J. Phys. Chem. 1993,97, 12320-12324
12320
Viscoelastic Properties of Gel-Emulsions: Their Relationship with Structure and Equilibrium Properties Ramon Pons,. Pilar Erra, and ConcepciC Solans Departamento de Tensioactivos, CID/CSIC c/Jordi Girona, 18-26, 08034 Barcelona, Spain
Jean-Claude Ravey and Marie-Jose S t M Laboratoire de Physico-Chimie des Colloides, LESOC, CNRS, URA 406 UniversitC de Nancy I, BP 239, 54506 Vandoeuvre les Nancy, France Received: June 14, 1993; In Final Form: August 30, I993O
Water-in-oil (w/o) highly concentrated emulsions form in ternary water/nonionic surfactant/hydrocarbn systems whose rheological behavior has been investigated using oscillatory measurements. The effects of temperature, volume fraction, oil-to-surfactant ratio, and brine salinity on the rheological properties have been studied. The shear moduli (GO) and relaxation times (eo) have been determined using a Maxwell model for a viscoelastic liquid. The shear moduli are a function of 7 (interfacial tension), 4 (volume fraction), and R32 (Sauter mean droplet radius). The relaxation times are proportional to 7 (continuous-phase viscosity) and inversely proportional to Go and 1 - 4 (continuous-phase volume fraction). The elastic component is related to the structure of the emulsions and the viscous element to the continuous-phase viscosity.
Introduction High-internal-phase-ratio emulsions (HIPRE) or highly concentrated emulsions are of the utmost technological One of the most important properties is their rheological behavior. Much work on the rheological behavior of HIPREs has been undertaken by Princen and co-worker~."~In their investigations they mainly studied the static rheological behavior, namely, static shear modulus and yield stress. They showed that the static shear modulus for a concentrated oil-in-water emulsion can be represented by the following e q ~ a t i o n : ~
where 7 is the interfacial tension, Rj2 is the Sauter mean droplet radius, 4 is the dispersed volume fraction, a is a constant with an experimental value of 1.769, and b is another constant with an experimental value of 0.712. This last constant is thought to be related to the maximum volume fraction of close-packed undistorted spheres. The emulsions used in their studies were oil-in-water highly concentrated emulsions with paraffin oil as the hydrophobic component and solutions of technical grade nonionic surfactants in water as the continuous phase.s In the present work we have studied the viscoelastic behavior of water-in-oil (w/o) concentrated emulsions which form in ternary water/nonionic surfactant/hydrocarbon systems. The nonionic surfactant used is of the polyethylene glycol ether type. These concentrated emulsions have been termed gel-emulsions due to their appearance and stiffness; they can contain a very large amount of water (i.e. 99.5% w/w) and a very low content ofsurfactant (Le. 0.1%w/w).*-15 They onlyformat a temperature higher than the HLB temperature of the corresponding ternary system, and their stability exhibits a maximum as a function of t e m p e r a t ~ r e Oil-to-surfactant . ~ ~ ~ ~ ~ ~ ~ ratio and electrolyte content of the aqueouscomponent are important stability factors, too.9J'JJ4 From a structural point of view these emulsions are formed of water droplets surrounded by a water-in-oil microemulsion, the composition of which is close to that of the system in equilibrium with excess water?J5 Earlier results obtained using phase diagram determinations and electron microscopy showed the system to be
* To whom correspondence should be addressed. e
Abstract published in Advance ACS Abstracts, October 15, 1993.
a water-in-(water-in-oilmicroemulsion) emulsion.9,lI In previous work we confirmed this structure by means of S A X S (smallangle X-ray diffraction), and we were able to determine mean droplet size values for gel-emulsions. Similar results referring to gel-emulsion structure were found in similar emulsions formed using perfluorinated nonionic s ~ r f a c t a n t s . ' ~Earlier J~ observations of their flow behavior prompted us to study their viscoelastic properties more systematically.
Materials and Methods Materials. Homogeneous tetraethyleneglycolhexadecyl ether (abbreviated as C16E04) and triethylene glycol dodecyl ether (C12E03) were obtained from Nikko Chemical Co. (Japan) and used as received. Pure n-decane and Pro Analysis NaCl were from Merck and used as received. Water was double distilled. Methods. Gel-Emulsion Reparation. Increasing amounts of aqueous phase were added to a mixture of oil and surfactant with vigorous stirring by means of a vibromixer. For all the samples the conditions of preparation were kept as close as possible. Equal amounts of gel-emulsion were prepared for each batch (12 g) in Pyrex tubes 18 mm in diameter. The system was stirred for the same length of time, 5 min, once addition of water was completed. Interfacial Tension. A spinning drop (Texas University) instrument was used. The original instrument was modified for better temperature control (hO.5 OC in the range 25-60 "C). Aqueous phase and continuous phase were the external and internal media, respectively. Rheology. A Carrimed stress-controlledrheometer fitted with plane-cone geometry was used. The system is temperature controlled to fO.1 OC. The plate is made of stainless steel. Two different cones were used: a polycarbonate cone 6 cm in diameter and a stainless steel cone 4 cm in diameter. No differences were observed due to the use of one or the other material; most experiments used the bigger cone due to its higher sensitivity. The emulsions were stirred for 5 min at the corresponding temperature of measurement in their Pyrex tubes, and then with a spatula a sample was placed on the plane, taking care not to shear it excessively. The plate was slowly raised, and once in place, the system was allowed to relax for 5 min. The samples did not evolve significantly during the measurement; typically a measurement tookabout 1 h. On a few occasions
0022-3654/93/209~-12320$04.00/0 0 1993 American Chemical Society
Viscoelastic Properties of Gel-Emulsions
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12321
the readings were erratic; in these cases, when the plate was lowered, the emulsion was completely stuck to the cone and on the plane an oily film could be seen. In a normal experiment, when the cone and plate were separated, part of the emulsion remained with the cone and part with the plane, indicating that the adhesion to both surfaces was good. First the linear viscoelasticityregion was determined by varying the stress applied at a given oscillation (w). Then frequency sweep measurements were performed in this linear viscoelasticity region. The resulting spectra, G’(storage modulus) and G’’ (loss modulus), were fitted with those of a simple Maxwell viscoelastic liquid model. Some systems could be also adequately fitted with a single solid element, but the Maxwell model was used because extrapolated values of relaxation time are thought to be significative for values lower than 500 s. Least squares was simultaneously applied on both G’ and G”. When G‘and G“ were fitted separately, small differences were found; these differences were relatively large only for systems in which relaxation times are extrapolated values. Then, from G’(w) and G”(w), the shear moduli GOand relaxation times 00 were evaluated according to the following equations (see ref 18 for instance):
& *
T
‘.
x
100 -
\
’..
+
10 -
+ Q:Q.
rxp. (1)
- Q W
uk.(l)
Q:Q* rxp. ’ “Q’;Q’
1 0.005
\
(2)
ak.(2)
0.05
0.5
5
FREQUENCY/Hz
cr’= co(weo)/( 1+
(3)
The characteristic relaxation time of the system, 00, is related to the shear modulus, GO,and the viscosity of the element, 70,by 00 = oo/Go (4) The value of the dynamic shear modulus, as in eq 2 4 , is identified with the static shear modulus in eq 1. It could be argued that this identification implies the use of a dynamic variable in a static model. We think that this identification is allowed, taking into account that although in the development of Princen’s model it is assumed that the films are in mechanical equilibrium, our experimental results show that G’is independent of the frequency for measurement times lower than the characteristic relaxation time of the systems. For systems behaving as pure elastic solids over the whole range of frequencies tested, this means that no dynamic effects occur from to 5 s.
Results and Discussion For a concentrated emulsion Princen5 found, based on o/w highly concentrated emulsions, the relationship between Go, 7 , R32, and 4 (eq 1). We have investigated whether this model also applies to w/o highly concentrated systems. The pseudoternary H20(or brine) /C 16E04/C1OH22 and HzO/C I 2EO@ loHz2 systems were chosen because the HLB temperature of both systems is around 0 “C and they form gel-emulsions with good stability at room temperature. The variables we considered were water or brine volume fraction, temperature, oil-to-surfactant ratio, and salinity. The last three parameters influence the oil/water interfacial tension and thus, indirectly, the droplet size. The dispersed-phasevolume fraction affects the droplet size, probably by two mechanisms: increasing the volume fraction reduces the surfactant available to forin interfacial films and, on the other hand, increases the overall viscosity of the system, making it more difficult to break the films to form small In Figure 1 two sets of C’ and G” values as a function of frequencyare shown;they correspond to measurementsperformed on the same system. The symbols represent experimental values, while the curves represent the fit of eqs 2 and 3. The parameters corresponding to the Maxwell element are shown in Table I. Two conclusions can be drawn from these results: (a) a single liquid element is an adequate representation of the viscoelasticbehavior, and (b) Go and 00 have very different reproducibility and
Figure 1. Example of G‘and G” reproducibility and fit of eqs 2-3. The with R = 1 and 99% water content at system is H~O/CLZEO~/CIOHZ~ 40 OC.
TABLE I: Parameters CO and 80 of Qs 2-3 Obtained from Replicated Frequency Sweep Experiments on the System H2O/C&O$CioH22 with R = 1 and 99%Water Content at 40 “C GO(€’a) 216 8 218 h 8
*
60
(B)
0.3 4.3 h 0.3 2.1
uncertainty characteristics. While GOcan be fitted with estimated errors of the order of their reproducibility, which are always lower than 5%, both the uncertainty and reproducibility of 80 are rather poor. The low accuracy in the determination of 00 is counterbalanced by the sensitivity of this parameter to the control parameters. The behavior we observed is in contrast with some experimental results on oil-in-water emulsions.’J9 From a theoretical point of view, highly concentrated emulsions present a solid-like behavior with a defined yield stress (under the condition of stickiness of the droplets of the dispersed phase). Our findings, with w/o emulsions, seem to indicate that this condition is not always fulfilled and some slip is possible. Muence of Composition. Although the general trends of the rheological behavior of gel-emulsions are independent of its composition,somedifferences are found in the quantitative aspects of this behavior. In Table I1 we list values of parameters a and b of eq 1 for different gel-emulsion compositions and various temperatures. The values of the radius used in these calculations were obtained by SAXS or optical microscopy and are shown in Table III;15the values of interfacial tension are shown in Table IV. We used the values of the droplet radius obtained at 30 “C, which is the temperature at which the rheological measurement was done. This fact implies that the values of the parameter a are, probably, overestimated at lower temperatures and underestimated at higher temperatures. In addition, interfacial tension measurements could not be made at 20 “C, and the values used were those obtained at 25 OC; this implies an underestimation of parameter a. We do not obtain a unique value of the parameter a for the series of gel-emulsions measured. The trend of variation of this parameter is not clear, however, and no significativedifferences are found with temperature (some variation could be masked by
12322 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993
Pons et al.
TABLE II: Fitted Parameters of Eq 1 for Various Gel-Emulsion Compositions system' Rb T(OC) salc(%) ad bd 0.54 f 0.36 0.68 f 0.86 0 C16E04 1.5 30 0.62 f 0.55 0 0.44 f 0.23 C16E04 1.5 40 1.41 f 0.71 0.75 f 0.6 20 1.5 C16E04 1.5 0.73 f 0.67 1.5 1.26 f 0.72 Cl6E04 1.5 30 0.73 f 0.64 1.5 1.27 f 0.70 C16E04 1.5 40 0.53 f 1.1 5 1.03 h 1.1 C1.&04 1.5 20 0.69 f 0.27 5 1.50 f 0.36 C16E04 1.5 30 0.93 f 0.19 0.68 f 0.23 5 C&04 1.5 40 1.84 f 0.50 0.69 f 0.31 15 C&04 1.5 20 1.55 f 0.51 0.71 f 0.38 15 Cl6E04 1.5 30 0.70 f 0.32 15 1.61 f 0.45 C16E04 1.5 40 0.30 f 0.09 0.74 f 0.36 0 Cl2E03 1.5 40 Surfactant used. Oil-to-surfactant ratio. NaCl content of the aqueous phase in weight percent. Best fit of eq 1. TABLE III: Mean Radius, &2 (in pm), Used in the Calculations, Obtained by SAXS and Optical Microscopy" d~ 0% NaCl* 1.5% NaClb 5% NaCF 15% NaClb ~
50 -
~~
0.986 0.972 0.93 0.86 0.77 0.73 0.67
1.6 1.1 0.5 1.6 1.5
1.7 1.2 0.4 0.5
1.2 1.O 0.8 0.7
2.0 1.4 1.5 0.55
0.4
0.5
1.1
0 20
25
TABLE I V Values of Interfacial Tension (in mN d), Used in the Calculations" C16E04 C12E03 temp ("C) 0% NaCl 1.5% NaCl 5% NaCl 15% NaCl 0% NaCl
40
0.35 0.65 1.15
0.5 0.8 1.8
0.5 0.8 1.8
0.8 1.1 1.6
50
55
e IS I
+e oxp*rlmental '
20 30 40
45
Figure 2. Go as a function of temperature: experimental and calculated values (eq 1). The system is H z O / C I ~ E O ~ / C Iwith O HR~=~ 1.5 and 99% w/w water content. The lines are drawn as a guide for the eye. 10000
~
35
TEMPERATURE ("C)
1.36
'C I O H ~ ~ / Cweight ~ ~ E ratio O ~ was 1.5. SAXS at 30 OC. Optical microscopy at room temperature.
30
.+oq.
6
2.3
'Oil-to-surfactant ratios are 1.5 for C16E04 and 1.O for ClzE03. The oil is decane and the external phase water or brine. the use in the calculations of the same droplet size, regardless of the temperature of measurement). The intervals for a 95% confidence are rather large, i.e. about 20% of the value for the best case, but in general the value found by Princen, a = 1.769,* is included in the calculated confidence intervals. The systems H~O/C&O~/C~O and H ~H~~ O / C ~ ~ E O ~ / C with ~ Ono HZ ~ salt present give values clearly lower than that obtained by Princen.s Similar sudden changes upon the addition of NaCl are observed in other properties of gel-emulsions, for example, the apparent yield stress of gel-emulsions in a system with C I ~ E O ~ / C I ~ H ~ ~ , doubled from 0%NaCl to 1.5% NaCl, to remain almost constant up to 15% NaC1.14 Stability and transparency are also greatly increased by addition of small quantities of NaC1.I4 The values 0.1 we find for constant b are well in the range expected from Princen's 20 25 30 35 40 45 50 55 results. TEMPERATURE ('C) Temperature Effect. The effect of temperature on the values Figure 3. Relaxation time as a function of temperature: experimental of shear moduli Go is shown in Figure 2 for an oil-to-surfactant and calculated values (eq 6) (same system as in Figure 2 ) . The lines are ratio of 1.5 and a 99% w/w dispersed phase. A maximum is drawn as a guide for the eye. obtained as a function of temperature. In the same figure Go calculated from eq 1 using a = 1.34 and b = 0.712 is also an oil-to-surfactant ratio equal to 1.5 and a volume fraction of represented. The existence of a maximum is the consequence of 0.986. Considering gel-emulsions as a Maxwell liquid, the the sharp increase of interfacial tension at low temperature relaxation time is related to viscosity and shear modulus as shown (increase of Go) and the concomitant increase of the droplet size in eq 4. The value of the viscosity involved in this equation can at high temperature (decreaseof Go). The value of the parameter be related to the viscosity of the continuous phase (7) of the a we obtain to relate the experimental data ranges from 1.1 to gel-emulsion. Interpreting the Maxwell liquid model literally, 1.7 (for the points of Figure 2), close to the value obtained by we should consider that the system is formed by two components, Princen, 1.769.s an elastic solid and a liquid of viscosity 70. In this analysis the In Figure 3 relaxation times are represented as a function of elasticity is due to the structure of elastic films and the viscosity temperature for the ternary system H ~ O / C & O ~ / C I Owith H~~ component is due to the shearing of the continuous phase. The
'
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12323 300 ++e oxpor~monta~
250
-
.+ eq. 1
200 -
IS0-
100 -
50 -
0' I 1 I I I I 0.65 0.7 0.75 0.8 0.85 0.9 0.95
1
9 Figure 4. Go as a function of volume fraction: experimental and fitted values for eq 1. The system is H ~ O / C I ~ E O ~ with / C ~R ~=H1.5~ at~ 40 OC. The lines are drawn as a guide for the eye.
viscosity, which we can obtain from rheological measurements, is a property of the whole system. Considering that only a part of the system is responsible for the loss, we should divide the viscosity of the continuous phase by the fraction of the system that is responsible for the relaxation time; then
and therefore, using eqs 4 and 5,
60 = c9/(Go(l - 4))
(6) where c is an adjustable parameter, 9 is the viscosity of the continuous phase, and 4 is the dispersed-phase volume fraction. We should note that the precise mechanism of relaxation could be due to the slip of the droplets of the dispersed phase and subsequent shearing of the continuous phase. This mechanism should lead to a result similar to the one obtained in eq 6. The dependence on the continuous-phaseviscosity should be the same, and the dependence on the volume fraction would be similar to the one exposed. An increase in the volume fraction would lead to an increase of the area of close contact between neighbor droplets and thus to an increase of the apparent viscosity. In Figure 5, values according to eq 6 are represented as well, using experimental shear modulus and continuous-phase viscosities; qualitatively, the agreement is good. Quantitatively, we should consider the value of constant c for different systems. The values obtainedforthe systems westudied (H20/C&04/C&22 with varying oil-to-surfactant ratio (R) and a volume fraction of 0.986) are 2200 for R = 2.3, 530 for R = 1.5, and 19 000 for R = 1.0 (for this oil-to-surfactant ratio the values of relaxation time are extrapolated; therefore the error in their determination is much bigger than for the other oil-to-surfactant ratios). From these values we cannot conclude whether c is a constant with a great dispersion due to experimental error or includes the variation of parameters not taken into account here. Volume Fraction Effect. In Figure 4 the shear modulus Go is represented as a function of the dispersed-phase volume fraction for the system H Z O / C ~ ~ E O ~ / C Iwith O H Zan~ oil-to-surfactant , weight ratio of 1.5 and at 40 O C . Fitted values from eq 1 are also represented in Figure 4. The agreement is fairly good. In some
0.65
0.70 0.75
0.80
0.85 0.90 0.95 1.00
v o h " fraction
9
Figure 5. Relaxationtime as a functionof volume fraction: experimental and calculated values (eq 6) (same system as in Figure 4). The lines are drawn as a guide for the eye.
gel-emulsions a maximum of GOas a function of the volume fraction is found. The appearance of a maximum is consistent with the values of droplet radius found by SAXS for certain systems15since a strong increase in droplet radius is obtained for gel-emulsions of volume fractions higher than 0.95. Concerning the values of parameter b, Princen's equation is only applicable for volume fractions higher than those for close packing of spheres. The parameter b is found to be close to the maximum volume fraction of close-packed spheres, b 0.75. Some influence of dynamic effects is expected for volume fractions close to this value because, in low volume fraction emulsions, the elastic response is not due to the mechanism described by Princen. Concerning the variation of relaxation time with volume fraction, experimental values and those calculated from eq 6 are represented in Figure 5. The system is HzO/CI&O~/CIOH~Z with R = 1.5 measured at 20,30, and 40 OC. The values of the parameter c are respectively 290, 1100, and 850. The variation of the parameter c could be due to the experimental errors. Although the dispersion of these results does not allow confirmation of this interpretation, the qualitative behavior is that predicted (eq 6). Results of the rheology of oil-in-water highly concentrated emulsions agree with this interpretation:20in these emulsions, the continuous-phase viscosity exhibits a maximum as a function of temperature, which is reflected in a maximum of the relaxation time. Oil-to-Surfactant Ratio Effect. The effect of the oil-tosurfactant ratio on the shear modulus is, again, the effect of two opposite trends. On the one hand the oil-to-surfactant ratio produces a minimum in the droplet radius for R = 1.5 in the system H ~ O / C ~ ~ E O ~ / CBesides, ~ O H ~interfacial ~ . ~ ~ tension increases as the oil-to-surfactant ratio increases;I5 therefore, as expected, experiment shows a maximum in Go as a function of theoil-tcwurfactant ratio. The three oil-to-surfactant ratios tested were 1.0, 1.5, and 2.3 Go is higher for R = 1.5 than for the other R values. This trend is obtained at all the temperatures tested (20-50 "C). Concerning the relaxation times, Go goes through a maximum and the viscosity decreases; therefore the relaxation times should
-
12324 The Journal of Physical Chemistry, Vol. 97,No.47, 1993
aJPa
Pons et al.
Conclusions
700 +
From the experimental results presented in this paper we can conclude that, in a first approximation, gel-emulsion behavior can be adequately described by a Maxwell liquid element. The solid element is related to the structure of the system, that is, a foam structure with characteristic droplet size, interfacial tension, andvolume fraction. Theviscouselement is related to theintrinsic viscosity of the continuous phase and its volume fraction. The parameters that lead to an increase of the interfacialtension (temperature, salinity, oil-to-surfactant ratio) produce analogous effects on the shear modulus, Le. an increase. However, the increase in interfacial tension also produces an increase in the droplet size, the total effect being the apparition of maxima. The same parameters that produce an increasein the interfacial tension also produce a decrease in the viscosity of the microemulsions in equilibrium with excess water, and the total effect results in a decreaseof the viscous element, reducing the relaxation time of the system.
T= 2OoC
Acknowledgment. Financial support from DGICYT (Grant PB 92/0102) and from the CSIC/CNRS Bilateral Cooperation Program are gratefully ackowledged.
-
0
5
10
15
20
% NaCl Figure 6. GOas a function of NaCl concentration. The systems are 99% w/w H20 or brine and C I ~ E O ~ / C I with O H ~R~= 1.5. The lints are
drawn as a guide for the eye. go through a minimum or decrease continuously. Although the experimental behavior is not perfectly clear, nevertheless it correspondsto a continuous decrease as the oil-to-surfactant ratio increases. Effect of the Salt Concentration in the Aqueous Phase. The addition of sodium chloride produces an increase in interfacial tension, while the effect on the droplet radius is almost negligible up to very high salt concentrations; see Tables I11 and IV. By application of eq 1 an increase in the elastic modulus is expected to reach a plateau or even go through a maximum at high NaCl concentration. Experimentally, the behavior qualitatively agrees with theseassumptions,asshowninFiyre6. Thelackofextensive data on the properties of the system does not allow us to determine quantitatively the degree of validity of this model. Concerning the relaxation times, an increase in the NaCl concentration of the aqueous phase implies a decrease of water content of the microemulsion in equilibrium with excess water. As a result a decrease of the viscosity should be produced that combined with the increase of GOwould lead to a decrease of relaxation times. These assumptions have been confirmed experimentally.
References and Notes (1) Sagitani, H.; Hattori, T.; Nabeta, K.; Nagai, M. Nippon Kagaku Kaishi 1983, 1399. (2) Ishida, H.;Iwama, A. Combust. Sci. Technol. 1984, 37, 79. (3) Bampfield, A.; Cooper, J. In Encyclopedia of Emulsion Technology; Becher, Ed.; Marcel Dekker: New York, 1988; Vol. 3, p 281. (4) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. (5) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986,112,427. ( 6 ) Princen, H. M. hngmuir 1988,4, 486. (7) Princen, H. M.;Kiss, A. D. J. Colloid Interface Sci. 1989,128, 176. (8) Solans, C.; Azemar, N.; Comelles, F.; Sdnchez-Leal, J.; Parra, J. L. PrmeedingsoftheXVIIIJornadas CEDIAID AID: Barcelona, 1986; p 109. (9) Kunieda, H.; Solans, C.; Shida, N.; Parra, J. L. Colloids Surl. 1987, 24, 225. (10) Solans, C.; Azemar, N.; Parra, J. L. Prog. Colloid Polym. Sci. 1988, 76. 224. (1 1) Solans, C.; Domhguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S.E. Colloid Polym. Sci. 1988, 266, 570. (12) Kunieda, H.; Yano, N.; Solans, C. Colloids Surf. 1989, 36, 313. (13) Kunieda, H.; Evans, D. F.;Solans, C.; Yoshida, M. Colloids Surf. 1990.47.35. (14) Pons, R.; Solans, C.;StCW, M.J.; Erra, P.; Ravey, J. C. Prog. Colloid Polym. Sci. 1992, 189, 110. (15) Pons, R.; Ravey, J. C.; Sauvage, S.;StCW, M.J.; Erra, P.; Solans, C. Colloids Sur/. 1993, 76, 171. (16) Ravey, J. C.; StCW, M . J. Physica B (Amsterdam) 1989,394, 156. (17) Ravey, J. C.; StCW, M. J. Prog. Colloid Polym. Sci. 1990,82, 218. (18) Courraze, G.; Grossiord, J. L. Initiation b la Rheologie; Technique et Documentation; Lavoisier: Paris, 1983; pp 1 9 4 3 . (19) Ebert, G.; Platz,G.; Rehage, H. Ber. Bunsen-Ges.Phys. Chem. 1988, 92, 1158. (20) Pons, R.; Solans, C.; Tadros, Th. F. Longmuir, submitted for
publication.
