Article pubs.acs.org/Langmuir
Imidazolium-Based Zwitterionic Surfactants: Characterization of Normal and Reverse Micelles and Stabilization of Nanoparticles Franciane D. Souza, Bruno S. Souza, Daniel W. Tondo, Elder C. Leopoldino, Haidi D. Fiedler, and Faruk Nome* Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis, Santa Catarina 88040-900, Brazil S Supporting Information *
ABSTRACT: This paper presents the physicochemical properties of micellar aggregates formed from a series of zwitterionic surfactants of the type 3-(1-alkyl-3-imidazolio)propane-sulfonate (ImS3-n), with n = 10, 12, 14, and 16. The ImS3-n dipolar ionic surfactants represent a versatile class of dipolar ionic compounds, which form normal and reverse micelles. Furthermore, they are able to stabilize nanoparticles in water and in organic media. Aqueous solubility is too low at room temperature to allow characterization of micellar aggregates but increases with addition of salts, allowing determination of aggregation number and cmc. As expected, these parameters depend on the length of the alkyl chain, and cmc values follow Klevens equation. In the presence of NaClO4, all ImS3-n micelles become anionoid by incorporating ClO4− on the micellar interface. A special feature of these surfactants is the ability to form reverse micelles and solubilize copious amounts of saline solutions in chloroform. 1H NMR and infrared spectroscopic evidence showed that the maximum water to surfactant molar ratio w0 achievable depends on the concentration and type of salt dissolved. Reverse micelles of the ImS3-n surfactants can be used to stabilize metallic nanoparticles, whose size may be tuned by the amount of water dissolved.
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also anion dependent,1,3,13,14 indicating that ion-specific incorporation by zwitterionic surfaces is relevant to understanding the behavior of biological membranes.1,3,13−15 The same phenomenon was also observed in micelles of 3(1-tetradecyl-3-imidazolio)propanesulfonate, ImS3-14, a zwitterionic surfactant which differs from sulfobetaines by the replacement of the ammonium by an imidazolium group.16 This surfactant allows preparation of palladium17 and gold18 nanoparticles in chloroform solutions containing small amounts of water, probably by a template effect caused by reverse micelles. Usually the shapes and sizes of reverse micelles are dependent on the water to surfactant ratio, which is defined in eq 1.19,20 The amount of water that can be solubilized is influenced by the structure of the surfactant, the nature of the solvent, and the presence of cosurfactants and dissolved salts.19,21 The properties of water in reverse micelles are strongly dependent on the water-to-surfactant concentration ratio (w0). In fact, at low w0, water molecules interact mostly with the surfactant polar head and counterions (or added salts), consistent with an organization similar to that of water present in hydrated salts or near the interface of biological membranes. As w0 increases, full hydration of the surfactant head is achieved
INTRODUCTION Zwitterionic surfactants are far less investigated than neutral and charged surfactants, despite the fact that they are generally mild and skin friendly, ideal to be used in the personal care industry. They resemble nonionic surfactants in that the corresponding micelles have no net charge. However, unlike nonionic micelles, in aqueous salt solutions zwitterionic micelles preferentially incorporate anions and become negatively charged, thereby interacting with cations under suitable conditions. This interesting behavior suggests that addition of salts modulate micellar properties, which in turn can be exploited to speed the rate of acid/base-catalyzed reactions.1−5 Methods such as NMR spectroscopy, conductivity, diazo trapping, kinetics, and potentiometry using ion-selective electrodes establish micellar preferences for anions.4,6 Strong evidence is also given by capillary electrophoresis which indicates that these zwitterionic micelles incorporate anions and become anionoid.7−12 This interaction is anion dependent and follows the Hofmeister series, and anions such as ClO4− bind much more strongly than small, strongly hydrated anions such as OH− and F−. Notably, for sulfobetaines this specific anion binding process correlates well with the hydration free energies of the anions.2−4 Once the anions have been incorporated on the zwitterionic micelles, H3O+ is attracted to the micellar surface, promoting acid-catalyzed reactions and protonation of hydrophobic organic bases. This phenomenon is © XXXX American Chemical Society
Received: December 10, 2014 Revised: March 3, 2015
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solubility of ImS3-16 could not be determined under the experimental conditions. The solubility of the ImS3-n series is considerably lower than that of the analogous SB3-n surfactants. For instance, a 0.27 M solution of SB3-14 is completely homogeneous at T > 0 °C. Clearly the ImS3-n surfactants show contrasting behavior since phosphocholines and sulfo- and carboxybetaines are very soluble in water.29 The lower solubility of the ImS3-n family has been explained in terms of the strong interactions between the sulfo and imidazolium groups in the crystal lattice.16,30 In aqueous solutions, addition of salts weakens intermolecular forces and increases the aqueous solubility of the ImS3-n surfactants, allowing measurements of surface tension at 25 °C (Figure 2A and Table 1). Surfactants with n = 10 to 14 are adequately water-soluble in 0.08 M NaCl, while ImS3-16 requires 0.12 M NaCl. The cmc values, measured of necessity in aqueous electrolyte, were found to be closely similar to those of other sulfobetaine surfactants: the log(cmc) values for the different ImS3-n surfactants plotted against the number of carbon atoms in the hydrophobic tail are shown in Figure 2B, where the data for the structurally related N-alkyl-N,N-dimethylammonio-1propanesulfonate (SB3-n) sulfobetaines have been included for comparison purposes. As expected, the cmc decreases logarithmically with the number of carbon atoms in the linear alkyl chain, nC, following the well-known Klevens equation15,32 (eq 2)
and additional water will resemble more bulk water, with reestablishment of the well-known three-dimensional structure of bulk water.20,22 Thus, water in reverse micelles will have properties ranging from those expected for bulk water (at high w0) to water molecules with restricted translational and rotational mobility, high viscosity, low polarity, fewer hydrogen bonds, and lower dielectric constant, at low w0.23−25 w0 =
[H 2O] [surfactant]
(1)
The size of reverse micelles can be easily modulated by changing w0, thus allowing its use as a nanoreactor for the synthesis of metal nanoparticles with uniform size, shape, and composition. Since the nanoparticles are stabilized with a monolayer of surfactant, their size is limited to that of the reverse micelle.26−28 Conversely, an increase of nanoparticle size can be achieved by increasing w0.28 Sodium bis(2ethylhexyl)sulfosuccinate (AOT), an anionic surfactant whose main advantage is that it does not need a cosurfactant, is the most commonly used surfactant in reverse micelle and reverse micelle-based nanoparticle synthesis studies.28 We herein report changes in properties such as cmc, aggregation number, aqueous solubility, and anion-binding ability, observed for the homologous ImS3-n series (n = 10, 12, 14, and 16 carbons in the hydrocarbon chain). More importantly, we fully characterize the formation of reverse micelles of the ImS3-n zwitterionic surfactants series in CHCl3 without the need for a cosurfactant, and we show how to use this surfactant system as nanoreactors for the synthesis and stabilization of palladium nanoparticles of controlled size.
log(cmc) = A − Bnc
(2)
In general, in Klevens equation, A values vary significantly and depend on the charge and nature of the headgroup and counterion. Conversely, B values are normally close to 0.3 for cationic and anionic surfactants, and close to 0.5 for nonionic and dipolar ionic surfactants.15 The results given in Figure 2B show that the Klevens relation holds for the ImS3-n and SB3-n sulfobetaine families, with correlation coefficients >0.999 in both cases. Closely related values of A (2.80 ± 0.08 for the ImS3-n series and 2.84 ± 0.11 for the SB3-n derivatives) and of B (0.48 ± 0.01 and 0.46 ± 0.01 for the ImS3-n and SB3-n series, respectively) were obtained. The experimental values of B are in the range expected for dipolar ionic surfactants and are very close to those reported for long chain carboxybetaines and amine oxide type surfactants.29 The aggregation numbers (Nagg) were determined by fluorescence quenching of pyrene in micellar media as shown in Figure 3. The experimental fluorescence intensity ratio vs quencher concentration follow eq 3:33−36
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RESULTS AND DISCUSSION Critical Micelle Concentration, Aggregation Number, and Solubility. Solubility and Krafft temperatures (KT) of surfactants depend on the alkyl chain length and the nature of the headgroups and counterions. For zwitterionic surfactants, the nature of the headgroup is very important and, interestingly, replacement of ammonium by an imidazolium group markedly decreases aqueous solubility.16 Figure 1 shows maximum solubilities as a function of temperature in pure water for ImS3-10, ImS3-12, and ImS3-14. While ImS3-14 surfactant shows a break at 56 °C, surfactants with shorter chain lengths show higher solubility at lower temperatures. Furthermore,
[Q ]Nagg ⎛I ⎞ ln⎜ 0 ⎟ = ⎝ I ⎠ [ImS3‐n] − cmc
(3)
where Q represents the stoichiometric concentration of the DPC quencher. As expected, the I0/I ratio increases with increasing concentration of DPC, and the values of Nagg are included in Table 1. The increase of Nagg with the increase in hydrophobic chain length follows the trend expected for zwitterionic surfactants, considering that Nagg values less than 100 are indicative of the formation of spherical micelles. Molecular lengths, estimated by DFT calculations, are also given in Table 1 for all ImS3-n surfactants, and we take this value as Rm for the corresponding micelle. A full description of
Figure 1. Temperature dependence of the solubilities of the ImS3-10 (★), ImS3-12 (●), and ImS3-14 (▲) surfactants. Data for ImS3-14 taken from ref 16. B
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Figure 2. (A) Surface tension vs log [ImS3-10] (■), [ImS3-12] (▲), [ImS3-14] (●) in 80 mM NaCl and [ImS3-16] (★) in 120 mM NaCl. Data for ImS3-14 taken from ref 16. Temperature 25 °C. (B) Dependence of the cmc on the number of carbon atoms in the hydrophobic tail group for the (●) ImS3-n and (■) SB3-n surfactants. Data for SB3-n taken from ref 31.
Table 1. Properties of the 3-(1-Alkyl-3imidazolio)propanesulfonate Surfactants surfactant ImS310a ImS312a ImS314a ImS316b a
γcmc (mN·m−1)
Nagg
molecular lengthc (Å)
0.03 ×
40.7 ± 0.7
45 ± 3
20.80
0.03 ×
38.8 ± 0.8
57 ± 2
23.55
0.03 ×
36.3 ± 0.7
65 ± 5
26.28
0.03 ×
33.4 ± 0.8
67 ± 5
28.07
cmc (M) 9.44 ± 10−3 9.06 ± 10−4 1.02 ± 10−4 1.15 ± 10−5
In 80 mM NaCl. bIn 120 mM. cEstimated by DFT calculations. Figure 4. Effect of added NaClO4 on the ζ-potential of (★) 0.1 M ImS3-10, (●) 0.01 M ImS3-12, and (▲) 0.01 M ImS3-14 micelles at 25.0 °C, 80 mM NaCl. Data for ImS3-14 taken from ref 16.
perchlorate incorporation was not achieved under the experimental conditions, whereas ImS3-16 solutions in NaClO4 are excessively viscous precluding electrophoretic measurements. The increase in ζ-potential is related to the incorporation of ClO4− anion at the micellar surface, which is higher than incorporation of Cl− since ClO4− is less hydrated than Cl−. This important property of zwitterionic micelles has been discussed in detail, and the effect observed with the ImS3-n surfactants closely follows the behavior observed for the SB3-n sulfobetaine micelles.13 Formation and Characterization of Reverse Micelles. The most frequently used solvents for the formation of reverse micelles are cyclohexane, n-hexane, and isooctane. However, ImS3-14 surfactant is not soluble in these solvents but is very soluble in chloroform. Addition of small aliquots of water to a chloroform solution of ImS3-14 resulted in a turbid mixture. On the other hand, addition of saline solutions (NaCl or NaClO4) leads to transparent mixtures, presumably due to formation of reverse micelles. The amount of saline solution that can be added to the reverse micelles depends on the concentration and type of salt added as shown in Figure 5A. As can be seen, increasing NaCl and NaClO4 concentrations allow an increase in w0 up to a maximum value, above which the solubility begins to decrease. Importantly, the increase in solubility promoted by NaClO4 is significantly larger than that observed from addition of NaCl, and the maximum w0, in the presence of NaClO4, is about 66%
Figure 3. Fluorescence intensity of pyrene (2 × 10−6 M) as a function of [DPC] in aqueous solution containing: (★) 0.01 M ImS3-10; (●) 0.01 M ImS3-12; (▲) 0.01 M ImS3-14, and (■) 0.001 M ImS3-16. The ImS3-16 solutions contain 120 mM NaCl, whereas for other surfactants, [NaCl] = 80 mM. Data for ImS3-14 taken from ref 16.
the theoretical calculation for the Im3-14 surfactant has been published,16 and the results for all the ImS3-n surfactants are similar in all respects, showing that the positive charge is not localized on the nitrogens but distributed over the H atoms of the imidazolium ring.16 Figure 4 shows the variation of micellar ζ-potentials as a function of [NaClO4] added to ImS3-n solutions in 80 mM NaCl. As can be seen, addition of NaClO4 increases the negative ζ-potential, with the maximum value depending on alkyl chain length. For ImS3-14, the maximum value, calculated by fitting the data to a Langmuir isotherm,1−3 is −70.1 mV, while for ImS3-12, it is −59.8 mV. The maximum ζ-potential for ImS3-10 could not be determined accurately since full C
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Figure 5. (A) Variation of maximum solubility of aqueous solutions of (●) NaClO4 and (□) NaCl as a function of salt concentration in solutions containing 0.05 M ImS3-14 in CHCl3 (lines drawn to guide the eye). (B) Cartoon model of NaClO4 interaction with ImS3-14 reverse micelles.
larger and reached at lower salt concentration for NaClO4. This is a convincing result which shows that the added salts are responsible for reducing electrostatic repulsion between ionic groups of the ImS3-14 surfactant and for making the interfacial film more rigid and stable, increasing the capacity to solubilize water pools in reverse micelles. The larger effect observed upon addition of the perchlorate anion is found to be parallel for the incorporation of anions in the aqueous micelles of SB3-14, where larger and less hydrated anions bind better to the micellar interface, promoting an increase in ζ-potential larger than that of hydrophilic, strongly hydrated anions.13 Similarly, the larger value of maximum w0 obtained for NaClO4 is consistent with a stronger interaction with the imidazolium moiety (Figure 5B), with free perchlorate anion distributed in the interior of the reverse micelle.37,38 Thus, as observed for normal micelles in water, the effect is also anion dependent. A more hydrophilic anion with a larger enthalpy of hydration, such as Cl−, favors solubilization in the water pool and contributes less to the interfacial region. After saturation of the interfacial film, any additional anion will promote thinning of the electric double layer formed in the interior of the reverse micelle and probably the micellar interface becomes very rigid, decreasing the capacity to solubilize water. DFT calculations using the PCM solvent model in chloroform were also employed to explain the inability to form reverse micelles in pure water. In CHCl3 we examined the relative energy of two conformers, labeled bent and ion-pair conformers. The most important difference between these conformers is that the latter shows an intramolecular hydrogen bond interaction among the sulfonate and imidazolium groups with an O···H interatomic distance of only 2.06 Å as shown in Figure 6. The results indicate that in CHCl3 the ion-pair conformer is 3.3 kcal mol−1 more favorable than the bent one, probably due to charge stabilization provided by the sulfonate− imidazolium electrostatic interaction, which occurs only in the ion-pair conformer. In this conformation the surfactant headgroup is less prone to interact with water, which explains why ImS3-14 in chloroform is unable to dissolve pure water. However, when salt is added, the ions interact with the charged groups, breaking the intramolecular interaction between the imidazolium and sulfonate groups and allowing formation of reverse micelles. The changes in chemical shift of the NMR signals of water and of the imidazolium hydrogens, shown in Figure 7, reflect the microenvironment of water and of the imidazolium group
Figure 6. Bent and ion-pair conformers of ImS3-14 optimized at the PCM/B3LYP/6-31+G(d,p) level.
as a function of added water. As can be seen in Figure 7A, at small w0, the 1H chemical shift of water is at about 2.7 ppm. This value is higher than the chemical shift of water in CDCl3 (1.56 ppm) and considerably lower than that of bulk water (4.79 ppm),39 indicating that the added water is involved in hydration of the surfactant headgroup and added ions. The water 1H chemical shift is not affected by salt type (Figure 7A), suggesting that the predominant interactions are between water molecules and the surfactant headgroup.40,41 On increasing the water concentration, the water signal is displaced to higher field, reaching chemical shifts closer to that observed for bulk water, presumably because a large proportion of water molecules are not interacting with the surfactant sulfonate group. It is important to note that the decrease in water chemical shift at lower w0 is a result of the strong interaction of water with the surfactant sulfonate group. Correspondingly, the chemical shifts of the imidazolium hydrogens are also significantly affected by addition of NaClO4 solution as shown in Figure 7B. Increasing the water content from zero to w0 = 10 promotes the major changes in chemical shift of the three 1H signals, indicating that addition of water modifies the micellar interface due to interaction with the polar headgroup of the ImS3-14 surfactant, effectively changing the environment around the imidazolium ring. For w0 > 10, there is less change in the imidazolium shifts, indicating that additional water increases the size of the water pool.41 The H1 downfield shift with decreasing w0 is in agreement with an increase of acid character of H1. On the other hand, signals corresponding to H2 and H3 show the opposite behavior, suggesting that ion-pair conformation becomes more important on decreasing the microemulsion water content. D
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Figure 7. (A) Variation of the 1HNMR water chemical shift as a function of w0 in 0.05 M ImS3-14 solution in CDCl3 due to addition of (●) 0.20 M NaClO4 or (■) 0.30 M NaCl solutions. (B) Changes in 1H NMR imidazolium chemical shifts as a function of 0.2 M NaClO4 added: (■) H1; (▲) H2; (●) H3.
to prepare gold nanoparticles with an average size of 4.3 ± 0.8 nm by mixing a reverse micelle solution containing 0.05 M ImS3-14, 0.20 M water, and 0.2 M AuCl3 in chloroform with a solution containing 0.05 M ImS3-14, 0.20 M water, and 2.0 M NaBH4, also in chloroform.18 By using a similar procedure, but with K2PdCl4 as the metal precursor and 2.0 M ascorbic acid as the reductant, 4.2 ± 0.9 nm Pd nanoparticles were also produced at w0 = 4.8.17 Another advantage to the synthesis of nanoparticles in reverse micelles is the possibility of controlling nanoparticle size by modifying the amount of water in the reverse micelle. Thus, we repeated the Pd nanoparticle synthesis following the procedure above but with w0 = 14.7 in order to produce larger nanoparticles. The transmission electron microscopy micrograph and size distribution of the nanoparticles prepared with w0 = 4.8 and 14.7 are shown in Figure 9. As can be seen, the nanoparticles are well dispersed on the grid based on TEM image analysis of ∼300 particles and showed average sizes of 4.2 ± 1.0 nm (w0 = 4.8) and 5.8 ± 2.0 nm (w0 = 14.7). Thus, increasing the size of the ImS3-14 micellar core increases the mean particle diameter which also leads to an increase in the polydispersity. Nevertheless, it should be noted that the particle’s shape remains almost unchanged and most are nearly spherical with more than 90% showing an aspect ratio in the range of 1.0 to 1.4 as given in Supporting Information. Additionally, it is important to mention that after chloroform evaporation, it is possible to redisperse the nanoparticles in chloroform and even into water solutions without signs of particle aggregation. Therefore, it is reasonable to assume that the colloidal stabilization mechanism is probably due to the presence of polar and apolar domains of the ImS3-14 surfactant, with the polar headgroup near the nanoparticle surface, while the alkyl chains are directed to the bulk solvent, hindering coalescence.
Similar effects on the water and imidazolium 1H chemical shifts are observed upon addition of 0.30 M NaCl or 0.35 M NaClO4 (Figures S13 and S14 in Supporting Information). Changing the size of the hydrophobic tail does not alter the observed results, and the behavior for ImS3-12 surfactant was identical to that described above for ImS3-14 (see Figure S15 in Supporting Information). Additional evidence was obtained by following the 13C signal of a solution containing 0.12 M ImS314 surfactant, where addition of an aqueous solution containing 0.2 M NaClO4 to obtain w0 = 16.8 promotes a significant change in the chemical shift of the carbon located between ring nitrogens, from 137.1 to 135.9 ppm (13C NMR spectra are given in Figure S16 of Supporting Information). Infrared absorption changes in the 3100 cm−1 to 3800 cm−1 range as a function of w0 provide additional details about water structure in the reverse micelles. Figure 8A shows representative spectra, obtained by addition of aqueous 0.2 M NaClO4 to 0.05 M ImS3-14. As can be seen, addition of salt solution leads to an increase in the infrared absorption in the region corresponding to the OH stretching and HOH bending modes of liquid water. From w0 = 2.2 to 12.0, the spectrum can be conveniently fitted by three Lorentzian functions at about 3690, 3606, and 3460 cm−1, whereas for w0 > 12.0, an additional peak, centered at 3244 cm−1, appears as shown in Figure 8B. Figure 8C shows the variation in the four peak areas as a function of w0. As can be seen, peaks at 3690 and 3606 cm−1 are slightly increased over the whole w0 range. Conversely, the peak at 3460 cm−1 suffers a rapid increase at w0 > 10 whereas the peak at 3244 cm−1 appears only at w0 > 13. These peaks can be ascribed to four different states of the water molecules. The peak at 3690 cm−1 can be attributed to water molecules in the interfacial region, which interact with the CHCl3 organic phase.42 Conversely, the peak at 3606 cm−1 can be assigned to water bound to the imidazolium and sulfo group of ImS3-14, as already observed for the sulfo group of AOT reverse micelles in the isooctane/water system. Finally, the peak at 3460 cm−1 can be related to that of water residing between the hydrated headgroup and the water pool, which appears at about 3244 cm−1. Synthesis of Metallic Nanoparticles in Reverse Micelles. Reverse micelles can be employed as nanoreactors for the synthesis of metallic nanoparticles due to the restricted environment offered by such system. In fact, ImS3-14 was used
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CONCLUSIONS The ImS3-n zwitterionic surfactants represent a versatile class of dipolar ionic compounds which form normal and reverse micelles without the need for a cosurfactant. Furthermore, they are able to stabilize nanoparticles in water and in organic media. At room temperature and in pure water the solubility of ImS3-n surfactants is very low, but in salt solutions solubility increases, allowing the characterization of ImS3-n micelles at room temperature. Micelle formation follows the Klevens rule; E
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ImS3-n surfactants can solubilize aqueous salt solutions, and the maximum w0 values obtained depend on the type and concentration of salt added. In fact, the presence of the imidazolium group allows formation of a more rigid and stable interfacial film, increasing the capacity to solubilize water. Importantly, formation of reverse micelles allows the synthesis of metallic nanoparticles with narrow size distribution and controllable diameter. Future studies intend to evaluate the catalytic activities of these nanoparticles in coupling and reduction reactions.
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EXPERIMENTAL SECTION
Synthesis of ImS3-n Surfactants. The synthesis of the imidazolium-based surfactants was performed by adaptation of a literature procedure16 and consists of the preparation of alkyl imidazoles followed by alkylation using 1,3-propane sultone. Synthesis of Alkyl Imidazoles. A solution of imidazole (16.3 g, 0.24 mol) in dry 1,4-dioxane (100 mL) was added to 150 mL of a suspension of oil-free sodium hydride (5.8 g, 0.24 mol) with stirring for 2 h at 90 °C. A solution of n-decyl (or the corresponding ndodecyl, n-tetradecyl, or n-hexadecyl) bromide (0.12 mol) in 1,4dioxane (100 mL) was then added dropwise to the reaction solution, and the mixture was stirred for 48 h at 90 °C. The solvent was evaporated in vacuo, giving a yellow residue that was dissolved in 200 mL of CH2Cl2 and washed three times with 50 mL of water. After removal of CH2Cl2, the oily residue was purified by column chromatography (silica gel) using n-hexane/ethyl acetate as eluent and giving the alkyl imidazoles as colorless oils in high yield (>90%). The 1H NMR spectra of the alkyl imidazoles are similar to that reported in the literature for n-dodecylimidazole,43 the only difference being the signal at 1.25 ppm which corresponds to 16, 18, 20, and 22 H for n-decyl, n-dodecyl, n-tetradecyl, and n-hexadecylimidazole, respectively. The NMR spectra are given in Supporting Information. Synthesis of ImS3-n Surfactants. A solution of 1,3-propane sultone (9.0 g, 0.074 mol) in acetone (80 mL) was slowly added to a round-bottom flask containing the corresponding 1-alkyl imidazole (0.067 mol) in acetone (80 mL) at 0 °C. The reaction mixture was then warmed to room temperature and stirred for 5 days, affording a white solid. Filtration gave a white powder that was washed four times with fresh acetone, filtered, and dried under vacuum, giving the zwitterionic surfactants in high yield (>85%). All compounds showed similar 1H NMR spectra, the only difference being the signal at 1.25 ppm which corresponds to 16, 18, 20, and 22 H for ImS3-10, ImS3-12, ImS3-14, and ImS3-16, respectively. The NMR and mass spectra are given in Supporting Information. Solubilities of ImS3-n. The solubilities of ImS3-n surfactants were determined by measuring solution turbidity caused by precipitation of surfactants in water. ImS3-n solutions were heated slowly until complete dissolution in a quartz water-jacketed cell with PTFE stopper. The temperature was kept constant for 1 h and then decreased gradually (0.1 °C min−1) using a thermostatic bath with a water circulator having a temperature control of ±0.1 °C. The solubility temperatures were taken by the drop in the light transmission at 450 nm versus temperature graph. Surface Tension Measurements. Surface tensions were measured by the du Noüy ring method on a Kruss K-20 EasyDyne tensiometer, at 25 °C. Before each measurement, the ring was briefly heated with a Bunsen burner until glowing. The vessel was cleaned with chromic-sulfuric acid and boiling distilled water and then flamed in a Bunsen burner. Surface tension measurements were repeated at least five times with 0.1 mN m−1 resolution. Fluorescence Measurements. Fluorescence quenching of pyrene by DPC was monitored in a Varian Cary Eclipse spectrofluorimeter. The low probe concentration (2 × 10−6 M) avoided excimer formation, and the maximum quencher concentration was 1.6 × 10−4 M. The [pyrene]/[micelle] and [quencher]/[micelle] ratios were low enough to ensure Poisson distributions. An excitation wavelength of 337 nm and an emission wavelength of 394 nm were used.
Figure 8. (A) Infrared absorption spectra of the reverse micelles formed by addition of 0.2 M NaClO4 to 0.05 M ImS3-14 in CHCl3. Bottom to top w0 = 2.2; 6.6; 11.0; 15.4; 19.8 and 22.1. Prior to analysis, the contributions of the pure ImS3-14 in CHCl3 spectrum was subtracted from all spectra. (B) Fitted curves for two representative w0. The original spectra are given in red, and the stars represent the sum of the Lorentz functions. (C) Variation in the area of the infrared absorption peaks given in part B as a function of w0: (▲) 3244 cm−1; (■) 3460 cm−1; (●) 3606 cm−1; (★) 3690 cm−1 (lines drawn to guide the eye).
therefore, cmc and aggregation numbers depend on the size of the alkyl chain, and the values obtained are indicative of formation of spherical micelles. Addition of NaClO4 to ImS3-n solutions changes the ζ-potential for ImS3-10, ImS3-12, and ImS3-14, indicating that ClO4− anion interacts strongly with the micellar interface. Analysis of these data allows comparison with SB3-n sulfobetaines, illustrates the importance of headgroup structure on aggregation behavior, and shows the extent to which the hydrocarbon chain affects ImS3-n surfactant properties. It is of fundamental importance to note that the ImS3-n surfactants, in contrast to sulfobetaines, are able to form reverse micelles in chloroform without the presence of a cosurfactant. F
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Figure 9. Transmission electron microscopy micrographs at 100 kV and size distribution of the Pd nanoparticles stabilized by ImS3-14 reverse micelles in chloroform prepared at w0 = 4.8 (top) and 14.7 (bottom). Additional images are given in Supporting Information. Variation of 1H NMR Signals of Water and Surfactant in Reverse Micelles. The changes in the chemical shifts of the water and aromatic protons of ImS3-12 and ImS3-14 were followed by 1H NMR in a Bruker 200 MHz spectrometer. A 0.05 M solution of ImS314 (or ImS3-12) was prepared in CDCl3 and several aliquots of a salt solution, prepared in D2O, were added, followed by NMR scan using typical parameters. Infrared Spectroscopy of Water in Reverse Micelles. The changes in the absorption bands of water of a 0.05 M ImS3-14 surfactant solution in CHCl3 were quantified by infrared spectroscopy in a Varian Excalibur 3100 FTIR using a demountable liquid cell with KBr window following spectral changes due to addition of small aliquots of NaClO4 water solution. Nanoparticle Synthesis in Reverse Micelles. The palladium nanoparticles were prepared by reduction of Pd2+ ions solubilized in reverse micelles. Briefly, two stocks solutions in 0.05 M ImS3-14 in CHCl3 were prepared containing equal volumes of aqueous stock solutions of 0.2 M K2PdCl4 or 2.0 M ascorbic acid, giving the required w0, followed by a rapid mixing. The metallic nanoparticles were characterized by transmission electronic microscopy (TEM), and the sample for TEM analysis was prepared by deposition of the chloroform nanoparticle dispersion on a carbon-coated copper grid. TEM analysis was performed with a JEOL JEM-1011 transmission electron microscope operating at 100 kV at LCME/UFSC (Florianópolis, Brazil). Computational Calculations. Density functional theory (DFT) calculations were performed at the B3LYP/6-31+G(d,p) level of theory using Gaussian 09 implemented in Linux operating systems. The water environment was simulated with the polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM)44 with radii and nonelectrostatic terms for Truhlar and coworkers’ SMD solvation model.45 The energy difference between the bent and ion-pair conformers corresponds to the Gibbs free energy.
Capillary Electrophoresis. Experiments were carried in an Agilent CE3D capillary electrophoresis system, with on-column diode-array detection at 25 °C, and electropherograms were monitored at 272 nm. Samples were introduced by hydrodynamic injection at 50 mbar/5 s. Fused-silica capillaries (Polymicro Technologies) of total length 60.0 cm, effective length 51.5 cm, and internal diameter 50 μm were used. The electrophoresis system was operated under normal polarity and constant 30 kV. The capillary was conditioned by flushes of 1 M NaOH (5 min), deionized water (5 min), and electrolyte solution (10 min). Between experiments, the capillary was reconditioned by a pressure flush with the electrolyte containing 3 mM sodium borate (2 min). Micellar mobility was monitored by following the migration of micellar-bound pyrene (1 μM), and acetone (0.1%) was used as the electroosmotic flow marker. The electrophoretic mobility of a zwitterionic micelle (μ, m2 V−1 s−1) is given by
μ=
Leff ⎛ 1 1 ⎞⎟ ⎜⎜ − E ⎝ tapp teo ⎟⎠
(4)
where E is the applied electric field strength (V m−1), Leff is the effective capillary length, and tapp and teo are migration times of the micelle and the electroosmotic flow, respectively. When the ζ-potential is not very high, the following Henry’s equation relates the ζ-potential of the micelle to its mobility2,16
ζm =
μη ε0εf (κR m)
(5)
where η is the viscosity of the medium, f(κRm) corresponds to Henry’s function, κ is the Debye−Hückel shielding parameter (m−1), Rm is the radius of a spherical zwitterionic micelle, and εo and ε correspond to vacuum permittivity and the relative permittivity of the solvent. Solubilitiy of Salt Solutions in Reverse Micelles of ImS3-14 Surfactant. The solubility of NaClO4 and NaCl salt solutions in reverse micelles was monitored by adding these salt solutions to a 0.05 M ImS3-14 solution in chloroform until a small increase in turbidity was detected by the increase in absorbance at 450 nm. The measurements were performed at 25 °C.
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ASSOCIATED CONTENT
S Supporting Information *
NMR, mass spectrometry and CHN results along with additional TEM images and aspect ratio graphs for the G
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(16) Tondo, D. W.; Leopoldino, E. C.; Souza, B. S.; Micke, G. A.; Costa, A. C.; Fiedler, H. D.; Bunton, C. A.; Nome, F. Synthesis of a New Zwitterionic Surfactant Containing an Imidazolium Ring. Evaluating the Chameleon-like Behavior of Zwitterionic Micelles. Langmuir 2010, 26, 15754−15760. (17) Zapp, E.; Souza, F. D.; Souza, B. S.; Nome, F.; Neves, A.; Vieira, I. C. A Bio-Inspired Sensor Based on Surfactant Film and Pd Nanoparticles. Analyst 2013, 138, 509−517. (18) Fernandes, S. C.; de Souza, F. D.; de Souza, B. S.; Nome, F.; Vieira, I. C. Gold Nanoparticles Dispersed in Zwitterionic Surfactant for Peroxidase Immobilization in Biosensor Construction. Sens. Actuators, B 2012, 173, 483−490. (19) Correa, N. M.; Silber, J. J.; Riter, R. E.; Levinger, N. E. Nonaqueous Polar Solvents in Reverse Micelle Systems. Chem. Rev. 2012, 112, 4569−4602. (20) Onori, G.; Santucci, A. IR Investigations of Water-Structure in Aerosol OT Reverse Micellar Aggregates. J. Phys. Chem. 1993, 97, 5430−5434. (21) De, T. K.; Maitra, A. Solution Behavior of Aerosol OT in Nonpolar-Solvents. Adv. Colloid Interface Sci. 1995, 59, 95−193. (22) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Interactions of Small Molecules with Reverse Micelles. Adv. Colloid Interface Sci. 1999, 82, 189−252. (23) Hasegawa, R.; Sugimura, T.; Suzaki, Y.; Shindo, Y.; Kitahara, A. Microviscosity in Water Pool of Aerosol-OT Reversed Micelle Determined with Viscosity-Sensitive Fluorescence Probe, AuramineO, and Fluorescence Depolarization of Xanthene Dyes. J. Phys. Chem. 1994, 98, 2120−2124. (24) Faeder, J.; Ladanyi, B. M. Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles. J. Phys. Chem. B 2000, 104, 1033−1046. (25) Sechler, T. D.; DelSole, E. M.; Deak, J. C. Measuring Properties of Interfacial and Bulk Water Regions in a Reverse Micelle with IR Spectroscopy: A Volumetric Analysis of the Inhomogeneously Broadened OH Band. J. Colloid Interface Sci. 2010, 346, 391−397. (26) Uskokovic, V.; Drofenik, M. Synthesis of Materials within Reverse Micelles. Surf. Rev. Lett. 2005, 12, 239−277. (27) Qi, L. Synthesis of Inorganic Nanostructures in Reverse Micelles. In Encyclopedia of Surface and Colloid Science, 2nd ed; Somasundaran, P., Ed.; Taylor & Francis Group: Cincinnati, 2006; Vol. 2, pp 6183−6207. (28) Lemyre, J. L.; Lamarre, S.; Beaupre, A.; Ritcey, A. M. A New Approach for the Characterization of Reverse Micellar Systems by Dynamic Light Scattering. Langmuir 2010, 26, 10524−10531. (29) Rosen, M. J. Surfactants and interfacial phenomena, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2004. (30) Reichert, W. M.; Trulove, P. C.; De Long, H. C. 3-(1-Methyl-3imidazolio)propanesulfonate: A Precursor to a Bronsted Acid Ionic Liquid. Acta Crystallogr. E 2010, 66, O591−U4799. (31) Farrukh, M. A.; Beber, R. C.; Priebe, J. P.; Satnami, M. L.; Micke, G. A.; Costa, A. C. O.; Fiedler, H. D.; Bunton, C. A.; Nome, F. Reactivity and Models for Anion Distribution: Specific Iodide Binding to Sulfobetaine Micelles. Langmuir 2008, 24, 12995−13000. (32) Klevens, H. B. Structure and Aggregation in Dilute Solutions of Surface Active Agents. J. Am. Oil Chem. Soc. 1953, 30, 74−80. (33) Tachiya, M. Application of a Generating Function to ReactionKinetics in Micelles - Kinetics of Quenching of Luminescent Probes in Micelles. Chem. Phys. Lett. 1975, 33, 289−292. (34) Infelta, P. P.; Gratzel, M. Statistics of Solubilizate Distribution and Its Application to Pyrene Fluorescence in Micellar Systems Concise Kinetic-Model. J. Chem. Phys. 1979, 70, 179−186. (35) Turro, N. J.; Yekta, A. Luminescent Probes for Detergent Solutions - Simple Procedure for Determination of Mean Aggregation Number of Micelles. J. Am. Chem. Soc. 1978, 100, 5951−5952. (36) Prieto, M. F. R.; Rodriguez, M. C. R.; Gonzalez, M. M.; Rodriguez, A. M. R.; Fernandez, J. C. M. Fluorescence Quenching in Microheterogeneous Media - a Laboratory Experiment Determining Micelle Aggregation Number. J. Chem. Educ. 1995, 72, 662−663.
palladium nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to INCT-Catálise, PRONEX, FAPESC, CNPq, and CAPES for their support of this work and to LME/LNLS and LCME/UFSC for technical support during electron microscopy work.
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