Langmuir 1991, 7, 2248-2252
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Temperature-DependentForces between Hydrophilic Mica Surfaces Coated with Ethyl(hydroxyethy1)cellulose Isabelle Pezron? Erwoan Pezron,? Per M. Claesson,+and Martin Malmsten'lt The Surface Force Group, Department of Physical Chemistry, The Royal Institute of Technology, S-10044 Stockholm, Sweden, The Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden, and Physical Chemistry 1, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden Received February 7, 1991. I n Final Form: March 27, 1991 The forces acting between hydrophilic mica surfaces across aqueous ethyl(hydroxyethy1)cellulose (EHEC) solutions have been investigated with the surface force technique. It was found that the adsorbed amount is rather low at these surfaces and that the adsorbed polymer molecules are easily squeezed out from the gap between the surfaces on compression at low temperatures (20 O C ) . The force-distance curve is purely repulsive at this temperature, and up to the point where the EHEC molecules are squeezed out, all forces were found to be reversible on compression decompression. The adsorbed amount increases somewhat with temperature, but remains quite low at 1temperatures. Moreover, it becomes increasingly difficult to squeeze out the adsorbed polymer molecules by applying an external force. It was found that the temperature dependence was not completely reversible over the incubation time used (20 h). On diluting the solution to roughly 104 wt % ,only a limited desorption occurs. The temperature dependence of the interaction after dilution is similar to that before dilution. A comparison of the forces acting between EHEC-coatedmica surfaces and EHEC-coated hydrophobicsurfaces demonstratesthe importance of the surface-polymer interaction.
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Introduction In many practical situations, polymers are used due to their interesting surface properties. This is the case in the design of sterically stabilized colloidal dispersions' and many biocompatible surfaces.2 Naturally, both the adsorption characteristics and the stabilizing effect of the adsorbed polymer depend on several physicochemical properties. These include for example the solvency of the polymer (often discussed in terms of the x parameter3) and the interaction between the polymer and the surface (described by x ~ ) ~There * ~ are, .however, also other properties of importance when discussing, e.g., steric stabilization. These include, among others, lateral interactions between adsorbed polymer molecules, surface mobility, and dynamic effects. Ethyl(hydroxyethy1)cellulose (EHEC) is a nonionic cellulose ether, which is obtained by chemical modification of cellulose with ethyl groups and oligo(ethy1eneoxide) chains. Like many other nonionic polymers and surfactants, EHEC shows a reversed temperature-dependent phase behavior and a lower consolute temperature.- The lower temperature phase boundary is often called the cloud point (CP). The cloud point of EHEC depends on the molecular weight and the degrees of substitution. It is also strongly affected by the presence of low-molecular-weight cosolutes, like salts, alcohols, and surfactants.**g + The Royal Institute of Technology and The Institute for Surface Chemistry. t Chemical Center. (1) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (2) Kim, S. W.; Feijen, J. Crit. Rev. Biocompat. 1986, I, 229. (3) Flory, P. J. Principles of Polymer Chemistry, Comell University P r w : Ithaca, New York, 1953. (4) Scheutjens,J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979,83,1619. (5) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1980,84,178. (6) Nonronrc Surfactants-Physical Chemistry, Surfactant Science Series; Marcel Dekker: New York, 1987; Vol. 23. (7) Bailey, F. E., Jr.; Koleake, J. V. Poly(ethylene oxide);Academic Press: New York, 1976. (8) Lindman, B.; Carleson, A.; KarlatBm, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990,32, 183. (9) Malmsten, M.; Lindman, B. Langmuir 1990,6, 357.
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From previous ellipsometry experiments, it has been concluded that the main driving force for adsorption of EHEC onto silanated silica surfaces of different hydrophobicity is a hydrophobic interaction between the polymer and the surface (although the expulsion from solution per se constitutes an important mechanism for adsorption in many of these systems), since both the adsorbed amount and its temperature dependence are much larger at a hydrophobic surface than a t a hydrophilic one.g The surface force technique,1° which allows measurement of the forces between two surfaces to be made as a function of their separation, has previously been used in order to study the forces acting between hydrophobic surfaces (mica coated by a Langmuir-Blodgett-deposited monolayer) coated with EHEC, inter alia as a function of temperature.11J2 It was found that EHEC adsorbs both extensively and strongly onto hydrophobic surfaces and that the adsorbed amount increases significantly when the cloud point is approached.12 Furthermore, it was found that the adsorbed layer contracts a t higher temperatures. The force was observed to be purely repulsive up to a few degrees above the CP. At temperatures well above the CP, the force was found to be attractive. The observed temperature dependence was rationalized inter alia by considering the orientation of the adsorbed polymers at the interface. In the present paper, we investigate the adsorption of EHEC onto hydrophilic muscovite mica surfaces and the interaction between these surfaces with the surface force technique. We present some results that highlight the effects of temperature on the adsorption as well as the effects of surface mobility on the interaction between weakly adsorbing surfaces. By comparison with previous results obtained with hydrophobic surfaces,12we are able to gain some information on the importance of the surfacepolymer interaction. (10) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans.
1 1978, 74, 975.
(11) Malmsten, M.; Claesson, P. M.; Pezron, E.; Pezron, I. Langmuir 1990, 6, 1572. (12) Malmsten, M.; Claesson, P. M. Langmuir 1991, 7, 988.
0 1991 American Chemical Society
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Forces between Hydrophilic Mica Surfaces Experimental Section Polymer. Ethyl(hydroxyethy1)cellulose (EHEC)was supplied by Berol Nobel AB, Sweden. The EHEC fraction used has an average molecular weight of 250 000 and a radius of gyration of about 800 A, as determined by light scattering. The molecular weight distribution is broad, as evidenced by GPC. The molar substitutionof ethyl groups and ethylene oxide groups is 1.4 and 0.9, respectively. The cloud point of this particular fraction is 39 O C on heating. Dry EHEC powder normally contains 3-5 wt 7% of NaCl (impurityfrom synthesis),and therefore the EHEC solutions were dialyzed against membrane-filtered water (Millipore, USA) for 5 days. As a dialyzing membrane, regenerated cellulose with a molecular weight cut-off of 6000 was used (SpectrumMedicalIndustries,USA). After dialysis the polymer was freeze-dried. Surface Force Measurements. Measurementsof the forces acting between two negatively charged, hydrophilic muscovite mica surfaces across EHEC solutions were carried out with a surface force apparatus. Both an apparatus of the type originally designed by Israelachvili and co-workers (Mark II)l0and an apparatus of the modified design by Parker et al.lSwere used in this set of experiments. The two interacting surfaces are glued onto curved, optically polished silica disks and mounted in a crossed cylindrical geometry. This geometry is experimentally suitable, and the measured force, F,,divided by the local geometric mean radius of the cylinders, R, is related to the free energy of interaction per unit area between two flat surfaces, Gf, as first demonstrated by Derjauginl' F,(D)/R = 2*G,(D)
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provided that the radius of the cylinders (about 2 cm) is much larger than the surface separation (D). The distance between the surfaces is determined with an accuracy of 0.2 nm by using multiple-beam interferometry. The force is measured, with a detection limit of about lo-' N, from the deflection of a doublecantileverspring supporting the lower surface. Due to some drift in the system, the detection limit of the force at 45 "C was somewhat higher. The experiments were performed in the following way: First, the contact position between the surfaces was determined in air. This position defines the zero separation. Then a droplet of a 0.1 wt 5% EHEC solution was placed between the surfaces,whereafter the forceswere measured as a function of surface separation and temperature. Finally, the apparatus was filled with water (final EHEC concentration about 1Wwt 7% 1, and the forceswere measured again at different temperatures. The temperature was measured with a thermistor inside the surface force apparatus. When a droplet was used, the temperature of the air surrounding the droplet was recorded. The temperature difference between the surrounding air and the droplet is regarded as small since the solution in the droplet was observed to become cloudy at the normal temperature (to within 1-2'). The adsorbed amount was determined from the thicknessand the refractive index of the adsorbed layer at a distance of separation of 50-100 A by using the formula of de Feijterl'j and dn/dc = 0.158 cmS/g. After any change of the conditions,the system was allowed to equilibrate for several hours.
Results The forces measured between two mica surfaces across
a 0.1 wt 7% aqueous EHEC solution are shown as a function of surface separation in Figure 1. Results from three different force measurements, performed after more than 20 h adsorption a t a temperature of 20 "C, are shown in order t o illustrate the reproducibility. At large distances, an electrostatic double-layer force, originating (13) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Reo. Sci. Instrum. 1989,60, 3135. (14) Derjaugin, B. V. Kolloid-2. 1934,69, 155. (15) de Feijter, J. A.; Benjamim,J.; Veer,F. A. Biopolymers 1978,17, 1759.
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Figure 1. Force measured between two mica surfaces (normalized by the radius of curvature) across a 0.1 7% EHEC solution as a function of surface separation at 20 "C. Results from three different force measurementsare shown in order to illustratethe reproducibility: ( 0 )forces measured on the first approach; ( 0 ) forces measured on the second approach at the same position; (w) forces measured in a second experiment using another pair of mica sheets. The solid line represents DLVO forcescalculated by using a nonretarded Hamaker constant of 2.2 X 10-20 J, interaction at constant charge with a surface potential at large separations of 50 mV, and a Debye length of 250 A. The insert shows the forces plotted on a logarithmic scale. from charges on the mica surfaces, predominates the interaction. The electrostatic part of the force curve can be fitted by forces calculated in the nonlinear PoissonBoltzmann approximation by using the method of Chan et al.I6 and by assuming interaction at constant charge, with a surface potential at large separations of 50 mV and a Debye length of 250 A. (Note that the constant-charge approximation constitutes an upper limit of the doublelayer interaction, as discussed previ~usly.*~J*) However, at distances below approximately 100 A a repulsive force stronger than that expected from Poisson-Boltzmann theory is observed. This additional repulsive force arises from steric interactions between adsorbed polymer molecules. Under a sufficiently high compressive load these are squeezed out from the gap between the surfaces. The force needed to accomplish this is rather small, varying between 2500 and 3000 pN/m. Once the force barrier is overcome the mica surfaces reach adhesive contact. The attractive force (at D = 0) measured on separation has a value of 20-25 mN/m, which is significantly lower than that observed in polymer-free dilute electrolyte solutions (about40 mN/m). The adsorbed amount, measured when the surfaces were 50-100 A apart, was found to be in the range 0.2-1 mg/m2. An increase in temperature to 35 "C (4 "C below the cloud point) results in an increased adsorbed amount (Figure 6). Furthermore, the force needed to squeeze out the adsorbed polymer molecules from the gap between the surfaces increases to about 4000 pN/m. The doublelayer force at 35 "C is weaker than that at 20 "C, but i t still dominates the long-range interaction (Figure 2).The surface potential at large distances is 40 mV and the Debye length is 250 A, is inferred by fitting DLVO theory to the measured forces. At 45 OC, 6" above the cloud point, no repulsive doublelayer force is observed, which indicates that the surface potential is below about 10-15 mV. At this temperature quantitative aspects of the force curves are uncertain, inter alia due to experimental difficulties caused by thermal drift. As can be seen in Figure 3, a repulsive force is observed on compression also at this temperature. The (16) Chan, D.Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (17) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (18) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531.
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Figure 4. Force measured between two mica surfaces (normalized by the radius of curvature) across a 0.1 % EHEC solution as a function of surfaceseparation at 20 OC: ( 0 )forcesmeasured before the system was heated to 45 OC; (U) forces measured after the system had been heated to 45 "C. The solid line has the same meaning as in Figure 1. 20000
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force is more repulsive on approach than on separation. On separation a weak minimum is observed in the force curve. However,the range of the force, the layer thickness, and the location of the minimum vary between experiments and between different positions on the surface. The main finding, however, is that it is not possible to squeeze out the molecules from the gap between the surfaces at this temperature. Instead, when the compressive force was increased to about 10 000-20 000 pN/m the surface started to flatten. At even higher forces, molecules located a t the edge of the flat region were pushed out, whereas those in the center were trapped between the surfaces. This resulted in characteristic bell-shaped and "irreversible" changes in the adsorbed layer. This phenomenon, which is known as elastohydrodynamic lubrication, has also been observed for some proteins on mica.lB After the system was cooled to 20 "C again, the forces were remeasured. After the system was heated, the repulsive double-layer force was essentially the same as that observed before heating. However, the force needed to squeeze out the polymers from the gap between the surfaces was larger than that before heating (Figure 4). The interaction properties change somewhat when the EHEC solution is diluted with pure water to about 10-4 w t 5% (Figure 5). The long-range double-layer force after dilution is characterized by a surface potential of 100 mV and a Debye length of 600 A. (The finding that the decay length is longer after dilution than before seems to indicate electrolyte impurities of a finite concentration in the (19) Blomberg, E.; Claesson, P. M.; Christenson, H.K. J. Colloid Interface Sci. 1990, 138, 291. (20)Roberta, A. D.; Tabor, D. h o c . R. SOC.London 1972, A325,323.
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Figure 2. Force measured between two mica surfaces (normalized by the radius of curvature) across a 0.1 % EHEC solution as a function of surface separation: ( 0 )forces measured at 20 OC;(m) forcesmeasured at 35 OC. The solid linesare theoretically calculated DLVO forcea, assuminginteractionat constant charge. The Hamaker constant is 2.2 X lem J, and the Debye length 250 A. The surface potential on noninteracting surfaces is 50 mV (upper line) and 40 mV (lower line).
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Figure 5. Force measured between two mica surfaces (normalized by the radius of curvature) immersed in a 1W% EHEC solution as a function of surface separation at (u) 20 "C and ( 0 ) 45 OC. The mica surfaces were first allowed to equilibrate in contact with a 0.1 % EHEC solution for 2 days before the solution was diluted with water. The solid line is a theoretical DLVO force curve calculatedby assuming interactionat constant charge, a nonretarded Hamaker constant of 2.2 X 10-20 J, a surface potential at large separations of 100 mV, and a Debye length of 600 A.
polymer sample, despite the extensive dialysis.) Forces due to adsorbed polymers are significant at distances below about 50-100 A. The force needed to squeeze out the adsorbed polymer molecules from the interaction zone varies with temperature and is about 2000-3000 pN/m a t 20 OC and close to 20 OOO pN/m at 45 OC. The adhesion force at mica-mica contact (35-45 mN/m) is the same as that observed in pure water (40 mN/m).
Discussion State of the Adsorbed Layer at 20 O C . One of the main findings in this study is that even though EHEC does adsorb onto hydrophilic mica surfaces, the adsorbed amount is quite low (Figure 6). Furthermore, at temperatures below the cloud point, it is easy to squeeze out the molecules from the interaction zone. The molecules may leave the contact zone by surface diffusion or by desorption. In either way, it is clear that EHEC molecules are weakly absorbed onto hydrophilic mica surfaces a t 20 "C. As long as the surfaces are kept away from mica-mica contact, similar forces are observed on approach and separation; i.e., the measurements are performed a t quasiequilibrium conditions. This means that until this point, the measurements are performed slowly compared to the time scale for desorption/adsorption, conformational rearrangements, and lateral translations. (It typically takes 30 min to collect data for a complete compression/decompression cycle.) These findings indicate that the adsorbed polymer molecules are quite mobile,
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which in turn indicates that EHEC adsorbs essentially a separate coils at this temperature. Another interesting finding is that the surface charge density, deduced by fitting forces calculated in the nonlinear Poisson-Boltzmann approximation to the measured forces, is significantly lower in the EHEC solution than in electrolyte solutions.10J7 This indicates that adsorption of the uncharged polymer reduces the surface charge density. A similar effect has been observed when nonionic surfactants21 adsorb onto mica surfaces. We propose that this reduction in surface charge density upon adsorption of nonionic species is caused by a lowering of the dielectric constant at the mica/solution interface on adsorption, rendering dissociation of surface sites less favorable. Effects of Temperature. With an increase in temperature, the adsorption and interaction properties change in several ways. First, with an increase in temperature, the adsorbed amount increases, which is well understood from theor95 and in line with previous f i n d i n g ~ . ~At J~ all temperatures, however, the adsorbed amount remains quite low (Figure 6). Second, with an increase in temperature, the observed surface charge density decreases. We propose that this is a consequence of the higher adsorbed amount, which further reduces the local dielectric constant, as compared to the case at 20 "C, resulting in a lower surface charge density (vide supra). Finally, well below the cloud point it is comparably easy to squeeze out the adsorbed moleculesfrom the interaction zone by applying an external force. This, together with the finding that quasiequilibrium conditions are fulfilled, indicates that, at this temperature, EHEC adsorbs as separate coils that are quite mobile on the surface. As the temperature is raised, however,lateral attractions develop between adsorbed EHEC coils, which makes it increasingly difficult to reach mica-mica contact. Above the cloud point this effect dominates to such an extent that molecules in the center of the contact area remain trapped between the surfaces even under a very high load (vide supra). Note that the main reason why the molecules remain between the surfaces on compression is an effective lateral attraction between polymer molecules and not primarily a consequence of a stong polymer-surface attraction. This may be of great technical importance, since it shows that it is possible to confine weakly adsorbing polymers at an interface, just by increasing the lateral polymer-polymer attraction. As discussed above, the quantitative aspects of the force curves are uncertain a t 45 "C, inter alia due to experimental difficulties caused by thermal drift. Hence, no further conclusions can be made on the basis of the present data. (21)Rutland, M.;Christenson, H.K.Langmuir 1990,6, 1083.
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Figure 7. Comparison of forces (obtained with droplet) actin between (0)hydrophilic mica surfaces and (X) hydrophobizej mica surfaces (mica coated by a deposited monolayer of eicosylamine and eicosanol)12at 20 OC. The solid line has the same meaning as in Figure 1. 10000
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In parenthesis, however, it ib interesting to note that there is a weak attraction (on separation) at this temperature, which is expected for worse than 0 conditions (CP = 39 "C).l The effects of solvency on the interaction force have previously been studied with hydrophobicized mica surfaces."J2 We note that after the system has been heated to 45 "C, the force barrier observed at 20 "C is larger than that observed before heating. This is a clear evidence that the temperature dependence is not completelyreversible (over a period of 20 h), which is in line with previous findings on hydrophobic s u r f a c e ~ . ~ J ~ As the EHEC solution is diluted to a concentration of about lo4 w t % ,there is some desorption, as indicated, e.g., by the increase in the adhesion force. The desorption, however, is not complete, as evidenced by the force curves. Hence, despite the low adsorption affinity (vide supra),some moleculesremain on the surface upon dilution with water, which is particularly clear when forces are measured at 45 "C. (Note that it is unlikely that an increase in temperature results in an increased adsorbed amount at the low bulk polymer concentration obtained after dilution (about 10"' w t ?6).g)
EHEC at Hydrophobic and Hydrophilic Surfaces. As can be seen in Figures 6-8, there are several major differences regarding adsorption and interaction properties between hydrophilic (bare) mica and hydrophobic mica (mica coated with a Langmuir-Blodgett layer of eicosylamine/eicosano111J2). First, the adsorbed amount is significantly lower on the hydrophilic surfaces than on the hydrophobic ones (although a slightly different EHEC fraction was used in the latter case12), which is in line with earlier ellipsometry experiments.9 Second, due to an increase in the tail and loop size, the higher adsorbed
2252 Langmuir, Vol. 7, No.10, 1991
amount at the hydrophobic surfaces results in a thicker polymer layer, and consequently in a more long-rangesteric interaction (cf. Figures 7 and 8). Last, but not least, we note that in the case of hydrophobicsurfaces the polymelsurface affinity is high enough to prevent the polymer molecules from being squeezed out from the interaction zone even at 20 "C. Considering all this, it is quite clear that the surface-polymer interaction is of considerable importance for the adsorption and interaction properties in sterically stabilized systems.
Conclusions EHEC adsorbs with a rather low adsorbed amount onto hydrophilic mica surfaces. Below the cloud point the polymer is easily squeezed out on bringing the surfaces together, indicating weak adsorption, probably as separate coils. At higher temperatures the adsorbed amount increases, although it remains quite low at all temperatures. At the same time, the importance of lateral
Pezron et al. interactions increases, as evidenced by the increase of the force needed to accomplish mica-mica contact. On dilution of the polymer solution to a final concentration of 10-4wt % ,some, but not all, of the adsorbed polymer molecules desorb. The comparison between results obtained for hydrophilic and hydrophobicsurfaces highlights the importance of the polymer-surface interaction and emphasizesthat not only the polymer-solvent interaction, but also the polymer-surface interaction, has to be considered in any application of polymers as stabilizing or flocculating agents.
Acknowledgment. Profeseor Bj6rn Lindman is gratefully acknowledged for valuable comments of the manuscript. This work was financed by Berol Nobel AB, Sweden, Kabi Invent, Sweden, and the Swedish National Board for Technical Development (STU). Registry No. EHEC, 9004-58-4;muscovite, 1318-94-1.
