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Langmuir 2001, 17, 28-34
Polymer-Surfactant Interactions in Dilute Mixtures of a Nonionic Cellulose Derivative and an Anionic Surfactant Erlend Hoff,† Bo Nystro¨m,*,† 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 August 14, 2000. In Final Form: October 25, 2000 The interaction between ethyl(hydroxyethyl)cellulose (EHEC) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in dilute aqueous solutions at different temperatures with the aid of viscometry. The results suggest that at low polymer concentrations the polymer-surfactant complexes are molecularly dispersed at the shear rates operating in the capillary viscometers used in this study. The delicate interplay between hydrophobic interactions and polyelectrolyte effects is demonstrated through the reduced viscosity data. At surfactant concentrations slightly above the critical aggregation concentration cac (the value of cac decreases with increasing temperature), the intrinsic viscosity features suggest a sharp collapse of the polymer-surfactant aggregates. At moderate amounts of surfactant, the molecular complexes expand due to amended thermodynamic conditions and enhanced electrostatic repulsion between chains decorated with SDS. At very high levels of surfactant addition, the small contraction of the molecular units is attributed to screening of the electrostatic interactions. At low surfactant concentrations (up to 4 mm), a temperature-induced shrinkage of the molecules is also observed. The very high values of the Huggins coefficient around cac suggest strong coil-coil interactions. The polymer-concentration-induced enhancement of the reduced viscosity around the cac is another indication of enhanced intermolecular interactions.
Introduction The interaction between ionic surfactants and nonionic water-soluble amphiphilic polymers has gained a growing interest1-7 in recent years because of the various industrial applications of such systems and their inherently interesting properties. Among the large number of systems investigated, those involving nonionic ethyl(hydroxyethyl)cellulose (EHEC) and ionic surfactants (e.g., sodium dodecyl sulfate; SDS) have attracted a great deal of interest in recent years.5,8-15 In the presence of an ionic surfactant, several studies have shown13,15-19 that the binding of the † ‡
University of Oslo. University of Lund.
(1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) De Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (3) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 123. (4) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 85, 1. (5) Lindman, B.; Carlsson, A.; Gerdes, S.; Karlstro¨m, G.; Piculell, L.; Thalberg, K.; Zhang, K. In Food Colloids and Polymers: Stability and Mechanical Properties; Walstra, P., Dickinson, E., Eds.; The Royal Society of Chemistry: London, 1993; pp 113-125. (6) Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (7) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77. (8) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005. (9) Lindman, B.; Carlsson, A.; Karlstro¨m, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (10) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (11) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelo¨f, L.-O. J. Phys. Chem. 1992, 96, 871. (12) Nystro¨m, B.; Lindman, B. Macromolecules 1995, 28, 967. (13) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (14) Kjøniksen, A.-L.; Nystro¨m, B.; Lindman, B. Macromolecules 1998, 31, 1852. (15) Kjøniksen, A.-L.; Nystro¨m, B.; Lindman, B. Langmuir 1998, 14, 5039.
surfactant to EHEC is strongly cooperative; that is, the surfactants form micelle-like aggregates upon binding to EHEC at the so-called critical aggregation concentration (cac), which is independent of polymer concentration and lower than the regular critical micelle concentration of the pure surfactant. For aqueous solutions of the present EHEC fraction in the presence of SDS, the value of cac has been reported17,19 to decrease with increasing temperature. The binding of surfactant to the polymer continues until the chains become “saturated”, whereafter normal free micelles start to form when the overall surfactant concentration is sufficiently high. Binary EHEC-water solutions exhibit phase separation at elevated temperatures8 (i.e., for EHEC the solubility decreases with an increased temperature), and this phenomenon is attributed to a less favorable interaction between the polymer and the solvent. The temperature where the solution phase separates is frequently referred to as the cloud point. The interaction of EHEC with an ionic surfactant such as SDS can give rise to charging effects of the polymer and in effect imparting polyelectrolyte properties to the nonionic polymer. The binding of SDS to EHEC results in a gradual rise of the cloud point temperature of the system; that is, the solubility of the polymer increases with increasing surfactant concentration.8,9,14 Various aspects on aqueous solutions of EHEC in the presence of an ionic surfactant have been reported by using different techniques such as conductivity,16,17,20,21 dynamic (16) Zana, R.; Binana-Limbele´, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992, 96, 5461. (17) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (18) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (19) Evertsson, H.; Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Langmuir 1996, 12, 5781. (20) Holmberg, C.; Sundelo¨f, L.-O. Langmuir 1996, 12, 883.
10.1021/la001175p CCC: $20.00 © 2001 American Chemical Society Published on Web 12/02/2000
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light scattering,15,22-24 equilibrium dialysis,19-21,25-27 fluorescence quenching,16,17,19,25,27,28NMR,17,29-32 titration calorimetry,13,18,33 and viscosity.11,13,18,20,22,23,26,27,32 However, most of these investigations have been carried out on moderately concentrated solutions, and systematic studies of how temperature and surfactant concentration affect the polymer dynamics in very dilute solutions are still lacking. The dynamical and rheological behavior of the amphiphilic EHEC polymer in aqueous solutions in the presence of an ionic surfactant is governed by an intricate interplay between intrachain, interchain aggregation, and electrostatic interactions. To understand the general dynamical features of these systems, and especially the effect of competition between inter- and intrachain association, we need information about their behavior in very dilute solution, i.e., at conditions where the polymer chains behave as individual coils. In this work we have carried out precision viscosity measurements on dilute (0.03-0.11 wt %) EHEC solutions at different temperatures in the presence of various amounts of the anionic surfactant SDS. The objective of this study is to gain insight into the factors that control the rheological behavior in dilute EHEC solutions under various conditions of temperature and surfactant addition. Experimental Section Materials and Solution Preparation. The EHEC sample used in this study is designated DVT 89017 and was supplied by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. This sample had a cloud point of ca. 37 °C15 (in 0.1 wt % aqueous solution without surfactant), the average degree of substitution of ethyl groups was DSethyl ) 1.9/anhydroglucose unit, and the molar substitution of ethylene oxide groups was MSEO ) 1.3/ anhydroglucose unit. The number-average molecular weight Mn for this polydisperse sample (Mw/Mn ≈ 2) is about 100 000. All these data, except the cloud point data, have been given by the manufacturer. The anionic SDS was obtained from Fluka and was used without further purification. The water was double distilled. The EHEC sample was dissolved, and the dilute solution was dialyzed against water for about 2 weeks to remove salt (impurity from the manufacturing) and other low molecular weight components and was thereafter freeze-dried. As the dialyzing membrane, regenerated cellulose with a molecular weight cutoff of 8000 (Spectrum Medical Industries) was used. After being freeze-dried, the polymer was redissolved in aqueous media with the desired SDS concentration. Samples were prepared by weighing the components, and the solutions were allowed to stand in a refrigerator for a few days and thereafter homogenized by stirring at room temperature for several days. All the measure(21) Holmberg, C.; Nilsson, S.; Sundelo¨f, L.-O. Langmuir 1997, 13, 1392. (22) Nilsson, S.; Sundelo¨f, L.-O.; Porsch, B. Carbohydr. Polym. 1995, 28, 265. (23) Goldszal, A.; Costeux, S.; Djabourov, M. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 112, 141. (24) Porsch, B.; Nilsson, S.; Sundelo¨f, L.-O. Macromolecules 1997, 30, 4626. (25) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1995, 273, 83. (26) Holmberg, C. Colloid Polym. Sci. 1996, 274, 836. (27) Evertsson, H.; Holmberg, C. Colloid Polym. Sci. 1997, 275, 830. (28) Medeiros, G. M. M.; Costa, S. M. B. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 119, 141. (29) Walderhaug, H.; Nystro¨m, B.; Hansen, F. K.; Lindman, B. J. Phys. Chem. 1995, 99, 4672. (30) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (31) Evertsson, H.; Nilsson, S.; Welch, C. J.; Sundelo¨f, L.-O. Langmuir 1998, 14, 6403. (32) Nilsson, S.; Thuresson, K.; Lindman, B.; Nystro¨m, B. Macromolecules, in press. (33) Wang, G.; Lindell, K.; Olofsson, G. Macromolecules 1997, 30, 105.
Langmuir, Vol. 17, No. 1, 2001 29 ments were carried out on dilute (0.03-0.11 wt %) EHEC solutions in the presence of various amounts of SDS (0-50 mm) at temperatures in the interval 10-30 °C. The polymer concentrations are well below the overlap concentration c*, estimated from c* ) 1/[η], where [η] is the intrinsic viscosity. The value of c* (at a temperature of 10 °C) varies roughly from 0.2 to 0.4 wt % as the surfactant concentration increases from 0 to 50 mm. There is a rise in the value of c* as the temperature increases from 10 to 30 °C. This effect is weak for most of the EHEC-SDS solutions except at surfactant concentrations around the cac, where there is a stronger increase of the value of c* with increasing temperature. Low Shear Rate Viscosity Measurements. Low shear rate solution viscosity in the range 0.07-140 s-1 was measured on a Bohlin VOR rheometer system using a double-gap concentric cylinder, which is applicable for low-viscosity liquids. The experiments were conducted at 20 °C, and the samples were prethermostated at this temperature. Capillary Viscometry. The efflux times of the dilute solutions were measured using a Ubbelohde ASTM capillary viscometer, equipped with a home-built photodetector device, connected to a stopwatch, which enabled us to measure efflux times of the order of 500 s with accuracy of (200 µs by detecting passage of the meniscus. The efflux time is kept long to minimize the need for applying kinetic corrections to the observed data. The experiments were carried out at different temperatures (temperature controlled to within (0.01 °C) in a thermostated bath with a long-time thermal stability of about (0.002 °C. The measurements were repeated at least three times at each temperature, starting with the lowest temperature (10 °C). To obtain high reproducibility, we found that the following procedures were important: (a) A standard procedure for cleaning and filling the viscometer was developed, and great care was exercised to avoid polymer adsorption onto the capillary walls and formation of air bubbles in the viscometer. (b) To avoid dust, all liquids and solutions that were introduced into the viscometer were filtered, and steps were taken in the selection of filters to ensure that the adsorption of polymer onto the filters was minimized and that correct concentration of the sample in the viscometer was used. (c) At certain temperatures and compositions of the polymer-surfactant mixtures, a time dependence of the viscosity (the viscosity decreased with time) was detected.34 In some cases it took more than 24 h before the time effect died out. In light of this, great care was exercised to ensure equilibration of the samples. Therefore, the viscometer was filled with solution and prethermostated for a long time prior to measurement. Time effects for the EHEC-SDS system have been reported previously.35 (d) The bulb and capillary of the viscometer were maintained in contact36 with the experimental liquid to avoid structural changes. (e) To perform measurements of high reproducibility, it is important to have a thermostat with longtime thermal stability and flow times should be measured with high accuracy. The measured efflux times are converted to reduced viscosity through the following expression:
ηred )
ηsp η - ηs t - t0 ) ≈ w ηsw t0w
(1)
where ηred is the reduced viscosity, w the polymer concentration (weight fraction), η the solution viscosity, ηs the solvent viscosity, ηsp the specific viscosity, t the efflux time of the solution, and t0 the efflux time of the solvent. The intrinsic viscosity is defined by
[η] ) lim wf0
ηsp w
(2)
The intrinsic viscosities are obtained by extrapolation of the reduced viscosity curves to zero polymer concentration using the Huggins’ equation: (34) Hoff, E. Unpublished results. (35) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1994, 272, 338. (36) Cohen, J.; Priel, Z.; Rabin, Y. J. Chem. Phys. 1988, 88, 7111.
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Langmuir, Vol. 17, No. 1, 2001 ηsp ) [η] + kH[η]2w + ... w
Hoff et al. (3)
where higher terms have been omitted and kH (Huggins’ coefficient) is a hydrodynamic measure of the intensity of the polymer-polymer interactions. A drawback of our experimental technique is that the measurements have been carried out at relatively high shear rates (depending on polymer and surfactant concentrations and temperature, average shear rates in the range 350-950 s-1 have been covered) in the capillary viscometer. The repetitions of some experiments with capillaries of different size suggested only a slight shear rate dependence of the viscosity. However, as will be discussed below, most of this effect has declined at the fairly high shear rates operating in the capillary viscometers utilized in this study.
Results and Discussion The shear rate dependence of the viscosity is a feature that is frequently observed in solutions of associating polymers. In Figure 1, the effect of shear rate on dilute (0.05 wt %) aqueous solutions of EHEC in the presence of various amounts of SDS is illustrated over a broad shear rate domain (measurements from both the Bohlin rheometer and the Ubbelohde viscometers are included). The results reveal a strong shear-thinning effect up to a shear rate of about 10 s-1 for the solutions with low levels with surfactant addition, and at higher shear rates only a weak shear rate dependence of the viscosity is detected. The weak shear rate effect is similar to that reported37 previously from capillary viscosity measurements on dilute aqueous solutions of a different fraction of EHEC in the absence of surfactant. The data at high shear rates (solid squares) are obtained from the Ubbelohde viscometers and represent the shear rate domain covered by our capillary viscometers. At high surfactant concentrations the shear rate dependence of the viscosity is less pronounced, probably due to the formation of nonaggregated coils. At high levels of surfactant addition, deaggregation of the EHEC chains occurs because enhanced adsorption of SDS to EHEC promotes electrostatic repulsion between adsorbed SDS clusters and amended thermodynamic conditions. These results suggest that, in the shear rate range where the present capillary viscometers operate, the association complexes are disrupted by the shear rate, and the polymer chains behave essentially as isolated coils. The concentration dependence of the reduced viscosity for EHEC in the presence of low levels of surfactant (see Figure 2a-e) and at high levels of SDS addition (see Figure 2f,g) is linear as described by eq 3. The validity of eq 3 is usually observed for nonionic and nonassociating polymers in dilute solutions. At low SDS concentrations, where no or very little surfactant is bound to the polymer, EHEC can be expected to behave as a nonionic polymer. A similar behavior can also be expected at high surfactant concentrations due to screening of electrostatic interactions. Effects from sodium ions, free micelles, and free SDS contribute to the total electrostatic screening. It is evident from Figure 3 that a more complex picture emerges at intermediate levels of surfactant addition (4-12 mm SDS). In EHEC solutions with 4 mm (Figure 3a) and 6 mm (Figure 3b) SDS the plots of the reduced viscosity versus polymer concentration exhibit upward curvatures at higher EHEC concentrations, and this trend of nonlinearity is gradually more accentuated as the temperature decreases. The linear representations obtained by plotting ηsp/w versus w2 (see the inset plots in Figure 3a,b) suggest that the second-order term in concentration is decisive (37) Manley, R. St. J. Ark. Kemi 1956, 9, 519.
Figure 1. Illustration of shear rate dependence (at 20 °C) of dilute (0.05 wt %) EHEC solutions with the indicated surfactant concentrations. The three solid squares at the highest shear rates represent the shear field covered by measurements from the capillary viscometers.
for the concentration dependence of the reduced viscosity. The deviation from linearity as predicted from eq 3 is probably due to an intricate interplay between hydrophobic interactions and polyelectrolyte effects. In dilute aqueous solution, the dimension of the amphiphilic EHEC chain, and accordingly its viscosity, is governed by two opposing effects: the Coulombic repulsive forces between adsorbed SDS clusters and the attractive interactions between its hydrophobic segments. The type of curvature displayed in Figure 3a,b is usually attributed to enhanced intermolecular associations, often observed38-41 in solutions of hydrophobically associating polymers. An explanation of the behavior depicted in Figure 3a,b may be that at low temperatures (improved thermodynamic conditions) the coils are expanded, and this effect facilitates intermolecular contacts as the polymer concentration increases. At higher temperatures, the coils shrink (poorer thermodynamic conditions), and this reduces the tendency of intermolecular contacts. For EHEC solutions with 8 mm SDS (Figure 3c), the results demonstrate an upsweep in reduced viscosity at low polymer concentrations, an effect that is well-known for polyelectrolytes. At a given surfactant concentration, the charge density of the polymer chains increases with decreasing polymer concentration and thereby the strength of the polyelectrolyte effect (expansion of the polymersurfactant complexes due to repulsive forces). The upsweep in reduced viscosity becomes gradually less marked as the temperature rises, probably due to enhanced attractive intramolecular interactions that reduce the polyelectrolyte effect. The intriguing downward curvature observed at the highest polymer concentrations in the presence of 12 mm SDS (Figure 3d) can be rationalized in the following scenario. At low EHEC concentrations (high values of the surfactant-to-polymer ratio), the saturation of polymer chains with surfactant results in an excess surfactant concentration (the ionic strength increases) in solution, whose counterions screen the electrostatic interactions and thereby offset the polyelectrolyte effect. At higher (38) Sivadasan, K.; Somasundaran, P. Colloids Surf. 1990, 49, 229. (39) L’Alloret, F.; Hourdet, D.; Audebert, R. Colloid Polym. Sci. 1995, 273, 1163. (40) Nishikawa, K.; Yekta, A.; Pham, H. H.; Winnik, M. A. Langmuir 1998, 14, 7119. (41) Cathe´bras, N.; Collet, A.; Viguier, M.; Berret, J.-F. Macromolecules 1998, 31, 1305.
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Langmuir, Vol. 17, No. 1, 2001 31
Figure 2. Variation (linear concentration dependence according to the Huggins’ equation) of the reduced viscosity as a function of polymer concentration for dilute EHEC/water/SDS solutions at the temperatures and surfactant concentrations indicated.
polymer concentrations, the surfactant-to-polymer ratio decreases, and the polyelectrolyte effect (curvatures in Figure 3d) comes into play. Effects of surfactant addition and temperature on the intrinsic viscosity for EHEC are depicted in Figure 4. The
intrinsic viscosity can be considered as a measure of the hydrodynamic volume ([η] ∝ S3/M, where S is the radius of gyration and M is the molecular weight of the polymer42) of the chains at infinite dilution and thus indirectly reflects the size of the molecules. The most prominent feature in
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Figure 3. Effects of polymer concentration, levels of surfactant addition, and temperature on the reduced viscosity for the EHEC/ water/SDS system. The inset plots illustrate the linear change of the reduced viscosity as a function of w2.
Figure 4. (a) Effect of surfactant concentration on the intrinsic viscosity for the EHEC/water/SDS system at the temperatures indicated. (b) Temperature dependence of the intrinsic viscosity for the EHEC/water/SDS system at the surfactant concentrations indicated.
Figure 4a is the dramatic drop of [η] at surfactant concentrations close to the cac. A drop of [η] during the formation of polymer-surfactant complexes has been (42) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
reported11,20,26 for other EHEC fractions in the presence of SDS. However, in contrast to the present study, [η] has been observed20 to pass through a maximum at low levels of SDS addition at elevated temperatures (25 and 30 °C). A close inspection of the location of the pronounced decrease of [η] in Figure 4 reveals a shift toward lower SDS concentrations as the temperature rises, and this behavior is consistent with the reported17 decrease of cac with increasing temperature for this EHEC fraction. The marked drop of [η] indicates a strong contraction of the polymer-surfactant entities at the onset of surfactant binding to the polymer. At this stage, micelles begin to form inside the macromolecule, accompanied by collapse of the polymer chain as a result of the cooperative binding of surfactant molecules.5 In a recent theoretical study,43 it was suggested that in a flexible polymer-surfactant system the cac is associated with a considerable change in polymer statistics, and polymer-surfactant interaction leads to an effective reduction in the second virial coefficient of the polymer. As a result of this, the polymer is predicted to undergo partial collapse at the cac. Furthermore, in a theoretical study44 of binding of small molecules (surfactant molecules) to semiflexible polymers (EHEC chains may be considered as semiflexible), it was argued that bound molecules may modify the local characteristics of polymer conformation, e.g., change its local stiffness. A modified chain stiffness may drive a coil-to-globule collapse. The results in Figure 4a show that the polymer-surfactant complexes exhibit a contraction, and this effect seems to be associated with the value of cac. In this context we may note a recent viscosity study45 on dilute aqueous solutions of a hydrophobically modified (hydroxypropyl)guar poly(43) Diamant, H.; Andelman, D. Europhys. Lett. 1999, 48, 170. (44) Diamant, H.; Andelman, D. Phys Rev. E 2000, 61, 6740. Diamant, H.; Andelman, D. Macromolecules 2000, 33, 8050. (45) Aubry, T.; Moan, M.; Argillier, J.-F.; Audibert, A. Macromolecules 1998, 31, 9072.
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mer in the presence of a nonionic surfactant (Triton X-100). For this system an abrupt drop of the intrinsic viscosity was observed at surfactant concentrations close to the critical micelle concentration of this surfactant in pure water. This effect was explained in terms of intense intramolecular hydrophobic association junctions. The pronounced drop of [η] observed for this nonionic polymersurfactant system suggests that this phenomenon is not demarcated to systems with a polyelectrolyte effect. It is interesting to compare the marked drop of the intrinsic viscosity for the EHEC-SDS systems with the features reported15 from a dynamic light scattering (DLS) study on dilute solutions (0.1 wt %) of the same EHEC fraction in the presence of SDS. In the DLS work, the hydrodynamic radius was found to pass through a maximum, located at the cac. This effect was ascribed to the formation of large polymer-surfactant clusters around cac, and deaggregation of the EHEC-SDS clusters was observed at higher levels of surfactant addition. An explanation of the difference in behavior can be that the association complexes break up when exposed to the shear field operating in capillary viscometers, while in DLS (at the considered polymer concentration) the intramolecular contraction is overshadowed by the intermolecular aggregation process. At higher surfactant concentrations, we can see that [η] rises at all temperatures, and this conformational expansion is probably a result of enhanced electrostatic repulsion and excluded-volume effects due to improved thermodynamic conditions. At this stage, the solution can be considered to consist of necklaces46 made of EHEC chains loaded with bound micelles. At very high levels of surfactant addition, a slight decrease of [η] can be discerned, and this shrinkage of the polymer-surfactant complexes can probably can be traced to electrostatic screening. The temperature dependence of the intrinsic viscosity at various levels of added surfactant is displayed in Figure 4b. The general picture that emerges is that [η] falls off with increasing temperature at low surfactant concentrations (0-4 mm); the effect seems to be more pronounced at surfactant concentrations around the cac, while at higher SDS concentrations [η] is virtually independent of temperature. The decrease of [η] with increasing temperature can be rationalized in terms of deteriorated thermodynamic conditions at elevated temperatures for EHEC solutions with low levels of surfactant. At low surfactant concentrations, the thermodynamic conditions become poorer as the temperature rises in the considered temperature interval (the cloud point is approached15), and as a result of weakened excluded-volume effect a contraction of the coils is expected. As discussed above, the shrinkage of the polymer-surfactant complexes is most marked close to the onset of surfactant binding to the polymer, and thus the temperature dependence of [η] should be most pronounced under these conditions. At higher levels of surfactant addition, the cloud point is far above15 the temperatures covered in this work, and hence the impact of temperature on the thermodynamic conditions and on [η] can be anticipated to be of less importance. For flexible macromolecules in good solvents the Huggins’ coefficient kH usually takes values of about 0.30.5.47 High values of kH are observed in poor solvents or in the case of enhanced coil-coil interactions. In studies of amphiphilic polymers, very high values of kH are used (46) Lindell, K.; Cabane, B. Langmuir 1998, 14, 6361. (47) Bohdanecky´, M.; Kova´r, J. In Viscosity of Polymer Solutions; Jenkins, A. D., Ed.; Elsevier: Amsterdam, 1982; Vol. 2.
Langmuir, Vol. 17, No. 1, 2001 33
Figure 5. Effect of surfactant concentration on the Huggins’ coefficient for the EHEC/water/SDS system at the temperatures indicated.
as an indicator of hydrophobic associations. The effects of surfactant concentration and temperature on the Huggins’ coefficient are displayed in Figure 5. The most salient feature is the marked peaks located at moderate surfactant concentrations. A close scrutiny of the data reveals that the peaks are more pronounced at elevated temperatures, and the position of the maximum of the peak is shifted toward lower SDS concentrations as the temperature is increased. The maxima coincide with the respective minima of the intrinsic viscosity (cf. Figure 4a). At very low SDS concentrations (up to 2 mm) and high levels of surfactant addition (8-50 mm) kH assumes “normal” values at all considered temperatures. These findings suggest that strong polymer-polymer attractive interactions prevail in the initial stage of surfactant binding to the polymer and that these interactions are strengthened at higher temperatures. At finite polymer concentrations, we may expect that elevated temperatures will promote the evolution of enhanced intermolecular hydrophobic interactions. The shift of the maximum of kH toward lower surfactant concentrations with increasing temperature has also been observed previously20 for a similar EHEC/ SDS system. However, in the above cited work the maximum was found to become more pronounced with decreasing temperature, which is in contrast to the present finding. This behavioral difference may be traced to differences between the EHEC samples used. We may note that the previous mentioned viscosity study45 on dilute aqueous solutions of a hydrophobically modified (hydroxypropyl)guar polymer also revealed a marked increase of Huggins coefficient at surfactant concentrations close to the critical micelle concentration of this surfactant. This effect was attributed to intermolecular hydrophobic associations. To gain further insight into the effect of polymer concentration on the viscosity properties, the reduced viscosity is plotted versus surfactant concentration in Figure 6 for various finite polymer concentrations and at different temperatures. As expected, the general behavior is reminiscent of that observed for the intrinsic viscosity (cf. Figure 4), except for a significant difference, namely the polymer concentration-induced effect on the reduced viscosity around 3-4 mm SDS. This feature seems to be strongest marked at 15 °C, but it is conspicuous also at the other temperatures. Since the magnitude of this effect is strengthened at higher polymer concentrations, the conjecture is that the effect is associated with enhanced
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Figure 6. Effect of surfactant concentration on the reduced viscosity for the EHEC/water/SDS system at the temperatures and polymer concentrations indicated.
intermolecular interactions. It appears that strong association complexes are formed at higher EHEC concentrations and that these are not disrupted by the shear rates operating in the capillary viscometer. At higher levels of surfactant addition (above 6 mm), the association aggregates break up and the concentration-induced effect disappears. The marked maxima observed at higher polymer concentrations in Figure 6a are similar to those reported11,13,20,26 for different EHEC fractions in the presence of SDS. Conclusions In this work, we have studied the effects of temperature and SDS addition on the viscosity properties of dilute solutions of the nonionic cellulose ether EHEC. The intricate interplay between hydrophobic associations and polyelectrolyte effects is evidenced by the reduced viscosity results. At low polymer concentrations, the polymersurfactant complexes seem to be molecularly dispersed at the shear rates, temperatures, and surfactant concentrations considered in this work. The EHEC polymer interacts with SDS at low levels of SDS addition and forms micelle-like clusters. The EHEC-
SDS complexes exhibit a collapse over a narrow surfactant concentration region slightly above the critical aggregation concentration. At low surfactant concentrations, a temperature-induced shrinkage of the molecules is detected, while at higher amount of surfactant addition no temperature dependence of the intrinsic viscosity is found. The molecules expand at moderate levels of surfactant addition due to improved thermodynamic conditions and enhanced electrostatic repulsions. At very high surfactant concentrations, a slight contraction of the polymer-surfactant complexes occurs, and this finding is attributed to screening of the polyelectrolyte effect. The Huggins’ coefficient increases with temperature and attains high values in the vicinity of the critical aggregation concentration. This behavior, which is typical for associating systems, reflects some intercoil interactions. Another evidence of intermolecular associations at finite polymer concentrations is the concentration-induced enhancement of the reduced viscosity observed at surfactant concentrations around cac. LA001175P
