Langmuir 1992,8,2140-2146
2140
Influence of Ionic Surfactants on the Viscoelastic Properties of Zwitterionic Surfactant Solutions H.Hoffmann,*ltA. Rauscher,? M. Gradzielski,fand S. F. Schulzt Lehrstuhl fiir Physikalische Chemie, Universitdt Bayreuth, Universithtstrasse 30, W-8580 Bayreuth, Germany, and Institute of Terrestrical Ecology, Swiss Federal Institute of Technology, Zurich, Grabenstrasse 3, 8952 Schlieren, Switzerland Received January 29,1992.I n Final Form: May 18,1992 The rheological and micellar properties of viscoelastic surfactant solutions of oleyldimethylamineoxide
(ODMAO)are studied as a function of excess ionic surfactants. The uncharged surfactant forme long
entangled rodlike micelles. The solutions are viscoelasticat small concentrations (19% weight) and behave as Maxwell fluids with a high zero shear viscosity qo which is determined by a single ahear modulus and a single structural relaxation time (TO = G?*T& The modulus shows the normal d n g behavior [GO= (C/C*)~.~] while the structural relaxation tune is concentration independent. The influence of both a cationic (tetradecyltrimethylammoniumbromide,C14(TMA)Br)and an anionicsurfactant (sodiumdodecyl sulfate, SDS)on Go, qo, and T~ are investigated. It is found that Go is little affected when the micelles are charged with the ionic surfactants while qo and T~ first increase and then decrease with increasing charge density. The rheological parameters are more sensitive to the cationic than to the anionic surfactant. At the maximum of 90, the structural relaxation times are longer than 100 8. It is concluded that these long times are not due to disentanglement processes as in polymer solutions, but have a different origin. The long times are probably the result of adhesive contacts between the long entangled rods. The adhesion energy could be hydrophobic in origin. In this model the structural relaxation time is determined by the adhesion energy of the contact. The charge density on the micelles controls the adhesion energy. At a critical charge density the bond energy disappears and the contact network is transformed into a system which consistsof stiff, overlapping rodlike micelles and which has a much lower viscosity. The conclusions are supported by results of electric birefringence and a few light scattering measurements. The electric birefringence data show a relaxation processin the millieecondtime region which is present for the conditions of the contact network and overlapping rods. With increasing ionic surfactant T~ passes over a maximum. 1. Introduction
In this paper the influence of charge on viscoelastic surfactants which consist of entangled giant rodlike micelles will be investigated.’T2 Most of the viscoelastic systems which have been studied until now in detail consisted of cationic surfactants in the presence of strongly binding counterions or in the presence of excess salt.3 In this work we investigate how excess charges on the network influence the properties of viscoelastic surfactants when no excess salt is added. We will start with a network of a zwitterionic surfactant and will replace increasing amounts of the zwitterionic surfactant with ionic surfactants, both cationic and anionic. For our studies we choose alkyldimethylamine oxides, since these compounds have been studied in detail and are known to aggregate into rodlike For high enough concentrations viscous solutions can be obtained. In particular the oleyldimethylamine oxide (ODMAO) is known to form viscoelastic solutions, which showthe recoil effect at moderate concentrations of less than 1 % .7 The introduction of charges will of course have an effect on the aggregation behavior of the systems, and it may no longer form entangled rods in the presence of the charges. This can be checked with various techniques, such as static and t
Universitilt Bayreuth.
* Swiss Federal Institute of Technology.
(1) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (2) Kem, F.; h a , R.; Candau, S.J. Langmuir 1991, 7,1344. (3) a) Shikata, T.; Hirata, H. Langmuir 1989,5,398. b) Imae, T. J. Phys. Chem. 1990,94, 5953. (4) Hoffmann, H.; Oetter, G.; Schwandner, B. h o g . Colloid Polym. Sci. 1987, 73, 95. (5) Imae, T.; Ikeda, S.Colloid Polym. Sci. 1984,262, 497. ( 6 ) Imae, T.; Ikeda, S.Colloid Polym. Sci. 1986,263, 756. (7) Hashimoto, K.; Imae, T. Langmuir 1991, 7, 1734.
dynamic light scattering, electric birefringence, and rheological measurements. We are aware of the problem which results from the basicity of the amine oxides. The oxygen of the surfactant has a weak basicity, and consequently a small fraction of the molecules carries a positive charge. The charge density resulting from this effect is nearly negligible in pure water. However, it becomes larger in the presence of anionic surfactants.8 2. Measurement Techniques 2.1. Sample Preparation. Tetradecyldimethylaminoxide (ClPMAO), hexadecyldimethylamine oxide (ClSDMAO), and oleyldimethylamine oxide (ODMAO)were solutionsof a content of about 25 % , received as a gift from Hoechst AG Gendorf, Germany. The solutions were freeze dried and for further purification recrystallized twice from acetone. The tetradecyltrimethylammonium bromide (C1dTMA)Br)was purchased by Merck and purified by recrystallizationfrom acetone with a small amount of ethanol. For mixtures of differentsurfactantsolutions at a constant total concentration, solutions of the single components were mixed at the desired composition and then heated. All solutions were left at rest for at least 24 h to come to equilibrium. In addition the samples for the light scattering experiments were centrifuged for 10 h at 3000g to remove dust
and small air bubbles.
2.2. Rheological Measurements. Rheological measurements have been carried out with a Rheometrica fluid rheometer RF7800. Themagnitudes of the complexviecosityIT*(, the storage modulus G’,and the loss modulus G” were measured in a frequency range between w = 1W2and w = 102 radls at 25 O C . Here G’ denotes the elastic response of the sample, and G” is the viscous part, which is dissipated during flow. To determinethe zero shear viscosity in the highly dilute concentrationregime, we used a oscillating capillary viscosimeter (PAAFt OCR-D). The structural relaxation time T, and the plateau storagemodulus Go (8) Rathman, J. F.; Christian, S. D. Langmuir 1990, 6, 391.
0743-7463/92/2408-2140$03.00/0 0 1992 American Chemical Society
Langmuir, Vol. 8,No. 9, 1992 2141
Properties of Zwitterionic Surfactant Solutions were determined from the frequency-dependent measurements by a non-least-squares fit of a Maxwell material according to eq 1.
x
o
l o 6 Ix ODMAO C,,DMAO C,,DMAO
*
(1)
2.3. Electric Birefringence Measurements. For the electric birefringence measurements we used the dc method with rectangular pulses with different pulse lengths and the pulsed ac method with a sinusoidalelectricfield. Details of both methods are summarized in refs 9-11. The mean relaxation times 7 were determined by a non-least-squares fit of the decay of the birefringence using a stretched exponential decay function according to eq 2. An(t) = Anst exp(-t/T)”
(2)
2.4. Dynamic and Static Light Scattering. Both static and dynamic light scattering data were carried out on the viscoelastic systems of ODMAO and ionic surfactants. Viscoelastic solutions are always in the semidilute concentration range in which both the scattering intensity and the diffusion constant do not reflect the properties of the individual particles, but are controlled by the ensemble and the mutual interaction. The measurements were taken with a Brookhaven goniometer and BI 8000 correlator at a wavelength of 632 nm and a temperature of 25 OC. Since highly viscoelastic solutions do not give the ensemble averages of the scattered light in small scattering volumes at data accumulation times smaller than the largest structural reIaxation time, static light scattering data were taken on rotating samples, and dynamic light scattering data were accumulated over 15-100 min and more at each scattering angle. Average scattering intensities and average apparent diffusion coefficients were extracted from the measurements at high scattering angles, which nevertheless represent “small scattering vectors” at this concentration region, compared to the intermicellar distances. To determine the apparent diffusion coefficient D,,, we calculated the field correlation functiongE(q,t) from the intensity correlation function according to eq 3. From the initial slope of (3) gE(q,t) we calculated the apparent diffusion coefficient D,, according to eq 4. In the static light scattering experiment we (4)
can describe the angle-dependent scattering intensity by eq 5, (5) where I,, denotes the scattering intensity of a monomer, cpi the number density of micelles of an aggregation number of NWg, F(q) the form factors of the micelles, S(q) the structure factor of the solution, and q the scattering vector: q = (4?mref/ho)sin (0/2) (6) Here nrefdenotes the refraction index of the solvent, the wavelength of the light source and 6 the scattering angle. The aggregation number can be expressed by the length L and the radius r of the micelles and by the head group area A of the monomer, Nagg= 2rrLIA. The overall monomer density rpo is given by eq 7. The form factor itself depends very strongly on
(9)Fredericq, E.;Houssier, C. Electric Dichroism and Electric Birefringence; Claredon Press: Oxford, 1973. (10)Schorr, W.; Hoffman, H. J. Phys. Chem. 1981,85,3160. (11)Angel, M.Ph.D. Dissertation, Universitat Bayreuth, 1985.
x
o o
c
io0
io*
101
103
c [mmol]
Figure 1. Zero shear viscosity qoas a function of the concentration for the different surfactants ODMAO, CuDMAO, and CleDMAO. the size and the shape of the micelles. For different shapes we get the following q dependence of the form factors:12 F(q) = 1
(very small)
F(q) = 1- (1/36)(qL)2 F(q) = r/qL
F(q) = 12/q2LI
(small rods)
(large rods) (large coils)
(8) (9) (10) (11)
In these equations 1 denotes the persistence length of the aggregates. For large micelles the effect of polydispersity is canceled by the form factor. For small q values first the coil-like structure dominates the scattering. With increasing q the form factor for rodlike segments takes over. Due to eqs 5,7,10, and 11, we therefore can estimate from a crossover of the q dependence of the scattering intensity at a scattering vector e: a persistence length 1, according to eq 12. 1, = 12/aqx
(12)
3. Rheological Results In Figure 1we show the zero shear viscosities of several alkyldimethylamine oxides (C,DMAO) as a function of the concentration. For the oleyl and the hexadecyl compounds the viscosities become rather high for moderately concentrated solutions of a few percent.’ This shows that the systems under these conditions are in the entanglement region. Detailed measurements on the C1DMAO have shown that this surfactant also forms rodlike micelles over an extended concentration r e g i ~ n . ~ The rods do not yet overlap, however, and as a consequence the viscosities are low up to about 100 mM. Generally it can be said that the rods begin to overlap when the viscosity becomes twice as high as the solvent viscosity. The rheological results in Figure 1reflect the general tendency of the aggregation behavior of surfactants to form larger and more stable rods with increasing chain length of the surfactants. The elastic properties of the more viscous solutions were determined from oscillating rheological measurements. Typicalresults for a 50 mM ODMAO solution are shown in Figure 2, where the magnitudes of the complex viscosity (lq*l), the storage modulus (G’), and the loss modulus (G”)are plotted against the frequency. The data can be fitted very well with a single Maxwell model. The zero shear viscosity 90 is thus (12)Berne, B. J.; Percora, R. Dynamic Light Scattering; V. Wiley: New York, 1976.
Hoffrnunnet ai.
2142 Langmuir, Vol. 8, No. 9, 1992
3 e
X CI,DMAO iOleyiDMAO
h
3
-*
E
X CPyCliNaSnl
10'
i
2
Y
h
3
v
-k
b
loo
I
h
3
v 0 10"
101
100
10'
1%
w (radhec)
Figure 2. Storage modulus G', loss modulus G", and the
magnitude of the complex viscoSity Iq*l as a function of the angular frequency o for a 50 mM solution of ODMAO at 25 O C .
Figure 3. Schematic drawing of the entanglement network in a viscoelastic surfactant solution with ita characteristic lengths k, 1, and m.
simply the product of the shear modulus and a structural relaxation time (70 = Go*7). The plateau value (Go) in Figure 2 is remarkably constant over a wide frequency region of several decades. Such results have previously been observed for ODMAO and for cationic systems in combination with strongly binding counterions.3J In temporary entanglement networks one would expect a wide spectrum of time constants for the disentanglement processes as in entangled polymer solution^.^^ Since this is not observed, it was concluded that the structural relaxation time is determined by a different mechanism, namely, the breaking and re-formation of the rods. Experimental evidence for such a mechanism has been observed, and the term kinetically controlled viscosities has been coined for such situation^.'^ On the basis of the experimental results and the theoretical treatments of viscoelastic surfactants, they are pictured to look like the sketch in Figure 3. The sketch contains three length scales which are marked k, I, and m. The length k is the mean distancebetween the rods. For charged systems thislength can be determined from the correlation peak of smallangle neutron scattering (SANS)measurements. It is approximentely given by k = r(d#)lf2where r is the radius of the rodlike micelles and # is the volume fraction. The length 1 is the average length between two entanglement contact points. A scale for this length can be calculated from the shear modulus assuming that the modulus is given by Go = ukT and v = ( l / l ) 3 . For moderately concentrated solutions one finds that I > k. The length m finally is the contour length between two knots, which (13) Cates, M.E. Macromolecules 1987,20, 2289. (14)Hoffiann, H.; U b l , M.;Rehage, H.Rogress and Trends in Rheology II; Supplement to Rheol. Acta 1988,26, 246-248.
1
I
I
IO'
10 2
103
c [mmol]
Figure 4. The shear modulus GOas function of the concentration for C&MAO, ODMAO,cet lpyridinium chloride with sodium salicylate,and C&9 + CI& (7:3). appears to be much larger than 1 if a simple interpenetrating network structure is assumed. It can be calculated from the totalrod length, the number density of the knots, and the functionality of the knots. If such a network is stretched, the resulting stress can be released by a disentanglement process or simply by a scission process of the arms.lS In this model the knots have a certain lifetime. Different microstructures for the knotsor network points are conceivable. They can con& of real entanglements in the visual sense, or they can perhaps consist of contacts between two rodlike micelles, which are held together by adhesion forces, which could be hydrophobic in origin. It is also conceivable that the contact point may coalesce, and this may lead to a real cross-linked network in which free roda with end cape no longer exist.l6 The arms between the knots may be coiled with a persistence length I,, which is determined by the inherent stiffness of the roda. For times which are shorter thanthe lifetime of the knots,the network can be compared to a real cross-linked network. The arms are attached to the contact points, but they can undergo translational and rotational diffusionproceeees. It is likely that their motion can be seen in electric birefringence measurements or in oscillating rheological measurements in the frequency range between 100 an loo0 Hz. All viscoelastic solutions which we have studied and which have a structural relaxation time in the long time range of many seconds show in addition a relaxation time (73) in the millisecond time r0gi0n.l~ Perhaps the process belonging to this motion can be looked at as a breathing of the network. In this model the viscoelastic network could change its mesh sizes on a time scale of a few milliseconds from the average nearest-neighbor distance to a distance which is determined by the separation of the knots to larger values. A particle with a dimension which is between k and 1 would then see a microviscosity which is given by G073,which is many orders of magnitude smaller than the macroscopic zero shear viscosity. The modulus in viscoelastic surfactant solution scales as for polymer solutions with a scaling factor of about 2.3. Resulta for the moduli of the alkylamine oxides are given in Figure 4. Figure 5 shows the longest structural relaxation time as a function of the concentration. For ODMAO the time constants are independent of the concentration. This behavior has never been observed (15)Cates, M. E.; Candau, J. J.Phys.: Condens. Matter lsoO,2,6869. (16)Privata communication by M. Cabs. We thank M. C a h for making a manuscript available to u8 before publication. (17) H o f f " , H.; Rehage,H.; Rauecher, A. Proceeding8 ofthe Nrtoschool, Nabsummer school, June 1991, Maratea,Italy; Chen, 8. H., Ed.
Langmuir, Vol. 8, No. 9,1992 2143
Properties of Zwitterionic Surfactant Solutions X
?
L
OleylDMAO
\
V
48:2 101
' ' '
1
IO'
1
103
102
*
IO
I
,
c [mmol]
,
, . ' ' ' , I
IO'
Figure 5. Structural relaxation time 71 from rheological experiments as a function of the concentration for Cl&MAO, ODMAO,and cetylpyridiniumchloride with sodium salicylate.
50.0
d%
Lu ,
'
".'"'I
,
'
'
' 1 ' 1
'
'
1
IO"
IO 2
'"1
'
"
'
'
'
1
10'
10
w (radisec)
Figure 7. The storage modulus G' as a function of the shear frequency w for various mixtures of ODMAO with SDS. The overall concentration was 50 mM; data were taken at 25 O C .
'lo W"s1 addition of C,,TMABr
addition of C,,TMABr
addition of SDS
c
IO'
IO"
I
IOIb
Figure 6. Zero shear viscosity 90 at 25 "C for different mixtures of ODMAO with C1dTMA)Bror SDS.The overall concentration of all samples was 50 mM. before on other surfactant systems. Such a behavior has however been observed on viscoelastic lecithin solutions in organic solvents in the presence of small amounts of water.'* The mechanism for this process is not clear. It is likely, however,that it is a kinetically controlled process. In order to study the influence of charges on the networks, we chose a 50 mM solution of the ODMAO and replaced increasing amounts of the zwitterionic surfactant by a cationic surfactant (tetradecyltrimethylammonium bromide, C1dTMA)Br) and an anionic surfactant (sodium dodecyl sulfate, SDS). The results for the zero shear viscosities are shown in Figure 6. For both ionic systems we find first an increase of the viscosity and then a decrease with increasing mole fraction. The effect of the SDS is however much stronger than for CdTMA)Br. The increase of the viscosity could be the consequence of an increase of rs,GO,or both parameters. As it is shown by dynamic measurements in Figure 7it is mainly the effect of rs while the modulus of the system remains constant. If the shear modulus represents the structure and the density of the network, it seems, therefore, that the structure of network points is not affected by excess charges on the network. This is a very remarkable and unexpected result. It could, of course, also be possible that the independence of GOon the charge density reflects a situation in which two effects are compensated by each other. For uncharged systems one would expect that the shear modulus changes linearlywith the osmotic modulus. When the network is charged, the osmotic modulus increases (18)a.) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. b.) Scartazzini, R.; Luisi, P. L. J.
Phys. Chem. 1988, 92,829.
b
'
1
1
b
2
+
4 amount at ionic surfactant ["A] '
'
'
'
k
3
'
loo
boi
Figure 8. Shear modulus GOand the structural relaxation time 7 for various mixtures of ODMAO with SDS or Cl,(TMA)Br (at 25 "C). The overall concentration of all samples was 50 mM. strongly with the charge density as will be seen in the section on light scattering experiments. In Figure 8 the moduli are shown for solutions of different mixing ratios for cationic and anionic surfactants. When the mole fraction of SDS is larger than 0.1, we notice that the zero shear viscosity breaks down. The transition occurs rather abruptly within a small mixing ratio. As will be shown by the electric birefringence results we still find rodlike micelles after the viscosity has dropped to about 100 mPa at x = 0.2. It then remains about constant to x = 0.3, and upon a further increase of the SDS content, it finally goes back to the solvent viscosity. For the cationic system the drop occurs at a smaller mixing ratio. The sharp drop in the viscosity seems to reflect the transition of the network structure to a situation in which individual charged rods are present. In the viscosity range of 100 mPa the rods seem still to overlap. It is interesting to note that for this mixing ratio the viscosity can practically be brought back to the level of the viscosity of the low charge density region by the addition of electrolyte. The modulus in these situations also has again a value which is close to the value of the uncharged systems. The results on the mixtures of ODMAO SDS reflect the general behavior of charged rodlike systems. The observed behavior is very similar to the behavior of the alkylpyridinium salicylates, where the shear modulus for a given surfactant concentration is independent of the counterion/surfactant ratio between 0.6 and 1.5.l At the smallest ratio the rods are still charged while they are neutral a t a ratio of around 1and actually carry a reverse charge at high ratios. The structural relaxation time is very much dependent on this ratio, while Go is constant. High viscosities are also found in mixtures of alkyldimethylphosphine oxides and ionic surfactants as it is shown in Figure 9. For this system the cationic and anionic sur-
+
Hoffmannet al.
2144 Langmuir, Vol. 8, No. 9, 1992
it
104
1
103
-
9 'Tu'
t
f
A
4
d
-
[msl
addition of SDS
\i
addition of C,4TMABr
k
l o C,,DMPO/C,,PyCI
1
1O'l0
10
20
30
40
--
50
amount of ionic surfactant [vol %]
Figure 9. Logarithmic plot of the magnitude of the complex
viscosity 11*I for mixtures of C&MPO with SDS or cetylpyridinium chloride. The overall concentration of the surfactants waa 1%.
factants have about the same effect on the viscosity. It should also be noted that alkylpoly(glyco1ethers) or alkyl glucosidesgive viscosity maxima when mixed with an ionic surfactant, and when the uncharged systemsform already rodlike micelles in the dilute solution^.^^ 4. Conclusions of the Rheological Data
The results show that the zero shear viscosity and the longest structural relaxation time pass over a maximum with increasing charge density, while the shear modulus and the short relaxation time in the millisecond region remain more or less constant. This result is difficult to rationalize. The introduction of the charge on the long rodlike micelles must affect their stiffness. Obviously this stiffening is reflected in the longest structural relaxation time but not in the number density of cross-links or the shear modulus. If network points were entanglement points, one would expect that stiffening of the rods would hinder the formation of entanglement points. But this is not the case. The relaxation time 73 as seen in the electric birefringence data is affected by the charge density and becomes almost 1 order of magnitude longer when the mole fraction is increased from 0.04 to 0.2. 5. Electric Birefringence
If an electric field is applied to the micellar solution, the resulting orientation gives rise to a birefringence signal, which decays with typical relaxation times, as soon as the field is turned off. Ionic surfactant solutions, in which rodlike micelles exist, give rise to at least four different relaxation processes with increasing surfactant concentration.20 In the dilute concentration range, where the rodlike micelles do not overlap, a single process can be detected which is due to the rotation of the rods (TI). At the crossover to the semidilute range a second process with opposite sign of the first one becomes visible. Its amplitude is larger than the amplitude of the first process, and consequently the sign of the birefringence signal turns also. Several explanations have been proposed for this process. The process can also be detected in polyelectrolytes.21 It is well established by now that during this process the charged micellesassume a partial alignment perpendicular (19)Tamori, K.; Esumi, K.; Meguro, K.; Hoffmann, H. J. Colloid Interface Sei. 1991,147, 33. (20) Hoffmann, H.; Krher, U.; Thurn, H. J. Phys. Chem. 1990,94, 2021. (21) Krher, U.; Hoffmann, H. Macromolecules 1990,24, 256.
to the applied electric field. This situation occurs only around the overlap concentration at low ionic strength. When the concentration is increased further, the 7 2 process disappears and a third process (73) appears in the millisecond time region which has again the same sign as the f i s t process but is several orders of magnitude slower. This process has been interpreted as a coupled translational rotational diffusion of the charged micelles in the semidilute range. It is superimposed by a fourth even slower process. This process is generally also seen in the rheological measurements. Both the 7 3 and the 7 4 processes seem to occur in all viscoelasticsurfactant solutionswhich have been studied. The time constants of the 73 process seem to be very insensitive to the chemistry of the systems. The process has been found for perfluorosurfactants,22 for double-chain cationic and for single-chain surfactants.= Thisprocess can also be detected in mixtures of the alkyldimethylamine oxides and ionic surfactants. Results for ODMAO and mixtures of it with &(TMA)Br and SDS are shown in Figure 10. Here we have given an average time constant for this relaxation process because in reality one observes a spectrum of such relaxation times centered around this value. In addition this time constant depends on the pulse length of the applied electric field. The shorter this pulse length the shorter the observed relaxation time. One fiids that upon addition of the ionic surfactant the time constant 73 first increases about an order of magnitude. After a certain ratio of ionic to nonionic surfactant is surpassed, a decrease of 7 3 begins. Again this decrease is observed earlier and more drastic in the case of the cationic surfactant. In Figure 11 we have plotted the amplitude of the static component of the birefringence in a dynamic electric birefringence for a 50 mM ODMAO solution as a function of the frequency. Ita amplitude monotonously declinines with increasing frequency. Therefore, it seems to be the case that this 73 process can only take place on a certain time scale described by this frequency. Corroborating this evidence is also the fact that in the transient electric birefringence experiments one does not observe full saturation of the birefringence signal even for pulse lengths of 10me;i.e., the processes that are necessary to take place for the 73 relaxation process require a time on the order of these 10 ms which corresponds to a frequency of 100 Hz in the dynamic experiment. As mentioned previously,it has been assumedthat the process is due to a hindered rotation of the rods. With this model it would be expected that the process should become slower (22) Angel, M.; Hoffmann,H.; Krher, U.; Thurn, H. Ber. BurwenGes. Phys. Chem. 1989,93, 184. (23) Krher, U. PbD. Dissertation, Universitet Bayreuth, 1990.
Properties of Zwitterionic Surfactant Solutions
Langmuir, Vol. 8, No. 9,1992 2146
la L
\
'O'
1
IO"
IO '
v
addition of SDS
2
addition of C,,TMABr
l
x
j0.40
- 0.35 2
- 0.30 .--I
- 0.25 ' Z - 0.20 3 C
-x C
70.15
I
,
20
,
1
,
I
1
I
,
I ,
6
8 IO2
2
. .
t
P
42 8
c
0
4
4 [nm-'I
[%I
Figure 11. Amplitude of the static component in a dynamic electric birefringence experimenton a 50 mM ODMAO solution aa a function of the frequency w (at 25 "C).
-1
50:O
I
,
0.00
IO IO amount of ionic surfactant [mmol]
Figure 12. Apparent diffusion coefficient D,, and reciprocal light scattering intensity I-' for various 50 mM mixtures of ODMAO and SDS or C14(TMA)Brat 25 "C.
2
%44*% 4
6
8
2
4
9 [nm-']
Figure 13. Double-logarithmic plot of the angular dependence of the light scattering intensity for various 50 mM mixtures of ODMAO with SDS (a) and with Cu(TMA)Br (b). q-', q-2,and qo(occurrence of small micelles) dependences are indicated by
the different dotted lines.
with increasing charge density. The charge density increases the effective volume of the rods and makes it more difficult for them to rotate. If we assume however that all rodlike micelles are part of the network structure, as seen in Figure 3, it is also conceivable that the process belongs to a diffusional process of the rods between the network points or the network points themselves. With increasing concentration, the density of the network points increases, the mean contour length between the network points decreases, and the time constants should become shorter. We do not have a proof for this mechanism of the 73 process, and there is no good theory available for the made assumption, but the model is a consistent description for the other experiments. If the 7 4 process in this picture becomes faster than 73, both processes can no longer be detected separately and an average rotation of the entangled rods can be detected. This may be tested very nicely by acceleration of the 74 process by the addition of a cosurfactant like hexanol to the solution. Under such conditions the 7 4 process can be shifted several orders of magnitude without any large effect on the 73 process.24 It is not very clear how the hexanol speeds up the 74 process. It is conceivable that some of the hexanol is already incorporated into the rods and the rods can break much faster. It is however also possible that the hexanol loosens the contacts and the network points can open and close much faster. There is also some evidence that the 73 process can be detected by oscillating rheological measurements.21 We have made oscillating flow birefringence measurements on some viscoelastic surfactants. These measurements revealed clearly a process in the (24) Hoffmann, H. Unpublished results.
millisecond time region. The time process was rather insensitive to the 7 4 time constant. 6. Light Scattering Data In Figure 12 the reciprocal average scattering intensity and the apparent diffusion constants are plotted against the mole fractions of the charges for both mixtures. The results show that the relative changes for both parameters are the same, which is expected, since both quantities are proportional to the osmotic modulus. Obviously they are controlled by the interaction between the particles. For the cationic mixtures both parameters change by a factor of 10 and reach their maximum values when about 10% of the zwitterionic surfactant is replaced by the cationic surfactant and do not show a parallel behavior any more. A comparison of the light scattering with the viscosity data indicates that the decrease in 111 and D occurs at about the same mole fraction of cationic surfactant, i.e., where the viscosity breaks down. The relation to the microscopic structure is probably an indication of a breakup of the network into individual rodlike micelles, which then become smaller with increasing mole fraction and reach their minimum size at the minimum. For even higher molar ratios the charge density on the then globular micelles increases and the interaction is controlled by the electrostatic repulsion and the screening of counterions. For a larger molar ratio D, therefore can be associated with individual globular micelles. For the mixtures with SDS the situation is similar; the transitions are however smeared over a larger mole fraction of SDS. The reason for this is that the incorporation of the negatively charged surfactants induces an uptake of protons on the amine oxide, which in turn reduces the charge density. For the
2146 Langmuir, Vol. 8, No. 9, 1992 same molar ratio of SDS,the charge density is much smaller than for CdTMA)Br and pairs of SDS-ODMAO are presumably formed. In dilute solutions it is customary to determine the persistence length of rodlike micelles from the angular dependence of the scattering i n t e n ~ i t y .Since ~ ~ ~ the ~~~ measurements were carried out in the semidilute range, the persistence length cannot straightforwardly be determined this way. We measured however the angular dependence of the scattering intensity and found an interesting result.26 There is a crossover from a q-2 to a q-l dependence for many solutions as shown in Figure 13. If we calculate a length scale from the q value of the crossover,we find a length in the range of around 4000 A. This should be compared to the mean distance of the network points from rheological measurements which is found to be in the region of lo00 A. For samples with high ionic surfactant content an angle-independentcomponent appears in the intensity data, which must be attributed to the occurrence of very small, probably spherical micelles. The apparent diffusion coefficientsrepresent data that belong to a strongly coupled motion of the network meshes. If the values are corrected by an average structure factor derived from static light scattering (details will be given in ref 26), diffusion coefficients of the short time motion of the meshes against the viscosity of water can be reproduced. If the results of the angle-dependent static light scattering are interpreted as a rod length, this would give a picture of interpenetrating stiff rod segments which touch and form bonds at distances much smaller than the rod length. On the first view it seems contradictingthat length scales larger than the mesh size should be detected in the (25) Porte, G.; Appell, J.; Poggi, J. J. Phys. Chem. 1980,84, 3106. (26) Detailed results will be published in a separate paper. (27) Claueen,T.M.;Vinson,P.K.;Minter, J.R.;Davis,H.T.;Talmon, Y.; Miller, W. G.J. Phys. Chem. 1992,96,474. (28) During the reviewing procedure of this paper, a paper on viscoelastic surfactant solution has been publishedn in which electron micrographs of entangled rode are shown. But even on the basis of these very clear electron micrographs, it is not unambiguously possible to differentiate between contacta and entanglements.
Hoffmann et al.
static light scattering experiment. But static light scattering measures a quantity different from those measured by dynamic light scatteringand rheologicalmeasurements. The latter measure the length scales of motion units and therefore give blob sizes and mesh sizes, whereas static light scattering probes the time-averaged structure of the micelles, which does not show features of knots that exist only during a small fraction of the total integration time. The detection of rodlike segments that are longer than the mesh size detected in other measurements, therefore, supporta the picture of temporal adhesive contact points between the micelles. 7. Conclusions Networks in viscoelastic surfactant systems can be formed from uncharged zwitterionic surfactants. These networks can be charged by replacement of some of the zwitterionic surfactant by ionic surfactants. The shear modulus remains constant in the mixed solutions, while the osmotic modulus increases with the charge density. Without excess salt the network breaks down if more than 10%of the zwitterionic surfactants are replaced by charged surfactants. For a higher mole fraction of charged surfactants the viscoelastic properties can be regained by addition of excess salt. For a given total surfactant concentration the shear modulus is remarkably independent of the conditions of the system. It does not depend on the charge density,on the chain length of the surfactant, or on excess salt. It is believed that the network points are not created by entanglement points but by adhesion bonds between rods,%which are in contact with each other. It is likely that the adhesion energy is controlled by hydrophobic bonding of the micellar interface. The longest structural relaxation time which controls the zero shear viscosity is determined by this bond energy. It is therefore very sensitive to charge density, excess salt, and in particular the chain length of the surfactant.
Acknowledgment. This work was sponsored by the DFG through the SFB 213 in Bayreuth. A.R. and S.S. thank the DFG for financial assistance.
