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Langmuir 1999, 15, 3242-3249
Effects of Adsorption of Low-Molecular-Weight Triblock Copolymers on Interactions between Hydrophobic Surfaces in Water K. Eskilsson,*,† B. W. Ninham,‡ F. Tiberg,§ and V. V. Yaminsky‡,| Physical Chemistry 1, Centre for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, Department of Applied Mathematics, Research School of Physical Engineering, Institute of Advanced Studies, The Australian National University, Canberra, ACT 0200, Australia, and Institute for Surface Chemistry, P.O. Box 124, S-114 86 Stockholm, Sweden Received October 20, 1998. In Final Form: January 20, 1999 In this work, we report on the interaction forces between hydrophobed silica surfaces immersed in polymer solutions. The polymers studied were a series of poly(ethylene oxide)-polytetrahydrofuranpoly(ethylene oxide) (PEO-PTHF-PEO) triblock copolymers and a poly(ethylene oxide) homopolymer. The interaction forces were measured by the interfacial gauge technique. We show how the interactions are changed by the adsorbed state of the copolymers. This depends on both the copolymer concentration and the adsorption time. Above a critical surface coverage, the interaction between approaching surfaces at first shows a steric repulsion due to overlap of the adsorbed polymer layers. This repulsion increases as the distance between the surfaces decreases. In this regime the energy-distance curve could be accounted for by the theory of grafted polymer brushes of de Gennes. However, for small surface-to-surface distances the interaction curves do not follow this prediction. Instead, the repulsion stabilized at a more or less constant level with decreasing intersurface separation. Finally, however, hard wall contact was established between the two surfaces. We infer that adsorbed copolymers to a large extent are expelled from the gap between the surfaces in this small surface-to-surface distance range. The force needed to expel copolymers from the intersurface gap was shown to be equal to the surface pressure at the solid-liquid interface. We also studied the influence of the rate of approach and the separation of the surfaces on the energy-distance curves. The process of expelling polymers from the surface-to-surface gap was shown to depend on the velocity of the approaching surfaces and the surface coverage. For high approach rates and/or large surface coverages, the lateral mobility of the polymers was such that it inhibited the expulsion process of polymers from the gap. However, rapidly repeated force curves, measured at a constant rate, and successively, were found to be perfectly reproducible.
Introduction Low-molecular-weight amphiphilic block copolymers exhibit phase behavior and adsorption properties reminiscent of both surfactants and polymers. The industrial importance of these molecules is motivation enough to investigate their properties in solution and adsorption at interfaces. Nonionic surfactants and copolymers are today frequently used to modify wetting properties, create protein resistant surfaces, and stabilize colloidal particles, foams, and emulsions. Success in such applications is often dependent on the interaction forces between surfaces that carry adsorbed copolymers. The traditional interferometric surface force apparatus (SFA) developed a few decades ago opened up the possibility of measuring forces between surfaces in solutions containing polymers or surfactants. Interactions between different kinds of surfaces immersed in polymer solutions have since then been studied quite intensively with the SFA technique. Most studies have been performed with adsorbed homopolymers or terminally attached polymers. Terminally attached polymers are nonadsorbing polymers that are chemical grafted to the surface or more commonly block copolymers anchored * To whom all correspondence should be addressed. E-mail,
[email protected]. Fax. 46-46-2224413. † Lund University. ‡ The Australian National University. § Institute for Surface Chemistry. | On leave from the Institute of Physical Chemistry, The Russian Academy of Science, Moscow.
preferentially at the surface by one of the blocks. It has been shown in many of these studies that uncharged polymers of high molecular weight adsorb irreversibly to many solid interfaces, that is, that these polymers cannot be expelled from the gap between the two surfaces. The interactions between surfaces covered by high-molecularweight polymers in both good and poor solvents are today relatively well understood; see reviews.1-3 Some investigations of interactions between surfaces covered by lowmolecular-weight polymers have also been performed.4-6 In these studies it has been inferred that the interaction forces have the same overall appearance as terminally attached high molecular weight polymers. It has, nevertheless, been shown that polymers are expelled from the gap between the surfaces under some conditions.5 The force-distance curves obtained during successive compressions, however, exhibited a large degree of hysteresis. Several studies of force interactions in surfactant systems have been published recently.7-17 These articles (1) Luckham, P. F. Adv. Colloid Interface Sci. 1991, 34, 191-215. (2) Klein, J. Pure Appl. Chem. 1992, 64, 1577-1584. (3) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces, 1st ed.; Chapham & Hall, London, ed. 1, 1993. (4) Ansarifar, M. A.; Luckham, P. F. Polymer 1988, 29, 329-335. (5) Schille’n, K.; Claesson. P. M.; Malmsten, M.; Linse, P.; Booth, C. J. Phys. Chem. B 1997, 101, 4238-4252. (6) Claesson, P. M.; Go¨lander, C. G. J. Colloid Interface Sci. 1987, 117, 366. (7) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500.
10.1021/la981469z CCC: $18.00 © 1999 American Chemical Society Published on Web 04/07/1999
Interactions between Hydrophobic Surfaces in Water
display some incompatible views on how to interpret the force data. The nature of the long-range attraction between hydrophilic surfaces covered by small amounts of oppositely charged surfactants is one topic that was greatly debated. Yaminsky and co-workers,13,15,18,19 analyzed force-distance curves between surfactant-covered macroscopic surfaces within the general framework of the Gibbs adsorption equation. They showed, for instance, that the hydrophobic attraction was due to an increasing adsorption of ionic surfactants at the oppositely charges surfaces with decreasing surface-to-surface distance. The understanding of the repulsive force interactions observed between surfactant-covered hydrophobic surfaces has also been improved during recent years. It is clear that adsorbed surfactant layers can be expelled from the intersurface gap when the surfaces are brought into contact. The height of the repulsive force barriers appears to be proportional to the surface pressures exerted by the adsorbed surfactant films.15 In the present work we have studied the effects of adsorbed poly(ethylene oxide)-polytetrahydrofuranpoly(ethylene oxide) (PEO-PTHF-PEO) triblock copolymers on interaction forces between hydrophobic surfaces. A corresponding study of forces between hydrophilic surfaces was published earlier.20 Experimental Section Surface Preparation. Melting one end of a 2 mm thick glass rod (Pyrex) provides a sphere with a diameter of several millimeters. Such spheres provided the substrates for the surface force experiments. Unlike the case for SFA interferometry, the scaling radii of such spheres coincide with the macroscopic radii, measured with high accuracy by a micrometer. A freshly molten glass sample was rinsed in water, dried in a stream of nitrogen, and placed in a desiccator in a saturated vapor of dimethyldichlorosilane (DMDCS) for 20 h. After the silane treatment the samples were rinsed repeatedly with chloroform, ethanol and water. The way the samples are prepared is very important for the final result.21 There is a continued discussion about the chemical status of silane layers.22 However, the surfaces prepared as above have been shown to be stable in pure water as well as organic solvents for many days.23 Surface Forces. Surface forces were measured with a solidstate sensor in the basic setup described earlier.15,24-27 The (8) Pashly, R. M.; McGuggian, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (9) Luckham, P. F.; Klein, J. J. Colloid Interface Sci. 1986, 117, 149158. (10) Kekicheff, P.; Christensson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (11) Herder, P. J. J. ColloidInterface Sci. 1990, 134, 346. (12) Rutland, M. W.; Waltermo, A° .; Claesson, P. M. Langmuir 1992, 8, 176. (13) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashly, R. M. Langmuir 1996, 12, 1936-1943. (14) Yaminsky, V. V.; Jones, C.; Yaminsky, F.; Ninham, B. W. Langmuir 1996, 12, 3531-3535. (15) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836-850. (16) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356. (17) Lachlan, M. G.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (18) Yaminsky, V. V. Langmuir 1994, 10, 2710-2717. (19) Yaminsky, V. V.; Christenson, H. K. J. Phys. Chem. 1995, 99, 5176-5179. (20) Eskilsson, K.; Ninham, B. W.; Tiberg, F.; Yaminsky, V. V. Langmuir 1998, 14, 7287. (21) Yaminsky, V. V. Colloids Surf. A 1997, 129-130, 415-424. (22) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (23) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid Interface Sci. 1993, 161, 91-100. (24) Parker, J. L. Langmuir 1992, 8, 176. (25) Parker, J. L.; Stewart, A. M. Prog. Colloid Polym. Sci. 1992, 88, 162. (26) Stewart, A. M.; Parker, J. L. Rev. Sci. Instrum. 1992, 63, 5626. (27) Stewart, A. M. Meas. Sci. Instrum. 1995, 6, 114.
Langmuir, Vol. 15, No. 9, 1999 3243 Table 1. Molecular Weight of the Polymer, Number of Ethylene Oxide Groups, n, Number of Tetrahyrdrofuran Groups, m, Value of the Critical Micellar Concentration, cmc and the Adsorbed Amount, Γp. Corresponding to the Ellipsometric Values at the Adsorption Plateau polymer
MW (g mol-1)
n
m
cmc (wt %)
Γp (mg m-2)
P224-28 11900 224 28 0.03 2.4 P146-28 8400 146 28 0.04 2.3 P172-14 8600 172 14 0.2 2.3 P128-14 6700 128 14 0.15 2.3 P50-14 3200 50 14 0.02 2.4 apparatus is referred to as the interfacial gauge. The hydrophobic samples installed in the gauge were immersed in a liquid in a quartz beaker. The polymer concentration was varied by small amounts of stock solution added to the sample beaker, and stirring was achieved by rotating the beaker. Forces were determined from measurements of the electric response of a piezoelectric sensor, which results from an external load induced by a magnet. This gives interaction forces between the two surfaces as a function of their mutual displacement as well as rate of displacement. The acquisition system enables collection of data points at a desired frequency (up to 50 kHz). Changing the period and the amplitude of the loading ramps alters the speed and the load of the samples. The surfaces were moved at a constant speed (≈60 nm/s) in all measurements unless specified otherwise. An advantage of using interferometric measurements of surface forces is that the absolute zero position can be directly observed. Instead, separation distances quoted here are relative to hard wall contact. This means that there may be polymers sandwiched between the surfaces; the actual zero position should then be shifted outward by a factor that is unknown. This, however, does not alter the form of the interaction before the contact. The obtained force-displacement curves are then transferred into energy-distance curves by the Derjaguin approximation. Polymers. Five different (ethylene oxide-tetrahydrofuraneethylene oxide) triblock copolymers, EOn/2THFmEOn/2, have been studied in this work. Depending on the chemical composition, these are referred to as P224-28, P172-14, P146-28, P128-14, and P50-14, respectively (see Table 1). The first number, n, refers to the number of EO groups, and the second, m, refers to the number of THF groups of the polymer. The triblock copolymers were produced by Akzo Nobel Surface Chemistry, by ethoxylating polytetrahydrofurane (PTHF) polymers of molecular weights of 1000 and 2000 g/mol, respectively. The latter were purchased from BASF. Important properties (i.e. molecular weight and critical micelle concentration (cmc)) of the copolymers have been characterized by NMR diffusion and fluorescence spectroscopy. A description of these characterization procedures is given in ref 28. A summary of the properties of these copolymers is presented in Table 1. The polydispersity index, Mn/Mw, of the copolymer samples is between 1.1 and 1.2 according to the manufacturer. The triblock copolymer samples are free of homopolymers, but there may be a small contamination by diblock copolymers. The PEO homopolymer with a molecular weight of 4000 g/mol (SERVA, Feinbiochemica) was used with no further purification. All aqueous solutions were prepared from Millipore water. The ethanol used was freshly distilled.
Results and Discussion As a basis for our surface force discussion, we need first to go through some details of the adsorption of copolymers at a single surface. The adsorptions of the PEO-PTHFPEO (Pn-m) copolymers and the PEO homopolymer are discussed in detail in refs 28-29. All copolymers were observed to form monolayers on hydrophobized silica. The middle tetrahydrofuran block was always preferentially anchored at the surface, resulting in the formation of PTHF trains. PEO segments are either anchored at the surface or protrude into the aqueous phase, depending critically on the surface coverage. The copolymer adsorption isotherms were well described by the conventional Langmuir expression, with a plateau adsorption value of about 2.4 (28) Eskilsson, K.; Tiberg, F. Macromolecules 1997, 30, 6323-6332. (29) Eskilsson, K.; Tiberg, F. Macromolecules 1998, 31, 5075-5083.
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Figure 1. Adsorption isotherms for the triblock copolymer P224-28 and the PEO homopolymer P90. The dashed line is the best fit to the Langmuir isotherm; the solid line is drawn to guide the eye. The data are taken from ref 20.
mg/m2. Since all copolymers have roughly the same plateau surface coverage (in mass per unit area), the surface area per molecule must increase more or less linearly with the molecular weight. The isotherm measured for the PEO homopolymer was a typical high-affinity type polymer isotherm.3 The plateau adsorption was about five times larger for the triblock copolymers than for the homopolymer. This is explained by accounting for the contribution of the short hydrophobic PTHF blocks in the copolymer. Representative adsorption isotherms for the copolymers and homopolymer are shown in Figure 1. The curves show the isotherms measured for the P224-28 copolymer (MW ) 11 900 g/mol) and the P90 homopolymer (MW ) 4000 g/mol), respectively. Interactions in Air and in Pure Water. Hydrophobic surfaces in air were observed to jump into adhesive contact at a surface-to-surface separation of about 13-14 nm. An adhesion energy, F/(2πR), of about 20-22 mN/m2 was calculated from the pull-off force needed to separate the two surfaces. The range of the attractive force was slightly smaller in water as compared to that in air. The jump into contact in water was observed at an intersurface distance of 10 nm (see Figure 2), that is, a few nanometer less than that in air. By contrast the adhesion energy in water was close to 40 mN/m. This is almost twice the measured value in air. In water, we also observed a small repulsive interaction for intersurface distances larger than the jumpin distance. The repulsion is due to the existence of residual electrical charges on the methylated glass substrates, which give rise to a weak double-layer interaction. It is well-known that residual charges often are present on silica after the surface has been methylated.30 However, the charge density of the hydrophobized surface is generally small as compared to that at the bare silica surface. This was also the case in our study. We remark that the measured jump-in distances and adhesion energies agree well with previously published data for the same type of surfaces.15,31 The measured jump-in distances (30) Blake, T. D.; Kitchener, J. A. Trans. Faraday Soc. 1972, 68, 1435. (31) Yaminsky, V. V.; Yusupov, R. K.; Amelina, E. A.; Pchelin, V. A.; Shchukin, E. D. Kolloidn. Zh. 1975, 37, 918.
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Figure 2. Energy-distance curves for four different concentrations of the triblock copolymer P224-28; the curve for the bare surfaces is displayed as a reference.
in both air and water are larger than expected for the given spring stiffness if van der Waals forces alone are responsible for the attraction between the solid surfaces.32 The origin of the extra attraction has been variously attributed to “water structure”, small air bubbles at the surface, cavitation and/or entanglement, and bridging by protruding polysiloxane chains. Bridging is consistent with the fact that the attraction is increased in both air and water.15,21 However, there is no firm proof that this is the likely adhesion mechanism. Capillary condensation and bubble formation (in air and water, respectively), for instance, may very well prove to be the dominant adhesion mechanism. More work is still needed in order to reach a good understanding of the interactions between hydrophobic surfaces in air and water. However elucidation of adhesion mechanism is not the aim of the present work. Therefore, we finish this discussion of interactions between bare hydrophobic surfaces by noting that there was a clear tendency for the surfaces to slow, about 1 nm before hard wall contact. This soft contact is due to the compressibility of the hydrophobic coating. Interaction in Polymer Solutions. We now turn our attention to some general observations on interactions between surfaces immersed in aqueous polymer solutions. We note first that force curves measured successively during rapidly repeated compressions and separation cycles were essentially identical. All the polymers studied, at all concentrations, shared this feature. Our results show that the adsorbed polymer layers are able to relax to their original nonperturbed state during the time the surfaces are separated. A complete approach-separation cycle lasts for about 1 min. However, interactions were not measured under perfect equilibrium conditions. We did indeed observe a reproducible dependence on the speed of surfaceto-surface approach. This will be further discussed below. The adsorption-desorption kinetics of the copolymer systems was slow compared to the time needed for a complete force curve measurement. The adsorptiondesorption processes due to molecular exchange of polymers between the adsorbed layers and the bulk solution could in fact be neglected during the measurements. We could therefore study situations at which the adsorbed (32) Yaminsky, V. V.; Ninham, B. W. Langmuir 1993, 9, 3618-3624.
Interactions between Hydrophobic Surfaces in Water
layer was not at equilibrium with the bulk. To check that adsorption equilibrium was reached, force-distance curves were always compared with corresponding curves measured after another hour of equilibration time. Systems were considered to be in equilibrium with the bulk solution when the force curves before and after this period were shown to be identical. Now, we proceed to discuss the interaction forces obtained in the presence of adsorbed homo- and copolymers, respectively. PEO Homopolymer Solutions. Measurements of the interactions in PEO homopolymer solutions were performed mainly to provide a bench mark for the copolymer systems. A comparison of the energy-distance curves measured in homo- and copolymer solutions is essential for understanding the importance of the middle hydrophobic PTHF block. The energy-distance curves during approach with adsorbed PEO turned out to be similar to the curves obtained in the absence of adsorbed polymers; that is, only a small electrostatic repulsion was observed prior to the adhesive jump into contact. The jump-in distance was observed to be about 8 nm, which is 2-3 nm shorter than the corresponding distance between bare surfaces. The polymer concentration was varied between 10-4 and 10-2 wt %, but only very small concentration effects were noticed. However, the depth of the adhesion minimum was decreased. This together with the change of the jump-in distance shows that some polymers were indeed adsorbed at the surface. The adhesion energy in water decreased from a value of F/(2πR) ) 40 mN/m in water to 28 mN/m at 10-4 wt % and 24 mN/m at 10-2 wt % of PEO homopolymer. The relatively small change in adhesion with concentration change was expected. The adsorption isotherm measured by ellipsometry shows that the surface has a saturated polymer coverage of roughly 0.5 mg/m2 already at the polymer concentration 10-5 wt % (see Figure 1). The absence of nonelectrostatic repulsion at distances larger than 8 nm indicates that these polymer layers are thin. The number density of polymer chains stretching further out from the isolated surface than 4 nm must therefore be small. This is a reasonable result, since an adsorbed layer thickness of 4 nm is almost twice the radius of gyration of a P90 homopolymer.33 The ellipsometrically determined adsorbed layer thickness for the PEO homopolymer at hydrophobized silica was only around 3 nm. Other studies34,35 also show that PEO polymers form relatively thin adsorbed layers at hydrophobic latex surfaces. Hence, we can conclude that steric repulsive interactions are not observed during the mutual approach of the two hydrophobic surfaces covered by the P90 polymer. The adsorbed polymer remaining in the gap after hard wall contact does, however, decrease the adhesive interaction between surfaces. Triblock Copolymer Solution. Figure 2 shows the energy-distance curves measured in aqueous solutions containing different amounts of the copolymer P224-28. As a reference, we also show the force curve measured in pure water. All measurements were performed after bulk equilibration. The energy-distance curves have the same characteristic features at all polymer concentrations. The force curves begin to diverge from the water reference curve as the distance between the surfaces becomes smaller. The deviation results from a larger repulsive force contribution. This begins at a distance that from here on is referred to as the steric force onset distance. This (33) Bhat, R.; Timasheff, S. N. Protein Sci. 1992, 1, 1133-1143. (34) Baker, J. A.; Berg, C. J. Langmuir 1988, 4, 1055-1061. (35) Killmann, E.; Maier, H;, Baker, J. A. Colloids Surf. 1988, 31, 51-71.
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Figure 3. Height of the repulsive barrier for four different concentrations of the triblock copolymer P224-28, as a function of adsorbed amount. The adsorbed amount is taken from Figure 1 and put into correspondence with the force data according to the concentration used.
distance should be roughly speaking equal to twice the adsorbed layer thickness. We define the onset distance as the distance where the repulsive energy measured for the polymer system is 0.05 mN/m larger than that at the corresponding distance on the reference curve. The repulsion continues to increase as the surface-to-surface distance decreases to values smaller than the onset distance. But close to the hard wall contact, we find that this increase in the repulsive interactions levels off and finally becomes almost constant with decreasing thickness. We will refer to the energy where the repulsive interactions level off as the barrier height. The steric force onset distance increased from 14.5 to 21 nm when the copolymer concentration was increased from 5 × 10-5 to 10-3 wt %. Above 10-3 wt %, no further change was observed in the force onset distance with increasing concentration. The high sensitivity of the interfacial gauge makes the measurements rather sensitive for small volume fractions of polymer tails protruding into the solution. We believe then that the onset distance should be a representative measure of the hydrodynamic thickness of the adsorbed layer at the isolated surface. The increase in layer thickness from 7.2 to 10.5 nm occurs when the adsorbed amount increases from 1 to 2.4 mg/m2. In our previous ellipsometric study, we found that at a surface coverage above 1 mg/m2 the adsorbed layer thickness increases only slowly with increased surface coverage due to interactions between the PEO chains.28 In Figure 3, we have plotted the adsorbed amount (obtained from the adsorption isotherm) versus the barrier height at different polymer concentrations. The barrier height in this concentration interval increases as the adsorbed amount increases. There also exists a critical adsorbed amount, above which the steric repulsive interactions dominate over attractive interactions. This observation agrees with the fact that we did not observe any steric repulsion between surfaces with adsorbed PEO homopolymers for which the plateau adsorption value was about 0.5 mg/m2. Hence, we note that, at these rather low coverages, surface force measurements provide little information about the adsorbed layer characteristics.
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At higher surface excess values, in the presence of the repulsive barrier, the height of the barrier is inferred to be a measure of the surface pressure exerted by the adsorbed copolymer layers. The very reason for the distance independent force, observed at short surfaceto-surface separations, is related to molecules being laterally displaced from their original positions in the gap. Copolymers become expelled from the intersurface gap, as the distance between the surfaces becomes substantially smaller than twice the adsorbed layer thickness of the copolymer. The force needed to expel the copolymers depends on the equilibrium surface pressure and also to some extent on the rate of surface-to-surface approach. The copolymers are forced out from the gap when the pressure between the surfaces exceeds the surface pressure outside the contact zone. The fact that the energy over this range is net repulsive rather than net attractive is due to the slow kinetics of the depletion process and the fact that not all copolymers are depleted between the surfaces. It was furthermore observed that the size of the depletion range decreased slightly as the adsorbed amount increased. It was also noticed that, for surfaces moved at higher speeds over the depletion range at lower surface coverages, the resistance is lower and the interaction therefore more attractive. All measurements in Figure 2 were performed with a measurement time-resolution of 100 points per second. On separation, the surfaces could be separated by almost 3 nm, before they jumped out of contact. This distance did not change much as the polymer concentration was increased, but the magnitude of the adhesion again decreased substantially. The decrease may be partially related to the increased surface pressure with increasing surface excess. But still the decrease is larger than expected from the changed surface pressure alone. This fact again indicates that some copolymers are trapped between the surfaces after the compression into hard wall contact. Influence of Surface Compression and Separation Rates. The rate at which the surfaces approach each other was found to be an important parameter that to some extent also influences the energy-distance curves. In all measurements presented so far, the rate of the moving surface was about 60 nm s-1. By changing the period and amplitude of the magnetic loading ramps, we were able to vary the rate of approach and separation. Energydistance curves obtained at different rates are shown in Figure 4. The measurements were carried out at two different P224-28 polymer concentrations, 5 × 10-5 and 1 × 10-3 wt %, respectively. It is clear from the graph that the repulsive barrier increases with an increasing approach rate. At the largest speeds and copolymer concentrations, the force curves do not stop increasing closer to the hard wall contact. Rather, the force continues to increase all the way into contact, but with a much smaller slope. These kinetic effects are related to the rate of lateral movement of the copolymers at the surface. When the surfaces are compressed rapidly, the copolymers do not have enough time to move out of the intersurface gap during the compression. This results in a continued increase of the barrier height. Ultimately this is a consequence of the fact that more copolymers remain in the gap under a certain force load. Finally, however, most of copolymer molecules are expelled, but higher loads are needed to do so. It should be emphasized that the increase of barrier height due to increased approach speed is small relative to the total barrier. Adsorption Kinetics and Surface Forces. In the above, we have discussed situations in which the adsorbed layer
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Figure 4. Energy-distance curves for different approach speeds. The curves are shown for concentrations 5 × 10-5 and 10-3 wt % of the triblock copolymer, P224-28.
Figure 5. The energy-distance curve at different times after an increase of the triblock copolymer P224-28 concentration from 10-4 to 10-3 wt %. The curve at time zero corresponds to the equilibrium energy-distance curve at 10-4 wt %.
is in equilibrium with the bulk solution. We proceed further to the relation between adsorption kinetics and the force of interaction, that is, interaction forces during situations in which the adsorbed layer was clearly not in equilibrium with the bulk. It was possible to study such situations because the adsorption rates of the copolymers at low bulk concentrations were slow compared to the time needed for a force measurement. In Figure 5, we show the energydistance curves that were obtained when force measurement runs that were performed first at a bulk P224-28 concentration of 10-4 wt % were then subsequently increased to 10-3 wt %. The curve at time zero corresponds to the equilibrium energy-distance curve at 10-4 wt % of copolymer. The other curves represent measurements at different times after the addition of copolymer. The energydistance curves obtained with increasing time are representative of the evolution of the forces with increasing
Interactions between Hydrophobic Surfaces in Water
Figure 6. Energy-distance curves for the different copolymers. All curves are obtained at equilibrium adsorption of polymers at 10-2 wt % concentration.
surface coverage. We can therefore follow the adsorbed layer during its continued buildup with time. As the adsorbed amount increases with time at constant bulk concentration, the height of the repulsive barrier increases. We also notice that the increase of the repulsive interaction with decreasing distance becomes steeper. Both these observations are in agreement with an increased polymer density in the surface region. We further observe that the general characteristics of the force-distance curves are similar to those obtained under equilibrium conditions. It is important to note that during this kinetic study we did not observe any hysteresis in consecutive compression and separation cycles. This shows that the dynamic surface pressure (acting at a given instance of time) can be measured by the interfacial gauge technique. The measurement is the surface analogue of a dynamic surface tension measurement at the liquid-vapor interface. Effects of Polymer Chemical Composition. It is of interest to study how force interactions are affected by changing the chemical composition and size of copolymer segments. For this purpose, a number of different Pn-m copolymers were used. All experiments were performed at a polymer concentration of 10-2 wt %. All systems were, furthermore, allowed to equilibrate before the measurements were performed. In Figure 6, we show the different energydistance curves obtained on approach. The shapes of all copolymer force curves are similar to that of the curve obtained for the largest copolymer P224-28, which has been discussed above. For the copolymers P146-28 and P50-14, we did not notice the distance indifferent repulsive force region obtained close to hard wall contact. The repulsion in this region continued to increase slowly with decreasing intersurface distance all the way into contact. This resembles the situation obtained during measurements with adsorbed P224-28 at higher approach rates. Our earlier ellipsometric measurements showed that the P146-28 and P50-14 copolymer systems form the most dense adsorbed layers. Hence, we can interpret the kinetic effect as being due to changing lateral mobility. The higher inner densities of P146-28 and P50-14 compared to those of other polymers result in a decreased lateral mobility. This result may prove useful for the optimization of sterically stabilized systems. Another observation that deserves to be mentioned is that the copolymers can be
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divided into two groups depending on the THF block length. For each group, the repulsive barrier increases steadily with the number of EO segments, both with regard to onset distance and the barrier height. Barrier Height in Relation to Surface Pressure. What is measured in a surface force experiment is essentially the compressibility of the adsorbed layer. Resistance to compression results in a repulsive force, while attractive interactions can arise if the amount of material is increased in the intersurface gap during a surface-to-surface separation decrease. Hydrophobic and van der Waals interactions also result in attractive interactions between the two surfaces. In our case (for adsorbed amounts and layer thicknesses significantly larger than about 0.5 mg/m-2 and 5 nm, respectively), repulsive interactions dominate the total force interaction during surface-to-surface approach. This repulsion is due to the overlap of the adsorbed copolymer layers. Attractive forces dominate the interaction between bare surfaces and surfaces with low copolymer coverage. We have also shown that the adsorbed polymers to a large extent are expelled from the intersurface gap at distances close to hard wall contact. However, the layers quickly re-form when the surfaces are separated, so that no hysteresis is observed between two consecutive force measurement cycles. This was also shown to be the case when the adsorbed layers were not in equilibrium with the surrounding bulk solution. The transport of polymer from the bulk is very slow compared to the time needed for two consecutive force runs.29 We can conclude from this that the adsorbed layers are disrupted on approach but retain the capacity to re-form in the contact region even without an efficient exchange with the bulk solution. The reservoir for this selfregulating system is simply the surface area outside the contact zone. In a Langmuir trough, a monolayer readjusts itself in response to changed surface area. The equilibrium density profile in the lateral direction is maintained without exchange with the bulk phase. The Gibbs equation can be used to understand this phenomenon. We will here use the same approach to interpret our force curves. The repulsive barrier height is assumed to represent the pressure needed to expel copolymers from the intersurface gap. This should correspond to roughly twice the surface pressure at the undisturbed solid-liquid interface. A simplified way to calculate the surface pressure is to combine the Langmuir isotherm with the Gibbs equation. The resulting Szyszkowski equation is strictly only valid for highly ideal systems.36,37 Nevertheless, it can often be successfully applied to describe more complex systems. The surface pressure is given by the following equation.
πsl ) γ° - γ ) RTΓm ln(1 + aC)
(1)
The constants Γm and a are obtained by fitting the experimental copolymer isotherms to the Langmuir equation. The calculated surface pressure at the nondisturbed interface (using eq 1) and half the value of the repulsive barrier for P224-28 are plotted in Figure 7 against the bulk concentration. We can see from the graph that the measured values of the barrier height agree relatively well with the surface pressure values calculated (by the Szyszkowski equation) from the independently measured adsorbed amounts. This may appear a little surprising due to the extreme simplifications used and the fact that we have already shown that there exists a force dependence of the approach speed of the surfaces (i.e. a (36) Von Szyszkowski, B. Z. Phys. Chem. 1908, 64, 385. (37) Jones, P.; Hockey, J. A. Trans Faraday Soc. 1971, 67, 2679.
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Eskilsson et al.
Figure 7. Surface pressure of the triblock copolymer P224-28 calculated from the Szyszkowski equation (open circles) and as half of the barrier height from Figure 2 (filled squares).
nonequilibrium effect). However, the effect of approach rate is relatively small compared to the absolute value of the barrier height. We conclude by noting that adsorption data can indeed be used to estimate the repulsive force barrier height between copolymer-covered hydrophobic surfaces. This is probably also true for other systems, such as surfactants. Barrier Shape. In an attempt to further understand the force curves and the process of lateral displacement of adsorbed polymers from the gap between two approaching surfaces, the repulsive part of the forcedistance curves was fitted to the theory of de Gennes. This was developed to interpret interactions between terminally grafted polymer brushes.1,38,39 The relation between the energy of interaction of tethered polymers and the distance between surfaces is given by the following equation.
E(D) ∝
([
]
9/4 kT (2L0) D7/4 + 5/4 3 s0 1.25(D) 1.75(2L0)3/4
[
2L0 2L0 1.25 1.75
])
(2)
In this theoretical formula, 2L0 is the onset distance for the steric repulsion, s0 is the spacing between two polymer chains, k is the Boltzmann constant, and T is the absolute temperature. The spacing between two polymer chains s0 can be approximated from the adsorbed amount determined by ellipsometry. We calculated the area per molecule and divided this by a factor of 2 to account for the fact that the copolymers have two PEO chains that are anchored preferentially to the surface by the THF block. The value so obtained was then used in eq 2 to fit the experimental data. The prefactor, which is missing in eq 2, was used as the fitting parameter (between 0.2 and 0.3 in our case). The experimental and fitted energydistance curves obtained for two different polymer concentrations (and surface coverages) are shown in Figure (38) de Gennes, P. G. Polymer adsorption. In Liquids at interfaces; Charvolin, J., Joanny, J. F., Zinn-Justin, J., Eds.; North-Holland: Amsterdam, 1990. (39) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189.
Figure 8. Experimental energy-distance curves and the fit by the theory of de Gennes (eq 2), for concentrations 10-4 and 10-2 wt % of the triblock copolymer P224-28.
8. The theoretical curve fits the experiment very well at large separation. At smaller separations, however, the fit breaks down. We infer that the fit begins to deviate when the polymers begin to be laterally forced out from the gap. Hence, as discussed earlier, as they move out of the gap between approaching surfaces, this results in an increase of s0, in the interaction zone, and thereby also in a decrease of the interaction energy. It is interesting to note that the theoretical energy versus distance curve obtained at the lower copolymer concentration (i.e. 10-4 wt %) can be fitted almost to the maximum height of the repulsive barrier, while the fit starts to deviate relatively early for the more concentrated sample. The reason for this is probably the fact that changes of s0 are relatively speaking larger at high surface coverages and large forces. Note the very strong dependence of the interaction energy on s0. Finally, we want to emphasize the fact that most of the force-distance curve characteristics can be accounted for by using eqs 1 and 2 for long and short surface-to-surface distance regimes, respectively. Conclusions We have come to a number of general conclusions regarding the interaction forces between hydrophobic surfaces covered by adsorbed low-molecular-weight triblock copolymers. We confirm the earlier described development of the adsorbed layer with coverage. This included a transition from a mixed pancake layer at low concentrations to a brush-type structure at higher concentrations. We have also shown that the polymers studied in this work do not act as stabilizers for hydrophobic colloids below a critical adsorbed amount and layer thickness. These are roughly 0.5 mg/m-2 and 5 nm, respectively. The copolymers become more and more efficient as the adsorbed amount increases. The force at larger intersurface distances at higher coverages was well described by the theory of de Gennes for terminally grafted polymer chains. We further show the maximum value of the steric repulsive barrier (obtained closer to hard wall contact) is equal to twice the surface pressure of the adsorbed polymer layer. This pressure can be estimated and calculated independently from the measured adsorp-
Interactions between Hydrophobic Surfaces in Water
tion isotherm by the use of the Szyszkowski equation. Close to hard wall contact, polymers are expelled from the gap between the surfaces. This process begins when the pressure between the surfaces exceeds the surface pressure at the solid-liquid interface. We have also shown that the surface forces exhibit a dependence on both the rate of the surface-to-surface approach and the volume fraction of polymers in the adsorbed layer. Higher speed and increased volume fraction result in an increase of the repulsive barrier. This rate dependence occurs because polymers become kinetically trapped in the intersurface gap. The adsorbed layers were also observed to re-form
Langmuir, Vol. 15, No. 9, 1999 3249
rapidly when the surfaces were separated. Rapidly repeated force curves were shown to be perfectly reproducible. This is an important observation in the context of colloidal stability, where the stabilizing polymer layers are subjected to repeated collisions. Acknowledgment. We thank the Swedish Research Council for Engineering Sciences (TFR) and the Swedish National Board for Technical Development (NUTEK) for financial support. LA981469Z
