Langmuir 2000, 16, 9099-9102
Sodium Octyl Sulfate/Cetyltrimethylammonium Bromide Catanionic Vesicles: Aggregate Composition and Probe Encapsulation† C. Caillet, M. Hebrant, and C. Tondre* Laboratoire de Chimie Physique Organique et Colloı¨dale, Unite´ Mixte de Recherche CNRS-UHP (UMR 7565), Universite´ Henri Poincare´ -Nancy I, B.P. 239, 54506 Nancy-Vandoeuvre Cedex, France Received June 8, 2000. In Final Form: July 12, 2000
Introduction A great deal of work has recently been focused on the so-called “catanionic vesicles”, which have been shown to form spontaneously from mixtures of single-tailed cationic and anionic surfactants.1-7 Many efforts were made to characterize the phase behavior of their aqueous mixtures and the structures obtained depending on the system composition.2-7 In many cases transmission electron microscopy was used to image the microstructure of the particles obtained. There is no doubt that the association of a cationic and an anionic surfactant through electrostatic attractions of their polar heads can mimic in some way the behavior of double-chain surfactants such as phospholipids. A large variety of systems were investigated, changing the respective lengths of the alkyl chains that will be paired or changing the nature of the polar heads. Fewer studies were concerned with “ion pair amphiphiles” in which the counterions, normally included in the surfactant mixture, were removed.8-10 A generally admitted feature is that precipitation of catanionic surfactant mixtures occurs under equimolar conditions. In fact, the examination of already published phase diagrams shows that vesicle formation usually takes place with an excess of one of the surfactants.2,5,6 One of the main applications of vesicular systems is their use as vectors for active molecules. The ionic nature of the surfactants used here seems to exclude biomedical applications but there are other situations where delivery systems could include this type of surfactant. Some results of entrapment experiments using catanionic vesicles were reported,1,5,9 but there is no clear view concerning their real capacity to encapsulate probe molecules and their subsequent release. In addition, one should be careful about the tendency to generalize the findings concerning one particular system. * To whom correspondence should be addressed. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. (1) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (2) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (3) Ambu¨hl, M.; Bangerter, F.; Luisi, P. L.; Skrabal, P.; Watzke, H. J. Langmuir 1993, 9, 36. (4) Zhao, G.-X.; Yu, W.-L. J. Colloid Interface Sci. 1995, 173, 159. (5) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (6) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95. (7) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski. J. A. J. Phys. Chem. 1996, 100, 5874. (8) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J. Am. Chem. Soc. 1990, 112, 1635. (9) Bhattacharya, S.; De, S.; Subramanian, M. J. Org. Chem. 1998, 63, 7640. (10) Chung, Y.-C.; Lee, H.-J. Bull. Korean Chem. Soc. 1999, 20, 16.
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Our objectives when starting the present work were to quantify the entrapment ability of different catanionic vesicles, especially the dependence on the characteristics of their amphiphilic films, and to evaluate the film stability through probe permeation experiments. The unexpected results obtained with sodium octyl sulfate (SOS)/cetyltrimethylammonium bromide (CTAB) vesicles prompted us to report these preliminary results. In relation to the preceding objectives, we made some analysis of the external aqueous phase in equilibrium with the mixed surfactant vesicles. From this analytical approach, which to the best of our knowledge was never reported thus far, we have gained some new insight into the composition of the vesicular aggregates. Experimental Section Cetyltrimethylammonium bromide (CTAB) was purchased from Fluka (purum ≈ 98%) and was twice recrystallized in methanol-diethyl ether. Sodium octyl sulfate (SOS) from ACROS (HPLC grade, 99%) was used as received. The vesicle dispersions in the case of surfactant mixtures (SM) were obtained by mixing equal volumes of 1.5% CTAB and 3.4% SOS. A certain equilibration time was necessary before use. To obtain the ion pair amphiphile (IPA), the surfactants were passed over a strongly basic or a strongly acidic resin (Merck) to obtain respectively CTAOH and HOS, which were then mixed in equimolar conditions. For the entrapment experiments the probe was added to each solution before mixing. The probes 5(6)-carboxyfluoresceine (CF), riboflavine (RF), and glucose were respectively from Eastman Kodak and Sigma for the latter two. Triton X-100 was from Fluka. The concentration of the fluorescent probes was determined with a Shimadzu RF540 spectrofluorimeter after standard curves had been established under conditions as close as possible to the experimental ones. The destruction of the vesicles to ensure complete release of the probes was obtained either by adding SOS (in the case of CF) or Triton X-100 (in the case of RF and glucose). The analysis of the glucose content was performed according to the procedure described in ref 11, which is based on the oxidation of glucose by glucose oxidase. This reaction is followed by a colorimetric determination (435 nm) using o-dianisidine. Standard curves were adapted to the different conditions used and a blank absorbance was systematically deduced. Size exclusion chromatography (SEC) was used to separate the vesicle-encapsulated probes from the free probes. A Bio-Rad (Biologic LP) apparatus, equipped for turbidity (or UV 254 nm) and conductivity detection, was used with a fraction collector (model 2128) and a double-trace recorder (Tracelab BD 41). The column was filled with Sephadex G-50 (Medium, Sigma) and the void volume was calibrated with 2000 kDa Blue Dextran. The following experimental conditions were applied: column length, 25 cm; diameter, 1.5 cm; flow rate, 1.1 mL/min. The analysis of the bromide content was performed by an electrochemical method based on a titration with a standard solution of 0.1 M AgNO3. A combined electrode, Ag/Ag+//K2SO4/ Hg2SO4/Hg (XM 950 from Tacussel), was used for that purpose. The sodium content of the solutions was obtained from atomic absorption spectroscopy (Varian AA-1275) at λ ) 589 nm, with an air/acetylene flame. The standards included a matrix (especially surfactants) as close as possible to the composition of the solution to be analyzed. Acid titration after ionic exchange on a strongly basic resin gave the total concentration of CTA+ and Na+. Subtracting the known concentration of Na+ gave the CTA+ concentration. The OS- concentration was obtained from HPLC, from which we could also obtain a second estimation of the Brcontent. The HPLC apparatus was composed of a Shimadzu LC10 ATVP pump, a Sedex 55 E.L.S. detector, and a Kromasil C18 column (25 cm). The eluent was a mixture of 80% methanol (11) Huggett, A. StG.; Nixon, D. A. Biochem. J. 1957, 66, 12.
10.1021/la000810o CCC: $19.00 © 2000 American Chemical Society Published on Web 09/19/2000
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Langmuir, Vol. 16, No. 23, 2000
Notes
Table 1. Analysis of the Two Compartments after Dialysis of a Vesicular Solution of Initial Composition: [SOS] ) 7.32 × 10-2 M; [CTAB] ) 2.06 × 10-2 M vesicle compartment [Na+] 3.6 × 10-2 M [CTA+] 1.96 × 10-2 M [Br-] 1.0 × 10-2 M (0.92 × 10-2 M)b [OS-] 4.7 × 10-2 M
aqueous compartment
deduced vesicle compositiona
3.5 × 10-2 M 0.16 × 10-2 M 1.06 × 10-2 M (0.92 × 10-2 M)b 2.7 × 10-2 M
≈0 1.8 ((0.1) × 10-2 M ≈0 2.0 ((0.1) × 10-2 M
a
The uncertainty takes into account the mass balances. b Values obtained from HPLC (see the text for the other cases).
(Merck) and 20% of an aqueous solution of 0.1% trifluoroacetic acid, adjusted to pH 8 with triethylamine. Dialysis experiments were carried out with a microdialysis device including eight microcells with two compartments of 1 mL each. Cellulose tubing (Visking), with an average pore diameter of 24 Å, separated the two compartments. For the determination of the continuous phase of the vesicular systems, 16 h with a gentle rotation of the block of cells was allowed for equilibration. For the ultrafiltration/centrifugation experiments, we made use of small centrifugating cells called Microcon-10 (Amicon-Millipore) containing 500 µL of vesicular dispersion. They included a membrane of regenerated cellulose (Amicon YM10, with molecular weight cutoff 10 000 Da). The samples were centrifugated for 6 min at 11 000 rpm (Sigma-201 M). Enough permeate (ideally 250 µL) was collected under these conditions to allow for the determination of the glucose content.
Results and Discussion Different probes were used to determine the entrapment capacity of the particles whether they were obtained by the simple mixture of the two surfactants (“SM” in the following) or after elimination of the counterions (“IPA” in the following). We used gel permeation chromatography to separate the probe-containing vesicles from the probes that remained free. These experiments required a good choice of the eluent, so as to be as sure as possible that the particles kept their integrity during their migration along the chromatographic column. This prompted us to determine, in the case of the SM system, the composition of the continuous aqueous phase in equilibrium with the particles. In fact, using this continuous phase as the eluent (see below) did not improve the procedure, but the result was found interesting by itself. The choice of the initial composition of the CTAB/SOS system was based on the phase diagram previously published by Yatcilla et al.7 This composition was taken in the middle of the large vesicle domain obtained with an excess of SOS, and it included 7.32 × 10-2 M SOS and 2.06 × 10-2 M CTAB. Microdialysis experiments were performed in which the vesicular dispersion was put on one side of a dialysis cell whereas a similar volume of pure water was introduced on the other side. We assumed that the vesicular particles will be retained by the semipermeable membrane, whereas the free molecules will equilibrate on both sides of the membrane, which implies a dilution by a factor of 2. Both compartments were analyzed according to the procedures described in the Experimental Section. The results obtained are given in Table 1. They verify quite satisfactorily the mass balance and electroneutrality requirements. It turns out that the composition of the colloidal particles themselves is very close to equimolar, although stable dispersions require the presence of an excess of one of the surfactants (SOS in the present case). On the basis of the values reported in Table 1, it can be inferred that the vesicles contain ≈47 mol % CTA+. This appears to be a confirmation of previous results deduced by Brasher et
al.12 from a SANS study, in which a value of 45 mol % CTAB was suggested. It is also in quite close agreement with a theoretical prediction from molecular-thermodynamic modeling performed by Yuet and Blanckschtein,13 who end up with a peak in the composition distribution corresponding to a mole fraction of 0.44. We can also notice that the almost perfect equilibration of both the Na+ and Br- counterions between the two compartments strongly suggests that ion pair amphiphiles were formed and this gave further encouragement to investigate the properties of IPA. Coming now to the entrapment experiments, we will just briefly mention the results obtained with two fluorescent probes (carboxyfluoresceine (CF)14 and riboflavine (RF)9), to give more emphasis on the results obtained with glucose.1,5 All three probes were chosen because of their hydrophilicity. With CF the measured encapsulation was extremely low (
