Kinetics of Induced Vesicle Breakdown for Cationic and Catanionic

Seyda Bucak,† Brian H. Robinson,* and Antonella Fontana‡. School of Chemical Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, U.K...
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Kinetics of Induced Vesicle Breakdown for Cationic and Catanionic Systems Seyda Bucak,† Brian H. Robinson,* and Antonella Fontana‡ School of Chemical Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, U.K. Received January 18, 2002. In Final Form: July 10, 2002 Cationic and catanionic surfactant vesicle systems have been perturbed by addition of a single-chain surfactant to take the system from a vesicle-stable region to a micelle-stable region. The first vesicle system is based on a double-chain cationic surfactant cetyldecyldimethylammonium bromide (C16C10DMABr) and the second vesicle system is based on a mixture of anionic and cationic single-chain surfactants, namely, sodium octyl sulfate (SOS) and cetyltrimethylammonium bromide (CTAB). In both systems, vesicle breakdown will involve the initial incorporation of the single-chain surfactant into the vesicle, followed by subsequent disintegration of the vesicle. The progress of reaction has been observed by monitoring turbidity changes using a stopped-flow or conventional spectrophotometer. The rate of breakdown of vesicles was found to be linearly dependent on the concentration of added single-chain surfactant, which provides the driving force for the breakdown reaction. The breakdown reaction rates could be changed by factors up to 100 by adjusting the concentration of the solutions.

Introduction Formation of vesicles from totally synthetic amphiphiles was first reported by Kunitake et al.1 in 1977. These vesicles were prepared by dispersing didodecyldimethylammonium bromide in water followed by sonication. Deguchi and Mino2 subsequently reported the formation of vesicles from dioctadecyldimethylammonium bromide, where sonication was again used to prepare the final vesicle dispersion. The existence of vesicles in both studies was confirmed by electron microscopy, together with 1H NMR and 13C NMR spectroscopy. Since this initial work, there have been many studies performed on synthetic bilayers.3-6a The main advantage of synthetic bilayers is that although they resemble phospholipid vesicles and therefore display the same characteristics, because of the absence of unsaturated chains they do not chemically degrade during or after their initial preparation. Another advantage of these synthetic systems is the net positive charge of these vesicles. This net charge on the surface of the vesicles causes “repulsion” between the aggregates, which is adequately described by the DLVO theory. As a result, the vesicles should not aggregate and thus have enhanced stability against flocculation. For these reasons, synthetic vesicles can be used as model membranes in studies designed to increase our understanding of mechanisms of vesicle formation and breakdown. Since the preparation of these model vesicles involved the use of sonication, only small vesicles were formed. The preparation of larger vesicles requires other methods.6b-d † Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. ‡ Present address: Dipartimento di Scienze del Farmaco, Universita “G. D’Annunzio”, Chieti, Italy.

(1) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860. (2) Deguchi, K.; Mino, J. J. Colloid Interface Sci. 1978, 65, 155. (3) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710. (4) Herrmann, U.; Fendler, J. H. Chem. Phys. Lett. 1979, 64, 270. (5) Kano, K.; Romero, A.; Djermouni, B.; Ache, H. J.; Fendler J. H. J. Am. Chem. Soc. 1979, 101, 4030. (6) (a) Kunitake, T.; Okahata, Y. Chem. Lett. 1981, 1397. (b) Angelova, M. I.; Dimitrov, S. D. Faraday Discuss. Chem. Soc. 1986, 81, 303. (c) Lasic, D. D. Biochemistry 1988, 256, 1. (d) Bergstrom, M.; Eriksson, J. C. Langmuir 1996, 12, 624.

Using dioctadecyldimethylammonium chloride (DODAC), Chaimovich et al.7,8 prepared large unilamellar vesicles by adapting the vaporization method of Deamer and Bangham.9 These vesicles were found to be suitable for use as models for permeability and transport studies through bilayers composed of synthetic amphiphiles. Later preparation methods included dicholoromethane injection, chloroform injection, and membrane extrusion, all of which lead to vesicles with different membrane permeability, size, and entrapment potentials. Carmona-Ribeiro et al.10a investigated counterion effects on the properties of cationic vesicles prepared using dioctadecyldimethylammonium chloride (DODAC), dioctadecyldimethylammonium bromide (DODAB), and dioctadecyldimethylammonium acetate (DODAAc). The largest vesicles, which had the lowest transition temperatures (Tm), were formed with bromide as counterion. Didodecyldimethylammonium hydroxide (DDAOH) vesicles were reported to form spontaneously.10b Vesicle formation was achieved upon dilution of concentrated (0.1 mol dm-3) solutions of surfactant. The nature of the counterion was also shown to be important in terms of vesicle formation and stability, where carboxylates were investigated.10c Polymerization of styrene in DODAB vesicles has also been achieved and found to enhance stability.11 Vesicles adopt a parachute-like morphology, with the polystyrene bead attached to the vesicle at a specific location. Spontaneous formation of stable vesicles has also been achieved by mixing anionic and cationic surfactants over a limited range of mixing ratios.12-18 This type of system was first characterized in detail by Kaler et al.12 Their (7) Ribeiro, C. A. M.; Chaimovich, H. Biochim. Biophys. Acta 1983, 773, 179. (8) Ribeiro, C. A. M.; Yoshida, L. S.; Sesso, A. Chaimovich, H. J. Colloid Interface Sci. 1984, 100, 433. (9) Deamer, D.; Bangham, A. D. Biochim. Biophys. Acta 1976, 443, 629. (10) (a) Nascimento, D. B.; Rapuano, R.; Lessa, M. M.; Ribeiro, C. A. M. Langmuir 1998, 14, 7387. (b) Talmon, Y.; Evans, D. F.; Ninham, B. W. Science 1983, 221, 1047. (c) Brady, E. J.; Evans, D. F.; Kachar, B.; Ninham, B. W. J. Am. Chem. Soc. 1984, 106, 4279. (11) Jung, M.; Hubert, D. H. W.; Bomans, P. H. H.; Frederick, P. M.; Meudijk, J.; van Herk, A. M.; Fisher, H.; German, A. L. Langmuir 1997, 13, 6877.

10.1021/la020064i CCC: $22.00 © 2002 American Chemical Society Published on Web 09/27/2002

Kinetics of Induced Vesicle Breakdown

Figure 1. Phase diagram for the ternary system SOS/CTAB/ water, T ) 25.0 °C: [CTAB] and [SOS] are given in wt %; M, spherical micelles; V, vesicles; L, lamellar phase; R, rodlike micelles; LC, liquid crystal phase; I, unresolved multiphase region. Adapted from ref 15.

approach provides a general method for preparing spontaneously formed vesicles of controlled size, surface charge, and permeability from commercially available surfactants. In particular, this method requires no mechanical or chemical perturbation. Mixing leads to spontaneous formation of vesicles which can be observed as a bluishturbidity after a period of seconds to minutes. These vesicles have been characterized by glucose entrapment studies12-15,17 and by using microscopic techniques such as polarizing microscopy,16 optical microscopy,17,20 cryogenic temperature transmission electron microscopy (cryoTEM),14,15,19-21 and freeze-fracture12,13,18,21 electron microscopy. It is possible to establish a phase diagram of the three-component system (anionic/cationic/water).12-17,22 Equimolar mixtures of anionic and cationic surfactants either form a lamellar phase or precipitate from solution.12-20,23 The anionic-rich vesicles tend to be less polydisperse and more unilamellar (it has been suggested that sometimes a small vesicle can be encapsulated in a large one at the concentrations employed).12-15 Vesicles reach equilibrium in about 3 months, and after this time no further change is observed.14,15 These vesicles are thought to be at a state of equilibrium; certainly they are stable for long periods of time, sometimes years.12-16 The fine structures present in the shaded regions of Figure 1 are not resolved in detail. For catanionic systems, the transition from vesicles to micelles is abrupt as shown in Figure 1, and as a result no coexistence of micelles with (12) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 247, 1371. (13) Kaler, E. W.; Harrigton, L. K.; Murthy, A. K.; J. Phys. Chem. 1992, 96, 6698. (14) Harrigton, L. K.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A. N.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (15) Yatcilla, M. T.; Harrigton, L. K.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski, J. A. N. J. Phys. Chem. 1996, 100, 5874. (16) Marquez, E. F.; Khan, A.; Miguel, M.; Lindman, B. J. Phys. Chem. 1993, 97, 4729. (17) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (18) Talhout, R.; Engberts, B. F. N. Langmuir 1997, 13, 5001. (19) Yaacob, I. I.; Bose, A. J. Colloid Interface Sci. 1996, 178, 638. (20) Marquez, E. F.; Oren, R.; Khan, A.; Miguel, M.; Lindman, B. J. Phys. Chem. 1998, 102, 6746. (21) Chiruvolu, S.; Israelachvili, E. N.; Xu, Z.; Zasadzinski, J. A. N.; Kaler, E. W.; Harrigton, L. K. Langmuir 1995, 11, 4256. (22) Caria, A.; Khan, A. Langmuir 1996, 12, 6282. (23) Nilsson, U.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1993, 97, 5654.

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vesicleshasbeenobservedinanysurfactantmixtures.13-15,24a However, other studies involving different surfactants and/ or preparation methods showed evidence for coexistence of micelles and vesicles.24b-e For the cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfate (SOS) system, using small-angle neutron scattering (SANS) to investigate the aggregate composition,25 it was found that a 2 wt % CTAB/SOS solution mixed in a 3:7 ratio provided a system with a bilayer composition of 40 mol % CTAB compared to an overall composition of 21 mol % CTAB. The bilayer thickness was found to be 22 Å. The effect of salt addition on catanionic vesicle systems was also investigated by Kaler et al.25,26 In the anionic-rich region, instead of progression to “flatter” microstructures with increasing salt concentration as found for lipids, a vesicle-to-micelle phase transition was observed. Hatton et al.27 performed kinetic studies on the SOS/ CTAB system. They described the formation of vesicles as a sequence of distinct processes, beginning with the formation of mixed micelles, passing through a stage where these micelles rearranged to form loose bilayer structures, culminating in the bending of the bilayers to form vesicle precursorsswhich then grow to give the final vesicle size. Recently, they suggested a model based on the initial formation of nonequilibrium vesicles and disks (floppy bilayer membranes) followed by vesicle growth by coalescence and fusion of vesicles. There have been several theoretical studies on the spontaneous formation of vesicles from anionic/cationic mixtures, and the basis of their enhanced stability has been discussed.29-35 Safran et al.29 showed that when two surfactants are mixed to form vesicles, the mixing process allows the formation of surfactant bilayers where the two monolayers have different surfactant compositions. The vesicular phase can be stabilized by the curvature energy, even in the limit of a large curvature elastic modulus. May et al.30a analyzed the free energy of a mixed “binary” bilayer in which the two amphiphilic components have identical headgroups but hydrophobic tails of different lengths They concluded that spontaneous vesicle formation in a bilayer composed of amphiphiles that differ only in their chain length could not take place unless the mixing was nonideal. These findings help to explain why vesicles can form over a wide range of mixing ratios and that they are more stable and more unilamellar in the case of asymmetric mixing of anionic and cationic surfactants. (24) (a) Soderman, O.; Harrigton, L. K.; Kaler, E. W.; Miller, D. D. Langmuir 1997, 13, 5531. (b) Hjelm, R. P.; Thiyagarajan, P. Alkan, H. J. Phys. Chem. 1992, 96, 8653. (c) Egelhaaf, S. U.; Schurtenberger, P. J. Phys. Chem. 1994, 98, 8560. (d) Heerklotz, H.; Binder, H.; Lantzsch, G.; Kloise, G.; Blume, A. J. Phys. Chem. B 1997, 101, 639. (e) Lichtenberg, D.; Robson, R. J.; Dennis, E. A. Biochim. Biophys. Acta 1983, 737, 285. (f) Almog, S.; Kushnir, T.; Nir, S.; Lichtenberg, D. Biochemistry 1986, 25, 2597. (25) Brasher, L. L.; Harrington, L. K.; Kaler, E. W. Langmuir 1995, 11, 4267. (26) Walker, S. A.; Zasadzinski, J. A. Langmuir 1997, 13, 5076. (27) O.Connor, A. J.; Hatton, T. A.; Bose, A. Langmuir 1997, 13, 6931. (28) Shioi, A.; Hatton, T. A. Paper to be submitted. (29) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354. (30) (a) May, S.; Ben-Shaul, A. J. Phys. Chem. 1995, 103, 3839. (b) Fogden, A.; Hyde, T. S.; Lundberg, G. J. Chem. Soc. Faraday Trans. 1991, 87, 949. (c) Fogden, A.; Daicic, J.; Kidane, A. J. Phys. II 1997, 7, 229. (31) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3818. (32) Bergstrom, M.; Erksson, J. C. Langmuir 1996, 12, 624. (33) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3819. (34) (a) Blankschtein, D.; Shilach, A.; Zoeller, N. Curr. Opin. Colloid Interface Sci. 1997, 2, 294. (b) Bucak, S. Ph.D. Thesis, University of East Anglia, May, 2000. (35) Engberts, B. F. N.; Kevelam, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 779.

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Fogden, Daicic et al.30b,c have considered in detail the bending energy of surfactant films and membranes. Zasadzinski et al.21,26 focused attention on the longterm stability of catanionic vesicles, and in particular the reason they do not aggregate even over periods of years. Addition of salt induces fusion and the formation of multilamellar structures by screening the repulsive interaction. Zasadzinski et al.26 concluded that the major contribution to bilayer repulsion and stability against aggregation in catanionic vesicles at low concentrations is simply the electrostatic double layer forces generated by the charged membranes, so for optimum stability added salt should be avoided. Blankschtein et al. concluded that catanionic vesicles were entropically favored and chain packing in the vesicle bilayer played an important role in determining the free energy of vesiculation.33 By application of a molecularthermodynamic theory, it was concluded that in the case of asymmetric chains, mixing is energetically favored, but in the case of more similar chains, mixing is entropically stabilized. Materials and Methods The surfactant cetyldecyldimethylammonium bromide (C16C10DMABr) was synthesized from N,N-dimethylhexadecylamine and 1-decyl bromide as described in our previous work.36 The appropriate microanalyses and NMR spectra of the product were characteristic of the compound. All other single-chain surfactants used (decyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and tetradecyltrimethylammonium bromide) were purchased from Fluka (purity g98%) and used without further purification. Cetyltrimethylammonium bromide (CTAB, purity g99%)) was purchased from Fluka and was used without further purification. SOS was purchased from Acros (purity g99%) and was purified further as suggested by Kaler et al.12-14 Structural characterization of vesicles was achieved using a combination of methods, including photon-correlation spectroscopy (PCS) and video-enhanced light microscopy. The PCS setup comprises an argon ion laser (of incident wavelength 488 nm) linked to a Spectra Physics laser control unit. A photomultiplier is located at a scattering angle of 90°. A Malvern K7025 photon correlator linked to a Malvern software program was used for data analysis. For microscope studies, a Zeiss Axiovert 25 inverted microscope with 6-V 25-W lighting, 5-fold lens turret, and phase contrast slide were used. The microscope is attached to a camera, and images are displayed on a Sony television screen and recorded by a Panasonic NVHS900B video. The camera is also connected to a Viglen PC, which is used to optimize, adjust, and store the data. Kinetic measurements were made using a HewlettPackard diode-array spectrophotometer and/or a Hi-Tech small-volume single-mixing stopped-flow instrument (dead time ∼5 ms) in absorbance (turbidity) mode at a wavelength of 300 nm. The stopped-flow is a SF52 stoppedflow unit (SFU) coupled to a SF-40C spectrophotometer control unit supplied by Hi-Tech Scientific Instruments. The solutions are thermostated by a flow circuit from a Haake F3 water bath into the SFU with temperature control to (0.1 °C. The spectrophotometer used is a Hewlett-Packard 8452A diode-array controlled by a Hewlett-Packard Vectra microprocessor with the temperature controlled by a Grant LTD6G water bath to (0.1 °C. (36) Robinson, B. H.; Bucak, S.; Fontana, A. Langmuir 2000, 16, 8231.

Bucak et al.

Analysis and Interpretation of the Kinetic Data. For the following reaction in an aqueous medium

vesicles + single-chain surfactant f mixed micelles a general rate expression would be

rate )

d[micelle] ) k[SCS]m[vesicle]n dt

(1)

The reaction described above is potentially complex since the reaction taking place is necessarily a multistep process. In addition, the vesicle dispersions tend not to be initially monodisperse, and therefore vesicles of different size coexist in the medium; these may well have different breakdown rates. Therefore, a phenomenological approach has been employed in an attempt to understand the kinetics of vesicle breakdown. The process of vesicle breakdown is monitored by measuring the turbidity change of the suspension by means of a UV-vis or stopped-flow spectrophotometer. A typical stopped-flow spectrum resembles a first-order exponential decay. Treating the data in this way allows the determination of an apparent first-order rate constant “kapp”, which can be used as a measure of the rate of the reaction. When the single-chain surfactant concentration is varied, different kapp values are obtained. The results in this paper are presented in terms of kapp expressed as a function of the added single-chain surfactant concentration. If kapp is substituted into the original rate expression, the following equation is obtained

rate ) kapp[vesicle]n

(2)

The profiles obtained from plotting kapp versus the concentration of the added single-chain surfactant are linear which implies that the reaction is first order with respect to the added single-chain surfactant; therefore

rate ) k[SCS][vesicle]n

(3)

The kinetic data for the breakdown process have been analyzed by this approach. Experimental Section The C16C10DMABr System. C16C10DMABr vesicle solutions were prepared by dissolving the solid in Anala water at room temperature, sonication not being needed for this system. Solutions were then maintained at a temperature of 25 °C for at least 2 h prior to use in kinetic experiments. The SOS/CTAB System. The vesicle stock solution I was prepared by mixing two 3 wt % aqueous solutions of SOS and CTAB in a 72:28 ratio, respectively. This solution (stock I) was then diluted by factors of 0.8, 0.6, 0.4, and 0.2 to make solutions of stock II, III, IV, and V, respectively. All the resulting solutions were then incubated at 25 °C for up to 3 months in order for the solutions to reach their final equilibrium state. A stock A vesicle solution (2.08 wt % SOS and 0.42 wt % CTAB) was also prepared. These solutions were incubated for a week or so at 25 °C.

Results and Discussion The Cationic Vesicle System. For a vesicle system containing only a single double-chain amphiphile, the bending curvature does not obviously appear to favor the formation of vesicles since the inner and outer layers of the vesicle contain the same molecular species. For spontaneous formation of vesicles, different bending curvatures of the bilayers are required. The problem of bending of the layers can be resolved to some extent by varying the packing and separation of the two chains of the double-tailed surfactant, which may optimize align-

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Scheme 1

ment to create local curvature; however, relatively large vesicles are expected to form as a result of this. The cationic surfactant with one long (C16) and one relatively short (C10) chain is easily solubilized in water due to the presence of the shorter chain, and this results in the vesicle bilayer being in the “fluid” state at temperatures close to ambient. Bending of the bilayer is likely to be facilitated by the asymmetry of the tails aligning the short chain in such a way as to ease the bending on “edges”, so that a more spherical structure can be predicted.22,36 The vesicles formed were polydisperse, as determined by photon correlation spectroscopy measurements.36 These vesicles were not extruded for the kinetic studies reported in this paper, as the focus of the work is on the breakdown of spontaneously formed vesicles. Compared with the di-C18 system, these vesicles were not very stable. This long-term instability was not investigated in detail, but a slow decrease in turbidity of the medium was observed over a period of days. As a result, all vesicle solutions were discarded 24 h after preparation. Some thermodynamic properties of this system were presented in a previous paper.36 The vesicles of C16C10DMABr did not show any tendency to undergo flocculation or fusion while being observed under an optical microscope. These vesicles, not being truly thermodynamically stable, display “hysteresis” effects during a heatingcooling cycle.34b This may indicate that vesicles can be kinetically trapped at certain local energy minima. It is worth noting that different protocols followed in the preparation of vesicles can then lead potentially to different vesicle structures/morphologies. Therefore, to obtain reproducible results it is essential to define these protocols prior to a given set of experiments and consistently apply the defined protocol. In particular, one might expect the kinetics of vesicle disintegration to depend on the size of the vesicles. When the vesicle suspension is mixed with an equal volume of the single-chain surfactant, initially it is expected that the single-chain surfactant intercalates into the vesicle bilayer resulting in vesicle growth.36 However, above a certain concentration of added single-chain surfactant, the vesicles break down spontaneously into mixed micelles. The main emphasis in this paper is on the breakdown kinetics of C16C10DMABr vesicles on addition of an N-alkyltrimethylammonium bromide single chain surfactant, and the primary aim is to study the effect of changing the concentration of the added single-chain surfactant on the vesicle breakdown kinetics. The protocol used for the experiments reported in this paper is as follows: The single-chain surfactant CnTAB, where n ) 10, 12, and 14, was added to the vesicle solution of C16C10DMABr in a 1:1 mixing ratio as shown in Scheme 1. For concentrations of added CnTAB lower than the critical breakdown concentration (cbc) for the vesicle-tomicelle transition, a fast increase (t1/2 ∼ 10-100 ms) in turbidity is observed due to incorporation of the singlechain surfactant into the vesicle bilayer. When the CnTAB concentration is high enough (i.e., above the cbc), fast vesicle breakdown occurs within ∼5 s, the rate being dependent on the nature and concentration of the singlechain surfactant.

Figure 2. Breakdown of 2 mmol dm-3 C16C10DMABr vesicles on addition of CnTAB (n ) 10, 12, 14), T ) 25.0 °C. Scheme 2

It can be seen from the results presented in Figure 2 that there exists an excellent linear correlation between the rate of breakdown and the concentration of CnTAB added. The gradients are as follows: C10TAB (cbc ) 45 mmol dm-3), 0.31 ((0.04) dm3 mmol-1 s-1; C12TAB (cbc ) 11 mmol dm-3), 1.4 ((0.21) dm3 mmol-1 s-1; C14TAB (cbc ) 1.25 mmol dm-3) 11.1 ((0.46) dm3 mmol-1 s-1. Vesicle breakdown can also be achieved by premixing vesicles of C16C10DMABr with the single-chain surfactant (CnTAB) such that the final composition is just inside the vesicle domain. Breakdown of vesicles was then induced on addition of further single-chain surfactant as depicted in Scheme 2: The rates of breakdown according to Scheme 2 were generally faster than those in the case of Scheme 1. The rate of breakdown is, again, strongly correlated to the concentration of the added single-chain surfactant that is again providing the “driving force” for vesicle breakdown as shown in Figure 3. The gradients are as follows; C10TAB, 4.2 ((0.2) dm3 mmol-1 s-1; C12TAB, 6.0 ((0.2) dm3 mmol-1 s-1; C14TAB, 23.3 ((2.2) dm3 mmol-1 s-1. The vesicle-to-micelle transition was induced by addition of single-chain surfactants (CnTAB). The cmc values of the CnTAB surfactants are related to the transition concentration for the breakdown as shown in Table 1. This is an expected result as the cmc can be described as a measure of the “hydrophobicity” of the surfactant. Surfactants with low cmc’s integrate into the vesicle bilayer much more readily than those with higher cmc’s since the efficiency of absorption into the bilayer will be related to the hydrophobicity. The rate of breakdown of C16C10DMABr vesicles by addition of CnTAB surfactant micelles increased with increase in the concentration of CnTAB. Addition of CnTAB is the “driving force” for the breakdown reaction. The larger the driving force, the faster the rate of the breakdown reaction. This is important because the reaction can be slowed by a factor of up to 300 by using a

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Bucak et al. Scheme 3

Scheme 4

Figure 3. Breakdown of mixed vesicles of 2 mmol dm-3 C16C10DMABr and x mmol dm-3 CnTAB on addition of more CnTAB (n ) 10, 12, 14), T ) 25.0 °C: x ) 20 mmol dm-3 (n ) 10); x ) 10 mmol dm-3 (n ) 12); x ) 1 mmol dm-3 (n ) 14).

Figure 4. Table 1 name of surfactant

cmcref (mmol/L)

critical breakdown concn (mmol/L)

C10TAB C12TAB C14TAB

67.6 12.4 3.8

45 ((3.8) 11 ((0.3) 1.3 ((0.2)

CnTAB solution with a concentration just above the transition region. In this region, the driving force would be just sufficient for the reaction to take place but the reaction will be slow. It can be concluded that there is a linear relationship between the driving force and the rate of the breakdown reaction. From Figure 4 a plot of log ∆kapp/∆[CnTAB] against log cmc gives a straight line with a slope of ca. -1, which suggests that the hydrophobicity of the added CnTABs contributes very significantly to the breakdown of C16C10DMABr vesicles. The C16TAB surfactant is somewhat anomalous because the concentrations used to break down the vesicles are greater than the cmc value, and its Krafft temperature is also above the working temperature. Breakdown kinetics with this surfactant are much slower than those for the shorter-chain surfactants. In the series C10TAB, C12TAB, and C14TAB, C14TAB is most effective as a vesicle-breaking agent. The “driving force” effect was much more pronounced than that for the other chain lengths, such that a small difference in concentration resulted in a large observed difference in reaction rates. The breakdown process involves the

incorporation of the single-chain surfactant into the vesicle bilayer as the initial step. When a certain critical concentration of single-chain surfactant is reached, mixed vesicles (C16C10DMABr/CnTAB) transform into mixed micelles, since the single-chain surfactant prefers to form micelles. Initial incorporation of the single-chain surfactant into the vesicle structure is likely to result in a change in morphology of the vesicles, which then leads to their breakdown. Because the final concentration of the system exceeds the transition concentration, the “driving force” concept dominates the kinetic considerations. The Catanionic System. Vesicle breakdown was achieved by mixing vesicle solutions with additional SOS at a sufficiently high concentration to take the system into the micelle-stable domain. Mixing was always carried out in a 1:1 ratio. The final solution was optically clear, which is entirely different to the bluish-turbid vesicle dispersion normally encountered. The breakdown of vesicles, therefore, can be followed by means of the turbidity change in a stopped-flow spectrophotometer. This is shown schematically in Scheme 3. This reaction scheme is shown schematically on the phase diagram shown in Scheme 4. Figure 4 shows the turbidity of the vesicle solutions as a function of time for different added SOS concentrations. It can be seen that as the concentration of added SOS is increased, the turbidity decrease is more abrupt. The reaction rates, calculated as previously described, are shown in Figure 5. The gradients for each plot are as follows: stock I, 380 ((22) dm3 mmol-1 s-1; stock II, 340 ((14) dm3 mmol-1 s-1;

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Langmuir, Vol. 18, No. 22, 2002 8293 Scheme 5

Scheme 6

Figure 5. Breakdown of stock IV vesicle solutions on addition of SOS at 25 °C, typical stopped-flow traces.

Figure 6. Breakdown of SOS/CTAB vesicle solutions on addition of SOS at 25 °C. The arrows corresponding to each set show the phase boundary for that solution.

Figure 7. Breakdown of vesicles (SOS/CTAB) on addition of 2.5 wt % SOS and various concentrations of CTAB.

stock III, 180 ((6.2) dm3 mmol-1 s-1; stock IV, 150 ((9.8) dm3 mmol-1 s-1; stock V, 137 ((8.5) dm3 mmol-1 s-1; The “driving force” for the breakdown reaction is the addition of SOS, which drives the mixed vesicle solution into a mixed micellar region. It is again evident that the reaction rates increase with increasing SOS concentration. The driving force can be applied in a different direction, again moving the solution into a micellar region. This is shown schematically in Scheme 5. This reaction scheme is shown schematically in Scheme 6.

The gradient is as follows: stock A, -12.9 (+-0.20) dm3 mmol-1 s-1. The graphs above clearly show the effect of the driving force on the reaction kinetics. It is noteworthy that when more SOS is added, the rate of breakdown of vesicles increases. In other words, the larger the “driving force”, the faster the reaction. Systems which are at true equilibrium are in a sense “ideal” model systems and lead to a better understanding about vesicle behavior. This type of system is therefore a better candidate for the study of spontaneous vesicle breakdown and the possible intermediates in vesicle-tomicelle transitions. SOS/CTAB vesicles reached their final equilibrium state in 3 months. During that period, the dispersion changes in terms of turbidity, size, and possibly morphology until the vesicles reach an energy minimum. These vesicles are then not “kinetically trapped” at a local minimum of energy. Kinetic experiments performed with vesicles before equilibrium is reached gave different results than those performed after equilibrium has been attained. This implies that significant changes to the vesicles are taking place over times up to 3 months. Induced breakdown of vesicles takes place at a much faster rate (milliseconds to seconds) than vesicle formation. Formation of vesicles can be induced on addition of a micellar solution of CTAB to a micellar solution of SOS. Formation has an initial fast phase followed by a relatively slow change, taking a few hours, settling down to a small change in rate which lasts up to months (3 months for this particular catanionic surfactant couple).27 With increase of the SOS concentration, the system is perturbed more toward a micellar region. This trend is evident in all experiments including those carried out with nonequilibrated samples. As the final concentration of solution approaches the phase boundary for the vesicleto-micelle transition, the rate constants for the breakdown reaction go down by a factor of ∼100.

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The “driving force” for vesicle breakdown was applied in two different directions in the plane of the phase diagram, but in both cases into the SOS-rich micellar region. Addition of SOS micelles (as shown in Scheme 5) took the vesicle solution diagonally downward into the micellar region, whereas addition of SOS/CTAB mixed micelles (as shown in Scheme 6) moved it horizontally across to the right into the micellar region. Although the direction of the driving force was different, the consequences of the “driving force” were essentially the same. ∆kapp/∆[SOS] is a measure which gives information on the extent to which each solution is affected by the driving force. On this basis, concentrated vesicle solutions appear to be more affected by the driving force than the more dilute systems. A small change in the driving force results in a bigger difference in concentrated solutions. When a dilute solution is perturbed by addition of SOS micellar solution, the vesicle solution first progresses through regions where vesicles still exist and then finally the vesicle lobe is exited. Therefore, the reaction will take place over a longer time scale and the driving force effect is less pronounced. In the last set of experiments, vesicle solutions were perturbed in a different direction upon the addition of a SOS/CTAB micellar solution. The driving force effect is essentially the same in the sense that the breakdown becomes faster as the final concentration of the solution is further away from the vesicle-micelle phase boundary. In a previous series of experiments, Brinkmann, Neumann, and Robinson investigated the thermodynamics and kinetics of the vesicle to mixed micelle transition for the anionic surfactant system sodium tridecylbenzene-

Bucak et al.

sulfonate (STBS)/sodium dodecyl sulfate (SDS) in the presence of 50 mmol dm-3 sodium chloride. In this study, the final concentration reached on perturbing the STBS system with SDS was located just inside the mixed micelle regime. The kinetics were characterized by a lag phase followed by a first-order decay associated with vesicle breakdown, the whole process taking place in the 10-100 s time range. The systems studied in this paper relate to much higher concentrations of added single-chain surfactant, and in many ways these systems are simpler to interpret. Conclusions Vesicle systems are complex to study kinetically. This mainly arises from the fact that they are usually not at thermodynamic equilibrium and they can possess high polydispersity. In this work, two different vesicle systems were studied. It has been shown that vesicle breakdown is first order with respect to addition of single-chain surfactant, when the added surfactant takes the system into the micelle stable region The reaction rate increases as the driving force or concenration of surfactant is increased. It has been shown that a rate decrease of a factor of up to 100 can be achieved by driving the vesicles only just outside the vesicle-stable region. This behavior is likely to apply to other vesicle-forming systems. Decreasing the vesicle breakdown rate is useful in investigating the intermediates present in the vesicleto-micelle transition, and small angle neutron scattering experiments in this area are under investigation. LA020064I