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Effects of Temperature, Surfactant, and Salt on the Rheological Behavior in Semidilute Aqueous Systems of a Nonionic Cellulose Ether Bo Nystro¨m,*,† Anna-Lena Kjøniksen,† and Bjo¨rn Lindman‡ Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway, and Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received January 10, 1996. In Final Form: April 22, 1996X Oscillatory shear experiments have been carried out on thermoreversible gelling and nongelling semidilute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) (at a constant polymer concentration of 1 wt %) in the presence of various amounts of sodium dodecyl sulfate (SDS) and at some different levels of NaCl addition. Depending on the concentrations of surfactant and salt, a temperature-induced sol-gel transition or only a viscosification of the solution was observed. For the gelling systems, the value of the gel temperature, determined by the observation of a frequency independent loss tangent, was found to be dependent on the composition of the system. At the gel temperature, a power law frequency dependence of the dynamic storage modulus (G′ ∼ ωn′) and loss modulus (G′′ ∼ ωn′′) was constantly observed with n′ ) n′′ ) n. Values of the viscoelastic exponent n in the range 0.3-0.4 were reported. The value of n, as well as the gel strength parameter S, was dependent on the composition of the system. The rheological properties of the nongelling systems were affected by temperature, surfactant, and salt. In the absence of salt, the network structure is disrupted at high surfactant concentations and the dynamic viscosity decreases. However, if salt is added at this stage an enhanced viscoelastic response is observed and the network structure is re-established. The rheological results of this work indicate that the effects of surfactant and salt counteract each other. The present results for both gelling and nongelling systems are analyzed in a model where the interplay between swelling (caused by the ionic surfactant) and connectivity (established by “lumps” or hydrophobic associations) is considered.
Introduction In recent years, aqueous systems containing nonionic polymers and ionic surfactants have received increasing attention because of their interesting fundamental properties and their importance in a number of industrial applications including detergents, pharmaceutical and paints.1-6 A typical, well studied example of this class of systems is aqueous ethyl(hydroxyethyl)cellulose (EHEC) in the presence of the anionic surfactant sodium dodecyl sulfate (SDS). In this case the interactions between polymer chains and surfactant give rise to the formation of mixed aggregates of surfactant and polymer. Depending on the polymer and surfactant concentrations, the aggregates contain segments belonging to one or more polymer chains. In semidilute polymer solutions these mixed aggregates induce a cross-linking of the polymer chains, leading to the viscosification or gelation of the system. During this process an associative polymer network is expected to form, with intriguing structural,7,8 dynamical,7,9 and rheological9,10 properties. †
University of Oslo. University of Lund. X Abstract published in Advance ACS Abstracts, June 1, 1996. ‡
(1) Evani, S.; Rose, G. D. Polym. Mater. Sci. Eng. 1987, 57, 477. (2) Polymers in Aqueous Media; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (3) McCormic, C. L.; Bock, J.; Schulz, D. N. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley-Interscience: New York, 1989; Vol. 17, p 730. (4) Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (5) Lindman, B.; Tomlin, J.; Carlsson, A. In Cellulosics: Chemical, Biochemical, and Material Aspects; Kennedy, J. F., Philips, G. O., Williams, P. A., Eds.; Ellis Horwood: Chichester, 1993; pp 319-324. (6) Goddard, E. D., Ananthapadmanabhan, K. P., Eds. Interactions of surfactants with polymers and proteins; CRC Press: Boca Raton, FL, 1993.
S0743-7463(96)00029-7 CCC: $12.00
Previous studies7,10,11 on semidilute aqueous systems of EHEC at a moderate level of surfactant addition have shown that these systems exhibit an interesting phenomenon of thermoreversible gelation; i.e., they gel upon heating and redissolve upon subsequent cooling. This type of physical gels is transparent and can probably be characterized as “soft” gels. Rheological measurements on semidilute EHEC systems, in the presence of a moderate amount (4 mmolal) of an ionic surfactant, have revealed10 that the incipient gelation temperature decreases with increasing polymer concentration and that the gel strength increases as the polymer concentration rises. At the same surfactant and polymer concentation, the gel strength in the presence of the anionic SDS was higher than that in the presence of the cationic surfactant cetyltrimethylammonium bromide. It has been reported9,12,13 from rheological studies on similar systems that the associative polymer network is disrupted at sufficiently high surfactant concentations. The above discussion indicates that the behavior of polymer-surfactant mixtures is governed by a subtle interplay between hydrophobic and hydrophilic interactions. In the presence of ionic surfactants, the strong interactions between polymer and surfactant may give rise to electrostatic interactions, which can play an important role in the overall interaction situation. In this (7) Nystro¨m, B.; Roots, J.; Carlsson, A.; Lindman, B. Polymer 1992, 33, 2875. (8) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (9) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (10) Nystro¨m, B.; Walderhaug, H.; Hansen, F. K.; Lindman, B. Langmuir 1995, 11, 750. (11) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloids Surf. 1990, 47, 147. (12) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. (13) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304.
© 1996 American Chemical Society
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case, the bound ionic surfactant endows an apparent polyelectrolyte character to the initially nonionic EHEC. This usually results14,15 in an increase in the cloud point temperature of the system; i.e., the solubility of the polymer increases. When salt (e.g. NaCl) is added to the EHEC-surfactant system, the electrostatic interactions will be screened and a cloud point depression is observed.14,15 These features suggest that factors such as temperature, polymer and surfactant concentrations, and salt addition will significantly affect the cross-linking situation of the associative polymer network. These effects are expected to have a crucial influence on the rheological characteristics of the system. In the present work we have carried out oscillatory shear experiments on semidilute aqueous EHEC (at a constant concentration of 1.0 wt %) at different temperatures in the presence of various amounts of SDS and at different levels of salt (NaCl) addition. The aim of this investigation is to gain a deeper understanding of the factors which govern the rheological properties of this type of associative system.
Chambon and Winter16 proposed a constitutive equation for linear viscoelasticity of incipient gels, which they called the gel equation
∫
t
(t - t′)-nγ˘ (t′) dt′
(1)
-∞
where σ is the shear stress, γ˘ is the rate of deformation tensor, n is the relaxation exponent, and S is the gel strength parameter (with dimensions Pa sn), which depends on the cross-linking density and the molecular chain flexibility. A more general version of this model has been developed,17 where the model proposed by Winter and Chambon constitutes a special case. The storage modulus G′ and the loss modulus G′′ at the gel point will both follow similar power laws in frequency
G′ ) G′′/tan δ ) SωnΓ(1 - n) cos δ
(2)
where Γ(1 - n) is the gamma function. The phase angle between stress and strain (δ) is independent of frequency but proportional to the relaxation exponent16
δ ) nπ/2
(3)
This result suggests that the power law behavior of the dynamic moduli can be expressed as
G′(ω) ∼ G′′(ω) ∼ ωn
n ) d(d + 2 - 2df)/2(d + 2 - df)
(4)
Theoretical models18,19 have been elaborated to rationalize values of the relaxation exponent in the physcial accessible range 0 < n < 1. In the theoretical advances, based on the fractal concept, the dynamic exponent n is associated with information about the molecular structure and connectivity of the incipient gel. The structure may be described by a fractal dimension df, which is defined by Rdf ∼ M, where R is the radius of gyration and M is the mass of a molecular cluster. On the basis of the percolation approach,20 the Rouse model,21-23 which assumes no hydrodynamic interaction between polymeric clusters, predicts (14) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005. (15) Lindman, B.; Carlsson, A.; Karlstro¨m, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (16) Chambon, F.; Winter, H. H. J. Rheol. 1987, 31, 683. (17) Friedrich, C.; Heymann, L. J. Rheol. 1988, 32, 235. (18) Muthukumar, M. Macromolecules 1989, 22, 4656. (19) Schiessel, H.; Blumen, A. Macromolecules 1995, 28, 4013. (20) Stauffer, D. Introduction to Percolation Theory; Taylor & Francis: London, 1985. (21) De Gennes, P.-G. C. R. Acad. Sci. Paris 1978, 286B, 131. (22) Martin, J. E.; Adolf, D.; Wilcoxon, J. P. Phys. Rev. Lett. 1988, 61, 2620.
(5)
Quite recently the relation between the viscoelastic and structural properties of systems of cross-linking polymers near the gel point was considered in the framework of a mechanical ladder model,19 which predicts a scaling law for the complex shear modulus with an exponent 0 e n e 0.5. It was shown19 that the parameter n is related to the spectral dimension ds (g1) of the fractal through the relationship
n ) 1 - ds/2
Background
σ(t) ) S
n ) d/(df + 2), and with d = 3 (the space dimension) and df ) 2.5 (percolation statistics) n assumes a value of 0.67. In the electrical analogy, a suggested24,25 isomorphism between the complex modulus and the electrical conductivity of a percolation network with randomly distributed resistors and capacitors yields a value of n ) 0.72. If we consider a situation where the strand length between cross-linking points of the incipient gel varies, one may anticipate that increasing strand length should enhance the excluded volume effect. In order to take this into account, Muthukumar18 suggested that if the excluded volume interaction is fully screened, the relaxation exponent can be expressed as
(6)
Before the rheological results are presented and discussed it may be worthwhile to first give some basic aspects on phenomena such as surfactant binding to EHEC, phase separation, and gelation of associating EHEC systems. The EHEC sample used in the present work can be characterized as a random copolymer with an irregular distribution of hydrophobic patches. We should note that this EHEC sample is different and displays different physical properties in aqueous solutions in the presence of an ionic surfactant than the hydrophobically modified EHEC (HMEHEC) used in recent studies.8,9 The hydrophobic modification of the latter polymer consists of branched nonylphenol groups grafted to the polymer backbone. In aqueous solutions the interaction of HM-EHEC with added surfactant occurs via the hydrophobic side groups and the hydrophobic alkyl chains of the added surfactant, thereby increasing intermolecular associations through the formation of mixed “micelles”. The EHEC polymer does not possess this type of specific “sites” for interaction, but in this case the hydrophobic interaction domains are rather irregular. We should also note that, in contrast to semidilute aqueous EHEC-ionic surfactant systems, the corresponding HMEHEC-surfactant solutions do not form gels at elevated temperature. Previous surfactant NMR self-diffusion measurements on thermally gelling systems of EHEC in the presence of, e.g., SDS26 have revealed a strong attraction between EHEC and the ionic surfactant, and the amount of surfactant bound to the polymer was roughly independent of temperature. As a consequence of this interaction, the solubility of the polymer in water increases14,15 (the cloud point is shifted toward a higher temperature than that in pure water) because the bound ionic surfactant endows an apparent polyelectrolyte nature to the originally nonionic EHEC. The thermal gelation of EHEC-surfactant systems is a rather general phenomenon with respect to the surfactant, because almost any surfactant can be used as long as it is ionic and has a sufficiently long hydrocarbon tail. It is noteworthy that gelation of EHEC solutions in the presence of a nonionic surfactant has never been observed. A recent small angle neutron scattering (SANS) study27 has analyzed inter alia the thermoreversible gelation of EHECsurfactant systems and advanced a new and attractive model for the gelation process. This model elucidates the intricate interplay between thermoreversible gelation and phase separation. This (23) Adam, M.; Lairez, D. In Physical Properties of Polymeric Gels; Cohen Addad, J. P., Ed.; John Wiley & Sons Ltd: Chichester, 1996; pp 87-142. (24) De Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (25) Alexander, S. J. Phys. (Paris) 1984, 45, 1939. (26) Walderhaug, H.; Nystro¨m, B.; Hansen, F. K.; Lindman, B. J. Phys. Chem. 1995, 99, 4672. (27) Cabane, B.; Lindell, K.; Engstro¨m, S.; Lindman, B. Macromolecules 1996, 29, 3188.
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Table 1. Gel Temperature of Gel-Forming Systems and Indication of Systems Which Are Not Gelling in the Considered Temperature Range gel temperatures (1 wt % EHEC) at SDS concentration given concentration of NaCl (mM)
3 mmolal
4 mmolal
6 mmolal
8 mmolal
10 mmolal
0 1 4
37 °C 33 °C no gel
37 °C 37 °C 34 °C
no gel 38 °C no gel
no gel no gel no gel
no gel no gel no gel
theme has been the subject of a recent theoretical paper.28 The scenario of Cabane et al.,27 which will be used to rationalize the present results, can be summarized in the following way. In the absence of surfactant, a temperature rise induces a macroscopic phase separation of the semidilute aqueous EHEC solution into a polymer-rich phase and an excess aqueous phase. When an ionic surfactant is added, the phase separation behavior of the random copolymer is modified, because the surfactant causes fragmentation of large domains of the polymer-rich phase into microscopic “lumps” which are stabilized by the adsorption of surfactant on the surfaces of these “lumps”. Each “lump” is formed by the loose association of polymer segments belonging to different polymer molecules, and the adsorption of ionic surfactant gives rise to a polyelectrolyte character of the “lump”. The SANS experiments on the EHEC systems indicated that elevated temperature favors the growth of the “lumps”, and the formation of these association entities occurs at the expense of the micellar organization. In this model, the “lumps” provide a permanent connectivity of the gel network, and the swelling ability of the system is caused by the electrostatic repulsions of the ionic surfactant. In order to have both swelling and connectivity of the system, we need to be close to phase separation, i.e. to have balance between repulsive (swelling) and attractive (connecting) forces. At high levels of surfactant addition the “lumps” break down (the connectivity of the system is lost), the network is disrupted, and micelles are formed at sufficiently high surfactant concentration. We should note that the cloud point (CP) in salt-free solutions of EHEC in the presence of, e.g., SDS increases with surfactant concentration.14,15,29 When salt (NaCl) is added, the CP of the EHEC-SDS solution decreases14,15,29 (this trend becomes gradually more pronounced with increasing salinity); hence, more surfactant is needed to raise the CP. In the framework of this model, addition of salt to aqueous EHECSDS systems will perturb the delicate interplay between “lumps” and surfactant mainly by weakening the repulsive forces between the “lumps”, and the gelling ability and strength are thereby affected. This suggests that the screening of the repulsive electrostatic forces between the charged polymer chains in the presence of salt gives rise to a reduction of the swelling effect, and more surfactant is needed to restore the swelling power. This is consistent with the findings27,30 that the presence of salt in EHEC-surfactant systems has the consequence of shifting the zones of gelation to higher surfactant concentations.
Experimental Section Materials and Solution Preparation. A sample of ethyl(hydroxyethyl)cellulose (EHEC, fraction DVT 89017) with a number average molecular weight (Mn) of approximately 80 000 and degrees of substitution of ethyl and hydroxyethyl groups of DSethyl ) 1.9 and MSEO ) 1.3, respectively, was obtained from Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. The polymer is polydisperse with a polydispersity index (Mw/Mn) of about 2. The values of DS and MS correspond to the average numbers of ethyl and hydroxyethyl groups, respectively, per anhydroglucose unit of the polymer. The cloud point of a 1 wt % aqueous EHEC solution without surfactant was observed to be 34 °C. The values of Mn, MS, DS, and CP were all given by the manufacturer. The surfactant SDS was purchased from Fluka and was used as received. (28) Tanaka, F.; Stockmayer, W. H. Macromolecules 1994, 27, 3943. (29) Carlsson, A. Ph.D. Dissertation, Lund University, 1989. (30) Lindell, K.; Engstro¨m, S. Int. J. Pharm. 1993, 95, 219; 1995, 124, 107.
Figure 1. Viscoelastic loss tangent as a function of temperature for the systems and frequencies indicated. The notion tg indicates the gel temperature. Dilute EHEC solutions were dialyzed against pure water for several days to remove salt (impurity from the manufacturing) and were thereafter freeze-dried. As dialyzing membrane, regenerated cellulose with a molecular weight cutoff of 8000 (Spectrum Medical Industries) was used. After freeze-drying, the polymer was redissolved in aqueous media with the desired SDS and NaCl concentrations. The samples were prepared by weighing the components, and the solutions were homogenized by stirring at room temperature for several days. All the measurements were carried out on semidilute (1 wt %) EHEC samples in the presence of various amounts of SDS and NaCl. The surfactant concentrations are always above the critical concentration for formation of polymer-bound micelles. Oscillatory Shear Experiments. These measurements were conducted in a Bohlin VOR rheometer system using, depending on the viscosity of the sample, a double-gap concentric cylinder, an ordinary concentric cylinder geometry (C 25; inner radius 12.5 mm), or a cone-and-plate geometry, with a cone angle of 5° and a diameter of 30 mm. The double-gap device is applicable for low-viscosity liquids. Since measurements were carried out at elevated temperatures (up to ca. 45 °C), a layer of silicone oil was always added to the sample to avoid evaporation of the solvent. In this work, the results from the oscillating sweep measurements in the approximate frequency domain 0.01-3 Hz are reported. The values of the strain amplitude were checked in order to ensure that all experiments were conducted within the linear viscoelastic region, where G′ and G′′ are independent of the strain amplitude. The instrument was equipped with a temperature control unit that was calibrated to give a temperature in the sample chamber within 0.1 °C of the set value. At each temperature the sample was allowed to equilibrate for about 1 h before measurements were commenced. In these experiments no disturbing hysteresis effects were observed.
Results and Discussion Let us now discuss the results of gelling and nongelling EHEC systems in the light of the scenario outlined in the Background section. In this study, the effects of surfactant and salt on the rheological properties of aqueous semidilute EHEC (1 wt %) systems have been investigated over an extended temperature range. It was found that some of the systems formed thermoreversible gels at elevated temperatures, while other systems did not exhibit any signs of gel formation over the considered temperature region. These results are summarized in Table 1. We will first discuss some characteristic properties of the gelling systems. The general feature observed for the gelling EHEC systems in Figure 1 is that the loss tangent, tan δ, decreases during the gel formation, indicating that the solutions become more and more elastic. This type of
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Figure 3. Plot of the gel strength parameter S (calculated from eq 2) and the viscoelastic exponent n as a function of SDS concentration for the indicated systems at their respective gel temperatures.
Figure 2. Frequency dependences of the storage modulus G′ and the loss modulu G′′ at different stages of the gel-forming process for the systems indicated. The curves have been shifted horizontally by a factor B of the value listed in the insert. The inset plots show the power law behavior of the dynamic moduli at the gel temperature.
behavior has frequently been observed for incipient gels of both physical and chemical natures. The gel temperature is determined by observation of a frequency independent value of tan δ obtained from a multifrequency plot of tan δ versus temperature. The gel temperatures obtained by this procedure are in good agreement with those measured with the test tube “tilting” method,31 where the gelation temperature was determined by tilting the test tube containing the solution. In this approach the temperature at which the solution no longer flows is (31) Wellinghoff, S.; Shaw, J.; Baer, E. Macromolecules 1979, 12, 932.
taken as the temperature of gelation. This method served to define the sol-gel transition temperature to about (1 °C. Figure 2 shows changes in the frequency dependence of G′ and G′′ at different stages during the gelation process. Measurements of G′ at lower temperatures than those displayed in Figure 2 revealed a large scatter in the experimental points, probably due to a weak viscoelastic response. The data in Figure 2 are shifted horizontally by a factor B of the value listed in the insert. The general trend for all systems is that at temperatures well below the gel temperature, G′ is smaller than G′′ at low frequencies and a liquid-type behavior is predominant. At the gel temperature, the G′ and G′′ curves become parallel and power laws in frequency are observed over a frequency domain of more than two decades (see the inset plots of Figure 2). At temperatures above the gel temperature, G′ increases rapidly and becomes larger than G′′. This behavior is characteristic of the solidlike state that evolves above the gel point. Figure 3 shows how different conditions of salt and surfactant concentrations affect the values of the power law exponent n and the gel strength parameter S for the incipient gels. Let us first discuss the findings of the parameter S (see Figure 3a), which has been determined with the aid of eq 2. In the absence of salt, gels are formed at 3 and 4 mmolal SDS, and the value of S increases only slightly. These surfactant concentrations are sufficiently high to produce a polyelectrolyte-type expansion of the EHEC chains, thereby leading to the required swelling of the polymer network. At lower surfactant concentrations the weak swelling effect will lead to macroscopic phase separation at elevated temperatures. At high surfactant concentrations no temperature-induced gelation occurs, which probably suggests that the connectivity required for gel formation is not fulfilled due to the disruption27 of the “lumps” at higher surfactant addition. At a constant salt concentration of 1 mM, the value of S increases with increasing level of surfactant addition, while at fixed SDS concentrations of 3 and 4 mmolal S seems to fall off with increasing amount of NaCl (see Figure 3a). These findings emphasize the intricate interplay between swelling and the connectivity properties of the gelling systems. The swelling effect of the polymer network caused by the ionic surfactant is counteracted by the addition of salt. It is obvious from the results in Figure 3a that the gel strength is governed by the competition between repulsive (swelling) forces and hydrophobic (connecting) associations.
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Figure 4. Effect of the addition of surfactant on the storage (open symbols) and loss moduli (solid symbols) (0.1 Hz) of 1 wt % EHEC at the salinities and temperatures indicated. The curves have been shifted vertically by a factor A of the value listed in the insert. The vertical trend of the data has not been changed by this action.
The relaxation exponent n is roughly located in the range 0.3-0.4 (see Figure 3b). We may also note that the value of n decreases slightly with increasing level of surfactant addition at a constant salt concentration of 1 mM. The results for the incipient gels in Figure 3b suggest that the value of the relaxation exponent depends on surfactant and salt concentrations. Moreover, a recent rheological study10 on aqueous EHEC-SDS and EHEC-CTAB systems revealed that n varied with the EHEC concentration and the type of surfactant. These findings as well as previous results on chemically16,32-39 and physically10,39-42 gelling systems suggest that there is not a unique molecular structure with a unique power law exponent corresponding to the incipient gel state. The general surmise is that the viscoelastic exponent is associated with structural and connectivity properties of incipient gels. The dependence of the parameter n on the structural properties can be rendered evident by the use of models based on fractal networks. The present values of n can be rationalized in the framework of the Muthukumar18 fractal model, which considers screened excluded volume interactions. On the basis of this model (see eq 5), values of n ) 0.30 and n ) 0.40 yield fractal dimensions of df ) 2.2 and df ) 2.1, respectively. It has been speculated37 that incipient gels with high values of the relaxation exponent have low fractal dimensions and are said to be “open”, while gels with lower n values have higher fractal dimensions and are “tight”. The present n values suggest a rather “tight” gel network. These results may also be interpreted in the framework of the mechanical ladder model of Schiessel and Blumen,19 where the (32) Chambon, F.; Winter, H. H. Polym. Bull. (Berlin) 1985, 13, 499. (33) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367. (34) Chambon, F.; Petrovic, Z. S.; MacKnight, W. J.; Winter, H. H. Macromolecules 1986, 19, 2146. (35) Winter, H. H. Encyclopedia of Polymer Science and Engineering, Supplement Vol.; John Wiley & Sons, Inc.: New York, 1989; p 343. (36) Valle´s, E. M.; Carella, J. M.; Winter, H. H.; Baumgaertel, M. Rheol. Acta 1990, 29, 535. (37) Scanlan, J. C.; Winter, H. H. Macromolecules 1991, 24, 47. (38) Muller, R.; Ge´rard, E.; Dugand, P.; Rempp, P.; Gnanou, Y. Macromolecules 1991, 24, 1321. (39) Mu¨ller, O.; Gaub, H. E.; Ba¨rmann, M.; Sackmann, E. Macromolecules 1991, 24, 3111. (40) Cuvelier, G.; Launay, B. Makromol. Chem., Macromol. Symp. 1990, 40, 23. (41) Lin, Y. G.; Mallin, D. T.; Chien, J. C. W.; Winter, H. H. Macromolecules 1991, 24, 850. (42) Carnali, J. O. Rheol. Acta 1992, 31, 399.
Figure 5. Effect of the addition of surfactant on the dynamic viscosity (0.1 Hz) of 1 wt % EHEC at the salinities and temperatures indicated.
fractal concept has been implemented. In this model, values of n smaller than 0.5 (see eq 6) indicate that the spectral dimension is greater than 1, and this may be attributed to an enhanced density of cross-links.19 In a recent study43 it has been argued that small values of n may be due to the presence of entanglement coupling in the gelling polymer system. The effects of temperature, surfactant, and salt on the dynamic moduli of a 1 wt % aqueous solution of EHEC at a frequency of 0.1 Hz are depicted in Figure 4. The general trend, at all salinities, is that both G′ and G′′ increase with increasing temperature, and G′ > G′′ at elevated temperature. In the absence of salt (Figure 4a) and for the lowest salt concentration (1 mM; Figure 4b), G′ and G′′ both fall off considerably with increasing surfactant addition at SDS concentrations above 6 mmolal. This trend is more pronounced at higher temperatures, where the interpolymer association is stronger at moderate levels of surfactant addition. This decrease in G′ and G′′ is attributed to the gradual disruption of the “lumps” with increasing surfactant addition, and thereby is the connectivity of the polymer network lost. A different pattern of behavior is observed at the highest salt addition (4 mM). (43) Koike, A.; Nemoto, N.; Takahashi, M.; Osaki, K. Polymer 1994, 35, 3005.
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Figure 6. Temperature dependences of the storage (open symbols) and loss (solid symbols) moduli (0.1 Hz) of 1 wt % EHEC at the salinities and surfactant concentrations indicated. The curves have been shifted vertically by a factor A of the value listed in the insert.
In this case a minimum in G′ and G′′ is located at a surfactant concentration of about 4 mmolal, followed by an increase at higher SDS concentrations. This feature seems to be similar at all temperatures. This effect of salt addition elucidates the intriguing interplay between electrostatic (swelling) and hydrophobic (connecting) interactions. At a low surfactant concentration and high salinity, the electrostatic interaction is screened, leading to large “lumps” and weak swelling effects of the system. This situation of weak swelling may suggest that the systems are close to phase separation, and this effect may give rise to the anomalous behavior observed in the initial parts of the G′ and G′′ curves given in Figure 4c. At higher levels of surfactant addition (smaller “lumps” and the thermodynamic conditions become better), the replusive forces will gradually dominate, producing a polyelectrolyte-type expansion of the EHEC chains, thereby leading to a swollen polymer network. This will lead to a structural enhancement of the polymer network, and the values of the dynamic moduli are expected to rise. The surfactant concentration dependence of the dynamic viscosity η′ at different temperatures and salt concentrations is given in Figure 5. These results are strongly reminiscent of those depicted in Figure 4, but in this plot it is easier to judge the temperature effect. The general trend at moderate surfactant concentrations is that η′ increases with increasing temperature, indicating stronger association effects at elevated temperatures. Again the results suggest a progressive disruption of the network structure at higher surfactant addition. At the highest salt addition (4 mM), the dynamic viscosity passes through a minimum at about 4 mmolal SDS, and at higher SDS concentrations a pronounced rise of η′ is detected. This enhancement of the dynamic viscosity indicates that the degree and strength of the structure within the EHEC network increase. Previous viscosity measurements44 on various EHEC-surfactant systems support the present observation that the viscosity enhancement is shifted toward higher surfactant concentations when salt is present. These findings suggest that the effects of surfactant and salt addition counteract each other. In other words, the swelling caused by SDS addition is counteracted by salt, and more surfactant is needed to restore the level of swelling. We may also note that the phase separation behavior of EHEC-SDS systems shows15 (44) Arwidsson, M. Personal communication.
Figure 7. Temperature dependence of the dynamic viscosity (0.1 Hz) of 1 wt % EHEC at the salinities and surfactant concentrations indicated.
that CP increases with surfactant addition and CP decreases with increasing salinity. The temperature dependences of the dynamic moduli at various surfactant concentrations and salinities are illustrated in Figure 6 for the EHEC (1 wt %) systems. In the absence of salt (Figure 6a) and at the lowest salt addition (1 mM) (Figure 6b) the characteristic features are similar. At surfactant concentrations of 3, 4, and 6 mmolal, the dynamic moduli increase with increasing temperature and G′ > G′′ at elevated temperatures. At higher levels of surfactant addition the general trend is that G′ < G′′ (liquidlike behavior), and at the highest SDS concentration (10 mmolal) both G′ and G′′ fall off strongly at temperatures above 25 °C. These results suggest that at low surfactant concentrations the network structure is strengthened with increasing temperature, with an enhanced viscoelastic response. At higher SDS concentrations the “lumps” disappear and the network connectivity is lost, leading to decreasing values of the dynamic moduli. The pronounced decrease of G′ and G′′ at elevated temperatures and high SDS concentrations suggests that the disruption process of the network is promoted by the enhanced thermal motion at high temperatures. It is interesting to note that a temperature increase strengthens the polymer network at low surfactant concentration, whereas at high levels of surfactant
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Figure 8. Plots of the effect of the frequency of oscillation on the dynamic viscosity of 1 wt % EHEC at the temperatures, surfactant concentrations, and salinities indicated.
addition the network disruption process is enhanced at elevated temperatures. In the presence of a high salt addition (4 mM) we observe quite different features than those at low salt addition (see Figure 6c). In this case the values of the dynamic moduli, above 4 mmolal SDS, increase with increasing surfactant concentration. At 4 mmolal SDS, G′ > G′′, and this tendency becomes more pronounced at higher temperatures. This anomalous behavior is analogous to that observed previously for the dynamic moduli (Figure 4c) and the dynamic viscosity (Figure 5). For the other surfactant concentations, G′′ > G′ at low temperatures, and at higher temperatures, G′ approaches G′′ or even becomes larger. The temperature dependences of the dynamic moduli seem to become weaker at higher levels of surfactant addition. These results at high salinity suggest an enhancement of the network structure at higher levels of surfactant addition and with increasing temperature. Again these findings elucidate the complex interplay between surfactant and salt effects on the overall interaction situation in these systems. While binding of surfactant to EHEC changes the polyelectrolytic character of the polymer and thereby the swelling properties of the network, salt addition will counteract this effect by screening the electrostatic interactions. The effects of temperature, surfactant, and salt on the dynamic viscosity are illustrated in Figure 7. At a low level (4 mmolal) of SDS addition (Figure 7a), the values of η′ for the salt-free solution and in the presence of 1 mM NaCl rise strongly (the increase is somewhat less pronounced for the 1 mM NaCl system) with increasing temperature up to about 40 °C. At high salinity, the values of η′ are considerably smaller than those at low salt contents, and there is practically no temperature dependence of η′. At 6 mmolal SDS (Figure 7b), a quite different pattern of behavior is observed. In this case the values of η′ are highest for the system with the highest salt concentration and decrease with decreasing salt addition. The dynamic viscosity data display a maximum at about 35 °C for the system with the highest salt content, while the maxima for the lower salt concentations are located at ca. 40 °C. At still higher levels of surfactant addition (see Figure 7c and d) the general features are the same but the difference between the values of η′ of systems with high and low salinity is strengthened. The maxima of the curves are roughly located at 25 and 20 °C at surfactant concentrations of 8 and 10 mmolal, respectively.
Furthermore, at these conditions the temperature dependence of η′ is much more pronounced at high salt addition than for the systems with lower salt concentrations. Again the results demonstrate the interesting interplay between salt (promoting “lumps”) and surfactant (swelling) effects. The general picture that emerges is that if the surfactant concentation is low, an excess of salt leads to a disruption of the network, whereas at high levels of surfactant addition a high salinity may give rise to structural enhancement of the polymer network and thereby increase the dynamic viscosity. The effect of the frequency of oscillation on the dynamic viscosity of 1 wt % solutions of EHEC containing varying amounts of SDS and NaCl is given in Figure 8. At the surfactant concentrations of 4 mmolal (Figure 8a) and 6 mmolal (Figure 8b) the general pattern of behavior of η′ is similar at all salinities. In these cases the results indicate that the polymer systems become more viscoelastic, i.e. displaying increased frequency dependence, typical of polymer network systems containing cross-linked or entangled chain networks, as the temperature is increased. However, for the system with a SDS concentration of 4 mmolal and a salinity of 4 mM the values of η′ are substantially smaller than the corresponding ones at lower salinity (see Figure 8a). In salt-free EHEC solutions at the highest (10 mmolal) level of surfactant addition (Figure 8c), the frequency dependence of η′ is considerably reduced, and the system displays rheological characteristics typical of polymer systems with little cross-linking effect or molecular entanglement. However, as the salinity increases, the system retrieves its enhanced viscoelastic response. These findings emphasize again the crucial interplay between the effects of surfactant and salt on the rheological properties. Conclusions In this study of semidilute aqueous systems of EHEC in the presence of various amounts of surfactant and salt, we have emphasized the complex interplay between surfactant and salt effects on the rheological properties. For the thermoreversible gelling systems, it is observed for the incipient gels that G′ parallels G′′ over an extended frequency domain and the moduli can throughout be described by power laws G′ ∼ G′′ ∼ ωn. The values of n are located in the range 0.3-0.4. The viscoelastic exponent, as well as the gel strength parameter, depends
3240 Langmuir, Vol. 12, No. 13, 1996
on the conditions of surfactant and salt. The rheological behavior of the nongelling systems also depends on the level of surfactant addition and on the salinity. A number of interesting rheological features have been found, and the competition between effects induced by surfactant and salt additions has been scrutinized. The results from both gelling and nongelling systems have been analyzed in the framework of a model where the interplay between swelling (caused by the ionic surfactant) and connectivity (made of “lumps” or hydrophobic associations) is considered. In the absence of salt, it seems that the maximum value of the viscoelastic response is determined by an “optimal” balance between repulsive (swelling) and attractive (connecting) forces. At high surfactant concentrations, the “lumps” are disrupted (the connectivity is lost) and the structure of the polymer network breaks down and the strong viscoelastic response is lost. However, if salt is
Nystro¨ m et al.
added (the electrostatic repulsions are screened and the “lumps” and the connectivity again come into play), a network of high strength may be reformed. This picture is consistent with the phase separation behavior of these systems, namely that CP in the absence of salt increases with surfactant concentration and the CP of a EHECsurfactant solution is decreased by salt addition. The results of this study elucidate the intricate interplay between surfactant and salt effects. The present findings indicate that these effects act in opposite directions on the viscoelasticity of the polymer systems. Acknowledgment. The authors are grateful for valuable discussions with L. Piculell, K. Thuresson, and H. Wennerstro¨m, Chemical Center, University of Lund. We thank Akzo Nobel AB for supplying the polymer. LA960029+
