pubs.acs.org/Langmuir © 2010 American Chemical Society
Concentration and Medium Micellar Kinetic Effects Caused by Morphological Transitions Marı´ a del Mar Graciani, Amalia Rodrı´ guez, Victoria Isabel Martı´ n, Gaspar Fernandez, and Marı´ a Luisa Moya* Departamento de Quı´mica Fı´sica, Universidad de Sevilla, C/Profesor Garcı´a Gonz alez 1, 41012 Sevilla, Spain Received July 19, 2010. Revised Manuscript Received October 24, 2010 The reaction methyl naphthalene-2-sulfonate þ Br- was investigated in several alkanediyl-R-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br- (with s=2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br- micellar solutions, with 30% weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration. In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic micellar effects in the whole surfactant concentration range.
Introduction Dimeric surfactants represent a new class of surfactants. They are formed by two amphiphilic moieties connected at the level of the head groups by a spacer.1,2 The possibilities of using dimeric surfactants to create new types of surfactants have recently been attracting increasing attention.3 The interest in such surfactants arises from their physicochemical properties that are better than those of conventional surfactants, such as much lower critical micelle concentrations (cmc), better wetting, greater surface tension lowering, and unusual morphologies. These properties could make them potentially useful in many fields of application, for example, in soil remediation, enhanced oil recovery, drug entrapment and release, and so forth.4 At concentrations above the cmc, dimeric surfactants tend to self-associate in water to form micelles whose characteristics depend on surfactant nature as well as on temperature.1,2 Additives affect the self-aggregation process and the features of the formed aggregates. This influence operates through variations in the chemical poten (bulk), tial of the surfactant molecules in the bulk phase, μsurfactant as well as in the chemical potential of the surfactant molecules in the micelles, μsurfactant (m). The importance of one or another effect depends on the nature of the additive. The changes caused *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Menger, F. M.; Keiper, J. N. Angew. Chem., Int. Ed. 2000, 39, 1906. (2) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205. (3) See, for example, (a) Viscardi, G.; Quagliotto, P.; Barolo, C.; Savaino, P.; Barni, E.; Fisicaro, E. J. Org. Chem. 2000, 65, 8917. (b) Faustino, C. M. C.; Calado, A. R. T.; García-Río, L. J. Phys. Chem. B 2009, 113, 977. (c) Stjerndahl, M.; Lundberg, D.; Zhao, H.; Menger, F. M. J. Phys. Chem. B 2007, 111, 2008. (d) Mishra, B. K.; Mukerhee, P.; Dash, S.; Patel, S.; Pati, H. N. Synth. Commun. 2009, 39, 2529. (e) Brito, R. O.; Marques, E. F.; Silva, S. G.; do Vale, M. L.; Gomes, P.; Araujo, M. J.; Rodríguez-Burgos, J. E.; Infante, M. R.; García, M. T.; Ribosa, I.; Mitjans, M. Colloids Surf., B 2009, 72, 80. (4) See, for instance, Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior and Applications; Zana, R., Xia, J., Eds.; Marcel Dekker Inc.: New York, 2004.
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by the additives can be made evident through the study of adequately chosen chemical reactions in the micellar solutions. Since the process between methyl naphthalene-2-sulfonate, MNS, and bromide ions meets the requirements,5 this process was investigated in several alkanediyl-R-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br- (with s=2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. It is worth noting that these dimeric surfactants are particularly interesting because they undergo morphological transitions when surfactant concentration increases,1,2 with the dimeric micelles changing shape from spherical aggregates into elongated ones. The surfactant concentration at which this morphological transition occurs is often referred to as “second cmc” (C*). The sphere-to-rod transition is followed by variations in the characteristics of the micellar aggregates which will affect the rate of the MNS þ Br- process. The additives used were the polar organic solvent 1,2-propylenglycol, which remains in the bulk phase,6 1-butanol, that distributes between the bulk and micellar pseudophases,7 and the surfactant N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the gemini surfactants.8 Most of the kinetic measurements in the presence of additives were carried out in 12-5-12,2Brmicellar solutions. Nonetheless, kinetic data obtained in other 12-s-12,2Br- micellar solutions showed that the kinetic effects provoked by the additives were similar for all the surfactants investigated. In this work, the use of an empirical equation was proposed for the quantitative rationalization of the kinetic micellar effects (5) Moya, M. L.; Graciani, M. M.; Rodrı´ guez, A.; Fernandez, G. Curr. Top. Catal. 2008, 7, 43–53. (6) Rodrı´ guez, A.; Graciani, M. M.; Fernandez, G.; Moya, M. L. J. Colloid Interface Sci. 2009, 338, 207. (7) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (8) Martı´ n, V. I.; Rodrı´ guez, A.; Graciani, M. M.; Robina, I.; Moya, M. L. J. Phys. Chem. B 2010, 114, 7817.
Published on Web 11/23/2010
DOI: 10.1021/la102857d
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Graciani et al. Scheme 1
Scheme 2
Second Critical Micelle Concentration, C*, Determination.
observed in the whole surfactant concentration range, below and above the morphological transition. This equation was also shown to be useful in the presence of the additives which alter not only the aggregation process but also the surfactant concentration range in which the spherical-to-rod transition takes place. It is interesting to point out that few kinetic studies have been carried out in dimeric micellar solutions undergoing morphological transitions,9-16 and, to the authors’ knowledge, this is the first attempt at quantitatively explaining the kinetic micellar effects caused by the changes in the micelle characteristics accompanying morphological transitions.
Experimental Section Materials. Pyrene-3-carboxaldehyde, P3C, was from Fluka. 6-Methoxy-N-(3-sulfopropyl)quinolinium, SPQ, was purchased from Molecular Probes, Inc. and used as received. N-Decanoyl N-methylglucamide, MEGA10 (see Scheme 1), and 1-butanol were from Fluka and used without further purification. 1,2-Propylene glycol, 1,2-prop, was from Aldrich. Dodecyltrimethylammonium bromide (DTAB) was from Aldrich. The synthesis of the dimeric surfactants (see Scheme 1) was done as described in ref 17. The surfactants were characterized by 1H NMR, 13C NMR, and elemental analysis (CITIUS, University of Seville), with the results being in agreement with those previously reported. Methyl naphthalene-2-sulfonate, MeNS, was synthesized following the method in the literature.18 Conductivity Measurements. Conductivity was measured with a Crison GLP31 conductimeter as described previously.8 Steady-State Fluorescence Measurements. Fluorescence measurements were done by using a Hitachi F-2500 fluorescence spectrophotometer. The temperature was kept at 303 K via a water flow cryostat connected to the cell compartment. (9) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36, 2346. (10) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262–3272. (11) (a) Brinchi, L.; Germani, R.; Goracci, L.; Savelli, G.; Bunton, C. A. Langmuir 2002, 18, 7821. (b) Bhattacharya, S.; Kumar, V. P. J. Org. Chem. 2004, 69, 559. (12) Liveri, M. L. T.; Lombardo, R.; Sbriziolo, C.; Viscardi, G.; Quagliotto, P. New J. Chem. 2004, 28, 793. (13) Qiu, L.-G.; Xie, A.-J.; Shen, Y.-H. Colloid Polym. Sci. 2005, 283, 1343. (14) Rodrı´ guez, A.; Graciani, M. M.; Bitterman, K.; Carmona, A. T.; Moya, M. L. J. Colloid Interface Sci. 2007, 313, 542. (15) Graciani, M. M.; Rodrı´ guez, A.; Moya, M. L. J. Colloid Interface Sci. 2008, 328, 324. (16) Rodrı´ guez, A.; Graciani, M. M.; Cordobes, F.; Moya, M. L. J. Phys. Chem. B 2009, 113, 7767. (17) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Caran, K. L.; Romsted, L. S. Langmuir 2000, 16, 9095. (18) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497.
18660 DOI: 10.1021/la102857d
The 110-6 M SPQ surfactant solutions were prepared in double distilled water. The fluorescence intensities were measured at 443 nm by the excitation at 346 nm, as indicated in ref 19. Excitation and emission slits were 5 and 10 nm, respectively, and a scan speed of 60 nm was used. Surfactant concentrations were well above the cmc. Study of the Polarity of the Micellar Interfacial Region. The 10-5 mol dm-3 P3C micellar solutions were prepared as in ref 20. Pyrene-3-carboxaldehyde was excited at 356 nm, and fluorescence spectra were recorded from 380 to 600 nm. A scan speed of 60 nm/min was used, and the excitation and emission slits were changed depending on the micellar media. The fluorescence maximum shown in the spectra is strongly dependent on the polarity of the medium.21 In order to check the reliability of the results, the fluorescence emission spectrum of P3C in a 0.1 M aqueous DTAB micellar solution was recorded at 303 K. The fluorescence maximum was 446 nm, in good agreement with literature data.21 Kinetic Measurements. The reaction MNS þ Br- (Scheme 2) was recorded at 326 by using Hitachi 3100 and Shimadzu 1800 UV-visible spectrophotometers. MNS was added in 10 μL of acetonitrile to 1 mL of the reaction solution, so that the organic substrate concentration in the reaction medium was 10-4 M. The bromide ions involved in the process come from the surfactant molecules, and no additional Br- was added. Kinetics were followed for more than five half-lives in all the micellar media. The observed rate constant was obtained from the slopes of the ln(At - A¥) against time plots, with At and A¥ being the absorbance at time t and at the end of the reaction, respectively. The A¥ value was experimentally obtained by letting the reaction finish. Each experiment was repeated at least twice, and the observed rate constants were reproducible within a precision better than 5%. The temperature was maintained at 303.0 ( 0.1 K using a water-jacketed cell compartment connected to a water-flow cryostat. Kinetics in 12-2-12,2Br- and 12-3-12,2Br- could not be done for surfactant concentrations higher than 0.04 and 0.07 M, respectively, because of solubility problems. The reactions of the organic substrates with water can make a contribution to the reaction MNS with Br-,22,23 although this contribution is not significant except at low surfactant concentrations. Kinetic data have been corrected, when necessary, from the spontaneous hydrolysis contribution as in ref 24.
Results and Discussion Figure 1 shows the dependence of the observed rate constant, kobs, for the reaction MNS þ Br- on surfactant concentration for the different pure 12-s-12,2Br- micellar solutions investigated. The kinetic data in the presence of additives and those corresponding to DTAB are displayed in Figure 2. The latter were obtained for the sake of comparison (DTAB can be considered as the monomeric counterpart of the dimeric surfactants). The kinetic data in (19) Kuwamoto, K.; Asakawa, T.; Ohta, A.; Miyagishi, S. Langmuir 2005, 21, 7691. (20) Zana, R. J. Phys. Chem. B 1999, 103, 9117. (21) Kalyanasundaram, K.; Thomas, J. K. J. Chem. Phys. 1977, 81, 2176. (22) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331. (23) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, R.; Bunton, C. A. Langmuir 1997, 13, 4583. (24) Rodrı´ guez, A.; Graciani, M. M.; Mu~noz, M.; Moya, M. L. Langmuir 2003, 19, 8685.
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Figure 1. Dependence of the observed rate constant, kobs/s-1, for the reaction MNS þ Br- on surfactant concentration in various aqueous dimeric micellar solutions at 303 K: (a) 12-2-12,2Br-; (b) 12-3-12,2Br-; (c) 12-4-12,2Br-; (d) 12-5-12,2Br-; (e) 12-6-12,2Br-; (f) 12-8-12,2Br-; (g) 12-10-12,2Br-; (h) 12-12-12,2Br-.
Figures 1 and 2 are presented in independent plots in order to make the visualization of the experimental results easier. Figures 1 and 2 Langmuir 2010, 26(24), 18659–18668
show that in all cases kobs increases upon augmenting the surfactant concentration. With the exception of 12-5-12,2Br- in the DOI: 10.1021/la102857d
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Graciani et al. Table 1. Critical Micelle Concentration, cmc, Micellar Ionization Degree, r, Second Critical Micellar Concentration, C*, and Wavelength of the Fluorescence Maximum of Pyrene-3-carboxaldehyde in the Micellar Phase, λmax(m), for Various Alkanediyl-r-ωbis(dodecyldimethylammonium bromide), 12-s-12,2Br-, Surfactants at 303 K surfactant
cmc (mM)
R
C* (M)
λmax(m)a
12-2-12,2Br0.95 0.17 0.016 438.8 0.99 0.22 0.036 439.2 12-3-12,2Br1.1 0.25 0.025 439.4 12-4-12,2Br 1.1 0.28 0.029 439.6 12-5-12,2Br0.99 0.31 0.028 440.1 12-6-12,2Br0.88 0.40 0.031 441.2 12-8-12,2Br0.59 0.45 0.028 441.6 12-10-12,2Br 0.36 0.45 0.023 442.1 12-12-12,2Bra [Surfactantm]=0.01 M; data corresponding to s=2, 5, 6, 8, 10, and 12 were taken from ref 8.
Figure 2. Dependence of the observed rate constant, kobs/s-1, for
the reaction MNS þ Br- on surfactant concentration in various aqueous dimeric micellar solutions in the presence of additives and in DTAB micellar solutions at 303 K: (a) 12-2-12,2Br-; (b) 12-5-12, 2Br-; (c) DTAB.
presence of 1,2-prop 30 wt %, and of DTAB micellar solutions, the observed rate constant does not reach a plateau at high surfactant concentrations, but it increases steadily when [12-s-12,2Br-] increases. The first observation can be explained by considering that both the MNS and Br- reagents are distributed between the bulk and micellar pseudophases. Therefore, the process is taking place simultaneously in the two pseudophases and the reaction rate is the sum of the two contributions. An increase in surfactant 18662 DOI: 10.1021/la102857d
concentration brings about a further incorporation of the organic substrate molecules into the micelles. As a consequence, the contribution of the reaction occurring in the micellar pseudophase increases and, given that the bromide ion concentration in this pseudophase is much higher than that in the bulk phase, the observed rate constant augments. One would expect kobs to reach a constant value when the surfactant concentration was high enough for the MNS molecules to be fully bound to the micelles. However, in most cases, no plateau is observed. This behavior can be explained by taking into account that in 12-s-12,2Br- micellar solutions an increment in surfactant concentration causes a sphere-to-rod transition. The change in size and shape of the micelles is accompanied, among other effects, by a diminution in the micellar ionization degree,19,25 which leads to an increment in the interfacial bromide ion concentration and thus in kobs. There are other changes following the morphological transition which can influence kobs. They will be considered below. cmc and r Values in the Micellar Reaction Media. In order to quantitatively rationalize the kinetic micellar effects observed, it is necessary to estimate the bromide ion concentration in the bulk and micellar pseudophases, where the process occurs. In order to do this, the critical micelle concentration, cmc, and micellar ionization degree, R, of the aggregates present in the micellar reaction media have to be known. These magnitudes were determined by means of conductivity measurements and applying Carpena’s method26 to the experimental specific conductivity data (see Figure 1 in the Supporting Information). The cmc and R values are listed in Tables 1 and 2. The dependence of these magnitudes on the spacer length has been previously discussed.8 The influence of additives on the cmc and R values of 12-212,2Br- and 12-5-12,2Br- surfactants can be rationalized by considering the effects of the additive on different contributions 27 to the Gibbs energy of micellization, ΔG. M 1,2-Propylene glycol remains in the bulk phase, lowering its polarity6 and making it a better solvent for the surfactant molecules. As a consequence, the Gibbs energy change accompanying the transfer of the surfactant hydrophobic chain from the bulk phase (water þ 1,2-prop) into the micellar interior diminishes (it is less negative), with this leading to an increment in the cmc.28,29 The presence of the (25) (a) Rodrı´ guez, A.; Graciani, M. M.; Mu~noz, M.; Robina, I.; Moya, M. L. Langmuir 2006, 22, 9519. (b) Rodríguez, A.; Graciani, M. M.; Angulo, M.; Moya, M. L. Langmuir 2007, 23, 11496. (26) Carpena, P.; Aguiar, J.; Bernaola-Galvan, P.; Carrnero Ruiz, C. Langmuir 2002, 18, 6054. (27) Camesano, T. A.; Nagarajan, R. Colloids Surf., A 2000, 167, 165. (28) Nagarajan, R.; Wang, Ch.-Ch. Langmuir 2000, 16, 5242. (29) (a) Moya, M. L.; Rodrı´ guez, A.; Graciani, M. M.; Fernandez, G. J. Colloid Interface Sci. 2007, 316, 787. (b) Rodríguez, A.; Graciani, M. M.; Moya, M. L. Langmuir 2008, 24, 12785.
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Table 2. Critical Micelle Concentration, cmc, Micellar Ionization Degree, r, Second Critical Micellar Concentration, C*, and Wavelength of the Fluorescence Maximum of Pyrene-3-carboxaldehyde in the Micellar Phase, λmax(m), for Various 12-s-12,2Br- (s = 2,5) Surfactants in the Presence of Additives at 303 K surfactant
cmc (mM)
12-2-12,2Br;-MEGA, Xdimeric = 0.9b 12-2-12,2Br;-MEGA, Xdimeric = 0.6b 12-5-12,2Br;-MEGA, Xdimeric = 0.8b 12-5-12,2Br;-MEGA, Xdimeric = 0.6b 12-5-12,2Br-, 1,2-prop 10 wt % 12-5-12,2Br-, 1,2-prop 30 wt % 12-5-12,2Br-, [1-butanol] = 0.1 M 12-5-12,2Br-, [1-butanol] = 0.2 M
1.0 1.4 1.3 1.6 1.3 2.7 0.96 0.80
R
C* (M) λmax(m)a
0.18 0.019 0.22 0.053 0.30 0.048 0.34 0.089 0.29 0.060 0.39 >0.1 0.33 0.060 0.40 0.025
439.2 442.8 441.3 442.9 440.1 445.9 440.6 439.9
a [Surfactantm] = 0.01 M. b Data taken from ref 8; 1,2-prop = 1,2propylene glycol.
organic solvent also affects other energetic contributions to ΔGM, mainly controls the cmc change.16,25 but the diminution in ΔGtransf The cmc increase results in an increment in the ionic concentration (the monomer concentration augments), which leads to a decrease in the electrostatic repulsions between the positively charged head groups of the dimeric surfactants through a screening effect. Besides, the average aggregation number of the dimeric aggregates is expected to decrease because of the presence of the organic solvent.6,16,25,28 The two effects are followed by a diminution in the electrostatic charge density at the micellar interfacial region, with this bringing about an increment in the micellar ionization degree. MEGA 10 molecules formed mixed micelles with the dimeric surfactants, as has been shown previously through circular dichroism.8 The experimental cmcs were found to be lower than those expected while considering an ideal behavior. This nonideal behavior was explained by considering the diminution in the electrostatic repulsions due to the presence of the nonionic component in the mixed aggregates.8 The spacer length was shown not to play an important role in the behavior of the 12-s-12,2Br--MEGA 10 reduction can also explain the increase binary systems. The ΔGelect in R following the addition of the nonionic surfactant. 1-Butanol distributes between the aqueous and the micellar pseudophases. The changes in the cmc and R caused by 1-butanol addition can be explained by considering that alcohol molecules intercalate between the dimeric surfactant molecules within the micelles, diminishing the electrostatic repulsions between the positively charged head groups. This decrease in ΔGelect promotes micellization and reduces the electrostatic charge density at the interfacial region, thus causing the cmc to decrease and R to increase. Micellar Growth in the Microheterogeneous Reaction Media. 12-s-12,2Br- surfactants show morphological transitions when surfactant concentration increases.1,2,8,16,25 Micellar growth is significant from the kinetic point of view since changes in the size and shape of micelles affect the rate of micelle-modified reactions.14 Therefore, information about the second cmc, C*, is necessary in order to rationalize the kinetic data. C* values were determined by using the method developed by Kuwamoto et al.19 Figure 3 shows the SPQ fluorescence quenching in aqueous 12-5-12, 2Br- micellar solutions in the absence and in the presence of 1,2-propylene glycol 10 wt %, at high surfactant concentrations, with [surfactant] well above the cmc (see Tables 1 and 2). The C* values are listed in Tables 1 and 2. Each second cmc value was repeated at least twice, and the precision is C* ( 0.002 M. The dependence of C* on the spacer length was discussed in a previous work.8 The ionic interactions and the additional extrapacking term to the deformation of the surfactant tails were considered in order to account for the experimental C* trend, Langmuir 2010, 26(24), 18659–18668
Figure 3. SPQ fluorescence quenching in aqueous 12-5-12,2Brmicellar solutions at high surfactant concentrations in pure water and in the presence of 1,2-prop 30% wt. T = 303 K.
although there are other size and shape dependent Gibbs energy .27 The extra-packing term accounts for the contributions to ΔGM packing constraints on the surfactant tails (as they are connected by the spacer) so as to pack within the micellar core. This term favors micellar growth, since it decreases as micellar size increases and makes the transition from sphere to cylinder easier. The electrostatic term is large in magnitude and limits micellar growth. With regard to the presence of additives, Table 2 shows that an increase in the weight percentage of 1,2-prop causes an increment in C*. In fact, in the presence of 30 wt % 1,2-prop, no morphological transition is observed (for [surfactant] < 0.1 M). The dependence of C* on 1,2-prop weight percentage can be discussed by taking into account that addition of 1,2-prop is expected to decrease the aggregation number.16,25,29 This size diminution brings about a decrease in the tail deformation Gibbs energy and in the extrapacking Gibbs energy contributions, making the sphere to rod transition less favorable. As a result, C* diminishes upon increasing the weight percentage of 1,2-prop. MEGA10 molecules incorporate into the dimeric micelles forming mixed aggregates. In contrast to the dimeric micelles, pure MEGA10 micelles do not show a tendency to micellar growth upon increasing surfactant concentration.30 It has been previously shown that addition of a spherical micelle-forming surfactant, such as MEGA10, to a treadlike micelle-forming surfactant, such as 12-s12,2Br- surfactants, results in the progressive inhibition of the capacity of the latter to form such micelles when the surfactants comicellized.31 This is in agreement with cryogenic transmission electron microscopy (cryoTEM) measurements carried out with DTABþ12-2-12 mixtures.32 The presence of 1-butanol molecules in the aqueous phase influences C* similarly to 1,2-propylene glycol, making the morphological transition less favorable. On the other hand, intercalation of 1-butanol molecules in the interfacial region reduces the electrostatic repulsions between the positively charged head groups. As a consequence, the transition from spherical to spherocylindrical micelles would be more favorable and C* would diminish. For 1-butanol 0.1 M, an increase in C* is observed as compared to pure water, thus pointing out that the changes in the bulk phase (30) Hierrezuelo, J. M.; Molina Bolivar, J. A.; Carnero Ruiz, C. J. Phys. Chem. B 2009, 113, 7178. (31) Rodrı´ guez, A.; Graciani, M. M.; Moreno-Vargas, A. J.; Moya, M. L. J. Phys. Chem. B 2008, 112, 11942. (32) Zana, R.; Talmon, Y. Nature 1993, 362, 228.
DOI: 10.1021/la102857d
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caused by the presence of the alcohol molecules in this phase control the second cmc variation. When 1-butanol concentration is 0.2 M, induced by the incorporation of the the reduction in ΔGelect alcohol molecules into the micellar pseudophase is the energetic contribution mainly controlling the second cmc variations. Kinetic Micellar Effects on the Reaction between Methyl Naphthalene-2-Sulfonate and Bromide Ions. Figure 1 shows the influence of the dimeric surfactant concentration on the process MNS þ Br-. The observed rate constant for the reaction can be written as33 k w Brw- þ k m =V m Brm- K m 2 k obs ¼ 2 1 þ K m 12-s-12, 2Brm k w2 Brw- þ k 2m Brm- K m ¼ ð1Þ 1 þ K m 12-s-12, 2BrmHere [Brw ] and [Brm] are the bromide ion concentrations in the aqueous and micellar pseudophases, respectively, referred to the total solution volume. Vm is the molar volume of the reactive region at the micellar surface, and Km is the equilibrium binding constant of the MNS molecules to the dimeric micelles. [12-sw m 12,2Brm] is the micellized surfactant concentration. k2 and k2 are the second order rate constants in the aqueous and micellar pseudophases (in mol-1 dm3 s-1), respectively, and (km 2 /Vm) = k2m (in s-1) is the second order rate constant in the micellar pseudophase written with the concentrations expressed as molar ratios, [Brm]/[12-s-12,2Brm]. In order to fit the kinetic data by using eq 1, the kw 2 value was experimentally determined, with its value being 1.6 10-4 mol-1 dm3 s-1 at 303 K. The micellized surfactant concentration can be calculated by using the cmc values listed in Table 1. Finally, the bromide ion concentrations in the aqueous and micellar pseudophases can be estimated by using eqs 2 and 3 - ð2Þ Brm ¼ ð1 - RÞ 12-s-12, 2Brm-
Brw- ¼ ½12-s-12, 2Brtotal - Brm-
ð3Þ
where R is the micellar ionization degree. From the fittings, the adjustable parameters, Km and k2m, would be obtained. These fittings will only render reliable adjustable parameters for kinetic data within the range cmc < [12-s-12,2Br-] < C*, where, for the surfactants investigated, R remains constant. Nonetheless, [Brw] and [Brm] can be calculated within the whole surfactant concentration range from the steady-state fluorescence data by using Kuwamoto’s method.19,14,34 Micellar growth leads to a decrease not only in the micellar ionization degree but also in the micropolarity and in the microviscosity of the interfacial region as well as in Vm and in the water content. This means that, for [12-s12,2Br-] > C*, Km and km 2 can vary upon increasing surfactant concentration. Since the goal of this Article is to discuss the kinetic micellar effects within the whole surfactant concentration range, the authors proposed the following procedure in order to do so. The kinetic data corresponding to the surfactant concentration range cmc < [12-s-12,2Br-] < C* were fitted by using eq 1 and a set of Km and k2m adjustable parameters was obtained for each of the dimeric surfactants. The dotted lines in Figure 4a and in Supporting Information Figure 2a-g correspond to these fittings. The lines have been drawn for the whole surfactant concentration (33) Bunton, C. A.; Yao, J.; Romsted, L. S. Curr. Opin. Colloid Interface Sci. 1996, 65, 125. (34) Asakawa, T.; Kitano, H.; Ohta, A.; Miyagishi, S. J. Colloid Interface Sci. 2001, 242, 284.
18664 DOI: 10.1021/la102857d
range in order to help the reader. The adjustable parameters obtained are listed in Table 3. Subsequently, [Brw ] and [Brm] values were calculated for the whole surfactant concentration range by taking the variations in R following micellar growth into account. Using these bromide ion concentrations and the Km and k2m values listed in Table 3, kRobs was estimated by using eq 1. The medium dashed lines in Figure 4a and in Supporting Information Figure 2a-g show the dependence of kRobs on surfactant concentration. One can see that, in all cases, consideration of R changes improves the agreement between the theoretical and the experimental rate constant values. However, the agreement is not good, which is not a surprise, since, as was mentioned above, micellar growth is followed by other changes in the aggregates characteristics not taken into account in eq 1. It is interesting to point out that micelles and other association colloids can influence the rates of micelle-modified reactions through two different effects. The concentration or depletion of reactants in the interfacial region has a major effect on the rates of bimolecular processes, such as the one investigated in this work. This is called the concentration effect. However, micelles also exert a medium effect that influences reactivity. This effect depends on the transfer of substrate from water to micelles, on the reaction mechanism, and on the properties of the interfacial region, such as local charge, polarity, and water content. Usually, the kinetic micellar effects observed in bimolecular processes are mainly due to the concentration of one or both reactants in the small volume of the micelles. However, there are processes for which large kinetic micellar effects, not connected to concentration effects, are operative.35 In the reaction studied, the difference between kw 2 and km 2 would account for the micellar medium effects in the dimeric micellar solutions assuming that no morphological transition occurs. When the changes in the micellar ionization degree caused by the morphological transition were taken into account, the concentration kinetic micellar effects provoked by micellar growth were considered. Therefore, the disagreement between the kRobs and the experimental rate constants has to be due to medium kinetic micellar effects provoked by micellar growth. The difficulty is that discussion of micellar medium effects is always qualitative because they are the result of many factors influencing reactivity simultaneously and it is not possible to obtain experimental information about how each of them affects reactivity. Because of this, the authors examined the suitability of various empirical equations for explaining the data and found that if kobs is expressed by eq 4, the kinetic data can be quantitatively fitted for all the micellar reaction media investigated. It has to be indicated that eq 4 is an empirical equation and was not deduced from eq 1. k w Brw- þ k 2m Brm- K m þ b 12-s-12, 2Brm- 2 ð4Þ k obs ¼ 2 1 þ K m 12-s-12, 2Brm In this equation, Km and k2m are the adjustable parameters summarized in Table 3. Solid lines in Figure 4a and in Supporting Information Figure 2a-g show the results of the fittings using eq 4. The values of the adjustable parameter b are listed in Table 3. One can see that the agreement between the theoretical and the experimental data are good for all the dimeric micellar solutions used as reaction media. The question is: has b any meaning? The authors proposed that b can be considered as an empirical parameter that involves the medium kinetic micellar effects caused by the changes in the aggregates characteristics following micellar (35) Rodrı´ guez, A.; Graciani, M. M.; Mu~noz, M.; Fernandez, G.; Moya, M. L. New J. Chem. 2001, 25, 1084 and references therein. .
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Figure 4. Results of the fittings of the kinetic data corresponding to the reaction MNS þ Br- in various dimeric micellar solutions in the absence and in the presence of additives at 303 K (see the text). Table 3. Fitting Parameters for the Reaction MNS þ Br- in DTAB and 12-s-12,2Br- Micellar Solutions in the Absence and in the Presence of Additives at 303 K [surfactant]
Km (dm3 mol-1)
-1 103 (km 2 /Vm) = k2m (s )
12-2-12,2Br12-3-12,2Br12-4-12,2Br12-5-12,2Br12-6-12,2Br12-8-12,2Br12-10-12,2Br12-12-12,2BrDTAB 12-2-12,2Br--MEGA, Xdimeric = 0.9 12-2-12,2Br--MEGA, Xdimeric = 0.6 12-5-12,2Br--MEGA, Xdimeric = 0.8 12-5-12,2Br--MEGA, Xdimeric = 0.6 12-5-12,2Br-, 1,2-prop 10 wt %a 12-5-12,2Br-, 1,2-prop 30 wt %a 12-5-12,2Br-, [1-butanol] = 0.1 M 12-5-12,2Br-, [1-butanol] = 0.2 M a 1,2-prop = 1,2-propylene glycol.
368 ( 35 350 ( 28 363 ( 59 354 ( 22 355 ( 35 207 ( 34 197 ( 21 122 ( 19 182 ( 14 276 ( 34 195 ( 31 380 ( 52 270 ( 28 211 ( 22 53 ( 2 152 ( 22 154 ( 23
1.82 ( 0.06 2.27 ( 0.05 2.1 ( 0.2 2.29 ( 0.03 7.2 ( 0.2 3.1 ( 0.2 2.1 ( 0.2 3.0 ( 0.06 1.27 ( 0.06 1.8 ( 0.1 1.61 ( 0.08 1.96 ( 0.07 1.68 ( 0.04 2.30 ( 0.07 2.27 ( 0.02 2.30 ( 0.07 3.1 ( 0.1
growth. Before going into the discussion of the b values listed in Table 3, some comments about Km and k2m values will be made. The equilibrium association binding constants of the MNS molecules to the dimeric micelles, Km, are similar for s=2, 3, 4, 5, and 6, within the experimental errors. For the spacers with s=8, 10, and 12, Langmuir 2010, 26(24), 18659–18668
102 b/s-1M-2 19 ( 1 9.6 ( 0.5 6.5 ( 0.3 2.1 ( 0.2 1.7 ( 0.8 1.3 ( 0.3 1.1 ( 0.2 1.3 ( 0.2 2.3 ( 0.6 1.3 ( 0.0.3 1.8 ( 0.0.4 0.82 ( 0.0.02 1.6 ( 0.3 1.3 ( 0.2 3.07 ( 0.02
Km seems to decrease, particularly for s=12. This could be related to an increase in the polarity of the interfacial region, where the organic substrate molecules are localized. In this regard, the wavelength of the fluorescence maximum of pyrene-3-carboxaldehyde in the micellar pseudophase (at [12-s-12,2Br-]=0.01 M), λmax(m), DOI: 10.1021/la102857d
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listed in Table 1 shows that an increase in the spacer length brings about an increase in the polarity of the interfacial region. On this basis, a decrease in Km would be expected upon augmenting s. Nonetheless, the localization of the P3C probe can change from one to another micellar medium, as can the MNS molecule site in the interfacial region vary. A reasonable conclusion is that, when comparing Km(s=2) to Km(s=12), a decrease in the equilibrium binding constant is observed due to an increment in the polarity of the interfacial region. The aggregation number of 12-2-12,2Brspherical micelles is substantially higher than that of 12-1212,2Br-.8,36 As a result, the intercalation of solvent molecules into the interfacial region is easier for s=12 than for s=2, and this increment in the interfacial water content leads to an increase in polarity. k2m does not seem to depend on the spacer length (see Table 3), with the exception of the value found in 12-6-12,2Br- micellar solutions, which is higher than those estimated for the rest of the dimeric micellar media. No plausible explanation has been found for this high k2m value. In order to get some information about the capacity of the dimeric micelles as catalysts for the reaction MNS þ Br- with respect to water, km 2 =k2mVm has to be estimated for the different micellar reaction media. Vm values for s=2, 3, 4, 5, 6, 8, 10, and 12 were 0.56, 0.58, 0.59, 0.60, 0.63, 0.66, 0.70, and 0.73 dm3 mol, respectively.37 The km 2 values calculated are within -3 mol-1 dm3 s-1, the range 110-3 mol-1 dm3 s-1 b(12-4-12,2Br-)>b(12-5-12,2Br-)>b(12-6-12,2Br-)> b(12-8-12,2Br-) ∼ b(12-10-12,2Br-) ∼ b(12-12-12,2Br-). All the dimeric micellar solutions investigated undergo a morphological transition upon increasing surfactant concentration. However, the tendency to micellar growth depends on the spacer length. Danino et al.36 have investigated the dependence of the aggregation number of 12-s-12,2Br-, with s=3, 4, 5, 6, 8, and 10, micelles on surfactant concentration by time-resolved fluorescence quenching. They found that the tendency to micellar growth follows the trend 12-3-12,2Br- > 12-4-12,2Br- > 12-5-12,2Br- > 12-612,2Br- ∼ 12-8-12,2Br- ∼ 12-10-12,2Br-. The dimeric surfactant with s = 2 could not be studied due to experimental problems. (36) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (37) Wetting, S. S.; Verral, R. E. J. Colloid Interface Sci. 2001, 235, 310. (38) (a) Bohme, K. D.; Young, L. B. J. Am. Chem. Soc. 1970, 92, 7354. (b) Bohme, K. D.; Mackay, G. I.; Pay, J. D. J. Am. Chem. Soc. 1974, 96, 4027. (c) Tanaka, K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J. Chem. 1976, 74, 1643. (d) Olmsted, W. E.; Braumen, J. I. J. Am. Chem. Soc. 1977, 99, 4219. (e) Henchman, M.; Paulson, J. F.; Hiel, P. M. J. Am. Chem. Soc. 1983, 105, 5509. (f) Dewar, M. J.; Storch, D. M. J. Chem. Soc., Chem. Commun. 1985, 94. (39) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216.
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However, the strong tendency to micellar growth shown by this dimeric surfactant can be examined by using rheology measurements.16,40 In fact, the viscoelastic behavior found for 12-2-12,2Brmicellar solutions was attributed to the entanglement of long and flexible aggregates. The fact that for s>2 no viscoelastic behavior is found in pure water points out that for s = 2 the tendency to micellar growth is stronger than for s > 2. This conclusion is also in agreement with cryoTEM measurements carried out in 12-s-12,2Br- micellar solutions by increasing surfactant concentration.36,41 These measurements also show that the growth of 12-12-12,2Br- micelles by augmenting surfactant concentration is slow. If the b values summarized in Table 3 are examined, one can see that b increases when the tendency of the surfactant to micellar growth also increases. This is a reasonable result, since a strong tendency to micellar growth will be accompanied by large changes in the characteristics of the interfacial region (the reaction site). Therefore, substantial medium kinetic micellar effects would be expected and, thus, high b values would be found. This result has to be consistent with that in the presence of additives. This point will be examined in the next section. Kinetic Micellar Effects on the Reaction between Methyl Naphthalene-2-sulfonate and Bromide Ions in the Presence of Additives. The expression for the observed rate constant depends on the nature of the additive. When an organic polar solvent, such as 1,2-propylene glycol, is added to the aqueous micellar solution one can write: - k bulk Brbulk þ k 2m Brm- K m 2 þ b 12-s-12, 2Brm- 2 ð5Þ k obs ¼ 1 þ K m 12-s-12, 2BrmHere the different terms have the same meaning as in eq 4. In this case, the authors used the term bulk phase instead of aqueous phase in order to consider that now the bulk phase is a water-1,2prop binary mixture, with a determined weight percentage of 1,2prop. The procedure used to fit the experimental kinetic data was similar to that in the absence of additives. The experimental kbulk values in 10 and 30 wt % 1,2-prop are 1.910-4 and 2.310-4 2 -1 mol dm3 s-1, respectively, at 303 K. The Km, k2m, and b values obtained from the fittings are listed in Table 3. The solid line in Figure 4b shows that for 10 wt % 1,2-prop the agreement between the theoretical and the experimental data is good. In the case of 30 wt % 1,2-prop, no morphological transition is observed for [12-5-12,2Br-]
