consistently larger, but again the relative changes in solubilization capacities associated with changing solution conditions were consistent for both methods of calculation. For all the counterions considered, the solubilizing power depended only weakly on the aggregation number (approximately as Nagg0.2), whereas the solubilization capacity scaled as Nagg1.8. Thus the increases in both Nagg and solubilization capacity offset one another, and the solubilizing power is nearly independent of any change in salt conditions. This is similar to the result, reported by Ikeda and Maruyama,1 that the solubilizing power of dodecylpyridinium chloride and dodecylpyridinium bromide micelles for the dye Sudan Red B in the presence of NaCl was constant, regardless of the micellar species or the ionic strength of the solution, as long as the micelles were spherical in shape. The data in Figure 5 indicate the microenvironmental polarity around the solubilized pyrene molecules. Since the I1/I3 ratio is sensitive to the microenvironmental polarity, I1/I3 decreases fairly abruptly at the cmc, whereupon pyrene molecules begin to partition from the polar aqueous pseudophase to the less polar micellar pseudophase. The cmc values from the I1/I3 ratio are consistent with those from UV-vis absorbance measurements, since both detect the onset of pyrene microphase partitioning. (28) Bales, B. L.; Shahin, A.; Lindblad, C.; Almgren, M. J. Phys. Chem. B 2000, 104, 256.

Effect of Electrolytes on Pyrene Solubilization

It is interesting to note that the microenvironment for pyrene in dodecyl sulfate micelles in 100 mM LiCl is more polar than in 100 mM NH4Cl or NaCl, as indicated by the statistically significantly higher I1/I3 asymptote at large surfactant concentrations (1.08 versus 1.05). This suggests that pyrene molecules solubilized in the smaller micelles that form in 100 mM LiCl solution are less protected from water molecules than they are in the larger micelles that form in 100 mM NaCl or NH4Cl solution. The microenvironmental polarity for pyrene in dodecyl sulfate micelles with 100 mM LiCl is similar to that in sodium dodecyl sulfate micelles in pure water (I1/I3 ) ∼1.08),23 for which the aggregation number is 78. The aggregation number is 135 in 100 mM NH4Cl, 105 in 100 mM NaCl, and only 80 in 100 mM LiCl. So it seems that the aggregation number primarily dictates the pyrene microenvironmental polarity. The microenvironmental polarity for pyrene in a dodecyl sulfate micelle with Nagg ≈ 80 is the same, regardless of whether it is an LDS micelle in 100 mM LiCl or an SDS micelle in salt-free water. The microenvironmental polarity is indistinguishable for micelles in 100 mM NaCl or NH4Cl, suggesting that as long as the aggregation number is greater than approximately 100, solubilized pyrene receives the same degree of protection from water. Similar comments may be made about the solubilizing power and solubilization capacity. For an aggregation number of approximately 80, the solubilizing power is 6.8 ( 0.1 × 10-3 pyrene/surfactant when those micelles come about by adding SDS to salt-free water, and 7.1 ( 0.1 × 10-3 pyrene/surfactant when they come about by adding either LDS or SDS to 100 mM LiCl solutions. These are within approximately 4% of each other and are close to the experimental error limits. The solubilization capacities are also comparable, 0.68 ( 0.01 in salt-free water and 0.9 ( 0.1 in 100 mM LiCl. The agreement here is not as close as it was for solubilizing power, but in both cases, these solubilization capacities are well below the capacities of micelles that have aggregation numbers above 100 (for 100 mM NaCl or NH4Cl). Conclusions For a constant surfactant concentration, adding 1:1 electrolyte to micellar dodecyl sulfate solutions increases the solubility of pyrene, simply because of the decrease in cmc. There is only a small effect on the solubilizing power, so the solubilization curves (such as Figure 1) merely shift to lower surfactant concentration when the ionic strength

Langmuir, Vol. 16, No. 26, 2000 10043

is increased. The weak sensitivity of the solubilizing power to an increase in the electrolyte concentration occurs because the increase in aggregation number (that decreases the number of new micelles formed for an increment in surfactant concentration) is compensated by an increase in micellar solubilization capacity. The solubilization capacity increases significantly when the aggregation number is increased, so the smaller number of micelles that exist in the presence of increased electrolyte concentrations are able to carry more pyrene. The counterion binding affinity for dodecyl sulfate micelles decreases in the order NH4+ > Na+ > Li+. The improved screening of interheadgroup repulsion causes the aggregation number to scale in the same order. In addition, the ability of the chloride salts of these cations to decrease the cmc also scales in the same order, i.e., cmc(NH4Cl) < cmc(NaCl) < cmc(LiCl). The pyrene solubilization capacity of dodecyl sulfate micelles is determined primarily by the aggregation number, so this property also scales in the order NH4+ > Na+ > Li+. The aggregation number of dodecyl sulfate micelles in 100mM LiCl is the same as that of sodium dodecyl sulfate micelles in the absence of added electrolyte, and they also have similar solubilization capacities. In addition, the pyrene microenvironmental polarity is the same for micelles that have the same aggregation number, independent of the salt conditions that are responsible for their having that aggregation number. Although there is a consistent discrepancy between solubilization capacities calculated directly from excimer fluorescence data alone versus those calculated by the Kolthoff and Stricks formula, both show the same trends in response to changing electrolyte conditions. It should be noted that the Kolthoff and Stricks calculation is usually based on the aggregation number measured in the absence of solubilizate, although it certainly does not need to be so. We have used the aggregation number measured in the presence of pyrene solubilizates. For the particular case of pyrene-loaded dodecyl sulfate micelles, the aggregation numbers may be slightly higher than they would otherwise be in the absence of pyrene. Acknowledgment. This material is based on work supported by the National Science Foundation under Grant No. CTS-9623849. LA0005560