pubs.acs.org/Langmuir © 2009 American Chemical Society
Stepwise Aggregation of Dimethyl-di-n-octylammonium Chloride in Aqueous Solutions: From Dimers to Vesicles Loı¨ c Leclercq,† Veronique Nardello-Rataj,*,† Mireille Turmine,‡ Nathalie Azaroual,§ and Jean-Marie Aubry† †
Universit e Lille 1, LCOM, Equipe Oxydation et Physico-Chimie de la Formulation, CNRS UMR 8009, B^ at. C6, F-59655 Villeneuve d’Ascq Cedex, France, ‡Universit e Pierre et Marie Curie-Paris 6, Laboratoire Interfaces et Syst emes Electrochimiques, CNRS, UPR15-LISE, Case 133, 4 place Jussieu, F-75252 Paris Cedex 05, France, and §Universit e Lille Nord de France, UDSL, Facult e de Pharmacie, UMR CNRS 8009, F-59006 Lille, France Received July 21, 2009. Revised Manuscript Received September 10, 2009
The self-aggregation of dimethyl-di-n-octylammonium chloride, in diluted aqueous solutions, was studied with various experimental and theoretical techniques: zetametry, conductimetry, dimethyl-di-n-octylammonium and chloride-selective electrodes, tensiometry, NMR spectroscopy (1H and DOSY), and molecular modeling (PM3 and molecular dynamic). The combination of the data obtained by these techniques led us to propose a stepwise aggregation process with increasing concentration: dimers (0.2-10 mM), bilayers (10-30 mM), and finally vesicles (>30 mM).
Introduction Molecular-recognition-directed self-assembly can lead to the formation of highly complex and fascinating structures with new and interesting properties.1 For example, the extent of supramolecular self-association and hence the properties resulting thereof can be externally controlled.2 Surfactants are archetypal examples of molecules that give supramolecular self-assemblies because the orientation and packing of the molecules can be controlled by changing the concentration or the temperature or by adding electrolyte. In other words, amphipilic molecules form aggregates above the critical aggregation concentration (cac) as a consequence of the hydrophobic effect.3-5 The commonest types of aggregates formed from amphiphiles are (i) micelles, (ii) cylindrical micelles, (iii) bilayers, (iv) vesicles, and (v) inverted micelles. Fortunately, there are systematic relationships between structure and the type of aggregates formed by a surfactant in aqueous medium, which is often well predicted by the packing parameter (P) (eq 1).6 P ¼
v ao l c
ð1Þ
*Corresponding author. E-mail:
[email protected]. (1) (a) Schneider, H. J.; Yatsimirsky, A. Principles and Methods in Supramolecular Chemistry; Wiley-VCH: Weinheim, Germany, 2000. (b) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: Chicheste, U.K., 2000. (c) Philip, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154–1196. (d) Lehn, J.-M.; Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (2) For recent examples of switchable supramolecular systems, see (a) Leclercq, L.; Schmitzer, A. R. J. Phys. Chem. B 2008, 112, 11064–11070. (b) Borovkov, V. V.; Hembury, G. A.; Inoue, Y. Angew. Chem., Int. Ed. 2003, 42, 5310–5314. (c) Xu, H.; Stampp, S. P.; Rudkevich, D. M. Org. Lett. 2003, 5, 4583–4586. (d) Matthews, O. A.; Raymo, F. M.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. New J. Chem. 1998, 1131– 1134. (e) Archut, A.; V€ogtle, F.; DeCola, L.; Azzellini, G. C.; Balzani, V.; Ramanujam, P. S.; Berg, R. H. Chem.;Eur. J. 1998, 4, 699–706. (3) (a) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1986. (4) Tascioglu, S. Tetrahedron 1996, 52, 11113–11152. (5) Savelli, G.; Germani, R.; Brinchi, L. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Dekker: New York, 2001; Chapter 8. (6) (a) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525–1568. (b) Israelachvili, J. N. In Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991, p 371.
1716 DOI: 10.1021/la9026706
Table 1. Principal Relationships between Packing Parameter (P) and Aggregate Structure in Aqueous Media
where v is the volume of the hydrophobic tail, lc is the chain length, slightly inferior to that of a fully extended one, and ao is the surface of the polar head. In Table 1, the aggregate structures as a function of P are summarized. From a general point of view, di-n-alkyl-dimethylammonium (e.g., di-n-dodecyl-dimethylammonium bromide [DDoDMA][Br]) is known to form vesicles readily (P ≈ 1).7 However, these doubletailed quaternary ammonium salts attract increasing attention because of their multiple applications (e.g., as biocides, phasetransfer catalysts, or ionic liquids8,9). They are well-known compounds and have been investigated for their surface and solution (7) (a) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860–3861. (b) Talmon, Y.; Evans, D.; Ninham, B. W. Science 1983, 221, 1047–1049. (c) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710–1714. (d) Miller, D. D.; Evans, D. F. J. Phys. Chem. 1989, 93, 323–333. (e) Miller, D. D.; Magid, L. J.; Evans, D. F. J. Phys. Chem. 1990, 94, 5921–5930. (f) Svitova, T. F.; Smirnova, Y. P.; Pisarev, S. A.; Berezina, N. A. Colloids Surf., A 1995, 98, 107–115. (8) (a) Olivier-Bourbigou, H.; Magna, L. J. Mol. Catal. A 2002, 182-183, 419–437. (b) MacFarlane, D. R.; Forsyth, S. A.; Golding, J.; Deacon, G. B. Green Chem. 2002, 4, 444–448. (c) Starks, C. M.; Liotta, C. L.; Halpern, M. PhaseTransfer Catalysis; Chapman and Hall: London, 1994. (d) Sasson, Y.; Neumann, R. Handbook of Phase Transfer Catalysis; Blackie Academic & Professional: London, 1997. (9) (a) Turmine, M.; Mayaffre, A.; Letellier, P. J. Colloid Interface Sci. 2003, 264, 7–13. (b) Santos, G. P., Jr.; Martins, C.; Fortuny, M. A.; Santos, F.; Turmine, M.; Graillat, C.; McKenna, T. F. L. Ind. Eng. Chem. Res. 2007, 46, 1465–1474. (c) Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992, 8, 782–787. (d) Funasaki, N.; Neya, S. Langmuir 2000, 16, 5343–5346. (e) Soltero, J. F. A.; Bautista, F.; Pecina, E.; Puig, J. E.; Manero, O.; Proverbio, Z.; Schulz, P. C. Colloid Polym. Sci. 2000, 278, 37–47.
Published on Web 09/30/2009
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behavior using various methods.10 Most of the investigations concern di-n-decyl-[DDDMA] and di-n-dodecyl-dimethylammonium [DDoDMA] and their longer homologues with various counterions.11 However, very little information has been reported on shorter chain lengths, such as for dimethyl-di-n-octylammonium ([DMDOA]).12 For example, the chloride derivative, abbreviated as [DMDOA][Cl], is poorly described in the literature, and the data are not always concordant (e.g. the cac determined by conductivity is 31,12b 41,12d or 70 mM12e). To our knowledge, there is surprisingly no data on surface tension measurements, commonly used to study the aqueous behavior of surfactants. The choice of [DMDOA][Cl] was dictated by the few available data points reported in the literature. Indeed, the expected highest cac, compared with longer analogues ([DDoDMA][Cl]), should favor the formation and the analysis of various aggregates over a wider range of concentration. To confirm this assumption and to collect more information about the aggregates of [DMDOA][Cl], we carried out some experiments in water as a function of its concentration using a homemade membrane electrode selective to [DMDOA] cations and a commercial electrode selective to [Cl] anions. Measurements of specific conductivity, zeta potential, surface tension, 1H NMR chemical shift, and diffusion coefficient (DOSY) have also been performed to corroborate the results. These experimental results have also been completed by theoretical computations to support our assumptions on the nature of the aggregates.
Figure 1. Specific conductivity (κ) plotted against [DMDOA][Cl] concentration at 25.0 °C. Scheme 1. Preparation of Dimethyl-di-n-octylammonium Chloride ([DMDOA][Cl])
Results Synthesis. As mentioned by many authors, high surfactant purity (>99.5%) is essential for the study of physicochemical properties in order to avoid undesirable modifications of the aggregation values.13 [DMDOA][Cl] was obtained by an SN2 reaction involving dimethyloctylamine and 1-octylbromide, followed by ion exchange with a hydroxide counterion, and finally the aqueous solution of [DMDOA][OH] was neutralized with an aqueous hydrochloride solution (Scheme 1).14 The product was characterized by 1H and 13C NMR and elemental analysis, which (10) (a) Adamczyk, Z.; Para, G.; Warszynski, P. Langmuir 1999, 15, 8383–8387. (b) Bakshi, M. S. Colloid Polym. Sci. 2000, 278, 1155–1163. (c) Skerjanc, J.; Kogej, K.; Cerar, J. Langmuir 1999, 15, 5023–5028. (d) Ruso, J. M.; Sarmiento, F. Colloid Polym. Sci. 2000, 278, 800–804. (e) Zielinski, R. J. Colloid Interface Sci. 2001, 235, 201–209. (f) Ranganathan, R.; Okano, L. T.; Yihwa, C.; Quina, F. H. J. Colloid Interface Sci. 1999, 214, 238–242. (g) Fujio, K.; Mitsui, T.; Kurumizawa, H.; Tanaka, Y.; Uzu, Y. Colloid Polym. Sci. 2004, 282, 223–229. (h) Galan, J. J.; Gonzalez-Perez, A.; Del Castillo, J. L.; Rodriguez, J. R. J. Therm. Anal. Calc. 2002, 70, 229–234. (i) Pal, O. R.; Gaikar, V. G.; Joshi, J. V.; Goyal, P. S.; Aswal, V. K. Langmuir 2002, 18, 6764–6768. (j) Ruso, J. M.; Attwood, D.; Taboada, P.; Mosquera, V. Colloid Polym. Sci. 2002, 280, 336–341. (11) (a) Fontell, K.; Ceglie, A.; Lidman, B.; Ninham, B. Acta Chem. Scand., Ser. A 1986, 40, 247–256. (b) Khan, A; Kang, C. J. Colloid Interface Sci. 1993, 156, 218– 228. (c) Caboi, F.; Monduzzi, M. Langmuir 1996, 12, 3548–3556. (d) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Kirby, S. D.; Engberts, J. B. F. N. Faraday Trans. 1997, 93, 453–455. (e) Haas, S.; Hoffmann, H.; Thunig, C.; Hoinkis, E. Colloid Polym. Sci. 1999, 277, 856–867. (f) Soltero, J. F. A.; Bautista, F.; Pecina, E.; Puig, J. E.; Manero, O.; Proverbio, Z.; Schulz, P. C. Colloid Polym. Sci. 2000, 278, 37–47. (g) Ono, Y.; Kawasaki, H.; Annaka, M.; Maeda, H. J. Colloid Interface Sci. 2005, 287, 685–693. (h) Feitosa, E.; Jansson, J.; Lindman, B. Chem. Phys. Lipids 2006, 142, 128–132. (i) Chidambaram, M.; Sonavane, S. U.; de la Zerda, J.; Sasson, Y. Tetrahedron 2007, 63, 7696–7701. (j) Svitova, T.; Smirnova, Y.; Yakubov, G. Colloids Surf., A 1995, 101, 251–260. (12) (a) Hiramatsu, K.; Kameyama, K.; Ishiguro, R.; Mori, M.; Hayase, H. Bull. Chem. Soc. Jpn. 2003, 76, 1903–1910. (b) Lang, J. J. Phys. Chem. 1982, 86, 992–998. (c) Rabie, H. R.; Vera, J. H. Langmuir 1996, 12, 3580–3584. (d) Milioto, S.; Bakshi, M. S.; Crisantino, R.; De Lisi, R. J. Colloid Interface Sci. 1993, 159, 354–365. (e) Delitala, C.; Marongiu, B.; Pittau, B.; Procedda, S. Fluid Phase Equilib. 1996, 126, 257–272. (f) Svitova, T. F.; Smirnova, Y. P.; Pisarev, S. A.; Berezina, N. A. Colloids Surf. A 1995, 98, 107–115. (13) (a) Lunkenheimer, K.; Miller, R. Tenside 1979, 16, 312–316. (b) Rabie, H. R.; Weber, M. E.; Vera, J. H. J. Colloid Interface Sci. 1995, 174, 1–9. (14) Nardello-Rataj, V.; Caron, L.; Borde, C.; Aubry, J.-M. J. Am. Chem. Soc. 2008, 130, 14914–14915.
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showed that the salt is very hygroscopic but very pure (>99.5%). Thus, the salt was kept in a glovebox under an argon atmosphere. Conductivity. Specific conductivity experiments were first carried out on aqueous solutions of [DMDOA][Cl] to confirm or invalidate the various cac’s (instead of cmc’s because the nature of the aggregates is not known) given in the literature (Figure 1). It is well known that the specific conductivity (corrected for solvent) is linearly correlated to the surfactant concentration in both pre- and postaggregation regions, with the slope in the preaggregation region being higher than that in the postaggregation region. The break is due to the binding of some of the counterions to the aggregate. In other words, the intersection point of these two straight lines corresponds to the cac, but the ratio between the two slopes gives the degree of ionization, R, of the aggregate. The degree of binding of the counterion, β, to the aggregate is (1 - R).15 As depicted in Figure 1, the conductivity gives an intercept at 32.0 ( 0.5 mM and a degree of binding of the counterion, β, of 0.46. It is noteworthy that these values are close to those obtained by Milioto et al. (41 and 0.42 mM, respectively)12d and are in very good agreement with that of Lang (31 mM).12b The value reported by Delitala et al. (70 mM) might be attributed to the presence of hydrophobic impurities in their product.12e Zeta Potential. To confirm the value of the cac observed by the specific conductivity, complementary experiments have been performed by zeta potential measurements (Figure 2). In spite of the fact that the data obtained are related to tiny aggregates at low concentration, there is a significant increase in the zeta potential (15) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley-Interscience: New-York, 2004.
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Figure 2. Zeta potential (ζ) plotted against [DMDOA][Cl] concentration at 25.0 °C.
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the surface tension reaches a limiting value that does not change appreciably even though the [DMDOA][Cl] concentration in the aqueous phase is further increased. This critical concentration in the solution coincides with the saturation of both the surface and the subphase by [DMDOA][Cl] molecules. Here, the surface tension decreases until reaching 12.0 ( 0.5 mM. It is noteworthy that this concentration corresponding to the surface tension minimum, usually called the cmc but named the cac here, is not in good agreement with the values obtained previously by conductivity or zetametry measurements. This apparent discrepancy can be explained by the existence of preaggregation before the cac reported above. Indeed, the value obtained by the previous methods is due to only a change in the ionization of the aggregates and if preaggregation occurs before 30 mM then the aggregates are highly ionized (i.e., not detected by conductivity). For the following discussion, the cac’s obtained by the surface tension and by the conductivity and zetametry measurements are represented, respectively, by cac1 (∼10 mM) and cac2 (∼30 mM). Some physicochemical parameters can be obtained with the surface tension data. For instance, the occupied molecular area (A) at the air/water interface can be calculated according to the Gibbs adsorption isotherm (eq 2).15 1 Dσ ðmol m -2 Þ Γ ¼ ð2Þ nRT D ln Ct Γ is equal to the number of moles of [DMDOA][Cl] adsorbed at the air/water interface per unit area, R is the gas constant, T is the temperature (298 K), Ct is the [DMDOA][Cl] salt concentration in the subphase, and n the dissociation parameter. Typically, n = 1 for nonionic species whereas n > 1 for ionic species depending on their state of dissociation. Dissociation parameter n = 2 follows from a formal thermodynamic derivation that considers the counterion to be an independently adsorbing species (i.e., the surfactant is completely dissociated).26 The [DMDOA][Cl] molecular area is deduced from Γ according to eq 3
Figure 3. Surface tension (σ) plotted against [DMDOA][Cl] concentration at 25.0 °C.
below the cac reported above. A maximum is reached at around 30 mM, from which there was an abrupt decrease in the zeta potential. The same profile has already been reported in the literature for dodecyl-trimethylammonium bromide [DoDTMA][Br].16 Authors concluded that the maximum in the zeta potential corresponds to the cac and the increase below the cac suggests the formation of preaggregates. The reduction of the zeta potential after the cac is explained by an increase in the number of the counterions adsorbed on the aggregate, in agreement with conductivity results (see above).16,17 Surface Activity. Variations of air/water surface tension (σ) for [DMDOA][Cl] aqueous solutions as a function of total [DMDOA][Cl] concentration in the aqueous subphase are shown in Figure 3. The profile of the σ versus log [DMDOA][Cl] curve is typical of a soluble amphiphilic surfactant. Indeed, at very low [DMDOA][Cl] concentrations, from 0.4 to 12 mM, the surface tension decreases almost linearly with increasing log [DMDOA][Cl], indicating that the molecules are being adsorbed at the air/water interface, significantly lowering the interfacial free energy. This behavior is limited to a maximum [DMDOA][Cl] concentration from which (16) Sabate, R.; Gallardo, M.; Estelrich, J. Electrophoresis 2000, 21, 481–485. (17) (a) Rodenas, E.; Dolcet, C.; Valiente, M.; Valeron, E. C. Langmuir 1994, 10, 2088–2094. (b) Cocera, M.; Lopez, O.; de la Maza, A.; Parra, J. L.; Esteerich, J. Langmuir 1999, 15, 2230–2233.
1718 DOI: 10.1021/la9026706
A ¼
1020 2 ½Å N Γ
ð3Þ
where N is Avogadro’s number. Linear regression analysis (correlation coefficient >0.99) of the σ versus log[DMDOA][Cl] straight lines led to a molecular area at the air/water interface of 88 A˚2 assuming a completely dissociated surfactant. (See the conductivity and zeta potential above.) For [DDDMA][L-lactate], Pernak et al. have found a molecular area of around 100 A˚2 with n = 2.18 Ion-Selective Electrodes. The term ion-selective electrode (ISE) is applied to electrodes with membranes that are specifically permeable to one or a few chemically related ions.19 Ion-selective electrodes are electrochemical half-cells in which a potential difference that is dependent on the concentration of a particular ion in solution arises across the electrode/solution interface. They can be used to determine, quantitatively and specifically, the cac, for example.9,20 The equilibrium electromotive force (emf) of aqueous solutions was recorded from 2 10-4 to 140 mM (Figure 4) using a commercial chloride electrode and a homemade [DMDOA]-membrane-selective electrode. This last electrode is (18) Cybulski, J.; Wisniewska, A.; Kulig-Adamiak, A.; Lewicka, L.; CienieckaRosleonkiewicz, A.; Kita, K.; Fojutowski, A.; Nawrot, J.; Materna, K.; Pernak, J. Chem.;Eur. J. 2008, 14, 9305–9311. (19) Bates, R. G. Electrode Potentials. In Treatise on Analytical Chemistry. Part I, Theory and Practice; Kolthoff, I. M., Elving, P. J., Eds.; John Wiley & Sons: New York, 1978; Vol. 1, pp 773-820. (20) (a) Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992, 8, 782–787. (b) Gharibi, H.; Razavizadeh, B. M.; Rafati, A. A. Colloids Surf., A 1998, 136, 123–132.
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Figure 4. Electromotive forces (emf) plotted against [DMDOA][Cl] concentration at 25.0 °C: for [DMDOA] (black dots) and [Cl] (red dots). (The dotted line represents the emf calculated from eq 9 with Kd = 166 ( 5, calculated with concentration in M.)
made of a pH glass electrode coated with a silicon polymer9a,9b containing [DMDOA][BPh4] as a cation exchanger.9c For the [DMDOA]-cation-selective electrode, the electrode displays pseudo-Nernstian behavior (eq 4) in the concentration range from 2 10-4 to 0.2 mM (i.e., the emf increases linearly with the logarithm of the [DMDOA][Cl] concentration). The slope (63.4 mV) is close to the value obtained for similar surfactants such as di-n-decyl-dimethylammonium bromide ([DDDMA][Br]) (58.3 mV)9d and the expected value (59.2 mV at 298 K). emf ¼ 324:6 þ 63:4 logðDMDOAÞ ðmVÞ
ð4Þ
We note that the emf exhibits a maximum for a total surfactant concentration very close to the point where the surface tension levels off (cac1 = 10 mM). However, the [Cl]-anion-selective electrode shows Nernstian behavior until 30 mM (eq 5; cac2); this value is in good agreement with the conductivity and the zeta potential measurements. (See above.) emf ¼ -26:5 -53:1 log½Cl ðmVÞ ð5Þ The ammonium-selective electrode shows that the emf deviates from those predicted by the Nernstian equation, particularly at concentrations just below the cac1. It is noteworthy that the corrections to obtain activity give the same profile curve. This observation has already been reported in the literature by Wyn-Jones et al. for various monoalkyl-ammonium and monoalkyl-pyridinium bromides.21 The authors ascribed this nonideal behavior to preaggregates as dimers. On the hypothesis of dimers in the concentration range between 0.2 and 10 mM, a dimerization constant can be calculated from the emf data. In this premicellar region, the experimental information available from the emf data is the variation of surfactant unimer concentration with the total surfactant concentration. In this region, we assume that the observed nonideal behavior is associated with the buildup of dimers (U2) from unimers (U) taking place via a bimolecular mechanism Kd
U þ U su U2 where Kd is the dimerization constant that can be calculated from eq 6: Kd ¼
½U2 ½U2
ð6Þ
(21) Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992, 8, 702–707.
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Figure 5. (a) 1H NMR chemical shift plotted against [DMDOA]-
[Cl] concentration and (b) partial 1H NMR spectra for various [DMDOA][Cl] concentrations (25.0 °C, D2O).
Figure 6. Diffusion coefficient (D) from DOSY experiments plotted against the [DMDOA][Cl] concentration at 25.0 °C.
The total concentration of [DMDOA][Cl] (Ct) is given by C t ¼ ½U þ 2½U2
ð7Þ
[U2] can be explicitly written and substituted into eq 6 so that the unimer concentration ([U]) is obtained as a function of the total concentration (Ct) and the dimerization constant (Kd). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 -1 þ 1 þ 8 K d C t ½U ¼ ð8Þ Kd 4 The unimer concentration is substituted into eq 4, assuming the complete dissociation of ionic species. (See above.) pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 1 -1 þ 1 þ 8 K d C t ð9Þ emf ¼ 324:6 þ 63:4 log Kd 4 DOI: 10.1021/la9026706
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Leclercq et al. Table 2. Physicochemical Parameters Obtained for [DMDOA][Cl] Aqueous Solutions by Various Techniques at 25.0 °C
technique
cac1 (mM)
cac2 (mM)
physicochemical parameters
32.0 β = 0.46 conductimetry n.d. 30.0 zetametry n.d.b tensiometry 12.0 n.d.a. 88 A˚2/molecule in the surface region c 30.0 Kd = 166 ( 5 ISE [DMDOA] 10.0 30.0 ISE [Cl] n.d.a 1 d H NMR n.d. 30.0 30.0 DOSY n.d.e a Not detected. b Not detected but evidence of aggregates before 30 mM. c Plausible dimers between ∼0.2 and 10 mM. d Not detected but evidence of aggregates between 3 and 30 mM. e Not detected but evidence of aggregates between 2 and 30 mM. a
Figure 7. Prevalent aggregates of [DMDOA][Cl] in water as a function of the concentration at 25.0 °C.
The emf data is calculated from eq 9 in order to fit the adjustable parameter (Kd) with an appropriate algorithm. The model of the dimer is in agreement with a dimerization constant (Kd) equal to 166 ( 5 (calculated with concentration in M; see the dotted line in Figure 4). 1 H NMR and DOSY. 1H NMR chemical shifts are very sensitive to aggregation changes with concentration.22 Figure 5 reports the chemical shifts of some [DMDOA][Cl] protons as a function of their concentration. It is noteworthy that the self-association is fast on the NMR timescale; therefore, the observed chemical shift is the weighted average of the shifts for all the species present at equilibrium (eq 10) δobs ¼
X
f i δi with f i ¼
i
xi C i Ct
ð10Þ
increased (i.e., no discontinuities observed at 10 mM). However, a discontinuity is observed for cac2 (30 mM). The most affected protons are the N-CH3, mainly for a concentration range higher than 30 mM; this observation supports the chloride anions’ proximity to the polar headgroup. Finally, diffusion-ordered NMR spectroscopy (DOSY) experiments were carried out. Indeed, this technique has proven to be advantageous in the study of small supramolecular assemblies.23 Our measurements have been performed between 0.5 and 1000 mM (Figure 6). For polydisperse samples, the diffusion coefficient observed (Dobs) reflects the weighted average of the diffusion coefficients of all of the species present in solution (eq 11) Dobs ¼
X i
f i Di with f i ¼
xi C i Ct
ð11Þ
where δobs represents the observed chemical shift, Ct is the total concentration, xi is the stoichiometric coefficient of the reagent in the ith species, δi is its chemical shift, and Ci is its equilibrium concentration. The NMR analysis clearly shows the two ranges of aggregation. Actually, for low [DMDOA][Cl] concentrations, NMR signals of N-CH3, N-CH2, and -CH3 protons undergo significant downfield shifts: a weak downfield shift is observed between 3 and 30 mM, whereas below 30 mM there is a much stronger downfield shift. These observations support the value of cac2 (30 mM) and the preaggregation (for concentrations below 30 mM and dimers before 10 mM) determined above. The preaggregation seems to be continuous as the concentration is
where Dobs represents the observed diffusion coefficient, Ct is the total concentration of the surfactant, xi is the stoichiometric coefficient of the reagent in the ith species, Di is its diffusion coefficient, and Ci is its equilibrium concentration. The interactions between [DMDOA][Cl] molecules were also evidenced by DOSY experiments. A significant decrease in the diffusion coefficient of [DMDOA][Cl] was observed when the concentration was increased, showing two ranges of behavior: (i) moderate, monotonous decrease in Dobs between 2 and 30 mM and (ii) a steeper decrease above 30 mM, in agreement with the cac2. (See below.) The moderate decrease observed between 2 and 30 mM is explained by continuous aggregation and confirms the 1H NMR. (See below.)
(22) (a) Xing, H.; Lin, S.-S.; Lu, R.-C.; Xiao, J.-X. Colloids Surf., A 2008, 318, 199–205. (b) Leclercq, L.; Schmitzer, A. R. J. Phys. Chem. A 2008, 112, 4996–5001.
(23) Hapiot, F.; Leclercq, L.; Azaroual, N.; Fourmentin, S.; Tilloy, S.; Monflier, E. Curr. Org. Synth. 2008, 5, 162–172.
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Figure 8. Snapshots of the MD simulation in water at 50 ps: (a) two, (b) four, (c) six, and (d) eight [DMDOA][Cl] molecules (in CPK green, chloride anions; in CPK blue, nitrogen atoms; and in ball-and-stick yellow, alkyl residues).
Above 30 mM, the association is collective and the decrease in Dobs is more important. This last situation is classical for surfactant systems.24
Table 3. Enthalpy Change (ΔH, kcal/mol),a COSMO Surface Area Gain (CSAG, A˚),a and Ionizationb for Various Aggregates of [DMDOMA][Cl] Un þUn
Discussion Table 2 summarizes the physicochemical data obtained for [DMDOA][Cl] in aqueous solutions by the various techniques presented above. It appears that all of the cac2 values are in perfect agreement. However, the tensiometry and the ISE measurements show a cac1 of around 10 mM. Moreover, the analysis of the emf data in the region between 0.2 and 10 mM (i.e., the non-Nernstian behavior) supports the formation of preaggregates as dimers. A similar result has already been reported in the literature by WynJones for various monoalkyl-ammonium and monoalkyl-pyridinium bromides.21 On one hand, Bunton et al. have recently shown that dimerization can occur for a double-tailed quaternary ammonium such as [DDoDMA][Cl].25 Such preaggregation (dimers etc.) has also been described for some gemini surfactants14 and for cationic oligomeric surfactants.26 As early as 1948, Ratson and co-workers reported the formation of floating aggregates of [DDoDMA][Cl], suggesting a driving force different from anion association.27 In recent work on [DDoDMA][Cl], Bunton et al. have ascribed the formation of preaggregates in a hydrophobic interaction between the alkyl tails of at least two molecules.25 In other words, both alkyl chains of two (24) (a) Bordes, R.; Vedrenne, M.; Coppel, Y.; Franceschi, S.; Perez, E.; RicoLattes, I. ChemPhysChem 2007, 8, 2013–2018. (b) Dozol, H.; Berthon, C. C. R. Chim. 2006, 9, 556–563. (c) S€oderman, O.; Stibls, P.; Price, W. S. Concept Magn. Reson. A 2004, 23, 121–135. (25) Gillitt, N. D.; Savelli, G.; Bunton, C. A. Langmuir 2006, 22, 5570–5571. (26) Laschewsky, A.; Wattebled, L.; Arotcarena, M.; Habbib-Jiwan, J.-L.; Rakotoaly, R. H. Langmuir 2005, 21, 7170–7179. (27) Ralston, A. W.; Eggenberger, D. N.; Du Brow, P. L. J. Am. Chem. Soc. 1948, 70, 977–979.
Langmuir 2010, 26(3), 1716–1723
n
ΔHU2n (kcal/mol)c
ΔH U2n =CSAGU2n
su U2n CSAGU2n (A˚)d
ionization (%)e
-11 -67 100 -41 -405 100 -63 -517 83 -82 -642 62 a Based on a PM3 calculation (RHF, MOPAC2009). b Based on MD simulation. c Calculated with eq 11. d Calculated with eq 12. e Chloride in the solvation surface/total chloride.
1 2 3 4
surfactant molecules are overlapped to generate a dimer. If dimerization occurs at concentrations below cac1 (10 mM), at surface saturation, then the bulk is probably rich in dimers and these dimers interact to form more and more complex aggregates (tetramers, hexamers, octamers, etc.). Because of the packing parameter (0.5 < P < 1) of the [DMDOA], most favorable aggregates are bilayers. It is noteworthy that this assumption is confirmed by the DOSY experiments because over a large concentration range between cac1 (10 mM) and cac2 (30 mM) diffusion coefficient shows a slight, continuous decrease, proving gradual aggregation. On the other hand, on the basis of the chloride-selective electrode, the conductivity, and the zeta potential results, these bilayers are always totally ionized except for cac2, which corresponds to an increase in the number of chloride ions adsorbed onto the aggregates. At cac2, the diffusion coefficient shows a steeper decrease classically observed in the formation of large aggregates such as vesicles.24a On the basis of this observation, the formation of vesicles (or large bilayers), already described for longer homologues,7 is assumed at concentrations higher than 30 mM (cac2, Figure 7). DOI: 10.1021/la9026706
1721
Article
Leclercq et al.
Figure 9. Structure obtained after PM3/COSMO geometry optimization for (a, b) dimers, (c, d) octamers of [DMDOA] ((a,c) ball and stick and (b, d) ESP surface projection).
To support our assumption of the gradual formation of dimers and more complex aggregates and to confirm the total dissociation of the ammonium cations and chloride anions before cac2 deduced from zeta and conductivity measurements, molecular modeling assuming the formation of dimers, tetramers, hexamers, and octamers via a sequential bimolecular stepwise mechanism has been performed. Un þ Un
ΔH U2n =CSAGU2n
su
U2n ðn ¼ 1, 2, 3, 4Þ
The molecular dynamic (MD) simulations of aggregates in aqueous solution were carried out in a cubic simulation box with periodic boundary conditions in all directions. Several 100 ps runs were performed with different numbers of unimers with different initial relative orientations. In most cases, the [DMDOA] molecules form dimers, tetramers, hexamers, or octamers within the first 20 ps of the simulation run and remain in a stable position (Figure 8). Indeed, these last aggregates are close to flexible bilayers. It is noteworthy that the counterions of dimers move toward the sides of the periodic box (Figure 8 and Table 3). This observation supports dimer and anion dissociation in aqueous systems. However, for higher aggregates some chlorides are near the aggregates that confirm the conductivity data (Table 3). After MD, the periodic box, the chlorides, and the water molecules are deleted, and the thermodynamic parameters for the [DMDOA] aggregates were assessed with semiempirical quantum calculations using the PM3 semiempirical method with COSMO water solvation parameters.28 The COSMO method (conductor-like screening model) is useful for determining the stability of various species in a water environment. Enthalpy and entropy changes (ΔH and ΔS, respectively) determine whether the reaction can take place spontaneously.29 The enthalpy (ΔH) is defined as presented in eq 11. ΔH U2n ¼ H U2n - 2 H Un
ð11Þ
The calculated ΔH for [DMDOA] aggregate formation is exothermic for all aggregates (Table 3). Moreover, the gradual aggregation is also entropy-driven via the hydrophobic effect. Pure water molecules adopt a structure that maximizes the (28) Stewart, J. J. P. Stewart Computational Chemistry, version 7.213W, http:// openmopac.net/. (29) Smith, M. B.; March, J. Mechanisms and Methods of Determining Them. In March’s Advanced Organic Chemistry, 6th ed.; John Wiley & Sons: New York, 2007.
1722 DOI: 10.1021/la9026706
entropy. The hydrophobic part of the [DMDOA] cation disrupts this structure and decreases the entropy, creating a cavity that is unable to interact electrostatically with the water molecules. When more than one cavity is present, the surface area of disruptions is high, meaning that there are fewer free water molecules. To counterbalance this, the water molecules expel the hydrophobic molecules together and form a cage structure around them with a smaller surface area than the total surface area of the various cavities.30 This maximizes the amount of free water and thus the entropy. The estimation of ΔS is difficult to predict by modelization, but an indirect method to compare the COSMO surface area (CSA) of the 2n and n aggregates was used. The COSMO surface area gain (CSAG) is defined as presented in eq 12. CSAGU2n ¼ CSAU2n -2 CSAUn
ð12Þ
The resulting aggregates have a smaller solvation surface area than the total solvation surface area formed by both unimers or both previous aggregates (Table 3). Therefore, the spontaneous and continuous aggregation is favored both for enthalpic and entropic considerations. On the basis of these thermodynamic considerations, we proposed that the dimerization is the first notable aggregate (
