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Competitive Solubilization of Phenol by Cationic Surfactant Micelles in the Range of Low Additive and Surfactant Concentrations :: Radhouane Chaghi, Louis-Charles de Menorval, Clarence Charnay, Gaelle Derrien, and Jerzy Zajac* Institut Charles Gerhardt, Equipe Agr egats, Interfaces et Mat eriaux pour l’Energie, CNRS UMR 5253, Universit e Montpellier 2, C.C. 1502, Place Eug ene Bataillon, 34095 Montpellier cedex 5, France Received October 16, 2008. Revised Manuscript Received December 27, 2008 Competitive interactions of phenol (PhOH) with micellar aggregates of hexadecyltrimethylammonium bromide (HTAB) against 1-butanol (BuOH) in aqueous solutions at surfactant concentrations close to the critical micelle concentration (CMC), BuOH concentration of 0.5 mmol kg-1, and phenol contents of 1, 5, or 10 mmol kg-1 have been investigated at 303 K by means of 1H NMR spectroscopy, titration calorimetry, and solution conductimetry. The solubilization loci for phenol were deduced from the composition-dependence of the 1H chemical shifts assigned to various protons in the surfactant and additive units. Since in pure HTAB solutions phenol is already in competition with Br-, addition of 1 mmol kg-1 NaBr to the system weakens the phenol competitiveness. The presence of butanol in the HTAB micelles causes phenol to penetrate deeper toward the hydrophobic micelle core. For higher phenol contents, the butanol molecules are constrained to remain in the bulk solution and are progressively replaced within the HTAB micelles by the aromatic units. The competitive character of phenol solubilization against butanol is well supported by changes in the thermodynamic parameters of HTAB micellization in the presence of both of the additives.
Introduction The dissolution of linear or cyclic alcohols into water by the action of surfactant micelles has attracted considerable attention and interest over several decades (refs 1-15 and references therein). When added in low or moderate amounts, alcohol molecules can distribute between the micelles and the surrounding aqueous phase in a way that is dependent mainly on their hydrophobic-hydrophilic character. They appear to inhabit mostly the outer portions of the surfactant micelles (i.e., the polar mantle, micelle-solution interface). Moderate and long-chain alcohols are commonly thought to pack within mixed micelles oriented in the same manner as the surfactant units, with the polar group *Corresponding author. E-mail:
[email protected]. Telephone: 33467143255. Fax: 33467143304. (1) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1–64. (2) De Lisi, R.; Milioto, S. In Solubilisation in Surfactant Aggregates; Christian, S. D., Scamehorn, J. F., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1995; Vol. 55, p 59. (3) Eda, Y.; Takisawa, N.; Shirahama, K. Langmuir 1996, 12, 325–329. (4) Thimons, K. L.; Brazdil, L. C.; Harrison, D.; Fisch, M. R. J. Phys. Chem. B 1997, 101, 11087–11091. (5) Del Castillo, J. L.; Suarez-Filloy, M. J.; Castedo, A.; Svitova, T.; Rodriguez, J. R. J. Phys. Chem. B 1997, 101, 2782–2785. (6) Foerland, G. M.; Samseth, J.; Gjerde, M. I.; Hoeiland, H.; Jensen, A. O.; Mortensen, K. J. Colloid Interface Sci. 1998, 203, 328–334. (7) Villeneuve, M.; Ikeda, N.; Motomura, K.; Aratono, M. J. Colloid Interface Sci. 1998, 208, 388–398. (8) Suratkar, V.; Mahapatra, S. J. Colloid Interface Sci. 2000, 225, 32–38. (9) Gonzalez-Perez, A.; Czapkiewicz, J.; Del Castillo, J. L.; Rodriguez, J. R. J. Colloid Interface Sci. 2003, 262, 525–530. (10) Gonzalez-Perez, A.; Galan, J. J.; Rodrıguez, J. R. Fluid Phase Equilib. 2004, 224, 7–11. (11) Benalla, H.; Zajac, J. J. Colloid Interface Sci. 2004, 272, 253–261. (12) Abou, V.; Benalla, H.; Meziani, M. J.; Zajac, J. Prog. Colloid Polym. Sci. 2004, 126, 30–34. (13) Mata, J. P.; Aswal, V. K.; Hassan, P. A.; Bahadur, P. J. Colloid Interface Sci. 2006, 299, 910–915. (14) Mata, J. P.; Majhi, P. R.; Kubota, O.; Khanal, A.; Nakashima, K.; Bahadur, P. J. Colloid Interface Sci. 2008, 320, 275–282. (15) Chaghi, R.; de Menorval, L. C.; Charnay, C.; Derrien, G.; Zajac, J. J. Colloid Interface Sci. 2008, 326, 227–234.
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being located in the hydrated headgroup domain and the hydrophobic tail pointing toward the nonhydrated inner core. Nevertheless, the depth of additive penetration into the micelle structure may be subject to change in function of the overall alcohol content in the system and the degree of packing of the surfactant units in the host aggregate. Based on modeling studies of the system octanol/sodium octanoate/water, Aamodt et al. claimed that the first incorporated octanol molecule per micelle was located in the inner core due to the large decrease in surface area per molecule in the micelle.16 According to recent investigations made on the solubilization of phenol by cationic micelles of hexadecyltrimethylammonium bromide (HTAB) by means of titration calorimetry and 1H NMR,15 the aromatic units were located preferentially in the headgroup region of cationic micelles by an enthalpy-driven solubilization mechanism, but some additional molecules were simultaneously forced to penetrate deeper toward the hydrophobic micelle core. It should be emphasized that this “solubilization” behavior, observed for low HTAB concentrations in the vicinity of the critical micelle concentration (CMC) and additive contents much below their critical phase-separation concentrations, was at variance with the solely “interfacial” phenol location within the cationic micelles of HTAB formed in a concentrated surfactant solution (50 mM HTAB and 0.01 M NaBr).13 Even such a subtle factor as solvent isotope effect was shown to alter the solubilization mechanism for both cyclic and linear alcohol additives,15,17 which called for much caution when interpreting the results coming from different measurements carried out on apparently the same systems. The list of potential uses of micellar solubilization, eagerly cited in numerous papers, includes advanced drug delivery systems, microreactors for a variety of chemical, biocatalytic and enzymatic reactions, or templating units in the preparation of nanosize :: (16) Aamodt, M.; Landgren, M.; Jonsson, B. J. Phys. Chem. 1992, 96, 945–950. (17) Candau, S.; Hirsch, E.; Zana, R. J. Colloid Interface Sci. 1982, 88, 428–436.
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particles or ordered nanopore materials.18-23 However, it is not common to admit that in most of these applications the main solubilizate has to compete with other species present in the system for solubilization sites in the surfactant-based structures. Fundamental research on competitive solubilization of two solutes that exhibit low or reduced water miscibility are rare, and it is mostly focused on application aspects of the resulting hybrid structures.24-26 The intention of the present work is to fill that gap through a systematic study of the competitive aspect of micellar solubilization in a model system. The solubilization phenomenon was investigated at low surfactant concentrations close to the CMC and additive contents were much below their critical phase-separation concentrations. The main concern here was to restrict the consideration mostly to the net effect of additive-micelle and additive-additive interactions by precluding drastic changes in the micelle size and shape (e.g., sphere-to-rod transition). One of the goals was to shed light on fundamental aspects of the application of micellar solubilization in the one-step preparation and functionalization of advanced porous materials.23 A novel idea was to insert the precursor of the future reactive center into spherical micelles which would serve both as “soft porogens” to create nanoscopic pore structure and as precursor-delivery “capsules” for targeted functionalization of the emerging internal surface. In the present study, alcohol solubilization by cationic micelles of hexadecyltrimethylammonium bromide was considered thoroughly in context of competition between two additives, phenol and 1-butanol. Both alcohols are moderately soluble in water (phenol = 84 g L-1 and butanol = 75 g L-1 at 298 K27,28) and thus may be partitioned between micelles and bulk. These molecules have comparable dipole moments but their octanolwater partition coefficients (KOW) are somewhat different (i.e., log KOW = 1.46, phenol; log KOW = 0.88, butanol29). The impact of phenol concentration on the extent and mode of solubilization was quantified at fixed butanol content in the aqueous phase. Proton nuclear magnetic resonance (1H NMR) was used as the main experimental technique in measuring the solubilization loci of the two additives and their mutual competition. This technique had been successfully employed to study competitive interactions of various counterions with cationic micelles in the range of high additive and surfactant concentrations.30-33 Despite the small concentrations used in the present work, the complete NMR spectra of various phenol/butanol/HTAB/deuterated water systems were recorded with a high precision owing to the presence of (18) Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999. (19) Lettow, J. S.; Han, Y. J.; Schmidt-Winkel, P.; Yang, P.; Zhao, D.; Stucky, G. D.; Ying, J. Y. Langmuir 2000, 16, 8291–8295. :: (20) Abarkan, I.; Doussineau, T.; Smaihi, M. Polyhedron 2006, 25, 1763–1770. (21) Narang, A. S.; Delmarre, D.; Gao, D. Int. J. Pharm. 2007, 345, 9–25. :: (22) Stubenrauch, C.; Wielputz, T.; Sottmann, T.; Roychowdhury, C.; DiSalvo, F. J. Colloids Surf. A 2008, 317, 328–338. (23) Derrien, G.; Charnay, C.; Zajac, J.; Jones, D. J.; Roziere, J. Chem. Commun. 2008, 3118–3120. (24) Reekmans, S.; Luo, H.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 628–637. (25) Jian, X.; Ganzuo, L.; Zhiqiang, Z.; Guowei, Z.; Kejian, J. Colloids Surf. A 2001, 191, 269–278. (26) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303–5308. (27) Achard, C.; Jaoui, M.; Schwing, M.; Rogalski, M. J. Chem. Eng. Data 1996, 41, 504–507. (28) Petritis, V. E.; Geankoplis, C. J. J. Chem. Eng. Data 1959, 4, 197–198. (29) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525–616. (30) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699–705. (31) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7919–7927. (32) Vermuthen, M.; Stiles, P.; Bachofer, S. J.; Simonis, U. Langmuir 2002, 18, 1030–1042. (33) Onoda-Yamamuro, N.; Yamamuro, O.; Tanaka, N.; Nomura, H. J. Mol. Liq. 2005, 117, 139–145.
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two external references.15 Furthermore, the micellization of HTAB in mixed H2O solutions and changes in the micellization thermodynamics upon phenol and butanol addition were monitored with the aid of solution conductimetry and titration calorimetry methods. Although some differences in size may exist between micelles formed in H2O and D2O,34 the conclusions with respect to the mechanism of competitive solubilization by cationic micelles drawn from the NMR study were subsequently exploited to rationalize the thermodynamic data. In particular, calorimetry could offer the complementary approach at the macroscopic level.
Experimental Section Materials. Cationic surfactant, hexadecyltrimethylammonium bromide (HTAB), phenol (PhOH, >99% purity), deuterated water (D2O, 100.0 atom %), and deuterated benzene (C6D6, 99.9% purity) were obtained from Sigma-Aldrich (France), whereas butan-1-ol (BuOH, >99% purity) was a Carlo Erba (France) product. All these compounds were used without further purification. Sodium bromide (NaBr, >99% purity) was obtained from Baeckeroot Labo and dried by a thermal treatment under vacuum. Water (H2O) was deionized and purified with a Millipore Super Q system. The pH of all aqueous solutions measured by using a Multilab 540 instrument in conjunction with a pH electrode ranged between 5.6 and 6.2. Methods. All solutions were prepared gravimetrically on a molality basis. For example, the molality of the surfactant solute was designated mHTAB. Special precautions were made to keep the temperature of preparation at 303 K to avoid precipitation of the surfactant. It is worth noting that phenol addition shortened the time of surfactant dissolving in the 0.5 mmol kg-1 BuOH solution in D2O during solution preparation. When PhOH content was increased from 1 to 10 mmol kg-1, the time necessary to obtain a homogeneous and limpid solution changed from a few minutes to about 10 s. Proton NMR spectra were obtained at 400.13 MHz with a digital resolution of 0.06 Hz/data point using a Bruker DRX 400 NMR spectrometer. The 90° pulse length was typically 10 μs, and the relaxation time (t1) was 6 s. Chemical shifts (δ) were determined with an absolute uncertainty of Δδ = (0.002 ppm relative to an external reference of residual protons in fully deuterated benzene (δ = 7.1577 ppm) with an aliquot of tetramethylsilane (δ = 0.0000 ppm) contained in Wilmad coaxial insert capillaries (1 mm o.d.). The number of scans recorded was 128 for all systems studied. The NMR measurements were performed at 303 ((0.1) K. The total surfactant concentration was maintained constant at either 0.25 or 1.5 mmol kg-1. In mixed additive-HTAB solutions, butanol was added to a given surfactant solution in D2O to reach a molality of 0.5 mmol kg-1 while the ratio HTAB/phenol was varied. Typical 1H spectra and peak assignments for the individual system components (HTAB and BuOH) are shown in Figure 1. The specific conductivity of H2O solutions was measured at 303 ((0.1) K as a function of surfactant concentration at a fixed additive content using a Multilab 540 conductimeter equipped with a LR 325/01 electrode and a thermostatted cell. The operating procedures and further data processing, enabling one to obtain the CMC, the degree of counterion binding to the micelle ( β), and the standard Gibbs free energy of micellization per mole of HTAB (ΔmicG°), are detailed in the Supporting Information. The enthalpy changes upon HTAB micellization in the presence of BuOH and PhOH were determined in dilution calorimetry experiments. For this purpose, a homemade Montcal microcalorimeter was used to date at 303 ((0.1) K.15,35 The operating procedures and data treatment leading to the determination of the standard molar enthalpy of micellization per (34) Berr, S. S. J. Phys. Chem. 1987, 91, 4760–4765. (35) Zajac, J. Colloids Surf. A 2000, 167, 3–19.
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Figure 1. Condensed formula of (a) hexadecyltrimethylammonium ion (HTA+) and (b) butan-1-ol (BuOH), and 1H NMR spectra and peak assignments for the HTAB/D2O and BuOH/D2O systems at 303 K. mole of HTAB (ΔmicH°) in a given environment are explained in the Supporting Information.
Results and Discussion Since phenol seems to partially displace bromide counterion from the Stern region of the cationic micelle, it is interesting to see how the addition of an extra electrolyte to the aqueous phase affects the solubilization mechanism. Figure 2 shows the PhOH concentration-dependence of T-CH3, R-CH2, β-CH2, and ω-CH3 proton resonances in the 1H NMR spectra for the PhOH/HTAB/NaBr/D2O system in comparison with the curves related to the aqueous solution including no extra electrolyte. The chemical shifts (δ) for given protons were identified in the NMR spectra of a 1.5 mmol HTAB solution containing 1, 5, or 10 mmol kg-1 PhOH and 1 mmol kg-1 NaBr. For the purpose of this comparison, the plots of δ versus mPhOH obtained for solutions of the same composition but without NaBr were taken from the previous paper.15 The trends in δ with mPhOH are expressed in terms of the relative chemical shift, Δrδ, calculated as follows: Δr δ ¼
ðδ0:25 -δÞ 100 ð%Þ δ0:25
where δ is the observed chemical shift for a given proton of the surfactant unit and δ0.25 is the chemical shift for the same proton in the reference spectrum recorded for a 0.25 mmol kg-1 HTAB solution in pure D2O (unmicellized HTAB species). It should be noted here that, at a surfactant concentration of 0.25 mmol kg-1 (below the CMC irrespective of the overall additive content in the system), no effect of additive on the chemical shift of the R/β-methylene and ω-methyl protons can be observed, since the differences are always within the experimental error (see Table I in the Supporting Information). The T-CH3 resonance signal occurs at somewhat higher field (the upfield shift is 3 times the experimental error in the absence of NaBr), which argues in favor of the hypothesis of nonbonding phenol-cation interaction already below the CMC (a high-field shift of a proton peak is commonly interpreted as being a ringcurrent-shifted one8,15,33,36,37). (36) Olsson, U.; Soderman, O.; Guering, P. J. Phys. Chem. 1986, 90, 5223–5232. (37) Bijma, K.; Engberts, J. B. F. N. Langmuir 1997, 13, 4843–4849.
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Figure 2. Dependence of the chemical shifts for different HTAB protons on the phenol concentration in the absence of extra salt (O)15 or in the presence of 1 mmol kg-1 NaBr (b). Positive values of Δrδ represent upfield shifts in δ. The overall surfactant content in these systems is 1.5 mmol kg-1. The solid lines are plotted to guide the eye.
When NaBr and PhOH are added to the 1.5 mmol kg-1 HTAB solution at the same proportion (i.e., 1 mmol kg-1), the ω-CH3 resonance remains essentially unchanged and the T-CH3, R-CH2, and β-CH2 signals are shifted less downfield (the related Δrδ values are less negative) compared to their positions in the 1H spectrum of the 1 mmol kg-1 NaBr/1.5 mmol kg-1 HTAB/D2O system. This means that some rare phenol species are located predominantly in the outer parts of the HTAB micelle. A quasi linear change in Δrδ is monitored for the main HTAB protons with further increasing phenol content in the system. The saturation of solubilization loci by the additive is thus gradual in the presence of NaBr, contrary to the step saturation scheme observed previously in the absence of extra electrolyte.15 Figure 3 presents the corresponding fragments of 1H NMR spectra showing the positions of resonance signals assigned to aromatic protons in meta, ortho, and para phenol positions. Changes in δ due to the appearance of micelles in the solution, addition of NaBr, or/and increase in PhOH content are generally small. Such small variations are extremely difficult to interpret, since the observed 1H chemical shifts represent average values.38 In the case of phenol, there exist at least three different contributions to the observed δ value, since not only does the aromatic additive distribute between the micelles and the surrounding aqueous phase, it may inhabit both the polar mantle and the hydrophobic micelle core. The insertion of aromatic molecules in a hydrophobic environment is known to induce an upfield shift of the corresponding proton peaks.39 Indeed, low-frequency-shifted resonances corresponding to the aromatic protons were reported by Hansen et al.40 for lipophilic phthalates either dissolved in organic solvents far less polar than D2O or forming emulsion droplets in D2O. Ma and co-workers41 observed the upfield shifts of protons in aromatic molecules of phenol trapped in the hydrophobic core of Pluronic micelles. For fixed concentrations of PhOH, the ortho-, para-, and meta-methylyne resonance signals (38) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183–222. (39) Fendler, J. H.; Fendler, E. J.; Infante, G. A.; Shih, P. S.; Patterson, L. K. J. Am. Chem. Soc. 1975, 97, 89–95. (40) Hansen, P. E.; Skibsted, U.; Nissen, J.; Rae, C. D.; Kuchel, P. W. Eur. Biophys. J. 2001, 30, 69–74. (41) Ma, J.-H.; Guo, C.; Tang, Y.-L.; Zhang, H.; Liu, H.-Z. J. Phys. Chem. B 2007, 111, 13371–13378.
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Figure 3. Selected fragments of 1H NMR spectra for various ternary and quaternary solutions in D2O at 303 K showing the positions of resonance signals assigned to aromatic protons in meta, ortho, and para phenol positions. The composition of the corresponding solution is marked for each spectrum; the molalities (mmol kg-1) are given in brackets. The signal intensity was amplified arbitrarily to better show the changes in the chemical shift.
in Figure 3 appear displaced toward higher applied field when HTAB micelles appear in the system. Thus, the “average” microenvironment of aromatic molecules seems slightly more hydrophobic. The intensity of this effect diminishes in the order para- > meta- > ortho-methylyne signal, which indicates that most of the solubilized phenol molecules should have their hydroxyl groups oriented outward. The upfield shifts are somewhat less marked on addition of 1 mmol kg-1 NaBr to the system, especially for the lowest PhOH content. This provides arguments against deep phenol penetration into the micelle core when the additive is put in extra competition with the bromide ion. On the basis of unexpectedly small changes in δ, one can also conclude that a large majority of PhOH units remain in a polar microenvironment, that is, in the bulk solution and micelle mantle region. 1 H NMR Measurements of Competitive PhOH Solubilization by HTAB Micelles against BuOH. The sensitivity of the NMR method was first tested on ternary and quaternary systems containing 0.25 mmol kg-1 HTAB (see Figure III in the Supporting Information). All changes in the signal position for various protons in butanol and surfactant units indicate the increased solute-solute interactions compared to those in binary solutions. It is also interesting to note that the chemical shift of residual HDO is sensitive only to the presence of butanol in the bulk aqueous phase (see Tables II and III in the Supporting Information). Phenol is thought to interact directly with the surfactant, leaving the bulk water properties essentially unchanged.41 Figure 4 illustrates modification of the 1H NMR pattern for ternary solutions of HTAB and BuOH in D2O when the surfactant concentration increases from 0.25 to 1.5 mmol kg-1. The proton resonances in butanol and surfactant units are clearly shifted downfield, with the effect being much more pronounced for the surfactant. This trend resembles qualitatively that observed for the surfactant protons upon micelle formation.15 It can be thus deduced that surfactant micelles appear in the solution and simultaneously the local environment of the butanol chain becomes somewhat more hydrophobic. These conclusions seem consistent with the previous findings that the solubilized alcohol molecules are oriented in the same manner as the surfactant units.1,2 The incorporation of the C4 chain into Langmuir 2009, 25(9), 4868–4874
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Figure 4. Selected fragments of 1H NMR spectra for the 0.5 mmol kg-1 BuOH solutions in D2O at 303 K illustrating the effect of surfactant micellization on the positions of resonance signals assigned to protons in surfactant and butanol units: (a) 0.25 mmol kg-1 HTAB and (b) 1.5 mmol kg-1 HTAB. The dashed lines refer to the 1.5 mmol kg-1 HTAB/D2O system. The signal intensity was amplified arbitrarily to better show the changes in the chemical shift.
the micelle interior should induce some local changes in the size of the micellar core involving a wider distribution of the terminal surfactant CH3 groups. This probably accounts for deshielding of the ω-methyl proton resonance upon butanol addition, since the ω-CH3 signal occurs here at somewhat lower applied field (at δ 0.909 ppm) compared to the related signal in the 1H NMR spectrum for the 1.5 mmol kg-1 HTAB/D2O system (at δ 0.888 ppm). Finally, the ω-CH3 peak appears in Figure 4 on the lefthand side of the Hd-CH3 resonance, with both signals partially overlapping each other. Another argument in favor of butanol solubilization by the HTAB micelles may be searched in the location of the residual HDO resonance in Figure 4. This signal is shifted downfield, indicating that some alcohol molecules have disappeared from the aqueous solution and been transferred to the pseudomicellar phase. The effects of phenol addition to solutions containing HTAB micelles and BuOH are illustrated in Figures 5 and 6. Figure 5 represents the 1H NMR spectra for three PhOH-containing quaternary solutions in comparison (over selected spectral regions) with that obtained with the appropriate ternary BuOH/HTAB/D2O system. In all solutions, the molality of the surfactant and that of butanol were maintained constant and equal to 1.5 and 0.5 mmol kg-1, respectively. The most important observation concerns the positions of the Ha, Hb, Hc-methylene and Hd-methyl resonance signals in the 1H spectrum related to the highest PhOH content: these positions are very close to those reported for the 0.25 mmol kg-1 HTAB/ 0.5 mmo1 kg-1 BuOH/1 mmol kg-1 PhOH/D2O system. This clearly indicates that butanol mostly remains in the bulk solution surrounding the HTAB micelles. The same conclusion can be drawn from the location of the residual HDO peak which matches that for the system containing unmicellized HTAB units where all butanol molecules are in the aqueous phase. A systematic shift of the R-CH2 and T-CH3 signals toward higher applied field with increasing phenol content may be taken as evidence for intercalation of aromatic molecules in the outer micelle parts, although direct BuOH-HTAB interaction is also capable of producing upfield shifts of the surfactant proton resonances (see Figure III in the Supporting Information). In the DOI: 10.1021/la803451q
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Figure 5. Selected fragments of
1
H NMR spectra for the 1.5 mmol kg HTAB/0.5 mmol kg BuOH/D2O system at 303 K illustrating the effect of PhOH addition on the positions of resonance signals assigned to protons in surfactant and butanol units: (a) no PhOH added, (b) 1 mmol kg-1 PhOH, (c) 5 mmol kg-1 PhOH, and (d) 10 mmol kg-1 PhOH. The dashed lines refer to the system containing unmicellized HTAB (0.25 mmol kg-1 HTAB + 0.5 mmol kg-1 BuOH + 1 mmol kg-1 PhOH). The signal intensity was amplified arbitrarily to better show the changes in the chemical shift. -1
-1
case of higher phenol contents (5 and 10 mmol kg-1), the extent of changes in the resonance position is more pronounced for the R-CH2 to such a degree that both the resonance signals partially overlap each other. It should be noted here that similar broadening of the 1H NMR bands was considered as argument for the sphere-to-rod-like micellar transition in the alkyltrimethylammonium salicylate/sodium salicylate/D2O systems.36,41 The formation of rod-shaped micelles is promoted by any effect leading to the decreased electrostatic repulsion among the cationic headgroups (e.g., an increased counterion binding). It is true that the increased concentration of electron-donating phenol molecules due to their specific adsorption inside the Stern layer of the cationic micelle may induce some subtle changes in the distribution of the surface charge density, mainly by decreasing locally the dielectric constant of the medium in this region. It is questionable whether this effect is strong enough to moderate effectively the electrorepulsive forces and make the HTAB micelles more compact. The “regular” shape of the conductivity and enthalpy curves in the premicellar and postmicellar regions (see Figures I and II in the Supporting Information), irrespective of the phenol content, argues against the sphere-to-rod transition with increasing PhOH solubilization. However, this is not an absolute proof, since the HTAB self-assembly structures formed in D2O and H2O are not really identical.15 It is also clear that phenol molecules penetrate deeper into the micelle core. First, this accounts for the γI-CH2 peak appearing on the right-hand side of the N-CH2 signal in Figure 5 (i.e., an important shift of the resonance signals assigned to some methylene groups of the HTAB chain in the direction of higher applied field). Second, the ortho-, para-, and meta-methylyne peaks in Figure 6 appear systematically upfield with increasing PhOH content, with the magnitude of the shifts being much larger than those observed in Figure 2. This means that the fraction of aromatic units located in a hydrophobic microenvironment is increased when phenol has to compete with butanol for the solubilization sites in the HTAB micelles. The competitive solubilization of alcohols depends on the overall ratio between PhOH and BuOH in the system. It can be 4872
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Figure 6. Effect of PhOH addition to the 1.5 mmol kg-1 HTAB
solution in D2O on the 1H NMR spectrum of the aromatic protons for the PhOH/BuOH/HTAB/D2O system at 303 K. The molalities (mmol kg-1) of the additives for each spectrum are given in parentheses. The signal intensity was amplified arbitrarily to better show the changes in the chemical shift.
seen in Figure 5 that changes in the position of the residual HDO signal with increasing PhOH content parallel those related to the butanol methyl resonances, which is a strong indication in favor of the possibility to predict the amount of BuOH in the bulk phase (the corresponding chemical shifts of the residual HDO in various systems are listed in Table III in the Supporting Information). When the overall phenol content in the system reaches a value of 1 mmol kg-1, the amount of butanol molecules solubilized by the HTAB micelles seems to increase. For this particular system, the solubilization of both alcohols is likely cooperative. Further addition of phenol causes an increase in its chemical potential in the system, thereby enhancing its affinity for the micelles. Since butanol is simultaneously “displaced” to the bulk solution, the solubilization phenomenon becomes strongly competitive. The pronounced upfield shifts in the location of R-CH2, T-CH3, and ortho-, para-, and meta-CH peaks corresponding to the 10 mmol kg-1 PhOH/BuOH/ HTAB/D2O system in Figures 5 and 6 indicate that not only numerous phenol units are present in the micelle core but also butanol is replaced by the aromatic additive in the polar mantle. The magnitude of these changes is greater than those reported for the 10 mmol kg-1 PhOH/HTAB/D2O system in Figures 2 and 3, which means that the extent of phenol solubilization is markedly increased in the presence of butanol. Rationalization of Thermodynamic Data Obtained by Micellization of HTAB in the Presence of Additives. The comparison of the 1H chemical shifts for various protons in surfactant and additive units in solutions of different composition has allowed identifying the solubilization loci of phenol within cationic micelles of HTAB and has revealed some tendencies in the solubilization pathway at the atomic level that may be further exploited to rationalize the results of thermodynamic study of the effect of alcohols addition on the HTAB micellization behavior in H2O. Of course, it should be kept in mind that the use of various solvents, that is, H2O and D2O, may lead to some differences mainly in the extent of solubilization, with the interactions of the additives with the surfactant host and between themselves being of essentially the same type.15,17 Table 1 includes all thermodynamic parameters of micellization inferred from the combined analysis of the data from conductimetry and calorimetry measurements. The addition of BuOH to the aqueous solution of HTAB induces a very small Langmuir 2009, 25(9), 4868–4874
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Table 1. Effect of PhOH and BuOH Addition on the Micellization Parameters of HTAB, as Inferred from Conductivity and Titration Calorimetry Measurements at 303 K: Critical Micelle Concentration (CMC), Degree ( β) of Counterion Binding to the Micelle, as Well as Standard Gibbs Free Energy (ΔmicG°) of Micellization, Standard Micellization Enthalpy (ΔmicH°), and Standard Micellization Entropy (ΔmicS°) per Mole of HTABa mPhOH (mmol kg-1)
CMC (mmol kg-1)
β
ΔmicG° (kJ mol-1)
ΔmicH° (kJ mol-1)
ΔmicS° (J mol-1 K-1)
0 0.91 ( 0.06 0.67 ( 0.04 -46.4 ( 1.0 -22.8 ( 1.7 77.9 ( 11 1.0 0.91 ( 0.06 0.60 ( 0.04 -44.4 ( 1.1 -16.2 ( 0.9 93.1 ( 12 5.0 0.75 ( 0.05 0.65 ( 0.04 -46.6 ( 1.0 -19.5 ( 1.2 89.4 ( 12 10.0 0.77 ( 0.05 0.64 ( 0.04 -46.2 ( 1.0 -13.5 ( 0.8 107.9 ( 15 a The BuOH molality is equal to 0.5 mmol kg-1. The absolute uncertainties in determining CMC, β, and ΔmicH° include contributions from both measuring procedures and further data processing.
(quite within the experimental error) change in the CMC of the surfactant and a decrease in the degree of Br- binding to the micelle (compare CMC = 0.93 ( 0.05 and β = 0.72 ( 0.04 for pure HTAB15). The effect on the β parameter is similar to that caused by the 5 mmol kg-1 PhOH content.15 Micelle formation in the presence of 0.5 mmol kg-1 BuOH is much more exothermic than that in pure HTAB/H2O solution (ΔmicH° = -7.1 ( 0.5 kJ mol-1 (ref 15)), indicating the increased BuOH-HTAB interaction upon additive transfer from aqueous phase to micelles. This trend is at variance with the decreased exothermicity of the micellization of sodium dodecyl sulfate (SDS) induced by the addition of BuOH.42 It is important to realize that the previous result was not obtained through direct calorimetry measurements, but it was inferred from the temperature-dependence of the molar conductivity versus concentration plots using van’t Hoff equation (this procedure is not strictly rigorous as being based on some approximations). Since there is little difference in the Gibbs free energy (ΔmicG°) between the BuOH/HTAB/H2O and HTAB/H2O systems in the present work, the enhanced exothermic character of micellization is clearly compensated by a decrease in the positive entropy (i.e., ΔmicS° changes from 132.7 to 77.9 J mol-1 K-1). Hence, an extra “ordering” phenomenon is expected to parallel the release of structured water upon micellization of HTAB when BuOH is inserted into the micellar structures. This ordering effect is likely due to the restricted degrees of freedom of the solubilized BuOH molecules that adopt an orientation parallel to the surfactant units inside the micellar aggregates. Such a strong orientation maximizes the interaction of BuOH with the surfactant, which probably accounts for the enthalpy effect stronger than that observed when a small amount of pure PhOH is added to the aqueous phase (ΔmicH° = -14.7 ( 0.8 kJ mol-1 for the 1 mmol kg-1 PhOH/HTAB/H2O system15). The greater affinity of butanol for the hydrated portions of the HTAB micelle is justified also on the basis of its more hydrophilic character compared to phenol.29 Simultaneous addition of butanol and phenol yields irregular changes in the thermodynamic parameters of HTAB micellization, depending on the overall PhOH/BuOH ratio. These changes may be interpreted in terms of PhOH-BuOH competition for the solubilization sites within HTAB micelles by taking into account the main conclusions drawn from 1H NMR measurements in the previous section. The location of phenol molecules either in the headgroup region or in the micelle core certainly leads to two, opposite in sign, contributions to the overall ΔmicH° value. It has been shown43 that the molar enthalpy of phenol transfer from water to octanol is negative, whereas that of transfer from water to cyclohexane is positive. Furthermore, the transfer of phenol units from water to nonaqueous solvents is accompanied by a significant increase in (42) Chauhan, M. S.; Kumar, G.; Kumar, A.; Chauhan, S. Colloids Surf. A 2000, 166, 51–57. (43) Dearden, J. C.; Bresnen, G. M. Int. J. Mol. Sci. 2005, 6, 119–129.
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entropy due to the release of structured water, with this positive entropy of transfer being much more important in the case of cyclohexane. On introduction of small amounts of PhOH into the system (1 mmol kg-1 PhOH), the value of β diminishes to about 0.6, whereas the CMC remains constant. The micellization of HTAB becomes less exothermic, and the positive entropy ΔmicS° increases. It is easy to accept that the penetration of phenol molecules into a partially dehydrated region of a HTAB micelle between the surfactant headgroups and the first segments of the hydrophobic tails (i.e., in the so-called palisade layer44) increases the efficiency of both the additives in displacing bromide counterions for the Stern layer of the HTAB micelle. The desorption of Br- and the solubilization of PhOH in a more hydrophobic microenvironment result together in an endothermic contribution to ΔmicH° and a positive contribution to ΔmicS°. When the phenol concentration increases from 1 to 5 mmol kg-1, there is a net decrease in the CMC accompanied by an increase in the β value. The micellization phenomenon is again more exothermic. These changes are consistent with the gradual exclusion of the solubilized butanol species from the HTAB micelles by phenol molecules which replace BuOH species in the outer micelle parts but also can penetrate deeper into the HTAB micelle (it is worth noting in Figure 6 that the “average” microenvironment of PhOH does not change a lot). Further phenol addition has little effect on the values of CMC and β, whereas the exothermicity of the micellization phenomenon is greatly diminished. With the increased incorporation of PhOH into the hydrophobic microenvironment, the micellization of HTAB becomes less exothermic and the concomitant entropy change, ΔmicS°, becomes more positive on a per mole basis.
Conclusions In the present work, the molar fraction of BuOH is about 0.00001, whereas the mole fraction of PhOH does not exceed 0.0002. For such low additive contents, the most relevant conclusions with respect to the solubilization loci within the HTAB micelles at atomic level can be drawn from the detailed analysis of the 1H NMR spectra, since the 1H chemical shifts appear very sensitive to nonbonding interactions and subtle changes in the local environment of surfactant units and additive molecules. The results of thermodynamic study of HTAB micellization in H2O for the same additive concentrations well support the general trends inferred from the 1H NMR measurements. With cationic surfactants in dilute aqueous solution near to the CMC, phenol molecules solubilized by micelles have to compete with other species present in the system. This competitive character of micellar solubilization has never been sufficiently emphasized before. The overall effect of such competitions (44) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; Chapter 3.
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depends on the proportions among all competitors within the micellar pseudophase. In pure HTAB solutions, aromatic additive is already in competition with the surfactant counterion for the Stern layer of the HTAB micelle. Addition of 1 mmol kg-1 NaBr to the system induces an extra competition between PhOH and Br-, thereby precluding deeper penetration of the former into the micelle. The intrinsic affinity of butanol for the cationic surfactant is higher than that of phenol, as evidenced by greater changes in the signal position for various additive and surfactant protons in the corresponding 1H NMR spectra, as well as by more negative enthalpy and less positive entropy of micellization. On addition of the aromatic additive to the 0.5 mmol kg-1 BuOH solution containing micellized surfactant, phenol molecules are forced to penetrate deeper toward the hydrophobic micelle core. For higher phenol contents in the system, there are strong indications that butanol molecules are “displaced” progressively to the aqueous phase and should be thus replaced by phenol species with a lower affinity for the HTAB aggregate. In the context of the new application of micellar solubilization mentioned in the Introduction, the main challenge is to protect the precursor of the reactive center from direct
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contact with the inorganic framework during the formation stage. When such a precursor is partially hydrophilic and shows a tendency to locate in the outer micelle parts, butanol may be added to the system to block these sites and make the precursor units enter the micelle core. It is worth noting that the use of heptanol is unable to produce such an effect.45 The results of the present study also show that it is important to optimize the proportion between both of the additives in the system to preclude the displacement of butanol molecules from the micelles. The solubilized butanol molecules may be then removed from the porous material together with the surfactant during calcination. Acknowledgment. The authors wish to thank Dr. Maryse Bejaud for her assistance in 1H NMR experiments. Supporting Information Available: Conductivity measurements and data processing, calorimetry measurements and data processing, and supplementary 1H NMR results. This material is available free of charge via the Internet at http:// pubs.acs.org. (45) Chaghi, R. Ph.D. Dissertation, University of Montpellier 2, Montpellier, 2007.
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