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Temperature Dependence of Hydrodynamic Properties and Surfactant-Polymer Interaction in Solution. The EHEC/ SDS/Water System Christina Holmberg and Lars-Olof Sundelo¨f* Physical Pharmaceutical Chemistry, Uppsala University, Uppsala Biomedical Centre, P.O. Box 574, S-75123 Uppsala, Sweden Received February 14, 1995. In Final Form: October 2, 1995X Hydrodynamic properties and interactions between ethyl(hydroxylethyl)cellulose (EHEC) and sodium dodecyl sulfate (SDS) in water solution are discussed as a function of temperature in a composition interval of the surfactant ranging from zero up to well above the critical micelle concentration (cmc) and for the polymer concentration from zero up to slightly above the concentration of critical overlap, at temperatures between 20 and 34 °C, which is the limit of phase separation. The results from viscometric measurements, equilibrium dialysis, cloud point determinations, and conductometric measurements show a strong interaction when small amounts of SDS are added to an EHEC solution. For solutions with polymer concentrations in the range between 0.15 and 0.25% and a total SDS concentration of about 2 mM, a large increase in reduced viscosity is observed. The reduced viscosity passes through a maximum, followed by a marked decrease for SDS concentrations >5 mM. Further additions of surfactant to the polymer solution result in about half the value of the reduced viscosity displayed by the pure polymer solution. The SDSEHEC composition, which shows an increase in reduced viscosity, corresponds to the onset of surfactant redistribution. These effects persist at elevated temperatures although they are shifted to somewhat lower SDS concentrations, but a second maximum in reduced viscosity develops upon raising the temperature when the concentration of the added SDS is in the vicinity of the surfactant cmc. At all temperatures the equilibrium dialysis shows that the redistribution increases with added SDS until normal micelles begin to form in the bulk solution. The intrinsic viscosity was found to decrease with increasing amount of redistributed SDS to EHEC. The investigation is part of a study concerning thermodynamic and hydrodynamic interactions in systems containing amphiphiles and uncharged polymers1-4 of interest for applications, for instance, in drug formulations.
Introduction The problem of interaction between amphiphiles and uncharged polymers has been treated by several investigators5-19 and a number of physicochemical methods have been utilized. However, the EHEC/SDS/water system is somewhat special due to the strong interactions both between polymer coils and between the polymer and the amphiphile.1-4,20 The interaction between EHEC and SDS is particularly important in a composition interval containing the normal cmc point of the amphiphile. This indicates a close connection between cluster (micelle) X
Abstract published in Advance ACS Abstracts, January 1, 1996.
(1) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelo¨f, L.-O. J. Phys. Chem. 1992, 96, 8. (2) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1994, 272, 338. (3) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1995, 273, 83. (4) Holmberg, C.; Nilsson, S.; Sundelo¨f, L.-O. Submitted for publication in Langmuir. (5) Lindman, B.; Thalberg, K. In Interaction of Surfactants with polymers and proteins, 1st ed.; Goddard, E. D., Ed.; CRC Press: Boca Raton, FL, 1993; Chapter 5. (6) Piculell, P.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (7) Gilany, T.; Wolfram, E. Colloids Surf. 1981, 3, 181. (8) Nagarajan, R. Colloids Surf. 1985, 13, 1. (9) Breuer, M. M.; Robb, I. D. Chem. Ind. 1971, 1, 530. (10) Sakamoto, N. Polymer 1987, 28, 288. (11) Goddard, E. D. Colloids Surf. 1986, 19, 255. (12) Shirahama, K.; Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987, 296, 96. (13) Lindman, B.; Carlsson, A.; Carlstro¨m, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (14) Winnik, F. M.; Winnik, M. A. Polym. J. 1990, 22, 482. (15) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (16) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (17) Gilanyi, T. Acta Chem. Scand. 1973, 27, 729. (18) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (19) Microdomains in Polymer solutions; Dubin, P., Ed.; Plenum Press: New York, 1985. (20) Carlsson, A. Nonionic cellulose ethers-Interactions with surfactants, solubility and other aspects. Dissertation, University of Lund, 1989.
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formation and the interactions observed.3 In previous publications the thermodynamic and hydrodynamic properties of the system have been investigated for constant temperature, i.e., at 20 °C.1,3 A molecular model has been suggested for the interaction proposing a clustering adsorption as the basic step, which at elevated polymer concentrations is followed by polymer metwork formation where the tie points consist of hydrophobic side chains solubilized in adsorbed clusters.3 The system also shows a pronounced time dependence of its properties. This has been thoroughly discussed in a previous paper.2 The system EHEC/water has a cloud point (CP), the temperature where the polymer phase separates, which decreases rapidly with increasing polymer concentration indicating in the binary case a balance between hydrophobic and hydrophilic effects. This strong temperature sensitivity of interaction is further promoted by the addition of amphiphile5,6 which by redistribution to the polymer chain changes the CP. Thus it becomes of interest to investigate the temperature dependence of the system properties. The most conspicuous macroscopic sign of interaction in the EHEC/SDS/water system is the dramatic increase in viscosity as the polymer concentration is increased in the presence of surfactant. Very high viscosities are observed for EHEC concentrations as low as 0.2 or 0.5%. At still higher polymer concentrations the system approaches the appearance of a gel. The viscosity effect is dramatically enhanced even at low polymer content by the addition of salt.20 Gelling occurs upon raising the temperature in solutions containing high EHEC concentrations and certain amounts of SDS.21 In the present study the viscosity of polymer-surfactant solutions with low EHEC concentrations has been measured, and knowing the amount of SDS redistributed to EHEC,4 the Huggins constant and the intrinsic viscosity for the polymer-surfactant complex could be calculated1 at © 1996 American Chemical Society
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different temperatures. On the basis of these parameters the temperature effects on the system properties are discussed in terms of first-order coil-coil interactions and the characteristics of individual polymer molecules. From a pharmaceutical point of view the EHEC/SDS/ water system is interesting since it provides a medium which could be a liquid at room temperature but a gel at body temperature (37 °C) if the chemical composition of the polymer is properly adjusted. Although such qualitative properties, important for many applications, have been known for some time, their physicochemical origin has remained unclear. In the set of papers already mentioned1-4 a molecular model was presented to serve as a basis for possible applications, for instance, in drug release formulations. Experimental Section Materials. The following chemicals were used: sodium dodecyl sulfate (SDS), 99.9% pure (used as supplied), Lot 733L488460, Batch 1.58553-100, Merck, Spa˚nga Sweden; radioactive SDS, 35S, Lot JC 1981, Batch 8803, Amersham, England; ethyl(hydroxyethyl)cellulose (EHEC) fraction CST-103 Mn ) 480 000, MSeo ) 0.7, DSethyl ) 1.5, L920 32, Berol Kemi, Stenungsund, Sweden. The EHEC stock solution, approximately 1.5% by weight, was freed from the remaining salt by dialysis in tube membranes (molecular weight cut-off approximately 10 000) from Union Carbide, Chicago IL. The stock solution of EHEC was filtered through 0.8 mm Millex-AA filters, Millipore SA, Molsheim, France. All solutions were made with MilliQ water (Millipore) as solvent. Preparation of Solutions. Viscosity. Solutions for viscosity studies at 20 °C were prepared by weighing the required amounts of the EHEC stock solution into approximately diluted SDS stock solutions. All samples were prepared 24 h in advance to let the time dependent effects settle.2 The stock solutions were prepared according to standard procedures described elsewhere.1 The solutions for measurements at 25-34 °C were put in a thermostated water bath immediately after mixing. These solutions were left for 48 h to attain equilibrium properties, since they showed time effects extending over a slightly longer period of time than those at 20 °C.2 Cloud Point and Conductivity. Solutions for cloud point and conductivity measurements were prepared as described above for viscosity at 20 °C. All samples were prepared 24 h in advance to let the time dependent effects settle.2 Dialysis. A dialysis experiment contained a polymer solution with a certain concentration and a set of SDS solutions. The polymer solution was prepared by weighing an amount of EHEC stock solution into water to the desired concentration. The surfactant solutions were prepared by diluting the SDS stock solution with appropriate amounts of water obtaining solutions with SDS concentrations from 1 to 40 mM. Small amounts of radioactive SDS was added until the activity equaled 50 000 dpm/mL. One such series was made for each polymer concentration at all temperatures. Methods. Viscosity. The viscometric measurements were carried out in ordinary Ostwald capillary viscometers with a flow time for water of approximately 100 s at 20 °C. The samples were thermostated in the viscometer in a water bath for 15 min before measurements were made. It was estimated that corrections for kinetic and end effects were unnecessary.22 Cloud Point. The cloud points of the EHEC and EHEC-SDS solutions were determined by visual observation in glass tubes and taken as the temperature when the last visible sign of clouding in the solution disappeared upon cooling. Conductometry. The solutions prepared for conductometry were taken directly from a rotating table, one at a time, and put straight into a thermostated, water jacketed compartment for measurement of the conductivity with a Metrohm conductometer and immersion-type measuring cell. The conductivity (4-6 µS/ cm) of the pure polymer solutions was subtracted from the measured values to obtain the conductivity of the amphiphile.4 (21) Carlsson, A.; Bogentoft, C.; Lindman, C.; Andersson, L. Proceed. Intern. Symp. Control. Bioact. Mater. 1991, 18, 445. Controlled Release Society, Inc. (22) Barr, G. A monograph of viscometry; Humphrey, M., Ed.; Oxford, University Press: London, 1931.
Figure 1. Cloud point temperature as a function of EHEC concentration in absence of SDS. Density. Solution densities were determined in a digital densiometer DMA 02C, Anton Paar K.G., A 8054 Graz, Austria, according to Kratky et al.23 The accuracy is better than (0.0015 kg/m3. Dialysis. The equilibrium dialysis experiments were carried out in specially designed cells, with retentate and dialysate compartments separated by a SPECTRA/POR membrane (molecular weight cut-off 12000-14000). The cell design was similar to the one used by Fishman and Eirich.24 Before being filled, the cells were thoroughly rinsed with deionized water. The water was then removed and the compartments were filled with the actual measuring solutions. Each series of experiments was performed with the same polymer concentration (constant polymer concentration) in the retentate compartment of the dialysis cells and surfactant solutions with concentrations ranging from 1 to 40 mM SDS into the respective dialysate compartments. For each composition (EHEC/SDS) the experiments were made in triplicate. The cells were left to equilibrate in an air thermostat for a week. Results from experiments performed to determine the time requested for the system to equilibrate showed that 4 days were sufficient, although 7 days were allowed as a precaution.4 The concentration of SDS in the EHEC-SDS and SDS solutions, respectively, were determined by scintillation counting. Test experiments indicated that the presence of EHEC did not cause an extinction of the scintillation, and the volumes of scint cocktail were chosen such as to avoid phase separation of the polymer.
Results Cloud Point. The EHEC/water system is, for the EHEC sample CST-103 used here, characterized by a lower consolute point somewhat above room temperature. Part of the phase separation curve is given in Figure 1 as determined by cloud point measurements. A limited portion of the polymer concentration axis is seen since the cloud point measurements were only performed up to about 1.5% (w/w), but cloud point curves of other EHEC fractions continue to flatten out as the polymer concentrations become larger.20 For the same polymer/solvent system the exact position of the phase equilibrium curve is determined by the molecular weight and by the molecular weight distribution of the polymer.25 Since the cloud point for the binary EHEC/water system showed a strong dependence on polymer concentration, the effect of added amphiphile was investigated for two EHEC concentrations, 0.05% and 0.20% EHEC, respectively, and (23) Kratky, O.; Leopold, H., Stabinger, H. Methods in enzymology; Hirs, C. H. W., Timasheff, S. N., Eds.; Academic Press: New York, 1973; Vol. 27. (24) Fishman, M. L.; Eirich, F. R. J. Phys. Chem. 1971, 75, 3135. (25) Flory, F. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (26) Bohdanecky, M.; Kovar, J. Viscosity of Polymer Solutions; Elsevier: Amsterdam, 1982.
Temperature Dependence of Surfactant-Polymer Interaction
Figure 2. Cloud point temperature as a function of total SDS concentration for EHEC solutions: O, 0.05% EHEC; 2, 0.20% EHEC.
the cloud point was determined as a function of SDS concentration. When the amphiphile SDS is added, the phase separation (cloud point) curve is shifted upward, initially at a rate proportional to [SDS]tot. After [SDS]tot
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≈ 4 mM, the increase becomes very steep, however. These curves seem to have the same shape for both polymer concentrations, as seen from Figure 2. There is an obvious difference in CP between 0.05% and 0.20% EHEC solutions containing the same amount of SDS. Intermediate polymer concentrations ranging from 0.075 to 0.13% showed results very close to that of 0.05%. Several experimental observations (see also below) tend to indicate that system properties will change in a pronounced way when the polymer concentration becomes higher than 0.10-0.15% (w/w). Viscosity. Experiments were performed with sets of constant polymer concentrations ranging from 0.05% to 0.25% (the latter exceeding the concentration of critical overlap, cp* ≈ 0.23% for the pure polymer1) and varying the SDS concentration from 1 to 12 mM and for tempertures in the range 20-34 °C. At elevated temperatures compositions with small amounts of SDS and high polymer concentrations were omitted to avoid phase separation. The reduced viscosity, ηred, for each SDS/EHEC/water solution was calculated according to standard procedures as described in refs 25 and 26 using as “solvent” viscosity that of the corresponding SDS/water solution. The viscosity data in Figure 3 show the dramatic evolution of the reduced viscosity for constant EHEC concentrations
a
b
c
d
Figure 3. (a) Experimental results for ηred (mL/g) in the EHEC/SDS/water system as a function of the total SDS concentration for different EHEC concentrations at 20 °C: b, 0.05% EHEC; 0, 0.10% EHEC; +, 0.15% EHEC; O, 0.20% EHEC; 2, 0.25% EHEC. (b) Experimental results for ηred (mL/g) in the EHEC/SDS/water system as a function of the total SDS concentration for different EHEC concentrations at 25 °C: O, 0.05% EHEC; +, 0.075% EHEC; 9, 0.10% EHEC; ×, 0.13% EHEC; 0, 0.18% EHEC; 2, 0.20% EHEC; b, 0.25% EHEC. (c) Experimental results for ηred (mL/g) in the EHEC/SDS/water system as a function of the total SDS concentration for different EHEC concentrations at 30 °C: O, 0.05% EHEC; +, 0.075% EHEC; 9, 0.10% EHEC; ×, 0.13% EHEC; 0, 0.18% EHEC; 2, 0.20% EHEC; b, 0.25% EHEC. (d) Experimental results for ηred (mL/g) in the EHEC/SDS/water system as a function of the total SDS concentration for different EHEC concentrations at 34 °C: O, 0.05% EHEC; +, 0.075% EHEC; 9, 0.10% EHEC; ×, 0.13% EHEC; 2, 0.20% EHEC.
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a
Figure 4. Equilibrium dialysis data for 0.10% EHEC, plotted as y (millimoles SDS redistributed per gram EHEC) versus [SDS]eq at different temperatures: +, 20 °C; O, 25 °C; 2, 30 °C; 0, 34 °C.
with varying amounts of SDS due to strong interactions in certain composition intervals, most marked in a middle range of SDS concentration. In the concentration interval with [SDS]tot between 3 and 6 mM the reduced viscosity is very sensitive to the polymer concentration, the amount of SDS present, and the temperature. This can be observed in parts c and d of Figure 3, where a maximum for the lower polymer concentrations at 30 and 34 °C develops when small amounts of SDS are present, and an inflection or a shallow minimum can be seen at 20-25 °C, parts a and b of Figure 3. The viscosity for higher polymer concentrations in this region shows a marked maximum and is shifted somewhat toward lower SDS concentrations with increasing temperature. At low SDS concentrations at 20 and 25 °C the reduced viscosity is rather insensitive to the amount of SDS present, whereas at high SDS concentrations the reduced viscosity is rather insensitive to changes in SDS concentration and temperature. Dialysis. Equilibrium dialysis experiments show how much surfactant is bound to the polymer as a function of the composition. The data presented in Figure 4 give the amount of millimoles of SDS “bound” per gram of polymer, y, compensated for the Donnan equilibrium, plotted versus the equilibrium concentration of SDS at various temperatures. The compensation for the Donnan effect was performed as described in ref 1. The redistribution curves for different polymer concentrations4 and temperatures show quite similar characteristics. The curve in Figure 4 may be divided into three regions along the SDS concentration axis. (I) At low surfactant concentrations no interactions occurs. (II) At 2 mM SDS the adsorption starts abruptly at a foot point followed by a steep increase of redistributed SDS, implying a cooperative mechanism,11 up to a maximum which differs slightly with polymer concentration and temperature. (III) After maximum adsorption, desorption seem to occur, possibly an effect of ordinary micelles being formed at this surfactant concentration, solubilizing the polymer. In region II where the steep increase in adsorption appears, a “breakpoint” can be observed. This breakpoint comes out more clearly if KD ) y/[SDS]eq is plotted versus log[SDS]eq, see Figure 5. KD is an apparent distribution coefficient of surfactant between the aqueous bulk solution and polymer phase.27 The breakpoint is more pronounced at 20 °C than at 30 °C. At this higher temperature it seems to disappear if polymer concentration, cp, is reduced. A rise in temperature causes a slightly decrease in the maximum amount of surfactant bound to the polymer, otherwise the adsorp-
b
Figure 5. (a) Redistribution coefficient, KD, plotted versus log[SDS]eq at 20 °C: +, 0.05% EHEC; 9, 0.10% EHEC; 2, 0.20% EHEC. (b) Redistribution coefficient, KD, plotted versus log[SDS]eq at 30 °C: +, 0.05% EHEC; 9, 0.10% EHEC; 2, 0.20% EHEC.
Figure 6. Conductivity, κ, of SDS/EHEC/water solutions as a function of total SDS concentration at 30 °C: +, pure SDS solution; 9, 0.10% EHEC; 4, 0.20% EHEC.
tion behavior does not seem to change much with the temperature in this interval. Conductivity. From conductivity measurements performed at different temperatures, see Figure 6 and Table 1, it can be concluded that the presence of the polymer will increase the degree of dissociation of the amphiphile. The degree of ionization of the surfactant-polymer
Temperature Dependence of Surfactant-Polymer Interaction
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Figure 7. Intrinsic viscosity (mL/g) as a function of y (millimoles of SDS redistributed per gram polymer) at different temperatures: b, 20 °C; 2, 25 °C; 4, 30 °C.
Figure 8. Huggins’ constant, kH (dimensionless unit) plotted versus y (millimoles of SDS redistributed per gram polymer) at different temperatures: b, 20 °C; 2, 25 °C; 4, 30 °C.
Table 1. Summary of Calculated Values of the Degree of Ionization, r, at Different Polymer Concentrations for the SDS/EHEC/Water System at 20 and 30 °C
Discussion
temp, °C
EHEC, %
R
20
0 0.05 0.10 0.15 0.20
0.39 0.75 0.74 0.70 0.69
temp, °C
EHEC, %
R
30
0 0.05 0.10 0.15 0.20
0.39 0.79 0.77 0.74 0.71
complex and normal micelles were calculated according to standard procedures.28-31 Data Treatment The dialysis data together with the reduced viscosity of the system were used to calculate the intrinsic viscosity, [η], of the SDS-EHEC complex. The intrinsic viscosity is a measure of the hydrodynamic volume of the polymer at infinite dilution and is obtained by extrapolating the reduced viscosity to zero polymer concentration. Since the surfactant-polymer complex is in equilibrium with free surfactant, the equilibrium composition of the complex will alter when the solution is diluted with regard to the polymer keeping the surfactant concentration constant. To calculate the intrinsic viscosity of the SDS-EHEC complex, the reduced viscosity, ηred, was recalculated to constant amount of redistributed SDS, i.e., constant y, and as a function of polymer concentration by combining the viscosity data from Figure 3a-c and dialysis results using the expression1
[SDS]tot ) cpy + [SDS]eq
(1)
The intrinsic viscosities obtained for constant y at different temperatures are plotted in Figure 7. At finite polymer concentrations the polymer-polymer interaction is of importance, and this effect can be illustrated by Huggins’ constant kH, defined by the following expression
ηred ) [η] + kH[η]2cp + ...
(2)
where higher order terms have been omitted. kH as calculated from (2)1,25,26 is seen as a function of y at different temperatures in Figure 8. Due to uncertainties in the extrapolation of the nonlinear curves normalized to constant y, the values of the intrinsic viscosities and Huggins’ constant contains an error of about 10-15%.1 (27) Shirahama, K.; Oh-Ishi, M.; Takisawa, N. Colloid Surf. 1989, 40, 261.
In this work the hydrodynamic behavior of the EHECSDS system at different temperatures has been investigated by means of viscosity. The results from these measurements may well be correlated to the amount of surfactant redistribution to the polymer and the cloud point of the system. At low SDS concentrations at 20 and 25 °C the reduced viscosity is rather insensitive to the amount of SDS present, but ηred decreases at higher temperatures in this interval. This might be caused by incipient phase separation as the cloud point is approached and the fact that elevated temperatures result in a higher population of the nonpolar conformers of the polymer which prefer to interact with themselves, and less of the polar ones which prefer to interact with water.32 The maximum in ηred occurs in the same SDS composition where lower values of CP can be observed for the system: see Figure 2. Also, the maximum in ηred appears at the same SDS composition where a sharp rise in viscosity occurs before gels are formed with polymer concentrations ≈ 1% when the temperature is raised.21 At high SDS concentrations the reduced viscosity is rather insensitive to changes in SDS concentration and temperature. In this range of SDS concentrations the cloud point of the system “exceeds” 100 °C, and for higher polymer concentrations the ability of EHEC to form a gel vanishes.21 The steepest change of cloud point with increasing [SDS] seems to occur in a region where the rise in viscosity has its maximum; cf. Figure 3. From a quasi-thermodynamic point of view, one could interpret this in the sense that the quality of the solvent increases and that the excluded volume will increase. On the other hand the hydrodynamic volumesat least at high dilution with respect to polymersseems to decrease since the intrinsic viscosity decreases when [SDS] increases above a certain threshold value. This is inferred by the [η] vs y curves in Figure 7. Such a seemingly contradictory result is presumably related to a change in the state of solvation. In water the EHEC molecules have a very expanded structure which might approach a free draining situation. If SDS is added, clustering adsorption occurs and the solvation changes at the same time as the chain may become bulkier. Hence (28) Lo¨froth, J. E.; Johansson, L.; Norman, A.-C.; Wettstro¨m, K. Prog. Colloid Polym. Sci. 1991, 84, 73. (29) Botre´, C.; Crescenzi, U. C.; Mele, A. J. Phys. Chem. 1959, 63, 650. (30) Abu-Hamidiyya, M.; Al-Mansour, L. J. Phys. Chem. 1979, 83, 2236. (31) Evans, H. C. J. Chem. Soc. 1956, 585. (32) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962.
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Figure 9. Equilibrium concentration of SDS in the dialysis experiments where y has its maximum as a function of polymer concentration.
there could be both a decrease in chain expansion and a change toward a nondraining situation. Both these effects will bring down the measured value of [η]. If the real molecular volume is decreased cannot be concluded from hydrodynamic measurements alone. The minimum in [η] seems to be matched by a maximum in hydrodynamic interaction intensity, i.e. a maximum in kH; see Figures 7 and 8. As temperature is increased, the decrease in [η] will be less pronounced and the increase in kH will be more moderate. At some intermediate value of the adsorption parameter (y ≈ 2) a maximum in [η] and a minimum in kH begin to develop. The explanation for this is unclear but it correlates well with the “breakpoint” at y ≈ 2 in the dialysis curves; see Figure 4. At this breakpoint the rate of increase of adsorption becomes very steep. As already noted there is only a slight change in dialysis equilibrium when the temperature is raised from 20 to 34 °C; see Figure 4. If the [SDS]eq value for the maximum in the dialysis curves (y vs [SDS]eq or KD vs log[SDS]eq) is plotted versus polymer concentration for the 20 °C results, the points will converge to the normal cmc for SDS when cp f 0, see Figure 9. Furthermore the curve in Figure 9 displays a deflection at the polymer concentration where one shifts to the “strong increase regime” in viscosity; see Figure 3a. This “strong increase regime” seems to be fairly independent of temperature but the increase in viscosity becomes somewhat smaller as temperature is raised to 25 °C and then it increases again at 30 °C. The increase in ηred (when plotted vs [SDS]tot) up to the maximum seems to correlate well with the increase in kH up to its maximum value. From Figures 10 and 11 it is seen that the entire build up of the high viscosity takes place over a very short concentration interval in amphiphile from the foot point and up to a value where y ≈ 1, which corresponds to a rather low adsorption of amphiphile to polymer. This indicates that the viscosity increase is not connected with the adsorption saturation of the chain and that it is counteracted by some competing process in much the same way as the redistribution of amphiphile to polymer reaches a maximum (see the dialysis curves) after which it decays. In the viscosity case the explanation could be that after a certain degree of adsorptionof amphiphile the electrostatic repulsion between chain elements becomes so strong that the interactive network breaks down. At 30 °C a shoulder develops at about y ) 2, i.e., at the breakpoint, in the ηred vs [SDS]tot curve. From the conductivity measurements it can be concluded that the presence of the polymer will increase the
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Figure 10. Viscosity data, ηred (mL/g), and results from dialysis experiments, y (millimoles SDS redistributed per gram polymer), at 20 °C as a function of the total SDS concentration for 0.20% (w/w) EHEC: 2, ηred; b, y.
Figure 11. Viscosity data, ηred (mL/g), and results from dialysis experiments, y (millimoles SDS redistributed per gram polymer), at 30 °C as a function of the total SDS concentration at 0.20% (w/w) EHEC: 2, ηred; b, y.
degree of dissociation of the amphiphile; see Figure 6 and Table 1.28-31 This effect could have some influence on the electrostatic interaction between chains to which amphiphile has been adsorbed. The ion dissociation increase slightly with temperature, indicating that the decrease in y might be due to a smaller cluster size rather than fewer clusters. Conclusions From the experimental observations some general results can be outlined for the system in question. In the dialysis equilibrium between the amphiphile and the polymer a maximum adsorption (redistribution) is achieved at a SDS concentration not too far from the normal cmc point. This maximum is shifted to lower [SDS]eq values at higher polymer concentrations. For the hydrodynamic interaction, however, as observed for the macroscopic viscosity, there is also a very pronounced maximum in, for instance, ηred. This maximum develops only if the polymer concentration exceeds a certain minimum value (≈0.1% (w/w) or slightly higher) and in a very constant [SDS]tot interval corresponding to a very low adsorption (redistribution). For the system in question the viscosity maximum develops and decays in the SDS composition region from the foot point up to an adsorption of about y
Temperature Dependence of Surfactant-Polymer Interaction
) 1. At this low value of y, [η] approaches its minimum, implying that the size of the polymer is of minor importance to the viscosity effect. If the temperature is increased, there seems to develop a second but much weaker maximum at a composition corresponding to the “breakpoint” in the dialysis equilibrium curves; see Figures 4 and 5. This second maximum corresponds closely to the system composition where the most pronounced time dependent effects were observed.2 The fact that the viscosity maximum only develops if the polymer concentration has a certain minimum value is not surprising since the effect must somehow be linked to a considerable polymer-polymer interaction. In a qualitative way this is comparable to the formation of a certain number of “tie points”. The fact that a maximum is observed can most likely be attributed to the presence of two opposing processes. In previous publications it has been suggested that the tie points mentioned consist of amphiphile clusters shared by one or several polymer molecules.1-4 The probability of formation of such shared clusters should be proportional to the product of concentration of sites with and without adsorbed clusters and should also depend on the average spatial distribution of polymer segments in solution to facilitate interaction. Since the polymer is uncharged but the adsorbed moiety is (negatively) charged, the first part of the redistributionswith low aggregation numbersswill probably create a structure quite similar to a normal polyelectrolyte. This will distribute the polymer segments more evenly in solution thus increasing the probability of interaction. From our cluster size studies it is also apparent that there is a strong tendency to increase the cluster size up to some plateau value. This can be achieved either by “multiple” adsorption to a single site or by the interaction
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of two or more sites with low adsorption. The latter process becomes more likely in a more concentrated polymer solution. If this picture is correct, the process for build up of the high viscosity is quite different from the mechanisms behind the adsorption maximum. The latter is clearly connected with a saturation process interrupted by the competing formation of normal micelles.4 The interruption of the viscosity increase could in the same way be interpreted as a decrease in the interaction probability when the polymer chains become more saturated. It is probable that the increased charge and the high degree of dissociation of the amphiphile in the clusters also contribute by Coulombic interaction. The second feature of interest in the analysis of the hydrodynamic interaction is the second maximum in viscosity which is observed to correlate with the “breakpoint”. This breakpoint is characterized by a marked increase in adsorption as [SDS] is increased. At 20 °C this effect is observed for all polymer concentrations, at 30 °C it seems to be weakened as cp decreases. Hydrodynamically this second maximum differs from the first in the respect that the “hydrodynamic volume” parameter [η] increases and that the interaction parameter kH decreases rather than increases. This could be a reflection of an increase in molecular volume due to rapid increase of charges which is balanced out by further addition of dissociated amphiphile. Acknowledgment. Financial support from the Swedish Natural Science Research Council and from the Swedish Research Council for Engineering Science is gratefully acknowledged. LA950112H
