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Steric and Bridging Forces between Surfaces Bearing Adsorbed Polymer: An Atomic Force Microscopy Study Simon Biggs Advanced Mineral Products Research Centre, School of Chemistry, The University of Melbourne, Parkville, Victoria 3052,Australia Received January 31, 1994. I n Final Form: June 17, 1994@ Direct measurement of the forces of interaction between a 10 pm radius sphere of zirconia with a flat polished zirconia ceramic plate in aqueous electrolyte solution has been performed. The interaction profiles of these surfaces after the addition of a high molecular weight (Mw = 750 000) polyacrylic acid were also recorded. The surface coverage of the polymer adsorbed at the solidaqueous interface was controlled by varying the added polymer concentration and by varying the equilibration time of the polymer with the two surfaces at large separations. At a long equilibration times (24 h) and higher polymer concentration (30pglL)a repulsive interaction force was observed between the surfacesupon their approach,this repulsive force beginning at twice the radius of gyration and increasing monotonically as the separation is decreased. At a lower polymer concentration (10 p g / L ) and short equilibration time (< 1h) the polymer was seen to bridge between the sphere and the flat plate. At the scan rates employed here, for the approach and separation ofthe surfaces,the bridgingpolymer remained adsorbed to both surfacestethering them together.
Introduction The control and prediction of the forces responsible for the stability or instability of dispersions of colloidal particles when interacting with each other are of great importances1 As a result, much effort has been put into devising experiments for measuring these forces. Perhaps the best known device so far utilized in these force studies is the so-called surface forces apparatus (SFAh2Over the last 15 years direct measurements using the SFA have led to a confirmation of the main features ofthe Derjaguin, Landau, Venvey, and Overbeek (DLV0)3,4theory of colloid stability. In addnition, various other forces have been observed and quantified including solvation5 and hydration force^.^^^ When a polymer is adsorbed or grafted onto the surface of a particle, it can lead to a range of possible interaction f0rces.l These may include a steric barrier between the particles, as well as bridging and depletion flocculation processes. Once again, many studies of the forces of interaction between mica surfaces in the presence of adsorbed polymer layers have been performed with the SFA.’-12 The different effects of using either polymer adsorbed in loops and or as a terminally anchored Abstract published in Advance ACS Abstracts, November 15, 1994. (1)Israelachvili, J. N.Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (2)(a) Tabor, D.;Winterton, R. H. S. Proc. R. SOC.London, Part A 1969.312. 435. (b) Israelachvilli, J. N.: Adams, G. E J. Chem. Soc., Faraday Trans. 1 1978,74,975. (3)Derjaguin, B.; Landau, L. Acta Physiochem. 1941,14,633. (4)Venvey, E.G. W.; Overbeek, J. Th., G. Theory ofthe Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (5)Pashley, R.M. J. Colloid Interface Sci. 1981,83,531. (6)(a) Pashley, R. M. J. Colloid Interface Sci. 1981,80, 153. (b) Israelachvili, J. N.; Pashley, R. M. In Biophysics of Water;Franks, F., Ed.; Wiley: New York, 1982;pp 183-194. (7) (a) Klein, J.; Luckham, P. Macromolecules 1984,17,1041. (b) Klein, J.; Luckham P. Macromolecules 1985,18, 721. (c) Klein, J.; Luckham, P. J. Colloid Interface Sci. 1987,117,149. (8)Klein, J. Adu. Colloid Interface Sci. 1982,16, 101. (9)Israelachvili,. J.N.:. Tirrel,. M.:. Klein,. J.:. AlmoaY -. Macromolecules 1990,17, 204. (10)(a) Taunton, H.J.; Toprakciaglu, C.; Fetters, L. J.; Klein, J. Macromolecules 1990.23. 571. (b) Ansifar. M. A.: Luckham. P. F. Polymer 1988,29,329. ’ (11)Costello, B. A. de L.; Luckham, P. F.; Tadros, Th. F. Langmuir 1992,8,464. (12)Costello, B. A.de L.; Kim, I. T.; Luckham, P. F.; Tadros, Th. F. Colloids Surf., A 1993,77,55. @
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chainlo as well as graft copolymersl1J2have been shown to agree quantitatively with theory.13 Recently, the use of atomic force microscope (AFM) for the measurement of interparticle forces has been described by a number of a ~ t h o r s . l ~ - Ducker ~O et al.19were the first to demonstrate the real utility of the AFM to surface force studies when they reported results for the interaction of a 3.5-pm silica sphere with a polished silica plate in aqueous electrolyte solutions. Since ths initial report the use of the AFM for studies of this type has been reported by many workers on a wide range of substrates. These studies have included a range of mineral oxides, for example, rutile20and zirconia,21as well as metal^^^,^^ and polymeric latex materials.23 In a particularly interesting development the forces of interaction between two polymer latex spheres have also been measureda2* The use ofthe AFM for analyzing the effects of a variety of additives on colloidal dispersions has also recently begun to expand rapidly. Rutland and SendenZ5have investigated the effects of adsorbing nonionic surfactants of the type C12E5 on colloidal silica; their data showed clearly the presence of a short range steric barrier. O’Shea et a1.26 reported a study of physisorbed copolymers of polystyrene-polyethylene oxide on mica in a variety of solvents with differing solvation properties for the copolymers, these authors did not however use a colloidal probe but used the commerically available silicon nitride tips. (13)De Gennes, P. G. Adv. Colloid Interface Sci. 1987,27,189. (14)Binning, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986,56, 930. (15)Dung, U.;Gimzewski, J. K.; Pohl, D. W. Phys. Rev. Lett. 1986, 57. 2403. (16)Goodman, F.0.; Garcia, N. Phys. Rev. B 1991,43,4728. (17)Hartmann, U.Phys. Rev. B 1991,43,2404. (18)Butt, H.4. Biophys. J. 1991,60,1438. (19)(a) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353,239. (b) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992,8,1831. (20)Larson, I.; Drummond, C. J.;Chan, D. Y. C.; Grieser, F. J.Am. Chem. Soc. 1993,115,11885. (21)Prica, M.;Biggs, S.; Grieser, F. In preparation. (22)Biggs, S.; Mulvaney, P. J. Chem. Phys. 1994,100,8501. (23)Karaman, M.E.:Meagher, 1993, - L.: Pashley, R. M. Langmuir 9,1220. (24)Li, Y. Q.;Tao, N. J.; Pan, J.; Garcia, A. A.; Lindsay, S. M. Langmuir 1993,9,637. (25)Rutland, M. W.; Senden, T. J. Langmuir 1993,9, 412. (26)O’Shea, S.J.;Welland, M. E.; Rayment, T. Langmuir 1993,9, 1826.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 1, 1995 157
Polymer Interaction between Colloidal-Sized Objects In a number of recent publications from this laboratory, rheological yield stress data for concentrated suspensions of colloidal zirconia in the presence of a variety of additives have been interpreted with respect to the short range dispersion forces operating in these systems.27 In one particular case the addition of high molecular weight polyacrylicacid (M, = 750 000)was seen to give anomalous behavior. This anomaly was suggested to be due to briding flocculation.28 The present work describes the use of the AFM for the study of polymer interactions between colloidal-sized objects. In particular the interpretation of polymer bridging interactions with the AFM is presented and discussed. To facilitate comparison with the published rheological yield stress data from this laboratory,28 a sample of polyacrylic acid (Mw= 750 000) adsorbed onto zirconia surfaces was investigated.
Experimemtal Section Materials. Reagents. All water was distilled and subsequently purified to Millipore Milli-Q quality. Potassium nitrate, potassium hydroxide, and nitric acid were of analytical grade and were not purified further before use. The polymer used was a polyacrylic acid with a molecular weight, M,, of 750 000 and was obtained from Aldrich Chemicals. The sample was used as supplied and was not purified further. The fully extended chain length for this sample of PAA based upon it’s M, value will be approximately 3.2 pm. A stock aqueous solution of the polymer was prepared with a concentration of 0.044 g L-l. After complete dissolution, under gentle agitation, measurements using this solution were generally performed within 48 h. The average pKa of a high molecular weight PAA is 6.3. Measurements were performed at a pH of about 5 (see below), at this pH the polymer will be only weakly dissociated. The radius of gyration RG for a sample of weakly dissociated polyacrylic acid (M,= 770 000) has been previously determined from light scattering to be 56 nm.29 It was assumed for the purposes of this investigation that this value approximates closely to that of the sample employed here. Colloidal Zirconia. Colloidal zirconiawas obtained from IC1 Z-Tech. The sample supplied was calcined at 850 “C before being spray dried. The colloidal particles were typically around 300 nm diameter. For use in AFM experiments a diameter of at least 2-3 pm is usually preferred for ease of manipulation (seelater). As a result of spray drying a wide size range of spherical, spray-dried agglomerates were also present in the supplied samples. Agglomerates of greater than 5pm were used in the AFM experiments. Polished Zirconia Plate. AFM measurements were made against a plate of pure zirconia ceramic polished to optical smoothness. The mean roughness of this plate, determined from an AFM image, recorded using a commerical silicon nitride cantilever, was found to be f 2 nm across an area of 1pm x 1 pm. This plate was cleaned by boiling for 10 min in ammoniacal H202 before rinsing in H20,followed by soaking for 24h in 0.1 M KNO3. Af’ter soaking, the plate was steamed for 3 h before being mounted immediately onto the AFM and sealed under the appropriate aqueous solution for study. (27) Leong, Y. K; Boger, D. V.; Scales, P. J.; Healy, T. W.; Buscall, R. J. C h m . Soc., Chen. Commun. 1993,640. (28) Leong, Y. K; Healy, T. W.; Boger, D. V.; Scales, P. J. Presented at the Proceedings of the 94th Annual Ceramic Society Meeting, 1992. (29) Orofino, T. A.; Flory, P. J. J. Phys. Chem. 1959, 63, 283.
Figure 1. Colloid probe. A spherical agglomerate of zirconia, calcined at 850 “C and attached to t h e commerical silicon nitride cantilever with a small a m o u n t of glue. T h e surface debris consists of s a l t crystals from t h e reaction solution after solvent evaporation.
Force Measurements with the AFM. The use of a n AFM for surface force measurements has been described previously by a number of other authors and will only be summarized here. Force-distance information was obtained from a Nanoscope I11 AFM (Digital Instruments) which was operated in the “forcemode”. In this operating mode theX-Y raster motion of the sample on the scanning piezoelectric crystal is suspended and the sample is moved toward and away from the cantilever in the Z direction by the application of a saw tooth voltage. The colloid probe was prepared by attaching a spraydried agglomerate of zirconia to one of the commerical cantilevers (Digital Instruments) with a small amount of resin (Shell Epikote 1002) according to the technique of Ducker et al.19 An example of such a probe is shown in Figure 1. The hollow structure of the spray-dried agglomerate is clearly visible. However, in the interfacial contact region the agglomerate approximates very well to a sphere. In a typical experiment, one of the colloidal probes (the radii are given in the figure legends) was mounted into the commerically available liquid cells (Digital Instruments) and was washed with ethanol before drying with a steady stream of nitrogen. After the zirconia plate was mounted atop the AFM-D piezo scanner, the liquid cell was mounted into the head unit and was attached to the scanner. The probe was then brought into close proximity with the surface. At this point, the solution for study was pumped into the liquid cell from a sealed bulk reservoir using a peristaltic pump. The detection of the cantilever bearing the colloid probe is monitored by the changes of a voltage from a split photodiode onto which is focused a laser beam that is reflected from the rear of the cantilever. The supplied software of the AFM generates a file that contains the ouput of the photodiode and the displacement of the sample on the piezocrystal. This raw data was then converted into real force versus separation data following the principle of Ducker e t al.,19which requires the definition of a position of zero force and a position of zero separation. Zero force is simply defined as operating at large separations where a change in the crystal position has no effect on the cantilever deflection. Zero separation is defined as being when the change in crystal position causes an equal change in the cantilever displacement, the so-called
158 Langmuir, Vol. 11, No. 1, 1995
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to the technique described by Larson et a l ? O and was found ' I to have a value of 6.0 x J. At the pH shown, the
0.01
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""""""' 50 60 70 80 Separation dlstance, D (nm)
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Figure 2. Force-distance profile for a zirconia probe (radius = 12pm)and a polished zirconia ceramicplate in M KNOs. The open circles are the experimental data points; the small filled symbols are fits to the nonlinear Poisson-Boltzmann equation according to the algorithm of Chan et al.31The upper and lower lines are for interaction at constant charge and constant potential, respectively. "he best fit parameters are Y = -40 mV and k-l = 30.4 nm. region of constant compliance. From a knowledge of these two positions and the slope of the region of linearity at constant compliance, we are able, using Hooke's law, to calculate the force as a function of separation. I n this study, all of the reported data were collected with a 100 pm long thin-legged triangular cantilever having a manufacturers spring constant of 0.36 N/m. Experimental determination of the spring constant using the technique of Senden and DuckeS0gave a value of 0.24 f0.01 N/m, this value being used in all of the data analysis reported here. Relevance of Force Data to Real Systems. The measurements reported here were recorded at scan rates of between 1.9 and 10 Hz and in a background electrolyte solution of M KN03. All of the reported data were obtained a t a scan rate of 4.88 Hz. At this scan rate, allowing for a typical scan size of 1000 nm (or total travel of 2000 nm inward and outward), the AFM drives the surfaces together a t a rate of about 10 pm s-l. Thus, a 30-nm double layer will be traversed in 3 x s. The diffusion rate of 300 nm zirconia colloids undergoing Brownian collisions may be taken as 3 x cm2 s-l. Double layer interactions should, therefore, occur over a time scale o f t ( ~ / D K ~ ) s. Thus, the scan rates used here approximate well to actual collision velocities.
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Results Zirconia-Zirconia Force Interactions in the Absence of Polyacrylic Acid. All measurements of polymer adsorption were performed between zirconia surfaces. Prior to each measurement the cleanliness of the system was verified by recording the force distance information for pure zirconia-zirconia interactions in aqueous electrolyte at three or four different pH values. A typical curve of the log of the normalized force as a function of distance is presented in Figure 2. The best fits to the experimental data were calculated under either constant surface charge or constant surface potential during interaction a t the limits of the Poisson-Boltzmann equation, when applied to the DLVO theory, using the technique of Chan et al.sl The van der Waals constant used here was calculated32from dielectric data according (30) Senden, T. J.; Ducker, W. A. Langmuir 1994,10, 1003. (31)Chan, D. Y.C.; Pashley,R. M.; White, L. R. J.Colloid Interface Sci. 1980, 77, 283. (32)Drummond, C. J. Personal communication.
force is repulsive over the entire range studied. Even a t small separations ( < 10 nm), where the van der Waals attractive contribution to the overall force profile might be expected to dominate, a n extra short range repulsive barrier is seen. At pH values close to the isoelectric point (iep) for zirconia (pHi,, = 7-7.5) a small jump into contact was observed. The jump-in distance was typically from 6 to 12 nm. This variability may reflect some surface roughness of the colloidal probe. It should be noted however that the presence of a jump-in at these pH values suggests that the absence of a jump-in a t higher pH as shown in Figure 2 is a real effect and is not related to roughness of the probe or surface. The experimentally determined potentials were compared to known electrophoretic potentials for an equivalent system. In all cases, the potential at any given pH was seen to be constant between many experiments. A full analysis of the AFM potential data as a function of pH when compared to the electrophoretic mobility data for zirconia will be presented elsewhere21 and will not be considered in detail here. The force profiles seen were very similar to those reported previously for other mineral oxide system^.'^,^^ As is typical for AFM studies operating in the force mode, only the data for the approach of the surfaces were used in the calculation of interaction potentials. The adherence data data on separation of the surfaces showed the large fluctuations in absolute values observed p r e v i o ~ s l which y ~ ~ were ~ ~ ~attributed ~ ~ ~ to rolling and bending of the cantilever. It is worth noting however that the form of the adherence curves for these "clean" surfaces was reproducible, the cantilever always returning rapidly from a zero separation position to a large separation ('10 nm) upon overcoming the force of adhesion. PolyacrylicAcid. a. General Comments. It wasnoted above that surface roughness ofthe probe led to difficulties in obtaining accurate jump-in distances close to the iep for this system. It is to be expected that for a large agglomerate of smaller spheres, as used here, there will be some roughness caused by the slight protrusion of one or more of the spheres relative to it's neighbors. As also noted above, this was insufficient to cause the total loss of a jump-in under the appropriate solution conditions. However, when the probe and surface are in contact, the exact contact radius is therefore unknown. This may lead to problems in normalizing the data with the radius of the probe, but only when the surfaces are in contact or a t very small separations. The two experimental conditions used here were carefully chosen such that, in one case, bridging interactions between the surfaces would be favored. In the other case, a higher polymer concentration and longer equilibration time were employed to ensure a higher polymer coverage of both surfaces and, thus, a lower probability of polymer bridging. In the first case, a 30 pug L-l polymer solution was allowed to fully equilibrate with the surfaces over a 24-h period. In this experiment, the surfaces were held a t a separation of 25 pm during the total equilibration time. In the second experiment, the probe was scanned toward and away from the surface continuously using the AFM "force mode" over a scan range of f 2 pm and a t a slow scan rate of 0.01 Hz. While this was happening, the polymer was added to the bulk electrolyte solution to give a concentration of 10 pg L-l. After circulation of the solution with a peristaltic pump for 60 min, force data were collected. In all cases the raw data were consistent over multiple force scans and a t multiple scan rates of between 1 and 10 Hz. All runs were performed a t pH
Polymer Interaction between Colloidal-Sized Objects
Langmuir, Vol. 11, No. I , 1995 159 1
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