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Langmuir 1998, 14, 5664-5666
Long Range Attractive Surface Forces Due to Capillary-Induced Polymer Incompatibility Ha˚kan Wennerstro¨m,* Krister Thuresson, Per Linse, and Eric Freyssingeas† Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received May 5, 1998. In Final Form: July 20, 1998 Observations of a novel case of capillary-induced phase separations (CIPS), which we judge to be of considerable practical significance, is reported. A long-range attractive force was observed between mica in mixed aqueous semidilute solutions of dextran and poly(ethylene oxide). The result was found to be similar to reported CIPS in liquid mixtures, and the origin of the long-range linear force can be related to the formation of a capillary condensate with a polymer composition different from the mixed polymer bulk solution.
Introduction Capillary condensation is a well-established source of a long-range attractive force between particles in a gas phase.1 Similarly capillary-induced phase separation in a liquid mixture has also received considerable attention. Of particular practical importance is the formation of a meniscus of water between hydrophilic surfaces in an apolar medium.2 In fact, in several different cases it has been observed that a confinement between surfaces has induced a phase separation, which in turn resulted in an attractive force. A number of hitherto observed examples of this general phenomenon of capillary-induced phase separations (CIPS) are illustrated in Figure 1. In this Letter we report observations of a novel case of CIPS which we judge to be of considerable practical significance. Solutions of polymer mixtures have a strong tendency to phase separate and form two liquid phases. This is usually referred to as polymer incompatibility.3 The phenomenon is caused by the fact that demixing at constant osmotic pressure is only prevented by the entropy of mixing of the polymer molecules, while the solubility in the solvent is dominantly caused by the entropy of the chain conformations. For long polymers the entropy of mixing molecules is small, and only a small (per monomer unit) opposing energy term is required to cause a phase separation.4 Mixed aqueous systems of dextran and poly(ethylene oxide) have for a long time been used for partitioning separation of biological macromolecules and organelles,5 and the phase behavior is well characterized and theoretically modeled.6 We have therefore chosen this system to document the existence of a capillary phase separation due to polymer incompatibility. For this purpose measurements using the surface force apparatus (SFA) were performed to determine the magnitude and the range of the attractive force. † Present address: Laboratoire de Physique, Ecole Normale Supe´rieure, 46 alle´s d’Italie, 69364 Lyon Cedex 07, France.
(1) Zimon, A. Adhesion of Dust and Particles; Plenum: New York, 1982. (2) Christenson, H. K.; Fang, J.; Israelachvili, J. N. Phys. Rev. B 1989, 39, 11750. (3) Flory, P. J. Principles of Polymer Chemistry, 13th ed.; Cornell University Press: Ithaca, NY, 1953. (4) Flory, P. J. J. Chem. Phys. 1942, 10, 51. (5) Albertsson, P.-A° . Partition of Cell Particles and Macromolecules, 3rd ed.; Wiley-Interscience: New York, 1986. (6) Sjo¨berg, A° .; Wennerstro¨m, H.; Tjerneld, F. Polymer 1986, 27, 1768.
Figure 1. Schematic representation of different cases of capillary-induced phase separation (CIPS). Please note that the figure is not drawn to scale, as in practice the radius of curvature of the interacting surfaces is much larger than dimensions of the formed meniscus.
Experimental Section Dextran with a molecular weight of 5 × 105 (dextran T 500, Pharmacia, Sweden) and poly(ethylene oxide) (PEO) with a molecular weight of 4000 (SERVA, Feinbiochemica) were used without further purification. After preparation, the polymer solutions were thoroughly stirred for an extended time (more than 12 h) to ensure complete dissolution and mixing. The electrostatic double layer repulsion was suppressed by adding KNO3 to a concentration of 5 × 10-3 M. The Debye length at this electrolyte concentration is 4.3 nm, and the double layer interaction gives a measurable repulsion in the SFA out to a separation of approximately 25 nm.7 We note that such a small concentration of added salt is not expected to change the quality of water as solvent for this type of polymers.5 The interaction between two mica surfaces was measured as a function of separation with an interferometric surface force apparatus, Mark IV, which has been (7) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153.
S0743-7463(98)00524-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/03/1998
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Table 1. Compositions and the Phase Separation Temperatures for the Two Investigated PEO-Dextran Mixtures PEO to dextran weight ratio
PEO (% wt)
dextran (% wt)
5 × 10-3 M KNO3 (% wt)
phase separation temperature, T* (°C)
0.223 0.106
2.71 1.80
12.13 16.95
85.16 81.25
15 ( 0.5 13 ( 0.5
described in detail elsewhere.8,9 We here just note that in order to measure as close to equilibrium conditions as possible, we applied a measurement procedure developed in our previous investigation.10 This takes into account the slow relaxation processes of polymer solutions within a confinement. The system was allowed to relax for a constant period of time, ∆t, after each change of the distance between the surfaces before the surface separation was measured. Several force curves were measured at different ∆t values. The value of ∆t was increased until the obtained force-distance profiles did not change anymore. No measurements were done before the (thermal) drift in surface separation was less than 10 Å/min. This was achieved about 24 h after sample injection. Around 20 °C the tendency for segregation of a PEO and dextran mixture increases with decreasing temperature. The measured phase separation temperatures of the bulk solution, T*, for the two investigated compositions are given in Table 1. Prior to injection into the SFA, the solutions were always passed through a filter (1.2 µm pores) to remove particle contamination. Samples were injected at temperatures well above T* (∆T ) 10 °C). For each sample, measurements were performed at three different temperatures, Texp, chosen such that the differential temperature, ∆T ) Texp - T*, was the same for both samples (∆T ) 10.5, 8.5, and 6.5 °C). In addition a sample containing only 5 wt % dextran was investigated at a temperature of Texp ) 19.5 °C. Results Figure 2 shows the measured force between two curved mica surfaces in a mixed PEO-dextran aqueous solution of composition 2.71 wt % of PEO, 12.13 wt % of dextran, and 5 × 10-3 M KNO3. On compression (filled symbols) a long-range attractive force is observed, which appears already at a surface separation of 400 nm. This is an order of magnitude larger than the dimensions of the polymer molecules. Furthermore, the force varies linearly with separation down to a separation of less than 100 nm, where it gradually changes into a steep repulsive force. The observed force curve shows the same qualitative behavior for a range of temperatures and for two different compositions. Table 2 summarizes these results by giving the intercepts and slopes of the fitted straight line for the different conditions. A large difference was observed between the force curves recorded on compression and separation. A long equilibrium time (many hours) was allowed before the compression was started and the compression rate was chosen such that the force profile (filled symbols in Figure 2) was invariant with respect to small changes of ∆t (see Experimental Section). This suggests that the compression was performed close to thermodynamic equilibrium between the surfaces. (8) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (9) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (10) Freyssingeas, E.; Thuresson, K.; Nylander, T.; Joabsson, F.; Lindman, B. Langmuir, in press.
Figure 2. Measured force vs surfaces separation between two mica surfaces immersed in an aqueous solution of 2.71 wt % PEO, 12.13 wt % dextran, and 5 × 10-3 M KNO3 at ∆T ) 6.5 °C. The filled symbols refer to compression, while the open ones refer to separation. Table 2. Parameters from a Linear Fit of Force vs Distance in the Outer Part Recorded upon Compression, Where the Force Is Attractive PEO to dextran weight ratio
∆T (°C)
slope (kPa)
intercept (mN/m)
RK (µm)
∆γ (mN/m)
0.223 0.223 0.223 0.106 0.106 0.106
10.5 8.5 6.5 10.5 8.5 6.5
1.09 1.30 1.75 0.69 1.08 1.79
-0.32 -0.49 -0.72 -0.18 -0.36 -0.66
0.29 0.38 0.42 0.26 0.33 0.37
0.025 0.039 0.057 0.014 0.029 0.053
The unfilled symbols in Figure 2 represents the force measured when separating the surfaces (at the same speed as during compression). In this case no measurable attractive force was observed except for the small attractive well around a surface separation of 50 nm. This indicates that the curve recorded on separation does not represent an equilibrium force. In a control experiment the force was also measured for a pure dextran solution using the same experimental procedure as in the mixed polymer system. No measurable attractive force could be detected as the surfaces are brought into contact (Figure 3). However, at small surface separation a short-range repulsive force similar to the one observed in the mixed system appeared. When surfaces are separated, an attractive minimum appeared at a separation of about 50 nm. This is a typical behavior for a polymer adsorbing from a good solvent with a transient bridging effect which appears when the surfaces are separated (as also observed for pure PEO solution11). Apart from the minima no measurable force could be observed on further separation. Discussion We observe an attractive force of considerable strength and with a range of hundreds of nanometers, which is unusually long-ranged even for a polymer system. It should be beyond doubt that the existence of such an attractive force has considerable practical consequences for systems where it appears. In fact, the present study was motivated by a similar observation of an attractive component to the force in a nominally single polymer (11) Klein, J.; Luckham, P. Macromolecules 1984, 17, 1041.
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Figure 3. Measured force vs surfaces separation between two mica surfaces immerse in an aqueous solution of 5 wt % dextran and 5 × 10-3 M KNO3 at T ) 19.5 °C. The filled symbols refer to compression, while the open ones refer to separation.
solution but where the polymer, ethyl hydroxyethyl cellulose (EHEC), has a considerable chemical polydispersity.10 An attractive force between curved surfaces that varies linearly with surface separation is typical for a force caused by capillary-induced phase separation12 and the mechanistically similar depletion attraction.12,13 With a range of the force of several hundreds of nanometers one can exclude the depletion mechanism as the relevant one. On the basis of the use of the Derjaguin approximation, one can readily derive an expression for the force generated when a phase β is condensing between the mica surface from a bulk phase R13
F = -4π{γ(mica/R) - γ(mica/β)} × R0 hgβR h 1) -4π∆γ 1 RK 2{γ(mica/R) - γ(mica/β)}
{
}
(
)
where h denotes the separation between the surfaces and in the second equality we have introduced the difference ∆γ in surface free energies and the generalized Kelvin radius14
RK ≡
2∆γ 2γRβ ) cos θ gβR gβR
The second equality involving the contact angle θ follows from Youngs equation. Here gβR denotes the free energy increase, per unit volume, of converting bulk phase R to (12) Petrov, P.; Olsson, U.; Wennerstro¨m, H. Langmuir 1997, 13, 3331. (13) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain: where Physics, Chemistry, Biology and Technology Meet, 2nd ed.; Wiley: New York, 1998. In press. (14) The definition of the Kelvin radius in ref 12 differs by a factor of 2 from that used here. The source of the difference is in the factor of 2 difference in the mean curvature of a sphere and a cylinder of the same radius.
bulk phase β. The closer to the coexistence of R and β, the smaller is gβR and the larger the range of the force. The two fitted quantities RK and ∆γ are also included in Table 2. That the generalized Kelvin radius increases as one approaches the phase boundary is indeed borne out by the experimental results. The surface free energies only differ marginally making ∆γ small. This should be expected for two aqueous polymer solutions and it has the advantageous aspect that the attractive force is weak enough to be measurable with our device. For CIPS in binary liquid mixtures one can typically only observe the onset of the phase separation since the force is so strong that it in practice causes mechanical instability in the measuring system.15 However, in more complex fluids a similar full force curve has been observed previously.12 We have at present no direct measurement on the composition of the phase that separates out between the surfaces. On theoretical grounds we expect that this phase should have a composition similar to the polymer solution concentrated in PEO that separates from the experimental solution at small additions of PEO or at lowering the temperature. Similarly it is clear from the force curve at short separations that polymers are adsorbed on the mica surface. This occurs also for pure dextran, where the polymer consequently has to be dextran. If this is the case also in the PEO-dextran mixed system is not settled at present. The surface activity shown by the PEO could in such a case be caused by its much shorter chain length (which implies a better packing close to the surface). Alternatively the PEO monomer unit has a considerably higher affinity to the surface and displaces the dextran in the mixed system. The striking irreversible character of the force measurements in the present study as well as in a previous study on EHEC10 was not found in the otherwise similar measurements on microemulsions.12 We have at present no detailed interpretation of this observation. As the separation between the surfaces changes maintaining equilibrium involves material transport to and from the gap region. When the surfaces are in contact, they flatten and some of the adsorbed polymer is forced away from the surface. It is conceivable that the readsorption of polymer on the surface is responsible for the slow relaxation time implicit in the lack of reversibility. Conclusion We report the first observation of a strong long-range attraction due to a capillary-induced phase separation caused by polymer incompatibility. This phenomenon should be rather common for mixed polymer solutions, and it can have large consequences for the colloidal behavior of particle suspension containing polymer mixtures. Further work is in progress to characterize the adsorbed polymer under different experimental conditions, to established the composition of the separating phase, to establish the source of the irreversibility in the force curve, and to analyze the CIPS phenomenon theoretically using a more molecular description of the polymer system. Acknowledgment. P.L. acknowledges the Swedish National Research Council (NFR) for financial support. LA9805241 (15) Christenson, H. K. Colloids Surf. A 1997, 124, 355.
