Langmuir 1996, 12, 5781-5789
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Temperature Effects on the Interactions between EHEC and SDS in Dilute Aqueous Solutions. Steady-State Fluorescence Quenching and Equilibrium Dialysis Investigations Hans Evertsson, Stefan Nilsson,* Christina Holmberg, and Lars-Olof Sundelo¨f Physical Pharmaceutical Chemistry, Uppsala University, Uppsala Biomedical Centre, Box 574, S-751 23 Uppsala, Sweden Received April 11, 1996. In Final Form: August 26, 1996X The interaction between the hydrophobic, nonionic cellulose derivative ethyl hydroxyethyl cellulose (EHEC; fraction CST-103) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied as a function of temperature from 20 to 50 °C in dilute aqueous solutions, i.e. a polymer concentration slightly below the critical overlap concentration (c*) and a surfactant concentration up to three times the normal critical micelle concentration (cmc). Methods utilized in this investigation include equilibrium dialysis and steady-state fluorescence quenching. The results show that the average aggregation numbers of the polymer-bound SDS clusters decrease with an increase in temperature although the magnitude of the effect is composition dependent and is most pronounced for compositions which give the largest cluster sizes. The adsorption of SDS to EHEC shows a break-point at an intermediate value of the adsorption isotherm above which the cooperativity increases. This break-point diminishes and disappears, i.e. the cooperativity decreases, as the temperature increases up to 50 °C. It is suggested that the mechanism responsible for these two steps in the adsorption process is at first adsorption of SDS to aggregated EHEC chains and then to a mainly deaggregated state of EHEC. The critical surfactant concentration where the adsorption to the polymer starts seems to be slightly shifted toward lower values as the temperature is raised from 20 to 50 °C. To summarize the results, the interaction between EHEC and SDS gets more intensive as the temperature is raised. Two fluorophore/quencher pairs, which previously have been used for determination of average aggregation numbers in aqueous surfactant and polymer-surfactant systems utilizing the steady-state fluorescence quenching technique, are compared. A good agreement between the two pairs is reported. Reference measurements of average aggregation numbers and adsorption isotherms for the PEO/SDS/water system are also given.
Introduction Complexes between ionic surfactants and nonionic polymers were identified more than 20 years ago and have since then been the subject of continued investigations. A number of reviews on this topic exist.1-5 The study of nonionic polymer-ionic surfactant systems is important both from a fundamental standpoint for obtaining an understanding of the mechanisms operating and structures formed in such solutions and from an applied point of view due to the numerous uses of these substances in, for instance, pharmaceutical formulations, cosmetics, and food products. In a series of publications6-9 the interaction between the nonionic cellulose ether ethyl hydroxyethyl cellulose (EHEC; fraction CST-103) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in this laboratory in dilute aqueous salt-free solutions, i.e. * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) 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. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (3) Robb, I. D. In Anionic SurfactantssPhysical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; Vol. 11, p 109. (4) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 13, 530. (5) Goddard, E. D. Colloids Surf. 1986, 19, 255. (6) Holmberg, C.; Sundelo¨f, L.-O. Langmuir 1996, 12, 883. (7) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelo¨f, L.-O. J. Phys. Chem. 1992, 96, 871. (8) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1994, 272, 338. (9) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1995, 273, 83.
S0743-7463(96)00345-9 CCC: $12.00
approximately up to the critical overlap concentration of the polymer (c*) and up to three times the normal critical micelle concentration (cmc) of the surfactant. The main goal has been to provide a basic understanding of this and similar systems and especially properties of importance for pharmaceutical applications. This particular EHEC fraction has been chosen mainly due to its hydrophobic character and promising possibility to form a hydrogel close to body temperature. A rather detailed picture of the interaction pattern has emerged, and a molecular model has been suggested proposing a clustering adsorption of micelle-like aggregates (clusters) of surfactant onto EHEC as the basic step. The size of these clusters increases with both surfactant and polymer concentration. In very dilute polymer solutions (well below c*) the clustering process is assumed to be an intramolecular phenomenon with multiple mixing of the hydrophobic parts of the polymer in the same surfactant cluster, leading to shrinkage of the polymer coil and a decrease in hydrodynamic volume. At elevated polymer concentrations (c* or higher) the surfactant clusters are shared intermolecularly, thus acting as tie points in a threedimensional polymer network, causing high viscosities of the solution and even gel formation. In a recent study by Nilsson et al.10 several physicochemical methods were combined in order to characterize a set of nonionic cellulose ethers (including several different EHEC fractions) concerning their molecular weight, molecular weight distribution, polydispersity, molecular size, magnitude of hydrodynamic and thermodynamic interaction, tendency to aggregate, etc. This study clearly showed that each fraction of, for instance, (10) Nilsson, S.; Sundelo¨f, L.-O.; Porsch, B. Carbohydr. Polym. 1995, 28, 265.
© 1996 American Chemical Society
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EHEC has to be considered as a, more or less, unique substance with specific properties which might affect details in the interaction mechanism with surfactants. Some semidilute aqueous solutions of relatively hydrophobic nonionic cellulose ethers and ionic surfactants have shown gel formation at increased temperatures.11 This effect was interpreted as conformational rearrangements and increased hydrophobicity of the polymer upon temperature elevation. Recently Kamenka et al.12 studied semidilute solutions of an EHEC fraction (related to the one used in this investigation) and SDS at elevated temperatures. They reported a strengthened binding with temperature as shown by a reduction in cooperativity and in the critical surfactant concentration where adsorption to the polymer starts, here referred to as c1, and a decrease in aggregation number for some compositions. In contrast to this, other studies in the dilute polymer regime of EHEC in this laboratory6 by means of viscometry and equilibrium dialysis and a calorimetric study by Wang and Olofsson13 did not detect any significant change of c1 or any other straightforward evidence for a strengthened interaction with SDS at increased temperatures. To deepen the understanding and description of the interaction mechanism on a molecular level between EHEC (fraction CST-103) and SDS in dilute aqueous solutions (cp ≈ c*), there will be presented in this paper a systematic investigation of the system properties with temperature as the variable, going from 20 to 50 °C, with the focus especially on the average aggregation numbers, the adsorption isotherms, and the micropolarity. The average aggregation numbers are obtained from a combination of steady-state fluorescence quenching and equilibrium dialysis. In order to verify the applicability of this technique to determine aggregation numbers, this paper also discusses and compares two fluorophore/ quencher pairs previously used for the determination of aggregation numbers in polymer-surfactant systems with the steady-state fluorescence quenching method. Reference measurements on adsorption isotherms and aggregation numbers are also presented for the well-known and extensively studied PEO/SDS/water system,1,5,14 and our results are compared with previous reports. Experimental Section Materials. Ethyl hydroxyethyl cellulose (EHEC; fraction CST-103, MSeo) 0.7, DSethyl ) 1.5) with a weight-average molecular weight (Mw) of approximately 1.9 × 105, a polydispersity index (Mw/Mn) of 2.1 as determined from size exclusion chromatography experiments (with LALLS and RI detection),10 and an intrinsic viscosity, [η], of 455 mL/g as determined from viscometry10 was obtained from Akzo Nobel AB, Stenungsund, Sweden. The cloud point (CP) of EHEC/CST-103 was observed in the interval 28-37 °C depending on the polymer concentration,8 and when SDS is added to the EHEC/water system, the CP increases with increasing SDS concentration.8 None of the solutions investigated in this study showed any signs of phase separation. The surface tension (against air) was equal to 37 mN/m for an 0.2% (w/w) aqueous solution of CST-103, as determined by the pendant drop technique.15 The standard procedure of preparing EHEC stock solutions is described in a previous paper.7 After preparation, the EHEC stock solution was rinsed from low molecular weight material and salts using a Spectra/Por tube dialysis membrane, Spectrum Medical Ind., (11) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloids Surf. 1990, 47, 147. (12) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (13) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (14) Winnik, F. M. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 367. (15) Persson, B.; Nilsson, S.; Sundelo¨f, L.-O. Carbohydr. Polym. 1996, 29, 119.
Evertsson et al. LA, with a molecular weight cutoff at 12 000-14 000. Dialysis was performed against Milli Q water (Millipore) during 1 week, and after that the stock solution was filtered through 0.8 µm filters (Millex-AA, Millipore, SA, Molsheim, France) for removal of undissolved polymer, microgels, and dust particles. The conductivity of the dialyzed EHEC stock solution was as low as 4-6 µS, which corresponds to a concentration of sodium chloride of approximately 0.03 mM. Finally, the concentration was determined by drying samples to constant weight at 105 °C. All EHEC-SDS solutions in this work were prepared by weighing the desired amounts of EHEC stock solution into appropriately diluted SDS solutions at least 24 h before the samples were used for experiments, in order to let the previously reported timedependent effects settle.8 EHEC concentrations are given in percent by weight and were 0.20% (w/w) for all experiments. SDS concentrations are calculated as moles per kilogram of solvent, but since all the solutions used in this study are dilute, SDS concentrations are given in the molar scale. Poly(ethylene oxide) (PEO) with a nominal molar mass of 300 000 was obtained from Janssen Chimica, Geel, Belgium. Analytical grade sodium dodecyl sulfate, SDS, was obtained from Merck, Darmstadt, Germany, the radioactive SDS (35S) was bought from Amersham, England, benzophenone (99+%), tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bipy)32+), and 9-methylanthracene (9-MA) (98%) were from Aldrich-Chemie, Steinham, Germany, and they were all used as supplied. Pyrene (98+%), from Acros Chimica, Belgium, was twice recrystallized from absolute ethanol. All solutions were prepared using Milli Q water. Equilibrium Dialysis. The equilibrium dialysis experiments were performed at 20, 30, 40 and 50 °C using the same type of dialysis cells used in previous works.6,9,16,17 The cell consists of two cell compartments, each of 2 mL, separated by a membrane (Spectra/Por with a Mw cutoff 12 000-14 000) according to the same principle as the one developed by Fischman and Eirich.18 Before use the cells were rinsed several times with deionized water and finally with Milli Q water. Care was then taken to remove all of this water. The polymer solution was placed in one of the cell compartments, and the SDS solution, in the other. The SDS solutions contained a small amount of 35S, enough to achieve an activity of approximately 25 000 cpm/mL. The cells were placed in an air thermostat at the desired temperature for 1 week before the 35S content on each side was determined by scintillation counting. Preliminary results indicated that equilibrium was reached after 48 h although 7 days was allowed as a precaution. It was shown by test experiments that the presence of EHEC on the polymer side did not cause an extinction of the scintillation. Cells to be equilibrated at higher temperatures were left at room temperature for 1 day before being thermostated in order to avoid clouding due to a low initial SDS concentration in the polymer compartment. Fluorescence Measurements. Steady-state fluorescence measurements were recorded on a SPEX Fluorolog-2 spectrofluorometer model FL1T2 in the “s”-mode with 0.5 mm excitationslits and 1.25 mm emission slits. The cell holder was thermostated using a circulating water bath and so were the solutions to be measured for at least 12 h before the experiments were performed. When pyrene was used as a probe, the EHEC-SDS solutions were prepared with filtered pyrene-saturated water containing cmc the molarity of normal micelles is given by
[micelles]n ) ([SDS]eq - cmc)/Nn
(4)
Finally the average aggregation number of EHEC-bound SDS clusters, Np, is calculated from the equation
Np ) ([SDS]tot - [SDS]eq)/[micelles]p
(5)
which is a combination of eqs 2-4. In these equations [SDS]eq should be interpreted as the molar concentration of nonbound SDS as obtained from the dialysis experiments and corrected for the Donnan effect. In previous investigations of the EHEC/SDS/water9 and HPMC/SDS/water16 systems it was shown that the clusters formed along the polymer chains were smaller than ordinary micelles and did grow in size (Np) with both increasing surfactant and polymer concentration up to a certain limiting value. A pronounced difference was observed between a solution very dilute (cp < c*) with respect to the polymer and a solution of higher concentra(51) Croonen, Y.; Gelade, E.; Zegel, M. V. d.; Auweraer, M. V. d.; Vandendrlessche, H.; Schryver, F. C. D.; Almgren, M. J. Phys. Chem. 1983, 87, 1426. (52) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (53) Moroi, Y.; Humphry-Baker, R.; Gra¨tzel, M. J. Colloid Interface Sci. 1987, 119, 588. (54) Malliaris, A.; Moigne, J. L.; Sturm, J.; Zana, R. J. Phys. Chem. 1985, 89, 2709.
Table 1. Summary of Experimental Data and Calculated Values for the EHEC/SDS/Water System from 20 to 50 °C [SDS]eq (mM) [micelles]p (mM) [SDS]tot (mM) 20 °C 30 °C 40 °C 50 °C 20 °C 30 °C 40 °C 50 °C 3.00 4.00 5.00 7.00 9.00 10.5 11.5 13.0 15.0 17.0 20.0
2.35 2.73 2.95 3.42 3.82 4.00 4.10 4.28 4.40 5.00 8.15
2.40 2.60 2.90 3.68 4.10 4.22 4.50 4.61 4.79 7.22 8.50
2.25 2.63 3.11 3.68 4.25 4.54 4.83 5.21 5.49 6.26 9.79
2.91 3.42 3.83 4.23 4.54 5.35 5.86 6.47 7.08 8.40 12.2
0.071 0.075 0.098 0.126 0.152 0.164 0.174 0.184 0.204 0.224 0.271
0.081 0.087 0.111 0.137 0.155 0.165 0.170 0.189 0.199 0.228 0.281
0.134 0.149 0.178 0.189 0.194 0.249 0.299 0.301
0.115 0.166 0.190 0.204 0.209 0.247 0.282 0.264
Np na [SDS]tot (mM) 20 °C 30 °C 40 °C 50 °C 20 °C 30 °C 40 °C 50 °C 3.00 4.00 5.00 7.00 9.00 10.5 11.5 13.0 15.0 17.0 20.0 a
9.2 17.0 20.5 28.5 34.3 39.6 42.6 47.3 51.4 53.7 43.9
8.0 14.6 18.5 24.0 31.7 38.2 41.1 44.2 46.5 42.9 40.9
24.8 31.8 33.5 35.3 40.1 38.1 35.9 33.9
24.2 26.9 27.1 27.6 31.3 32.0 30.5 29.7
120 95 77 62 52 48 45 43 38 35 27
96 90 71 57 51 48 46 41 39 34 26
58 52 44 42 40 31 26 23
68 47 41 38 38 32 26 19
Number of glucose units per polymer-bound SDS cluster.
tion (cp > c*). These findings were interpreted9,16 in terms of an intramolecular clustering process operating at the very low polymer concentrations (cp < c*) leading to shrinkage of the polymer coil while at higher polymer concentrations (cp > c*) the clustering process tends to become intermolecular in nature; i.e., one cluster is shared by more than one polymer chain, creating a threedimensional network with high solution viscosity. An increase in the size of the polymer-bound surfactant clusters with surfactant concentration has also been reported for the hydroxypropyl cellulose (HPC)/SDS/ water25 and the PEO/SDS/water26,35,55 systems. Table 1 gives values of [SDS]tot, [SDS]eq, [micelles]p, and Np (as defined above) for aqueous 0.20% EHEC solutions. It also contains the number n, which gives the calculated average distance between the surfactant clusters along the EHEC backbone as polymer glucose units per SDS cluster. Figure 5 gives Np as a function of [SDS]tot for different temperatures while Figure 6 shows Np as a function of temperature for two specific EHEC/SDS/water compositions. In Figure 5 it can be seen that Np increases with [SDS]tot from the c1-value towards a plateau value or a weakly developed maximum. When the temperature increases, Np decreases, which is in qualitative agreement with the findings by Kamenka et al.12 for the related EHEC/SDS/water system in the semidilute regime and by van Stam et al.55 for the PEO/SDS/water system. However, the magnitude of this effect is composition dependent and is small but significant at the lowest SDS concentrations while it is most pronounced at the highest [SDS]tot investigated. As can be seen in Table 1 there is also both an increase in the number of polymer-bound SDS clusters ([micelles]tot) and consequently a decrease of the average distance between them (n) with an increase in temperature. These observations are consistent with the mechanism described above which suggests an intensive and low cooperative binding of SDS onto aggregated EHEC chains at elevated temperatures. As (55) van Stam, J.; Almgren, M.; Lindblad, C. Prog. Colloid Polym. Sci. 1991, 84, 13.
Interactions between EHEC and SDS
Figure 5. Average aggregation numbers of polymer-bound SDS clusters in 0.20% EHEC/water solutions as a function of the total SDS concentration: b, 20 °C; O, 30 °C; 2, 40 °C; 4, 50 °C.
Figure 6. Average aggregation numbers of polymer-bound SDS clusters formed in 0.20% EHEC/water solutions as a function of temperature: b, 10.5 mM SDS; O, 15 mM SDS.
already mentioned above, it has been shown37,38 that the hydrophobicity of EHEC increases with temperature due to an increased presence of nonpolar conformers. A similar mechanism has been suggested36 to operate in aqueous solutions of PEO. Thus certain cellulose derivatives such as EHEC, HPC, MC, and HPMC as well as PEO become more hydrophobic as the temperature increases, which is also shown by the existence of an upper critical solution temperature (cloud point temperature) above which aqueous solutions of these substances phase separate.8,16,56 However, it should be noted that this is not a general phenomenon of all nonionic polymers. For example PVP shows no clouding behavior and remains polar even at higher temperatures.39 As the temperature increases, there is both a dehydration50 and a decrease in polarity of EHEC, giving effects such as an increased tendency for self-aggregation of this polymer, a decreased cooperativity above the break-point and a decrease in the maximum amount of surfactant bound to the polymer (see Figures 3 and 4 above), an increase in the number of polymerbound clusters, leading to a reduced distance between them along the contour length of the polymer chain (see Table 1), and, in general, a decrease in the size (Np) of these polymer-bound clusters (see Figures 5 and 6). Thus, at elevated temperatures the polymer-surfactant inter(56) Bailey, F. E.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976.
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Figure 7. Average aggregation numbers of polymer-bound SDS clusters formed in 0.20% EHEC/water solutions as a function of the total SDS concentration at 20 °C. b: Ru(bipy)32+ is used as probe, and 9-methylanthracene, as quencher. O: Pyrene is used as probe, and benzophenone, as quencher.
action could be regarded as becoming more intensive in its nature with an increasing number of surfactant clusters adsorbed and distributed onto aggregated polymer chains rather densely. Fluorescence quenching techniques are among the most reliable ones for the determination of average aggregation numbers of surfactant micelles, and they have the advantage of being insensitive to intermicellar interactions.30 Good reviews exist14,30 which describe these techniques in connection to both surfactant and polymersurfactant systems. In recent steady-state fluorescence quenching studies of nonionic cellulose ether/SDS interactions in dilute aqueous solutions performed in this laboratory9,16 and by others25 aggregation numbers were determined with pyrene as the fluorophore and benzophenone as the quencher. In this paper we compare the results found with this pair with the results found with the Ru(bipy)32+/9-MA pair. Figure 7 shows the average aggregation number of polymer-bound SDS clusters, Np, as a function of the total SDS concentration, [SDS]tot, for identical aqueous 0.20% EHEC solutions at 20 °C determined with the Ru(bipy)32+/ 9-MA and pyrene/benzophenone pairs, respectively. Evidently both these sets of experiments give, within experimental error, identical results for Np, in the EHEC/ SDS/water system. Both pairs give also almost the same aggregation number of normal (not polymer bound) SDS micelles, Nn, and the values were found to be in good agreement with those from the literature (Nn ) 607019,52,53). In order to further evaluate the reliability of the steadystate fluorescence quenching method, as used by us, some reference measurements were performed on the wellknown and extensively studied PEO/SDS/water system.1,5,14 Figure 8 gives the adsorption isotherm of SDS for aqueous solutions of PEO (0.20%) at 20 °C. The binding of SDS to the hydrophilic polymer PEO is a highly cooperative, one-step process with an abrupt onset of interaction at a specific SDS concentration (c1 ) 5.0 mM) marking a very steep increase up to a maximum followed by a slight declining region. This is a typical behavior of a polymer-surfactant system which gives a clear-cut cac. The high cooperativity of the adsorption isotherm originates probably from the facts that PEO does not selfaggregate in aqueous solutions (see the discussion above) and that PEO and SDS interact intensely. Evidently also the salt-free PEO/SDS/water system has almost the same
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Figure 8. Equilibrium dialysis results for the PEO/SDS/water system at 20 °C and 0.20% PEO presented as y (millimoles of SDS bound per gram of PEO) as a function of the equilibrium SDS concentration.
Evertsson et al.
quencher. It should be noted, however, that van Stam and co-workers used the classical model assumption of a cac value above which all surfactant molecules added to the solution bind to the polymer until saturation in contrast to our methodology utilizing the equilibrium dialysis technique to get knowledge about the true binding or “redistribution situation”. However, in this case it is clearly seen that the high cooperativity of the interaction between PEO and SDS makes the assumption valid and the calculations of Np correct. As already described above, our investigation shows a difference in cooperativity between a very dilute polymer solution (cp < c*) and a solution of higher concentration (cp > c*) for both the EHEC/SDS/water9 and the HPMC/SDS/water16 systems which necessitates good information concerning the adsorption in order to calculate reliable values of Np. From the data presented one may conclude that the steady-state fluorescence quenching method, utilizing both the fluorophore/quencher pairs pyrene/benzophenone and Ru(bipy)32+/9-MA, works well and gives good determinations of average aggregation numbers (clusters) in aqueous polymer/surfactant systems such as the one studied here. This also verifies the validity of earlier results obtained in this laboratory.9,16 Conclusions
Figure 9. Average aggregation number of SDS clusters formed in 0.20% PEO/water solutions as a function of the total SDS concentration.
general shape of dialysis redistribution as has been found for some other nonionic polymer/surfactant systems6,7,9,16,45-47 (see above). It should be noted that this binding isotherm obtained from equilibrium dialysis experiments deviates from the ones published by other investigators of the PEO/SDS system.1,57,58 However, all these studies were made with a “swamping excess” of NaCl (0.1 M) in the solutions to suppress the Donnan effect, thus also changing the system properties and interaction mechanism markedly.17,37,59,60 Figure 9 presents the average aggregation numbers (Np) of the SDS clusters formed on PEO determined with the steady-state fluorescence quenching technique and calculated with the same methodology as has been used in this study of the EHEC/SDS/water system (and others9,16) and already described above. Np increases gradually upon increasing the SDS concentration from c1 up to a weak maximum following qualitatively the same pattern as the EHEC/SDS/water system. The absolute values of Np are in good agreement with the values recently reported by van Stam et al.,55 who studied the PEO/SDS/water system with a time-resolved fluorescence quenching technique using pyrene as a probe and dimethylbenzophenone as a (57) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976, 54, 450. (58) Shirahama, K. Colloid Polym. Sci. 1974, 252, 978. (59) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Langmuir 1986, 2, 536. (60) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloid Polym. Sci. 1988, 266, 1031.
The hydrophobic, nonionic cellulose ether EHEC interacts with the anionic surfactant SDS in aqueous solutions in a way that SDS starts to adsorb at a specific surfactant concentration (c1) to the EHEC chain in a cooperative manner as small micelle-like clusters. These polymer-bound clusters grow in size with both surfactant and polymer concentration up to a certain limiting value. The solubility of EHEC in pure water decreases with temperature due to dehydration and conformational rearrangements of the polymer chain and a decreased free energy of the polymer-polymer interaction while addition of small amounts of SDS enhances the solubility dramatically. The results presented in this paper show that the temperature affects some basically important features of the EHEC/SDS/water system such as the magnitude of the adsorption of SDS to EHEC (total amount of SDS adsorbed per mass unit of EHEC), the degree of cooperativity in the interaction, the size and number of the polymer-bound surfactant clusters, and the micropolarity sensed by pyrene inside the clusters. It is found that the average aggregation number of these clusters and the magnitude of the SDS adsorption generally decrease as the temperature increases in the system. Furthermore, upon elevation in temperature the “small” clusters formed at low SDS concentrations become more penetrable to water and the index of micropolarity (I1/I3) increases. One can clearly distinguish, for temperatures up to 30 °C, a break-point in the adsorption isotherm at an intermediate value of the adsorption, above which the cooperativity increases, indicating that there are two modes in the adsorption mechanism of SDS to EHEC, first to aggregated EHEC chains and then to a mainly deaggregated state of EHEC. This break-point diminishes and disappears as the temperature is raised. It has also been found that the surfactant concentration where adsorption to the polymer starts (c1) and the cooperativity of this binding decrease as the temperature increases. Overall, the picture that emerges of the EHEC/SDS complex at higher temperatures and in dilute solutions can be visualized in the following way: At 20 °C, the SDS clusters adsorb to hydrophobic sites on EHEC. As the adsorption of SDS continues, there is a simultaneous deaggregation of the EHEC chains, thus increasing the total number of
Interactions between EHEC and SDS
available hydrophobic, cluster-binding sites. At elevated temperature the free energy in the polymer-polymer interaction decreases, giving rise to an increasing tendency to polymer aggregation in addition to a decrease in hydration of the polymer chains. Thus, for higher temperatures the polymer-surfactant interaction becomes more intensive in its nature compared to the case for 20 °C with an increasing number of surfactant clusters distributed onto aggregated EHEC chains, as has been demonstrated as a decrease in c1 and cooperativity of the
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surfactant binding, a decrease in cluster size (Np), and an increase in the number of polymer-bound clusters per volume unit. Acknowledgment. This work has been financially supported by the Swedish Natural Science Research Council and the Swedish Council for the Engineering Sciences. LA960345O
