VOLUME 101, NUMBER 30, JULY 24, 1997
© Copyright 1997 by the American Chemical Society
LETTERS Preparation and Properties of Ionically Charged Lamellar Phases That Are Produced without Shearing M. Bergmeier, H. Hoffmann,* and C. Thunig UniVersita¨ t Bayreuth, Physikalische Chemie I, 95440 Bayreuth, Germany ReceiVed: April 3, 1997; In Final Form: June 3, 1997X
It is shown that an ionically charged LR-phase can be prepared without shear by mixing a small amount of methyl formate to a L3-phase from alkyldimethyl aminoxide, hexanol, and water. Within a few hours the methyl formate hydrolyzes to formic acid and methanol. The formic acid protonates the aminoxide surfactant head groups and triggers the transformation of the L3- to the LR-phase. The resulting LR-phase is a lowviscosity classic LR-phase with stacked bilayers. By shearing or simply by shaking of a thus produced sample of low viscosity, it is transformed instantaneously into a highly viscoelastic phase with densely packed multilamellar vesicles. At present, it is not clear which form of the LR-phase is thermodynamically stable.
Introduction Ternary phase diagrams of surfactant/cosurfactants and water show a rich phase behavior.1 Phase diagrams are usually established by admixing of the components with efficient mixing aids. The samples are then left to equilibrate at constant temperature. If the phases do not change with time, it is usually assumed that the phases are at thermodynamic equilibrium and the micellar structures in these phases are equilibrium structures. This investigation shows that this might not necessarily always be the case. In particular, the macroscopic properties and the morphology of LR-phases in ternary phase diagrams that are prepared by mixing of the components may depend on the history of the preparation of the samples. In the recent years there has been a large interest in vesicle phases in which the vesicles form spontaneously when surfactants are mixed with surfactants or cosurfactants.2-4 A general method of preparing such phases consists of mixing an L3-phase or a classic LR-phase with stacked bilayers with a few percent of an ionic surfactant.5 For surfactant phases with a few percent of surfactant, the LR- and the L3-phases are low viscous phases while the vesicle phase is highly viscoelastic and has a yield stress value. The viscoelastic properties are a result of the dense X
Abstract published in AdVance ACS Abstracts, July 1, 1997.
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packing of the polydisperse multilamellar vesicles and the stiffness of the bilayers. The transformation of the extended bilayers in the LR- and L3-phases to the vesicles by the addition of ionic surfactants can be explained by the Helfrich theory.6,7 The theory predicts that with increasing charge density on the bilayers the bending constant first increases and then levels to a constant value while the Gaussian modulus is negative and increases with the charge density.7 The system must therefore switch from the extended bilayers in the LR-phase to the vesicles. The vesicle phase is believed to be a thermodynamic stable phase. Experimental investigations showed that the L3-phase is extremely sensitive to charges. In L3-phases from nonionic surfactants or zwitterionic surfactants and cosurfactants, the transition to a LR-phase occurs already when 3% of the uncharged surfactant is replaced by ionic surfactants.2,8 For mesophases that consist of zwitterionic surfactants, the charging of the bilayers can also be enforced by protonation of the head groups of the surfactants. While it is believed that the vesicles in the charged bilayer phase are thermodynamically stable species, experimental investigations show that the macroscopic properties depend somewhat on the preparation of the samples.3 It is therefore conceivable that the microstructures in the samples are dependent on the shear conditions during the preparation. © 1997 American Chemical Society
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Figure 1. (left) Uncharged LR-phase from 100 mM C14DMAO and 220 mM hexanol; (middle) prepared from an L3-phase with 100 mM C14DMAO and 235 mM hexanol that was mixed with 10 mM formic acid; trapped bubbles were removed by centrifugation; (right) same sample as in the middle, but without centrifugation; note the trapped bubbles (yield value).
We therefore have looked for a method of preparing the charged phases without having to mix and to apply shearing forces to the samples. The solution to this problem was found in a kinetic reaction that protonates and charges the bilayer but otherwise has no effect on the system. The reaction that is used for the protonation is the hydrolysis reaction of methyl formate to formic acid and methanol. Formic acid is a strong enough acid to protonate alkylaminoxides or other zwitterionic acids, and methanol in small concentrations hardly has an effect on mesophases.9 The rate constant of the reaction is in a very convenient range so that the ester can be mixed with the starting phases and the protonation can proceed in times between minutes and a day depending on the conditions like temperature, pH, and concentration. For our investigation we used the ternary system tetradecyl dimethylaminoxide, hexanol, water. The phase diagram of this system has been studied in detail.2 For low surfactant concentration and increasing cosurfactant concentration the sequence of the single-phase regions is L1, LRl, LRh, and L3. The LRl-phase is a vesicle phase, and the LRhphase is the classic stacked bilayer phase. For the experiment we used the L3-phase with 100 mM surfactant and 235 mM hexanol. In order to demonstrate the charging of the bilayers by the hydrolysis reaction of methyl formate we also could have used the LRh-phase. In that case it would have been necessary to mix the methyl formate into the LRh-phase, which could have affected the size and the orientation of the domains of the LRphase, and again the experiment could have been affected by shear, which was to be avoided. In order to avoid this we used the L3-phase, an isotropic phase with a Newtonian flow behavior, and a short structural relaxation time. Because of its short structural relaxation time and its isotropic nature, this phase has none of the problems of an liquid crystalline mesophase. The L3-phase remains in the isotropic sponge state for several minutes after it has been mixed with a small amount of the methyl formate. With the L3-phase the conditions for the hydrolysis are thus well defined. It is a low-viscosity, slightly turbid, isotropic phase that shows weak flow birefringence under shear.10 Results When 10 mM of formic acid is added to the L3-phase under stirring in order to achieve microscopic mixing, one obtains a highly viscoelastic vesicle phase with a yield value. The vesicle
phase is completely transparent and shows a strong birefringence but without domains like in the oridinary LRh-phases. Samples of the LRh and the vesicle phase that is obtained by mixing formic acid into the L3-phase are shown in Figure 1, where the samples are viewed through polarizors. The LRhphase shows the birefringence pattern that is typical for a multidomain situation. The samples in Figure 2 show a L3-phase that had 10 mM of methyl formate added and increasing time intervals had passed since the mixing. We notice that the birefringent domain pattern that is typical for the LRh-phase develops after minutes. The pattern becomes stronger with time, but the size of the domains do not change with time. Even after 1 day the samples look very much the same as the LRh-phase and very different from the vescile phase in Figure 1. This simple experiment shows already that the two samples that are chemically identical must have different structures. We have convinced ourselves that the presence or absence of 10 mM of methanol in the two phases has no influence on the properties of the phase. It is interesting to note that we find no evidence for a macroscopic phase separation with time in the sample of Figure 2. For a slow increase of the acidity, we should expect a two-phase L3/LRregion in between the L3- and the LR-phases. Obviously the two-phase region is passed so quickly in the experiment that the sample does not have enough time for macroscopic phase separation. In order to determine the rate of the hydrolysis of the ester, we monitored the conductivity and the pH of the solution and with the help of calibration curves determined the ester concentration as a function of time. In Figure 3 the change of pH with time is given for neutral conditions, which we have in the experiments. The half-time of the reaction at room temperature is about 10 h. The described experiments demonstrate unambigously that the final states that are reached when formic acid is mixed with the L3-phase and when methyl formate is dissolved into the L3phase and then the hydrolysis reaction is allowed to proceed and reach equilibrium are not the same. The birefringence pattern of the two phases is very different. Obviously only one of the phases is in equilibrium, while the other one must be trapped in a metastable situation. In order to find out which of the two states is the thermodynamic stable state, we kept both samples for several months and looked at their appearence with
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Figure 2. Development of the birefringence with time in a sample of 100 mM C14DMAO, 235 mM hexanol, and 10 mM methyl formate: (from left to right) 18 min, 30 min, 1 h, and 1 day.
Figure 3. Change of the pH with time in the sample of Figure 2.
and without polarizers. We did not notice any change in the optical appearance of the two phases. In the next experiment we compared the rheological properties of the two phases. The results are shown in Figure 4 where rheograms of the two phases are compared. As expected from previous measurements, the phase that was produced by mixing the L3-phase with formic acid showed the typical behavior of a viscoelastic fluid with a yield value. The storage modulus G′ was frequency independent and about an order of magnitude larger than the loss modulus. The loss modulus had about the same value as for the case when the L3-phase had been protonated with HCl.11 The other phase however was a lowviscosity phase with very weak viscoelastic properties in which G′ and G′′ were more than 2 orders of magnitude lower than in the stirred sample. In the next experiment we measured the rheological properties of the sample with time after the methyl formate was added to the L3-phase. The results are shown in Figure 5. The results show an abrupt increase of the viscosity from less than 1 to 8 mPas at around 20 min and thereafter a continuous increase to the final value of about 30 mPas. In the final experiment we let the sample reach equilibrium in the rheometer and then sheared the solution for about half a minute at a well-defined shear rate and, after this exposure to shear, measured again the rheological properties of the sample by oscillating measurements. The results of these measurements are shown in Figure
Figure 4. Rheograms of two samples with 100 mM C14DMAO, 235 mM hexanol, and 10 mM methyl formate: (a) the sample had been at rest for 1 day but shaken before the measurement; (b) the sample had been at rest for 1 day and not been shaken.
6. We notice that the viscosity of the system depends on the shear rate to which the sample was exposed. With increasing shear rate the viscosity increases in a s-shaped manner. For low shear rates between 1 and 50 s-1 the viscosity changes very little; actually it decreases somewhat with increasing shear rate. In the range between 50 and 200 s-1 it increases abruptly and reaches a constant value for even higher shear rates. For these shear rates the solution has about the same rheological properties as the sample that was prepared by mixing of formic acid with the L3-phase. The results in Figure 6 demonstrate clearly that
5770 J. Phys. Chem. B, Vol. 101, No. 30, 1997
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Figure 5. Change of the rheological properties of a L3-phase from 100 mM C14DMAO, 235 mM hexanol, and 10 mM methyl formate with time. The rheological measurements were recorded with a Paar Instrument (OCR-D).
Figure 6. Development of the viscoelastic properties of an unsheared phase of 100 mM C14DMAO, 235 mM hexanol, and 10 mM methyl formate with shear. The sample was 1 day old. The viscoelastic properties were determined after the sample was exposed to periods of half a minute to different shear rates.
a well-defined critical shear rate exists for the stacked bilayer phase above which it is transformed into vesicles. The transition is discontinuous. The critical shear rate for the investigated system is much higher than the critical shear for the systems that have been investigated by D. Roux et al.12 In our preliminary experiments we have applied the different shearing rates for periods of half a minute. We do not know whether these time periods were long enough to reach a stationary state. The influence of shearing time at constant shear rate will be studied in the future. We know, however, already that the size of the vesicles for our charged systems depends on the shear rate. Increasing shear rates decrease the size of the vesicles.13 In this respect the studied systems behave in the same way as the systems that have been studied before at higher concentration and completely different conditions.12 The transformation of the low viscosity kinetically produced LR-phase into a highly viscoelastic vesicle phase can be demonstrated in a spectacular little experiment. For the experiment the LR-phase is best prepared in a fixed standing cylinder by mixing a small amount of methyl formate to the L3-phase of the alkylaminoxide. The sample is then left to equilibrate for a day. The glass vessel can then be tilted slowly to the right and to the left to show that the fluid in the container flows easily and has a low viscosity. When the sample is briefly shaken the fluid is transformed instantaneously into a highly viscoelastic fluid with a yield stress what can easily be seen on
Figure 7. Freeze fracture TEM micrographs that were prepared from the same sample: (a, top) the L3-phase from 100 mM C14DMAO, 250 hexanol, and 10 mM methyl formate in the presence of 20% glycerol in the water; (b, middle) the same sample after 1 h hydrolysis; (c, bottom) the same sample after 1 day and the sample being exposed to shear.
the recoil of bubbles that have been dispersed and trapped in the fluid by the shaking of the sample. Micrographs of the different morphologies in three samples that were obtained by FF-TEM are shown in Figure 7. Figure 7a shows the L3-phase, Figure 7b the LR-phase that was kinetically produced by the hydrolysis reaction, and Figure 7c the vesicle phase that was obtained by shaking the chemically produced phase. Discussion It is interesting to compare the results of this investigation with the results of the shearing experiments on uncharged systems that were carried out by D. Roux.12 These experiment were carried out on a classic LR-phase that had been prepared
Letters in the conventional way. The LR-phase was stabilized by undulation forces. It was observed that this LR-phase is aligned by the shear forces, and for shearing rates above a threshold value it is transformed into a multilamellar vesicle. At even higher shearing rates the vesicles are transformed into a completely aligned lamellar phase that was free of defects. So far we have not been able to observe the transition of the vesicle phase to the defect-free LR-phase. It is, however, conceivable that this transition can occur also but at shear rates that were not reached in this investigation. The threshold shear rate can depend on the interaction of the bilayers. It would thus be interesting to study the threshold value as a function of the charge density of the bilayers. The experiments have shown that the charged LR-phases from ternary systems can be prepared in two reproducible different states: in a low-viscosity classic LR-phase with domains of stacked bilayers and in a highly viscoelastic vesicle state. Both phases are stable for long times, and it is impossible to conclude which state is the more stable one in the thermodynamic sense. Obviously one phase has some small excess energy with respect to the other one. However there is a large activation energy between the phases that cannot be overcome by thermal energy. The low-viscosity phase can easily be transformed into the highly viscoelastic phase, but the viscoelastic phase cannot be transformed back into the classic LR-phase by a simple manipulation. Actually, we would like to mention that the phase can even be transformed into a third reproducible state.13 Under high shear rates of several thousand inverse seconds, the multilamellar vesicles are transformed into unilamellar vesicles that have a long lifetime. Conclusions It can be concluded that the morphologies and the properties of apparantly stable thermodynamic surfactant phases depend
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5771 on the history of their preparation. We have shown this for a phase that can be prepared by mixing the different components or by having one component being produced by a chemical reaction. In this example a simple hydrolysis reaction for the production of an acid was used to ionically charge a system. There are many more possibilities available to change the composition of surfactant phases by chemical reactions, and thus many surfactant phases can be produced by chemical reactions without shearing forces. In the future we likely will see more such experiments. It is demonstrated for the first time that an ionically charged LR-phase in which the large separation of the bilayers is maintained by electrostatic interaction can be prepared in the stacked bilayer state. This state is reached when shear during the preparation is avoided and the charging of the bilayers is accomplished in the solution at rest by a chemical reaction. References and Notes (1) Laughlin, R. G. The aqueous phase behaVior of surfactants; Academic Press: London, San Diego, 1994. (2) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, (11), 3972-3981. (3) Porte, G.; Oberdisse, J.; Couve, C.; Appell, J.; Berret, J. F.; Ligoure, C. Langmuir 1996, 12, (5), 1212-1218. (4) Kaler, E.; Herrington K. L.; Murtly, A.; Jasadzinsky, I. A. N. J. Phys. Chem. 1992, 96, 6698. (5) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Il NuoVo Cimento 1994, 16 (9), 1373-1390. (6) Helfrich, W.; Winterhalter, M. J. Phys. Chem. 1992, 96, 327330. (7) Lekkerkerker, H. N. W. Physica A 1990, 167, 984. (8) Strey, R.; Schoma¨cker, R. J. Phys. Chem. 1994, 98, 3908-3912. (9) Rathman, J. R.; Christian, Sh. D. Langmuir 1990, 6, 391. (10) Hoffmann, H.; Miller, C. A.; Gradzielski, M.; Kra¨mer, U.; Thunig, C. Colloid Polym. Sci. 1990, 268, 1066-1072. (11) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Faraday Discuss. 1995, 101, 319-333. (12) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 9, 1427. (13) Hoffmann, H.; Gradzielski, M.; Bergmeier, M., in preparation.
