Effect of Surfactant on Structural and Thermodynamic Properties of

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Langmuir 1995,11, 3730-3736

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Effect of Surfactant on Structural and Thermodynamic Properties of Aqueous Solutions of Hydrophobically Modified Ethyl (hydroxyethyl)cellulose Krister Thuresson,? Bo Nystrom,*lt Geng Wang,§ and Bjorn Lindmant Physical Chemistry 1 and Thermochemistry, Chemical Centre, University of Lund, P.O. Box 124, 5’-221 00 Lund, Sweden, and Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315, Oslo, Norway Received April 11, 1995. I n Final Form: July 11, 1995@ Solution properties of ethyl(hydroxyethy1)cellulose (EHEC) and a hydrophobically modified analogue (HM-EHEC) in water in the presence of various amounts of sodium dodecyl sulfate (SDS) have been investigated by cloud point (CP) measurements, titration microcalorimetry, and static light scattering. The CP experiments revealed that the hydrophobically modified polymer is less soluble than the unmodified EHEC at low concentrations of SDS, while at higher surfactant concentrations the solubility properties ofthe two polymers are practically the same. The resulting enthalpic titration curves from the calorimetric measurements on 0.25%polymer solutions at 25 “C consist of a pronounced endothermic peak (moderate SDS concentration) followed by a shallow exothermic one at higher surfactant concentrations. The prominent endothermic peak observed for both systems indicates a strong polymer-surfactant interaction. From the shape of the curves, the onset of the saturation stage of the binding process was estimated to be 20 mm for both polymers. The degree of surfactant binding t o the polymer was found, at moderate SDS concentrations, to be the same for EHEC and HM-EHEC. The light-scatteringresults for both the EHEC/ SDS and HM-EHEC/SDS systems revealed a decreasing angular dependence ofthe reduced inverse scattered intensity function S(O)/S(q)(q is the wave vector) with increasing surfactant concentration. It is observed at moderate levels of surfactant addition that the correlation length 6 decreases more strongly for the EHEC/SDS system than for the HM-EHEC/SDS system. By plotting the static light scattering data in the form S(qt)/S(O)versus the dimensionless parameter q6 a universal picture emerged. In the regime q6 > 1, the power law S(q) q - l . 6 was observed for both systems.

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Introduction The behavior of hydrophobically associating watersoluble polymers’ is the presence of ionic surfactants is governed by a n intricate interplay between hydrophobic, hydrophilic, and ionic interactions. Although, the detailed mechanisms behind these phenomena are often not well-understood, the hydrophobic interactions are in many cases anticipated to play a dominant role. For instance, the effect of intermolecular interactions among hydrophobic groups is expected to be of great importance for the unusual rheologica12-12features observed in aqueous systems containing cellulose derivatives with incorporated hydrophobic side groups and ionic surfactants. In semi~~~

’ Physical Chemistry 1, University of Lund.

* Department of Chemistry, University of Oslo.

of Lund. Abstract published in Advance ACS Abstracts, September 15, 1995. (1)Macromolecular Complexes in Chemistry and Biology, Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (2) Shaw, K. G.; Leipold, D. P. J. Coat. Technol. 1985, 57, 63. (3)Carlsson, A,; Karlstrom, G.; Lindman, B. Colloids Surf. 1990,47, 147. (4) Duleh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251. (5) Sivadasan, K.; Somasundaran, P. Colloids Surf. 1990, 49, 229. ( 6 )Zugenmaier, P.;Aust, N. Makromol. Chem.Rapid Commun. 1990, 11, 95. (7)Tanaka, R.; Meadows, J.; Williams, P. A,; Phillips, G. 0. Macromolecules 1992, 25, 1304. (8) Lindman, B.; Carlsson, A,; Gerdes, S.; Karlstrom, 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. (9) Pisa’rcik, M.; Bakos’, D. Acta Polym. 1994, 45, 93. (10) Nystrom, B.; Lindman, B. Macromolecules 1995,28, 967. (ll)Nystrom, B.; Walderhaug, H.; Hansen, F. K.; Lindman, B. Langmuir 1995, 11, 750. (12) Nystrom, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. 5 Thermochemistry, University @

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dilute polymer solutions, the interactions between hydrophobically modified polymers and surfactants can result in network formation and a rapid increase in solution viscosity. Quite recently,13 it has been argued that associating polymers in the semidilute regime can form a transient network of clusters or “flowers”connected by multiple bridges. In a recent study,12 dynamic light scattering and oscillatory shear experiments were carried out on aqueous solutions of a hydrophobically modified ethyl(hydroxyethy1)cellulose (HM-EHEC) and its unmodified analogue (EHEC)in the presence of various amounts of the anionic surfactant sodium dodecyl sulfate (SDS). For both the HM-EHEC/SDS and EHEC/SDS systems the rheological and hydrodynamic features were observed to be strongly dependent upon the level of surfactant addition, with at first a n increase and then a decrease in the values of the physical parameters. These effects are significantly more pronounced for solutions of the hydrophobically modified polymer. In order to gain further insight, from another angle of approach, into the nature of the complex polymersurfactant interactions, we have carried out cloud point (CP), microcalorimetric titration, and static light scattering (SLS)measurements on the same systems, namely aqueous solutions ofEHEC and HM-EHEC in the presence of various amounts of SDS. These experiments will yield information about the thermodynamic and structural properties of the systems. It is interesting to find out if the strong polymer-surfactant interactions that gave rise to the characteristic features observed in the above cited study12 on dynamical and rheological properties also are reflected in these measurements, or if a different pattern of behavior emerges. (13) Semenov, A. N.; Joanny, J.-F.;Khokhlov, A. R. Macromolecules 1995,28, 1066.

0 1995 American Chemical Society

Effect of Surfactant on EHEC and HM-EHEC

Langmuir, Vol. 11, No. 10, 1995 3731

Experimental Section Materials. The EHEC and HM-EHEC samples were manufactured by Akzo Nobel AB, Stenungsund, Sweden. Both the unmodified EHEC (CP (water) 65 "C)and the hydrophobically modified one (HM-EHEC) (CP (water) * 50 "C) are ethyl(hydroxyethy1)celluloseethers with the same molecular weight (ca. M , 100 000) and the degrees of substitution of ethyl and hydroxyethyl groups are DSethyl= 0.6-0.7 and MSEO= 1.8, respectively. Thevalues ofDS and MS correspond to the average number of ethyl and hydroxyethyl groups per anhydroglucose unit of the polymer. The values of M,, DS, and MS were all given by the manufacturer. The values of CP (1w/w %) of these samples have been measured in this work (see Figure 1). The HM-EHEC polymer is equivalent to the EHEC sample, but with branched nonylphenol chains grafted to the cellulosebackbone. The degree ofnonylphenol substitution was determined to be 1.7 mol % (ca. 6.5 groups per molecule) relative to the repeating units ofthe polymer by measuring the absorbance of the aromatic ring in nonylphenol at a wavelength of 275 nm with a Shimadzu UV-160 spectrophotometer. Phenol in aqueous solutionwas used as a calibration standard. In order to remove unreacted nonylphenol, the HM-EHEC sample was stirred with acetone overnight, after which the extract was removed and a new portion of acetone was added. The whole procedure was repeated four or five times. M e r extraction the powder was dried from acetone and an aqueous solution (ca. 1w/w % of polymer) was prepared. Impurities that were insoluble in water were allowed to settle duringcentrifugation (2h) at 10 000 rpm. The clear supernatant phase was separated from the pellet and low molecular weight impurities, such as salt, in the supernatant were removed by dialysis against Millipore water in a Filtron Ultrasette device. The dialysis procedure was continued until the conductivity of the expelled water showed no further decrease. This occurred after about 70 h of dialysis and at a conductivityof approximately 2 pS/m. After dialysis the aqueous solution was freeze-dried, leaving a dry polymer sample that was stored in a desiccator prior to use. The same purification procedure, except for the extraction with acetone, was used with EHEC. The anionic surfactant SDS was obtained from Fluka (SLS measurements) and BDH Chemicals Ltd (99%)(CP and calorimetricmeasurements) and was used as received. Samples were prepared by weighing the components, and the solutions were homogeneized by stirring at room temperature for several days. Cloud Point Measurements. The cloud points (CP)of EHEC (1.0 w/w S ) and HM-EHEC (1.0 w/w %) in the absence and in the presence of various amounts of SDS were measured in glass tubes (10 mm in diameter), which were sealed. The solutions were allowed to equilibrate at each temperature and the temperature where phase separation occurred was measured both in increasing and decreasing runs with visual detection. The average value was taken to represent the phase separation temperature. The precision of the determinations was usually better than =kl "C. Calorimetric Measurements. These experiments were made using the commercialmicrocalorimetricmeasuring channel of the 2277 TAM Thermal Activity Monitor system1*(Thermometric AB, Jarfalla, Sweden) in a home-built high-precision thermostatic bath. An insertion vessel of 3 mL volume made of high-grade stainless steel was utilized. The calorimetrictitration experiments consisted of series of consecutive additions of concentrated surfactant solution (10 wlw %) to the calorimeter vessel initially containing 2.7 gofwater or polymer solution. The liquid samples were added in portions of 7 pL from a gas-tight Hamilton syringe through a thin, stainless-steel capillary tube. Amicroprocessor-controlled motor-driven syringe drive was used for the injections. The fast titration procedure was used with 6 min between each inje~ti0n.l~ The dynamic correction method employed to deconvolute the potential signals was based on the simple Tian eq~ati0n.I~ The calorimetry measurements were made at 25.0 f 0.1 "C. Static Light Scattering Measurements. The scattering process allows us to explore a system on a length scale of q-l, ~

~~

+EHEC/SDS -C-

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501 d

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5

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, 10

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Figure 1. Cloud point as a function of surfactant concentration for the systems indicated. The polymer concentration is 1w/w %. The solid curves are only guides for the eye.

where q is the wave vector [q = 4x71 sin(0/2)/L,where i is the wavelength of the incident light in a vacuum, 0 is the scattering angle, and n is the refractive index of the solution]. The wave vector range covered in this study was 9.5 x lo6 < q(m-l) < 2.3 x lo7. We used a standard, laboratory-built light scattering spectrometer capable of both absolute integrated scattered intensity and photon correlation measurements at different scattering angles. The SLS experiments were performed using an ALV (Langen-FRG) apparatus with a home-built automatic goniometer table (measurements were carried out in steps of 4") and the signal was processed by ALV light scattering electronics controlled by the on-line program ODIL. A Spectra-PhysicsModel 2020 argon ion laser was operated at the wavelength 514.5 nm, with vertically polarized light, and the output intensity of the beam was adjusted with the aid of high-quality neutral density filters (MellesGriot) ofvarious transmittances, depending upon the scattered light intensity level of the sample solution. Polarizers were used in front of and behind the cell in order to insure a w configuration. The cell was held in a thermostat block filled with refractive index matching silicone oil, and the measurements were performed at a temperature of 25.0 i 0.05 "C. The sample solutionswere filtered in an atmosphere of filtered air through, depending on the viscosity of the solution, 0.45 or 0.8 pm filters (Millipore)directly into precleaned 10 mm NMR tubes (Wilmad Glass Co.) of highest quality. All the SLS experiments were carried out in the semidilute regime at a constant polymer concentration of 1.0 w/w %. In the present experimental configurationa detectiongeometry was used where a vertical slit, instead of a pinhole, is placed in front of the photomultiplier tube. In this case the absolute quantityR,(q), which is the excess Rayleigh ratio with vertically polarized incident and scattered beams, can be determined from R,(q) = hZ*(q),where Z*(q)is the excess scattered intensity and h = Rvv,benzenensold(z*benzenenbenzene). A value O f R,(90") = 3.21 x cm-' reportedl6 for benzene at 25 "C and 514.5 nm was employed in this study. The optical constant K (em3mol g-2) where N A is was calculated from K = (4x2n2/N~A4)(8n/&)2, Avogadro's constant, (anlac) is the refractive index increment, and c is the mass concentration (mass/volume). The values of the refractive index increment were close to 0.14 mug for both systems (EHECBDS and HM-EHEC/SDS)at all SDS concentrations. The reduced scattered intensity, Kc/R,(q) was calculated and values of this quantity in the limit q = 0 were determined from a Guinier plot,17that is, a plot of ln(Kc/R,(q)) versus 42.

Results and Discussion Thermodynamic Properties and Interaction. In Figure 1 partial phase diagrams are shown for the systems EHEC (1.0 W/W%)/SDSand HM-EHEC (1.0 W/W %)/SDS as a function of surfactant concentration. On adding SDS the cloud point (CP)falls off initially for both systems, but

~

(14) Suurkuusk, J.;Wadso, I Chem Scr. 1982, 20, 155. (15)Backman, P.; Bastos, M.; Hall&, D.; Lonnbro, P.; Wadso, I. J. Bzochem. Bzophys. Methods 1994, 28, 85.

(16) Dubois, M.;Cabane, B. Macromolecules 1989, 22, 2526. (17)Guinier, A.;Fournet, G. Small Angle Scattering of X-rays, J . Wiley & Sons: New York, 1955.

3732 Langmuir, Vol. II, No. 10, 1995

the drop in CP is more pronounced for the hydrophobically modified polymer and the minimum is located a t a lower surfactant concentration than that for the EHEC/SDS system. This trend is probably due to hydrophobic interactions between surfactant molecules and the hydrophobic groups on the HM-EHEC polymer. Actually, there are observations18that suggest that for HM-EHEC the critical aggregation concentration (cac), where the onset of surfactant binding to the polymer occurs, is very low or nonexistent. These findings suggest that the binding of surfactant to the polymer starts at a lower SDS concentration for the hydrophobically modified polymer. In this case the binding of surfactant molecules to the polymer is more or less noncooperative. This situation may arise if the hydrophobic tails can act as hydrophobic binding sites for the surfactant molecules. In the case of the unmodified EHEC in the presence of SDS the cac is 3-3.5 mm18 a t the present temperature of measurement. Values of cac in the range 1.5-3.5 mm have been observed19*20 for different EHEC samples. The general trend is that a less polar EHEC sample yields a lower cac. The most conspicuous feature in Figure 1is that a t low surfactant concentrations phase separation occurs a t a much lower temperature for the HM-EHEC/SDS system than for the EHECISDS system. This is probably a trivial effect that can be attributed to the introduction of hydrophobic side groups on HM-EHEC, making it less water-soluble.21 At higher surfactant concentrations (above ca. 2 and 4 mm SDS concentrations for the modified and unmodified polymer, respectively) there is a strong monotonic increase in CP for both systems, i.e., the solubility of the polymers increases. This effect can be ascribed22*23 to the binding of surfactant to the polymers, thereby imposing repulsive interactions between different polymer chains. In this process a polymer-ionic surfactant complex with a polyelectrolyte character is formed. The phase separation behavior is governed24by a delicate balance between electrostatic effects and hydrophobic interactions. We may note that at surfactant concentrations above ca. 8 mm the data of CP for the two systems practically coincide. This tendency suggests that the hydrophobic effect of the modified polymer loses its importance and the two polymer-surfactant systems exhibit a similar phase separation behavior. The findings in Figure 1may be interpreted in the following way. At low surfactant concentrations the hydrophobic effect of the micellelike aggregates dominates over the electrostatic effect and the CP decreases. This tendency is more pronounced for the HM-EHEC sample probably due to the noncooperative binding of surfactant to this polymer a t low SDS concentrations. At higher surfactant concentrations the hydrophobic tails are gradually hidden in the interior of the aggregates and the electrostatic effects prevail. We may also note that the strong decrease of the dynamic viscosity, observed l 2 a t high SDS concentrations of the HM-EHEC/SDS system, supports the view that the hydrophobe tails are embedded in clusters. The pronounced increase of CP observed for the two systems a t higher surfactant concentrations is a typical sign of polymers with strongly binding ionic surfactants. (18)Thuresson, K.; Soderman, 0.; Hansson, P.;Wang, G., submitted for publication. (19) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J . Phys. Chem. 1994,9R, 6785. (20)Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (21)Thuresson, K.; Karlstrom,G.; Lindman, B. J . Phys. Chem. 1995, 99, 3823. (22)Goddard, E. D. Colloids Surf. 1986, 19, 255. (23) Karlstrom, G.; Carlsson, A.; Lindman, B. J . Phys. Chem. 1990, 94, 5005. (24) Lindman, B.; Carlsson, A.; Karlstrom, G.; Malmsten, M. Adu. Colloid Interface Sci. 1990,32, 183.

Thuresson et al.

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Figure 2. The degree of surfactant bindingI8 (NMR selfdiffusion measurements) to the polymer as a function of surfactant concentration for the systems indicated. The polymer concentration is 1 w/w %.

In this context, it is interesting to find out if there is a difference in the level of SDS binding to the polymer for the unmodified and the hydrophobically modified EHEC. The degree of surfactant binding to a polymer can be determined by, e.g., surfactant NMR self-diffusion measurements.25-28 In the analysis of the results it is assumed as a first approximation that the surfactant is either bound to the polymer (as either monomers or clusters) with a diffusion coefficient equal to the polymer diffusion coefficientDb or is free in the solution and diffises as single monomers with a diffusion coefficient Df.This view constitutes a two-sites model, and in its crudest form the observed diffusion coefficient Dohsis the populationaveraged mean of the self-diffusion coefficients for the two states, i.e.,

(la) where p is the fraction of the total amount of surfactant adsorbed to the polymer. Equation la may be rewritten as

In a recent study18 self-diffusion measurements were carried out on the present systems. Figure 2 displays the degree of surfactant binding ( p )of the systems EHEC (1.0 w/w %)/SDS and HM-EHEC(l.O w/w %)/SDS over an extended surfactant concentration range. There is no detectable difference in the level of surfactant binding of the two polymers a t SDS concentrations above the cac (3-3.5 mm) of EHEC, and the value of p increases with increasing surfactant concentration to approach the saturation stage of the binding process at high SDS concentrations. It is generally expected that the presence of hydrophobic groups on the polymer should make the polymer chains more inclined to binding of surfactant. In this work, no such an effect is exposed in the considered SDS concentration range. However, we may note that in a recent study*$ it was observed a t very low SDS concentrations that the degree of surfactant binding to the polymer is higher for HM-EHEC than for EHEC. Recent studies20,29>30 have revealed that titration microcalorimetry may yield detailed information about (25) Carlsson, A.; Karlstrom, G.; Lindman, B. J . Phys. Chem. 1989, 93, 3673. (26) Zhang, K.; Jonstromer, M.; Lindman, B. J . Phys. Chem. 1994, 98, 2459. (27)Soderman, 0.; Stilbs, P. Prog. Nucl. Reson. Spectrosc. 1994.26, 445.

(28) Walderhaug, H.; Nystrom, B.; Hansen, F. K.; Lindman, B. J . Phys. Chem. 1995,99,4672.

Langmuir, Vol. 11, No. 10, 1995 3733

Effect of Surfactant on EHEC and HM-EHEC I

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Figure 3. Calorimetric titration curves at 25.0 "C from additions of 10 wt % SDS to the systems indicated. "he solid curves are only guides for the eye.

polymer-surfactant interactions. In Figure 3, results of the calorimetric titration of 0.25 wt % solutions of EHEC and HM-EHEC with 10 wt % SDS solution a t 25 "C are depicted, where the obtained enthalpy changes calculated per mole of added SDS in each injection are plotted against the total concentration of SDS. The corresponding curve from dilution of the SDS solution in pure water is also included in the figure. In the 10 wt % solution, SDS will be in micellar form and the dilute curve shows that when the final concentration is below the cmc (8 mM), the added micelles break up to give monomers. At final concentrations above the cmc, only the micelles in the solution were diluted. The enthalpy of micelle formation of SDS a t the cmc is nearly zero a t 25 "C. Differences between the dilution curve in water and the titration curves in the polymer solutions are ascribed to SDS-polymer interactions. We may note that the curves in Figure 3 share the essential features of the corresponding titration curves for the hydrophobically end-capped poly(ethy1ene oxide) (PEO)and the parent PEO in a previous Titration curves for unmodified EHEC and PEO have the same shape.30 The effect on the polymer behavior ofintroducing a small amount of hydrophobic groups is probably quite analogous in the two cases. The hydrophobic groups tend to aggregate by themselves in pure water and the initially added SDS is solubilized in these aggregates. However, as the concentration of the hydrophobic groups is low, the aggregates soon become saturated. The enthalpic effect will also be low and seen as a n endothermic shift of the titration curve a t low SDS concentrations. When the SDS concentration exceeds the cac, SDS will start to aggregate on the polymer chain in a related way as on the unmodified polymer. The cac and the peak maxima indicating the onset of surfactant aggregation have been observed to be essentially independent of polymer c ~ n t e n t . The ~ ~ effect ,~~ of hydrophobic moieties in Figure 3 is only evident a t low SDS concentrations (below the cac of the EHEC/SDS system), where the titration curve representing the hydrophobic HM-EHEC sample starts a t a higher level than that ofthe unmodified polymer. This result indicates that the aggregation of SDS a t low surfactant concentrations is not the same for the two polymers. This trend suggests that the process of surfactant binding to the polymer is different for the unmodified and the modified cellulose derivative. However, as the SDS concentration increases, the influence of the hydrophobic groups will fade away, as their concentration is low and the titration curves practically coincide (see Figure 3). This pattern of behavior is reminiscent of that observed from the cloud (29) Persson, K.;Wang, G.;Olofsson, G. J.Chem. Soc.Faraduy Trans. 1994, 90, 3555. (30) Wang, G.; Olofsson, G. J.Phys. Chem. 1996, 99, 5588.

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Figure 4. Calorimetric titration curves at 25.0 "C from additions of 10wt % SDS to the systems indicated. "he polymer concentration is in all cases 0.25%. The solid curves are only guides for the eye.

point measurements of these polymers a t higher SDS concentrations (see Figure 1). Another notable feature in Figure 3 is the emergence of the exothermic peak a t higher SDS concentrations. We ascribe the decrease in the observed enthalpy to give a n exothermic peak (relative to the dilution curve in water) to changes in the composition of the aggregates. It is expected that as the SDS concentration increases the number of SDS monomers per aggregate will increase. The concentration where the titration curves in the polymer solutions join the water curve can be considered to be the saturation concentration where free SDS micelles start to form.30 This concentration (20 mm) is the same for the two EHEC samples (0.25% polymer) in Figure 3. In order to further elucidate the factors that govern the shape of the titration curve, calorimetric titration data of other EHEC samples with different hydrophobicity have been included in Figure 4. The samples are designated CST 103 (DSethyl= 1.6, MSEO= 0.7, CP(water) = 37 "C, andM, = 100 OOO), DVT89017 (DSethyl= 1.9, MSEO= 1.3, CP(water) = 34 "C, andM,, = 80 0001, and E230G (DSethy1 = 0.8, MSEO= 0.8, CP(water) = 62 "C, and M, = 100 000). We may note that, a t 25 "C, the height of the endothermic peak in the titration curve is more pronounced as the hydrophobicity of the EHEC samples (without branched nonylphenol chains) increases. The enhancement of the hydrophobicity is also reflected in the lowering of the cac20,30(see Figure 4). The anionic surfactant SDS can interact both with the ethyl end groups and the EO groups of the EHEC samples. It is generally found that the more hydrophobic the EHEC samples, the more pronounced the endothermic peak. However, it should be noted that the height as well as the shape of the endothermic peak is also strongly dependent on t e m p e r a t ~ r e . ~ ~ Static Light Scattering. The properties of the light scattered by polymer solutions depend on the concentration regime and on the relative magnitude of the characteristic length scale explored in the light scattering experiment. In semidilute solutions (as in the present study), i.e., above the concentrationc* where the polymer chains begin to overlap, the characteristic length is the correlation length E that decreases with increasing polymer concentration. In this concentration regime the solution may be visualized as a transient network with a certain average mesh size 5.31,32 In the Guinier region, i.e. if q t -= 1, the angular distribution of scattered intensity is observed and the (31) de Gennes, P.-G. Scaling Concepts in Polymer Physics, Cornel1 University Press: Ithaca, 1979. (32) Nakayama, T.;Yakubo, K.; Orbach, R. L.Reu. Mod. Phys. 1994, 66, 381.

Thuresson et al.

3734 Langmuir, Vol. 11, No. 10, 1995 normalized inverse scattered intensity function is given by a Lorentzian scattering law of the form31

+

S(O)/S(q)= 1 q 2 t 2

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EHEClSDS

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where S(q) = R,(q)/KcM and S(0)= RT/M(an/&),with R the gas constant, T the temperature, and (an/&)the inverse osmotic compressibility. Semidilute solutions ofthe present associating systems may be viewed as transient networks formed by more or less interpenetrating clusters with a wide distribution of sizes. This situation may be pictured in the framework of the percolation (linking some objects a t random). In the regime q5 > 1, the length scale 4-l is associated with more local properties of the system (one sees inside of the clusters) and the scattered intensity depends strongly on the length scale. In this case the scattered intensity is directly due to local space correlation between monomers in a volume q-3 and is proportional to the number of monomers in this volume. The angular dependence can give us a direct access to the fractal dimension df of the clusters. From a n analogy with a fractal self-avoiding walk of randomly branched polydisperse macromolecules in semidilute solutions a percolation model has been elaborated34which predicts a power law behavior of S(q)

7 8 mmolal SDS 13.4 mmolal SDS 20 7 mmolal S

1

+

where35,u dt(3 - z). Here df = 2(d 2)/5 = 2 (d = 3 is the space dimension) is the exponent of a single swollen percolation cluster, and 5 = 2.2 is the exponent which characterizes the distribution of cluster sizes in the percolation In Figure 5 the square of q dependences of the normalized inverse scattered intensity (see eq 2) a t different levels of surfactant addition for the systems EHEC/SDS and HM-EHEC/SDS are depicted. For both systems, the angular dependence is strongest for the solutions without surfactant and in this case a tendency of curvature is observed in both graphs. The general trend for the EHEC/SDS system is that as the level ofsurfactant addition increases a weaker q2 dependence is detected. For the hydrophobically modified polymer a more complex picture emerges (see Figure 5b). The most striking feature is the strong q2 dependence observed for the solution of the highest surfactant addition. These findings are probably directly related to the magnitude of the correlation length (see the discussion below). In this context we may note that in a previous light scattering on an aqueous thermoreversible system of EHEC (1wt %) in the presence of cetyltrimethylammonium bromide (4 mm), the q2 dependence of the normalized inverse scattered intensity function was found to decline as the gel zone was approached. This result seems to indicate that as the degree of crossbinding or level of chain association increases the effective mesh size of the network decreases. By plotting the inverse of the scattered intensity function as a function ofq2a t small scattering angles the correlation length 5 can be determined with the aid of eq 2. Effects of the addition of SDS on of 1wt % solutions of EHEC and HM-EHEC are given in Figure 6 . In the absence of surfactant the value of 6 is practically the same for both (33) Stauffer, A.; Coniglio, A.; Adam, M. Adu. Polym. Sci. 1982,44, 104. (34) Daoud, M.; Leibler, L. Macromolecules 1988,21, 1497. (35) Martin, J. E.; Ackerson, B. J. Phys. Rev. A 1985,31, 1180. (36) Bouchaud, E.; Delsanti, M.; Adam, M.; Daoud, M.; Durand, D. J.Phys. (Paris) 1986,47, 1273. (37) Nystrom, B.; Roots, J.;Carlsson, A,; Lindman, B. Polymer 1992, 33, 2875.

1 4

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Figure 5. The q2 dependenceof the reduced inverse scattered intensity function for the polymers and SDS concentrations indicated. The polymer concentration is 1w/w %. The solid curves are only guides for the eye. t EHEC/SDS 4J- HM-EHEC/SDS

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Concentration of SDS (mmolal)

Figure 6. The effect of surfactant addition on the correlation length of the systems indicated. The polymer concentration is 1w/w %. The solid curves are only guides for the eye. polymers, but as the SDS concentration increases the value of 5 falls off more strongly for the unmodified polymer. For both systems 5 passes through a minimum, which is located a t about 10 mm for HM-EHEC and somewhat lower for EHEC. At higher surfactant concentrations the value o f t increases for the HM-EHEC/SDS system, while 5 for the EHEC/SDS system is almost constant. These results suggest that the networks of both systems are undergoing structural reorganization as the amount of surfactant addition increases. In this process electrostatic effects, but especially enhanced hydrophobic associations will probably play an important role. The conjecture is that, due to strong hydrophobic associations, the number of bridged3connecting the clusters composing the network increases when SDS is added a t moderate surfactant concentrations. The effect ofthis structural reorganization may be that the average spacing between the chains, the mesh size, decreases as the level of surfactant addition

Langmuir, Vol. 11, No. 10, 1995 3735

Effect of Surfactant on EHEC and HM-EHEC

0

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A EHECISDS (2 mmolal) EHEC/SDS (5 mmolal) EHECISDS (10 mmolsl) EHEC/SDS (25 mmolal)

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EHECISDS (5 mmolal) EHECISDS (8 mmolal) 0 EHEC/SDS ( I O mmolal) X EHEC/SDS (25 mmolal)

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HM-EHECMlater HM-EHEC/SDS (4.1 mmolal) HM-EHECISDS (4.4 mmolal) HM-EHECISJS (7.8 mmolal) HM-EHECISDS ( I O 4 mmolal) HM-EHEC/SDS (13 4 mmolal) HM.EHEC/SDS (20 7 mmolal)

01

1 q (m.7

Figure 7. A log-log plot of the q dependence of the scattered intensity function for the systems indicated. The polymer concentrationis 1w/w %. The solidlines represent the best fits of the experimental data (see eq 3).

rises. This process seems to be optimal a t about 10 mm for the HM-EHEC/SDS system and somewhat lower for the EHEC/SDS system. At higher surfactant concentrations a gradual “debridging“ or breakdown of chain associations is expected to come into play and should increase. This effect can be anticipated to be more pronounced for the hydrophobically modified polymer (see Figure 6). This untieing process is usually attributed7to the “solubilization”of the polymer-bound hydrophobes and as a result the average number of bound hydrophobes per surfactant aggregate is expected to decrease. It is known from rheological12 experiments on the same systems that at higher SDS concentrations (above 10 mm) the viscosity enhancement observed at moderate surfactant concentrations declines strongly. This trend is significantly more pronounced for the HM-EHEC/SDS system. It is possible that this difference is reflected in the different values of E for the systems observed a t SDS concentrations above 10 mm (see Figure 6). In this context we may also note that in a recent studylaon inter alia fluorescence measurements it was found that the number of hydrophobe zones for the HM-EHEC/SDS system was practically constant a t SDS concentrations up to 2-3 mm. Above this concentration there was a strong increase in the concentration of hydrophobe zones. In Figure 7 the intensity profiles in form of log-log plots ofS(q) versus q are displayed for the systems EHEC/ SDS and HM-EHEC/SDS in the presence of various amounts of surfactant. The solid lines represent the best fit of the experimental data with eq 3. For both polymer systems the q dependence is strongest in the absence of surfactant, yielding the same value ofp = 1.55 f0.06. As the SDS concentration increases, a complex angular dependence of S(q) emerges, where the apparent values

e

95

Figure 8. Plot of the quantity S(qt)/S(O)as a function of the dimensionless scaling variable q t for the systems indicated. The polymer concentration is 1w/w %. The solid lines indicate the scaling law behavior at q t > 1.

ofp become unrealistically low. This anomalous behavior can be rationalized in terms of crossover effects associated with the transition from the domain qe < 1to the regime qE > 1. In the homogeneous regime qe < 1,the scattering intensity function is virtually independent of q, but in the inhomogeneous regime qE > 1, the scattering intensity function can be described by a power law (see eq 3). By plotting38 the reduced scattered intensity function (S(qE)/S(O)) as a function of the reduced length scale qE (dimensionless quantity), the experimental points condense onto a single curve (see Figure 8). This is a universal plot that should be independent of the chemical nature of the polymer, its molecular mass, and its concentration. This type of representation constitutes a manifestation of the self-similarity of the polymer conformations, which is preserved a t length scales smaller than in semidilute polymer solutions. In Figure 8 the influence of crossover effects is clearly demonstrated. The straight lines (LA = 1.6) represent the power law behavior a t qE > 1. This value of p coincides with that found experimentally for semidilute polymer solutions and gelling polymer systems of various natures. The same (38)Martin, J. E.; Wilcoxon, J. P. Phys. Reu. A 1989,39,252. (39)Bastide, J.;Leibler, L.; Prost, J. Macromolecules 1990,23,1821. (40)Schossler, F.; Daoud, M.; Leibler, L. J . Phys. (Paris) 1990,51, 2373. (41)Mendes, E.; Linder, P.; Buzier, M.; Boue’, F.; Bastide, J. Phys. Rev. Lett. 1991,66,1595. (42) Shibavama, M.: Tanaka, T.; Han, C. C . J . Chem. Phys. 1992,97, 6829. (43)Zielinski, F.; Buzier, M.; Lartigue, C.; Bastide, J.;Boue’, F.Progr. Colloid Polym. Sci. 1992,90, 115. (44)Lal, J.; Bastide, J.; Bansil, R.; B o d , F. Macromolecules 1993, 26,6092. (45) Falcao. A.N.: Skov Pedersen.,J.:. Mortensen, K. Macromolecules ~~, 1993,26,5350. ’ (46)Dahmani,M.; Skouri, M.; Guenet, J. M.; Munch, J. P.Europhys. Lett. 1994,26,19. ~

~~

3736 Langmuir, Vol. 11, No. 10, 1995 value of p has also been reported for other complex systems,38s47-53 e.g., dilute tetramethoxysilicon gels, saltinduced fast aggregation of monodisperse polystyrene spheres, and microemulsion-based gelatin gels. These findings emphasize the universal character of the q-1.6 law in the description of the q dependence ofthe scattering intensity function of complex systems in the q t > 1regime.

Summary and Conclusions The interactions, phase behavior, and structural properties of aqueous solutions of HM-EHEC and EHEC in the presence of various amounts of SDS have been investigated. A number of characteristic features have been observed. Measurements showed that the CP at low surfactant concentrations is much lower for the hydrophobically modified polymer, while at higher SDS concentrations CP increases and the values of CP for the two systems approach each other. This trend indicates that the hydrophobically modified polymer is less soluble a t low SDS concentration, but at higher levels of surfactant addition, the solubility properties of both polymers are practically the same. The amount of surfactant bound to the polymer was high for both EHEC and HM-EHEC, and no difference in the degree of surfactant binding between the polymers could be traced. The calorimetric experiments at 25 "C yielded enthalpic titration curves consisting of an endothermic peak followed by an exothermic peak at higher surfactant concentrations. (47)Lindsay, H. M.; Lin, M. Y.; Weitz, D. A,; Sheng, P.; Chen, Z.; Klein, R.; Meakin, P. Faraday Discuss. Chem. SOC.1986,83,153. (48)Martin, J. E.; Wilcoxon, J.;Adolf, D.Phys. Rev. A 1987,36,1803. (49)Chu, B.;Wu, C.; Wu, D. Macromolecules 1987,20, 2642. ( 5 0 )Daoud, M.; Martin, J. E. In The Fractal Approach to Heterogeneous Chemistlv: Avnir. D.. Ed.: John Wilev & Sons: New York. 1989. (51) Carpineti,"M.; FeAi, F.;Giglio, M.;Paganini,E.; Perini, U:Phys. Rev. A 1990,42,7347. (52)Carpineti, M.; Giglio, M.; Paganini, E.; Perini, U. Progr. Colloid Polym. Sci. 1991,84,305. (53)Quellet, C.; Eicke, H.-F.; Sager, W. J . Phys. Chem. 1991,95, 5642.

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Thuresson et al.

For both polymer-surfactant systems a pronounced endothermic peak was detected, indicating polymersurfactant interactions. The peak height of the endothermic curve as well as its onset appeared to be related to the hydrophobicity ofthe EHEC samples. With the aid of the exothermic curves the onset of the saturation stage of the binding process was estimated to be 20 mm. The static light scattering measurements on the systems EHEC/SDS and HM-EHEC/SDSrevealed that the angular dependence of the reduced inverse scattered intensity function decreases as the surfactant concentration increases. We observed a stronger reduction of the correlation length with increasing SDS concentration for the EHEC/SDS system. The picture that emerged is that structural reorganization of the polymer network is associated with the level of surfactant addition. A universal description of all the light scattering data was obtained by plotting S(qt)/S(O)as a function of the reduced dimensionless scaling variable 45. In the regime qLj > 1,a power law in the form S(q) q-", withp = 1.6, was observed for both the EHEC/SDS and HM-EHEC/ SDS systems. This value of the exponent is in agreement with that reported for other systems of complex nature. The results obtained from this experimental study did not reveal the same type of drastic differences between the EHEC/SDS and HM-EHECISDS systems as those found in the previous dynamic and rheological investigation12on the same systems. This seems to indicate that difference in the strength of interaction in polymersurfactant systems is often more easily exposed in dynamic and rheological measurements.

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Acknowledgment. The authors are grateful for valuable discussions with Gerd Olofsson. K.T. and G.W. thank The National Board for Industrial and Technical Development (NUTEK). K.T. also thanks Akzo Nobel for financial support. We thank Akzo Nobel for supplying the polymers. L4950287R