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Surface Forces and Deformation at the Oil-Water Interface Probed Using AFM Force Measurement Patrick G. Hartley,‡,§ Franz Grieser,*,‡ Paul Mulvaney,‡ and Geoffrey W. Stevens† Advanced Mineral Products Special Research Centre, School of Chemistry, and Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia Received December 24, 1998. In Final Form: May 25, 1999 The immobilization of droplets of a hydrocarbon liquid (n-decane) on a flat solid surface has allowed the forces of interaction between a silica colloidal particle and the hydrocarbon-water interface to be measured using the atomic force microscope. Results are presented which demonstrate that nonpolar surfaces prepared in this way acquire a significantly negative diffuse layer potential in electrolyte solutions, as indicated by force distance relationships which obey DLVO theory at large separations. At smaller separations, deviations from DLVO theory are observed, followed by an attractive force which causes the instantaneous engulfment of the silica particle by the droplet. The presence of trace quantities of the anionic surfactant sodium dodecyl sulfate (SDS) radically alters the forces between the two surfaces, creating a significant barrier to engulfment, and an apparent “softening” of the fluid interface.
Introduction An understanding of the surface forces acting between solid-water and oil-water interfaces is a critical factor in the control of the adhesion and/or transfer of particulate material from aqueous to nonaqueous media. Among many applications, this is of particular importance in mineral processing, solid-fluid separation, and biomedical technologies. Surface forces are also implicated in the control of adhesion of “soft colloids”, such as emulsions, to solid surfaces, which represents a major challenge in the understanding of flow through porous media and is of interest to a broad range of industries, including those involved in oil recovery. Historically, the interactions between deformable and nondeformable surfaces have been described by measurements of isolated properties, such as the macroscopic contact angle. Electrophoretic measurements have also shown that both air-water and oil-water interfaces acquire a significantly negative ζ potential.1-4 Such measurements provide a qualitative assessment of the likely long-range interactions operating in these systems but are unable to describe the response of a deformable interface to the force field of an approaching particle or surface. Despite the great advances made in force measurement between solid surfaces beginning with the work of Derjaguin,5 Tabor and Winterton,6 and Israelachvili,7 there remains a paucity of experimental data concerning * Author for correspondence. † School of Chemistry. ‡ Department of Chemical Engineering. § Current address: CSIRO Molecular Science, Bag 10, Clayton, South Victoria 3169, Australia. (1) Taylor, A. J.; Wood, F. W. Trans. Faraday Soc. 1957, 53, 523. (2) Anderson, P. J. Trans. Faraday Soc. 1959, 55, 1421. (3) Aronson, M. P.; Petko, M. F.; Princen, H. M. J. Colloid Interface Sci. 1978, 65, 296. (4) Churaev, N. V.; Ershov, A. P.; Espova, N. E.; Iskandarjan, G. A.; Madjarova, E. A.; Sergeeva, I. P.; Sobolev, V. D.; Svitova, T. F.; Zakharova, M. A.; Zorin, Z. M.; Poirier, J.-E. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 91, 97. (5) Derjaguin, B. V.; Abrikossova, I. I. J. Chem. Solids 1958, 5, 1. (6) Tabor, D.; Winterton, R. H. S. Nature 1968, 219, 1120.
the interactions between deformable interfaces. This is predominantly due to experimental complexities associated with the high-resolution measurement of separation between such surfaces. A few exceptions do exist. Using an oil droplet attached to a flexible microcapillary, the disjoining pressure between the oil droplet and an oil-water interface has been measured as a function of their separation in anionic surfactant solutions.8 The results were found to be consistent with an electrostatic repulsion between the interfaces. Similar techniques were used earlier to explore the interactions between model deformable biological surfaces.9-12 A novel technique based on alignment of paramagnetic particles in a magnetic field has also been developed to directly measure the forces between emulsion droplets. Repulsive electrostatic forces in solutions containing charged surfactants were again observed.13 Studies of the surface forces operating between solid and deformable interfaces have been similarly rare. Recently, the atomic force microscope has been used to measure surface forces between colloidal probes and airwater (bubble) interfaces.14-16 Typically, micrometer-sized silica or glass spheres have been employed, with the interactions often monotonically repulsive in water or simple electrolyte solutions.14,16 Adhesion between such probes and interfaces following separation was attributed to the formation of a three-phase line when the surfaces are in contact.16 Increasing the hydrophobicity of the colloid probe was found to result in attractive forces in all cases, (7) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1978, 74, 975. (8) Aveyard, R.; Binks, B. P.; Cho, W.-G.; Fisher, L. R.; Fletcher, P. D. I.; Klinkhammer, F. Langmuir 1996, 12, 6561. (9) Evans, E. A.; Skalak, R. Mechanics and Thermodynamics of Biomembranes; CRC Press: Boca Raton, FL, 1980. (10) Evans, E.; Ritchie, K.; Merkel, R. Biophys. J. 1995, 68, 2580. (11) Evans, E. A.; Leung, A. J. Cell Biol. 1984, 98, 1201. (12) Francis, G. W.; Fisher, L. R.; Gamble, R. A.; Gingell, D. J. Cell Sci. 1987, 87, 519. (13) Mondain-Monval, O.; Leal-Calderon, F.; Bibette, J. J. Phys. II 1996, 6, 1313. (14) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109. (15) Ducker, W. A.; Xu, Z. G.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (16) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3721.
10.1021/la9817563 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/19/1999
Surface Forces at the Oil-Water Interface
with instantaneous engulfment frequently observed. Addition of anionic surfactant has also been shown to generate a purely repulsive force between a negatively charged silica particle and air bubble.15 Using thin film techniques, the interactions between silica surfaces and air-water17,18 or oil-water3,4 interfaces have been investigated. In general, the results suggest that stable wetting films usually result from electrical double layer repulsion between the solid-water and air-water (or oil-water) interfaces. Upon addition of anionic surfactants, equilibrium film thicknesses increase due to further adsorption of the charged species at the air-water interface,17 indicating a more stable, wetting film. Recent theoretical work has shown the importance of considering the deformation associated with interactions in these “soft” systems.19 The interaction between a macroscopic fluid drop and a solid interface has been considered theoretically and experimentally by Miklavcic and others,20-23 with particular emphasis on understanding the role of surface forces in modulating the topology of the mercury-aqueous interface. The response of the interfacial shape to a variety of interaction force laws was predicted and observed, indicating the preeminent role of deformation in these systems. The atomic force microscope has also been employed to investigate the interactions between solid-liquid and liquid-liquid interfaces. An initial study of the interactions between a silica colloid probe and an n-decane interface using the atomic force microscope was performed in our laboratory.24 A short-range attraction was observed as the probe approached the interface. This attraction was subsequently eliminated by the addition of a surfactant to the system. Snyder and others25,26 have also developed colloid probe AFM for investigating solid/oil-water interactions, with the particular aim of modeling the oil-assisted agglomeration process used in paper processing. Hydrophobic colloid probes were found to become partially engulfed in an organic phase. The wetting/dewetting of similar probes was observed by microscopy. The effect of surfactant additives on the engulfment was also explored; however, a detailed assessment of the surface forces present in the system was not possible. In the study presented here, we have developed a simplified experimental setup in order to expand on our initial observations.24 This has allowed us to investigate in detail the effects of both electrolyte and surfactant concentration on the interactions between a silica colloid probe and the n-decane-aqueous interface and to compare semiquantitatively the measurements with theory. The relationship between the force-distance profiles and measurements of contact angles and interfacial tensions is also assessed, with a view to correlating surface forces with bulk interfacial phenomena. (17) Read, A. D.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 30, 391. (18) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1974, 49, 249. (19) Danov, K. D.; Petsev, D. N.; Denkov, N. D.; Borwankar, R. J. Chem. Phys. 1993, 99, 7179. (20) Bachmann, D. J.; Miklavcic, S. J. Langmuir 1996, 12, 4197. (21) Horn, R. G.; Bachmann, D. J.; Connor, J. N.; Miklavcic, S. J. J. Phys.: Condens. Matter 1996, 8, 9483. (22) Miklavcic, S. J.; Horn, R. G.; Bachmann, D. J. J. Phys. Chem. 1995, 99, 16357. (23) Miklavcic, S. Phys. Rev. E 1996, 54, 6551. (24) Mulvaney, P.; Perera, J. M.; Biggs, S.; Grieser, F.; Stevens, G. W. J. Colloid Interface Sci. 1996, 183, 614. (25) Snyder, B. A.; Aston, D. E.; Berg, J. C. Langmuir 1997, 13, 590. (26) Aston, D. E.; Berg, J. C. J. Pulp Paper Sci. 1998, 24, 121.
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Materials Colloidal silica in the size range 4-6 µm was obtained from Allied Signal, Illinois. Preparation was carried out by a modified Sto¨ber process. The properties of this silica are described in detail elsewhere.27 Silica plates employed in contact angle determination, obtained from H. A. Groiss Ltd., were vitreous Suprasilgrade fused silica and were polished to optical smoothness. AFM imaging of the surfaces indicated an RMS roughness of 20 times larger than that of a typical colloid probe, and thus the approximation is still valid. We note, however, that this is certainly not the case following deformation of the interface. For the purposes of the current study, however, we have not addressed this problem, and all measured forces have been normalized by the radius of the spherical colloid probe in line with the Derjaguin approximation. (b) Presentation of Force-Distance Data. In conventional colloid probe measurements (which employ solid surfaces), the position at which the tip deflection becomes coupled to the motion of the surface, that is, the onset of “constant compliance”, is generally used to represent zero separation. In experiments involving deformable interfaces, however, the constant compliance region is a complex function involving both the deflection of the AFM tip and the interface itself. (29) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (30) Hartley, P. G. In Colloid Polymer Interactions; Dubin, P. L., Farinato, R., Eds.; John Wiley & Sons: New York, 1999. (31) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992.
Previous AFM workers,15,16 investigating the interaction forces between solid colloid probes and air-water (i.e., bubble) interfaces, developed a technique which involves extracting a distance calibration for the deflection of the AFM cantilever by performing measurements in the constant compliance regime between the tip and a solid surface. This value is then used to determine the force-displacement curve between the same tip and the deformable interface. The constant compliance found between the tip and the deformable interface can then be used to extract an effective spring constant for the interface using the relationship:15,32
Kint ) Ks
- 1) (∆h ∆c
-1
(2)
where Kint represents the effective spring constant of the interface, ∆h the distance traveled by the piezo drive, ∆c the deflection of the cantilever over that distance, and Ks the measured spring constant of the AFM cantilever. In these earlier measurements, the separation was then assumed to be relative to the constant compliance regime found between the tip and the deformable interface. The value of separation obtained in this way is, of course, highly speculative, because the exact separation at which constant compliance occurs is a function of environmental conditions. In the presence of weak repulsive forces, it is expected to occur at small surface separations, with the converse being true for strong repulsions. Also, for interfaces of different deformabilities, it occurs at different separations even when the surface force-separation relationship has identical form and magnitude. For these reasons, generation of force-distance curves using this procedure has an unfortunate consequence; by effectively ‘removing’ the constant compliance regime during scaling of the data, the changes in the elasticity of the interface during an experiment become invisible. In the experiments presented here, we have chosen a different route. The calculation of forces remains unchanged. That is, the cantilever deflection against a solid surface is used to derive forces at a given position using the measured spring constant (see earlier). However, in our analysis, rather than scaling the separation data to zero at the onset of a constant compliance region in the force-distance relationship between the solid and deformable surface, we choose to scale to a known, and weak, force. Zero “relative separation” is the point at which a deflection of the cantilever first becomes observable. In the data presented here, the force/sphere radius value chosen to normalize the data was 0.01mN/m in all cases. Thus zero relative separation corresponds to the displacement at which a force of 0.01 mN/m was first measured. In this way, the true separation remains undefined, but the relative separation, that is, the position of the colloid probe relative to the undeformed interface, is approximated more closely. It should be noted that the deflection of the cantilever is included in the calculation of relative separation. Contact Angle Determination. Contact angles for n-decane droplets on silica and Melinex film surfaces were obtained in aqueous solutions by mounting the surface of interest in a glass vessel (with a suitable optical window) that faced downward, above a syringe needle attached to a micrometer-driven syringe containing n-decane. The glass vessel was filled with the solution of interest, and a droplet of the n-decane was then introduced upward toward the surface to obtain the receding (aqueoussilica) contact angle. The advancing contact angle was obtained by withdrawing solution from the syringe. A video camera was used to collect images of the droplet shape, which were relayed to a thermal printer (Mitsubishi P67E). These images were then scanned into CorelDraw V7.0 software, and the contact angles were ascertained by zooming in on the 3-phase line and using the inbuilt “angular dimension” tool. In all cases, several droplets were measured at different positions on the surfaces. Measurement of Interfacial Tensions. Interfacial tensions were measured using the pendant drop technique in an FTÅ 200 (Portsmouth VA 23704) interfacial tensiometer. This method (32) Rabinovich, Y. I.; Yoon, R. H. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 93, 263.
Surface Forces at the Oil-Water Interface
Figure 1. Measurement of surface forces between a silica colloid probe and an n-decane-aqueous interface in various concentrations of NaNO3, pH 5.6. The filled and open symbols denote separate experiments. The downward arrows correspond to the position at which the colloid probe and cantilever tip became engulfed by the n-decane droplet, at which point deflection data became immeasurable. The significance of Fdef is discussed in the text. The solid lines associated with each 1:1 electrolyte concentration represent the predicted decay length for the interactions calculated from the theoretical Debye lengths (κ-1) for the solutions: 10-4 M/30.52 nm, 10-3 M/9.65 nm, 10-2 M/3.05 nm. uses the Bashforth-Adams technique to solve the LaplaceYoung equation from the optically determined droplet equatorial diameter.33
Results and Discussion Forces in 1:1 Electrolyte Solutions. Figure 1 shows the force versus relative separation profile for silica colloid probes approaching n-decane interfaces in pure water and in different concentrations of a 1:1 electrolyte. At low forces, the interactions were characterized by a repulsion which varied exponentially as a function of electrolyte concentration. Comparison of the decay lengths of the interactions with the theoretical Debye lengths for the known electrolyte concentrations yields excellent agreement at these low force regions of the interaction. The observations that the interactions in simple electrolytes are repulsive at low forces and that their decay lengths are on a scale with the Debye length in solution have an important implication. These observations suggest that the negatively charged silica colloid probe experiences a repulsive force due to the presence of a negative diffuse layer potential on the oil-water interface. This observation is consistent with measurements of electrophoretic mobilities of oil droplets, which suggest that oil-water interfaces carry a significantly negative ζ potential.34 We note that these repulsive forces are in contrast to earlier measurements from this laboratory24 which showed attractive forces and adhesion between the interfaces in dilute electrolyte. This may reflect refinements in the experimental procedure since this original work was carried out. At higher electrolyte concentrations, and at larger forces, a second interaction regime becomes apparent and is observable as a deviation away from the purely exponential force-distance behavior toward a simple linear relation(33) Padday, J. F. In Surface and Colloid Science; Wiley: New York, 1969. (34) Anderson, P. J. Trans. Faraday Soc. 1959, 55, 1421.
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ship. The point at which this first becomes apparent is denoted Fdef, and its significance is discussed in detail later. In this region, the measured decay length no longer agrees with the theoretical Debye length. In the presence of electrolyte, the onset of this linear region was followed by an instantaneous disappearance of the deflection signal on the AFM photodiode, indicating engulfment of both the particle and the apex of the AFM cantilever by the n-decane droplet. At a higher electrolyte solution concentration of 0.1 M (data not shown), all repulsions were removed, and the particle was instantaneously engulfed by the interface. Two extremes of behavior could give rise to this phenomenon. The first possibility is that the deviation toward linearity is a true reflection of the force versus separation behavior, that is, the colloid probe continues to move toward the interface, and the force versus “true” separation relationship deviates from the exponential behavior observed at lower forces. The second explanation is that the inherent deformability of the interface is coming to the fore. Here, the increasing force between the colloid probe and the interface may be causing the interface to deform and “wrap” around the particle. In this way, it is possible that the true separation between the colloid probe and the interface is constant, with the interface simply driven away as the particle is pushed toward it. In this case, then, the force at which deviation occurs (denoted Fdef in the figure) corresponds to the onset of deformation of the interface. In reality, it is likely that both explanations are partially true and that the switch between true separation change and pure deformation is not instantaneous. An indication of this is the engulfment of the particle at high forces. If the true separation between the particle and the interface was constant, the magnitude of the surface forces would be constant also, and there would be no opportunity for engulfment to occur. The rapidity of the engulfment process is demonstrated by the extremely low number of data points (maximum of 1) recorded between the force maximum and the disappearance of the photodiode signal. At typical data acquisition rates (approximately 1000 points per second), we estimate engulfment to occur in less than 2 milliseconds. This is equivalent to the “induction time”, which represents the time required for the thinning of the aqueous film between the solid and n-decane phases, and appears somewhat shorter than that measured in an earlier study.25 Use of a high-speed digital or analogue data acquisition technique will allow better observation of this engulfment process in future studies. In different experiments, the magnitude of the maximum force prior to engulfment varied considerably, yet the force versus relative separation curves always followed the same path. On no occasion was a stable configuration adopted between the particle and the interface, which would indicate attachment of the particle to the interface via the formation of a three-phase line on the surface of the colloid probe. Such attachment has been observed in studies of the interactions between silica particles and an air-water interface and has important implications for colloid flotation. The major difference between the experimental system presented here and these earlier studies14-16 is that the Hamaker constant for the silicawater-air bubble interaction is strongly negative,35 leading to a van der Waals repulsion between the interfaces. For the silica-water-decane system, on the other hand, the Hamaker constant is positive, implying that the van (35) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3.
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Table 1. Advancing (θa) and Receding Contact Angles (θr) Subtended by Aqueous Solutions on Solid Surfaces between an n-Decane Droplet and the Solid Surface surface
θr
θa
water 10-3 M NaNO3 water
medium
Melinex Melinex silica
107 ( 7° 113 ( 7° -
10-4 M NaNO3 10-3 M NaNO3 10-2 M NaNO3 10-3 M NaNO3 + 10-3 M SDS
silica silica silica silica
10- 3M NaNO3 + 10-3 M SDS
Melinex
94 ( 7° 90 ( 1° 0° (three-phase line not formed) 26 ( 10° 26 ( 10° 28 ( 11° 0° (three-phase line not formed) 96 ( 5°
Table 2. Calculation of Effective Spring Constants for n-Decane Interfaces in Various [SDS] Compared with Interfacial Tensionsa slope of n-decane-aqueous linear region effective spring interfacial tension [SDS]/(M) (nm/nm) constant (mN/m) (mN/m) 10-5 10-4 10-3 10-2 0.3
2.08 2.49 3.79 6.74 14.11
62 45 24 12 5
48 46 33 8 -
a All solutions also contained 10-3 M NaNO . Note that the slope 3 of the linear region is equivalent to ∆h/∆c in eq (2).
der Waals force between the colloid probe and the n-decane interface is expected to be attractive, albeit weakly.24 However, an attractive van der Waals interaction goes only part of the way in explaining the engulfment process. It can only explain how a three-phase line might be produced between the silica particle and the n-decane droplet. For the observed complete engulfment to occur, the wetting characteristics in these systems must be understood in more detail. The analysis of the contact angles, subtended by aqueous electrolyte solutions, between decane droplets and silica surfaces is shown in Table 1. As these data show, a stable wetting film of water was formed between the decane droplet and the silica surface in the absence of electrolyte. Upon addition of electrolyte, however, this film was easily ruptured, and a three-phase line was obtained. These observations may be readily explained by recourse to the AFM surface force measurements. If we assume that the disjoining pressure between the silica and the n-decane interface is predominantly due to an electrostatic repulsion between them, then in water, the range of this interaction is maximal because the Debye length is largest. In this case, therefore, we expect no wetting of the silica by the decane, and this is indeed what was observed. The addition of a simple (nonsurfactant) electrolyte, meanwhile, results in a reduction in the Debye length, yet causes only minimal changes in the aqueous-decane interfacial tension (see Table 2). Given the simple electrostatic interpretation of the disjoining pressure outlined above, the decane droplet is therefore allowed to approach the silica surface more closely. Such an approach could allow short-range attractive forces (e.g. van der Waals interactions) to overcome the repulsive forces, leading to a silica-aqueous-decane three-phase line and a finite contact angle. This is precisely what was observed for contact angle measurements conducted with electrolyte present in the aqueous medium (see Table 1). Once again, a comparison between the AFM force measurements and contact angles may be made. The formation of a three-phase line would lead to partial engulfment of the silica colloid probe by the n-decane interface. In this case, complete engulfment was observed.
40 ( 10° 35 ( 20° 47 ( 20° 121°
Two factors could facilitate this engulfment. The first is the additional force applied by the sphere due to the deflection of the cantilever as it is driven toward the interface. Additional forces associated with film drainage as the sphere approaches the interface may further act to force the sphere into the oil phase following aqueous film rupture. The second factor is the equilibrium wetting behavior of the silica by water in n-decane, which is accessible from the contact angles, as described above and given in Table 1. We note that this contribution is expected to be larger in the water-on-silica-in-oil case than in the earlier wateron-silica-in-air interface experiments15 because the silicawater-decane contact angle (∼30°) is considerably larger than the silica-water-air contact angle (
