W

J . Phys. Chem. 1994,98, 2613-2623

2613

Influence of Charges on Structure and Dynamics of an O/W Microemulsion. Effect of Admixing Ionic Surfactants Michael Gradzielski' and Heinz Hoffmann Lehrstuhl fur Physikalische Chemie I der Universitiit Bayreuth, 95440 Bayreuth, Federal Republic of Germany Received: July 8, 1993; In Final Form: December 14, 19939

The effect of charges on the properties of oil-in-water (O/W) microemulsions has been studied. These investigations started from an aqueous system made up from a zwitterionic surfactant (alkyldimethylamine oxide) and hydrocarbon. The O/W droplets in this system can be charged by the partial substitution of the nonionic surfactant by either a cationic or an anionic surfactant. The properties of these charged microemulsion aggregates were studied by static and dynamic light scattering, viscosimetry,and interfacial tension measurements. The charge density on the droplets can dimply be fixed a t a desired value by the composition of the surfactant mixture. The light scattering experiments showed that the aggregates remain constant in size over a large concentration regime (0.1-30 wt %) and in addition their size is only very little influenced by the admixture of ionic surfactant. The scattering behavior of the cationically substituted microemulsions could adequately be described by a simple random phase approximation (RPA) model that contains a hard-sphere interaction with an additional DLVO-potential term that accounts for the electrostatic repulsion. Deviations toward lower scattering intensities were o b s e q d for the anionically substituted microemulsions; Le., this system behaves asymmetrically with respect to cationic and anionic surfactant substitution. These deviations result both from an increased effective hard-sphere radius (larger hydration shell) and from a more effective electrostatic interaction. This effect is not particular for the system studied, but similarly observed for a microemulsion based on another nonionic surfactant (Brij 96). Dynamic light scattering experiments showed an increase of the diffusion coefficient with rising ionic content of the system. This increase is entirely due to the decreasing structure factor and only little influenced by the hydrodynamic factor, which is slowly decreasing with increasing ionic content. The diffusion coefficient of the charged microemulsions exhibits a maximum at -0.03 volume fraction.

1. Introduction

Microemulsions are thermodynamically stable multicomponent fluids, normally composed of an aqueous component, an oily component, an amphiphile as emulsifying agent, and frequently a cosurfactant (usuallyan alcohol of intermediate chain length). Their structural entities are much smaller than the wavelength of light which is the reason for their transparency. Such systems usually form spontaneouslyand have been known since the 19409.2 One can distinguish three different basic structural types of microemulsions: hydrophobic aggregates in water (oil-in-water, O/W), hydrophilic aggregates in oil (water-in-oil, W/O), and finally bicontinuous structure^.^ In all these cases, the surfactant forms a film at the internal interface that separates the water and oil domains. The presence of the cosurfactant is often required in order to lower the interfacial tension of this interface because a low interfacial tension is essential for the formation of microemulsions. For O/ W microemulsions mostly nonionic surfactants have been employed. However, they can also be produced with ionic surfactants, although those normally possess a much higher interfacialtension4than their uncharged counterparts. Therefore, in ionic systems, normally the presence of various amounts of cosurfactants is required in order to achieve microemulsification.~~6 In this work, O/W microemulsions based on the zwitterionic alkyldimethylamineoxides tetradecyl- and oleyldimethylamine oxide (TDMAO and ODMAO) were investigated with respect to the structural and dynamical changes that occur upon the substitution of the nonionic surfactant by an ionic surfactant, Le., by the process of charging the microemulsion droplets. Previous work7 has shown that these zwitterionic surfactants exhibit properties which could be described as being somewhere in between those of a typical nonionic and those of a typical cationic surfactant of equal chain length. For instance, their Abstract published in Advance ACS Abstracts, February 1, 1994.

0022-3654/94/2098-26 13304.50/0

cmcvalues are 0.121 X 10-3and 5.6 X 10-3 mmol/L,89respectively, and no cloud point is observed as it is commonly found for nonionics.1° In the binary surfactant systems, spherical micellar aggregates are formed just above the cmc and rodlike micelles at higher concentrations. The length of the rods increases with increasing con~entration.~ This effect is much more pronounced for the ODMAO where the rods already overlap in a 10 mM solution and which therefore displays viscoelastic properties already at these fairly low concentrations.ll Further investigations showed that the solubilization of hydrocarbon into these rodlike micelles induces a rod-to-sphere transition whereby globular microemulsion dropletsare formed.'* Such a behavior has similarly been observed for other nonionic surfactants.13 Upon further increaseof the amount of solubilized hydrocarbon, these droplets increase slightly in size until the solubilization capacity of the respective system is reached. Normally, this existence region of the microemulsion droplets is relatively small; Le., the size of the droplets is restricted to a narrow range for a given system. For the case of ODMAO, always a much larger amount of hydrocarbonis required in order to achieve the rod-to-sphere transition than in the case of the TDMAO, e.g., for decaneapproximatelya 5 times larger amount. At the same time thesolubilization capacity of ODMAO is about 2.5 times larger than that of TDMA0.14 For the ternary system amine oxide (TDMAO or ODMAOO/ decane/water, it has been shown before by investigations of concentrationsseries at constant surfactant to decaneratio (which was kept close to the solubilization capacity and well above the one of the rod-to-sphere transition) that these O/W microemulsions contain globular droplets that areof constant size irrespective of the total concentration (Le., surfactant plus decane) up to concentrations of -300 g/L. The light scattering and SANS data were in good agreement with a model that assumes simple hard-sphere interactions between these droplets.15 From such an analysis it was concluded that the droplet radius was 32 and 0 1994 American Chemical Society

2614 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 50 A for the TDMAO and ODMAO system, respectively.l6 In both cases a hydration shell of thickness 2.5-3 Acould be inferred from the data. The effective hard-sphere interaction radius is larger than the droplet radius by this hydration shell. Furthermore, SANS experiments were carried out on a diluted sample, where one assumed the effects of the structure factor to be negligible. Therefore, the angular dependence of the scattered intensity should only be determined by the particle form factor and the data showed that the microemulsion aggregates possess an unusually low degree of polydispersity of less than 10% for the mean radius ( u I 0.1R).16 Such a polydispersity is relatively small for a self-aggregatingcolloidalsystem. However, a similarly small value of 12% has recently been reported for a W/O microemulsion in the system AOT/hexane/H20.17 From all this it may be concluded that the amine oxide/ hydrocarbon system represents a good example of a simple and very well-defined model system of an O/W microemulsion made up from globular droplets. This renders it a suitable system for the study of systematic variations of the interactions between these droplets and how they influence the structural and dynamic properties of the microemulsion. For instance, these interactions could become varied by the substitution of the amine oxide by another surfactant. For our purposes it seemed especially interesting to substitute the zwitterionic by an ionic surfactant, thereby introducing repulsive forces of Coulomb type between the aggregates. By doing so we are able to vary this interaction continuously and study its effect on structure and dynamics of the system.

Gradzielski and Hoffmann

t ~ ine io-*

2.5 2.0 1.5 1.o 0.5 0.0 0 0 ~

2.1. MaterialsandPreparationof thesamples. The tetradecyl (TDMA0)- and oleyl (0DMAO)- dimethylamine oxides were obtained as a gift from Hoechst and recrystallized twice from acetone before use. The Brij 96 was purchased from Fluka Chemie AG and used without further purification. The decane was also obtained from Fluka Chemie AG in p.A. quality. Tetradecyltrimethylammonium bromide (TTABr) and chloride (TTACl) were purchased in 99% purity from Aldrich, and sodium sodecyl sulfate (SDS) used in cryst. reinst quality from Serva Feinstchemikalien. Sodium myristate from Fluka Chemie AG was employed in purum quality without further purification. Finally, sodium tetradecyl sulfate (TDS) was a gift from Henkel AG. All microemulsions were prepared by simply mixing together the required amounts and homogenizing the mixture by means of a magnetic stirring device at slightlyelevated temperature (50 "C). In all cases, homogeneous transparent solutions were obtained and allowed to equilibrate at least for 2 days before measurements commenced. 2.2. Methods. Static light scattering experimentswere carried out with a Chromatix KMX-6 at a wavelength of 632.8 nm. All solutions were filtered through a 0.2-rm filter to remove dust and other impurities. The refractive index increment was determined in a Chromatix KMX-16 differential refractometer that allows for a precission of up to seven decimals. Dynamic light scattering experiments were performed using a Malvern digital correlator K 7023. As a light source an Ar laser (Spectra Physics, 2 W) with a wavelength of 488 nm was employed. The rheological propertieswere measured with an oscillatingcapillaryviscosimeter (Paar OCR-D). Interfacial tension data were obtained by the spinning drop method18 (Site 2, ITE-Engineering) after the respective samples were saturated and equilibrated with decane (against which the interfacial tension was measured). 3. Static Light Scattering

According to preceding investigations,l6 the microemulsion droplets in the amine oxide/hydrocarbon system exhibit an unusually small polydispersity. In addition, their interaction can readily be described by a simple hard-sphere potential even up

50 50

100 100

150 150

200 200

250 250

300 300 c in g / l

Figure 1. Rayleigh factor Re at 25 OC for microemulsions formed from various mixtures of TDMAO and TTABr with decane. The molar ratio of surfactantto decanewas always kept at 3.2:l. Molar content of TTABr (with respect to the total surfactant): (0)0% (pure TDMAO); ( X ) 1.25%; (0)2.5% ( 0 )5 % (A)7.5% (V) 10%; (0)15% (fitted curves as solid lines).

TABLE 1: Fit Parameters R, R, and Charge z(RPA) for Various Mixtures of TDMAO and 'TTABR and Charge ~0 As Calculated from tbe Composition (for an Assumed Radius of 31.7 A, x(TTABR/x(surf.) 0

2. Experimental Section

cm-'

0.0125 0.025 0.050 0.075 0.10 0.15

a/eo 0

2.87 5.75 11.50 17.3 23.0 34.5

z(RPA)/eo 0

2.44 5.54 11.40 17.7 21.5 30.2

R (A)

(RPA) 31.7 32.6 32.8 32.3 33.3 31.6 31.0

R, (A)

@PA) 34.0 35.3

35.6 34.9 36.2 33.8 33.5

to high volume fractions of more than 0.35 and their size is independent of concentration. These properties render them to bea suitable model system for the study of theeffectsof systematic variations in the interactions in such microemulsions. One way to change these interactions would be through the substitution of the zwitterionic amine oxide by an ionic surfactant. By perturbing the system in such a way one would expect to switch from the formerly uncharged microemulsion droplets to charged aggregates interacting via a screened Coulomb potential. The charge on the aggregates would simply be determined by way of thecompositionof the surfactant mixtureand therefore thecharge density on the droplets would be fixed in an easily controllable fashion. In order to investigate this effect, the cationic trimethyltetradecylammonium bromide (TTABr), and the anionic sodium tetradecyl sulfate (TDS) were employed as ionic surfactants. Here it is interesting to note that by doing so one is able to continuously change the charge on the aggregates from zero to a desired value without disturbing the system otherwise and as will be shown without changing the droplet size very much. 3.1. Mixtures of TDMAO and TTABr. In order to study the structural properties of these charged microemulsion systems, static light scattering experiments were carried out on microemulsions based on TDMAO as surfactant. For that purpose, concentration series were investigated where the molar ratio of surfactant to decane was kept constant at 3.2:l. As a first step the effect of the cationic surfactant TTABr on the properties of themicroemulsionsystemswasstudied. In Figure 1 the scattering intensity as a function of the total concentration (surfactant + decane) is given for systems that contained 0, 1.25, 2.5, 5, 7.5, 10, and 15 mol % TTABr (of the total surfactant). Similar scattering curves as for the uncharged systems are observed but the higher the content of ionic surfactant the lower the scattering intensity and its maximum is shifted to higher concentrations.

Structure of O/ W Microemulsions

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2615

The reason for this behavior is the increased electrostatic repulsion between the droplets. The repulsive interaction increases the ordering of the aggregates in the microemulsion,which is reflected by a diminished structure factor and therefore results in a decreased scattering intensity. For the highest concentrations the intensity becomes almost independent of the content of ionic surfactant in the mixtures. At these high volume fractions of about 0.35 the main contribution to the repulsion between the aggregates simply arises from the excluded volume, i.e., the hardsphere interaction, because at this point the system is already densely packed. Therefore, the structure factor becomes practically independent of the particle charge and the similarscattering intensities at this point indicate that the size of the aggregates should be the same for all the mixtures. In the next step we attempted to account for the experimental scattering data by means of a theoretical model that includes the electrostatic interaction of the aggregates. Several approximations are known for the calculation of the structure factor for such a system and here we chose a particularly simple model, the random phase approximation (RPA),19*20that leads to an analytical expression for S(0). For the case of charged colloids (with an effective hard-sphere diameter Den)the RPA starts from the well-known hard-sphere model and treats the electrostatic interaction as a perturbation to that potential. In doing so one uses the hard-sphere fluid as a reference system (characterized by CHS (4)) and accounts for the additional interaction by means of a screened Coulomb potential2' (eq l), which arises from the Debye-Huckel approximation22 and should describe the potential accurately for the case that the Debye screening length K is larger than the particle diameter L4n.23 Then in the RPA the additional term to the direct correlation function c(q) is simply given by a Fourier transform of the potential (eq 2).24 (It should be noted here that V(r)has the dimension of an energy, while U(q),being its Fourier transform, has the dimension energy times volume. This means that the direct correlation function c(q), as first introduced by Ornstein and Z e r ~ ~ i kpossesses e , ~ ~ the dimension of a volume.) Using this result the structure factor S(q) can in general be calculated from eq 3.19 Because the wavelength of light is much larger than the largest dimension of the aggregates to be investigated, only S(0) is required, as given by eq 4, where the CarnahanStarling expression26was used to describethe hardsphere reference fluid. With this the Rayleigh ratio of the light scattering intensity is given by eq 5 and this model was used to fit the experimental data by a least-squares routine. The fit parameters in this procedure were the radius of the aggregates R, the effective interaction radius R, (& = 2R,), and now in addition the charge z of the aggregates. The Debye screening length K was assumed to be fixed by the concentration of the counterions. uDLVO(r)

= rcDef:$O

eft(l/r) exp(-(r-

(l)

where e is the dielectric constant of the medium (=ereO). c(q) = cHS(q)

=

- UDL.VO(q)/kT

(2)

1 1 - 'Nc(q)

(3)

where IN is the number density of the particles. s(o)-' =

(1

+ 29)2 - 4Q3 + Q4

+

(1 - @)2

where Q is the volume fraction and &, given by21

the surface potential

where zeo is the charge of the particle and K the Debye screening length

where Z is the ionic strength.

where is the refractive index of the solvent, NA the Avogadro constant, A the wavelength of the incident light, dn/dc the refractive index increment, cs the weight concentration of the solute, S(0) the structure factor (accounts for interparticle interferences), and Mw the molecular weight of the aggregate. The fit curves of the RPA model are plotted as solid lines in Figure 1 and are in good agreement with the experimental data. The corresponding fit parameters are given in Table 1 and this analysisshows that the charged aggregates too remain of constant size irrespectiveof the concentration. The radii R and R, remain constant and the obtained effective charges z(RPA) are close to the ones (ZO)predicted from the composition of the aggregates, if one assumes a constant radius of 31.7 A. For samples with more than 10 mol 3'6 ionic content, the obtained charges started to deviate toward smaller values than those predicted according to the composition. This effect seems to be associated with the onset of the counterion condensation as was further corroborated by measurements of the electric conductivity.'6 This finding is also in good agreement with experimentalinvestigationson binary surfactant mixtures of a nonionic and an ionic surfa~tant.2~-~* In these investigations the effective onset of the counterion condensation was observed between 10and 20 mol % ionic surfactant. Thesevaluesagree also well with calculationsfor systemsof similar size in the absence of added electrolyte that predict the effect of counterion condensation to become significant for an ionic surfactant content higher than -20 mol %.29 The good agreement between the charge values obtained from the RPA model and zo implies that this analysis of the concentration dependent static light scattering data is a suitable way of determining the charge or respectivelythe surface potential of a colloidal aggregate. Therefore, this method is somehow complementary to the conventional method that determines the surface potential via the electrophoretic mobility of the respective particle,30as normally done by means of electrophoretic light scattering.31 At this point it should be mentioned that it was also tried to fit the scattering data with the help of the structure factor of charged colloids according to the mean spherical approximation (MSA). The MSA also leads to an analytic expression for the structure factor32533and has frequently been used for the analysis of angle-dependent scattering data of micellar ~ystems,3~-35 polymer latices,36 or charged silica particles." However, the use of S(0) from the MSA always yielded fits of the scattering data that were by far not as good as those obtained with the RPA model just described; i.e., they showed systematic deviations (too high values for low concentration and too low values for high concentrations). Especiallyfor the particle radius unrealistically small values were obtained. For some reason the MSA model seems to be less suitable for the description of the experimental data. 3.2. Mixtures of TDMAO and TDS. In the next step static light scattering experiments were performed on concentration series where now a certain constant amount of TDMAO was substituted by the anionic surfactant TDS. The observed

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994

2616

f

RQ in

3.54

TABLE 2

cm-'

Commition (Usinn R,)

0

0.0125 0.025 0.075 0.125

15 1 J 10

05

00 0

50

100

150

250

200

300 c in g/l

Figure 2. Rayleigh factor Re at 25 O C for microemulsions formed from various mixtures of TDMAO and TDS with decane. The molar ratio of surfactant to decane was always kept at 3.21. Molar content of TDS (with respect to the total surfactant): (0)0% (pure TDMAO); (X) 1.25%; (0)2.5%; (0) 7.5%; (A) 12.5% (fitted curves as solid lines). A

304

RQ in

cm-'

a O

4

X

d

1

e

00

0

50

100

Fit Parameters and Charge z(RPA) for Various

Mixtures of TDMAO and TDS and Assumed Particle Radius R, (from Figure 4) and Charge As Calculated from the

P'Bab,

3.04

Gradzielski and Hoffmann

150

200

250

300

1

c in g/l

F w e 3. Rayleigh factor Re at 25 OC for microemulsions (x(surfactant)/ x(decane) = 3.2:l)that contained 2.5 mol % (with respect to the total

TDS; (A)SDS; (X) sodium surfactant) of various ionic surfactants: (0) myristate; ( 0 )TTABr; (e)'M'ACl. For comparison: (0)pure TDMAO, (+) 5 mol % TTABr and 5 mol % TDS.

scattering curves (Figure 2) are in principle similar to those in the cationic case, but a more careful inspection reveals that for the same degree of ionic substitution the scattering intensity is always significantly lower than in the case of the cationic substitution by TTABr (compare Figure 3). Moreover, one finds that even for the highest concentrations the scattering intensity still does not approach a common value but decreases with increasing TDS content, which is in contrast to the observation in the cationic case. This effect may better be seen in a direct comparisonwith the data for a TTABr-containingmicroemulsion, where always 2.5 mol % of the TDMAO was substituted by an ionic surfactant (Figure 3). In addition, it is evident that the effect of lower scattering intensitiesis not specificfor the particular surfactant TDS, but similarly observed for other anionic surfactants like SDS and sodium myristate. The same is true for cationic substitution where an analogous scattering behavior as for the TTABr is observed in the case of the TTACI; Le., the scattering properties of thecharged microemulsionsareonly little influenced by the nature of the counterion but predominantly determined by the fact whether the surfactant head group is anionic or cationic. Obviously there exists a dissymmetry with respect to anionic or cationic surfactant substitution, a fact that should not be expected from simple physical reasons, i.e., reversing the sign of the charge of the aggregates should not influence their scattering properties, but should be related to specific, chemical properties of the ionic surfactants. We are not aware that such

2.95 6.08 20.1 36.4

2.86 6.80 30.0 55.5

31.7 32.0 32.3 33.3 34.3

34.0 35.6 36.6 38.9 43.9

an asymmetrical scattering behavior has been reported before. Finally, it should be noted that the simple hard-sphere behavior is recovered for surfactant mixtures that contain identicalamounts of cationic and anionic surfactant (+ in Figure 3: 5 mol % TTABr and 5 mol % TDS). Of course the different scattering behavior leads to the result that the description of the experimental data of the TDS system is by far not as satisfactorily possible as in the case of the cationic substitution. The fits are significantly worse and tend to lead to too small radii and at the same time too high charges for the aggregatesareobtained. Thequestion arises how one may account for this different behavior of the oppositelycharged microemulsion droplets? Usually very valuable information regarding the properties of micellar solutions and especially of their microemulsions can be gained from the knowledge of the interfacial tension of the respective surfactant system against the hydrocarbon to be solubilized. The interfacial tension for various mixtures of TDMAO with TTABr or SDS as measured against decane has been reported before.16 Upon the admixture of the cationic surfactant a continuous increase of the interfacial tension was observed, whereas in the case of the mixtures with the anionic SDS a much different behavior was found. There a broad minimum of the interfacial tension can be seen at roughly equimolar ratios. This minimum indicates strong synergistic effects that occur on mixing the zwitterionic amine oxide with the anionic surfactanL8 The amine oxide head group itself has a weak basicity that will be enhanced by the presence of the electric field of the anionic head groups.*J8 This again leads to an increased protonation of the amine oxide and possibly to the formation of ion pairs. Anyway from the theory of microemulsions it is wellestablished that thevalueof the interfacial tension should directly be related totheabilityoftherespectivesurfactant mixture to solubilize the correspondinghydrocarbon. The lower the value of the interfacial tension the more hydrocarbon can be solubilized and the larger will be the microemulsiondropletsthat are f0rmed.3~ Considering the interfacial tension data one would expect to have larger droplets present in the case of the TDS mixtures than in the pure TDMAO systems, whereas the lower light scattering intensity would make one inclined to assume the presence of smaller aggregates in these solutions. In order to sort out this discrepancy, some light scattering measurements were made on samples of constant total concentration of 36 mM surfactant (TDMAO + TTABr or TDS) and 11.25 mM decane, but now in 100 mM KCl solution instead of pure water. The presence of the electrolyte should result in a shielding of the electrostatic repulsion. At the chosen salinity the electrostatic effect should practically vanish (Debye screening length K = 9.6 A at 100 mM concentration of an 1:l electrolyte such as KC1) and one ought to come back to the hard-sphere case. In Figure 4 the scattering intensity for these microemulsions is given as a function of the content of ionic surfactant. As can be seen, only a very small decrease of the scattering intensity and therefore of the particle size, which is proportional to it, occurs upon the addition of the TTABr whereas a discernibleincrease takes place with increasing TDS content. It should be mentioned here that the same trend was also observed using NaBr as shielding electrolyte; i.e. this

Structure of O/W Microemulsions

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2617

14f R@ in

cm-'

P

6

2' TD S l " " " " ' 1 " " " " '

TTABr ' " " ' " ' l " ' ' ~ '

effect should not be due to specific interactions between the ionic head groups and the counterions. The main effect of the excess electrolyte is just to shield the electrostatic interaction between the aggregates, without changing them much. From this experiment, it is evident that the presence of the anionic TDS leads to a slight increase in the microemulsion droplet size, but how does that agree with the deviation toward lower scattering intensities for the electrolyte-free concentration series that is observed experimentally (Figure 3)? It certainly cannot be the result of smaller aggregates in the case of anionic substitution, but requires a different explanation. In order to explain the experimental scattering data by the RPA model described above, this Rayleigh factor in 100 mM KCl solution was taken for the calculation of the particle radius R,,for a given content of TDS in the surfactant mixture; Le., this measurement was used to obtain the particle radius as a function of the content of anionic surfactant. This assumed radius R,, was now kept fixed in the fitting procedure and only R, and z(RPA) used as fit parameters. The corresponding theoretical curves are plotted as solid lines in Figure 2, and the obtained values together with the particle radius R,, and ZO, as calculated from the composition, are listed in Table 2. It is observed that with increasingrelative content of TDS an increase of the effective hard sphere radius R,occurs that is more pronounced than that ofR,. At thesame timeone finds that thevalues for theaggregate charge are consistently larger than those expected according to the composition, especially for high TDS content. Both these effects are the result of a significantly larger repulsive interaction for the TDS substituted microemulsions that has to be present in order to explain the lower scattering intensities in comparison to the TTABr case. Obviously some increased repulsion between the aggregates is present that seems to result both from a larger hard-sphere interaction radius as well as from a more effective electrostatic repulsion. A larger hard-sphere radius one could imagine to be due to a larger hydration shell. This appears to be quite reasonable because the sulfate head groupof the TDS should be more strongly hydrated than in comparison the amine oxide group or the trimethylammonium group. This ought to be that way because only in thecaseofthesulfategroup(orfor that matter thecarboxyl group in the sodium myristate) strong hydrogen bonding of water molecules can occur at the head group of the surfactant, Le., because only here strong 0-H bonding is possible. In order to check this assumption, the viscosity was measured for the same systems used for the light scattering experiments. In Figure 5 the relative viscosities for the pure nonionic system and for systems containing 7.5 mol 9% of TTABr or TDS are shown as a function of the volume fraction of the surfactant plus hydrocarbon (calculated from their weight fraction with an aggregate density of 0.851 g/mL, as extrapolated from density

l " " l " " l " " l " ~ ' " " ' " " ' l ' " l ' '

000 005 010 015 020 0 2 5 0 3 0 0 3 5 Figure 5. Relative viscosity vr at 25 OC as a function of the volume fraction @ for microemulsions of constant surfactant/decane molar ratio of 3.2:l. (0)Pure TDMAO (0)7.5 mol % TTABr; ( 0 ) ;7.5 mol 5% TDS (fitted curves as solid lines).

TABLE 3 Fit Parameter $&/$E for the Thomas Model for Differently Substituted Microemulsion Systems with Constant Molar Surfactant/Decane Ratio of 3.21 and Ratio of Hydrodynamic Radius to Particle Radius pure x(TTABr)/x(surf.) x(TDS)/x(surf.) system TDMAO = 0.075 = 0.075 1.408 1.600 bedbe 1.418 1.170 1.123 1.121 Rh/Ro measurements). The relative low viscosities, even for very high volume fractions of up to 0.35, again indicate that spherical particles should be present in the solutions because anisometric aggregates would lead to much higher values. However, whereas theviscosities of theuncharged and the cationicsystemsarealmost exactly equal, much larger values are observed in the anionic case. This behavior is evidencefor a much higher effectivevolume fraction in the anionic system, i.e., a significantly larger effective hydration shell of the particles, which parallels the findings of the static light scattering measurements. The viscosity data also compare well with an empirical expression (eq 6) given by Thomas@for the relative viscosity qr of hard-sphere suspensions qr = 1

+ 2.5@,, + 10.05(DCf~+ 0.00273 e ~ p ( 1 6 . 6 @ , ~ (6) ~)

where @,ais the effective volume fraction. The data were fitted with this expression and the results are shown as solid lines in Figure 5. Good agreement with theexperimentaldata is observed and one obtains a value for the ratio of the effective volume fraction to that of the 'dry" aggregate O,rr/Oe (or equivalently a ratio between the hydrodynamic radius Rh and the particle radius Ro) as fit parameter, which is given in Table 3 for the corresponding systems. For the case of the TDS substituted system a hydrodynamic radius can be deduced that is about 5% larger than that for the other two systems, Le., for the anionically substituted microemulsions there really are larger steric repulsive forces (of hard-spheretype) present. There is alsogood agreement between this value for Rh/Ro (= 1.170) and the ratio of R,/R,, (=1.168) as obtained from the light scattering data (Table 2). From this one may concludethat the hydrodynamicradius relevant to the viscosity and the effective hard-sphere interaction radius of the static light scattering experiment are closely related and practically identical for these microemulsion droplets. However, this larger interaction radius would by far not be sufficient to account completely for the larger repulsive forces that must be present in the TDS containing microemulsions. If it is included in the analysis of the scattering curves, the deviations toward lower scattering intensities cannot be fully accounted for and some additional repulsion has to be present. Within the RPA model this results in a too large estimate of the charges on the aggregates. 3.3. Microemulsions Based on ODMAO. Similar results as for the TDMAO based microemulsions could also be obtained

Gradzielski and Hoffmann

2618 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994

(

/

0,0004.~ 0.000

0 000

Figure 6. Rayleigh factor Re at 25 OC for microemulsions based on ODMAO (x(surfactant)/x(decane) = 10:9): ( 0 ) pure ODMAO (0) 1.25 mol 96 TTABr; (0)2.5 mol 96 TTABr; (X) 2.5 mol 96 TDS (fitted curves as solid lines). TABLE 4: Fit Parameters R,R, and Charge z(RPA) for Various Mixtures of ODMAO and lTABr or TDS and Charge q,As Calculated from the Composition for an Assumed Radius of 50.2 A. pure x(TTABr)/x, x(TTABr)/x, x(TDS)/x, system ODMAO = 0.0125 = 0.025 = 0.025 50.2 50.8 49.3 51.0 R (A) 53.4 53.8 52.9 56.4 R (A) r(RPA)/eo 7.60 13.5 16.0 role0 a

-

7.48

15.0

0 II

-04-0

-

15.0

Corresponds to a surfactant aggregation number of 598.

for systems based on the longer chain amine oxide ODMAO. For the ODMAO a higher solubilization capacity for hydrocarbon has been reported.'6 Because of this and also because of the fact that the ODMAO has a significantly longer alkyl chain than the TDMAO, much larger microemulsion droplets are formed with the ODMAO. For the concentration-dependent investigations a molar ratio of 10:9 for 0DMAO:decane was chosen. Again surfactant mixtures with different ion content of TTABr (1.25 and 2.5 mol 3' %) or TDS (2.5 mol %) were employed and their static light scattering properties are depicted in Figure 6. The behavior is very similar to that encountered for the TDMAObased systems before. Again for the TDS-substituted systems a significantly lower Rayleigh factor is observed in comparison to the positively charged microemulsions. As before, the intensity profile for the TTABr-containing systems can well be explained within the RPA model (eqs 4 and 5) whereas this is possible only to a less satisfactory extent for the TDS system. As for the TDMAO-based microemulsionsin the caseof TTABr substitution all three parameters (R, R,, and z(RPA) were varied freely, whereas for the TDS-containing system it was again necessary to keep R fixed (at the value of 51 .O A as it was obtained from staticlight scattering on a diluted sample, Le., 19.5mM ODMAO/ 0.5mM TDS/ 18 mM decane, in 50 mM KCl solution) and only the other two parameters were varied. The obtained fit curves for this model are plotted as solid lines in Figure 6, and the corresponding fit parameters are given in Table 4. At this point it is worth mentioning that the obtained radius for the ODMAO microemulsion of 50.2 A is in excellent agreement with a radius of 49.9 A that has been found previously for an identically composed system of total concentration of 1 wt % (surfactant + decane) in D2O by means of a SANS measurement.16 The lower scattering intensity for the anionically substituted microemulsions (for both the ODMAO and the TDMAO systems) has to have its origin in a larger repulsivity between the aggregates because their size is only little influenced by the admixture of the ionic surfactant as was verified by static light scattering experiments (compare Figures 4 and 12). Now here we can only speculate how this additional repulsion in the case of the anionic

IO ' Figure 7. Schematic drawing of the interface surfactant/water for the case of cationic, TTABr, (a) and anionic, TDS, (b) substitution. surfactant as compared to the cationic surfactant comes about. A conceivable possibility would be that the more hydrophilic head group of the anionic surfactant protrudes further into the surrounding aqueous solution as is sketched in Figure 7. This would locate the charges further outside the microemulsion droplets and should lead to an increased electrostatic interaction between them, because now the charges of different aggregates have to come intocloser contact, while thecenter-of-mass distance remains unchanged. Such an explanationwould also be supported by the fact that a higher degree of surface roughness has been observed for SDS micelles in comparison to alkyltrimethylammonium micelles,41 which means that for the pure SDS micelles the head groups protrude further into the aqueous surroundings. 3.4. Microemulsions Based on Brij 96. In order to verify the assumption that this additional repulsion is not due to a specific interaction between TDS and the zwitterionic surfactant we investigated O/W microemulsions with a different nonionic surfactant, the Brij 96, a decaethylene oxide-oleyl ether. For the Brij 96 the interfacial tension of the mixtures with TTABr or TDS shows in both cases a very similar monotonous increase with increasing ionic content (Figure 8). No synergistic effect, as in the case of the zwitterionic surfactant, is observed here, but a conventional mixing behavior seems to be the case. The y value for the pure Brij 96 system is also in good agreement with interfacial tension data for similar systems, like for instance (212EOs/dodecane (y = 3.1 mN/m).42 Again, concentration series at constant molar surfactant to decane ratio were investigated by means of light scattering in order to study the structural ordering effects caused by the electrostatic repulsion. The chosen surfactant to decane molar ratio was 1:l. For these experiments, microemulsions were prepared that contained 1.25 and 5 mol % (with respect to the total surfactant content) of TTABr or TDS, respectively. As is to be expected, the scattering intensity of the charged micro-

Structure of O/W Microemulsions

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2619 1

, Rg~(ionic) ~

e

Re (nonionic)

.nmO

0 0 .

n~

1.o

,"e

0

4

0.24 x(ion)

0.01 I , 00 0.1 0.2 0.3 0.4 0.5 Figure& Interfacial tension y from the spinningdrop method for various mixtures of Brij 96 with TTABr (0)and TDS ( 0 )measured against decane at 25 OC. The total concentration of surfactant was always 50 7

3

I

I

>

,

,

r

,

,

7

r

,

l

,

,

,

l

,

*

mM. Rein l / c m ,n..---a*

00010~

Figure 9. Rayleigh factor Re at 25 O C of the static light scattering for microemulsions of the system Brij 96/decane with various degrees of

ionic surfactant substitution as a function of the total concentration (surfactant + decane). The molar ratio of surfactant to decane was always kept constant at 1:l. Degree of substitution: (0)pure Brij 96; (+) 1.25 mol%TTABr;(X)1.25mol%TDS;(O)5mol%TTABr;(O) 5 mol 9% TDS.

i 0

4 0 tl

0.0

3 10

100

c in g/l

Figure 11. Ratio betwan the scattering intensities of charged microemulsions in comparison to thoseof the correspondingunchargedsystem. Thecontentofionicsurfactantwasalways1.25mol%(oftotalsurfactant). The molar ratio of surfactant:decane was 3.21 (for the TDMAO) and 1:l (forBrij96). (O)TTABr(withTDMAO);(O)TDS(withTDMAO); (m) TTABr (with Brij 96); ( 0 )TDS (with Brij 96).

in the order nonionic < cationic < anionic system. This might be explained by the fact that the ionic head groups will have a tendency to be surrounded by their hydrated counterions. This will create a kind of osmotic pressure that leads to a swelling of the decaethylene oxide shell and thereby to a small increase of the hydrodynamic radius of the aggregates. However, for the anionic substitution this effect is much less pronounced than in the case of the TDMAO-based systems and one may conclude that the corresponding microemulsion droplets possess about identical effective hard-sphere radii. Acomparison ofthe relative reduction ofthescattering intensity which is brought about by the charging process shows that it is similar for the TDMAO and for the Brij 96 case and therefore only little dependent on the effect of the larger head group hydration in the case of the TDS-substituted TDMAO microemulsions. In order to illustrate this point in Figure 11 the ratio of the scattering intensity of a microemulsion with 1.25 mol 96 ionic substitution to that of the corresponding unsubstituted microemulsion is given both for the TDMAO and for the Brij 96 system. For the intermediate concentration regime (10-1 50 g/L) it is observed that the reduction is somewhat larger in the case of the Brij 96-based microemulsions. However, at the same time the relative differencebetween the cationicallyand the anionically substituted microemulsions is in both cast8 very similar (Re(anionic)/Re(cationic) 0.8) and only slightly smaller for the Brij 96 cast. All this leads to the conclusion that the lower scattering intensity in the case of the anionic surfactant should first be specific for it and secondly be caused mainly by an increased effective electrostatic repulsion.

-

8 @

1 ' " ' 1 " ' " " " ' 1 ' ' ' ' 1 *

0.00 0.05 0.10 0.15 0.20 0.25 0.30 Figure 10. Relative viscosity vt as a function of the volume fraction 0 of surfactant plus hydrocarbon (25 O C ) , The molar ratio of surfactant to decane was 1:l for all microemulsions. ( 0 )Pure Brij 96; (0)5 mol 5% TTABr; ( 0 )5 mol 5% TDS.

emulsions is sharply reduced in comparison to the uncharged systems (Figure 9). However, as in the case of the amine oxide the intensity for the TDS substituted systems is always lower than that of the cationic system although to a slightly smaller extent. For these microemulsions the effect of increased head group hydration of the anionic surfactant should not result in a significant increase of the effective interaction radius because here the largedecaethylene oxide head group of the Brij 96 should determine the steric repulsion of the droplets. This assumption was corroborated by viscosity measurements (Figure 10) that showed less significant differences in the relative viscosities of the variously substituted Brij 96 microemulsions than for the TDMAO microemulsions. The viscositiesare slightly increasing

4. Dynamic Light Scattering

In this part of the paper, some results from dynamic light scattering experiments on these charged microemulsion droplets shall be described. For these studies the ODMAO system was chosen because of its higher scattering intensity that allowed for more precise measurements. In order to study the effect of the ionic substitution on the dynamical properties of these microemulsions, a series was employed that always contained 50 mM of surfactant and 45 mM of decane. The static light scattering intensity both for the aqueous system and in 50 mM KCI solution (in order to shield the electrostatic repulsion) is given in Figure 12. Here the Rayleigh ratio is plotted for the case of cationic and anionic substitution as a function of the ionic content. From the measurements in 50 mM KC1 solution it can be concluded that the size of the aggregates remains almost constant and is nearly independent of the content of ionic surfactant. Again, a slight increase of the size is observed upon addition of the anionc TDS. In the aqueous solutions the scattering intensity is

2620 The Journal of Physical Chemistry:Vol. 98, No. 10, 1994

Gradzielski and Hoffmann

-.

I

4 3

.n'

1 ---/.--a,'TD S 2/ , , , , , , 0 -3 -2

,; ', 1 ,

-1

,

0

--a ,

TTABr

! TDS

-.;.u ~ - , ,~

,

1

0

2

Cion in mM

"

'

l

-2

"

'

l

"

-1

'

~

0

,

(0).

-010

40 60 80 T in ps Figure 13. Logarithmic plot for the autocorrelation functions G(7) of microemulsions with various content of cationic surfactant TTABr at 25 OC. All microemulsions contained 50 mM surfactant (ODMAO + TTABr) and 45 mM decane. (0) 0 mM TTABr; (0)0.25 mM TTABr; (0)0.5 mM TTABr; ( 7 ) 1.0 mM TTABr; (A) 1.5 mM TTABr; (+) 2.0 mM TTABr.

~

1 c l O n in

Figure 12. Rayleigh factor Re for microemulsions composed of 50 mM surfactant/45 mM decane as a function of the degree of ionic substitution of the surfactant. The microemulsions are based on ODMAO where a certain amount qm was substituted by an ionic surfactant (TTABr or TDS). In the diagram the positive sign at the abscissa is valid for the cationic TTABr and the negative sign for the anionic TDS. The and in 50 mM KCIsolution measurements were obtained in pure water (0)

0

~

-3

I

,

2

mM

Figure 14. Effective collective diffusion coefficient Dem (0) and hydrodynamicfactor H(0) ( 0 )as a function of the ionic content c h for the same systems as in Figure 12, measured by dynamic light scattering in pure water (A = 488 nm; 8 = 90°; T = 25 "C). the microemulsions (Figure 14). For a molar content of 4% a rise by a factor of -4 is found. A cumulant analysis, Le., a fit of a quadratic or a cubic polynomial to the logarithm of G(T), showed only very minor systematic deviationsfrom linearity. The second cumulant K2 = (qZAD)* which can be regarded as a measure for the polydispersity of the respective aggregatesu was always in the range A D / D < 0.1 (a more precise determination was not feasible in a meaningful way, as it has been shown that a determination of values smaller than 0.15 from experimental data is not sensibly possible45). This is a further indication of the small polydispersity of the microemulsion droplets as has already been deduced from the SANS experiments mentioned above.16 It is well established that the effective diffusion coefficient in general is influenced both by the static structure factor S(q),as well as by a hydrodynamic factor H(q)*947 (eq 8, where DOis free

20

significantly reduced with increasing ionic content as a result of the electrostatic repulsion (similar to what has been observed in the static light scattering experiments before). In this intensity curve a slight asymmetry between anionic and cationicsubstitution is to be noticed. This is due to the fact that the alkylamine oxide is not a perfectly nonionicsurfactant but possesses a weak basicity (eq 12). This autoprotonation leads to a small positive charge on the aggregates which then will become neutralized by the addition of a small amount of anionic surfactant. From a comparison of the data of the electrolyte-free and the saltcontaining solutions, one can arrive at the additional factor that is contained in the structure factor because of the electrostatic repulsion (if one assumes only steric hard sphere interactions to be of relevance in the 50 mM KCl solutions). In the dynamic light scattering experiments, fairly simple, monoexponential autocorrelation functions were recorded for all the samples investigated (Figure 13). They decayed the faster the higher the ioniccontent. From the initial slope of the intensity autocorrelation function G(T)the effective diffusion coefficient D,rr was calculated according to eq 7.43 1 dlnG(7) De, = -lim 2 q z ~ dT

(7)

Accordingly, Doff becomes larger with increasing ionic content of

particle diffusion coefficient). For the case in question here, the size of the aggregates is much smaller than the wavelength of the light used in the experiments. Because of this one may to a good approximation use the low q - limit expressions for the corresponding factors, Le., S(0) and H ( 0 ) . However, the static structure factor may be determined from the static light scattering measurements. If one assumes the electrostatic repulsion to be completely shielded by the presence of the 50 mM electrolyte (Debye length at 25 OC; K = 13.5 A) only steric, i.e., hard sphere, repulsion should remain present in the systems. In that case the experimental structure factor can be calculated from the ratio between the scattering intensities without (Re) and with added electrolyte (Re,m). Furthermore, one has to take into considerationthe hard-sphere structure factor S~s(0)for the given concentration (eq 9 ) , which for instance can be done by way of the CarnahanStarling expression26 (volume fraction = 0.03 1 77 -Sw(O) = 0.777). Finally, the free particle diffusion coefficient Do can be estimated from a measurement of the pure ODMAO/decane system in 50 mM KCl solution. If one takes the experimental value of 4.77 X cm2/s, again assumes pure hard-sphere interaction and accounts for the concentration dependence with an expression obtained theoretically for the effective collective diffusion coefficient of such systems (eq lO),4* one arrives at a value of 4.56 X le7 cm2/s for DO.This value corresponds to a hydrodynamic radius Rh of 52.1 A via the Stokes-Einstein relation (eq 11) which is in good agreement with theresults of thestaticlight scattering experiments because it should include the hydration shell of the aggregate and therefore correspond to radius R8 in Table 4.

~

~

Structure of O/W Microemulsions

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2621

(9)

4

D in 10-’cm2/s

‘

,

where Re is the Rayleigh ratio of the sample in pure water and Re,Hs is the Rayleigh ratio of the sample in 50 mM KCl. De, = Do(l

+ 1.454@)

(10)

2:

where r) is viscosity of the solvent. With this information and together with the experimental structure factor Sexp(0), one is now able to calculate the hydrodynamicfactor fromeq 8 and the results areshown together with the values for the effective diffusion coefficients in Figure 14 as a function of the ionic substitution of the microemulsions. It is found that H(0) is always smaller than unity and decreases moderately with increasing ionic content of the microemulsions to about half the value of the pure amine oxide system for the highest ionic content. Similar values for H(0) of 0.3-0.7 were also obtained by means of dynamic light scattering experiments on charged silica particles in an ethanol/toluene solvent mixture.37 The hydrodynamic effect of the charging process on the microemulsions would therefore tend to slow down the collective diffusion. However, the decrease of H(0) is much smaller than that of the static structure factor and is not able to counterbalance its effect on the diffusion coefficient. Therefore, the increase of the diffusion coefficient Defris entirely due to the static structure factor and only little influenced by the hydrodynamic effects. The effect of the charging process is much more significant for their structural ordering than for their hydrodynamic interactions. Exactly such a behavior has also been predicted theoretically49 for charged spheres and these calculations show also only a weak dependence of H(0) on the charge they carry. Finally,dynamic light scattering experimentswere also carried out in order to investigate the concentration dependence of the effective collective diffusion coefficient of the charged microemulsions in comparison to their uncharged counterparts. Dcrr was obtained as described before and is plotted in Figure 15 for a pure ODMAO/decane microemulsion and one that contained 1.25 mol % (of total surfactant) TTABr as a function of the volume fraction (obtained from the concentration considering an aggregate density of 0.824 g/mL, as derived from density measurements). A clearly different behavior is observed in both cases, Den is always larger (for higher concentrations, -2025%) in thecaseof thecharged microemulsion. However, whereas for the pure ODMAO system a small monotonous increase of Den is observed, a pronounced maximum occurs for the charged microemulsion at a volume fraction of ~ 3 % Such . a behavior is in good correspondence with theoretical calculations for the diffusion properties of charged colloids that predict just such a maximum around this volume fraction49 under comparable conditions. Similar results have also been reported for charged micellar systems. Here aqueous solutions of decyl-, dodecyl-, and tetradecyltrimethylammonium bromide were investigated by means of dynamic light scattering and in all cases a maximum of the collective diffusion coefficient around a volume fraction of 0.03-0.05 was observed.50~~1 However, it should be noted that the charge density of these micelles was of course much higher than for the microemulsions studied in this investigation because here only a small percentage of the surfactant present was of ionic type. This might also explain why the relative extent of this maximum was more pronounced in the case of the micelles, i.e., a factor of -3-5 between maximum and minimum of Den in comparison to a factor of -2 for the microemulsion studied here (Figure IS). The Occurrence of this maximum for the collective diffusion coefficient can be explained by the fact that the electrostatic

Q O

,

,

,

I

,

~

/

~

,

,

I

I

I

,

,

I

I

II I,

,

I

I

~I

I

I

I

I

I

This factor of 0.73 deviates markedly from the theoretically predictedvalue of 1.454. An explanation for thisdeviation might be that there is some attractive interaction of the van der Waals type present between the microemulsion droplets. Such an attraction is known to reduce the concentration-dependent factor of D,fl significantly and the existence of such van der Waals interactions in micellar systems has been deduced from dynamic light scattering experiments.52 CH3

I

Cl4Hm-N-0

I

CH3

CH3

I

+ H20 a C14Ha-N-O*-H I

+ OH-

(12)

CH3 ff2C Kb= -

1-a

For very low concentrations, deviations of the observed effective diffusion coefficients towards larger values than the linear relationship may be noted in Figure 15. This increase of Denwith decreasing total goncentration is presumably due to the fact that the zwitterionic amine oxide is not absolutely nonionic but as a weak base (for comparison, pK. of Cl2DMAO 4.953) can become charged by protonation (eq 12). Even if only a very small percentage of less than 0.1% of the molecules becomes charged (as is to be expected from the pKa value), this effect can be of significance because at the same time the Debye screening length is very large. This effect is only observable at low concentrations as has been found by static light scattering M

2622 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994

Gradzielski and Hoffmann

amine oxide by a cationic or an anionic surfactant. In the cationic case, the interactions are well described by a simple random phase approximation for the structure factor that takes into account 2 the electrostatic repulsion by means of a screened Coulomb potential. The particle charges obtained from this model are in 1 good agreement with those expected from the particle size and the composition of the surfactant mixture. Only for a relatively 0.5 high content of ionic surfactant of more than 10 mol % the effect of counterion condensation starts to be discernible. Therefore, the use of such surfactant mixtures is a convenient way to adjust the charge density (or for that matter the surface potential) of 0.1 these colloidal aggregates in an easily controllable fashion to any I desired value without otherwisedisturbing the respective system. 0.05 However, the RPA model does not describe the anionic microI " , ' I emulsions equally well. Analogous results have been obtained 1 5 10 50 both for microemulsions based on TDMAO and ODMAO. c in g/l The different behavior in the anionic case is due to a number Figure 16. Rayleigh factor Re of the system TDMAO/decane (molar of reasons. First, a slight growth of the droplets occurs that is ratio 3.2:l) as a function of the total concentration in the pure water ( 0 ) in agreement with a synergistic effect which is observed in the and 50 mM KCl solution (0)at 25 O C . The solid line corresponds to interfacial tensions for the respective surfactant mixtures. In the hard sphere fit (compare Table 1). addition,a larger interactiondiameter of the aggregatesis observed as was verified by viscosity measurements. Presumably this is experiments on TDMAO/decane microemulsions. In Figure 16 due to a more extended hydration shell, which is caused by strong systematically increasing deviations toward smaller scattering hydrogen bonding of the water molecules at the head group of intensities are to be observed with decreasing concentration the anionic surfactant. Finally, a more effective electrostatic whereas the ideal hard-sphere scattering behavior (solid line in repulsion has to be present in the anionic microemulsions, as has Figure 16) was regained for identical measurements in 50 mM been evidencedby identical experiments on similar systems based KCl. From this it can be concluded that the effect responsible on another nonionic surfactant (Brij 96). Here again a different for the lower scattering intensities has to be of electrostatic origin. scattering behavior is observed for the case of cationic or anionic However, this effect is only of importance for concentrations of surfactant substitution. However, in the Brij 96 system no less than 1 wt % because above this concentration no difference synergistic effect between anionic and nonionic surfactant is of the Rayleigh factor can be seen for the aqueous and the 50 mM present and the effective hard-sphere radius of the aggregates KC1 solutions. The decreasing importance of the electrostatic should mainly be determined by the large oligoethylene oxide effect is to be expected because the Debye screening length K head group. Therefore, the difference in the scattering properties should be proportional to c-II2 (compare eq 4b) whereas the here should be entirely due to a larger effective electrostatic interparticledistancedshould be proportional t o d 3 . Therefore, interaction for anionic substitution. This increased interaction K (which corresponds to the distance over which the electrostatic might be caused by the fact that in the anionic case the charged force is effective) should diminish faster than d with increasing head groups protrude further into the surrounding aqueous total concentration which explains why the electrostatic effect solution,thereby increasingthe mutual repulsion of the aggregates. becomes less and less important. Moreover, for the amine oxide The effective collective diffusion coefficient (as obtained from the fraction a of the protonated species (compare eq 12) should dynamic light scattering experiments) for the charged microbe given by eq 13a (Ostwald's dilution laws4)and with the amine emulsions increases with rising ionic content. This is mainly due oxide being only a weak electrolyte it should approximately be to the decreasingstructure factor whereasthe hydrodynamicfactor proportional to c-lI2 (eq 13b). This means that also the charge on the microemulsion droplets should be proportional to cl/* in these systems is shown to be only little influenced by the charges on the aggregates. Only a small decrease of the hydrodynamic and, of course, this effect will also lead to a decreasing importance factor with rising charge is seen. With increasing concentration of electrostatic interactions in these systems with increasing a maximum of the effective collective diffusion coefficient is concentration. observed at low volumefraction (-0.03). However,thediffusion is always somewhat faster than for the corresponding uncharged 5. Conclusions microemulsion. For higher concentrations the effect of charging O/W microemulsions based on the zwitterionic surfactant the microemulsion droplets on the structure factor and the alkyldimethylamineoxide were studied with respect to the effect hydrodynamic factor becomes very similar which leads to the that a charging process has on the structure and dynamics of the result that the collective diffusion coefficient is only little inrespective microemulsion droplets. For the pure amine oxide/ fluenced by the charge on the aggregates. decane system nearly uncharged aggregates are present. Only for very diluted samples the charging process caused by the References and Notes autoprotonation of the amine oxide surfactant is of significance (1) Prince, L. M.Microemulsions: Theory andPractfce;Academic: New to the scattering properties of the respective samples. Their size York, 1977. remains constant up to very large concentrations of -30 wt % (2) Hoar, T. P.; Schulman, J. H. Nature 1943, 152, 102. (3) Scriven, L. E. Nafure 1976, 263, 123. and their interactions can be described by a simple hard-sphere (4) Oetter, G.; Hoffmann, H. J. Dispersion Sci. Technol. 1989, 9,459. model. Moreover, only a comparatively low polydispersity of ( 5 ) Cebula, D. J.; Harding, L.; Ottewill, R. H.; Pusey, P. N. Colloid less than 10% has been found for the droplets. Polym. Sci. 1980, 258, 973. (6) Magid, L.J.;Triolo, R.; Jones, R. M.; Johnson, J.S., Jr. Chem. Phys. Upon substitution of the amine oxide by an ionic surfactant Lett. 1983, 96, 669. charged microemulsion droplets are obtained. Their charge (7) Hoffmann, H.; Oetter, G.; Schwandner, B. Progr. Colloid Polym. density is simply determined by the surfactant composition. The Sei. 1987, 73, 95. ( 8 ) Phnecker, G . Dissertation, Universiat Bayreuth, 1991. size of the droplets is only very little influenced by the admixture (9) Imae, T.; Ikeda, S . J. Colloid Interface Sci. 1984, 98, 363. of the ionic surfactant and again they are of constant size over (10) Nakagawa, T.; Shinoda, K . In Colloidal Surfactants; Shinoda, K., the whole existence region of the LI-phase. Interestingly, the Nakagawa, T., Tamamushi, B., Isemura, T., Eds.; Academic: New York, scattering properties differ for the two cases of substitution of the 1963; pp 129ff. I

1

'

.

Structure of O/W Microemulsions (1 1) Hoffmann, H.;Rauscher, A.; Gradzielski, M.;Schulz,S. F. Lungmuir 1992,8, 2140. (12) Oetter, G.; Hoffmann, H. Colloids Surf. 1989, 38, 225. (13) Siano, D. B. J . Colloid Interface Sci. 1983, 93, 1. (14) Gradzielski, M. Dissertation, Universitit Bayreuth, 1992. (1 5) Gradzielski, M.; Hoffmann, H.; Oetter, G. ColloidPolym. Sci. 1990, 268, 167. (16) Gradzielski, M.; Hoffmann, H. Ado. Colloid Interface Sci. 1992,42, 149. (17) Ricka, J.; Borkovec, M.; Hofmeier, U. J . Chem. Phys. 1991, 94, 8503. (18) Cayas, J. L.; Schechter, R. S.; Wade, W. H. In Adsorption at Interfaces; ACS Symposium Series 8; American Chemical Society: Washington, DC, 1975; p 234. (19) Hansen, J. P.; McDonald, J. R. TheoryofSimple Liquids;Academic Press: London, 1986. (20) Andersen, H. C.; Chandler, D. J. Chem. Phys. 1970, 53, 547. (21) Verwey, E. J. W.; Overbeek, J. T. G. Theory ofthe Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (22) Debye, P.; Hiickel, E. Z . Phys. 1923, 24, 185. (23) Stigter, D.; Hill, T. L. J . Phys. Chem. 1959, 63, 551. (24) Baba-Ahmed, L.; Benmouna, M.; Grimson, M. J. Phys. Chem. Liq. 1987, 16, 236. (25) Ornstein, L. S.;Zernike, F. Proc. R. Acad. Sci. (Amsterdam) 1914, 17,793. (26) Carnahan, N. F.; Starling, K. E. J . Chem. Phys. 1969, 51, 635. (27) Treiner, C.; Amar Khodja, A.; Fromon, M. J . Colloid Interface Sci. 1989, 128,416. (28) Bucci, S.; Fagotti, C.; Degiorgio, V.; Piazza, R. Lungmuir 1991, 7, 824. (29) Belloni, L. Chem. Phys. 1985, 99, 43. (30) von Smoluchowski, M. In Graetz, Handbuch der Electrizitiit und des Magnetismus; Barth: Leipzig, 1921; Vol. 2.

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2623 (31) Ware, B. R. Ado. Colloid Interface Sci. 1974,4, 1. (32) Hayter, J. B.; Penfold, J. Mol. Phys. 1981,42, 109. (33) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (34) Hayter, J. B.; Hayoun, M.; Zemb, T. ColloidPolym. Sci. 1984,262, 798. (35) Hayter, J. B.; Penfold, J. J . Chem. Soc. Faraday Trans. I 1981,77, 1851. (36) Jayasuriya, D. S.;Tcheurekdjian, N.; Wu,C. F.; Chen, S. H.; Thiyagarajan, P. J. Appl. Crystallogr. 1988, 21, 843. (37) Phillipse, A. P.; Vrij, A. J . Chem. Phys. 1988, 88, 6459. (38) Rosen, M.J.; Zhu, 8. Y. J. Colloid Interface Sci. 1984, 99, 427. (39) Murphy, D. S.;Rosen, M. J. J. Phys. Chem. 1988,492, 2870. (40) Thomas, D. G. J . ColloidSci. 1965, 20, 267. (41) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (42) Rosen, M. J.; Murphy, D. S . Lungmuir 1991, 7 , 2630. (43) Berne, B. J.; Pecora, R. Dynamic Light Scattering, John Wiley & Sons, Inc.: New York, 1976. (44) Provencher, S . W. Makromol. Chem. 1979, 180, 201. (45) Aragon, S.R.; Pecora, R. J. Chem. Phys. 1976, 64, 2395. (46) Pusey, P. N. J . Phys. 1975, A8, 1433. (47) Ackerson, B. J. J. Chem. Phys. 1978, 69, 684. (48) Cichocki, B.; Felderhoff, B. U. J . Chem. Phys. 1988.89, 1049. (49) Genz, U.; Klein, R. Physica 1991, A171, 26. (50) Walrand, S.;Belloni, L.; Drifford, M. J. Phvs. 1565. _ (Fr.) . 1986.47. , (51) Chatenay, D.; Urbach, W.; Messager, R.; Langevin, D. J. Chem. Phys. 1987, 86, 2343. (52) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85. 711. (53) Rathman, J. F.; Scheuing, D. R. ACS Symp. Ser. 1991, No. 447, 123. (54) Ostwald, W. Z . Phys. Chem. 1888, 2, 270.