Langmuir 1994,10, 1151-1154
1151
Effect of 1-Butanol on Micellization of Sodium Dodecyl Sulfate and on Fluorescence Quenching by Bromide Ion Danil A. R. Rubio, Din0 Zanette, and Faruk Nome* Departamento de Quimica, Universidade Federal de Santa Catarina, 88040-970 Florian6polis, Santa Catarina, Brazil
Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, California, 93106 Received August 16, 1993. In Final Form: December 2 7 , 1 9 9 P
The increase of the fractional micellar ionization, a,of sodium dodecyl sulfate, SDS, on addition of 1-butanol, BuOH, has been estimated conductimetrically. Calculations were made with the ratio of the slopes of conductivityagainst [SDSI above and below the critical micelle concentration,cmc, and also by application of Evans' equation which includes the aggregationnumber determined by fluorescencequenching. The method of slopes gives very high values of a,especially at high [BuOH]. The aggregation number of 0.05 M SDS decreases from 65 in water at 25 "C to 22 in 0.98 M BuOH. The cmc of SDS decreases on addition of BuOH and goes through a shallow minimum at 0.55 M BuOH. The quenching of the fluorescence of naphthalene by B r in SDS is markedly increased by addition of BuOH and NaCl which permit incursion of B r into the anionic micelles. Introduction Structures of aqueous ionic micelles are perturbed by moderately hydrophobic alcohols,land surfaces of alcoholmodified micellesshould be similar to those of oil-in-water microemulsions that have alcohols as cosurfactants.2 Incorporation of alcohols and similar nonionic solutes decreases micellar charge densities and affinities for counterions.l-3 As a result, the fractional micellar ionization, a,is increased; Le., the fractional micellar coverage, 0 = 1-a,is decreased.3 Counterion concentrations at the surface decrease, consistent with NMR spectrometric evidence4and the results of trapping experiments? Overall rates of bimolecular micellar-enhanced ionic reactions also decrease, and analysis of these data requires information on the concentrations of counterions and substrates a t micellar surfaces, which are treated as reaction regions distinct from the aqueous medium.6 This pseudophase model is widely applied, and rate enhancements of bimolecular reactions of counterions are caused largely by their concentration in a small volume at the micelle-water interface.6 We have applied this model to S Nreactions ~ of B r and C1- with methyl naphthalene-2-sulfonate in alcohol-modified micelles,4J and we plan to examine 9 Abstract
published in Aduance ACS Abstracts, March 1,1994. (1) (a) Zana, R.; Yiv, S.; Strazialle, C.; Lianos, P. J. Colloid Interface Sci. 1981,80,208. (b) De Lisi, R.; Genova, C.; Testa, R.; Liveri, V. T. J. Solution Chem. 1984, 13, 121. (2) (a) Da Rocha Pereira, R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990. 94. 366. (b) Mackav. R. A. J. Phvs. Chem. 1982. 86. 4756. (c)
x.;
Burhide, B. A.;'bier, B. Mackay, R: A.; Durst, H. D.; Longo, F. R. J. Phys. Chem. 1988,92,4505. (3) (a) Larsen, J. W.; Tepley, L. B. J. Colloid Interface Sci. 1974,49, 113. (b) Bunton, C. A.; de Buzzaccarini,F. J.Phys. Chem. 1982,86,5010. (c) Tominaga, T.; Stem, T. B. Jr.; Evans, D. F. Boll. Chem. SOC.Jpn. 198g,53, 7g5. (4) Bertoncini, C. R. A.; Nome, F.; Cerichelli, G. J. Phys. Chem. 1990, 94,5876. (5) Chandhuri, A.; Romsted, L. S. J. Am. Chem. SOC.1991,113,5052. (6) (a) Romsted, L. 5. In Micellization, Solubilization and Microemuleiom; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (b) Romsted,L. S.roSurfactantsinSolution;Mittrrl,K.L.,Lindman, B., E&.; Plenum Press: New York, 1984; Vol. 2, p 1015. (c) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979,83, 1944. (d) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S.Acc. Chem. Res. 1991,24, 357. (7) Bertoncini, C. R. A.; Neves, M. de F. S.; Nome, F.; Bunton, C. A. Langmuir 1993,9, 1274.
hydrogen ion catalyzed hydrolyzes in micelles of sodium dodecyl sulfate modified by 1-butanol (BuOH).~We therefore need physical information on this system. Reactions of co-ions are inhibited by micelles, but they are not completely suppressed because there is a small, but finite, co-ion concentration at the micellar surface which increases on addition of electrolyte.0 We were interested in the extent to which added BuOH could affect concentrations of both counterions and co-ions at the surface of micelles of sodium dodecyl sulfate (SDS). Changes in a! with added BuOH should monitor the concentration of counterion in SDS micelles, and we used changes in fluorescencequenching of naphthalene by B r lo to follow changes in its concentration at the micellarsurface induced by BuOH. We plan to calculate a! from conductivity data by using Evans' equation." This treatment requires knowledge of the micellar aggregationnumber, N,and the criticalmicelle concentration, cmc, in the experimental conditions. Almgren and Swarup12measured aggregation numbers of SDS in the presence of several alcohols, including BuOH, but their experiments were at 22 "C, so we used their fluorescence quenching method a t 25 "C. Values of cmc were given by breaks in plots of conductance against [SDSI We also measured the distribution of BuOH between water and SDS by the solubility method of Gettins et al.l3
.
(8) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A., following paper in this issue. (9) (a) Quina, F. H.; Politi, J. M.; Cuccovia, I. M.; Martine-Franchetti, S. M.; Chaimovich, H. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, p 1125. (b) Armstrong, C.; Gotham, W.; Jenninp, P.; Nikles, J.; Romsted,L. S. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum Press: New York, 1989; Vol. 9, p 197. (c) Naacimento, M. da G.; Lezcano,M. A.; Nome, F. J.Phys. Chem. 1992,96,5537. (d) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851. (e) Blasko, A.; Bunton, C. A.; Armstrong, C.; Gotham, W.; He, Z. M.; Nikles, J.; Romsted,L. S. J.Phys. Chem. 1991,95,6747. (10) Hautala, R. R.; Schore. N. E.; Turro, N. J. Am. Chem. SOC.1973, 95,-5508. (11)Evans, H. C. J. Chem. SOC.1956,579. (12) Almgren, M.; Swarup, S.J. Colloid Interface Sci. 198S,91, 256. (13) Gettins, J.; Hall, D.;Jokling, P. L.; Rowing, J. E.; Wyn-Jones, E. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 1957.
0743-7463/94/2410-1151$04.50/00 1994 American Chemical Society
Rubio et al.
1152 Langmuir,Vol. 10,No. 4, 1994
Table 1. Effect of BuOH on the Aggregation Number of
SDS. [BuOHl,M N [BuOHl,M N a
0 66 0.44 28
0.055 57 0.55 24
0.11 51 0.67 22
0.22 45 0.67 22
.027
40 0.87 25
0.33 39 0.98 22
0.38 34
At 25 "C with 0.05 M SDS.
Table 2. Effwt of BuOH on u and the cmc of SDS. [BuOH], cmc, [BuOHl, cmc, M mM ub M mM ub 0 8.W 0.21 (0.40) 0.646 4.2 0.42(0.74) 0.665 4.7 0.46(0.80) 0.109 6.0 0.27 (0.60 0.219 0.328 0.437
104[MA] , M
Figure 1. Ratio of luminescence intensities of Ru(bipy)2+ in the absence and presence of quencher as a function of the MA concentration, in aqueous solutions of 0.05 M SDSwithout BuOH (0)and in the presence of 0.273 M (n),0.438 M (A), and 0.764 M (w) BuOH.
Experimental Section Materials. Purification of SDShas' been described.% There was no minimum in a plot of surface tension against [SDSI. Naphthalene and 9-methylanthracene, MA, from Aldrich were purified by sublimation. Tris(bipyridyl)ruthenium(II) was prepared as described.14 Other reagents were commercial samples and were redistilled or recrystallized as necessary. Methods. Surface tension was measured on a Fisher Model 20 tensiometer, and conductivitieswere measured with an Analion C-701 meter. Absorbances were measured on a Shimadzu 210 A, Beckman DU-65, or Aminco DW-2000 instrument, and fluorescence measurements were on an Aminco SPF-500C spectrofluorimeter. Solubilities of BuOH were measured by gasliquid chromatography (GLC). Aggregation numbers of SDS were estimated from changes in the fluorescence intensity of tris(bipyridyl)ruthenium(II) by addition of MA at 613 nm as described by Almgren and Swarupt2 on the basis of the method of Turr0.~5 The binding constant, KND,of naphthalene to micellized SDS, eq 1, was measured by following the solubility in mixtures of
KNAP
[NAPMI = [NAP,]([SDS] - cmc)
SDS and B U O H . ~ The . ~ ~ quantities in brackets are molarities in terms of total solution volume, and subscripts W and M denote aqueous and micellar pseudophases, respectively. The binding constant of BuOH to SDS was measured as described for cetyltrimethylammonium bromide, CTABr.' Good Stem-Volmer plotale were obtained for the quenching of the fluorescenceof naphthalene by B r in the presence of BuOH or NaCl.
Results and Discussion Aggregation Numbers. All measurements were carried out at 25 "C and 0.05 M SDS with 5.4 X 1V M Ru(bipy)g2+and 9-methylanthracene (MA) (0.2-1.04 mM). Plots of ln(Io/l)against [MA] were linear (Figure 1shows representative data), where IOand I are the fluorescence intensities, respectively, in the absence and presence of (14) Fackler, J. P., Jr. Inorganic Synthesis: John Wiley & Sons: New York, 1982; Vol. 21, p 127. (16)Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978,100,5951. (16)Web, C. H. Introduction to Moleculur Photochemistrv: John Wdey: New York, 1972.
6.4 5.0 4.6
0.33 (0.65) 0.38 (0.66) 0.40 (0.70)
0.764 0.874
5.0 8.1
0.46(0.86) 0.46 (0.90)
At 25 O C . Calculatd with Eva" equation;s values in parentheses are from the method of elopes. e Literaturevalues vary slightly with the method and investigator." The value from a plot of surface tension against log [SDS]is 8.08 X 1 V M.
the quencher, and the mean surfactant aggregationnumber N is given by the slope of the plot (eq 2).12J6 = [SDSI - cmc
The values of N (Table 1) are similar to, but slightly lower than, those determined by Almgren and Swarup with ca. 0.033 M SDS at 22 OC.12 We took the cmc in the presence of BuOH as the concentration of monomeric SDS, and uncertainty in this value creates errore in N which Almgren and Swarup estimate as ca. 10%. Critical Micelle Concentrations. The cmc of SDS, determined conductimetrically at 25 OC, goes through a minimum with increasing [BuOHl (Table 2). The value in water of 8.0 X 1o-S M agrees with literature values." Moderately hydrophobic alcohols in low concentration promote micellization,probably by inserting at the micellar surface and reducing unfavorable water-hydrocarbon contacts. At higher concentrations these alcohols destabilize micelles by displacing water from the surface, therefore decreasing its effective dielectric constant, increasing head group repulsions, and disrupting surfactant packing. Micellar Incorporation of BuOH. The solubility of BuOH in aqueous SDS solutions were measured at 25 OC, andthebindingconstant,K (eq3),wascalculatedfollowing [BUOHMI
K = [BuOH,l([SDSl - cmc + [BuOH,])
(3)
the method of Gettins et all3 The value of K = 1.08 M-1 is slightly lower than K = 5.4 M-l,l& calculated by Hayase and Hayano using vapor pressure measurements, but similar to other values obtained by solubility measurements like the value of K = 0.8 M-' reported by Zana et al. for LiDS18b and to that in CTABr.4JS Fractional Micellar Ionization. Conductometric determination of a has been based on either of two general methods. The simplest is the 'method of slopes" in which a is the ratio of the slopes of plots of conductance against [surfactant] above and below the cmc.lB The inherent (17) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueow Surfactant Systems; National Bureau of Standards: WE&ington, DC,1971. (18) (a) Hayaw, K.; Hayano, S. J. Colloid Interface Sci. 1978,63,448. (b) Muto, Y.; Yoda, K.; Yoehida, N.; Eeumi, K.; Meguro, K.; BinanaLimbele, W.; Zana, R. J. Colloid Interface Sci. 1989,190,166. (19) h a , R. J. Colloid Interface Sci. 1974,46, 372.
Micellization of Sodium Dodecyl Sulfate
assumption is that the contribution of the micelle to the conductance is the same as that of an equivalent number of monomeric ions, the sum of whose charges equals the micellarcharge. This assumption is least satisfactory when a is high, and values of a calculated by using this method are higher than those calculated in terms of the micellar contribution to conductance, estimated by using Evans' equation." This equation was derived for aqueous spherical micelles on the assumption that the micellar radius, and therefore the Stokes-Einstein mobility of the micelles, can be related directly to the aggregation number. This assumption is not strictly correct as applied to micelles that contain BuOH, which decreases N , and the relation between N and the radius then underestimates the latter so that we overcorrect for the micellar contribution to conductance. As a result this treatment will underestimate the decrease of a on addition of BuOH, but the error should not be large because under all conditions the calculated micellar contribution to conductance is less than 15 % of that of the free ions. There will be partial compensation between the uptake of BuOH and expulsion of water from the micellar surface, on the basis of an analogy with the results with cationic micelle^.^ The relation between conductance and [surfactant] is given by
Langmuir, Vol. 10, No. 4,1994 1153 Table 3. Effect of BuOH on the Binding Constant of Naphthalene to SDS Micelles. [BuOHl,M 0 0.324 0.764 0.874 KNAP,M-' 1321 933 671 661 At 25 "C.
1l0
02
04 06 [NaBr] ,M
08
Figure 2. Effect of BuOH on the quenching by B r of the fluorescence of naphthalene in 0.05 M SDS at 25 OC: ( 0 )no BuOH, (m) 0.11 M, ( 0 )0.44 M,and (0)0.98 M BuOH.
where SIand SZare slopes of plots of specific conductance against [surfactant] below and above the cmc and AX is the equivalent conductance of the counterion, Na+. Equation 4 is solved for m, the number of bound counterions so that m/N = 1 - a. The two sets of values of a are given in Table 2. There are large differences between conductometric values of a determined by the use of Evans' equationll and the method of slopes.19 A comparative analysis of the two methods of calculation has been given by Nieuwkoop and Snoei," and our value from the method of slopes agrees with theirs. This value of 0.40 in aqueous SDS (Table 2) is much higher than most values in the literature which are closer to the value of 0.21 from Evans' equation than to that of ca. 0.49.21 Values of a depend upon the method of measurement as pointed out by Gunnarsson et al.;22for example, a careful analysis of dynamic light scattering data by Corti and DigiorgioZ3gives a = 0.37 for SDS in water. However, the results of different methods should be self-consistent as regards the effects of added alcohols, for example. Co-ion Incorporation. The fluorescence of naphthalene (NAP) is quenched by B r which provides a method for monitoring the uptake of B r by micelles of SDS in the presence of BuOH. The binding constant, K N A was ~ , estimated by following the solubilization of naphthalene by SDS by monitoring ~ on the absorbance at 274 nm. Values of K N Adecrease addition of BuOH, because it increases the solubility of naphthalene in water (Table 3). However, even with 0.8 M BuOH, over 97% of the naphthalene is micellar incorporated in 0.05 M SDS. Addition of NaBr will increase this value by "salting-out" naphthalene from water. (20) Van Nieuwkoop, J.; Snoei, G. J. Colloid Interface Sci. 1986,103, 417. (21) Romsted, L. S. PbD. Thesis, Indiana University, Bloomington, IN, 1975. (22) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1980,84,3114. (23) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711.
The fluorescence of naphthalene in water is effectively quenched by B r , and plots of relative fluorescence intensities in water with added NaBr, &I, are linear up to 0.8 M NaBr. Addition of BuOH to water slightly increases the fluorescence yield of naphthalene by 28% and 45% with 0.5 and 0.8 M, respectively. In the absence of BuOH, fluorescence quenching by B r is essentially completely suppressed by 0.05 M SDS, and a plot of l o / l against [NaBrl has zero slope. Addition of BuOH increases the quenching (Figure 2). On the basis of results in water, BuOH in the micelle should increase the fluorescence yield, so the increase in quenching must be due to incursion of B r into the micellar pseudophase. Values of the quenching constant KD,M-l, in 0.05 M SDS at 25 "C as a function of [BuOHl are 0,0.04,0.22, and 0.41 for 0, 0.11, 0.44, and 0.98 M BuOH, respectively. The effect of BuOH on the quenching of the fluorescence of naphthalene in SDS is qualitatively similar to that of added NaC1. Added electrolytes reduce the micellar surface electrical potentialZ2*"and increase incursion by co-ions.8 Experiments with mixtures of NaBr and NaCl were carried out at 46 "C because of solubility problems at 25 "C. Plots of l o / l increase linearly with NaBr, up to 0.6 M in 0.05 M SDS. The slope is zero with no added salt, as at 25 "C, and increases to 0.19 and 1.7 M-' in 1 and 2 M NaC1, respectively. Added NaCl increases the aggregation number of SDS micellesF5but the zero slope of a plot of l o / l against [NaBrl shows that changes in micellar structure induced by NaBr are not of major importance. Effect of BuOH on Micellar Interactions with Coions and Counterions. The effect of organic solutes on the interaction of micelles and counterions is well established.ll2 The charge density of micelles is decreased by moderately hydrophobic alcohols, which decrease the electrostatic attraction for counterions. Specific micelleion interactions are important in aqueous solution, as (24) (a) Bunton, C. A.; Moffatt, J. R. J.Phys. Chem. 1986,90,538. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988, 92, 2896. (25) Missel, P. J.; Mazer, N. A,; Benedek, G. B.; Young, C. Y.; Corey, M. C. J.Phys. Chem. 1980,84, 1044.
1154 Langmuir, Vol. 10, No. 4, 1994
shown by the dependence of ion-exchange parameters on the size and polarizability of counterions.6 There is less evidence on ionic competition for alcohol-modified micelles, but with BuOH-modified cationic micelles, B r displaces C1-, as in water.7 Addition of organic solutes to aqueousmicelles typically increases Addition of BuOH to micelles of cetyltrimethylammonium bromide ((CTA)Br)decreases concentrations of B r at the micellar surfaces, Le., increases LY and reduces NMR line widths of B r ; 4it also decreases the trapping ability of B r in dediazonization.6 The effects are similar with (CTA)Cl. There is limited evidence on micelle-co-ion interactions. Anionic micelles in water inhibit bimolecular reactions of anions,6*26e.g., OH-and IO4-, but there is a residual reaction due to incursion of co-ions which is increased by addition of electrolyte. Corresponding results are observed with hydrogen ion catalyzed reactions in aqueous cationic micelles.M Our fluorescence results with added sodium salts are consistent with kinetic data in anionic micelles. The electrolyte reduces the micellar surface potential, which reduces co-ion repulsion, and this effect, plus increased total concentration of co-ion, gives a very large increase in the co-ion concentration at the surface. For counterions the effect of reduced surface potential is offset by the increase in the total counterion concentration, and as a result concentrationsat the surface increase only modestly. The situation is slightly different for addition of nonionic (26)Blesko, A.; Bunton, C. A.; Wright, S.J. Phys. Chem. 1993,97, 5435.
Rubio et al.
solutes. The decrease in charge density decreases the attraction for counterions and repulsion of co-ions. These descriptions neglect the role of ion-specific interactions, which are of major importance in water! and are present, but are less well characterized, in alcohol-modifiedmicellar system^.^ The description of micelles as a pseudophase distinct from solvent water implies that there is a physical boundary between the pseudophases. Thismodel is useful in treating many micellar phenomena, e.g., effects on reaction rates and equilibria, but a more realistic description is that concentrations of ionic and nonionic solutes vary monotonically with distance from the micellar surface.B* When the concentration gradient is very steep, solute distributions will approximate to the step function, as implied by a pseudophase description. This approximation should be satisfactory for very hydrophobic dilute solutes which locate almost wholly in the micelles and for dilute co-ions whose concentrations at micellar surfaces are very much lower than in water. With increasing ionic concentrations in the aqueous pseudophase, concentrations of co-ions at micellar surfaces increase sharply, and those of counterions modestly, so in the limit of high [electrolyte], ionic concentrations in water and micelles should converge. Acknowledgment. Support of this work by CNPq (Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gic0, Brazil), FINEP (Financiadora de Estudos e Projetos), and the National Science Foundation, Organic Chemical Dynamics and International Programs, is gratefully acknowledged.
