Langmuir 1998, 14, 7287-7291
7287
Interaction between Hydrophilic Surfaces in Triblock Copolymer Solution K. Eskilsson,*,† B. W. Ninham,‡ F. Tiberg,§ and V. V. Yaminsky‡ Physical Chemistry 1, Center 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 July 17, 1998. In Final Form: September 28, 1998 We report on the interactions between fused quartz surfaces immersed in solutions of low molecular weight nonionic homopolymers and triblock copolymers. The polymers studied were a poly(ethylene oxide) (PEO) homopolymer and a triblock copolymer of the type poly(ethylene oxide)-poly(tetrahydrofuran)poly(ethylene oxide). The surface force measurements were performed by the interfacial gauge technique. In water, a strong repulsive force was observed at short surface separations. This force has its origin in electrosteric interactions imposed by formation of a gel layer on silica. This interaction was totally suppressed when small amounts of polymers were adsorbed or when water was substituted for ethanol. Adsorbed polymers induce a long-range steric repulsion. However, in solutions containing homopolymers, when the distance between the surfaces decreases below 3 nm, the repulsive interaction levels off. Here the force remains more or less constant with decreasing surface-to-surface distance. Adsorbed polymer molecules are in this region to a large extent expelled from the gap between the two approaching surfaces. In presence of small amounts of triblock copolymer surface aggregates, we observed an attractive interaction at intersurface distances smaller than approximately 10 nm. An adhesion between the surfaces can also bee seen during separation of the two surfaces in this region. The attraction is caused by surface aggregates, which bridge the surface-to-surface gap. When adsorption is increased above a certain value, this attraction vanishes and the interaction becomes purely repulsive at all surface-to-surface distances. When ethanol was used as solvent instead of water, the interaction did not show long-range patterns but the copolymers induced adhesion between the surfaces.
Introduction Adsorption behavior of nonionic surfactants and block copolymers with relatively hydrophilic poly(ethylene oxide) blocks has been studied extensively in recent years. It has been demonstrated that both surfactants1-4 and copolymers5 form surface aggregates (e.g. micelles) above some well-defined bulk concentration, commonly referred to as the critical surface aggregation concentration (csac). It is of academic and practical interest to understand how these adsorbed aggregate structures affect interactions between surfaces. In this work, we have studied interactions between quartz surfaces immersed in aqueous solution of a triblock (ethylene oxide-tetrahydrofuranethylene oxide) copolymer. For comparison, we have also studied the interaction between surfaces with adsorbed poly(ethylene oxide) (PEO) homopolymers, which obviously do not self-assemble at solid surfaces or in bulk solutions. The homopolymer is a useful reference system, which helps us to distinguish between more sophisticated effects of surface self-assembly and ordinary polymer adsorption related interactions. A low molecular weight PEO homopolymer (Mw 4000) that has roughly the same * To whom all correspondence should be addressed. E-mail:
[email protected]. Fax: 46-46-2224413. † Lund University. ‡ The Australia National University. § Institute for Surface Chemistry. (1) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (2) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (3) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (4) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (5) Eskilsson, K.; Tiberg, F. Macromolecules 1998, 31, 5075.
size as the end blocks of the copolymers was used as the reference system. Interaction between mica surfaces immersed in PEO homopolymer solutions was among the first polymer systems studied by the surface force apparatus.6-8 The interaction between the PEO-covered surfaces at saturation was in these studies reported to be repulsive at all distances. However, attractive interactions were observed for high molecular weight polymers at partial coverage. This attraction was attributed to polymer bridging. Recently, Braithwaite et al.9,10 studied interaction of glass surfaces in solutions containing PEO homopolymers using atomic force microscopy (AFM). Their results were very similar to the earlier results presented for mica surfaces. However, they also showed that the speed of surface-tosurface approach was an important parameter and that the adsorbed polymer layers formed at equilibrium were easily perturbed by rapid repeated compressions and separations. The force curves changed strongly over a series of repeated measurements. Note that polymers of relatively large molecular weights, ranging from 40 000 to about 700 000, were used in these studies. The molecular weights of the polymer studied in this work were all smaller than 12 000. Hence, the polymers used (6) Israelachvili, J. N.; Tandon, R. K.; White, L. R. J. Colloid Interface Sci. 1980, 78, 432. (7) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041-1048. (8) Luckham, P. F.; Klein, J. J. Colloid Interface Sci. 1986, 117, 149158. (9) Braithwaite, G. J. C.; Howe, A.; Luckham, P. F. Langmuir 1996, 12, 4224-4237. (10) Braithwaite, G. J. C.; Luckham, P. F. J. Chem. Soc., Faraday Trans. 1997, 93, 1409-1415.
10.1021/la980903u CCC: $15.00 © 1998 American Chemical Society Published on Web 11/19/1998
7288 Langmuir, Vol. 14, No. 25, 1998
Eskilsson et al.
in this study have a larger lateral mobility at the surface. This increased mobility allows the adsorbed polymers to respond more rapidly to perturbations caused by their interactions. In this paper, we also make the first attempt to describe effects of copolymer surface aggregation on surface forces. This has to our knowledge not been studied previously. Experimental Section Surface Preparation. The substrates for the surface force experiments were prepared by melting a sphere at the end of a 2 mm diameter quartz rod (Suprasil 2, from Heraeus). The solidified droplet, with a diameter of 3-4 mm, has an ideally smooth surface.11 Unlike for SFA interferometer, the scaling radii of such spheres coincide with the macroscopic radii measured with high accuracy by a micrometer. Instability of silica surfaces in water is an important problem.12 Surface decomposition of silicon dioxide into polysilicic acids results in formation of a diffuse layer of silica gel. Properties of such softened lyophilic interfaces depend to a large degree on their history.12 Surface Forces. Forces and displacements were measured with a solid-state sensor13-16 in the basic set up described earlier.17 The apparatus is referred to as the interfacial gauge. The freshly melted quartz samples were installed in the gauge and immersed in liquid in a quartz beaker. The bulk concentration of polymers was varied by additions of a stock solution into the beaker. Stirring was achieved by rotating the beaker with the surfaces immersed. The forces at different surface-to-surface distances were measured by applying an external magnetic load and monitoring the electric response of the piezoelectric sensor. This gives interaction forces between the two surfaces as a function of their mutual displacement and speed. Speeds and loads were varied by changing the period and the amplitude of the loading ramps. The surfaces were moved at a constant speed (≈15 nm/s) in all measurements unless specified otherwise. The data acquisition system enables collection of points at a desired frequency (up to 50 kHz). The load and the displacement are at the normal to the surfaces at contact so that shearing movement is avoided. A disadvantage of using a solid state sensor is that absolute zero position cannot be directly inferred. Instead, all separation distances quoted here are relative to hard wall contact. Polymers. The (ethylene oxide-tetrahydrofurane-ethylene oxide) triblock copolymer studied in this work is referred to as P224-28. The first number refers to the number of EO groups and the second to the number of THF groups. The triblock copolymer was produced by Akzo Nobel Surface Chemistry, by ethoxylating PTHF polymer (2000), which was purchased from BASF. The polymer purification procedure and the method used to determine the average n/m ratio (NMR) and the cmc, of the copolymer (dye solubilization and absorption measurements) have previously been described.18 The polymer has a molecular weight of 11 900 and forms micelles in bulk solution with a fairly distinct cmc (0.02 wt %). No homopolymers are present, but there may be a small contamination in the form of diblock copolymers. The PEO homopolymer (Mw 4000, SERVA, Feinbiochemica) is referred to as P90, and was used without further purification. Solutions were prepared with Millipore water containing 0.2 mM NaBr (if not otherwise stated). The ethanol used in some experiments was freshly distilled.
Results and Discussion Interactions in Aqueous Solutions Containing Only Salt. Experiments were started by measuring the (11) Yaminsky, V. V.; Yusupov, R. K.; Amelina, E. A.; Pchelin, V. A.; Shchukin, E. D. Colloid J. USSR 1975, 37, 918-925. (12) Yaminsky, V. V.; Ninham, B. W.; Pashly, R. M. Langmuir 1998, 14, 3223. (13) Parker, J. L. Langmuir 1992, 8, 176. (14) Parker, J. L.; Stewart, A. M. Prog. Colloid Polym. Sci 1992, 88, 162. (15) Stewart, A. M.; Parker, J. L. Rev. Sci. Instrum. 1992, 63, 5626. (16) Stewart, A. M. Meas. Sci. Instrum. 1995, 6, 114. (17) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836-850. (18) Eskilsson, K.; Tiberg, F. Macromolecules 1997, 30, 6323-6332.
Figure 1. Energy vs distance curves for two quartz surfaces immersed in 0.2 mM NaBr solutions. The dotted line represents the best fit to the theoretical electrostatic double layer force with a surface potential of -75 mV and Debye length of 23 nm.
Figure 2. Energy vs distance curves for surfaces immersed in 0.2 mM NaBr solution with two different concentrations of the P90 homopolymer.
interaction between bare quartz substrates in air and in 0.2 mM NaBr solution, respectively. The force curves thus obtained serve as references for the interaction curves obtained in the polymer systems. No interaction was observed in air before the surfaces jumped into contact from at a distance of about 7 nm. A large pull-off force measured on separation indicates that the surfaces are smooth and uniform. Figure 1, shows the interaction on approach between two bare quartz surfaces immersed in 0.2 mM NaBr. The dotted line represents the nonlinear Poisson-Boltzmann fit. The electrostatic double layer force was best fitted with a Debye length of 23 nm and a constant surface potential of -75 mV. The measured force deviates from the fit below 4 nm of the surface-to-surface distance. At this distance the measured repulsion starts to increase faster than expected by the electrostatic double layer theory. An interesting observation is that this repulsion becomes suppressed by very small amounts of the polymer adsorbed at the surfaces (see Figure 3). A similar phenomenon occurred when the water was sub-
Interaction between Hydrophilic Surfaces
Figure 3. Energy vs distance curves at equilibrium for three different concentrations of the P224-28 triblock copolymer. The curve for the bare silica surfaces in the 0.2 mM NaBr solution is shown as a reference.
stituted for ethanol. No adhesion was observed in pure NaBr solution. The “non-DLVO” repulsion at small separations is known under the name of hydration or structural force. The origin of this repulsion is still disputed.19-21 A similar force has been reported between mica surfaces at separations smaller than 1.5 nm.22 The latter force occasionally showed oscillations with periodicity of about the size of the water molecule. No such behavior has, however, been observed between silica surfaces. We infer that this extra repulsion is due to compression of a gellike layer at the silica surface. The force is thus believed to have an electrosteric origin and results from interactions between partly dissolved silica polymers containing acidic groups.12 The fact that this steric interaction becomes suppressed by adsorption of small amounts of organic polymers or by immersion of the surfaces in an organic solvent (ethanol) is clear from our measurements. The most straightforward explanation of this effect would be that the resulting less polar environment induces dehydration and collapse of the silica gel layer. Interaction in Aqueous Solutions Containing PEO Homopolymer. Surface force measurements in PEO homopolymer solutions are important for comparisons with the corresponding PEO-based copolymers. In our earlier ellipsometric adsorption study of this PEO polymer,5 it was shown that the adsorbed amount slowly increases with increasing bulk concentration over a very wide concentration range. Even at high concentrations, the adsorption was low (i.e. below 0.25 mg/m2). This agrees well with results of other studies.23 Figure 2 shows energy vs distance curves obtained in aqueous solutions in the presence and absence of PEO homopolymer. To ensure that equilibrium conditions were reached, the force curves were always compared to force measurements performed under identical conditions an hour later. The system was considered to be in equilibrium (19) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404-408. (20) Israelachvili, J. N.; Wennerstro¨m, H. Nature 1996, 379, 219255. (21) Ducker, W. A.; Senden, T. J.; Pashly, R. M. Nature 1991, 353, 239-241. (22) Israelachvili, J. N.; Pashley, R. M. Nature 1983, 306, 249-250. (23) Trens, P.; Denoyel, R. Langmuir 1993, 9, 519.
Langmuir, Vol. 14, No. 25, 1998 7289
when the force curves before and after this period were shown to be identical. It is important to note that we did not observe any hysteresis in the interaction for rapid repeated compressions and separations. Note that a relatively large hysteresis (i.e. interaction patterns changed from run to run) was reported in some earlier work, but the molecular weights of the PEO polymers used in these investigations were much larger.7,9 The interaction that we observe is, however, not purely static. There is a clear dependence on the speed of approach of the surfaces, as will be further discussed below. The contribution of the electrostatic double layer force to the total surface interaction was not affected by the presence of adsorbed polymers. However, at shorter surface-to-surface distances, the steric repulsion between adsorbed polymer layers dominates over the electrostatic interaction. On continued approach, the steric force increases until the surface-to-surface distance is reduced to 2-3 nm. From here on, the repulsion as a function of distance ceases to increase. Instead, a relatively stable value is retained until the hard wall contact is established. The divergence from the electrostatic double layer repulsion is, as mentioned, due to steric interactions between the adsorbed polymer layers. For both of the two polymer concentrations shown in Figure 2, the experimental force curves begin to diverge from the electrostatic reference curve at separations of about 15 nm. The magnitude of the steric repulsion does, however, increase with increasing polymer concentration (and consequently increasing adsorbed amount). The increase of the steric repulsive force can be understood by considering the osmotic pressure increase with increasing polymer concentration (and adsorbed amount), which also is followed by an increase of the surface pressure exerted by the adsorbed layer. Our earlier studies show that the adsorption of the P90 homopolymer increses from about 0.08 mg/m2 at a bulk polymer concentration of 1 × 10-4 wt % to roughly 0.15 mg/m2 at 0.01 wt %. The force onset distance (≈15 nm) can be related to the polymer conformation in the adsorbed layer. The radius of gyration, Rg, of the PEO polymer used in this study has been reported to be about 2.6 nm.24 This value can be compared to the distance at which the adsorbed polymer layers are observed to begin to interact, ≈6Rg. Note that 4Rg would be the onset distance if we compressed two layer consisting of PEO random coils, each with an effective interaction distance of 2Rg. Our results indicate that polymers interact at distances larger than the radius of gyration. This implies that steric interactions due to the compression of dilute tails extending further than Rg are sufficiently large to dominate the force interaction between the silica surfaces at large intersurface distances. An extended range of the steric repulsion (i.e. >4Rg) may, however, also be related to the preferential adsorption of larger polymer chains at the surface. This is possible, since the polymers are naturally polydisperse. The effect can, indeed, be quite large, since the surface-to-volume ratio is small in our experimental system. The relatively small surface coverage of PEO chains at the silica surface means that the stretching of polymer chains due to surface crowding is insignificant. We finally note that ratio of the force onset distance to the radius of gyration is in the same ballpark as has previously been reported for PEO polymers adsorbed on mica.7 For adsorption of large PEO polymers on silica, steric interactions have been observed at distances as large as 12Rg.9 (24) Bhat, R.; Timasheff, S. N. Protein Sci. 1992, 1, 1133-1143.
7290 Langmuir, Vol. 14, No. 25, 1998
The stabilization of the repulsion at shorter distances indicates that polymers are expelled from the gap between the surfaces in this region. The distance where the repulsion levels off is for the PEO homopolymer sample close to Rg. We believe that the polymers are forced out from the gap when the energy of the steric repulsive interaction between the surfaces exceeds the surface pressure outside the contact zone. The small adsorbed amount and small size of the polymers studied suggest that these are not to a large degree entangled at the surface and, thus, retain a high level of lateral mobility. Strong entanglements and inhibited lateral transport of polymers in the surface plane could prevent the layers from being expelled from the gap between the surfaces even when the interaction energy exceeds the surface pressure. When the speed was increased 10 times from 15 to 150 nm/s, the repulsion did not stabilize in the short intersurface distance region. Instead, it continued to increase all the way into hard wall contact. This is due to the relatively slow kinetics of the depletion process and the fact that not all polymers are depleted between the surfaces. The time needed for the polymers to move away from the contact zone is therefore on the same order as the approaching speed of the surfaces. Such steric dynamic repulsion that substitutes for attractive interactions might very well arise in colloidal systems. Here lies important information about the mode of reversible bridging. As mentioned earlier, no adhesion was observed between bare quartz substrates in a 0.2 mM NaBr solution. However, bridging interactions are clearly present in the PEO polymer solution. This becomes evident by considering the forces monitored as the surfaces are moved out of contact. The force curves obtained during this process show adhesive interactions at short distances. This indicates that not all polymers are depleted from the contact zone during the compression step. Bridging interaction, hence, occurs between the surfaces covered by remaining polymers while these are at short separation or in contact. This adhesion does not depend on speed of approach, although increased speed leads to a shorter contact time between the surfaces. The kinetic and thermodynamic aspects of the adhesion requires further investigation in order to be fully understood. Interaction in Aqueous Solutions Containing Triblock Copolymers. The ellipsometric adsorption isotherm5 for the P224-28, copolymer shows that the adsorbed amount initially increases slowly with increase of copolymer concentration. However, a more cooperative increase of adsorption is observed in the concentration region around 10-4 wt %. The isotherm then passes through a maximum around the cmc, which for this polymer is around 0.02 wt %. It has been concluded5 that formation of surface aggregates is responsible for the increase of the adsorbed amount above values observed for PEO homopolymers. The driving force for the aggregation is a tendency to minimize the contact between the more hydrophobic (PTHF) middle part of the copolymers and water. The occurrence of a maximum in the adsorption isotherm is indicative of polydispersity of the copolymer system.5 When block copolymers are added to the NaBr solution, the energy-distance curves change much more dramatically with an increase of the polymer concentration than was observed for the PEO homopolymer system. In Figure 3, we show different force-distance curves that were measured at three different copolymer concentrations. The curve in the absence of copolymers is also shown as a reference. At the lowest copolymer concentration, repul-
Eskilsson et al.
Figure 4. Energy vs distance curves during approach of the surfaces. The force curves were measured at different times after the addition of the P224-28 copolymer. The bulk copolymer concentration was 10-4 wt %.
sive interactions were only detected at short surface-tosurface distances. At this concentration the surfaces can be moved from five to two nm separation without change of the interaction energy. A nonelectrostatic repulsive interaction first becomes noticeable at a 2 nm surface separation. This repulsion continues to increase until hard wall contact is established. When the copolymer concentration was increased first to 10-3 and later to 10-2 wt %, the interaction became progressively more repulsive at all intersurface distances smaller than 15 nm. At 10-4 wt % copolymer concentration, we enter the cooperative adsorption region of the adsorption isotherm.5 This indicates that copolymer aggregates have started to form at the surface. The adsorption kinetic at this low copolymer concentration is very slow, which enabled us to study how the force-distance curve changed with time (i.e. with increasing adsorbed amount). Figure 4 shows this time evolution for the 10-4 wt % copolymer sample. At short times we observe an attraction at 10 nm separation. This attraction does, however, become progressively weaker with time. In fact it is substituted with a distance-independent repulsive force over the entire depletion range after adsorption equilibrium is reached. The origin of the attraction is most likely bridging interactions. PEO chains tethered at the surface aggregate core can easily bridge the surface gap at these intersurface distances. The bridging attraction should be reduced with increasing coverage, since the available surface for PEO chains to adsorb at opposing surfaces is decreased. This notion is supported by the results, which show clear adhesion minima for the two shortest adsorption times. In contrast, no adhesion was observed after 21 h of adsorption time, and furthermore, no hysteresis on approach and separation was observed after this time. It is a fact that the repulsion temporarily stabilizes as the surface-to-surface distance decreases to values below 10 nm. This shows that the osmotic pressure and the copolymer volume fraction in the gap is relatively stable in this regime. The increasing repulsion, which is finally observed in the vicinity of the hard wall contact, is observed at all polymer concentrations. It is due to compression of thin polymer layers preferentially adsorbed in the gap. At higher polymer concentrations, when adsorbed layers at
Interaction between Hydrophilic Surfaces
Langmuir, Vol. 14, No. 25, 1998 7291
For copolymers that have been adsorbed from an aqueous solution (10-2 wt %) and then quickly replaced by ethanol, a similar result was obtained. This shows that the polymers are partly desorbed rapidly in ethanol. When the ethanol was replaced with water, we obtained a curve that was very similar to that observed in aqueous solutions of low polymer concentrations (10-4 wt %). The measurements are all shown in Figure 5. To achieve the same desorption effect with water would take 20 h of continuous stirring and rinsing.5 Conclusions
Figure 5. Energy vs distance curves during approach of the surfaces for (a) bare silica surfaces in 0.2 mM NaBr, (b) surfaces after a 10-2 wt % polymer solution was replaced by ethanol, and (c) surfaces when ethanol was finally replaced by a 0.2 mM NaBr aqueous solution.
isolated surfaces become more developed, this repulsion is observed from larger distances. The force needed to reach hard wall contact is nevertheless relatively small at all copolymer concentrations according to low surface pressure on surfaces at isolation. With regard to these statements, it should again be emphasized that no hysteresis was observed between two rapidly repeated runs. The polymer layer therefore rapidly relaxes back to its original configuration after the compression. Desorption of the Polymers and Interaction in Ethanol. Only a very weak electrostatic double layer repulsion is observed when bare silica surfaces approach each other in ethanol. Moreover, no adhesion could be detected in ethanol. There was further no sign of the steric repulsion, which was observed for the bare surfaces at small distances in water. The experiment performed in ethanol also show that a very small amount of polymers adsorbs at silica from ethanol. No polymer induced steric repulsion was observed, the surfaces did not interact until a surface-tosurface distance of 7 nm was reached. From this point and onward, an attraction was observed, which resulted in a jump of the surfaces into hard wall contact. Changing the polymer concentration from 10-4 to 10-3 wt % did not change the interaction.
The paper presents experimental studies of interactions between hydrophilic silica surfaces with adsorbed homo and triblock copolymers with relatively low molecular weights. We point out differences and similarities in the interaction-distance curves between silica surfaces imersed in solutions of different polymer concentrations. The force curves are related to the adsorption behavior and the structure of the different polymers at the solid-liquid interface. For both types of polymers, repulsive interactions were observed at large surface separations where the adsorbed polymer layers start to overlap. At shorter surface separations, the homopolymers are to a large extent depleted from the surface gap resulting in a relatively large distance-independent force regime which is prolonged until hard wall contact between the surfaces is established. For the homopolymer, we further observe relatively small changes in the magnitude of the interaction energies with concentration. This is consistent with the corresponding small increase in adsorbed amount in this concentration region. For surfaces with adsorbed triblock copolymers, the interaction on approach changes more significantly, from being attractive at short separations and low adsorbed amounts to a distance-independent force region with a small repulsive interaction and finally to much more pronounced repulsion at all separations. This evolution is also consistent with adsorption data. The force barrier height (defined as the force observed in the distance-independent force regime) appears, therefore, to correlate well with the surface pressure exerted by the adsorbed film at large surface-to-surface separations. A more detailed discussion about the relation between surface forces and surface pressures will be presented in a coming study, where the interactions between hydrophobic surfaces with adsorbed copolymers is presented. The attraction at low surface coverages is due to bridging of the surfaces by surface micelles. This disappears when the adsorbed amount increases. LA980903U
