Adsorption and Desorption of Unmodified and Hydrophobically

Hydrophobically Modified Ethyl(hydroxyethyl)cellulose on. Polystyrene Latex Particles in the Presence of Ionic. Surfactants Using Dynamic Light Scatte...
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Langmuir 2000, 16, 4478-4484

Adsorption and Desorption of Unmodified and Hydrophobically Modified Ethyl(hydroxyethyl)cellulose on Polystyrene Latex Particles in the Presence of Ionic Surfactants Using Dynamic Light Scattering Rolf Andreas Lauten, Anna-Lena Kjøniksen, and Bo Nystro¨m* Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315, Oslo, Norway Received November 8, 1999. In Final Form: January 27, 2000 The effect of addition of anionic (sodium dodecyl sulfate, SDS) or cationic (cetyltrimethylammonium bromide, CTAB) surfactant on the hydrodynamic thickness of layers of unmodified (EHEC) and hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), adsorbed on anionic polystyrene latex particles (PSL), was studied by dynamic light scattering. The hydrodynamic layer thickness of EHEC increases in the presence of SDS or CTAB at low surfactant concentration due to the formation of polymersurfactant clusters on the particle surfaces, followed by a reduction of this layer at higher levels of surfactant addition. The maximum in hydrodynamic thickness is observed at the critical aggregation concentration (cac) of the respective EHEC-surfactant complex, and the desorption is virtually complete around the critical micelle concentration of the pure surfactant. The interaction peak is more pronounced with SDS than in the presence of the cationic surfactant. For the HM-EHEC/SDS/PSL and HM-EHEC/CTAB/PSL systems, the hydrodynamic thickness decreased monotonically with increasing surfactant concentration, and the hydrodynamic radius of PSL does not pass through a maximum. The absence of an interaction peak for the adsorption of the hydrophobically modified polymer is attributed to a very low or nonexistent cac for the HM-EHEC-surfactant complexes. An increase or decrease in temperature for the EHEC/ SDS/PSL and HM-EHEC/SDS/PSL systems had no noticeable effect on the maximum of the peak and other adsorption features.

Introduction The interaction between ionic surfactants and nonionic water-soluble hydrophobically modified polymers (amphiphilic) is currently the subject of extensive investigations in view of the number of formulations and processes in which they are utilized simultaneously, and the topic has been recently reviewed.1-5 Amphiphilic polymers that usually contain a small amount of hydrophobic groups (typically less than a few mol %) have attracted great attention because of their use as rheology modifiers in a number of applications, including foods, cosmetics, and latex paints.6,7 Ethyl(hydroxyethyl)cellulose (EHEC) belongs to a class of amphiphilic water-soluble polymers with reduced solubility in water at elevated temperatures,8 and the polymer associates in aqueous solution and is known to * To whom correspondence should be addressed. (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) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 85, 1. (3) Lindman, B.; Carlsson, A.; Gerdes, S.; Karlstro¨m, G.; Piculell, L.; Thalberg, K.; Zhang, K. In Food Colloids and Polymers: Stability and Mechanical Properties; Walstra, P., Dickinson, E.; Eds.; The Royal Society of Chemistry: London, 1993; pp 113-125. (4) Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (5) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77. (6) McCormick, C. L.; Bock, J.; Schulz, D. N. Water-soluble polymers. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley-Interscience: New York, 1989; Vol. 17, pp 730-784. (7) Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; American Chemical Society: Washington, DC, 1991. (8) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005.

interact strongly with ionic surfactants.9-12 The associative behavior of this polymer is related to the presence of two different substituents, namely a hydrophobic (ethyl) and a hydrophilic (2-hydroxyethyl) substituent, randomly distributed along the cellulose backbone. Recently, the solution behavior of EHEC and a hydrophobically modified analogue (HM-EHEC) in the presence of ionic surfactant has attracted a great deal of interest, and characteristics such as binding13,14 of surfactant to the polymers and cloud points,15-17 as well as structural,18 dynamical,19 and rheological19,20 properties of these systems have been reported. Several interesting features dealing with the interaction between polymer and surfactant were observed. For instance, the viscosity measurements19 revealed significant polymer-surfactant interaction for both EHEC and HM-EHEC, but the interaction peak, observed in the viscosity, was more pronounced and located at a lower surfactant concentration for the hydrophobically (9) Lindman, B.; Carlsson, A.; Karlstro¨m, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (10) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (11) Nystro¨m, B.; Lindman, B. Macromolecules 1995, 28, 967. (12) Walderhaug, H.; Nystro¨m, B.; Hansen, F. K.; Lindman, B. Prog. Colloid. Polym. Sci. 1995, 98, 51. (13) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (14) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (15) Thuresson, K.; Lindman, B. J. Phys. Chem. B 1997, 101, 6460. (16) Joabsson, F.; Rose´n, O., Thuresson, K.; Piculell, L.; Lindman, B. J. Phys. Chem. B 1998, 102, 2954. (17) Thuresson, K.; Joabsson, F. Colloids Surf., A: Physicochem. Eng. Aspects 1999, 151, 513. (18) Thuresson, K.; Nystro¨m, B.; Weng, G.; Lindman, B. Langmuir 1995, 11, 3730. (19) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (20) Thuresson, K.; Lindman, B.; Nystro¨m, B. J. Phys. Chem. B 1997, 101, 6450.

10.1021/la991460c CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000

Ethyl(hydroxyethyl)cellulose on Polystyrene Latex

modified polymer. At high surfactant concentrations, the viscosifying effect decays, and the difference in behavior between EHEC and HM-EHEC disappears. The onset of aggregation of surfactant in the presence of polymer is characterized by a critical aggregation concentration (cac) that is lower than the regular critical micelle concentration of the pure surfactant. The cac value is independent of polymer concentration. The binding of surfactant to the polymer continues until the chain becomes “saturated”, whereafter normal free micelles start to form when the overall surfactant concentration is sufficiently high. The saturation level depends on the polymer concentration. It is known that above cac, ionic surfactants cooperatively bind to the slightly hydrophobic EHEC molecules, forming mixed aggregates.21-26 A related cooperative binding is also detected for HM-EHEC, but in addition, a noncooperative binding region13,15 of surfactant is observed at surfactant concentrations well below the cac of the EHEC-surfactant complex. In the noncooperative adsorption process, the surfactants bind to the hydrophobic domains originating from the attached hydrophobic tails of HM-EHEC. The cooperativity discussed here is usually interpreted in the framework of a model in which the surfactant and the polymer both contribute to form mixed micelles. The cac concept was recently studied theoretically27 by considering dilute aqueous solutions containing a polymer and surfactant. The result from this investigation suggests that the onset of polymer-surfactant self-assembly always starts at a lower concentration (cac) than that required for surfactant-surfactant self-assembly (cmc). The above cited work also addresses the behavior for a hydrophobically modified polymer. If hydrophobic side chains are attached to the polymer backbone, the polymer may already be partially collapsed by itself and should not show any further instability upon addition of surfactant. In this case, it is argued that no sharp onset of surfactant binding to the polymer (cac) is expected, and the binding to such a chain should progress gradually as a function of surfactant concentration. These results are consistent with the experimental findings for several systems. From the aspects given above it is obvious that an extensive knowledge of solution behavior of EHEC and HM-EHEC in the presence of ionic surfactants has been accumulated from studies with various experimental techniques. However, our understanding of the adsorption properties of these polymers onto colloidal particles in the presence of a surfactant is rather restricted.28 The adsorption of polymers and surfactants, and their effects on the adsorbed layer thickness at the solid/liquid interface, are important in understanding the mechanisms of stabilization and flocculation of dispersions and emulsions. The interactions between polymer and surfactant and the morphology of polymer-surfactant complexes are expected to affect29-37 the adsorption behavior on colloidal particles. (21) Zana, R.; Binana-Limbele´, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992, 96, 5461. (22) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (23) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (24) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (25) Evertsson, H.; Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Langmuir 1996, 12, 5781. (26) Kjøniksen, A.-L.; Nystro¨m, B.; Lindman, B. Langmuir 1998, 14, 5039. (27) Diamant, H.; Andelman, D. Europhys. Lett. 1999, 48, 170. (28) Malmsten, M.; Lindman, B. Langmuir 1990, 6, 357. (29) Argillier, J.-F.; Ramachandran, R.; Harris, W. C., Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242.

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The EHEC and HM-EHEC polymers consist of hydrophilic regions and hydrophobic microdomains. This structure suggests that these polymers should adsorb onto hydrophobic substrates and thereby impart stability by a steric effect. Measurements have shown28 that EHEC has a strong affinity not only for hydrophobic surfaces but also for hydrophilic surfaces. Although the polymer adsorption of EHEC on colloidal particles is well established, there is a lack of studies dealing with the adsorption properties of EHEC and HM-EHEC on substrates in the presence of a surfactant. In this paper, we report results from dynamic light scattering (DLS) measurements on the adsorption of EHEC or HM-EHEC onto monodisperse anionic polystyrene latex (PSL) particles in the presence of the anionic surfactant sodium dodecyl sulfate (SDS) or with the cationic surfactant hexadecyltrimethylammonium bromide (CTAB). DLS is a powerful technique in the determination of the z-average of particle diameters present in a suspension, and the thickness of an adsorbed layer can, in principle, be calculated from the size difference between coated and uncoated particles. The principal aim of this work is to survey the effect of surfactant addition on the hydrodynamic thickness of the absorbed layer of these polymers. The binding of an ionic surfactant to the polymer endows an apparent polyelectrolyte character to the originally nonionic EHEC and HM-EHEC polymer. In light of this, we would expect that an intricate interplay between electrostatic effects and hydrophobic interactions should govern the adsorption features. However, the results seem to indicate that the value of the cac concentration is a more crucial factor for the characteristic features of the adsorption process than the electrostatic forces. Experimental Section Materials and Solution Preparation. Both the unmodified and the hydrophobically modified polymer were supplied by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. The unmodified polymer is an ethyl(hydroxyethyl)cellulose (EHEC). The hydrophobically modified polymer (HM-EHEC) is equivalent to the EHEC sample with the same molecular weight (Mw ≈ 100 000) and degrees of ethyl and hydroxyethyl substitutions, DSethyl ) 0.6-0.7 and MSEO ) 1.8, respectively, but with a low number of branched nonylphenol groups grafted to the polymer backbone. The degree of substitution was determined to be 1.7 mol % (corresponding to approximately 6.5 groups per molecule), relative to repeating units of the polymer, by measuring the absorbance of the aromatic ring in nonylphenol at a wavelength of 275 nm with a Shimadzu UV-160 UV-vis spectrometer and using phenol in aqueous solution as a reference. The values of DS and MS correspond to the average number of ethyl and hydroxyethyl groups per anhydroglucose unit of the polymer. The values of Mw, DS, and MS were all given by the manufacturer. A schematic representation of the chemical structures of the polymers is displayed in Figure 1. Before use, the polymers were purified as described elsewhere38 and stored in a desiccator. The (30) Tanaka, R.; Williams, P. A.; Meadows, J.; Phillips, G. O. Colloids Surf. 1992, 66, 63. (31) Shubin, V. Langmuir 1994, 10, 1093. (32) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45. (33) Cosgrove, T.; Mears, S. J.; Thompson, L.; Howell, I. ACS Symp. Ser. 1995, 615, 196. (34) Otsuka, H.; Esumi, K.; Ring, T. A.; Li, J.-T.; Caldwell, K. D. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 116, 161. (35) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4997. (36) Giasson, S.; Weitz, D. A.; Israelachvili, J. N. Colloid Polym. Sci. 1999, 277, 403. (37) Otsuka, H.; Ring, T. A.; Li, J.-T.; Caldwell, K. D.; Esumi, K. J. Phys. Chem. B 1999, 103, 7665. (38) Thuresson, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1995, 99, 3823.

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Lauten et al. (to ensure that qRh < 1, i.e., to probe the diffusive mode) and at temperatures from 15 to 30 °C with an ALV 5000 multiple-τ digital correlator. If the scattered field is assumed to have Gaussian statistics, the measured intensity autocorrelation function g2(q,t) is directly related to the theoretically amenable first-order electric field correlation function g1(q,t) by the Siegert relationship g2(q,t) ) 1 + B|g1(q,t)|2, where B is an instrumental parameter. The correlation functions were recorded in the real-time “multiple-τ” mode of the correlator, in which 256 time channels are logarithmically spaced over an interval ranging from 0.2 µs to almost 1 h. The accumulation time is typically about a few minutes. Concerning the scattering intensity, the contribution of the latex spheres dominates at all the considered conditions. In this work, as well as in other DLS investigations40,41-43 on colloidal systems, the decays of the correlation functions have been found to be well described by a stretched exponential

g1(t) ) exp[-(t/τfe)β] Figure 1. Structural representation of EHEC and HM-EHEC. cationic CTAB and anionic SDS were both obtained from Fluka and were used without further purification. The monodisperse anionic polystyrene latex particles were prepared by F. K. Hansens group at the University of Oslo. The particles were synthesized by emulsion polymerization under a stream of nitrogen at 80 °C using sodium dodecyl sulfate as emulsifier and potassium peroxosulfate as initiator. The procedure employed in the preparation of the polystyrene particles has been described in detail elsewhere.39 The latex particles were dialyzed for two weeks, with frequent changes of the dialysis water, in a cellulose dialysis tubing. The zeta potential is about -60 mV, and the negative zeta potential is due to sulfate groups on the particle surface. The stability of colloidal suspensions was checked over several weeks by measurements of the mutual diffusion coefficient to ensure that irreversible aggregation of polystyrene spheres does not take place. From DLS, the hydrodynamic radius was determined to be 87 nm. In each experiment, the PSL particles were dispersed by ultrasound before mixing with the polymer solutions. All solutions were prepared by weighing the components, allowing for complete dissolution before mixing with the particles. The solutions were filtered directly into precleaned 10-mm NMR tubes (Wilmad Glass Company) of the highest quality in a protected atmosphere of filtered air. Finally, the tubes containing the clarified solution were sealed. The polymer-surfactant solution was always added to the PSL suspension, and the mixture was homogenized. For similar systems, it has been found40 that the order of mixing the components does not affect the results. In all experiments, the particle and polymer concentrations were kept constant at 0.001 and 0.01 wt %, respectively. At these concentrations, the latex particles are fully covered with polymer.40 The surfactant concentration was varied in the range 0-50 mmolal. Dynamic Light Scattering. The beam from an argon ion laser (Spectra Physics Model 2020), operating at 488 nm with vertically polarized light, was focused onto the sample cell through a temperature-controlled chamber (temperature-controlled to within ( 0.05 °C) filled with refractive-index-matching dibuthyl phthalate. In light scattering experiments, we probe a wave vector q ) (4πn/λ)sin(θ/2), where λ is the wavelength of the incident light in a vacuum, θ is the scattering angle, and n the refractive index of the medium. The value of n was determined for each solution and at each measurement temperature at λ ) 488 nm by using an Abbe´ refractometer. In this investigation, the full homodyne intensity autocorrelation function was measured mostly at a scattering angle of 30° (39) Hansen, F. K.; Evenrød, A. Fremstilling av seed for MPP; DYNO Industries: Lillestrøm, Norway, 1983. (40) Kjøniksen, A.-L.; Joabsson, F.; Thuresson, K.; Nystro¨m, B. J. Phys. Chem. B 1999, 103, 9818.

(1)

where τfe is some effective relaxation time and β (0 < β e 1) is a measure of the width of the distribution of relaxation times. The values of β at all experimental conditions are close to 0.9, suggesting a narrow distribution of relaxation times. The mean relaxation time is given by

τf ) (τfe/β)Γ(1/β)

(2)

where Γ(β-1) is the gamma function of β-1. The correlation functions were analyzed by using a nonlinear fitting algorithm (a modified Levenberg-Marquardt method) to obtain best-fit values of the parameters τfe and β, appearing on the right-hand side of eq 1. A fit was considered satisfactory if there were no systematic deviations in the plot of the residuals of the fitted curve. From the mean relaxation time τf, the mutual diffusion coefficient D is readily obtained from the expression 1/τf ) Dq,2 and the apparent average hydrodynamic radius Rh of the particles is related to D via the Stokes-Einstein relationship:

Rh )

k BT 6πηD

(3)

where kB is Boltzmann’s constant, T is the absolute temperature, and η is the viscosity of the water-surfactant medium. The hydrodynamic thickness of the adsorbed polymer layer, δh, can be calculated by subtracting the bare particle radius from that of the polymer-covered particle.

Results and Discussion Adsorption or desorption of dispersant molecules on the surfaces of colloidal particles should affect their hydrodynamic radius, and this should be measurable with DLS. It will be shown that δh in the presence of surfactant is subject to interference from that surfactant, which can either promote or prevent adsorption. The results presented below are concerned with the adsorption and desorption of EHEC and HM-EHEC on anionic PSL particles at various levels of SDS or CTAB addition. In this work, the surfactant concentration simply refers to the total concentration, that is, the surfactant adsorption to particles and polymer has not been taken into account. One could also utilize the concentration of free surfactant in the system, but this quantity is more difficult to determine. However, since the particle (0.001 wt %) and polymer (0.01 wt %) concentrations are both low, the total (41) Phillies, G. D. J.; Richardson, C.; Quinlan, C. A.; Ren, S. Z. Macromolecules 1993, 26, 6849. (42) Ngai, K. L.; Phillies, G. D. J. J. Chem. Phys. 1996, 105, 8385. (43) Phillies, G. D. J.; Lacroix, M. J. Phys. Chem. B 1997, 101, 39.

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Figure 2. Schematic depiction of the adsorption of the negatively charged polymer-SDS complex (left) and the positively charged polymer-CTAB complex (right) on anionic PSL particles.

and free surfactant concentration should be fairly close to each other. A schematic representation of the adsorbed polymer-surfactant complex in the presence of an anionic or a cationic surfactant is depicted in Figure 2. This illustration shows that the polymer may be negatively or positively charged, depending on the character of the surfactant. On increasing the surfactant bulk concentration, progressive binding of surfactant to the nonionic polymer leads to an increase in the net negative (SDS) or positive (CTAB) charge of the macromolecule. This difference in sign of the charged polymer-surfactant complexes may influence the adsorption-desorption properties of the system. However, we have not been able to establish any impact of the electrostatic interaction. At the experimental conditions considered in this work, the hydrodynamic thickness of the adsorbed layer was found to be time-independent. This suggests that equilibrium adsorption is reached within a few minutes. This observation is consistent with that reported40 from a recent DLS study of PSL particles immersed in aqueous solutions of EHEC and HM-EHEC. However, we should point out that for PSL particles immersed in solutions of HM-EHEC at SDS concentrations in the interval 2-4 mmolal, a rapid and significant aggregate growth was observed. This effect is not the theme of the present paper, but this issue will be addressed in a forthcoming publication.44 Therefore, measurements in this SDS concentration range in the presence of HM-EHEC have been avoided in this work. The effects of temperature and SDS addition on the hydrodynamic radius of PSL without polymer and in solutions of EHEC are shown in Figure 3. In suspensions without polymer, the overall picture that emerges is that SDS has no significant effect on the hydrodynamic size of PSL at the considered temperatures. However, at 25 °C there seems to be a slight increase in Rh at very low levels of SDS. We have not observed the rise in Rh of bare particles with increasing SDS, as was reported previously,45 but our result rather conforms with that of Mears et al.35 The variation in the hydrodynamic radius of the polymersurfactant complex-covered particles with increasing levels of surfactant addition is also illustrated in Figure 3. The general trend, at the considered temperatures, is that Rh passes through a maximum and then decreases with increasing surfactant concentration. The maximum is located at a surfactant concentration of about 2-3 mmolal, which is close to the value of cac (3-3.5 mmolal) reported13 for solutions of the same EHEC batch in the presence of SDS. It has been reported from some studies22,25 on aqueous EHEC/SDS solutions that a rise in temperature lowers the cac, while in other investigations,24,25 (44) Lauten, R. A.; Kjøniksen, A.-L.; Nystro¨m, B. To be submitted for publication. (45) Brown, W.; Zhao, J. Macromolecules 1993, 26, 2711.

Figure 3. Effects of temperature and surfactant addition on the hydrodynamic radius for the EHEC/SDS/PSL system. The solid curves are shown as guides for the eye.

the temperature had no noticeable effect on cac. In the present work, we could not, within experimental accuracy, establish a temperature influence on the position of the maximum. The marked rise of Rh at low surfactant concentrations is probably due to the formation of polymer-surfactant clusters at the solid-liquid interface. This finding is consistent with a recent DLS study26 on dilute aqueous solutions of EHEC with SDS (without PSL particles), in which the hydrodynamic radius was found to go through a maximum at about 2-3 mmolal SDS and the average cluster size decreased at higher levels of surfactant addition. This observation supports our surmise that the enhanced polymer-surfactant interaction around the cac gives rise to the formation of polymer-surfactant clusters at the surfaces of the particles. At higher surfactant concentrations, desorption of the polymer occurs, and this process appears to be complete, or nearly complete, around the critical micelle concentration (cmc, 8 mM) of the pure surfactant. We should note that in the above-cited DLS work on dilute EHEC/SDS solutions, complete deaggregation of the EHEC chains was reported around this SDS concentration. In view of this, our conjecture is that the desorption process is a result of the fact that the polymerbound micelles effectively increase the hydrophilicity of the EHEC chains and that it is more favorable for the polymer to be in the bulk solution with SDS than on the particle surfaces. A schematic representation of the desorption process of the polymer-surfactant complexes is displayed in Figure 4. It may be argued that these results do not necessarily prove that the polymer is desorbed from the substrate but that the EHEC conformation is strongly flattened on the PSL surface. However, this scenario is unlikely because the thermodynamic conditions of the system are amended as the surfactant concentration increases, and thus a contraction of the hydrodynamic thickness is not expected. Another effect that may promote desorption is the electrostatic repulsive forces between anionic PSL particles and SDS decorated EHEC chains. However, as will be discussed below, similar features are

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Figure 5. Effects of temperature and surfactant addition on the hydrodynamic radius for the HM-EHEC/SDS/PSL system. The solid curves are shown as guides for the eye. Figure 4. A schematic illustration of the desorption of the polymer-surfactant complex as the surfactant concentration increases (1 f 3).

observed in the presence of a cationic surfactant (CTAB), which may indicate that the electrostatic situation does not play a crucial role in the desorption behavior. It has been reported31,33,35 for other polymer/colloid/surfactant systems that around the cmc, almost all the polymer has desorbed from the surface. A related problem has recently been addressed in a neutron reflectivity and surface tension study,46 in which the effect of SDS on the adsorption behavior of poly(N-isopropylacrylamide) at the air-water interface was scrutinized. In the presence of SDS below the cac, the polymer adsorption is unaffected, while above the cac, the polymer is gradually displaced from the surface. It was argued that the loss of polymer from the surface is related to the complexation of polymer with surfactant in solution. In a recent DLS and small-angle neutron scattering study35 on the poly(ethylene oxide)/SDS/PSL system, the hydrodynamic layer thickness appeared to increase strongly at high levels of surfactant addition, suggesting that polymer with some bound micelles adsorbs at the interface in an extended form. This type of behavior has not been observed for the present systems, but Rh rises only slightly at higher surfactant concentrations. The reason for this difference is not clear but may be attributed to the different natures of the present and the previously studied polymer/surfactant/PSL systems. The surfactant-induced desorption process for the HMEHEC/SDS/PSL system at different temperatures is displayed in Figure 5. The general trend, at all temperatures, is that Rh falls off with increasing SDS concentration up to about 10 mmolal, and, at higher levels of surfactant addition, Rh becomes practically constant. Despite the fact that the thermodynamic conditions of (46) Jean, B.; Lee, L.-T.; Cabane, B. Langmuir 1999, 15, 7585.

Figure 6. Comparison of changes in the hydrodynamic radius for the systems EHEC/SDS/PSL and HM-EHEC/SDS/PSL with increasing surfactant concentration at 25 °C. The solid curves are shown as guides for the eye.

the system become worse at elevated temperature, it is not possible to detect a significant effect of temperature on the apparent hydrodynamic radius. We may note that in the absence of surfactant, the hydrodynamic thickness is larger for the hydrophobically modified analogue than for EHEC (see Figure 6). This is due to the stronger adsorption capacity of the hydrophobically modified polymer on PSL particles. It appears that not even at very high surfactant concentration (50 mmolal) does a complete desorption occur. This is probably due to the existence of some hydrophobic tails that are not desorbed from the surfaces of the particles. To facilitate a direct comparison of the pattern of behavior of the systems containing EHEC and HM-EHEC, a plot of δh versus SDS concentration at 25 °C is depicted in Figure 6. It is obvious that these systems exhibit different features at low and moderate surfactant concentrations, while at higher levels of surfactant addition, the difference gradually disappears. Our conjecture is that the difference in desorption behavior between EHEC and the hydrophobically modified ana-

Ethyl(hydroxyethyl)cellulose on Polystyrene Latex

Figure 7. Effect of CTAB addition on the hydrodynamic radius for the following systems at 25 °C: (a) EHEC/CTAB/PSL and (b) HM-EHEC/CTAB/PSL. A logarithmic scale is used on the abscissa to establish the maximum in Rh for the EHEC/CTAB/ PSL system. The solid curves are shown as guides for the eye.

logue is associated with the onset of surfactant binding to the polymer. It has been shown13 that the binding of SDS to the unmodified EHEC has a fairly pronounced cac (3-3.5 mmolal), while binding to the hydrophobically modified EHEC starts early (at very low amounts of surfactant) and is noncooperative up to a concentration of about the cac of the unmodified EHEC/SDS system. For the HM-EHEC/SDS system, hydrophobic domains, originating from the polymers’ hydrophobic tails, exist already at very low SDS concentrations. Since the binding of surfactant to HM-EHEC begins at a very low SDS concentration, we will probably not be able to discern an interaction peak of the kind observed for the unmodified polymer. Due to the binding of surfactant to the hydrophobically modified polymer at very low levels of surfactant addition, the polymer starts to desorb at a low SDS concentration. To elucidate how altered electrostatic interactions and the value of cac affect the adsorption and desorption features of these systems, we also carried out experiments in the presence of the cationic surfactant CTAB, which possesses a much lower cmc (1 mM) and cac than the anionic SDS. The effects of surfactant addition on the adsorption-desorption properties of the systems EHEC/ CTAB/PSL and HM-EHEC/CTAB/PSL at 25 °C are shown in Figure 7. The effect of CTAB on the hydrodynamic size of PSL is similar to that observed with SDS. For the EHEC/ CTAB/PSL system, the general behavior of Rh is reminiscent of that of the EHEC/SDS/PSL system, but for the former system, the maximum of the peak is shifted toward a lower surfactant concentration (0.3 mmolal) and the interaction peak is less pronounced. To the best of our knowledge, no cac has been reported for the present EHEC batch in the presence of CTAB. However, for EHEC samples with different degrees of substitution in the presence of CTAB, microcalorimetric titration experiments24 revealed cac values of 0.3 mmolal and 0.7 mmolal for the EHEC samples designated Bermocoll CST 103 and Bermocoll E230G, respectively. In addition, a cac value of 0.5 mmolal has recently been reported15 for the same batch as that utilized in this work in the presence of hexadecyltrimethylammonium chloride (CTAC), which is a surfactant with very similar properties to CTAB. Since

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the cmc of CTAC is 1.3 mmolal,47 we expect that the cac value of this EHEC fraction with CTAB (cmc is 1.0 mmolal) is somewhat lower than 0.5 mmolal. This result suggests that the location of the maximum of the Rh peak for the EHEC/CTAB/PSL system is close to the cac value. This is another indication of the close correlation between the value of cac and the location of the maximum of Rh. The fact that the maximum of the interaction peak is less marked in the presence of CTAB than for the EHEC/SDS/ PSL system is probably due to a weaker polymersurfactant interaction at the particle surface in the presence of a cationic surfactant. This weaker polymersurfactant interaction in the presence of a cationic surfactant as compared with an anionic surfactant is a well-known phenomenon that has been observed for EHEC-surfactant systems9,11,24,48 as well as for other polymer-surfactant systems.24,48 In similarity to the EHEC/SDS/PSL system, we can see that also with CTAB, the desorption seems to be completed at the cmc (1 mmolal) of this surfactant. In the case of the HM-EHEC/CTAB/PSL system, the desorption process is reminiscent of that observed in the presence of SDS, with a gradual decrease of Rh with increasing surfactant concentration. A complete desorption appears to occur already at a low (1 mmolal) level of CTAB addition, which is close to the cmc of this surfactant. Conclusions The interaction of ethyl(hydroxyethyl)cellulose (EHEC) or its hydrophobically modified analogue (HM-EHEC) with an anionic (SDS) or a cationic (CTAB) surfactant at the polystyrene latex (PSL)/liquid interface was investigated with the aid of dynamic light scattering. The hydrodynamic layer thickness of EHEC on anionic PSL particles passes through a maximum, the maximum is more pronounced with SDS than in the presence of CTAB, and it appeared that complete desorption of the polymer occurred around the cmc of the respective surfactant. The maximum of the peak is located at the cac of the respective EHEC/surfactant pair. Experiments on the EHEC/SDS/PSL and HM-EHEC/SDS/PSL systems were carried out at different temperatures. However, the temperature had no noticeable effect on the adsorption-desorption behavior of the polymer as probed with DLS. The peak is attributed to the formation of polymer-surfactant clusters on the PSL particles. The conjecture is that at higher surfactant concentrations, the binding of surfactant to the polymer effectively increases the hydrophilicity of the chain, and it is more favorable for the polymer to stay in the bulk solution than anchored to the particles; hence, desorption occurs. DLS measurements on the systems (HM-EHEC/SDS/ PSL and HM-EHEC/CTAB/PSL) containing the hydrophobically modified polymer revealed, for both systems, that the hydrodynamic thickness of the adsorbed layer decreased monotonically as the surfactant concentration increased. The lack of an interaction peak for these systems was ascribed to a very low or nonexistent cac of these polymer-surfactant complexes. In the absence of surfactant, the hydrodynamic thickness of the adsorbed layer was larger for the hydrophobically modified polymer. On increasing the surfactant (SDS or CTAB) bulk concentration, progressive binding of surfactant to the (47) Mukerjee, P.; Mysels, K. J. CMC of Aqueous Surfactant Systems; National Standards Reference Data Series; U.S. National Bureau of Standards, U. S. Goverment Printing Office: Washington, DC, 1971. (48) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276.

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polymer leads to an increase in the net negative (SDS) or positive (CTAB) charge of the macromolecule. The similar adsorption-desorption features observed with anionic or cationic surfactant suggest that the sign of the charge of the polymer-surfactant complexes is not decisive for the polymer adsorption onto negatively charged PSL surfaces.

Lauten et al.

Acknowledgment. The authors are grateful for valuable discussions with Fredrik Joabsson and Krister Thuresson, University of Lund. We thank Krister Thuresson for the preparation and the supply of the polymers. LA991460C