Langmuir 1999, 15, 1657-1659
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Forces Measured between Latex Spheres in Aqueous Electrolyte: Non-DLVO Behavior and Sensitivity to Dissolved Gas R. F. Considine,† R. A. Hayes,*,‡ and R. G. Horn Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia Received July 16, 1998. In Final Form: December 1, 1998 An atomic force microscope was used to measure the forces acting between two polystyrene latex spheres in aqueous media. The results show an electrostatic repulsion at large separations which is overtaken by an attractive “hook” that pulls the two spheres into contact from a considerable range (20-400 nm), much larger than could be expected for a van der Waals attraction. The range of operation of this attraction varies from one experiment to another and is not correlated with electrolyte concentration. However, the range is found to decrease significantly when the level of dissolved gas in the water is reduced.
Introduction Colloidal dispersions of monodisperse polystyrene latex spheres are important in products such as paints and have also been widely studied as “model” colloids.1 There is ample evidence that latex colloids in aqueous liquids are stabilized by electrostatic forces, so it is commonly assumed that the classical DLVO theory of colloid stability, which is based on a competition between electrostatic repulsions and van der Waals attractions,2 is applicable to these systems. However, there is a dearth of direct force measurements between latex spheres to either validate3 or cast doubt4 on this assumption. In recent years methods have been developed for using an atomic force microscope (AFM) to measure forces between a colloidal particle and a flat plate5 or between two colloidal particles6,7 immersed in aqueous media. We have used these methods to measure forces between polystyrene latex spheres in aqueous electrolyte solutions. Experimental Section Surfactant-free sulfate-stabilized latices of nominal diameter 6 µm were purchased from Interfacial Dynamics Corp. (Portland, OR). Qualitatively similar results were also obtained with amidine-stabilized latices. Use of two small spheres and slow scan rates (1 Hz) ensured that hydrodynamic drag forces were negligible. The methods of Cleveland et al.8 and Jaschke and * To whom correspondence may be addressed. † Present address: CSIRO Molecular Science, Private Bag 10, Clayton South MDC, Victoria 3169, Australia. ‡ Present address: Philips Research, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands. E-mail:
[email protected]. philips.com. (1) Wu, X.; Van de Ten, T. G. M. Langmuir 1996, 12, 3859. (2) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (3) Li, Y. Q.; Tao, N. J.; Pan, J.; Garcia, A. A.; Lindsay, S. M. Langmuir 1993, 9, 637. (4) Karaman, M. E.; Meagher, L.; Pashley, R. M. Langmuir 1993, 9, 1220. (5) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (6) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114. (7) Muster, T. H.; Toikka, G.; Hayes, R. A.; Prestidge, C. A.; Ralston, J. Colloids Surf., A 1996, 106, 203. (8) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403.
Butt9 were used to calibrate the AFM cantilever stiffness and z-piezoscanner, respectively, while the radii of the particular spheres under investigation were determined from optical and scanning electron microscopy. High-purity water was obtained from an Elgastat UHQ-PS system and sodium chloride at 99.999% purity purchased from Aldrich.
Results and Discussion Imaging the spheres with a regular AFM tip revealed that the latex sphere surfaces have significant roughness. There are undulations and asperities on various scales, with the largest peaks being about 20 nm high and typically separated laterally along the surface by about 10 times their height. Root mean square roughnesses over a 1 µm × 1 µm area were in the range 8-18 nm for individual spheres. A roughness on this scale immediately raises questions about the applicability of DLVO theory to these colloidal particles. Figure 1 shows a typical force vs separation curve. More than 2000 such curves have been measured, for electrolyte concentrations ranging from 10-4 to 1 mol/L. For NaCl concentrations less than 0.01 mol/L, all of the force curves have the following characteristic features: (i) At large separations, there is a region of repulsion in which the force decays exponentially with separation. Since the exponential decay length always matches (within experimental error) the Debye length calculated from the electrolyte concentration, it is concluded that this is an electrical double-layer repulsion.2 (ii) There is a maximum in the repulsive force, occurring when the spheres are still separated by a considerable distance (as much as 400 nm). The maximum does not vary very much, ranging from 0.2 to 0.5 mN/m for several pairs of sulfate-stabilized spheres measured over 2 orders of magnitude in electrolyte concentration. There appears to be no correlation between the magnitude of the force maximum and the surface separation at which it occurs. (iii) From the force maximum the spheres are pulled into close proximity by an attractive force which appears abruptly, as if the spheres are suddenly hooked together when they reach a certain separation. The range of this attractive “hook” is large and is highly variable between different pairs of spheressanywhere from 20 to 400 nm, (9) Jaschke, M.; Butt, H.-J. Rev. Sci. Instrum. 1995, 66, 1258.
10.1021/la980900h CCC: $18.00 © 1999 American Chemical Society Published on Web 02/12/1999
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Figure 1. A typical force curve measured between two sulfatestabilized latex spheres of radii R1 ) 3.5 µm and R2 ) 3.2 µm, as a function of the minimum separation D between their surfaces, when they are immersed in an aqueous solution of 1 × 10-4 mol/L NaCl. Force F is normalized by the harmonic mean radius R ) (1/R1 + 1/R2)-1. The zero of separation is taken as the position reached when the spheres are pressed together under a large force (>10 mN/m). An electrical doublelayer repulsion is discernible at a separation of 50-70 nm, and as indicated by the arrow, the spheres are pulled in abruptly from the force maximum (at around 50 nm in this case) to approximately 10 nm from contact.
although it remains constant for repeated force measurements on any particular pair of spheres making contact at a particular position. Such a large range is not consistent with a van der Waals attraction, which would be immeasurably small for separations greater than 10 nm, nor is the variability in range consistent with the longrange attractive forces that have been measured between hydrophobic surfaces in the surface force apparatus, since these have always been characterized as smoothly varying (exponential or double-exponential) functions of surface separation.10,11 (iv) At small separations there is a short-range repulsion, indicating that the surfaces of the latex spheres are somewhat compressible. The range of the repulsion is 1520 nm at low electrolyte concentration (10-4 mol/L) and decreases as electrolyte concentration increases, reaching π/2 within the gas phase) and they could form capillary bridges between the surfaces with negative mean curvature. The negative Laplace pressure inside such bridges would result in a significant attractive force that would appear as soon as the bubbles coalesce.19 We note that the same mechanism has been proposed independently in a contemporaneous report by Carambassis et al.20 showing similar force curves measured between a hydrophobized glass sphere and hydrophobized silicon wafer in the AFM. The possibility of tiny bubbles existing at or adjacent to hydrophobic surfaces has been suggested long ago in the Russian flotation literature21 and more recently by Vinogradova et al.,22 and the stability of bubbles attached to surfaces has been discussed by several authors.23-26 Capillary bridges formed by water vapor have previously been postulated by Christenson and Claesson27 as a possible source of the long-range attraction measured between hydrophobic surfaces, and Parker et al.28 have reported evidence for a long-range attraction arising from coalescence of bubbles attached to hydrophobic surfaces. If the attractive “hook” is attributable to gas bubbles attached to latex surfaces, one would expect that the size of such bubbles, and hence the range of the force, would be reduced if the level of gas dissolved in the water were reduced. This effect is demonstrated in Figure 3, where it is shown that the range of the attractive force is dramatically reduced when water is degassed, then it returns to its original range (in about 1 h) when the water is resaturated with air. To obtain these results, gas was evacuated from a closed vessel containing water which was simultaneously sonicated. With the aid of a peristaltic pump, water was pumped continuously from the sealed vessel through an AFM fluid cell (Digital Instruments) which had been completely sealed using silicone sealant. About 1 h later the measured force showed a markedly reduced range of the “hook” and a slightly greater force maximum, and this force curve was reproducible over a (18) Seebergh, J. E.; Berg, J. C. Colloids Surf., A 1995, 100, 139. (19) Christenson, H. K. J. Colloid Interface Sci. 1985, 104, 234. (20) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357. (21) Klassen, V. I.; Mokrousov, V. A. An Introduction to the Theory of Flotation; Butterworth: London, 1963. (22) Vinogradova, O. I.; Bunkin, N. F.; Churaev, N. V.; Kiseleva, O. A.; Lobeyev, A. V.; Ninham, B. W. J. Colloid Interface Sci. 1995, 173, 443. (23) Eriksson, J. C.; Ljunggren, S. Langmuir 1995, 11, 2325. (24) Kralchefsky, P. A. Langmuir 1996, 12, 5951. (25) Attard, P. Langmuir 1996, 12, 1693. (26) Ryan, W. L.; Hemmingsen, E. A. J. Colloid Interface Sci. 1998, 197, 101. (27) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390.
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Figure 3. Effect of removing dissolved gas from the aqueous solution. As in Figure 2, the long-range repulsive force is plotted on a logarithmic scale (here R1 ) 3.6 and R2 ) 3.7 µm, [NaCl] ) 2 × 10-4 mol/L). Before the water is degassed (+), the attractive “hook” snaps the surfaces together from a force maximum at a separation of about 150 nm; after the water is degassed the hook distance decreases to about 30 nm (×). About an hour after the water is reexposed to air at atmospheric pressure, the force maximum returns to near where it was originally (b). Saturating the water with nitrogen gas produces the same result as exposure to air.
period of several hours while the flow-through of degassed water was maintained. When normal air-exposed water was returned to the fluid cell (via the same pumping system) the force curve returned to its original position after a further hour. Conclusion These results clearly demonstrate that while there is a double-layer repulsion acting between polystyrene latex spheres having charged groups attached to the surfaces (and stabilizing colloidal dispersions of such spheres), the full interaction cannot be described by DLVO theory. In particular, there is an attractive force which overcomes the double-layer repulsion at a large separation. The range at which the attraction becomes effective is quite varied and may well be associated with the local roughness of the surfaces. If this is the case, then the usual assumption that the force between colloidal spheres scales with their radius would not be valid. This may account for the observation of Ottewill and Shaw29 that the stability ratio of latex dispersions does not follow the expected variation with particle size. Surface roughness (and pinning of the putative gas bubbles and bridges) would also account for the recent observation of Velegol30 that latex spheres thought to be held together in a secondary minimum cannot rotate freely. There is clear evidence that the range of the attractive force depends on the amount of gas dissolved in the water which, we suggest, favors an explanation in terms of gas bubbles attached to the somewhat hydrophobic and rough surfaces. Whether this explanation could account for the long-range attraction measured between other hydrophobic surfaces remains open to conjecture (Carambassis et al.20 suggest that it does). Previous attempts31 to investigate whether this attraction is affected by dissolved gas have not produced definitive results. However, the present method of flowing degassed water continuously through the experimental chamber may have produced a clearer result because it was more effective in maintaining a low level of dissolved gas throughout the measurement period. LA980900H (28) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (29) Ottewill, R. H.; Shaw, J. N. Discuss. Faraday Soc. 1966, 42, 154. (30) Velegol, D.; Anderson, J. L.; Garoff, S. Langmuir 1996, 12, 4103. (31) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736.
