Selective Adsorption of Poly(ethylene oxide) onto a Charged Surface

Dec 7, 2007 - We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively c...
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Langmuir 2008, 24, 1570-1576

Selective Adsorption of Poly(ethylene oxide) onto a Charged Surface Mediated by Alkali Metal Ions† Liraz Chai,‡ Ronit Goldberg,‡ Nir Kampf,‡ and Jacob Klein*,‡,§ Materials and Interfaces Department, Weizmann Institute of Science, RehoVot 76100 Israel, and Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, Oxford OX1 3QZ, United Kingdom ReceiVed August 14, 2007. In Final Form: October 23, 2007 Using a surface force balance, we have measured normal and shear interactions between mica surfaces across pure water and across 0.1 M aqueous solutions of LiNO3, NaNO3, KNO3, and CsNO3, both prior to adding polymer and following addition of 1.5 × 10-4 w/w poly(ethylene oxide) (PEO, Mw ) 170 kD) and overnight incubation. Our results reveal that while the PEO adsorbs strongly from the KNO3 and CsNO3 solutions, unexpectedly it does not adsorb at all from the LiNO3 and NaNO3 salt solutions. We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively charged mica surface of the etheric -O- group on the ethylene oxide monomer for the case of the more weakly hydrated K+ and Cs+, but not for the case of Na+ or Li+ with their more strongly bound water. A simple model relating the electrostatic energy changes occurring upon such liganding to the experimentally measured hydration energies of the different alkali metal ions supports this attribution.

1. Introduction Adsorption of flexible polymers from solution onto solid surfaces is used to control interfacial properties including steric stabilization or flocculation,1,2 surface tension and biocompatibilization, and wear and lubrication properties.3-5 Such adsorption occurs whenever the polymer segments adhere to the surface in preference to the solvent molecules:2,6 At the molecular level, the mechanisms promoting segmental attachment include attractive van der Waals (or dispersion) forces, counterion release entropy7 (in the case of polyelectrolytes adsorbing on oppositely charged surfaces), dipolar or hydrogen-bonding-type attraction,8,9 or specific chemical interactions with surface binding sites.10 Poly(ethylene oxide) (PEO), which is uniquely soluble in both organic and aqueous media,11 is especially widely used as a surface modifier. It is ubiquitous in biological and biomedical applications (when it is referred to as poly(ethylene glycol) or PEG), including osmotic pressure modification and drug encapsulation,12 and as a cell- and protein-repellent coating and in antifouling applications.13-16 On the experimental side, in addition to many other studies,11,17 the surface force balance †

Part of the Molecular and Surface Forces special issue. * To whom correspondence should be addressed. E-mail: jacob.klein@ weizmann.ac.il. ‡ Weizmann Institute of Science. § University of Oxford. (1) Napper, D. H. Polymeric stabilization of colloidal dispersions; Academic Press: London, 1983. (2) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (3) Klein, J. Annu. ReV. Mater. Sci. 1996, 26, 581. (4) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U.-I.; Nakamura, K.; Kawaguchi, H. Nat. Mater. 2004, 3, 829. (5) Klein, J.; Kumacheva, E.; Perahia, D.; Mahalu, D.; Warburg, S. Faraday Discuss. 1994, 98, 173. (6) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (7) Shafir, A.; Andelman, D.; Netz, R. R. J. Chem. Phys. 2003, 119, 2355. (8) Blin, J.-L.; Leonard, A.; Yuan, Z.-Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 2872. (9) Pattanayek, S. K.; Juvekar, V. A. Macromolecules 2002, 35, 9574. (10) Raviv, U.; Frey, J.; Sak, R.; Laurat, P.; Tadmor, R.; Klein, J. Langmuir 2002, 18, 7482. (11) Bailey, F. E.; Koleske, J. V. Poly(EthyleneOxide); Academic Press: New York, 1976. (12) Maggi, L.; Segale, L.; Torre, M. L.; Machiste, E. O.; Conte, U. Biomaterials 2002, 23, 1113.

(SFB) has been extensively used to investigate interactions between PEO layers adsorbed onto opposing mica sheets:18-21 norrmal and shear interactions between such layers have been investigated across both organic and aqueous solvents, and they have demonstrated both overall steric repulsion as well as longranged attractive interactions attributed to bridging,22,23 and modification of frictional forces by the adsorbed layers.24 In this paper, we examine in more detail the adsorption of PEO from aqueous solutions onto a charged solid, using as in previous experiments the model surface of mica: This is crystallographically smooth (cleaving along the {001} plane), and in aqueous environments it loses K+ ions from its surface to become negatively charged. In particular, we extend a recent brief communication25 where it was shown that, although PEO does not adsorb onto mica from the solution of the polymer in pure water (i.e., purified water with no added salt, so-called conductivity water), it does adsorb from a high concentration KNO3 solution. Direct evidence for adsorbance or nonadsorbance of the polymer was obtained from force vs surface separation D profiles: In Figure 1, we reproduce the normal and shear force profiles between mica surfaces (Fn(D) and Fs(D), respectively) across conductivity water both with and without added PEO, where the identity (within scatter) of the profiles revealed unambiguously the nonadsorbance of the polymer. The evidence for PEO adsorbance from a 0.1 M KNO3 solution was likewise obtained from the clear differences in the force (13) Perret, E.; Leung, A.; Morel, A.; Feracci, H.; Nassoy, P. Langmuir 2002, 18, 846. (14) Roosjen, A.; Kaper, H. J.; vanderMei, H. C.; Norde, W.; Busscher, H. Microbiology 2003, 149, 3239. (15) Ryan, P. L.; Foty, R. A.; Kohn, J.; Steinberg, M. S. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4323. (16) Huang, N. P.; Michel, R.; Voros, J.; Textor, M.; Hofer, R.; Rossi, A.; Elbert, D. L.; Hubbell, J. A.; Spencer, N. D. Langmuir 2001, 17, 489. (17) Cross, J., Ed. Nonionic surfactants; Marcel Dekker: New York, 1987. (18) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041. (19) Klein, J.; Luckham, P. F. J. Colloid Interface Sci. 1990, 141, 593. (20) Kuhl, T.; Guo, Y.; Alderfer, J. L.; Berman, A. D.; Leckband, D.; Israelachvili, J. N.; Hui, S. W. Langmuir 1996, 12, 3003. (21) Kuhl, T. L.; Berman, A. D.; Hui, S. W.; Israelachvilli, J. N. Macromolecules 1998, 31, 8258. (22) Klein, J. Macromolecules 1978, 11, 852. (23) Klein, J.; Luckham, P. F. Nature 1984, 308, 836. (24) Raviv, U.; Tadmor, R.; Klein, J. J. Phys. Chem. B 2001, 105, 8125. (25) Chai, L.; Klein, J. J. Am. Chem. Soc. 2005, 127, 1104.

10.1021/la702514j CCC: $40.75 © 2008 American Chemical Society Published on Web 12/07/2007

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NaNO3, and CsNO3. Remarkably, we find that while PEO adsorbs readily onto mica from both 0.1 M KNO3 and 0.1 M CsNO3 solutions, it does not adsorb at all from either 0.1 M LiNO3 or 0.1 M NaNO3 solutions. This unexpected difference is tentatively attributed to the different nature of the hydration shells about the ions, which favor PEO-mica attachment via ligand formation with the larger ions (K+ and Cs+) but not with the smaller ones (Na+ and Li+, as well as H+, the hydrated counterion in salt-free water). In section 2, we describe briefly the experimental procedure and then present the data, where both normal and shear force profiles point unambiguously either to adsorption or nonadsorption of the polymer. The final section discusses the origins of the different liganding efficiencies of ions in the alkali metal series. 2. Materials and Methods

Figure 1. (A) Normal force Fn(D)/R vs distance D profiles between mica surfaces across purified (conductivity) water, both in the absence (empty symbols) and presence of added PEO (following overnight incubation in the polymer solution), where R is the mean curvature of the mica surfaces. The inset zooms in on the jump-in region. The cartoon illustrates the SFB configuration. (B) Shear force Fs vs time traces measured directly taken from the SFB for the mica surfaces as they slide past each other. The top trace (a) is the back and forth lateral motion ∆x0 of the top surface, while traces (b) and (c) are the forces transmitted to the lower surface in the absence (c) and presence (b), respectively, of the added polymer in solution. There is no shear force (within the noise) in either case until the surfaces jump into contact, confirming the absence of any PEO adsorption (adapted from ref 25).

Figure 2. Schematic of the suggested25 attachment of PEO to mica via a hydrated K+ ion.

profiles in the absence and presence of the polymer in the salt solution. In that study,25 it was suggested that, at the high salt concentration, the hydrated metal ions (K+) act as ligands between the negatively charged mica surface and the etheric -O- on the PEO, which is also slightly negatively charged, as reproduced in Figure 2. Here, we develop this idea and examine also the surface attachment of PEO onto mica from aqueous solutions of LiNO3,

Poly(ethylene oxide) of weight-averaged molecular weight Mw ) 170 K was purchased from Polymer Laboratories Ltd. and used as received. According to the manufacturer’s data, the two end groups were t-butyl and hydroxyl, and the polydispersity was Mw/Mn ) 1.02. Aqueous salt solutions were prepared using cesium nitrate 99.999%, potassium nitrate 99.999%, sodium nitrate 99.995% (all purchased from Aldrich), and lithium nitrate 99.995% (purchased from Merck), which were used as received from fresh jars that were kept in a desiccator after opening. Water used was tap water subjected to the following steps: a reverse osmosis treatment (twice) and filtration through two polypropylene filters (of mesh size 25 µm and 2 µm) and a charcoal filter. The filtered water was then treated with either a three step Milli-Q system comprised of a reverse osmosis stage (RiOs), a UV treated reservoir, and a Milli-Q Gradient A10 purification system, or a Barnstead Nanopure system. Using this cleaning procedure, we obtained (conductivity) water with total organic compound 3-4 ppb (Milli-Q) or < ∼1 ppb (Barnstead) and specific resistivity of at least 18.2 MΩ cm. The pH of the water was 5.5 (as measured with a Merck pH paper) due to dissolved CO2 from the air. The surface force balance (SFB) used (shown schematically in Figure 1 inset), the cleaning protocols, and the experimental procedure have been described in detail previously.26 The SFB is capable of measuring normal and shear forces, Fn(D) and Fs(D), respectively, between curved mica surfaces (radius of curvature R ≈ 1 cm) as a function surface separations D in the range from 500 nm or more to contact, with a resolution in D of approximately (2-5 Å. It can in particular measure shear stresses over a large range of shear (or sliding) velocities and with uniquely high sensitivity and resolution,26 a feature which strongly complemented the standard normal force profiles in this study in determining whether polymer adsorbed onto the surfaces. All experiments were carried out according to the following procedure: after mounting the back-silvered mica sheets in the SFB, the air-contact positions of the interference fringes of equal chromatic order (FECO) were measured, and then pure water was added and the normal and shear interaction profiles were determined. In particular, the cleanliness of the experiments was ensured by observing the jump-in of the surfaces in the conductivity water to an adhesive contact some 0.8 ( 0.5 nm closer together than their air-contact position.27 This reveals the removal under water of a few angstroms of an air-adsorbed contaminant layer from each mica surface (in cases where such adhesive contact and removal of the contaminant layer was not observed, experiments were aborted). Following these controls, we carried out experiments with PEO either in salt-free water or in 0.1 M salt solutions. In the former case, the salt-free water was removed from the SFB cell and replaced by a 150 ((10%) µg/mL PEO solution in conductivity water, on which normal and shear force profiles were carried out. In the latter case, (26) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 6996. (27) Perkin, S.; Chai, L.; Kampf, N.; Raviv, U.; Briscoe, W. H.; Dunlop, I. E.; Titmuss, S.; Seo, M.; Kumacheva, E.; Klein, J. Langmuir 2006, 22, 6142.

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Figure 3. Normal force vs distance profiles Fn(D)/R (normalized in the Derjaguin approximation) between mica surfaces across polymer-free aqueous 0.1 M CsNO3 (empty symbols) and KNO3 (full symbols, reproduced from ref 25). Profiles on the left are prior to adding PEO, while those on the right were taken following overnight incubation in 150 µg/mL PEO added to the electrolyte solution. The dotted line is a summary of data from ref 18 for PEO (M ) 160K) in 0.1 M KNO3, and the solid lines are a guide to the eye. 0.1 M ((15%) salt solution was added to the cell, and normal and shear profiles were carried out in the salt solution as a control. This was then replaced by a PEO/0.1 M salt solution with a polymer concentration of 150 ((10%) µg/mL, on which normal and shear force profiles were carried out following overnight (15 ( 3 hours) incubation in the polymer solution. The PEO solutions were prepared by dissolving PEO in either water or the 0.1 M salt solutions at room temperature and stirring at 40 °C for 1-2 h and then allowing them to cool to room-temperature prior to introduction into the SFB cell. Liquids were introduced by opening the SFB cell in a laminar flow hood, separating the surfaces (to ∼1 cm), and slowly pouring in liquid solutions without using a syringe. This method of exchanging liquids avoids possible contamination and reduces the accuracy of the absolute separation measurements from (0.2-0.3 nm to (0.50.6 nm, due to shifts in the optical fringe positions on repositioning the apparatus. Results are shown from several different contact points within each experiment and up to 4 or more different experiments depending on the salt system. During an experiment, the room was kept at a constant temperature (within (0.5 °C) with the temperature varying between 24.5 and 27 °C in different experiments. Dynamic light scattering (DLS) was used to evaluate the hydrodynamic radii of the polymer in the different solutions. A concentration of 1.5 mg/mL PEO in pure water solutions and in 0.1 M salt solutions was prepared as above and filtered through a 0.2 µm HT Tuffryn filter (Pall Corporation, Ann Arbor, MI); the polymer concentration was a factor of 10 higher than that for the SFB experiments to enable a sufficient scattering signal. The samples, enclosed in 10 mm diameter borosilicate glass vials, were placed in a toluene-filled vat that was kept at 25 °C. Argon ion laser light (Lexel/Spectra Physics, λ ) 514.5 nm) was scattered from the samples and detected at 90° by an ITT FW130 photomultiplier. The photoelectron count- time autocorrelation function was calculated using a BI2030AT digital correlator (Brookhaven Instruments) and used to calculate the diffusion coefficient. Applying the StokesEinstein equation to the translational diffusion coefficients yielded an average value of the hydrodynamic radius.

3. Results Normal force profiles across 0.1 M KNO3 and across 0.1 M CsNO3, both in the polymer-free electrolyte solution and following addition of PEO, are shown in Figure 3. Prior to adding the polymer, the interaction for both salts is short-ranged, as expected from the Debye length at the high salt

Figure 4. (A) Normal force vs distance profiles Fn(D)/R between mica surfaces across polymer-free aqueous 0.1 M LiNO3 (empty symbols) and following overnight incubation in 150 µg/mL PEO added to the electrolyte solution (full symbols). The solid lines are from Figure 3 for interactions following incubation in PEO solutions in aqueous 0.1 M KNO3 and CsNO3. (B) As for (A) but in 0.1 M NaNO3 solutions (polymer-free (empty symbols) and following overnight incubation in 150 µg/mL PEO added to the electrolyte solution (full symbols)).

concentration, and is in line with earlier work.28-30 Following overnight incubation in the PEO solution in the 0.1 M aqueous electrolytes, there is a marked shift outward of the repulsion for both salts. The resulting Fn(D)/R profile for the PEO/KNO3 case (normalized in the Derjaguin approximation) is very similar to earlier profiles in this system (dotted curve), with the onset of interactions above the noise level setting on at D ≈ 80-85 nm. The corresponding profile for the PEO/CsNO3 case is similar, though, within the greater scatter in the data, the onset of repulsion appears somewhat further out at D ≈ 100-120 nm. Figure 4 shows the corresponding normal force profiles for the LiNO3 (Figure 4A) and NaNO3 (Figure 4B) cases, both with and without the added PEO. Similarly to the CsNO3 and KNO3 electrolytes, the pure LiNO3 and NaNO3 profiles both show the short-ranged repulsive interaction associated with the short Debye length at this salt concentration, and the hydration repulsion28 regime at D < 2-3 nm. Following overnight incubation in the PEO solution, however, there is no significant change (within the scatter) in the normal force profiles in either of the salts in Figure 4. (28) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (29) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press Limited: London, 1992.

SelectiVe Adsorption of PEO onto a Charged Surface

Figure 5. Traces of (b-e) shear forces Fs transmitted to the lower mica surface with time as the upper surface is made to move back and forth (∆x0), as in trace (a), across the different polymer-free salt solutions in the high-compression (or small surface separation) regime, showing the very low (within the noise) shear force signal in all cases.

Further insight into the adsorption behavior of PEO is obtained from the shear force profiles. Figure 5 shows the shear traces measured directly from the SFB as the upper surface slides past the lower one (see schematic in Figure 1) across the different 0.1 M aqueous electrolyte solutions prior to adding polymer. In the cases of all four different salt solutions, there is little shear response even at the D values shown, D < ∼2 - 3 nm, in line with extensive earlier reports for shear across highly confined NaCl solutions.27,30 In contrast, following overnight incubation in the PEO solution, as shown in Figure 6 for correspondingly high pressures between the surfaces, the behavior of the mica sheets in the LiNO3 and NaNO3 solutions is very different from that in the KNO3 and CsNO3 solutions. Figure 6A shows typical low- and high-pressure shear force traces for sliding in NaNO3 (similar also to LiNO3), where the friction remains very low (within the noise level) at all loads accessed in this study, while in KNO3 solution (similar also to CsNO3) the low shear forces at low loads increase to high values when sliding at higher loads. Figure 6B compares the frictional traces at correspondingly high loads for all four salt solutions, showing the much higher frictional dissipation in the potassium and cesium electrolyte solutions. In Figure 7, we summarize the shear force behavior as a function of surface separation for all four salt solutions following addition of PEO and overnight incubation, based on traces such those as in Figure 6. Finally, to examine the effect of the different salts on the PEO segmental interactions, we carried out DLS measurements. Table 1 shows the hydrodynamic diameter 2RH of the PEO molecules thus measured in the different 0.1 M aqueous salt solutions, as well as in conductivity water. (30) Raviv, U.; Klein, J. Science 2002, 297, 1540.

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Figure 6. Traces of (b-e) shear forces Fs transmitted to the lower mica surface with time as the upper surface is made to move back and forth (∆x0), as in trace (a). (A) Traces (b) and (c) show the response across a PEO/0.1 M NaNO3 solution (typical also of LiNO3) following overnight incubation at (b) low loads and at (c) high loads; both are within the noise level in the signal. Traces (d) and (e) show the corresponding response across a PEO/0.1 M KNO3 solution (typical also of CsNO3) following overnight incubation at (d) low loads and at (e) high loads. The high load case shows a clear frictional response. (B) Traces (b-e) show the shear response at high loads for all four alkali metal salts M+ following overnight incubation in PEO/MNO3 solutions, showing a clear frictional response only for incubation in solutions of the larger ions.

Figure 7. Summary of the shear forces as a function of surface separation, and at shear velocities 200 ( 20 nm/s in all cases, in the different PEO/salt solutions following overnight incubation. The shaded regions cover the scatter of the data in the various systems, while the inset zooms in on the very low shear forces following overnight incubation in PEO/LiNO3 and PEO/NaNO3 solutions.

4. Discussion and Conclusions The marked changes in interaction ranges of the normal force profiles in Figure 3, before and after adding PEO, show the clear adsorbance of PEO from 0.1 M KNO3 and CsNO3 solutions. At the same time, the absence of any such change in Figure 4 reveals unambiguously the nonadsorbance (within scatter) of the polymer from the LiNO3 and NaNO3 solutions. Conclusive evidence for the adsorbance or nonadsorbance of PEO is seen also from the shear profiles. Figure 5 shows that in the absence of polymer the shear forces at low D values, generally corresponding to high compressions, are within the noise level of the shear-response signal. This has been observed previously across 0.1 M NaCl solutions27,30 (though reported here for the first time for lithium

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Table 1. Hydrodynamic Diameter of PEO (170 K) at Concentration 1.5 mg/mL in Water and in 0.1 M Aqueous Salt Solutions, Obtained from Dynamic Light Scattering Measurements polymer/salt solution

PEO hydrodynamic diameter 2RH (nm)

PEO/water PEO/LiNO3 PEO/NaNO3 PEO/KNO3 PEO/CsNO3

25.8 ( 0.6 24.3 ( 4 25.8 ( 2 25.2 ( 0.6 26.4 ( 2.5

and potassium ions and for the NO3- anion) and has been extensively discussed.31 It has been attributed to the ball-bearinglike behavior of the hydrated ions, arising from a combination of the tenacious nature of the hydration sheath together with the rapid exchange rate (or relaxation time) of the hydration layer. Once the mica surfaces have been incubated following addition of PEO to the salt solutions, there is again a very marked difference between the Li+ and Na+ solutions and the K+ and Cs+ ones. Figure 6A compares low- and high-compression shear traces between the mica surfaces in the sodium and potassium salts (typifying the “small” and “large” alkali metal ions) following overnight incubation with PEO. For the NaNO3 solution, the two traces are indistinguishable within the scatter, revealing that in both cases the hydrated metal ions are mediating the sliding, that is, that there is no polymer on the surfaces: we note that even trace amounts of adsorbed polymer are expected to result in significant frictional drag due to bridging effects of the polymer.24 For the KNO3 solution, in contrast, the friction is characteristic of that between adsorbed polymer layers: low at low loads but high under strong compression, as discussed in detail elsewhere.24,32 Figure 6B shows high-compression shear traces for all four systems, highlighting the qualitative differences between the Li+ and Na+ solutions and the K+ and Cs+ ones, where the former show no signs of PEO adsorbance, in contrast to the latter. Figure 7 summarizes the shear data and shows the large qualitative differences between the Li+ and Na+ solutions on the one hand and the K+ and Cs+ solutions on the other. The larger range and magnitude of the shear forces with the adsorbed PEO in the CsNO3 solution deserve comment, and they are briefly discussed at the end of this section. Thus, the clear conclusion from these normal and shear interaction profiles is that PEO does not adsorb at all, within the scatter, onto the negatively charged mica surfaces from 0.1 M LiNO3 and NaNO3 aqueous solutions, but it does adsorb strongly from aqueous 0.1 M KNO3 and CsNO3 solutions. What is the origin of this behavior, given the chemical similarity of the alkali metal series? To answer this, we note the following. Because of the importance of this polymer noted in the Introduction, and also because of the similarity of the process to the binding of cations to crown ethers,11 a considerable body of work has been carried out on the interaction, either direct or indirect, of alkali metal (AM) ions with poly(ethylene oxide).33-38 One interesting effect exhibited by PEO in organic and aqueous solutions is its retention of the etheric segments as chelating agents for alkali ions, with sodium and potassium in particular.17 This was used by Kuhl et al.21 to explain the long-ranged electrostatic repulsion between phospholipid bilayers confining PEO chains. It is clear not only that the AM series cations can bind well to PEO,11,35,39 but also that within the series different ions appear to bind differently to (31) Leng, Y.; Cummings, P. T. Phys. ReV. Lett. 2005, 94, 026101. (32) Chai, L.; Klein, J. Macromolecules, in press.

PEO, as revealed by several approaches.40,41 These include their effect on the PEO/water phase separation behavior37 and on the conductivity of PEO/aqueous salt solutions,42 the effect of different AM ions on PEO/surfactant interactions,38 and the complexation of the AM ions with the PEO both in organic and in aqueous solvents.11,21,33,34 To our knowledge, however, the present study is the first to demonstrate the qualitatively different effects of AM ions in binding PEO onto a charged surface. In the mechanism proposed earlier for liganding of PEO to mica,25 the oxyethylene segments attached not via the usual mechanism of van der Waals attraction or by H-bonding of the polymer backbone -O- to surface -OH groups,8,9 but rather via the formation of a ligand with the hydrated or partly hydrated K+ ions. These ions are themselves localized near the charge sites on the mica surface, as illustrated in Figure 2. The origin of the selective adsorbance of PEO in the presence of the different AM ions should therefore be sought in their different efficiency as ligands either to the mica or between the PEO segments themselves. One possibility of the latter effect is that, in the presence of Li+ or Na+ ions, the polymer chains form larger aggregates, thereby reducing the concentration of available free polymer in solution to a level which would suppress the adsorption, which might account for the nonadsorbance observed with these ions. On the face of it, this is unlikely, as it has been shown by studies37 of the cloud point depression that the effect on the association of PEO is least for lithium and sodium salts (compared with corresponding potassium salts). However, we examined this more directly via dynamic light scattering, and, as seen in Table 1, there is little indication of large-scale aggregation of PEO in any of the salt solutions. The similarity of the hydrodynamic radii in all cases (including salt-free water) is also in line with the observation37 that the effect of the different salts on intramolecule bonding and on the segment/solvent interaction parameters is small at this salt concentration and temperature. The implication, therefore, is that one of the water molecules in the hydration sheath, whose dipolar orientation is such that the (slightly negative) oxygen faces the K+ ion, may be displaced by the etheric -O- on the PEO to enable a sufficiently close approach of the latter to the positive ion charge for efficient liganding to occur. This tentatively suggests that the different efficiency of the AM ions may be related to the ease of having the etheric -O- replace a hydration water molecule on the ion. To develop this further, we examine the energies associated with hydration of the different AM ions as well as with the PEO monomer, as shown in Table 2. The overall hydration free energy per ion needs to be divided by the total number of water molecules in the first hydration shell, known as the coordination number, to yield the hydration + free energy gain ∆Ghyd 1 |M associated with the presence of each (33) Awano, H.; Ono, K. Chem. Lett. 1982, 11, 149. (34) Awano, H.; Ono, K. Bull. Chem. Soc. Jpn. 1983, 56, 1715. (35) Sartori, R.; Sepulveda, L.; Quina, F.; Lissi, E.; Abuin, E. Macromolecules 1990, 23, 3878. (36) Bailey, F. E.; Callard, R. W. J. Appl. Polym. Sci 1959, 1, 56. (37) Florin, E.; Kjellander, R.; Eriksson, J. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2889. (38) Cooke, D. J.; Blondel, J. A. K.; Lu, J. R.; Thomas, R. K.; Wang, Y.; Han, B.; Yan, H.; Penfold, J. Langmuir 1998, 14, 1990. (39) Armand, M. B. Annu. ReV. Mater. Sci. 1986, 16, 245. (40) Belder, D.; Warnke, J. Langmuir 2001, 17, 4962. (41) Quina, F.; Sepulveda, L.; Sartori, R.; Abuin, E. B.; Pino, C. G.; Lissi, E. A. Macromolecules 1986, 19, 990. (42) Bordi, F.; Cametti, C.; Biasio, A. D. J. Phys. Chem. 1988, 92, 4772. (43) Nightingale, E. R. J. Phys. Chem. 1959, 63, 1381. (44) Table 1.11 in ref 48 unless otherwise indicated. (45) Table 1.8 in ref 48 unless otherwise indicated. (46) Zhan, C.-G.; Dixon, D. A. J. Phys. Chem. A 2001, 105, 11534.

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Table 2. Size and Hydration Characteristics48 and Hydration Energies49 of Hydrated Proton and Alkali Metal Ions in This Studya

ion H+ Li+ Na+ K+ Cs+ PEO -O-

ionic radius43 i rM +, nm

hydrated radius43 h rM +, nm

coordination number44

hydration energy45 (expt) (kJ/mol)

∆Ghyd 1 , hydration energy/ coordination water (kJ/mol)

EM+/H2O, from eq 2 (kJ/mol)

(0.028) .06 .095 .133 .169

.28 .38 .36 .33 .33

446 4-6 4-7 5-10 6-12

-109846 -510 -410 -337 -283

-274 -(85-128) -(59-103) -(33.7-67) -(24-47)

(-216) -101 -64 -45 -36

(147)

-3447

-34

a The coordination number refers to the mean number of water molecules in the first hydration shell. The final column is from eq 2, using a single value50 M+ ) 16 and the other parameters as in the text. Values for the ethylene oxide monomer are given in the last row.

bond lengths used in the model. Figure 8, bottom, shows also the final configuration when the etheric -O- on the EO monomer has displaced the hydration water molecule about M+ to form a ligand. An approximate model treats the energetics of the initial and final states as follows: The electrostatic interaction energy of the hydrogen/(etheric -O-) bond in the initial state is given by EEO,

EEO ≈ (δeEO)(δeH)/4πEO0aEO

(1)

where δeEO is the (negative) charge on the etheric oxygen on the EO monomer, δeH is the (positive) charge on the water hydrogen bonding to the etheric -O-, EO is an effective dielectric constant, and aEO is the separation between the charges (length of the H-bond, top left, Figure 8). Similarly, the energy EM+/H20 associated with one water molecule/M+ bond is Figure 8. Schematic illustration of the model described in the text, where the hydrated ethylene oxide (EO) monomer (top left, as suggested in Figure 6 of ref 47, with energy given by EEO, eq 1 in text) and the hydrated M+ ion localized at the negative charge on the mica surface (top right with energy given by EM+/H2O, eq 2 in text) combine (lower half) to give the EO attached to the M+ as shown (with energy given by EM+/H2O, eq 3 in text). The charges and bond dimensions used in the model are indicated.

water molecule in the shell about the ion M+. This is tabulated in the penultimate column of Table 2. Also given (last row of Table 2) are values for the coordination number and hydration energy of the ethylene oxide (EO) monomer and the corresponding hyd value ∆Ghyd 1 |EO. It is evident that the magnitudes of ∆G1 are + + much higher for the hydrated Li and Na ions, so that one would expect that perturbing the first hydration shell about these ions would be much more difficult (costly in terms of free energy) than for the cases of the hydrated K+ and Cs+ ions. Qualitatively, this would act to suppress a close approach of the etheric -Ogroup on an EO monomer to the hydrated Li+ and Na+ ions, relative to its close approach to the hydrated K+ and Cs+ ions. This may therefore suppress the liganding and thus the PEO adsorption in the former case, as is observed. We examine this idea more quantitatively with the help of Figure 8. This shows schematically the initial configurations of the EO monomer (with its hydrogen-bonded etheric -Ogroup47) and the hydrated M+ alkali metal ion localized near the negative charge on the mica surface, indicating the charges and (47) Lusse, S.; Arnold, K. Macromolecules 1996, 29, 4251. (48) Saluja, P. P. S. In International ReView of Science, Electrochemistry, part 1; Bockris, J. O. M., Ed.; Physical Chemistry Series 2; Butterworths: London, 1976; Vol. 6, pp 1-51. (49) Aqvist, J. J. Phys. Chem. 1990, 94, 8021.

EM+/H20 ≈ e(δeO)/4πM+0aM+/O

(2)

where e (electronic charge) is the (positive) charge on the M+ ion, δeO is the (negative) charge on the water oxygen bonding to M+, M+ is an effective dielectric constant, and aM+/O is the separation between the charges (typically comparable to the ionic i of M+, as the water oxygen is presumed adjacent to radius rM+ the ion as illustrated in Figure 8). Once the displacement of the water molecule by the EO monomer has occurred (Figure 8, bottom), the resulting electrostatic interaction energy is given by EM+/EO,

EM+/EO ≈ e(δeEO)/4πM+0aM+/EO

(3)

where aM+/EO is the separation between the charge e on M+ and the charge δeEO on the etheric -O-. These expressions include only electrostatic effects, but they capture, we believe, the essential features of the free energy gain through the hydration of the various charges as discussed below. The net energy change ∆G on going from the initial (top) to the final (bottom) configuration in Figure 8, that is, on going to the liganded configuration of the EO oxygen to M+, is given by

∆G ≈ EM+/EO - EM+/H20 - EEO

(4)

where we have ignored the small entropic contribution to the free energy change due to the release of the two water molecules (50) The value of the effective dielectric constant at the mica-water interface is likely to be intermediate between that of mica ( ) 5-7) and that of water ( ) 80). Indeed, the value of  we ( ) 16) use is close to that estimated ( ) 13) from a very different study of ion-release kinetics; see Raviv et al. J. Chem. Phys. 2002, 116, 5167.

1576 Langmuir, Vol. 24, No. 4, 2008

Chai et al.

(this is of the order of kBT/released water molecule ≈2.5 kJ/mol, which is small compared to the hydration energies ∆Ghyd 1 /water molecule). We may now assign values to the parameters in the model: (a) We take the charge separation aM+/O to equal the ionic radius i + rM + (as in column 1 of Table 2); (b) the charge separation aM /EO (once the water molecule is displaced by EO, Figure 8, bottom) is likely to be larger than the ionic radius, because the etheric -CH2-O-CH2- is considerably bulkier than a water molecule; however, its value must lie between the ionic radius and the hydrated radius (columns 1 and 2 of Table 2), and we approximate i h it as the mean of the two, aM+/EO ≈ (rM + + rM+)/2; (c) we take 51-53 the effective dielectric constants in eqs 1-3 to be equal, that is, EO ) M+; and (d) we set the (positive) charge |δeH| on the H of the water molecule to equal half the (negative) charge |δeO| on the water oxygen molecule, that is 2|δeH(H2O)| ) |δeO(H2O)| (since the water is overall neutral). Last, the relation of the electrostatic energies calculated in eqs 1-3 to the hydration energies ∆Ghyd 1 in Table 2 is of particular interest. In the final column of Table 2, we evaluate the energies EM+/H20 (eq 2) for the different ions, using the known values of the parameters and an effective dielectric constant51 M+ ) 16. We see that in all cases the calculated values are well in the range of the experimentally derived hydration energies ∆Ghyd 1 . Despite the very simple nature of our model, where we neglect all but the primary electrostatic interactions between the adjacent charges, we believe that this agreement reflects the broad validity of our approach.54 Indeed, it is consistent with more detailed calculations55 which show that the electrostatic interactions dominate the hydration energies (relative to the entropic ones). This suggests that within the scope of our model we may take the electrostatic interactions in eqs 1 and 2 to be roughly equal to the hydration energies per coordination water molecule in the penultimate + column of Table 2, that is, we take EEO ≈ ∆Ghyd 1 |EO, and EM /H20 +. Making these substitutions in eqs 1, 3, and 4 and ≈ ∆Ghyd | M 1 rearranging gives finally i h ∆G ≈ ∆Ghyd 1 |EO[(2e(aEO)/((rM+ + rM+)δeH) - 1] -

∆Ghyd 1 |M+ (5) The term in the square brackets includes the charge δeH on a water hydrogen atom, which is known,56,57 δeH(H2O) ≈ 0.35e, while the hydrogen-bonding separation aEO at the etheric -Oon the EO monomer may be taken at its typical value of 2 Å. Substituting in eq 5 yields i h hyd ∆G ≈ ∆Ghyd 1 |EO[(1.14/(rM+ + rM+)) - 1] - ∆G1 |M+

(6) i h where (rM + + rM+) (first and second columns in Table 2) is in nanometers. Equation 6 is the relation we need to estimate the free energy change associated with ligand formation as indicated in Figure 8. Substituting into eq 6 the values of the radii (first and second columns in Table 2) and the hydration energies per coordination

(51) Dielectric constants at interfaces may differ from their bulk values; see footnote 50 and refs 52 and 53. (52) Stern, H. A.; Feller, S. E. J. Chem. Phys. 2003, 118, 3401.

Table 3. Energy Change ∆G (eq 6) for Displacement of a Water Molecule by the Etheric -O- as in Figure 8 ion M+

∆G (kJ/mol) (from eq 6)

Li+ Na+ K+ Cs+

+31 to +74 +9 to +53 -15 to +18 -20 to +3

hyd + water molecule ∆Ghyd 1 |M and ∆G1 |EO (penultimate column in Table 2) gives the values summarized in Table 3. Despite the scatter arising from the uncertainty in the values of the coordination number in Table 2, we see at once that for M+ ) Li+ or Na+, ∆G . 0, that is, formation of the ligand as illustrated in Figure 8 would be energetically unfavorable and thus suppressed. In contrast, for M+ ) K+ or Cs+, ∆G < 0 over much of the range of estimated values, so that ligand formation could well be favored. This is indeed in line with our observations of nonadsorbance in the Li and Na salt solutions and of strong adsorbance from the K and Cs salt solutions. We remark, finally, that the value of ∆G for the Cs+ ion (Table 3), which is the lowest of the series, may also explain the rather large shear force response when PEO-bearing surfaces rub against each other in CsNO3 (Figure 6) as well as the thicker adsorbed layer of PEO in 0.1 M CsNO3 (Figure 3): we tentatively attribute this to the possible effect of the Cs+ ions acting as stronger ligands not only between the PEO and the negatively charged mica surface, but also, weakly, between the EO monomers themselves as they slide past each other in the shear measurements (though too weakly to perturb their mean solution dimension, see Table 1). To summarize, we have shown that PEO readily adsorbs onto a negatively charged (mica) surface from 0.1 M CsNO3 or KNO3 but not from 0.1 M LiNO3 or NaNO3. We attribute this to the formation of ion ligands between the etheric -O- on the ethylene oxide monomer and the hydrated alkali metal ions localized at the negative charges on the mica surface. A simple quantitative model suggests that this is indeed in line with the relevant ionic sizes, level of hydration, and hydration energies, and arises because the stronger binding of the water of hydration to the smaller ions makes such ligand formation unfavorable. Our findings may have interesting implications for the understanding and control of ubiquitous PEO interactions with surfaces and with biological molecules.

Acknowledgment. We thank Bob Thomas and Sam Safran for useful discussions, Ellen Wachtel for her help with the light scattering, and the Israel Science Foundation, the Petroleum Research Fund (Grant 45964-AC7), and the Minerva Foundation for financial support. This research was made possible in part by the historic generosity of the Harold Perlman Family. LA702514J (53) Teschke, O.; Ceotto, G.; de Souza, E. F. Phys. Chem. Chem. Phys. 2001, 3, 3761. (54) A fully detailed model for the interactions is beyond the scope of this experimental paper. Using dipole-dipole or dipole-charge expressions rather than those in eqs 1-3 is problematic, since the dipole dimensions are comparable to or larger than the separation axy and do not in any case qualitatively change the conclusions using only the leading charge-charge interaction terms as we have done. (55) Hribar, B.; Southall, N. T.; Vlachy, V.; Dill, K. A. J. Am. Chem. Soc. 2002, 124, 12302. (56) Martin, F.; Zispe, H. J. Comput. Chem. 2004, 26, 97. (57) Gregory, J. K.; Clary, D. C.; Liu, K.; Brown, M. G.; Saykally, R. J. Science 1997, 275, 814.