A Vesicle Phase That Is Prepared by Shear from a Novel Kinetically Produced StackedLr-Phase

Jingcheng Hao,† Heinz Hoffmann,* and Klaus HorbaschekLehrstuhl fur Physikalische Chemie I, UniVersitat 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 tetradecyldimethylamineoxide (C14DMAO) and dodecylethoxysulfonic acid [CH3(CH2)11(CH2CH2O)2.5SO3H, Texapon N70-H] whereno salt in the mixed solutions is formed from the combination of the two surfactants. First, we mixed C14-DMAO and Texapon N70-H where the total concentration was 100 mM and the mole fraction varied from 0to 1 for each component. With increasing mole fractions of Texapon N70-H we observe a L1-phase, a viscousL1-phase, a two-phase L1/LR-region where the low birefringent LR-phase is on the top of the L1-phase, andafter the two-phase L1/LR-region a single low birefringent but viscoelastic LR-phase, and finally at almostequal mole fraction a precipitate. On further increasing Texapon N70-H mole fractions, the sequence of thephases 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-phasehas a more or less frequency-independent storage modulus. Freeze-fracture electron micrograph results showthat small unilamellar and multilamellar vesicles coexist in the LR-phase. The size range of the small unilamellarvesicles is from 20 nm to the big ones with diameters larger 1.0µm, and the multilamellar vesicles arerelatively small in quantity (the largest multilamellar vesicles are 3.5µm or so). It is also demonstrated thata classic LR-phase as opposed to a vesicle phase is produced in the new cationic and anionic surfactant mixedsolutions by a different preparation route in which the LR-phase is prepared without shear by mixing a smallamount of methyl formate to a L1-phase from C14DMAO and sodium dodecylethoxysulfate [CH3(CH2)11(CH2-CH2O)2.5SO3Na, Texapon N70]. By this route, one obtains the stacked LR-phase and no vesicles. In the secondpathway, methyl formate hydrolyses to formic acid that protonates the amine oxide headgroups of C14DMAOto the cationic surfactant (C14DMAOH+) and thus triggers the transformation of the L1-phase to the LR-phase. The LR-phase formed by the simple hydrolysis has stacked bilayers and can be transformed into vesicleby the shearing forces that occur when the samples with the classic LR-phase are turned upside down a fewtimes. The LR-phase and vesicle phase have different macroproperties. This has been demonstrated byrheological measurements, SANS and FF-TEM. Our experimental results show that spontaneously formedvesicles that are often formed in cationic and anionic surfactant mixtures may be the result of shearing forcesthat occur during the mixing process of the two components. It is furthermore shown that the LR-phase andthe vesicle phases that are formed by the chemical reaction have different macroscopic properties from thesystems that were prepared from the zwiterionic surfactant (C14DMAO) and Texapon N70-H acid. In thelatter situation, the vesicle phases do not contain excess salt and the ionic charges on the vesicles are notshielded. As a consequence, the vesicle phases are strongly viscoelastic and have a yield stress that is largeenough to suspend small dispersed air bubbles in the solutions.

Introduction

Cationic-anionic surfactant mixtures have been studied ingreat detail.1-8 The mixtures show a large synergism in theirbehavior. As a function of the mixing ratio of the cationic andanionic surfactants one observes all possible micellar structuresthat can be formed in surfactant systems. Recently, even flatnanodiscs of cationic and anionic surfactant system have beenobserved for the first time.9 One often finds in the mixtures aprecipitate at the equimolar concentrations and vesicle phaseswhen one of the components is in small excess. The vesiclesare therefore ionically charged. Their charge is, however,

shielded by the excess salt formed from the counterions of thesurfactants that are present in the system. In such a situation,in the moderately concentrated solution of a few percent ofsurfactant the electrostatic interaction between the bilayers isweak, the compression modulus of the systems is small, andthe systems have low viscoelastic properties.

It is generally assumed that the vesicles in mixtures of cat-ionic and anionic surfactants do form spontaneously when theright mixing ratios are established and the vesicles are ther-modynamically stable species. The experimental results formany systems have also been supported by theoretical mod-els.10,11 However, there are still recent publications where theexistence of vesicles as thermodynamic stable species is doubtedand where it is assumed that given enough time the vesicleswill transform or condense into a LR-phase.12 In recent years

* To whom correspondence should be addressed. E-mail: heinz.hoffmann@uni-bayreuth.de. Fax: 0049-921-552780.

† Permanent address: Department of Chemistry, Shandong NormalUniversity, Jinan 250014. E-mail: hjcheng@pub.online.jn.sd.cn.

10144 J. Phys. Chem. B2000,104,10144-10153

10.1021/jp000394g CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 10/11/2000

there also has been a considerable amount of work on vesiclesystems in shear flow. It is now well-known that the sizedistribution of vesicles can be adapted by the strength of theshear rate.13-15

Actually, large multilamellar vesicles can be transformed tosmall unilamellar vesicles under high shear.16 It also has beenshown that LR-phases can be transformed to vesicle phases andthe thus produced vesicles do not relax to LR-phases when theshear is stopped. One simple method to produce vesicles is bymixing a L3-phase with an ionic surfactant.17 In practically allsystems where vesicles were observed, the systems wereprepared by mixing of two surfactant solutions. This means thatthere was shear involved. It is therefore conceivable that thevesicles were not produced by the thermodynamic conditionsin these systems but by the shear that was due to the mixing.Recently, it actually was shown that a normal LR-phase isobtained when the bilayer of L3-phase are ionically charged bya chemical reaction and not by mixing.17 For this reason wewere looking for a way to produce cationic and anionic mixtureswithout shear. We hoped to be able to do this by producingone component for the cationic/anionic combination by achemical reaction in a solution which is at rest and does notflow.

In the present investigation, we report a new type of cationicand anionic surfactant mixture where no salt is formed fromthe combination of the surfactants. We have found that the novelunilamellar and multilamellar vesicles are formed in mixturesof zwitterionic surfactants like alkyldimethylamine oxide andalkylethoxysulfonic acid. Mixtures of the two surfactants showthe typical behavior of cationic and anionic systems. Thesurfactant acid protonates the zwitterionic surfactant and acationic surfactant is formed that recombines with the anionicsurfactant formed by releasing protons. Starting from thezwitterionic surfactant we observe with increasing concentrationsof the sulfonic acid a viscous L1-phase, a two-phase L1/LR-region, a LR-phase, and finally a precipitate region. The phaseboundaries have been determined by conductivity measurementsand by visual inspection of the samples with and withoutpolarizers. Rheological measurements show that the LR-phaseis a highly viscoelastic phase with a yield stress value.Surprisingly, it shows practically little birefringence.

Additionally, one main purpose of this investigation is alsoto demonstrate that vesicles in aqueous mixtures of cationic andanionic surfactants1 do not form spontaneously but may be theresult of shearing forces that are due to the mixing of thecomponents. In order to demonstrate our idea, a series ofexperiments were carried out in which the cationic surfactantwas produced by a hydrolysis reaction in the absence of shear,and where in combination with the anionic surfactant the LR-phase was produced. For our investigation we used the mixedsolutions of the zwitterionic surfactant (C14DMAO) and sodiumdodecylethoxysulfate (Texapon N70) which is a L1-phase. Thereaction used for the protonation of C14DMAO is the hydrolysisreaction of methyl formate to formic acid and methanol. Theformic acid is a strong enough acid to protonate amine oxidesinto cationic surfactants, and methanol in small concentrationshas hardly an effect on lyotropic mesophases.18 The rate constantof the reaction is in a very convenient range so that the estercan be mixed with the starting L1-phase and the protonationproceeds in times between minutes and a day depending on theconditions like temperature, pH, and concentrations.

In the system C14DMAO/Texapon N70-H/water, the zwitter-ionic surfactant C14DMAO is charged by the acid of TexaponN70-H to a cationic surfactant and then to the combination of

the cationic and anionic surfactants. The system is thereforewithout excess salt.

However, in the formation of the LR-phase induced by hydrolysisreaction in the system C14DMAO/Texapon N70/HCOOCH3/water, zwitterionic surfactant C14DMAO was charged by theformic acid formed by the hydrolysis reaction of methyl formateto the cationic surfactant which then reacts to the cationic andanionic surfactant combination. In this route, the system hasthe excess salt of sodium formate.

Experimental Section

Chemicals. The used surfactant tetradecyldimethylamineoxide (C14DMAO) was a gift of the Clariant AG Gendorf andwas delivered as a 25% solution. It was crystallized twice fromacetone 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 ofHenkel (Dusseldorf) and used without further purification.Methyl formate of Merck and 1.0 M HCl solution of Merckwere both in P.A. quality. D2O (99.8% isotropic purity) wasused for the SANS experiments was obtained from Euriso-top,Groupe CEA, Saint Aubin (France), and water was doublydistilled.

Experimental Methods. We prepared the Texapon N70-H-[CH3(CH2)11(CH2CH2O)2.5SO3H] stock solutions from TexaponN70[CH3(CH2)11(CH2CH2O)2.5SO3Na] solutions (120 mM) bystrong acidic cationic exchanger (Ion exchanger I, Merck) atroom 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 stocksolution, the only detectable impurity in the stock solutions wasat ,0.1%, the ion exchange with hydrogen is.99.9%.

The phase diagrams of 100 mM C14DMAO and the variablealkylethoxysulfonic acid (or Texapon N70/HCOOH) concentra-tions were established by observing the mixed solutions in thecalibrated test tubes at 25°C. The samples were homogenizedby mixing and then they were allowed to equilibrate for at least4 weeks at 25°C.

The conductivity measurements of the samples were per-formed on a microprocessor conductivity meter LF 2500 CONof WTW (Germany).

Vesicle Phase from a Stacked LR-Phase J. Phys. Chem. B, Vol. 104, No. 44, 200010145

The rheological measurements were performed with a BohlinCS 10 rheometer. A double gap system was used to avoid evap-oration effects. The lowest possible stress value amounts to 3mPa. The viscoelastic properties were determined by oscillatorymeasurements from 0.001 to 10 Hz, whereby alternatively thestrain amplitude or the stress amplitude can be kept constant.

For the freeze-fracture transmission electron microscopy (FF-TEM), a small amount of sample was placed on a 0.1 mm thickcopper disk covered with a second copper disk. The coppersandwich with the sample was frozen by plunging this sandwichinto liquid propane which had been cooled by liquid nitrogen.Fracturing and replication were carried out in a freeze-fractureapparatus (Balzers BAF 400, Germany) at a temperature of-140 °C. Pt/C was deposited at an angle of 45°. The replicaswere examined in a CEM 902 electron microscope (Zeiss,Germany).

The small-angle neutron scattering (SANS) experiments wereperformed at the Hahn-Meitner Institute, Berlin (Germany), onthe instrument V4.19 A wavelength of 6.0 Å was chosen, andsample to detector distances of 1, 4, and 15.85 m wereemployed. The data were recorded on a 64× 64 two-dimensional detector. They became afterward radially averagedand converted into absolute units, i.e., the differential crosssection, by comparison with the scattering of H2O standard andthe proper correction for detector background and the scatteringof the empty cell.20,21

All the experiments were described in the following weredone 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 whenwe mixed C14DMAO and Texapon N70-H. The total concentra-tion was 100 mM, and the mole fraction varied from 0 to 1 foreach component. With increasing Texapon N70-H we observea L1-phase, a viscous L1-phase, and a two-phase L1/LR-regionwhere the low birefringent LR-phase is on the top of the samples.After the two-phase region we find a single low birefringent

but viscoelastic LR-phase and finally at almost equal molefraction a precipitate is formed. On further increasing theTexapon N70-H concentrations the sequence of the phases isreversed. The sequence in our system is the same as alreadyobserved for many cationic-anionic combinations of surfac-tants,9,22,23which shows that our system has the typical behaviorof cationic and anionic systems. In a second series of samples,100 mM C14DMAO solutions are titrated with Texapon N70-Hstock solution. In this series the C14DMAO concentrationremains constant. The sequence of phases is shown in Figure2. From 0 to 20.5 mM of Texapon N70-H concentrations, oneobserves a single transparent L1-phase that becomes viscous withincreasing of Texapon N70-H. Between 20.5 and 26.5 mM, wenote macroscopic separation into an upper birefringent LR-phaseand an isotropic viscous L1-phase at the bottom. Between 26.5and 54 mM, there is a single, more or less transparentviscoelastic LR-phase with the typical features of vesicle phases.Surprisingly, the LR-phase shows practically no birefringenceat rest, tilt, or flow of the phase. Above 54 mM TexaponN70-H, small particles of a precipitate are formed which coexist

Figure 1. Phase diagram of the mixtures of C14DMAO and TexaponN70-H, the total concentration is 100 mM. The conductivity data (b) isincluded 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.

Figure 2. Phase diagram for the mixtures of 100 mM C14DMAO andincreasing amounts of CH3(CH2)11(CH2CH2O)2.5SO3H (Texapon N70-H) at 25°C. Conductivity data (b) and the phase volumes (O) of thetwo phases are included in the phase diagram.

10146 J. Phys. Chem. B, Vol. 104, No. 44, 2000 Hao et al.

with the LR-phase. These samples do not separate into twophases. Finally, when the concentrations of Texapon N70-H G80 mM, a thick precipitate appears at the bottom. The precipitateis very interesting. On heating, the precipitate disappears at 45°C, and at higher temperatures, the samples are transformedinto the same LR-phase that are present in the neighborly LR-region. We have characterized the different phases with variousphysicochemical methods and some results are shown duringthe following discussion. The phase behavior results with orwithout polarizers are demonstrated in Figure 3. The thirdsample contains small dispensed bubbles that do not rise. Thisshows that the LR-phase has a yield stress value that is largeenough to suspend small air bubbles in the phase.

ConductiVity Measurement. The conductivity data of thesamples are included in Figures 1 and 2 of the two-phasediagrams. One should note that the different phase boundariescan well be determined by the conductivities. In Figure 1, onthe higher C14DMAO concentration side, with increasingconcentrations of Texapon N70-H, the conductivities increasein the L1-phase (see the inset) and decrease in the L1/LR two-phase region. The conductivities decrease to a minimum andhave more or less constant values in the LR-phase. However,on the higher Texapon N70-H concentration side (XC14DMAO <

0.25, the precipitate region appears in the 0.25< XC14DMAO <0.525), the conductivities all increase with the concentrationsof Texapon N70-H in the LR-phase, the two-phase L1/LR region,the viscous L1-phase and L1-phase. In Figure 2, one alsoobserves that with increasing concentrations of Texapon N70-Hthe conductivities rise more or less linearly in the L1- and viscousL1-phase up to 20.5 mM in which the two-phase region isreached and then decrease in the two L1/LR-phase region. Theresults show that the cationic surfactant formed from the zwitter-ionic surfactant C14DMAO because of the protonation of theamine oxide group as soon as the Texapon N70-H acid is added.When the LR-phase is reached, the conductivities decrease tothe minimum even though more Texapon N70-H is added andare about the same values as 100 mM C14DMAO solution. Thisis caused by the transformation of the L1-phase to the LR-phase.

Rheological Properties. The rheological behavior of thesolutions with 100 mM C14DMAO and increasing amounts ofTexapon N70-H is illustrated in Figure 4 (the sequence of phasesis also indicated) using the complex viscosity (|η* |) at afrequencyν ) 0.01 Hz. The both L1-phases are low viscoussolutions. The viscosity increases with increasing of TexaponN70-H concentrations. The increase is particularly pronouncedupon approaching the two-phase boundary. This can be ex-plained by the fact that at low Texapon N70-H concentrationsmixed CH3(CH2)11(CH2CH2O)2.5SO3

--+HOAMD14C sphericalmicelles are present, which at higher Texapon N70-H concentra-tions grow to rodlike micelles.24-28 The macroscopic propertiesof the LR-phase can be characterized by the complex viscosity(|η* |, ν ) 0.01 Hz). It is found that the|η* | are much higherthan those in the viscous L1-phases and have more or lessconstant values (240 Pa or so). The interesting rheologicalproperties of the LR-phases can also be characterized byoscillatory shear experiments. A rheogram for a sample 100mM C14DMAO/30 mM Texapon N70-H is given in Figure 5,which shows a situation which is similar as for many viscoelasticfluids. This fluid behaves like a Bingham fluid with a yieldstress value which can be seen from the fact that the storagemodulusG′ remains more or less constant about 20 Pa over thewhole investigated frequency range which is typical for a vesiclesystem.16,29,30The loss modulusG′′ decreases somewhat withincreasing frequency and is about 10 times smaller thanG′, i.e.,the difference betweenG′ andG′′ is similar to the other vesiclesystems.16,29,30

The samples with the precipitate at 25°C are very interesting.The precipitate dissolves at 45°C, for higher temperatures thesamples become less turbid, and above 50°C a fluid birefringent

Figure 3. Phase behavior of the system 100 mM C14DMAO andincreasing amounts of Texapon N70-H at 25°C, (a) without polarizersand (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.

Figure 4. Viscosity data for the mixtures of 100 mM C14DMAO andincreasing amounts of CH3(CH2)11(CH2CH2O)2.5SO3H (TexaponN70-H) at 25°C. Viscosity data (b) and the phase volumes (O) of thetwo phases are included in the phase diagram.

Vesicle Phase from a Stacked LR-Phase J. Phys. Chem. B, Vol. 104, No. 44, 200010147

phase without precipitate is formed. The temperature-dependentphase behavior of a sample with 100 mM C14DMAO and 60mM Texapon N70-H between 25 and 50°C with polarizers isshown in Figure 6. The results of the temperature dependenceare very similar to the results of the CTAHNC system.23 Themacroscopic properties of the precipitate sample for 100 mMC14DMAO and 60 mM Texapon N70-H dissolved at 50°C werecharacterized by an oscillatory shear experiment, and therheogram is given in Figure 7. The situation is similar to thecase in Figure 5 of the LR-phase. The structures in these samplesthat are novel unilamellar and multilamellar vesicles aredemonstrated in the following FF-TEM experiments.

FF-TEM Micrographs. After having characterized the mac-roscopic properties of the LR-phase, it is now interesting toinvestigate in some more detail the structures by FF-TEM. InFigure 8a,b, we see electron micrographs of the samples with100 mM C14DMAO, and 30 and 40 mM Texapon N70-H. Fromthe micrograph of 30 mM Texapon N70-H, Figure 8a, one canobserve that fairly small unilamellar and multilamellar vesiclescoexist. Some larger multilamellar vesicles with a quite limitednumber of shells and the size with less than 1µm diameter arepresent, but they are relatively rare. The diameter of the small

unilamellar vesicles is about 20 nm. Figure 8b clearly showsthat unilamellar and much larger multilamellar vesicles alsocoexist and the multilamellar vesicles are still relatively smallin quantity. The sizes of the unilamellar vesicles are in the rangeof 100 nm. The biggest multilamellar vesicles are 3.5µm or soand the interlamellar spacing between the bilayers is ratheruniform and in the range of about 600 Å.

Figure 5. Rheogram of the oscillatory shear experiment for a samplewithin the LR-phase of 100 mM C14DMAO and 30 mM TexaponN70-H.

Figure 6. Temperature-dependence phase behavior of a sample with100 mM C14DMAO and 60 mM Texapon N70-H between 25 and 50°C with polarizers. The precipitate at 25°C dissolves at highertemperature and forms a LR-phase.

Figure 7. Rheogram of the oscillatory shear experiment for the samplesfrom precipitate of 100 mM C14DMAO and 60 mM Texapon N70-H at50 °C. the rheogram is the situation which is completely similar toFigure 5 in the LR-phase.

Figure 8. Freeze-fracture electron micrographs of the samples 100mM C14DMAO and 30 mM (a, top), and 40 mM (b, bottom) TexaponN70-H (in an aqueous solution containing 20 wt % glycerol to avoidcrystallization).

10148 J. Phys. Chem. B, Vol. 104, No. 44, 2000 Hao et al.

The LR-Phase Produced by Chemical Reactions.In thissection we describe the preparation and the properties of theLR-phase that is produced by a chemical reaction. In principle,we would have liked to prepare the same phase by a chemicalreaction that we have discussed in the previous section. ThisLR-phase had no excess salt. Unfortunately, we could not thinkof a route which could have produced the same phase in whichno mixing was involved and where we could have started froma well-defined state. Instead, we found a route with which itwas possible to prepare the LR-phase but where the final LR-phase contained equimolar excess salt (sodium formate). Forcomparison of this phase with a normally produced phase weshow first the phase behavior and the phases that are producedin the normal way by mixing of the components and finally thephases that are produced by the chemical reaction where thefinal compositions of the two phases are identical. In one casewe mix the formic acid directly with the components. In thesecond route we produce the formic acid by a chemical reactionin the stagnant system.

Phase BehaVior of C14DMAO/Texapon N70/HCOOH/WaterSystem.The phase diagram for 100 mM C14DMAO as a functionof the concentration of Texapon N70 with equimolar concentra-tions of HCOOH at 25°C is shown in Figure 9. We can seethat with increasing anionic surfactant and formic acid concen-trations one can find that the classic isotropic L1-micellar phaseis followed by a viscous L1 micellar phase. A two-phase L1/L1

region is observed between 12.5 and 17.3 mM Texapon N70. Inthe two-phase region, the upper phase is transparent, and thevolume fraction decreases with the increasing anionic surfactantconcentration, the lower phase is highly viscous and clear.Above 17.3 mM Texapon N70 concentration, a narrow three-phase L1/L1/LR appears. The concentration range of surfactantfor this L1/L1/LR three-phase region is very small and lies at17.8 mM. The upper phase is the LR-phase; it has a lowviscosity, is slightly turbid, and is strongly birefringent. Themiddle phase is clear and has highly viscous properties whilethe lower phase is clear and has a low viscosity. The three-phase region is stable over fairly long periods of time (manymonths) and does not change appreciably during this time.Another two-phase L1/LR region appears following the three-phase region. In the two-phase L1/LR region, the lower phaseoccupies the large volume fraction; it is clear and has viscousproperties. The upper phase is slightly turbid, has low viscosity,and is strongly birefringent. After the two-phase L1/LR, one findsthe LR-phase region and the LR-phase region ranges from about18.5 to 37.5 mM anionic surfactant concentrations. The LR-phases are of low viscosity, turbid, and birefringent. For higher

anionic surfactant concentrations, the LR-phase region is fol-lowed by a large precipitate region up to the anionic/cationicsurfactant ratio of 1. The properties of the precipitate regionare also worth noting. At the lower part of the test tube, thesample have similar properties to the LR-phase region; however,the upper part of the test tube contains the precipitate. Withincreasing temperature the precipitate transforms into a singletransparent phase with highly swollen multilamellar vesicles.

Rheological Properties.We compared the rheological proper-ties of the LR-phases with and without salt, but otherwise ofthe same composition. Figure 10 shows a rheogram for a LR-phase with 100 mM C14DMAO, 30 mM Texapon N70, and 30mM HCOOH (the LR-phase in Figure 9). The LR-phase containssodium formate and is a turbid and birefringent solution. Therheogram shows that the LR-phase is a low viscoelastic phasebecause of the excess salt. The complex viscosity decreases overthe whole frequency range from 0.001 Hz with a slope of-1.The storage modulus,G′, and loss modulus,G′′, are almostfrequency independent. The storage modulus value is about anorder of magnitude higher than the loss modulus. Whencomparing Figure 10 with Figure 5, where the LR-phase doesnot contain salt but otherwise has the same composition, wefind that the salt-free system is very clear, not birefringent anda highly viscoelastic phase with a storage modulus of 20Pa anda yield stress value. The yield stress values of the LR-phasescan easily be recognized by the fact that small air bubbles thatare dispersed in the samples do not rise for a long time (severalweeks). It is noteworthy that the viscoelastic properties of thesalt-free system are more than 1 order of magnitude larger thanthat for the salt system.

The C14DMAO/Texapon N70/HCOOCH3/H2O System.In thekinetic production of the LR-phase, we added 30 mM methylformate to the 100 mM C14DMAO and 30 mM Texapon N70

solution which is still L1-phase. Methyl formate slowly hy-drolyses into formic acid and methanol and formic acid whichprotonates the amine oxide of C14DMAO into the cationicsurfactant.

The mixture of 100 mM C14DMAO and 30 mM TexaponN70 in the L1-phase is an isotropic, clear and low viscositysolution. The solution remains in the L1-state when methylformate is added to reach a concentration of 30 mM. Methylformate is a hydrophilic compound and solubilizes in the waterdomains of the mixed micellar solution. The samples of theL1-phase that is the mixture of C14DMAO and Texapon N70,and the LR-phase formed after the completion of the hydrolysisat rest and the vesicle formed by shaking the LR-phase are shown

Figure 9. Phase diagram and conductivities (b) for the system of 100mM C14DMAO/Texapon N70/HCOOH/water as a function of theTexapon N70 or HCOOH concentration at 25°C. Figure 10. Rheogram of LR-phase with 100 mM C14DMAO, 30 mM

Texapon N70, and 30 mM HCOOH.

Vesicle Phase from a Stacked LR-Phase J. Phys. Chem. B, Vol. 104, No. 44, 200010149

in Figure 11 when the samples are viewed with and withoutpolarizers. The LR-phase without shaking (middle sample) showsthe birefringent pattern which is typical for a multidomainsituation. The LR-phase with shaking by hand is more turbidthan that without shaking, less birefringent, and very much thesame as the sample of 100 mM C14DMAO, 30 mM TexaponN70, and 30 mM HCOOH (the LR-phase in Figure 9).

In Figure 12, a series of different states of a sample are shownto which 30 mM methyl formate was added and increasing timeintervals had passed since the mixing. We notice that 45 minafter the methyl formate is added to the original L1-phase thesolution is still a low viscous L1-phase and then develops intoa viscous L1-phase (below 90 min), then into the two-phaseL1/LR region where the birefringent LR-phase is on the top ofthe sample (above 120 min) and finally after more than 5 hinto the birefringent LR-phase, which can easily be observed inthe solution at rest between crossed polarizers.

The birefringence domain pattern of the single LR-phase doesnot change a lot with time. Even after 1 day the sample looksvery much the same and very different from the vesicle phaseformed by shaking in Figure 11 (left sample). The hydrolysisreaction of methyl formate is completed in about 1 day.17 Theprocess of the hydrolysis of methyl formate in solution can be

checked by measuring the pH and the conductivity. The decreasein pH and the increase in conductivity of the solution as afunction of time are given in Figure 13.

The above-described experiments demonstrate unambiguouslythat the final two states that are reached after the completion ofthe hydrolysis with shaking by hand and without shaking atrest are not the same. The birefringence pattern and macroscopicproperties of the two phases are very different. Obviously onlyone of the two phases is in the equilibrium while the other onemust be trapped in a metastable state. However, we kept bothsamples at 25°C for several months to look at their appearancewith and without polarizers, and we did not notice any changein the optical appearance of the two phases.

Rheological Properties of the LR-Phase at Rest and AfterShaking. In the next experiment we measured the viscosity ofa L1 sample with time after methyl formate was added to theoriginal L1-phase. The result is shown in Figure 14. The resultshows that the viscosity of the system increases from less than6 to 40 mPa s after 45 min and then abruptly to 10 Pa s ataround 90 min. After 90 min, the viscosity of the systemdecreases with time until around 5 h later and the viscosityreaches the final value of about 100 mPa s. The viscosity changeof the system reflects the phase transformation caused by thehydrolysis of methyl formate, i.e., from the original low viscousL1-phase, viscous L1-phase, the two phase L1/LR region and thebirefringent LR-phase.

In another rheological experiment, we compared the rheo-logical properties of two samples which were in the LR-phase,one formed by the hydrolysis of methyl formate without shaking,and one in the vesicle phase formed by stirring the LR-phase.The results are shown in Figure 15a,b where rheograms of thetwo samples are compared. The vesicle phase that was producedafter the completion of the hydrolysis at rest for 1 day but shakenbefore the measurement showed much the same situation asthe LR-phase that was prepared with 100 mM C14DMAO, 30mM Texapon N70, and 30 mM HCOOH (Figure 10). The twophases have the same composition. The two phases are all lowviscoelastic phases because of the excess salt; they are turbidand little birefringent fluids and contain densely packed mul-tilamellar vesicles as is demonstrated in the following FF-TEMand SANS experiments. The complex viscosity (|η* |) of thevesicle phase formed by chemical reaction by stirring decreasesover the whole frequency range from 0.001 Hz with a slope of-1. The storage modulus,G′, and loss modulus,G′′, are almostfrequency independent. The rheological properties are verysimilar as those in Figure 10 from the system 100 mM C14-DMAO, 30 mMTexapon N70, and 30 mM HCOOH. Therheogram (Figure 15a) of the LR-phase that was shaken wasvery different from that of the unstirred phase (Figure 15b).For the unstirred phase, the viscosity decreases less over thetotal frequency and has much lower viscoelastic properties inwhich G′, andG′′ were more than 1 order of magnitude lowerthan those in the stirred phase. One should notice thatG′ andG′′ all increase over the whole frequency range.

FF-TEM Micrographs of the LR-Phase at Rest and AfterShaking.After the macroscopic properties of the two sampleswhich were prepared by hydrolysis of methyl formate have beencharacterized, the next experiments will be done to investigatein some more detail the different microstructures of the twosamples by using FF-TEM and SANS. Visual inspection of thesamples with and without polarizers shows that differentmicrostructures must exist in the samples after they have beenmanipulated differently. In order to identify the differentmicrostructures for the two samples, we first investigated freeze-

Figure 11. Left sample: L1-phase that is the mixture of C14DMAOand Texapon N70. Middle sample: the LR-phase formed after thecompletion of the hydrolysis at rest. Right sample: the vesicle phaseformed by shaking the LR-phase. The upper row is without polarizerand the lower row is with polarizer.

10150 J. Phys. Chem. B, Vol. 104, No. 44, 2000 Hao et al.

fracture electron micrographs from the samples, and the resultsof the two samples are shown in Figure 16a,b. Figure 16a showsa micrograph of the LR-phase formed after the completion ofhydrolysis of methyl formate without shearing. One can observethe typical pattern for stacked bilayer of the normal LR-phase.This micrograph thus clearly shows that the combination ofanionic surfactant and cationic surfactant formed by the pro-tonation from zwitterionic surfactant (C14DMAO) in the L1-

phase in the absence of shearing forces leads to a normal LR-phase and not to a vesicle phase. Figure 16b shows that thevesicles were produced by shear. Obviously, the vesicle phaseshave been produced from the normal LR-phase by the shearingforces in the shaking procedure. Since the shear rates in theshaking procedure are not well-defined, the size distribution ofthe vesicles is polydisperse. The FF-TEM results of the twosamples clearly demonstrated that the generally held assumptionthat stable vesicles formed spontaneously in cationic and anionic

Figure 12. A series of different states of a sample with 30 mM methyl formate added to 100 mM C14DMAO and 30 mM Texapon N70 L1-phasewithout stirring and with increasing time intervals since the mixing. Observation with crossed polarizers.

Figure 13. Decrease in pH and the increase in conductivity of thesolution as a function of time with the system 100 mM C14DMAO, 30mM Texapon N70, and 30 mM methyl formate.

Figure 14. Change of the rheological properties of a L1-phase from100 mM C14DMAO, 30 mM Texapon N70, and 30 mM methyl formatewith time.

Figure 15. Rheograms of two samples from 100 mM C14DMAO, 30mM Texapon N70, and 30 mM methyl formate: (a, top) the samplehad been at rest for 1 day but shaken before the measurement; (b,bottom) the sample had been at rest 1 day and not been shaken.

Vesicle Phase from a Stacked LR-Phase J. Phys. Chem. B, Vol. 104, No. 44, 200010151

surfactant mixtures may be the result of shearing forces thatoccur during the mixing process of the two components.

SANS Measurements. The FF-TEM results of the two sampleswere also confirmed from scattering measurements as following.At first, we monitored the transformation of the L1-phase tothe LR-phase by a SANS measurements. For this purpose asample was prepared in D2O that was located in L1-phase (100mM C14DMAO and 30 mM Texapon N70). To this sample then30 mM methyl formate was added and the progress of thehydrolysis reaction was monitored continuously by SANSmeasurement until the scattering curves do not change with time.

The scattering curves for the L1-phase with and withoutmethyl formate are shown in Figure 17 (curves I and II). Thescattering curve in the presence of methyl formate was recordedabout 5 min after the methyl formate had been added to theL1-phase. Both scattering curves are very similar and show thetypical scattering features of an ionically charged L1-phase,namely a broad interaction peak atq ∼ 0.04 Å. When onecompares the scattering curves (curves I and II), one observesthat the scattering intensities with methyl formate are muchlower than that without methyl formate. This could be the resultof the strong interaction between the anionic surfactant[CH3(CH2)11(CH2CH2O)2.5SO3

-] and cationic surfactant (C14-DMAOH+) that is formed by a minute protonation, due to theonset of the hydrolysis reaction. The scattering curves of theLR-phase formed by the hydrolysis of methyl formate at rest

and the vesicle phase formed by stirring the LR-phase are verydifferent (curves III and IV). Curve IV is the typical pattern ofvesicle phase and there is a weak peak at very smallq value,q∼ 0.01 Å, a mean spacing of 628 Å between the vesicleaggregates can be calculated. Curve III is the scattering curveafter the completion of the hydrolysis of methyl formate, whichis different from curve IV and shows that the La-phase has adifferent structure as the vesicle phase ( curve IV). As obtainedfrom FF-TEM, it consists of stacked bilayers.

The results of SANS scattering curves with time, after methylformate was added into the L1-phase, are shown in Figure 18.With passing time, however, before the LR-phase appears, thebroad interaction peaks of the L1-phase shift to the smallerqvalues. In relation to theqmax for the L1-phase immediately afteradding of methyl formate, the broad peak after 75 min hasshifted to about3/4 of qmax. With further evolving time the peaksbecome even broader and shift somewhat to even smallerqvalues and disappear eventually when the LR-phase is reached.The scattering curves in the LR-phases do not much change withtime.

Conclusion

We have studied a new cationic and anionic surfactantmixture system where no salt is formed from the combination

Figure 16. Freeze-fracture TEM micrographs that were prepared fromthe same samples: (a, top) 100 mM C14DMAO, 30 mM Texapon N70,and 30 mM methyl formate after 1 day hydrolysis without shearing;(b, bottom) the same sample after 1 day hydrolysis and shaking beforemeasurement.

Figure 17. SANS scattering curves of the system 100 mM C14DMAO,30 mM Texapon N70 in D2O: (I) the original L1-phase (b), (II) directlyafter addition of 30 mM methyl formate (O), (III) after more than 7 h(4), (IV) completion of hydrolysis of methyl formate and after shaking(vesicle phase,2).

Figure 18. SANS scattering curves at different time with the system100 mM C14DMAO, 30 mM Texapon N70, and 30 mM methyl formatein D2O.

10152 J. Phys. Chem. B, Vol. 104, No. 44, 2000 Hao et al.

of the surfactants. We have found that unilamellar and multi-lamellar vesicle phases are formed in mixtures of alkyldi-methylamine oxide and alkylethoxysulfonic acid. Mixtures ofthe two surfactants show the typical behavior of cationic andanionic systems. The formation of vesicle phase in the newcationic and anionic surfactant mixture solutions is mostremarkable, and the LR-phase has a yield stress value that islarge enough to suspend small air bubbles in the phase.

The classic LR-phase as opposed to the vesicle phase in thenew cationic and anionic surfactant mixture solutions is alsodemonstrated by the preparation of the LR-phase without shearby mixing a small amount of methyl formate to a L1-phasefrom C14DMAO and sodium dodecylethoxysulfate [CH3(CH2)11-(CH2CH2O)2.5SO3Na, Texapon N70]. The LR-phase formed bythe simple hydrolysis has stacked bilayers and can be trans-formed into vesicle by the shearing forces that occur when thesamples with the classic LR-phase are turned upside down afew times. Our experimental results show that the generally heldassumption that the vesicles that form spontaneously in cationicand anionic surfactant mixtures may be the result of shearingforces that occur during the mixing process of the twocomponents.

The vesicle phases that are formed by the chemical reactionwith the combination of the cationic and anionic surfactantshave different macroscopic properties from the systems that areprepared from the zwitterionic (C14DMAO) and Texapon N70-Hacid. In the latter situation, the vesicle phases do not containexcess salt and the ionic charges on the vesicles are not shielded.As a consequence, the vesicle phases are strongly viscoelasticand have a yield stress which is large enough to suspend smalldispersed air bubbles in the solutions.

Acknowledgment. J.H. is an Alexander von HumboldtFoundation Fellow, and he thanks the Alexander von HumboldtFoundation which gave him a chance for doing research workunder the guidance of Professor Dr. Heinz Hoffmann inPhysikalische Chemie I, Universita¨t Bayreuth, Germany.

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