J. Phys. Chem. B 2009, 113, 7767–7779
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Water-Ethylene Glycol Cationic Dimeric Micellar Solutions: Aggregation, Micellar Growth, and Characteristics As Reaction Media Amalia Rodrı´guez,† Marı´a del Mar Graciani,† Felipe Cordobe´s,,†,‡ and Marı´a Luisa Moya´*,† Departamento de Quı´mica Fı´sica and Departamento de Ingenierı´a Quı´mica, UniVersidad de SeVilla C/ Profesor Garcı´a Gonza´lez 1, 41012 SeVilla, SPAIN ReceiVed: February 17, 2009; ReVised Manuscript ReceiVed: April 22, 2009
Effects of ethylene glycol (EG) addition on the micellization and on the micellar growth in two aqueous didodecyl dicationic dibromide surfactant, 12-s-12,2Br- (s ) 2, 6) solutions, with the weight percentage of EG up to 50%, have been investigated. An increment in the amount of EG makes the aggregation process less spontaneous due to the water-EG mixtures being better solvents for the cationic dimeric surfactant molecules than pure water (solvophobic effect). Results show that C*, the surfactant concentration where the sphere-to-rod transition occurs, increases when EG content in the bulk phase increases. The amount of the organic solvent influences C* principally through the decrease in the hydrocarbon/bulk phase interfacial tension (air/bulk phase surface tension) caused by its presence. Changes in the aggregation number, in the micropolarity, in the microviscosity, and in the rheological behavior accompanying micellar growth were studied in the water-EG micellar solutions. Kinetic studies provide information about the characteristics of the dimeric micelles as microreactors. Kinetic data also show that an increase in the surfactant concentration leads to micellar growth. Introduction At concentrations above the cmc (critical micelle concentration) surfactants tend to self-associate in water to form micelles. The micelles are generally spherical or spheroidal at concentrations slightly above the cmc.1 For several surfactants, micelles tend to grow and, in this process (morphological transitions), change shape when an appropriate parameter is modified. Thus, an increase in concentration or temperature2 brings about micellar growth. The surfactant concentration above which the morphological transition from spherical micelles into elongated ones occurs is often referred to as “second cmc” (C*).3 The study of the means to control the shape of micelles has always been an important topic of research in surfactant science, of both academic and applied interest. Indeed, micelle shape determines to a large extent the properties of surfactant solutions, in particular their rheology4 and the dynamics of micelles.5 Also, the solubilization of water-insoluble compounds by micellar solutions, which is at the basis of many applications of these systems, depends on micelle shape.6 The growth of spherical micelles into elongated ones has been investigated extensively from the theoretical viewpoint.1,7-9 Recently, a new model has been developed by Romsted et al. in order to account for the balance of forces controlling morphological transitions of association colloids.10 This model provides a qualitative explanation for the balancing force to the solvophobic effect that depends on short-range, specific ionpair and hydration interactions within the interfacial region. The solvophobic effect, in which amphiphile tails minimize their contact with water (the bulk phase) and drive amphiphile aggregation, can be changed by varying the characteristics of the bulk phase.11,12 To do so, organic solvents that remain in * To whom correspondence should be addressed. E-mail:
[email protected]; homepage: www.grupos.us.es/coloides. † Departamento de Quı´mica Fı´sica. ‡ Departamento de Ingenierı´a Quı´mica.
the bulk phase (they do not incorporate into the micelles) can be used. The addition of polar organic solvents to aqueous micellar solutions will alter the tendency of the amphiphile molecules to avoid the contact with the solvent and, therefore, it is expected to affect the value of surfactant concentration at which aggregation occurs (cmc) as well as micelles characteristics such as the micellar ionization degree, the aggregation number, and the polarity and solvent content in the interfacial region.13-18 Therefore, the presence of organic polar solvents alters the surfactant concentration range in which a sphericalto-rod transition will take place.19 Among the surfactants showing morphological transitions when surfactant concentration increases are the didodecyl dicationic dibromide dimeric surfactants with different methylene spacer lengths, 12-s-12,2Br(where s is the number of methylene groups).20 This work was meant to investigate the effects of ethylene glycol (EG) addition on the aggregation and micellar growth of two didodecyl dicationic dibromide dimeric surfactants, 12-2-12,2Br- and 12-6-12,2Br-; that is, those with two and six methylene groups in the spacer. It is worth noting that interesting effects on covalent incorporation of EG or its oligomers as part of the spacer or other segment of several bis-cationic gemini surfactants have been previously reported.21,22 Dynamic and static fluorescence, conductivity, spectroscopic, surface tension, rheology, and kinetic measurements were used in order to obtain information about the water-EG dimeric surfactants solutions. Kinetic measurements also provide information about the characteristics of the dimeric cationic micelles as microreactors. Experimental Section Materials. The syntheses of gemini surfactants were done as described in ref 23. The surfactants were characterized by 1 H NMR, 13CNMR, and elemental analysis (CITIUS, University of Seville), with the results being in agreement with those previously reported. Methyl naphthalene-2-sulfonate (MeNS)
10.1021/jp901457d CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
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was synthesized following the method in the literature.24 Methyl 4-nitrobenzenesulfonate (MNBS) was from Fluka. 6-Methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 1,3dipyrenylpropane (P3P) were purchased from Molecular Probes, Inc. and were used as received. Ethylene glycol and tetrahydrofurane were from Fluka and were used without further purification. Reichardt’s dye and pyrene were from Aldrich; the latter was purified before use by methods reported in the literature.25 Dodecylpyridinium chloride was from Aldrich and was recrystallized from acetone before use. Conductivity Measurements. Conductivity was measured with a Crison GLP31 conductimeter, connected to a water-flow cryostat maintained at 298.2 ( 0.1 K. A dispenser Crison Buret 1S was programmed for adding the adequate quantities of a concentrated surfactant solution in order to change [surfactant] from concentrations well below the cmc, up to at least 2-3 times the cmc concentration. This method allows one to obtain a large number of experimental conductivity data, the estimation of the cmc being more accurate. The conductimeter was calibrated with KCl solutions of the appropriate concentration range. Steady-state Fluorescence Measurements. Fluorescence measurements were made by using a Hitachi F-2500 fluorescence spectrophotometer. The temperature was kept at 298.2 ( 0.1 K by a water-flow cryostat connected to the cell compartment. The 1 × 10-6 M SPQ surfactants 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 26. The (1-2) × 10-6 M P3P surfactant solutions were prepared following the method in the literature.27 The appropriate amount of a P3P stock solution in tetrahydrofuran was injected in the investigated water-EG surfactant solution, which was then stirred at 30 °C for at least 12 h. The injected volume amounted to less than 0.3% of the volume of the solution, and this small amount of tetrahydrofuran has no effect on the measurements. The emission spectra of P3P solutions were registered between 350 and 550 nm; the excitation wavelength was 346 nm. The intensities of the monomer emission (IM) and the excimer emission (IE) were recorded at the wavelength corresponding to the first vibronic peak of the monomer, located near 378 nm, and that of the excimer at around 490 nm, respectively. Time-resolved Fluorescence Quenching Measurements. These measurements were carried out by using a FL920 fluorescence lifetime spectrometer from Edinburgh Instruments using the time-correlated single-photon counting technique at 298.2 K. The method uses a fluorescence probe, pyrene, and a quencher of that fluorescence probe, dodecylpyridinium chloride. The decay of the probe fluorescence was determined first in the absence of quencher, at a low [pyrene]/[micelles] molar concentration ratio ( C*. Besides, two emerging features can be stressed: (i) the deviation is larger, for a given micellar reaction media, for 12-2-12,2Br- than for 12-6-12,2Br-; (ii) the deviation is larger the lower the percentage of EG present in the micellar solution is. These observations can be explained if one considers that an increase in [12-s-12,2Br-] leads to micellar growth. The sphere-to-rod transitions are accompanied by a decrease in the micellar ionization degree, as was mentioned above. For instance, in the case of 12-3-12,2Br- micellar solutions, R is equal to 0.21 from [surfactant] just above the cmc up to ≈0.022 M, and it decreases to 0.13 for [surfactant] up to 0.1 M.56b The R diminution brings about an increase in the bromide ions interfacial concentration and, as a consequence, [Br-m] increases, and so does kobs. Micellar growth leads to a decrease in the interfacial water content, which is followed by a decrease in the polarity of the interfacial region (see Table 4). MNBS + Br- is a process in which charge is dispersed in the transition state61 and, as a consequence, a diminution in the polarity of the reaction site will favor the reaction.61 This is shown by the k2bulk values in water-EG homogeneous mixtures (see Table 4). Taking this into account, k2m is expected to increase upon increasing [12s-12,2Br-], since an increase in the dimeric surfactant concentration in water results in a decrease in polarity of the interfacial region (the reaction site in the micellar pseudophase). Morphological transitions can affect Km. Polarity of the bulk phase is nearly independent of [12-s-12,2Br-], but an increase in [12-s-12,2Br-] would result in a diminution of the interfacial
Rodrı´guez et al. region polarity. Therefore, Km is expected to increase when the dimeric surfactant concentration increases and, as a consequence, the contribution of the reaction occurring in the interfacial region increases, thus augmenting kobs. Finally, Vm could vary upon changing [12-s-12,2Br-]. The change in volume upon micellization is primarily due to the change in the water structure involved in the hydrophobic effect and to the change in electrostiction of the polar part of the molecule, modulated by the value of the micellar ionization degree.62,63 A further change in the structure of the micelles, in which solute-solute van der Waals interactions are mainly involved,63 is expected to give rise to a small change in Vm. In any case, if Vm varies, a decrease in the molar volume when spherocylindrical micelles form would be expected, due to the crowding of the head groups at the micelle surface. m Taking the variations in [Brm], k2 , Km, and Vm accompanying micellar growth into account, kobs is expected to increase upon increasing dimeric surfactant concentration if a morphological transition occurs. This explains the discrepancy between the theoretical and the experimental kinetic data shown in Figure 8 at high surfactant concentrations, where the tendency of the dimeric micelles to grow is strong. Micellar growth follows the trend 12-2-12,2Br- > 12-6-12,2Br-23,this work and this could account for the difference between ktheor and kobs following the same trend. With regard to the influence of EG content on the difference between kobs and ktheor, C* values in Table 1 show that the tendency to micellar growth upon increasing surfactant concentration diminishes when the ethylene glycol weight percentage in the system augments. Therefore, on the basis of the reasons mentioned above, the difference between kobs and ktheor is expected to diminish when EG wt % increases, as is observed. The reaction between methyl naphthalene-2-sulfonate (MNS) and Br- follows the same mechanism as that of MNBS + Br-, the only difference being the higher hydrophobicity of MNS as compared to that of MNBS.56b Because of this, Km(MNBS) < Km(MNS), and for micellar solutions where no morphological transitions occur, kobs for the process MNS + Br- reaches a plateau at surfactant concentrations lower than if the organic substrate was MNBS. On this basis, the authors studied the reaction MNS + Br- in aqueous 12-2-12,2Br- and 12-6-12,2Br- micellar solutions with the scope of showing an additional kinetic evidence of the morphological transitions. Figure 9 show the kinetic results. In this figure the dotted lines correspond to the expected trend if no sphere-to-rod transition happens (these lines were calculated as in the case of the MNBS + Br- reaction). One can see in Figure 9 that kobs begins to deviate from the expected trend at about 0.020 and 0.040 M for 12-2-12,2Br- and 12-6-12,2Br- micellar solutions, respectively. Therefore, from the kinetic results, the occurrence of morphological transitions can be inferred, with them taking place at surfactant concentrations in agreement with the C* values listed in Table 1. Characteristics of 12-2-12,2Br- and 12-6-12,2BrMicelles As Microreactors in the Presence and in the Absence of Ethylene Glycol. Data summarized in Table 5 can provide some information: (i) the binding equilibrium constant, Km, is higher for 12-6-12,2Br- than for 12-2-12,2Br-; (ii) Km decreases when EG wt % increases; and (iii) k2m is higher for 12-6-12,2Br- than for 12-6-12,2Br. The decrease in Km upon increasing wt % of EG can be explained by considering that when the amount of EG increases, the bulk phase becomes a better solvent for the organic substrate than water. Besides, an increase in EG wt % results in an increase
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Figure 9. Dependence of kobs for the reaction MNS + Br- on surfactant concentration in aqueous 12-s-12,2Br- micellar solutions (s ) 2,6 methylene groups). T ) 298.2 K. (a) 12-2-12,2Br-; (b) 12-6-12,2Br-. Dashed lines are the result of fitting the kinetic data corresponding to the surfactant concentration range previous to the sphere-to-rod transition ([surfactant] < C*).
in the polarity of the interfacial region (see Table 4). These two factors contribute to diminishing the affinity of the MNBS molecules for the dimeric micelles and, consequently, Km decreases. This Km diminution is one of the principal factors responsible for the reaction rate being slower the higher the EG content in the micellar reaction media is. Table 5 also shows that Km(12-6-12,2Br) > Km(12-2-12,2Br) and k2m(12-6-12,2Br) > k2m(12-2-12,2Br). These results could be accounted for by assuming that the reaction site in 12-6-12,2Br- micelles is less polar than that in 12-2-12,2Br- micelles. However, chemical trapping results in aqueous 12-s-12,2Br- micellar solutions are consistent with the interfacial region of the more packed 12-2-12,2Brmicelles being less polar than that of the 12-6-12,2Brmicelles.23 A possible explanation, similar to that proposed for the ET values listed in Table 4, is that the organic substrate
molecules are localized further away from the micellar interior in 12-2-12,2Br- micelles than in 12-6-12,2Br2 micelles. Consequently, Km as well as km would be expected to be higher in 12-6-12,2Br micellar solutions than in 12-2-12,2Br- micellar solutions. To get information about the dimeric surfactants ability as catalysts on the reaction studied, k2m values should be estimated from k2m and Vm values. From the work of Wetting et al.,64 values of 0.56 and 0.63 mol dm-3 for Vm corresponding to 12-2-12,2Br- and 12-6-12,2Br- spherical micelles were considered. Taking these Vm values into account and the k2m values listed in Table 5, one obtains 3.9 × 10-3 and 20.2 × 10-3 mol-1 dm3 s-1 for 12-2-12,2Br- and 12-6-12,2Br-, respectively, in pure water. For DTAB micelles a value of 1.2 × 10-3 mol-1 dm3 s-1 was obtained.13 These values have to be compared to that obtained in water and equal to 4.5 × 10-4
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mol-1 dm3 s-1. The above k2m values show that the reaction is faster in dimeric and DTAB micelles than in water. Previous kinetic studies have also shown the potential of gemini micelles as catalysts for various bimolecular chemical reactions.23,65,66 The main factors involved in the catalysis of the reaction investigated in this work by the cationic micellar aggregates would be the electrophilic interactions of the ammonium head groups and the forming nitrobenzenesulfonate ion and disruption of the hydration shell of the bromide ion.67 Conclusions In this work, effects of the EG addition on the micellization and on the micellar growth in two aqueous didodecyl dicationic dibromide surfactants, 12-s-12,2Br- (s ) 2,6) solutions, with the weight percentage of EG up to 50%, have been investigated. From the experimental results the following conclusions can be drawn: (1) An increase in the amount of ethylene glycol results in an increase in the critical micelle concentration. This is due to the water-EG mixtures being better solvents for the surfactant molecules than pure water, which leads to the transfer of the surfactant tail from the bulk phase into the micellar core and that of the alkyl chains in the head groups and in the spacer from the bulk phase into the micellar surface (or deeper if the chains are sufficiently long) to be less favorable when the amount of ethylene glycol in the mixture increases. Since the micellar ionization degree is nearly independent of EG content, the diminution in the Gibbs energy of micellization is controlled by changes in the cmc upon varying EG wt %. (2) An increment in the 12-s-12,2Br- concentration where the sphere-to-rod transition occurs, C*, is observed when EG content in the bulk phase increases. The amount of the organic solvent influences C* principally through the decrease in the hydrocarbon/bulk phase interfacial tension (air/bulk phase surface tension) caused by the presence of the organic solvent. (3) Kinetic studies, together with fluorescence, rheology, micropolarity, and microviscosity investigations, can provide useful information about the occurrence of morphological transitions in dimeric cationic micellar solutions. (4) Dimeric cationic 12-s-12,2Br- micelles are better catalysts than water for the reaction between methyl 4-nitrobenzenesulfonate and bromide ions. Acknowledgment. This work was financed by the DGCYT (grant BQU2002-00597) and Consejerı´a de Innovacio´n, Ciencia y Empresa de la Junta de Andalucı´a (FQM-274, P07-FQM03056). References and Notes (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Degiorgio, V. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M. Eds.; North-Holland: 1985; p 303. (3) See, for instance: Structure and Flow in Surfactant Solutions; Herb, C., Prud‘homme, R. Eds.; ACS Symp. Ser. No. 578; American Chemical Society: Washington, DC, 1994. (4) (a) Bernheim-Groswasser, A.; Zana, R.; Talmon, T. J. Phys. Chem. B 2000, 104, 4005. (b) Lu, T.; Huang, J.; Li, Z.; Jia, S.; Fu, H. J. Phys. Chem. B 2008, 112, 2909. (5) (a) Lang, J.; Zana, R. J. Phys. Chem. 1986, 90, 5258. (b) Lang, J.; Zana, R.; Candau, S. J. Annuli Chim. 1987, 77, 103. (c) Kahlweit, M. Pure Appl. Chem. 1981, 53, 2060. (d) Cui, X.; Yiang, X.; Liu, A.; Mao, S.; Liu, M.; Yuan, H.; Luo, P. ; J. Phys. Chem. B 2008, 112, 2874. (e) Yang, X.Y.; Chen, H.; Cheng, G.-Z.; Mao, S.-Z.; Liu, M.-L.; Luo, P.-Y.; Du, Y.-R. Colloid Polym. Sci. 2008, 286, 639. (6) Hoffmann, H.; U lbricht, W J. Colloid Interface Sci. 1989, 129, 388.
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