Langmuir 1990, 6, 1572-1578
1572
Temperature-Dependent Forces between Hydrophobic Surfaces Coated with Ethyl(hydroxyethy1)cellulose Martin Malmsten,? Per M. Claesson,*Jv§Erwoan Pezron,§ and Isabelle Pezrone Physical Chemistry 1, Chemical Center, PO-Box 124, S-221 00 Lund, Sweden, Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden Received February 6, 1990. I n Final Form: April 25, 1990 The forces acting between hydrophobic surfaces coated with a layer of a nonionic polymer, ethyl(hydroxyethy1)cellulose(EHEC),were studied inter alia as a function of surface separation and temperature. EHEC adsorbs strongly at hydrophobic surfaces, and the force measurements were performed with a constant adsorbed amount. The force vs distance curves were reversible; i.e., the same forces were measured on approach and separation. The surface interaction is very sensitive to temperature. At room temperature, the forces are purely repulsive. At 44 "C (5 OC above the cloud point), the force curve remains monotonically repulsive but less long-range. Hence, the EHEC layer contracts due to the poor solvency at this temperature. The fact that no attraction is observed despite that the temperature is above the cloud point is rationalized by considering that the most hydrophilic segments are oriented toward the bulk solution. Consequently, the local X-parameter for the stabilizing moieties is, for this chemically heterogeneous polymer, different from the average X-parameter. However, an elastic repulsion, due to the loss of conformational entropy, also contributes significantly to the repulsive force at this temperature. A t an even higher temperature (55 "C), a strong, but rather short-range, attractive force was observed. The temperature dependence of the force curves was found to be completely reversible at the incubation times used (approximately 15 h) Introduction In recent years, t h e physicochemical properties of polymers a t interfaces have received increasing attention. T h i s is p a r t l y d u e t o major b r e a k t h r o u g h s , b o t h experimental and theoretical, but also to the increasing importance of these systems in many practical situations. Hence, the use of polymers as stabilizers in, e.g., paints and foodstuff is widespread.' Also in medicine, the importance of these systems is well recognized, and tremendous efforts have been devoted to the development of biocompatible materials2 and to t h e targeting of p h a r m a c e u t i c a l p r e p a r a t i o n s by m e a n s of, e.g., polysaccharides,3just to mention a few applications in this field. Ethyl(hydroxyethy1)cellulose (EHEC) is a nonionic cellulose ether, consisting of a cellulose backbone which is substituted with ethyl groups and oligo(ethy1ene oxide) chains (Figure 1). Like many other ethylene oxide containing polymers and surfactants, EHEC shows a reversed temperature-dependent phase behavior.44 Hence, a t low temperature, aqueous EHEC solutions are clear, isotropic one-phase systems, whereas EHEC solutions at higher temperatures separate into two phases (Figure 2). T h e lower temperature phase boundary a t a certain polymer concentration is often called the cloud point (CP). The cloud point of EHEC, of course, depends on the
* To whom correspondence should
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Physical Chemistry 1, Chemical Center. * The Royal Institute of Technology. 5 Institute for Surface Chemistry. (1)Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (2)Kim, S. W.;Feijen, J. Crit. Reu. Biocompat. 1986,1 , 229. (3)Sunamoto, J.; Iwamoto, K. In Crit. Reu. Ther. Drug Carrier Syst. +
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( 4 ) Shinoda, K. Principles of Solution and Solubility; Marcel Dekker: New York, 1978. ( 5 )Nonionic Surfactants-Physical Chemistry; Schick, M. J., Ed.; Surfactant Sci. Ser., Val. 23;Marcel Dekker: New York, 1987. (6) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide);Academic Press: New York, 1976.
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molecular weight and on the degree of substitution of the two substituents, but it is also sensitive to the presence
0 1990 American Chemical Society
Forces between Layers of Cellulose of small amounts of different cosolutes, e.g., salts, alcohols, and surfactants. In particular, a very strong synergistic effect on the cloud point in the presence of both an electrolyte and an ionic surfactant has been studied extensively.'+ Another interesting property of the aqueous EHEC/ionic surfactant systems is the reversible formation of stiff gels at higher temperatures.lOJ1 Previously, t h e adsorption properties of E H E C , especially regarding the effects of temperature, cosolutes, and surface hydrophobicity, have been studied by in situ ellipsometry.l2 I t was found that both the adsorbed amount and the conformation of the adsorbed polymers are sensitive to temperature changes and to the presence of cosolutes. As the temperature increases, the adsorbed amount as well as the average film density increases. A t a subsequent temperature decrease, a completely reversible decrease in both the adsorbed amount and the average film density follows at short incubation times, whereas a marked hysteresis appears a t longer incubation times. Also in respects other than temperature changes (for example, addition of electrolyte or alcohol), both the adsorbed amount and the conformation of the adsorbed polymer molecules follow the expectations from observations of solvency. In the same study, the effects of surface hydrophobicity on the adsorption properties of EHEC were studied with a surface hydrophobicity gradient technique. It was found that hydrophobic interactions between the polymer and the surface constitute the main driving force for adsorption. It was also found that the adsorbed amount increases more rapidly with increasing temperature at a hydrophobic surface than at a hydrophilic. One of the major drawbacks of ellipsometry, the technique used in the previous study, is that it does not give detailed information on the conformation of the adsorbed polymers. A method which, however, does provide this information is the surface force technique.'3J4 This method, which allows measurements of the force acting between two surfaces as a function of their separation, has previously been used to measure steric interactions between homopolymers in good15and poor solvents16 in aqueous17 and nonaqueous18 solutions. A few studies, such as the work done on poly(2-vinylpyridine)/poly(tert-butylstyrene) by Ansarifar and L u ~ k h a m and ' ~ ~the ~ ~work done on poly(viny1-Zpyridine)/polystyrene by Hadziioannou et a1.,21,22 have been devoted to block copolymers. In a study by Gotze et (using a slightly different technique), the (7) Carlsson, A.; Karlstrom, G.; Lindman, B. Langmuir 1986,2,536. (8)Karlstrom, G.; Carlsson, A,; Lindman, B. J. Phys. Chem. 1990,94, 5005. (9)Carlsson, A.; Karlstrom, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988,266, 1031. (10)Carlsson, A.; Karlstrom, G.; Lindman, B. Colloids Surf. 1990,47, 147. (11)Carlsson, A.; Lindman, B.; Karlstrom, G. I n New Functionalization Developments in Cellulosics and Wood-Fundamentals and Applications; Ellis Harwood: Chichester, 1989. (12)Malmsten, M.; Lindman, B. Langmuir 1990,6,357. (13)Israelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans. 2 1978,74,975. (14)Parker, J. L.; Christenson, H.K.; Ninham, B. W. Reu. Sci. Instrum. 1989,601 , 3935. (15)Luckham, P. F.;Klein, J. Macromolecules 1985.18,721. (16)Klein, J. J. Chem. SOC.,Faraday Trans. 1 1983,79,99. (17)Klein, J.; Luckham, P. F. Macromolecules 1984,27, 1041. (18)Israelachvili, J. N.;Tirrell, M.; Klein, J.;Almog, Y. Macromolecules 1984,17,204. (19)Ansarifan, M.A.; Luckham, P. F. Polymer 1988,29,329. (20)Ansarifan, M. A.; Luckham, P. F. Polym. Commun. 1988,29,177. (21) Hadziiannou, G.; Patel, S.; Granick, S.;Tirrel, M. J. Am. Chem. SOC.1986,208,2869. (22)Patel, S.;Tirrel, M.; Hadziiannou, G. Colloids Surf. 1988,31,157. (23) Gotze, Th.; Sonntag, H. Colloids Surf. 1988,32, 181.
Langmuir, Vol. 6, No. 10, 1990 1573 interactions between adsorbed layers of methyl(hydroxypropy1)cellulose ( M H P C ) were studied in aqueous solutions. Force measurements have shown that the forces acting between ethylene oxide surfactants (adsorbed on hydrophobic surfacesN or in the lamellar phasez5)become less repulsive (or even a t some separations attractiveZ4)at higher temperatures. It has also been shown that adsorbed layers of ethylene oxide oligomers become more compact a t elevated temperatures.26 In this investigation, the surface force technique has been employed in order to obtain additional information on the adsorption properties of EHEC a t hydrophobic surfaces and on the'temperature-dependent interactions in the EHEC/water system. (A separate paper will deal with the forces between EHEC layers weakly bound to hydrophilic surfaces.) The force vs distance measurements are of technical interest since EHEC is used as a stabilizer in a variety of colloidal dispersions and as a means of making various substrate surfaces biocompatible. They are also of a fundamental interest since it is not very well known how surfaces or polymers that contain both hydrophilic and hydrophobic groups interact in aqueous solutions and how the forces between sterically stabilized surfaces changes as the temperature is increased above the cloud point of the stabilizing polymer.
Experimental Section Polymers. The polymer used here, EHEC, is a nonionic cellulose ether, which was supplied by Berol Nobel AB, Sweden. The EHEC fraction used has an average molecular weight of 250 000, as determined by light scattering. The polydispersity of this EHEC fraction is very large, as evidenced by GPC (results not shown). The radius of gyration is about 800 A, as determined by light scattering. The degree of substitution of ethyl groups is equal to 1.4 and the molar substitution of ethylene oxide groups is equal to 0.9. The cloud point (CP) of this particular fraction is 39 "C on heating. Dry EHEC powder normally contains 3-5 w t % of NaCl (impurity from synthesis), and therefore the EHEC solutions were dialyzed against membranefiltered water (Millipore, USA) for 5 days. As a dialyzing membrane, regenerated cellulose with a molecular cutoff of 6000 was used (Spectrum Medical Industries, USA). After dialysis, the polymer was freeze-dried. Preparation of the Hydrophobic Surfaces. All surface preparations were carried out in laminary flow cabinets. The deposition of the (uncharged) hydrophobic monolayer was performed with a computerized Langmuir trough system (KSV Chemicals, Helsinki). The mica surfaces were immersed in a water-filled all-Teflon trough. A 1:l mixture of eicosylamine and eicosanol (EA/EO) was dissolved in a chloroform/ethanol mixture (49:l). This solution was added dropwise onto the air/ water interface. After the solution was allowed to evaporate for 10 min, the monolayer was compressed to a surface pressure of 30 mN/m, corresponding to an area/molecule of about 20 Az. The mica surfaces were then pulled out of the solution at a speed of 5 mm/min while the surface pressure was kept constant. After deposition (transfer ratio LO), the surfaces were immediately mounted in the surface force apparatus. The advancing and receding contact angle on the hydrophobic surface was about 112" and 80°,respectively. Surface Force Experiments. Direct measurements of the forces acting between two hydrophobic substrate surfaces coated with EHEC were carried out with a surface force apparatus. Both an apparatus of the type originally designed by Israelachvili and co-workers (Mark 11)'s and an apparatus of mod(24)Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H.K. J . Chem. SOC.,Faraday Trans. I 1986,82,2735. (25)Carvell, M.; Hall,D. G.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discus. Chem. SOC.1986,81,223. (26)Claesson, P. M.; Golander, C. G. J. Colloid Interface Sci. 1987, 117,366.
1574 Langmuir, Vol. 6, No. 10, 1990
Malmsten et al.
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ified design by Parker et al.14 were used i n this set of experiments. We found that the force distance profile could be determined with the same accuracy with the two apparatuses. The two interacting surfaces are mounted in a crossed cylinder 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 between two flat surfaces, Gf,as first demonstrated by DerjauginZ7 F,(D)/R = ~ T G ( D ) ~ (1) 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 from deflections of a double variable cantilever spring supporting the lower surface. The detection limit is about lo-' N. The experiments were performed in the followingway: First, the contact position between the hydrophobic surfaces without any adsorbed EHEC was determined in air. This position defines the zero separation. Then a droplet of 0.1 wt % EHEC solution was placed between the surfaces, whereafter the force was measured as a function of the surface separation. Finally, the apparatus was filled with water, the adsorbed polymer layer was allowed to equilibrate according to the new situation (EHEC concentration about wt %), and the forces were measured again at different temperatures. In order to obtain equilibrium conditions as far as possible,'* the system was always allowed to adjust for several hours after changing the conditions. After equilibration, the force curves were measured during several compression/decompression cycles, and at several positions on the surfaces. The reproducibility of the force vs distance curves was good.
Results The forces measured between hydrophobic substrate surfaces across a 0.1 5';) EHEC solution are illustrated in Figure 3. A repulsion, which increases monotonically with decreasing surface separation, is observed at distances below 800 A (no double-layer forces are observed in this system). The same forces are observed on approach and (27) Derjaugin, B. V. Kolloid Z. 1934, 69, 155. (28) As inferred from enzymatic degradation rates (communicationwith
the manufacturer). (29) Cohen St&rt, M. A.; Scheutjens, J. M. H. M.; Fleer, G . J. J.Polym. Sci., Polym. Phys. Ed. 1980, 18, 1559.
D (A) Figure 4. Normalized force as a function of surface separation between two hydrophobic substrate surfaces coated with EHEC (adsorption from 0.1% solution for 4 h, followed by dilution with water to about lo4%) across water. Unfilled squares represent the forces measured on approach and filled squares forces measured on separation. The temperature was 25 O C . Note that the same forces are measured on approach and on separation.
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Figure 5. Comparison of the forces acting between hydrophobic surfaces coated with EHEC in a 0.1 SL EHEC solution (unfilled (unfilled squares). circles) and after dilution to about on separation. This is important since it shows that the measurements are carried out sufficiently slow to assure quasi-equilibrium conditions. With quasi-equilibrium, we mean that desorption is too slow for adjusting the adsorbed amount during the approach of the surfaces whereas "equilibrium" within the adsorbed layer is established quickly compared to the speed of approach. The force curve shown in Figure 3 was measured after approximately 2 h of adsorption (identical forces were measured after 4 h of adsorption). Hence, this incubation time is sufficient to ensure that adsorption equilibrium has been obtained. After 4 h, the aqueous EHEC solution was diluted to a final bulk polymer concentration of lo4 wt % . The forces observed after dilution are illustrated in Figure 4. The interaction remains, at room temperature, monotonically repulsive at distances below about 800 A, and the forces are measured under quasi-equilibrium conditions, as evidenced by the lack of hysteresis in the compression/ decompression cycle. The adsorbed layer is slightly more compact after dilution (Figure 51, whereas t h e layer thickness obtained under a very high force is the same
Langmuir, Vol. 6 , No. 10, 1990 1575
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Figure 8. Normalized force as a function of surface separation between two hydrophobic substrate surfaces coated with EHEC (adsorptionfrom 0.1 % solution for 4 h, followed by dilution with water to about l O - 4 % ) across water. Unfilled circles represent the forces measured on approach and filled circles forces measured on separation. The temperature was 55 "C. above the cloud point, some new features appear in the force curve (Figure 8). A weak repulsion is observed at distances between 300 a n d 190 A. From the latter separation, an attraction pulls the EHEC-coated surfaces together. On further compression, a "hard wall" is encountered which indicates that the adsorbed layer is very compact. On separation, a strong attractive force keeps the EHEC layers together. The fact that the surface separation has to be increased to over 200 A before the surfaces come apart indicates that molecules from the opposing layers either associate physically (2' > CP) or become slightly entangled. T h u s , contrary t o t h e e x p e r i m e n t s previously discussed, we d o a t t h i s temperature observe a slight hysteresis in the force curve at distances between 250 and 150 A.
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Figure 7. Comparison of the forces acting between hydrophobic surfaces coated with EHEC (adsorption from 0.1 7% solution for 4 h, followed by dilution with water to about lo4%)across water. The forces were first measured at 25 "C (unfilled squares) and then again after heating the solution to 44 O C (unfilled triangles). One day later, the forces were first measured a t 25 "C (filled squares) and then at 44 O C (filled triangles). within about 10 A. Hence, no, or a t least very few, molecules desorb upon dilution with water. The forces observed at 44 OC, which is slightly aboue the cloud point, are illustrated in Figure 6. As can be seen, the forces acting between the adsorbed EHEC layers are purely repulsive. The forces remain completely reversible during a compression/decompression cycle. Note that at this temperature no repulsion is observed until the surfaces are about 200-300 8,apart, as compared to 800 A a t room temperature. The strong contraction of the EHEC layer, which results from the decreasing solvency with incresing temperature, is illustrated in Figure 7. The layer thickness, determined at a high force, is the same a t both temperatures, indicating that no desorption or additional adsorption has occurred as a result of the change in temperature. Note that the change in surface interaction with temperature is reversible (Figure 7 ) . On raising the temperature to 55 "C,a temperature well
Discussion General Considerations. In order to understand our experimental findings, it is necessary to consider the possible interactions in the system. These interactions are of two major kinds, namely, elastic a n d osmotic interactions. The elastic interaction results from the loss of conformational entropy on approaching the two surfaces, which in turn results from a reduction in the available volume for each polymer chain. The osmotic interactions, which are also commonly referred to as mixing interactions, arise from the increase in the polymer concentration on compressing the two surfaces. This increase is favorable if x > 0.5 (x being t h e Flory-Huggins interaction parameter30) but unfavorable if x C 0.5, resulting in an attractive and a repulsive force, respectively. The force a t x = 0.5 is still expected to be repulsive, at least in the absence of bridging, due to the elastic repulsion. We are quite aware t h a t this subdivision of t h e interaction force can be disputed, especially since it is somewhat arbitrary. It could perhaps also be argued that the conformational entropy, giving rise to the elastic (30) Flory, P. J. Principles of Polymer Chemistry;Cornel1 University Press: Ithaca, New York, 1953. (31) Scheutjens, J. M. H. M.; Fleer, G.J. J. Phys. Chem. 1979,83,1619. (32) Scheutjens, J. M. H. M.; Fleer, G.J. J.Phys. Chem. 1980,84,178. (33) de Gennes, P A . Macromolecules 1981, 14, 1637. (34) de Gennes, P . 4 . Macromolecules 1982, 15, 492. (35) Klein, J.; Pincus, Ph. Macromolecules 1982, 15, 1129.
1576 Langmuir, Vol. 6, No. 10, 1990 repulsion, is coupled to the mixing entropy, especially in the low interpenetration limit, rendering the elastic and the osmotic components impossible to separate. However, considering the complexity of the system, we still think that the subdivision of the interaction force into separate contributions is useful, since it gives us a tool for the analysis of our experimental findings. The main features of this analysis are not markedly affected by t h e subdivision. For a block or comb (graft) copolymer like EHEC, the concept of a single global X-parameter has only a limited value. It may be used with an apparent success when discussing bulk properties and clouding phenomena. However, it is important to recognize that this parameter only describes a n average property, which is n o t appropriate when, for instance, steric stabilization is discussed. In this case, orientational effects may render the composition of the outer part of the adsorbed layer different from t h a t of t h e inner part. T h e relevant parameter is then the local solvency conditions for the stabilizing moieties. Hence, not only the temperature and the solvent composition but also orientational effects of the polymer at the interface are expected to influence the forces significantly. When osmotic interactions are discussed, it is important to consider whether the polymer layers interpenetrate or not. This is especially important for heterogeneous polymers. Whether or not interpenetration or compression or both occur of course depends on the segment density distribution. If the adsorbed layer is very dense, it is possible that compression is preferred to interpenetration. If, on the other hand, the polymer layer is less dense, interpenetration is probably preferred. In the former case, the osmotic interaction is given by the local solvency condition of the stabilizing segments, while in the latter case it depends on the global average solvency conditions for the polymer molecules. However, it is very likely that t h e two processes discussed above t a k e place simultaneously, one being more significant than the other, primarily depending on the segment density distribution in the adsorbed layer. S t a t e of Adsorption. Effects of Dilution. It has previously been observed that hydrophobic attraction between the EHEC polymer and the surface is the main driving force for adsorption.12 Hence, EHEC adsorbs extensively to hydrophobic surfaces, whereas both the absolute value and the temperature increment of the adsorption are much lower at a hydrophilic surface. It is clear that the most hydrophobic moieties, the ethyl groups, will be preferentially adsorbed onto the hydrophobic surface, whereas the hydrophilic regions will orient toward the bulk, causing not only the segment density but also the distribution of the substituents to vary with the distance from the surface. This effect is enhanced by the fact that EHEC is not evenly substituted but that these molecules actually have some block character.28 As has been discussed above, the forces acting between hydrophobic surfaces coated with EHEC are the same on approach and separation (Figures 3 and 4). From this, we infer that the adsorption of EHEC polymers is sufficiently strong for preventing desorption resulting from repulsive polymer-polymer interactions at short separations. Hence, the experiments are performed at a constant adsorbed amount, rather than a t a constant chemical potential. Taking t h e thickness of the polymer layer a t high compressional force, and measuring the average refractive index between the surfaces, it is possible to calculate the
Malmsten et al. adsorbed amount.36 This analysis gives r i= 6 mg/m2 at 25 "C. This is, of course, a very high adsorbed amount, especially compared to what was previously found on hydrophobized silica surfaces (r = 2 mg/m2 a t 25 "C). One of the reasons for this could be the rather low optical contrast between the adsorbed layer and the surrounding polymer solution, which could give rise an underestimation of the adsorbed amount in the ellipsometry experiment. The present measurements do not suffer from this, since we measure the adsorbed amount at small distances of separation. Moreover, the present surfaces are more hydrophobic than those used previously,12 which could contribute as well. Considering further the small layer thickness (about 50 A on each surface) and the short-range interaction (the repulsion reaches 1000 pN/m a t a separation of 500 A), which is considerably smaller than the radius of gyration for this EHEC fraction (RG= 800 A), it is quite clear that the adsorbed polymer layer is quite compact, even a t this temperature. When the solution is diluted roughly 1000-fold there is an initial small change in surface interaction. The range of the repulsive force decreases slightly whereas the layer thickness obtained at a high compression is roughly the same. Hence, only a small fraction of the EHEC molecules is desorbed on dilution. This observation is consistent with previous ellipsometry results, which have shown that the adsorbed amount does not change much on dilution.12This behavior is very frequently referred to as the polymer molecules being irreversibly adsorbed a t the surface. However, this behavior is only a consequence of a finite polymer polydispersity and the preferential adsorption of higher molecular weight polymer molecules.29 Therefore, the polymer molecules cannot be claimed to be irreversibly adsorbed in a thermodynamic sense. However, from a purely phenomenological point of view, it can be said that the polymer molecules are indeed irreversibly adsorbed. From the surface force measurements, we may go one step further and conlcude that not only the adsorbed amount but the conformation of the polymer as well is rather insensitive to dilution on a hydrophobic surface. E f f e c t s of T e m p e r a t u r e . Before t h e effects of temperature on the forces acting between EHEC-coated surfaces are discussed, it is important to briefly consider the relation between the cloud point (which is t h e "measure" of solvency used in this paper) and the 0temperature. It is well-known that phase separation in the case of infinitely long polymer chains takes place at O - c o n d i t i ~ n s .For ~ ~ shorter polymer chains, phase separation takes place a t slightly worse than 0-conditions. Thus, in this particular case, where the solvency decreases with increasing temperature, this means t h a t t h e measurements carried out above the cloud point correspond to worse than 0-conditions for the polymer as a whole. The local solvency for some moieties may, however, still correspond to better than @-conditions. The forces measured a t 44 "C (5 "C above the cloud point) are repulsive at all separations. However, the range of the force has decreased considerably, compared to room temperature (Figure 7). A repulsion of 1000 p N / m is encountered at a separation of 150 8,a t 44 "C but already a t 500 A a t room temperature. T h e adsorbed layer thickness measured at a very high force is the same at both temperatures, which shows that the decrease in the range of the force at higher temperatures is not due to a decrease in the adsorbed amount but is caused by a contraction of (36)de Feijter, J. A.; Benjamins, J.; Veer, F.A. Biopolymers 1978, 17, 1759.
Langmuir, Vol. 6, No. 10, 1990 1577
Forces between Layers of Cellulose the adsorbed polymer layer, accomplished by an increase in the train fraction and decreasing lengths of the loops and the tails.31-32 This is further supported by analysis of the adsorbed amount, which gives r = 6 mg/m2 a t 25 "C and I' = 7 mg/m2 a t 44 "C, the difference being within the experimental uncertainty. Our results indicate that the adsorbed layer a t high temperatures is extremely compact, with essentially no compressibility. The contraction is so strong that it almost resembles the much disputed coil-globule transition,' although here in two dimensions. Taking the thickness a t high compressional force in conjunction with the adsorbed amount, one finds t h a t the average polymer concentration in the adsorbed layer is extremely high (>70 wt 1, the precise value, however, depending on the assumed value of the (unknown) average density of the adsorbed layer. Thus, even if this parameter is probably rather uncertain, there is no doubt that the polymer layer is extremely compact at 44 "C. Hence, the poor solvency condition at 44 "C results in a more compact adsorbed layer but not in a net attraction between the EHEC-coated surfaces. We suggest that one important reason why no attraction is observed at 44 "C, despite the fact that the cloud point is as low as 39 "C, is that there is an orientation of hydrophilic groups toward the solution, which lowers the local x-parameter in the interaction region. Of course, since the polymer layer is compact and nearly incompressible, an elastic repulsion contributes significantly to the observed repulsive force at this temperature as well. The relative importance of the two interactions discussed above is unknown, but it is probable that the elastic forces will dominate at short distances, due to the high segment density. An analogous behavior has previously been reported in dispersion science, where it has been observed that it is actually possible to prepare dispersions which are stable in worse than 6-conditions for the stabilizing polymers. This phenomenon, which is commonly referred to as "enhanced steric stabilization", is lucidly discussed in the book by Napper.' However, to our knowledge, this is the first time that a similar behavior has been observed in direct surface force measurements. At 55 "C, which is 16 "C above the cloud point, an attractive force is observed (Figure 8). The reason why an attractive force is present at 55 "C, but not at 44 "C, is probably that the @-temperaturefor the stabilizing segments is higher than the average @temperature. (In fact, the slight repulsion observed a t large distances may indicate that not even a t this temperature do all parts of the polymer experience bad solvency conditions. However, since an elastic repulsion may also be significant a t this temperature, this conclusion is not unambiguous.) The fact t h a t the attractive region of the force curve (on approach) is narrow (between 190 and 150 A) is another indication that the adsorbed layer is extremely compact. The compactness of the layer does, however, not completely prevent chain interpenetration and/or bridging due to the association of polymer molecules adsorbed at different surfaces. This is evidenced by the fact that the surfaces stick together on separation and do not jump apart until they are a t a separation of slightly above 200 A. Hence, on separation there are two contributions to the attractive force, an attractive osmotic force, which also was observed on approach, and an attractive elastic force due to entanglement and/or physical association of polymer molecules adsorbed a t different surfaces. Note t h a t ordinary bridging effects must be extremely unlikely, considering the high segment density.
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Figure 9. Normalized forces measured between EHEC-coated
hydrophobic substrate surfaces as a function of surface separation plotted on a log-log scale. Unfilled circles represent the forces in 0.1 % EHEC solutions, unfilled squares the forces after dilution with water (to about at 25 " C , and unfilled triangles after dilution at 44 "C. It is interesting to note that the temperature dependence of the force curves is completely reversible (Figure 7) a t the incubation times used here (approximately 15 h). This may seem somewhat contradictory to some of the results obtained in the previous ellipsometry study, where it was found, on performing temperature cycle experiments, that the temperature dependence was not completely reversible a t incubation times of 8 h a t every temperature.'2 However, in the earlier experiments, the adsorbed amount was allowed to vary strongly with the temperature (the bulk polymer concentration was 0.1 wt %), in contrast to the present experiments, which were performed a t a n essentially constant adsorbed amount (vide supra). Thus, the two studies do not monitor the same process, which makes the comparison difficult. In fact, recent surface force measurements, performed a t a bulk E H E C concentration of 0.25 w t %, thus allowing the adsorbed amount to vary with the temperature, show t h a t the adsorption under these conditions is not completely thermorever~ible.~~ The Force Law. It is interesting to note the functional form of the force curves. In none of the experiments does an exponential distance dependence fit the measured forces. Instead, different power laws describe t h e interaction rather well over a large distance regime (Figure 9). A t 25 " C , the force decays roughly as D-2.5,before as well as after dilution, which seems to further strengthen the notion of an essentially constant adsorbed amount, as well as an unchanged conformation of the adsorbed polymer molecules, on dilution. The decay a t 44 "C is considerably faster (0-7.A t both temperatures, however, t h e dependence of t h e force on t h e separation is significantly stronger than what is predicted from theory. Applying the Derjaguin approximation, one would expect F D-2 for irreversibly bound polymers, both a t good solvency conditions (in the distant region of interaction)sS and for the inner part of the force curve a t poor solvency condition^.^^ A t smaller distances of separation, one would expect F D-1.25a t good solvency condition^.^^ This discrepancy between theory and experimental results is (perhaps) surprising. However, in the theoretical models it is assumed that the force is of purely osmotic origin whereas in t h e experiments, particularly a t high
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1578 Langmuir, Vol. 6, No. 10, 1990
temperatures, the segment density is sufficiently high for elastic forces to be of considerable importance. In fact, the strong variation of the force with surface separation seems to indicate that elastic forces predominate in this system.
Conclusions In this study, it was found that EHEC adsorbs strongly onto hydrophobic surfaces in a compact conformation. The a d s o r p t i o n is sufficiently s t r o n g for p r e v e n t i n g displacement of adsorbed molecules as the surfaces are pushed together. T h e “equilibrium” conformation is obtained rapidly compared t o t h e time scale of the experiments. Hence, the same forces are measured on approach and separation. The forces a t room temperature are purely repulsive. On diluting the polymer solution from 0.1 to wt ?1,the force changes very little, indicating only minor changes in the adsorbed amount as well as of
Malmsten et al. the conformation of the adsorbed polymer molecules. Slightly above the cloud point, the forces remain purely repulsive, but the adsorbed layer contracts significantly. At even higher temperatures, an attractive force is observed over a small distance regime. The temperature dependence of the forces cannot be explained by considering the global X-parameter. Instead, orientational effects due to the preferential adsorption of hydrophobic segments and orientation of hydrophilic segments toward the solution, as well as elastic interactions, have to be taken into account.
Acknowledgment. Professor Bjorn Lindman is gratefully acknowledged for valuable comments on 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; EA, 10525-37-8;EO, 62996-9.
