Langmuir 1996, 12, 3045-3054
3045
Normal and Reverse Vesicles with Nonionic Surfactant: Solvent Diffusion and Permeability Ulf Olsson* Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Kazuyoshi Nakamura† and Hironobu Kunieda Department of Physical Chemistry, Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan
Reinhard Strey Max-Planck-Institut fu¨ r Biophysikalische Chemie, Postfach 2841, D-37018 Go¨ ttingen, Germany Received January 19, 1996. In Final Form: March 14, 1996X Normal and reverse vesicle systems with nonionic surfactants were investigated. The solvent selfdiffusion coefficient was measured using the 1H-NMR Fourier transform pulsed gradient spin-echo technique. In the case of normal vesicles, the water solvent is found to exchange rapidly between the inside and outside of the vesicles on the experimental time scale (≈0.1 s). Here, only an average self-diffusion coefficient can be measured from which the fraction of entrapped water can be determined. In the reverse vesicle case, we observe either a fast or a slow exchange, depending, on the oil and the bilayer composition. In particular we have investigated a semipermeable membrane system, where with a solvent mixture, one of the two types of solvent molecules exchanges fast while the other exchanges slowly on the experimental time scale. Here, the lifetime of a solvent molecule inside the reverse vesicles was found to depend on the composition of the mixed reverse bilayers, leading to an observed transition from fast to slow exchange conditions when varying the bilayer composition. In the slow exchange case, the self-diffusion coefficients of solvent molecules on the outside and inside of the vesicles, where the latter reports on the vesicle self-diffusion coefficient, are in principal resolved. From the bimodal type of decay of the spin-echo amplitude, it is also possible to determine directly the fraction of solvent molecules entrapped inside the vesicles.
1. Introduction Vesicles are closed shells of surfactant or lipid bilayers in a solvent, with typical sizes in the range of 100-1000 Å.1-3 They are important model systems for biological membranes and for fluid surfaces in general. They are also receiving increasing attention in connection with various applications, notably drug delivery.4,5 In the past vesicles were mainly considered in aqueous lipid dispersions. Later, however, they have also been encountered with simple surfactants.6,7 Recently, it has also been demonstrated that vesicles, like all other surfactant selfassembly structures, can occur in both normal and reverse geometries.8,9 With normal vesicles we here refer to the classical structure where a normal bilayer, composed of * To whom correspondence should be addressed: e-mail
[email protected]. † Present address: Department of Living Science, Faculty of Education, Niigata University, Igarashi Nino-cho 8050, Niigata 950-21, Japan. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (2) Lasic, D. D. Liposomes: From Physics to Applications; Elsevier: Amsterdam, The Netherlands, 1993. (3) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain: Where Physics and Biology Meet.; VCH: New York, 1994. (4) Fendler, J. Membrane Mimetic Chemistry; Wiley: New York, 1983. (5) Liposomes: From Biophysics to Therapeutics; Ostro, M. J.; Ed.; Dekker: New York, 1987. (6) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (7) Szo¨nyi, S.; Cambon, A.; Watzke, H. J.; Schurtenberger, P.; Wehrli, E. In The structure and Conformation of amphiphilic Membranes; Lipowski, R., Richter, D., Kremer, K., Eds.; Springer-Verlag: Berlin, Heidelberg, 1992; pp 198-201. (8) Kunieda, H.; Nakamura, K.; Evans, D. F. J. Am. Chem. Soc. 1991, 113, 1051.
S0743-7463(96)00056-X CCC: $12.00
two monolayers with their hydrophobic parts facing each other, forms a closed shell in an aqueous solvent. Reverse vesicles, on the other hand, which are formed in apolar solvents, are composed of a reverse bilayer, where the hydrophilic parts of the two monolayers are facing each other. The existence of thermodynamically stable vesicle phases is a difficult problem which has been debated extensively in the literature. The classical phospholipid vesicles are normally formed by sonicating lamellar dispersions in the two-phase region with a “bound” lamellar phase in equilibrium with excess water. These vesicles are generally considered to be only metastable. With time, which may still be very long, they relax to a stable two-phase equilibrium. Stability has been claimed in a few cases. However, the problem of distinguishing stability from metastability is evident. The absence of macroscopic phase separation is not a sufficient criteria for stability, as for example is well-known in emulsion technology. Thermodynamic stability implies a stationary and reproducible size and shape distribution. Comparing different vesicle preparations could be one possible way of distinguishing stability. We will present such a study below. In this paper we report on solvent self-diffusion, studied by the PGSE NMR technique,10,11 in some different vesicle systems with nonionic surfactants. The objectives of the paper are essentially 2-fold. We investigate the vesicle (9) Kunieda, H.; Nakamura, K.; Davis, H. T.; Evans, D. F. Langmuir 1991, 7, 1915-1919. (10) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1-45. (11) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Clarendon Press: Oxford, 1991.
© 1996 American Chemical Society
3046 Langmuir, Vol. 12, No. 12, 1996
Olsson et al.
Figure 1. Pulse sequence in the classical PGSE experiment. A π/2 rf pulse followed by a π rf pulse at time τ, produce a spin echo at time 2τ. Two gradient pulses of duration δ and amplitude G are placed on each side of the π rf pulse with a time separation, ∆. In the present experiments, the second half of the echo was Fourier transformed, and the attenuated amplitudes of the various resonances were determined from the peak amplitudes in the frequency spectrum.
systems with respect to their stability and the average vesicle size. The latter is obtained by measuring the amount of solvent contained inside the vesicles, while information on stability is gained by monitoring how this property varies with time. In addition, by investigating different systems we find different conditions for the solvent diffusion depending on the membrane permeability. 2. Experimental Section Materials. Sucrose monoalkanoate (fatty acid contents 10 wt% C14, 40 wt % C16, and 50 wt % C18, abbreviated as DKE) was supplied from Dai-ich Kogyo Seiyaku Co. The monoester content is above 95%. Homogeneous tetraethylene glycol dodecyl ether (C12E4) and hexaethylene glycol hexadecyl ether (C16E6) were obtained from Nikko Chemicals Co. Extra-pure-grade decane was obtained from Sigma. Cyclohexane (99.5% purity), hexamethyldisilane (HMDS), and hexadecane (99% purity) were obtained from Merck AG. All chemicals were used without further purification. Distilled and ion exchanged water was used. Sample Preparation. In the study of normal vesicles, samples were weighed into small tubes equipped with a screwcap. The solutions were then equilibrated in the micellar phase at lower temperatures. The samples were rapidly transferred as micellar solutions into 5 mm NMR tubes which were immediately flame sealed. In the reverse vesicle study, appropriate amounts of DKE, C16E6, and decane were put into a small vial (10 mm diameter). The mixture was warmed at about 50 °C in a water bath and well mixed with a vortex mixer. After mixing, samples were sonicated at 30.0 °C with a high-intensity ultrasonic processor (600 W model, Sonics & Materials Inc.) equipped a small titanium tip and the resultant samples were translucent bluish solutions. Optical microscopy confirmed that the reverse vesicle size is smaller than the optical microscopic scale. The samples for NMR measurements were transferred to 5 mm NMR tubes and were stored in a thermostat water bath at 30.0 °C. Self-Diffusion Experiments. The PGSE sequence (Figure 1) for measuring self-diffusion coefficients is based on the basic spin-echo sequence where a 90° rf-pulse at time t ) 0, followed by a 180° rf-pulse at t ) τ gives rise to a refocusing of the transverse magnetization at t ) 2τ of magnitude I(2τ) ) I(0) exp{-2τ/T2}, where I(0) is the initial magnetization and T2 is the transverse relaxation time (assuming exponential relaxation). In the PGSE sequence two field-gradient pulses are placed on either side of the 180° rf-pulse with duration δ and separation ∆. Translational diffusion in between and during the field-gradient pulses results in an incomplete refocusing and the echo intensity is given by10,11
I ) I0 exp{-(2τ/T2 + kD)}
(1)
k ) γ2G2δ2(∆ - δ/3)
(2)
where
Here, I0 is the equilibrium magnetization, γ is the magnetogyric
ratio, G is the magnitude of magnetic field gradient, and D is the self-diffusion coefficient. The experiments in this report were performed on a modified JEOL FX60 spectrometer operating at 60 MHz (1H) and equipped with an external 2H field frequency lock. The temperature was controlled by a thermostated air flow, with a stability better than (0.5 °C, and measured by a calibrated copper-constantan thermocouple. In our PGSE sequence, ∆ ) τ, which is of the order of 0.1 s. The echo intensity is measured in the Fourier transformed spectrum of the second half of the echo, where individual resonances can be resolved. Self-diffusion experiments were carried out by following the echo decay as a function of δ, keeping ∆ fixed. The receiver coil has a slightly different characteristic in the presence of gradients. Hence the initial magnetization, I(0), was treated as a fitting parameter in the analysis rather than measured independently. 3. Solvent Diffusion in Unilamellar Vesicle Solutions. The diffusion behavior of solvent molecules in a vesicle solution depends on the ratio τ1/τd, where τ1 is the average lifetime of a solvent molecule inside the vesicle and τd is the diffusion (or mixing) time inside the vesicles. τd can be defined as the time at which the root mean square displacement of a bulk diffusion process equals the radius, R, of the vesicle, i.e.
τd )
R2 0 6Dsolv
(3)
0 where Dsolv is the bulk solvent diffusion coefficient. With a typical solvent diffusion coefficient of 10-9 m2 s-1, τd has a value of ≈10-6 s for a vesicle radius of 1000 Å and 10-8 s for a 100 Å vesicle. The average lifetime, τ1, depends on the vesicle size and the permeability of the membrane for solvent molecules. The permeability is often expressed in terms of a permeability coefficient, P, which in fluid phospholipid membranes has a value of 10-4-10-3 cm s-1.12,2 The relation between lifetime, permeability, and vesicle size is given by2
τl ) R/3P
(4)
Here R is the vesicle radius and we have assumed a spherical shape. For an arbitrary shape, R/3 should be replaced by the volume-to-area ratio. With the values of P quoted above, the lifetime becomes 0.01-0.1 s for a vesicle radius of 1000 Å. In the limit τl . τd, the solvent molecules inside the vesicles experience the confinement and collide with and are reflected by the membrane a large number of times before being able to penetrate the membrane into the continuous solvent. In this case the solvent molecules inside the vesicles diffuse with the vesicle, while those on the outside will experience an obstruction of their Brownian diffusion paths by the presence of the vesicles, depending on the volume fraction of vesicles. In the other limit, τl ≈ τd the membranes do not constitute a barrier for the solvent molecules, for example, if the membranes are highly perforated, like a mesh. In this case, the obstruction will depend on the surfactant concentration and at low concentrations (a few percent) the solvent diffusion will be essentially unhindered with a diffusion coefficient similar to that in the neat solvent. Confining ourselves to the case when τl . τd (significant entrapment), the outcome of the PGSE experiment will depend on ratio of τl to τexp, where τexp is the time scale of the experiment (the observation time). In the PGSE experiment τexp ) (∆ - δ/3) ≈ ∆, where ∆ is the time between the two gradient pulses and δ is the gradient pulse duration. In the case τd , τl , τexp, the experiment will only yield an average diffusion coefficient of the solvent molecules. On the other hand when τd , τexp , τl, the diffusion coefficients corresponding to the inside and the outside solvent molecules, respectively, can be resolved. As mentioned above, τd is of the order of 10-6 s or shorter for vesicle radii ( 0) are predicted. κjb can be expressed in terms of the spontaneous curvature, H0, of the monolayer polar/apolar interface16
κjb ) 2κjm + 4κmlH0
(19)
where l is the distance from the bilayer midplane to the polar/apolar interface. With nonionic surfactants of the ethylene oxide type (CmEn, sometimes also abbreviated CmEOn or RmEOn), H0 can be tuned over a wide range from positive to negative values by simply varying the temperature.19,20 H0 > 0 at lower temperatures and becomes 3 × 10-5 cm‚s-1.
Langmuir, Vol. 12, No. 12, 1996 3049
Figure 4. Echo attenuation of water as a function of k ) (γGδ)2(∆ - δ/3) at 32.4 °C. Curve a is the echo attenuation from a sample containing 2.0 wt % C12E4 and SDS with a SDSto-C12E4 molar ratio of 1/610. The sample was heated rapidly from the L1 phase at ≈10 °C to the measuring temperature (32.4 °C) without shaking. The sample showed streaming birefringence. Curve b shows the echo attenuation from the same sample after additional treatment in a vortex mixer (25 °C) for about 15 s. Curve c is the echo attenuation in pure water, which is shown for comparison.
There are two particularly important observations that we would like to stress here: (i) the results depend on how the sample was prepared, and (ii) the data collected 1 week after the sample preparation are essentially identical to those collected on the day of preparation. From this we can draw the conclusion that the vesicle size distribution depends on the way of preparation and it is very unlikely that any of the two samples corresponds to a thermodynamic equilibrium. Relaxation to whatever equilibrium state is a very slow process. The addition of a trace amount of SDS allows for an additional way of preparing a sample in the LR+ region. Due to the reduction of the L1′ + L1′′ two-phase area a sample can be rapidly heated from the L1 phase at lower temperatures to the LR+ region without macroscopic phase separation in the L1′ + L1′′ region. A sample containing 2.0 wt % C12E4 and 0.0026 wt % of SDS (corresponding to a SDS-to-C12E4 molar ratio of 1/610) was prepared in H2O and properly mixed and equilibrated in a 5 mm NMR tube in the isotropic L1 phase at lower temperatures (using cold tap water of ≈10 °C). The sample was then transferred without shaking to a second water bath equilibrated at 32 °C. This “temperature jump” resulted in a homogeneously bluish sample, which when observed through crossed polarizers showed a streaming birefringence when being tapped. The sample was left at 32 °C for about half an hour after which a PGSE experiment was performed at the same temperature. The data are shown in Figure 4, where we for comparison also show data from a reference sample of pure H2O. After the PGSE experiment the sample was homogenized in a vortex mixer, which made the streaming birefringence disappear, and then a second PGSE experiment was performed. The data from this second PGSE experiment are also shown in Figure 4. As is seen, the intensity decay from both treatments of the surfactant solution is exponential with a decay constant significantly different from pure water. A slight difference between the two treatments can also be observed. The relative diffusion coefficients are D/D0 ) 0.51 in the unshakened case and D/D0 ) 0.64 after treatment in the vortex mixer. This shows again that the vesicle size distribution depends on the preparation of the sample. Treatment with the vortex mixer gives a smaller vesicle size than with only a temperature jump from the L1 phase.
3050 Langmuir, Vol. 12, No. 12, 1996
Olsson et al.
Figure 5. Echo attenuation of water as a function of k ) (γGδ)2(∆ - δ/3) at 32.4 °C. Curve c is the echo attenuation from a sample containing 2.0 wt % C12E4 and SDS with a SDSto-C12E4 molar ratio of 1/630. The sample was homogenized and equilibrated at 45 °C in the lamellar phase and then brought to the measuring temperature of 32.4 °C. The sample was left to rest at 32.4 °C for 1 h before measurements. The sample showed static birefringence. Curve d is the calculated echo attenuation for a powder lamellar phase. Curve b shows the echo attenuation from the same sample after treatment in a vortex mixer for about 15 s. Curve a is the echo attenuation in pure water, which is shown for comparison.
As an illustration we also present results from a sample that has static birefringence. At higher temperatures (≈45 °C) a stable lamellar phase is formed over a large dilution range. A sample with similar composition as above, 2.0 wt % C12E4 in H2O and an SDS-to-C12E4 molar ratio of 1/630, was prepared in a 5 mm NMR tube at 45 °C in the lamellar phase. At this temperature the sample showed static birefringence, which remains even if the sample is treated in a vortex mixer or stirred with a magnetic stirrer. After homogenization in the lamellar phase at 45 °C, the temperature was lowered to 32 °C without shaking the sample. After 1 h at 32 °C the sample still showed static birefringence. A PGSE experiment performed on the birefringent sample showed a nonexponential decay, as can be seen in Figure 5 (trace c). After this experiment, the sample was homogenized in a vortex mixer and the PGSE experiment was repeated. The treatment in the vortex mixer removed the static birefringence and the echo decay (trace b in Figure 5) was now exponential with D/D0 ) 0.60, similar to the value found for the other vortex mixed sample above. In Figure 5 we also show the echo decay from pure water (trace a). For a lamellar phase with negligible membrane penetration the local diffusion is two-dimensional and the echo decay of a powder sample is given by11
I/I0 ) e-kD
∫01 dx e-kD x
0
0 2
(20)
with D0 being the free diffusion coefficient. A calculated line, using D0 of pure water, is shown in Figure 5 (trace d). The echo decay from the birefringent sample differs significantly from the calculated line in being slower at lower k and faster at higher k, indicating that the sample does not have a classical powder lamellar structure. In the limit of k f 0, eq 20 becomes 0
I/I0 ) e-k(2/3)D
(21)
Hence, with a slope of -(2/3)D0 in a semilog plot. The birefringent sample on the other hand shows a slope at low k with a smaller magnitude ≈-0.4D0. One explana-
Figure 6. Partial phase diagram of the ternary C12E4-HMDSwater system. The C12E4-to-HMDS weight ratio is kept constant to 2/1. The L1 phase is a microemulsion similar to the micellar phase of the binary C12E4-water system (see Figure 2) and LR is a lamellar liquid crystalline phase and LR+ is a region where vesicles can be formed. The L1 phase coexists at lower temperatures with an excess oil phase (L + O).
tion of this difference is that the lamellar phase contains liposomes. However a more detailed analysis requires additional experiments and is beyond the scope of the present study. 4.2. Vesicle Formation in the C12E4-WaterHMDS System. As mentioned above, it was not possible to measure the surfactant self-diffusion in the C12E4 vesicle system due to short transverse relaxation times for the surfactant protons. In order to have a probe for the surfactant bilayer, we therefore added a hydrophobic molecule, hexamethyldisilane (HMDS), to swell the bilayers. This molecule has 6 equivalent methyl groups and hence 18 equivalent protons, so that all the proton intensity is collected in a single peak in the NMR spectrum. The surfactant-to-oil weight ratio was kept constant at 2/1. With the constant C12E4/HMDS ratio, the phase behavior (we only investigated higher water contents) become very similar to the C12E5/decane/water system,22 studied recently. A schematic phase diagram, plotted as temperature versus the concentration of C12E4, is shown in Figure 6. With increasing temperature we see the usual sequence of phases. A microemulsion phase, L (similar to the micellar phase, L1, of the binary system), is stable at lower temperatures. Increasing the temperature, the system is transformed to a lamellar phase, LR, and a further increase in temperature results in the L3, or sponge, phase. The LR phase has a finite swelling capacity, and at lower surfactant concentrations there is a region, LR+, similar to the binary C12E4-water system, where vesicles may be formed. For the self-diffusion studies we used heavy water (D2O) rather than H2O in order to simplify the detection of the low concentration HMDS. In this case the measured water diffusion corresponds to the diffusion of trace amounts of HDO present in D2O as an impurity, and also produced by hydrogen exchange with the surfactant terminal -OH group. The phase diagram of Figure 6 was however produced with H2O, and switching to D2O corresponds to shifting the temperature scale by approximately -2 °C. For example the upper phase boundary of the microemulsion phase is shifted to ≈15 °C, and the lower limit of LR+ is below 20 °C. In Figure 7 the echo decays of water and HMDS, respectively, from a sample containing 1.25 wt % C12E4 (22) Olsson, U.; Schurtenberger, P. Langmuir 1993, 9, 3389-3394.
Vesicles with Nonionic Surfactant
Figure 7. Echo decay from water and HMDS respectively as a function of k ) (γGδ)2(∆ - δ/3) in a log-log plot. The sample consists of 1.25 wt % C12E4 and 0.625 wt % HMDS and the experiment was performed at 20 °C. For water we have used ∆ ) 0.14 s, while for HMDS ∆ ) 1.0 s.
and 0.625 wt % HMDS are shown. For water we have used ∆ ) 0.14 s while for HMDS ∆ ) 1.0 s. The sample was prepared in the LR+ region at 20 °C by first a temperature jump from the L phase and then treating it with a vortex mixer. The echo decays are plotted on a log-log scale and the best exponential fits are shown as solid lines. As is clearly seen, the diffusion coefficients of water and HMDS are very different. In fact, the diffusion coefficient of HMDS is 3 orders of magnitude smaller than that of water, clearly demonstrating that the HMDS molecules are confined to closed domains and very large aggregates. The water diffusion coefficient is Dw ) 1.1 × 10-9 m2 s-1, which corresponds to Dw/D0w ) 0.69, and for HMDS we obtain DHMDS ) 1 × 10-12 m2 s-1. From the Dw/D0w value, we obtain with eq 8 a vesicle volume fraction of Φv ) 0.23, which incorporated in eq 9 gives a radius R ) 1600 Å (Φs ) 0.013 and ls ) 15 Å). From DHMDS we can also calculate a radius, using the StokesEinstein relation together with the interaction term (eqs 16 and 17). Using Φv ) 0.23 in the interaction term we obtain R ) 700 Å, approximately a factor of 2 smaller than the value obtained from the water diffusion. The discrepancy can partly be explained by a broad size distribution. With polydisperse spheres the interaction term Ψ(Φv) is expected to increase (while it also will be different for different sizes in the ensemble) similarly as the viscosity of a hard sphere suspension is lower for a polydisperse system compared to a monodisperse one. This however cannot be the only explanation since we need to see Ψ ≈ 1 in order to obtain the same radius from the HMDS diffusion. An additional effect may be that we have contributions from a finite water solubility of HMDS. While the solubility may still be low, the diffusion coefficient of individual HMDS molecules in water is expected to be of the order of 10-9 m2 s-1. Hence, we only need a fraction of 1/1000 solubilized in the water in order to have a contribution from these of 10-12 m2 s-1. In spite of the quantitative discrepancy we are able to conclude that the present system indeed forms large vesicles with a size of the order 1000 Å. 5. Reverse Vesicles Recently, the counterstructure, reverse vesicles, has been found to occur in several different systems with nonionic surfactant.8,23-29 Here the local structure is a (23) Kunieda, H.; Kanei, N.; Uemoto, A.; Tobita, I. Langmuir 1994, 10, 4006-4011. (24) Kunieda, H.; Nakamura, K.; Olsson, U.; Lindman, B. J. Phys. Chem. 1993, 97, 9525-9531.
Langmuir, Vol. 12, No. 12, 1996 3051
Figure 8. Partial schematic phase diagram of the C16E6DKE-water-decane system at 30 °C. The total surfactant concentration is 3.0 wt % and the diagram is plotted as water content versus the fraction of DKE in the surfactant mixture. The L2 phase, present at lower fractions of DKE is an oil rich microemulsion phase which at the presently low surfactant concentration is expected to contain essentially surfactant. At higher DKE fractions and lower water content the L2 phase coexists with a concentrated lamellar phase (L2 + LR). Inside this two-phase region, the shaded area indicates where stable reverse vesicles may be prepared. Redrawn from ref 29.
reverse bilayer where a small amount of water also is needed to stabilize the reverse bilayer structure. One such system which gives reverse vesicles is the surfactant mixture of a sucrose ester surfactant (DKE) and C16E6 in decane, when also a small amount water is added.29 A partial schematic phase diagram (redrawn from ref 29) of this system at 30 °C is shown in Figure 8. Here, the total surfactant concentration is fixed at 3 wt % and the weight fraction DKE/(DKE + C16E6) and the water concentration are varied. In this system, reverse vesicles can be formed in the L2 + LR two-phase region, and long-time stability is obtained for a low but finite water concentration and intermediate weight fractions of DKE/(DKE + C16E6) (shaded area in Figure 8). 5.1. Reverse Vesicles in the DKE/C16E6-WaterDecane System. In the above mentioned system we prepared reverse vesicles in decane with two different surfactant-to-water ratios (S/W ) 15 and 5, respectively) at two different surfactant concentrations (3.0 and 6.0 wt %, respectively). The DKE/C16E6 weight ratio in the surfactant mixture was kept constant at 60/40. The samples were sonicated and stored at 30 °C. This reverse vesicle system, with decane as solvent, has been studied previously by other techniques. Slow Exchange. Figure 9 shows (I/I0) of the methylene proton signal as a function of k for a sample containing 3.0 wt % surfactant (DKE/C16E6 with weight ratio 60/40) and 0.2 wt % of water in decane at 30 °C. For a comparison is also shown the echo decay from pure decane. As is clearly seen in this semilogarithmic plot, the echo intensity from the vesicle solution decays with (at least) two wellseparated rate constants. The fast mode, having a relative amplitude of about 0.98, decays with approximately the same rate as pure decane. The slow mode, having a small (25) Kunieda, H.; Nakamura, K.; Infante, M. R.; Solans, C. Adv. Mater. 1992, 4, 291. (26) Kunieda, H.; Makino, S.; Ushio, N. J. Colloid Interface Sci. 1992, 147, 286. (27) Kunieda, H.; Yamagata, M. J. Colloid Interface Sci. 1992, 150, 277. (28) Nakamura, K.; Machiyama, Y.; Junieda, H. J. Jpn. Oil Chem. Soc. (UKAGAKU) 1992, 41, 480. (29) Nakamura, K.; Uemoto, A.; Imae, T.; Solans, C.; Kunieda, H. J. Colloid Interface Sci. 1995, 170, 367-373.
3052 Langmuir, Vol. 12, No. 12, 1996
Olsson et al.
Figure 9. Decay of the relative alkyl methylene echo intensity (filled triangles) as a function of k ) (γGδ)2(∆ - δ/3) in a semilog plot. The sample contains 3.0 wt % surfactant and 0.2 wt % water in decane. The surfactant is a DKE/C16E6 mixture with weight ratio 60/40 and the temperature is 30 °C. ∆ ) 0.14 s and G ) 0.107 T m-1. The echo decay in pure decane (open circles) is shown for comparison.
relative amplitude of about 0.02, decays with a rate which is approximately 2 orders of magnitude smaller. The presence of (at least) two well-separated modes shows that the exchange of decane molecules between inside and outside of the reverse vesicles is slow on the experimental time scale (≈0.1 s). The possibility that the slow mode corresponds to surfactant molecules can be ruled out since the surfactant in the reverse bilayer has a much shorter T2 than decane and its contribution to the spin echo is therefore essentially zero. The T2 relaxation times of the protons associated with the fast and slow modes, respectively, are, however very similar. In Figure 10 we compare the methylene and methyl resonances at k ) 0 (δ ) 0) and k ) 63.5 × 109 s m-2 (δ ) 0.050 s and ∆ ) 0.14 s). At k ) 0 the signal is dominated by molecules associated with the fast mode, having the much higher amplitude, while at k ) 63.5 × 109 s m-2 the fast mode has decayed to zero and the signal corresponds to molecules with the slow diffusion mode only. The resonances are slightly broader at k ) 63.5 × 109 s m-2, which is expected since for the solvent molecules inside the vesicles a larger fraction of the decane molecules are perturbed by the interface. The difference is however small and we can exclude the possibility of any surfactant contribution to the intensity recorded at k ) 63.5 × 109 s m-2. In Figure 11 we have plotted, on a log-log scale the decay of I/I0 as a function of k for the same sample. The data points correspond to four different experiments (combinations) with two different values of ∆ (∆ ) 0.14 and 0.28 s, respectively) and two different values of G (G ) 0.107 and 0.256 T/m, respectively). For larger values of ∆ the signal-to-noise ratio became too low to accurately detect the slow mode, which has a low amplitude. The data points from the two different values of ∆ overlap well, confirming that the T2 relaxation times are very similar for the fast and slowly diffusing species. As clearly seen in Figure 11 we can accurately follow the full decay of the fast mode, while we due to instrumental limitations only can follow the initial decay of the slow mode. The data can be reasonably well fitted to a biexponential decay (eq 15), and the resulting fit is shown in Figure 11 as a solid line. Here we also show the two individual modes as dashed lines. The values of the three different parameters in the fit are D1 ) 1.45 × 10-9 m2 s-1, D2 ) 8 × 10-12 m2 s-1, and P ) 0.016.
Figure 10. Comparison of spectra recorded in the PGSE experiment for two different values of δ. In spectrum a, k ) 0 while in spectrum b k ) 63.5 × 109 s m-2 (δ ) 0.050 s). ∆ ) 0.14 s in both cases. The sample composition is the same as in Figure 9. The similar band width in the two cases demonstrates that both spectra are due to decane. In (a) the intensity correspond mainly (98%) to decane molecules outside the reverse vesicles while in (b) the intensity is dominated by decane molecules inside the vesicles.
Figure 11. Relative echo amplitude (I/I0) of decane as a function of k ) (γGδ)2(∆ - δ/3) on a log-log plot in a solution of reverse vesicles. The data points correspond to four different experiments (combinations) with two different values of ∆ and two different values of G: (4) ∆ ) 0.14 s, G ) 0.107 T/m; (0) ∆ ) 0.28 s, G ) 0.107 T/m; ()) ∆ ) 0.14 s, G ) 0.256 T/m; (O) ∆ ) 0.28 s, G ) 0.256 T/m. I0 was for experiments with ∆ ) 0.14 fitted from a biexponential decay. The data obtained with ∆ ) 0.28 were then fudged to the ∆ ) 0.14 data. The full line is a biexponential fit to the data (see text) with the two separate modes being shown by the broken lines.
From the P value we can determine the volume fraction of vesicles as Φv ) PΦo + Φs + Φw, which gives Φv ) 0.040. The outer radius, Rout, of the vesicles can be estimated from the core-shell model (eq 11) to 170 Å, where we have used d ) 44 Å, as determined in the lamellar phase at higher surfactant concentration. Rout can also be
Vesicles with Nonionic Surfactant
Langmuir, Vol. 12, No. 12, 1996 3053
estimated from the slow mode diffusion coefficient, using the Stokes-Einstein relation (eq 16). Neglecting excluded volume effects, the D2 value above is consistent with Rout ≈ 200 Å. Of these two estimates, the latter is the most accurate. D1 is very close to the free diffusion coefficient of decane (D0 ) 1.49 × 10-9 m2 s-1), which is consistent with the low value of Φv. With eq 6 Φv can be determined from D1/D0 according to
(
Φv ) 2
)
D0 -1 D1
(22)
which inserting the values for D1 and D0 gives Φv ) 0.055. However D1 in this case is very close to D0 (low Φv) and the determination of Φv is therefore less accurate. This can be seen if we consider the absolute relative differential of Φv.
| | dΦv Φv
max
)
(|
| |
|)
∂Φv 1 ∂Φv dD0 + dD1 ) Φv ∂D0 ∂D1 D0 dD1 D0 + D0 - D1 D0 D1
(| | | |)
(23)
Hence in the present case, the relative uncertainty in Φv becomes approximately 40 times the sum of the relative uncertainties of D1 and D0. Since the latter uncertainties are together a few percent, the relative uncertainty in Φv is over 100%. In principle it would be possible to measure the lifetime, τ1, of the decane molecules inside the reverse vesicles by increasing ∆ and determining at which value a transition from a biexponential to a single exponential echo decay occurs. On the present instrument, this was however not possible. The echo amplitude decreases essentially exponentially with ∆, and already for ∆ ≈ 0.5 s (at which slow exchange could still be observed) the slow mode (which here has a small amplitude of ≈2%) was almost hidden in the noise. We can therefore only conclude here that the lifetime, τ1, is at leastlonger than 0.5 s for which a slow exchange and R ) 200 Å implies P < 10-6 cm s-1. 5.2. Stability of Reverse Vesicles. We investigated the stability of the reverse vesicles for four samples in the DKE/C16E6/water/decane system. The compositions correspond to two different surfactant concentrations, 3.0 and 6.0 wt %, respectively, with a DKE/C16E6 weight ratio of 60/40, and two different water-to-surfactant weight ratios: 1/15 and 1/5, respectively. The samples were sonicated and stored at 30 °C and self-diffusion experiments were performed on the samples once per week, over a time period of 8-9 weeks. The self-diffusion experiments were performed with ∆ ) 0.14 s and with the two different gradient strengths, 0.107 and 0.256 T/m, respectively. In all experiments a slow exchange between decane molecules between inside and outside of the vesicles were observed. In Figure 12, we have plotted the vesicle outer radius, Rout, calculated from the amplitude P of the slow diffusion mode as a function of time. Similar values could be evaluated from the slow diffusion coefficient, D2, assuming a biexponential decay. The slow mode amplitude P and the corresponding vesicle radius is stable over a long period of time. Essentially no growth or decomposition is observed over the 2 months investigated here. The uncertainty in P and radius is estimated to (10-15% (it is high because P is low). The vesicles with 3 wt % surfactant and W/S ) 1/5 have a slightly larger radius (250 ( 25 Å) than the other compositions which all have a similar radius (150200 Å).
Figure 12. Variation of vesicle radius, calculated from the relative amplitude of the slow diffusion mode, as a function of time for four different compositions: (O) 3 wt % surfactant, W/S ) 1/15, where W/S is the water-to-surfactant weight ratio; (9) 3 wt % surfactant, W/S ) 1/5; (b) 6 wt % surfactant, W/S ) 1/15; (4) 6 wt % surfactant, W/S ) 1/5. The surfactant is a DKE/C16E6 mixture with weight ratio 60/40. Storage and measurement temperature was 30 °C.
Figure 13. Relative echo amplitude (I/I0) of cyclohexane and hexadecane as a function of k ) (γGδ)2(∆ - δ/3) in a reverse vesicle solution with mixed oil. The surfactant is a DKE/C16E6 mixture of weight ratio 60/40 and with total concentration 3.0 wt %. The water concentration is 0.5 wt %. The oil is a cyclohexane/hexadecane mixture of weight ratio 25/75. ∆ ) 0.14 and G ) 0.107 T/m. Different diffusion behaviors are observed for cyclohexane and hexadecane.
5.3. Semipermeable Reverse Membranes. The permeability of a reverse bilayer membrane to oil molecules is expected to depend on the chain length of the oil and on the reverse bilayer composition. This was investigated at 30 °C in a system where the oil was a cyclohexane/hexadecane mixture with a weight ratio of 25/75. The overall surfactant and water concentrations were kept constant at 3.0 and 0.5 wt %, respectively. The surfactant was a mixture of DKE and C16E6, as in the decane experiments presented above. A mixture of two oils has been used previously to investigate the oil diffusion process in a microemulsion.30 In Figure 13 we show the relative echo attenuation of cyclohexane and hexadecane, respectively, as a function of k for a surfactant mixing ratio of 80/20 (w/w DKE/C16E6). As clearly seen, we observe a different behavior from the two oil molecules. The cyclohexane intensity decays exponentially with k while the echo from hexadecane (30) Olsson, U.; Nagai, K.; Wennerstro¨m, H. J. Phys. Chem. 1988, 92, 6675-6679.
3054 Langmuir, Vol. 12, No. 12, 1996
Olsson et al.
We note that the lifetime of an oil molecule inside a vesicle depends both on the radius and the permeability coefficient, P (eq 3). The vesicle radii in the two experiments shown in Figures 13 and 14, respectively, are very similar as can be seen from the similar weight factor of the slow mode in the hexadecane diffusion. Hence we can describe the difference as decrease in P as we increase the fraction of DKE in the bilayer composition. Conclusions
Figure 14. Relative echo amplitude (I/I0) of cyclohexane and hexadecane as a function of k ) (γGδ)2(∆ - δ/3) in a reverse vesicle solution with mixed oil. The surfactant is pure DKE with a concentration of 3.0 wt %. The water concentration is 0.5 wt %. The oil is a cyclohexane/hexadecane mixture of weight ratio 25/75. ∆ ) 0.14 and G ) 0.107 T/m. Both oils show a bimodal type decay.
decays as a sum of (at least) two significantly different decay rates. This means, that the two oils are in different exchange regimes. Cyclohexane exchange rapidly (τ1 , τexp) while hexadecane is in the slow exchange regime (τ1 . τexp) on the experimental time scale (τexp ≈ 0.1 s). Hence, on this experimental time scale, we have essentially a semipermeable membrane. Fits to the data points, which are shown as solid lines in Figure 12, gave D ) 8.0 × 10-10 m2 s-1 for cyclohexane and D1 ) 5.3 × 10-10 m2 s-1, D2 ) 2 × 10-12 m2 s-1, and P ) 0.98 for hexadecane. The diffusion coefficients can be compared with those in a pure binary oil mixture. In this case D/D0 ) 0.85 for cyclohexane while D1/D0 ) 0.95 for hexadecane. The lower D/D0 for cyclohexane is consistent with that this value is an average between inside and outside but is slightly lower than what is expected with a P value of 0.98. Lowering the DKE/C16E6 ratio to 60/40 resulted in similar semipermeable behavior as shown in Figure 13. However, in the case of pure DKE as surfactant, the reverse bilayer membrane is impermeable also for cyclohexane. The echo attenuation from this sample is shown in Figure 14. A similar bimodal decay is observed for both cyclohexane and hexadecane with a common weight factor, consistent with a slow exchange for both oils.
In this paper we have investigated some vesicle systems, both normal and reverse, with nonionic surfactant. Our main observations can be summarized as follows. We have shown that self-diffusion experiments are useful in demonstrating and characterizing vesicles. For normal vesicles, we observed a fast exchange of water between inside and outside of the vesicles on the ≈0.1 s experimental time scale, even when the bilayer was swollen by oil. Here the reduction of the water self-diffusion coefficient was used to measure the vesicle volume fraction. The observation of a very low oil diffusion coefficient in the three-component system confirmed the presence of vesicles. For the reverse vesicle case, either a slow and fast exchange could be observed, depending on the oil and the reverse membrane composition. For a particular solvent mixture, the reverse bilayer membrane could be permeable or impermeable for one of the solvent species on the experimental time scale. The normal vesicles in water were found to be large, of the order of 1000 Å. The size distribution depends on the preparation but has a long relaxation time. The reverse vesicles prepared by sonication were found to have radii in the range 200-300 Å. Acknowledgment. We thank Keichi Fukuda for his help with the diffusion measurements in the C12E4/HMDS/ water system and Mr. M. Akimaru (Nihon Surfactant Co.) for supplying the C16E6 nonionic surfactants. U.O. acknowledges financial support from the Swedish Natural Science Research Council (NFR). K.N. and H.K. acknowledge financial support from the Ministry of Education, Science and Culture of Japan (Scientific Research No. 03453005) and ICI Japan. R.S. is indebted to Professor M. Kahlweit for his support. LA9600560
