10144
J. Phys. Chem. B 2000, 104, 10144-10153
A Vesicle Phase That Is Prepared by Shear from a Novel Kinetically Produced Stacked Lr-Phase Jingcheng Hao,† Heinz Hoffmann,* and Klaus Horbaschek Lehrstuhl fu¨ r Physikalische Chemie I, UniVersita¨ t Bayreuth, D-95440 Bayreuth, Germany ReceiVed: February 1, 2000; In Final Form: July 27, 2000
We have studied a new cationic and anionic surfactant system which consists of tetradecyldimethylamine oxide (C14DMAO) and dodecylethoxysulfonic acid [CH3(CH2)11(CH2CH2O)2.5SO3H, Texapon N70-H] where no salt in the mixed solutions is formed from the combination of the two surfactants. First, we mixed C14DMAO and Texapon N70-H where the total concentration was 100 mM and the mole fraction varied from 0 to 1 for each component. With increasing mole fractions of Texapon N70-H we observe a L1-phase, a viscous L1-phase, a two-phase L1/LR-region where the low birefringent LR-phase is on the top of the L1-phase, and after the two-phase L1/LR-region a single low birefringent but viscoelastic LR-phase, and finally at almost equal mole fraction a precipitate. On further increasing Texapon N70-H mole fractions, the sequence of the phases is reversed again. In the LR-phase, the rheological measurements show that the complex viscosities (|η*|, ν ) 0.01 Hz) are much higher than those in viscous L1-phase, and furthermore show that the LR-phase has a more or less frequency-independent storage modulus. Freeze-fracture electron micrograph results show that small unilamellar and multilamellar vesicles coexist in the LR-phase. The size range of the small unilamellar vesicles is from 20 nm to the big ones with diameters larger 1.0 µm, and the multilamellar vesicles are relatively small in quantity (the largest multilamellar vesicles are 3.5 µm or so). It is also demonstrated that a classic LR-phase as opposed to a vesicle phase is produced in the new cationic and anionic surfactant mixed solutions by a different preparation route in which the LR-phase is prepared without shear by mixing a small amount of methyl formate to a L1-phase from C14DMAO and sodium dodecylethoxysulfate [CH3(CH2)11(CH2CH2O)2.5SO3Na, Texapon N70]. By this route, one obtains the stacked LR-phase and no vesicles. In the second pathway, methyl formate hydrolyses to formic acid that protonates the amine oxide headgroups of C14DMAO to the cationic surfactant (C14DMAOH+) and thus triggers the transformation of the L1-phase to the LRphase. The LR-phase formed by the simple hydrolysis has stacked bilayers and can be transformed into vesicle by the shearing forces that occur when the samples with the classic LR-phase are turned upside down a few times. The LR-phase and vesicle phase have different macroproperties. This has been demonstrated by rheological measurements, SANS and FF-TEM. Our experimental results show that spontaneously formed vesicles that are often formed in cationic and anionic surfactant mixtures may be the result of shearing forces that occur during the mixing process of the two components. It is furthermore shown that the LR-phase and the vesicle phases that are formed by the chemical reaction have different macroscopic properties from the systems that were prepared from the zwiterionic surfactant (C14DMAO) and Texapon N70-H acid. In the latter situation, the vesicle phases do not contain excess salt and the ionic charges on the vesicles are not shielded. As a consequence, the vesicle phases are strongly viscoelastic and have a yield stress that is large enough to suspend small dispersed air bubbles in the solutions.
Introduction Cationic-anionic surfactant mixtures have been studied in great detail.1-8 The mixtures show a large synergism in their behavior. As a function of the mixing ratio of the cationic and anionic surfactants one observes all possible micellar structures that can be formed in surfactant systems. Recently, even flat nanodiscs of cationic and anionic surfactant system have been observed for the first time.9 One often finds in the mixtures a precipitate at the equimolar concentrations and vesicle phases when one of the components is in small excess. The vesicles are therefore ionically charged. Their charge is, however, * To whom correspondence should be addressed. E-mail: heinz.
[email protected]. Fax: 0049-921-552780. † Permanent address: Department of Chemistry, Shandong Normal University, Jinan 250014. E-mail:
[email protected].
shielded by the excess salt formed from the counterions of the surfactants that are present in the system. In such a situation, in the moderately concentrated solution of a few percent of surfactant the electrostatic interaction between the bilayers is weak, the compression modulus of the systems is small, and the systems have low viscoelastic properties. It is generally assumed that the vesicles in mixtures of cationic and anionic surfactants do form spontaneously when the right mixing ratios are established and the vesicles are thermodynamically stable species. The experimental results for many systems have also been supported by theoretical models.10,11 However, there are still recent publications where the existence of vesicles as thermodynamic stable species is doubted and where it is assumed that given enough time the vesicles will transform or condense into a LR-phase.12 In recent years
10.1021/jp000394g CCC: $19.00 © 2000 American Chemical Society Published on Web 10/11/2000
Vesicle Phase from a Stacked LR-Phase there also has been a considerable amount of work on vesicle systems in shear flow. It is now well-known that the size distribution of vesicles can be adapted by the strength of the shear rate.13-15 Actually, large multilamellar vesicles can be transformed to small unilamellar vesicles under high shear.16 It also has been shown that LR-phases can be transformed to vesicle phases and the thus produced vesicles do not relax to LR-phases when the shear is stopped. One simple method to produce vesicles is by mixing a L3-phase with an ionic surfactant.17 In practically all systems where vesicles were observed, the systems were prepared by mixing of two surfactant solutions. This means that there was shear involved. It is therefore conceivable that the vesicles were not produced by the thermodynamic conditions in these systems but by the shear that was due to the mixing. Recently, it actually was shown that a normal LR-phase is obtained when the bilayer of L3-phase are ionically charged by a chemical reaction and not by mixing.17 For this reason we were looking for a way to produce cationic and anionic mixtures without shear. We hoped to be able to do this by producing one component for the cationic/anionic combination by a chemical reaction in a solution which is at rest and does not flow. In the present investigation, we report a new type of cationic and anionic surfactant mixture where no salt is formed from the combination of the surfactants. We have found that the novel unilamellar and multilamellar vesicles are formed in mixtures of zwitterionic surfactants like alkyldimethylamine oxide and alkylethoxysulfonic acid. Mixtures of the two surfactants show the typical behavior of cationic and anionic systems. The surfactant acid protonates the zwitterionic surfactant and a cationic surfactant is formed that recombines with the anionic surfactant formed by releasing protons. Starting from the zwitterionic surfactant we observe with increasing concentrations of the sulfonic acid a viscous L1-phase, a two-phase L1/LRregion, a LR-phase, and finally a precipitate region. The phase boundaries have been determined by conductivity measurements and by visual inspection of the samples with and without polarizers. Rheological measurements show that the LR-phase is a highly viscoelastic phase with a yield stress value. Surprisingly, it shows practically little birefringence. Additionally, one main purpose of this investigation is also to demonstrate that vesicles in aqueous mixtures of cationic and anionic surfactants1 do not form spontaneously but may be the result of shearing forces that are due to the mixing of the components. In order to demonstrate our idea, a series of experiments were carried out in which the cationic surfactant was produced by a hydrolysis reaction in the absence of shear, and where in combination with the anionic surfactant the LRphase was produced. For our investigation we used the mixed solutions of the zwitterionic surfactant (C14DMAO) and sodium dodecylethoxysulfate (Texapon N70) which is a L1-phase. The reaction used for the protonation of C14DMAO is the hydrolysis reaction of methyl formate to formic acid and methanol. The formic acid is a strong enough acid to protonate amine oxides into cationic surfactants, and methanol in small concentrations has hardly an effect on lyotropic mesophases.18 The rate constant of the reaction is in a very convenient range so that the ester can be mixed with the starting L1-phase and the protonation proceeds in times between minutes and a day depending on the conditions like temperature, pH, and concentrations. In the system C14DMAO/Texapon N70-H/water, the zwitterionic surfactant C14DMAO is charged by the acid of Texapon N70-H to a cationic surfactant and then to the combination of
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10145 the cationic and anionic surfactants. The system is therefore without excess salt.
However, in the formation of the LR-phase induced by hydrolysis reaction in the system C14DMAO/Texapon N70/HCOOCH3/ water, zwitterionic surfactant C14DMAO was charged by the formic acid formed by the hydrolysis reaction of methyl formate to the cationic surfactant which then reacts to the cationic and anionic surfactant combination. In this route, the system has the excess salt of sodium formate.
Experimental Section Chemicals. The used surfactant tetradecyldimethylamine oxide (C14DMAO) was a gift of the Clariant AG Gendorf and was delivered as a 25% solution. It was crystallized twice from acetone and characterized by the melting point (130.2-130.5 °C) and cmc (1.4 × 10-4 M). Sodium dodecylethoxysulfate [CH3(CH2)11(CH2CH2O)2.5SO3Na, Texapon N70] was a gift of Henkel (Du¨sseldorf) and used without further purification. Methyl formate of Merck and 1.0 M HCl solution of Merck were both in P.A. quality. D2O (99.8% isotropic purity) was used for the SANS experiments was obtained from Euriso-top, Groupe CEA, Saint Aubin (France), and water was doubly distilled. Experimental Methods. We prepared the Texapon N70-H[CH3(CH2)11(CH2CH2O)2.5SO3H] stock solutions from Texapon N70[CH3(CH2)11(CH2CH2O)2.5SO3Na] solutions (120 mM) by strong acidic cationic exchanger (Ion exchanger I, Merck) at room temperature. Na+ cannot be detected by K[Sb(OH)6](Na+ + [Sb(OH)6]- f Na[Sb(OH)6] V) in the Texapon N70-H stock solution, the only detectable impurity in the stock solutions was at ,0.1%, the ion exchange with hydrogen is .99.9%. The phase diagrams of 100 mM C14DMAO and the variable alkylethoxysulfonic acid (or Texapon N70/HCOOH) concentrations were established by observing the mixed solutions in the calibrated test tubes at 25 °C. The samples were homogenized by mixing and then they were allowed to equilibrate for at least 4 weeks at 25 °C. The conductivity measurements of the samples were performed on a microprocessor conductivity meter LF 2500 CON of WTW (Germany).
10146 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Hao et al.
Figure 1. Phase diagram of the mixtures of C14DMAO and TexaponN70-H, the total concentration is 100 mM. The conductivity data (b) is included in the figure, and the phase diagram on the higher C14DMAO concentrations is inset in the figure. The precipitate region appears in 0.25 < XC14DMAO < 0.525.
The rheological measurements were performed with a Bohlin CS 10 rheometer. A double gap system was used to avoid evaporation effects. The lowest possible stress value amounts to 3 mPa. The viscoelastic properties were determined by oscillatory measurements from 0.001 to 10 Hz, whereby alternatively the strain amplitude or the stress amplitude can be kept constant. For the freeze-fracture transmission electron microscopy (FFTEM), a small amount of sample was placed on a 0.1 mm thick copper disk covered with a second copper disk. The copper sandwich with the sample was frozen by plunging this sandwich into liquid propane which had been cooled by liquid nitrogen. Fracturing and replication were carried out in a freeze-fracture apparatus (Balzers BAF 400, Germany) at a temperature of -140 °C. Pt/C was deposited at an angle of 45°. The replicas were examined in a CEM 902 electron microscope (Zeiss, Germany). The small-angle neutron scattering (SANS) experiments were performed at the Hahn-Meitner Institute, Berlin (Germany), on the instrument V4.19 A wavelength of 6.0 Å was chosen, and sample to detector distances of 1, 4, and 15.85 m were employed. The data were recorded on a 64 × 64 twodimensional detector. They became afterward radially averaged and converted into absolute units, i.e., the differential cross section, by comparison with the scattering of H2O standard and the proper correction for detector background and the scattering of the empty cell.20,21 All the experiments were described in the following were done at 25 °C unless specified otherwise. Results and Discussion C14DMAO/Texapon N70-H/Water System. Phase BehaVior. In Figure 1, we would like to show the sequence of phases when we mixed C14DMAO and Texapon N70-H. The total concentration was 100 mM, and the mole fraction varied from 0 to 1 for each component. With increasing Texapon N70-H we observe a L1-phase, a viscous L1-phase, and a two-phase L1/LR-region where the low birefringent LR-phase is on the top of the samples. After the two-phase region we find a single low birefringent
Figure 2. Phase diagram for the mixtures of 100 mM C14DMAO and increasing amounts of CH3(CH2)11(CH2CH2O)2.5SO3H (Texapon N70H) at 25 °C. Conductivity data (b) and the phase volumes (O) of the two phases are included in the phase diagram.
but viscoelastic LR-phase and finally at almost equal mole fraction a precipitate is formed. On further increasing the Texapon N70-H concentrations the sequence of the phases is reversed. The sequence in our system is the same as already observed for many cationic-anionic combinations of surfactants,9,22,23 which shows that our system has the typical behavior of cationic and anionic systems. In a second series of samples, 100 mM C14DMAO solutions are titrated with Texapon N70-H stock solution. In this series the C14DMAO concentration remains constant. The sequence of phases is shown in Figure 2. From 0 to 20.5 mM of Texapon N70-H concentrations, one observes a single transparent L1-phase that becomes viscous with increasing of Texapon N70-H. Between 20.5 and 26.5 mM, we note macroscopic separation into an upper birefringent LR-phase and an isotropic viscous L1-phase at the bottom. Between 26.5 and 54 mM, there is a single, more or less transparent viscoelastic LR-phase with the typical features of vesicle phases. Surprisingly, the LR-phase shows practically no birefringence at rest, tilt, or flow of the phase. Above 54 mM Texapon N70-H, small particles of a precipitate are formed which coexist
Vesicle Phase from a Stacked LR-Phase
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10147
Figure 4. Viscosity data for the mixtures of 100 mM C14DMAO and increasing amounts of CH3(CH2)11(CH2CH2O)2.5SO3H (Texapon N70-H) at 25 °C. Viscosity data (b) and the phase volumes (O) of the two phases are included in the phase diagram.
Figure 3. Phase behavior of the system 100 mM C14DMAO and increasing amounts of Texapon N70-H at 25 °C, (a) without polarizers and (b)with polarizers. From left to right: 15 mM (viscous L1-phase), 22 mM (L1/LR-phase), 30 mM (LR-phase), and 85 mM (thick precipitate) Texapon N70-H.
with the LR-phase. These samples do not separate into two phases. Finally, when the concentrations of Texapon N70-H G 80 mM, a thick precipitate appears at the bottom. The precipitate is very interesting. On heating, the precipitate disappears at 45 °C, and at higher temperatures, the samples are transformed into the same LR-phase that are present in the neighborly LRregion. We have characterized the different phases with various physicochemical methods and some results are shown during the following discussion. The phase behavior results with or without polarizers are demonstrated in Figure 3. The third sample contains small dispensed bubbles that do not rise. This shows that the LR-phase has a yield stress value that is large enough to suspend small air bubbles in the phase. ConductiVity Measurement. The conductivity data of the samples are included in Figures 1 and 2 of the two-phase diagrams. One should note that the different phase boundaries can well be determined by the conductivities. In Figure 1, on the higher C14DMAO concentration side, with increasing concentrations of Texapon N70-H, the conductivities increase in the L1-phase (see the inset) and decrease in the L1/LR twophase region. The conductivities decrease to a minimum and have more or less constant values in the LR-phase. However, on the higher Texapon N70-H concentration side (XC14DMAO
