Surface and Capillary Forces Encountered by Zinc Sulfide

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Langmuir 2005, 21, 5882-5886

Surface and Capillary Forces Encountered by Zinc Sulfide Microspheres in Aqueous Electrolyte Graeme Gillies, Michael Kappl, and Hans-Ju¨rgen Butt* Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received January 26, 2005. In Final Form: April 22, 2005 The colloid probe technique was used to investigate the interactions between individual zinc sulfide (ZnS) microspheres and an air bubble in electrolyte solution. Incorporation of zinc ions into the electrolyte solution overcomes the disproportionate zinc ion dissolution and mimics high-volume-fraction conditions common in flotation. Determined interaction forces revealed a distinct lack of long-ranged hydrophobic forces, indicated by the presence of a DLVO repulsion prior to particle engulfment. Single microsphere contact angles were determined from particle-bubble interactions. Contact angles increased with decreasing radii and with surface oxidation. Surface modification by the absorption of copper and subsequently potassium O-ethyldithiocarbonate (KED) reduced repulsive forces and strongly increased contact angles.

Introduction The vast majority of commercially available zinc is sourced from the mining of sphalerite. Sphalerite is cubic crystalline zinc sulfide with, generally, iron and/or other metal impurities. In the mining industry, separation of sphalerite from gangue material takes place via froth flotation. This process involves introducing fine air bubbles into an aqueous dispersion of crushed ore. The hydrophobic sphalerite particles attach to the rising air bubbles and gather as froth. The widespread use of zinc throughout the world has resulted in numerous studies of aqueous ZnS dispersions. Throughout the literature, one observes a large variation in determined isoelectric point of ZnS. Notably, Moignard et al.1 showed that the IEP was dependent on the solids concentration and the equilibration conditions. Both dispersions of small volume fractions and equilibration under acid conditions gave isoelectric points at low pH, whereas high volume fractions and equilibration under alkaline conditions increased the isoelectric point. It is now well established that the behavior of ZnS in solution is governed by oxidative mineral dissolution (eqs 1-3). Typical studies of colloidal phenomena, including microelectrophoresis, light scattering, and settling studies, use volume fractions between 10-3 and 10-5. Under these dilute conditions, the surface chemistry of ZnS is controlled by eqs 1-3 and the resultant surface is largely influenced by the amount of adsorbed hydrolysis products.2 That is, eqs 1-3 indicate that alkaline pH values and larger volume fractions will result in an interfacial chemistry that resembles the zinc oxide/ zinc hydroxide interface of aqueous zinc oxide dispersions. 2ZnS f xZn2+ aq + Zn(1-x)S

(1)

2-y ]aq Zn2+ aq + yOH f [Zn(OH)y

(2)

[Zn(OH)y2-y]aq f [Zn(OH)y2-y]ads

(3)

In contrast, the colloid probe technique relies on individual particles immobilized on a reflective cantilever * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Moignard, M. J.; Dixon, D. R.; Healy, T. W. Proc. Australas. Inst. Min. Metall. 1977, 263, 31.

and occasionally on an adjacent surface. Thus, it follows that the volume fraction will vary, between 10-9 and 10-12 for a 50 µL and a 15 mL fluid cell, respectively. Under these ultradilute conditions, eq 1 will proceed strongly to the right, whereas the formation and subsequent adsorption of zinc hydroxide species (eqs 2 and 3) will not proceed in any measurable quantity. This results in a sulfur-like (i.e., zinc deficient) interface, sulfur particles are hydrophobic possessing a contact angle of 85° 3 with surface electrical properties defined by eq 4.2 Ssurface + H+ aq f SHsurface

(4)

In this article, particle-plate and particle-bubble interactions in the presence of additional zinc ions are reported. The presence of additional zinc ions is intended to mimic the ionic behavior of high-volume-fraction dispersions. Asymmetric micas or silicasZnS interactions have been reported by several authors.2,4-6 Atkins and Pashley4 reported the interactions between a ZnS microsphere and mica with varying electrolyte concentrations. They revealed that electrical double-layer forces tended to the constant charge limit but observed a distinct lack of van der Waal forces except at electrolyte concentrations above 0.1 M. In a similar experiment Toikka et al.2 showed that the interactions between a ZnS microsphere and a silica plate in 1 mM electrolyte solution were monotonically repulsive between pH 4 and 10. Xu et al.6 report interactions between a silica sphere and macroscopic grains of ZnS, thus investigating interactions with a volume fraction 3 orders of magnitude larger than the former mentioned studies. Unlike the former study, ZnS-silica interactions were attractive within 20 nm below pH 8. Throughout the literature, there have been several reports of particle-bubble interactions; in particular, Butt and co-workers7-11 have investigated different samples (2) Toikka, G.; Hayes, R. A.; Ralston, J. Colloids Surf., A 1998, 141, 3. (3) Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1990, 11, 88. (4) Atkins, D. T.; Pashley, R. M. Langmuir 1993, 9, 2232. (5) Toikka, G.; Hayes, R. A.; Ralston, J. J. Chem. Soc., Faraday Trans. 1997, 93, 3523. (6) Xu, Z.; Chi, R.; Difeo, T.; Finch, J. A. J. Adhes. Sci. Technol. 2000, 14, 1813. (7) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109. (8) Preuss, M.; Butt, H.-J. Langmuir 1998, 14, 3164.

10.1021/la050226l CCC: $30.25 © 2005 American Chemical Society Published on Web 05/21/2005

Surface Forces Encountered by ZnS Microspheres

including silica-, polystyrene-, and thiol-treated surfaces, as well as investigating surfactant-modified surfaces and the influences of line tension. Nguyen and co-workers12,13 examined drive rate dependence (hydrodynamics) and the interactions of polyethylene microspheres and treated silica microspheres in aqueous solution. Notably, Fielden et al.14 give a detailed account of particle-bubble interactions under conditions where hydrodynamics are negligible. Fielden et al. revealed that clean hydrophilic silica does not coalesce with a bubble in aqueous solution. Instead, they observed a monotonically repulsive interaction indicating a stable film separating the interfaces. Furthermore, by assuming the deformation of the bubble was linear and the change in bubble shape was negligible, the normalized force versus interfacial separation showed qualitative agreement with predicted DLVO forces for rigid bodies. When silica microspheres were rendered hydrophobic, either by treatment with octadecyltrichlorosilane (OTS) or dehydroxylation, interactions revealed an initial repulsion immediately prior to a jump-in. The jump-in feature is typical of all AFM particle-bubble studies and is a result of microsphere engulfment within the bubble where the onset of capillary forces dominate the interaction. In flotation, it is common practice to modify the surface chemistry of metal sulfides with dithio compounds such as potassium O-ethyldithiocarbonate (KED) to improve flotation efficiency. These compounds adsorb via a reaction whereby thio groups reduce the interacting metal ion. The adsorption of KED onto zinc sulfide is relatively slow compared to that of copper or lead sulfides, as the relevant metal ions are more readily reduced. Both copper and lead can be adsorbed into a zinc sulfide matrix; in doing so, particles are said to be ‘activated’ since the subsequent adsorption of KED is promoted.15 Therefore, we included in this study measurements of the interaction of copperactivated ZnS microspheres with a bubble in the presence of KED. Experimental Section Materials. Analytical grade Zn(NO3)2‚6H2O, Cu(NO3)2, KNO3, KOH, HNO3, ethanol, thioacetamide, and KED were obtained from Sigma-Aldrich Chemie (Steinheim, Germany). The KED was recrystalized from acetone prior to use. Deionized water further purified with a Satorius Arium 611 VF water purification system was used exclusively. Silica substrates were fashioned from polished silicon plates that possessed outer layers of silica (∼500 nm) formed by roasting in a diffusion oven. Polysilicon cantilever arrays were fabricated by a standard lithography process (IMM, Mainz, Germany). The backsides of the cantilevers were then coated with a 25 nm gold layer using Baltec MED 020 high-vacuum sputtering device. Spring constants were determined on the basis of a resonance method.16 Both silica and ZnS microspheres were mounted to the apex of a cantilever using a now-standard process17,18 with Shell Epikote 1004 as the adhesive. By using an array of (9) Preuss, M.; Butt, H.-J. J. Colloid Interface Sci. 1998, 208, 468. (10) Preuss, M.; Butt, H.-J. Int. J. Miner. Process. 1999, 56, 99. (11) Yakubov, G. E.; Vinogradova, O. I.; Butt, H.-J. J. Adhes. Sci. Technol. 2000, 14, 1783. (12) Nguyen, A. V.; Nalaskowski, J.; Miller, J. D. J. Colloid Interface Sci. 2003, 262, 303. (13) Nguyen, A. V.; Nalaskowski, J.; Miller, J. D. Miner. Eng. 2003, 16, 1173. (14) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3721. (15) Rashchi, F.; Sui, C.; Finch, J. A. Int. J. Miner. Process. 2002, 67, 43. (16) Hutter, J. L.; Bechhoefer, J. Rev. Sci. Instrum. 1993, 64, 1868. (17) Butt, H.-J. Biophys. J. 1991, 60, 1438. (18) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239.

Langmuir, Vol. 21, No. 13, 2005 5883 cantilevers, we were able to investigate up to six subsequent microsphere-bubble interactions against the same bubble. Monodispersed silica microspheres were supplied by Merck Monomer (Darmstadt, Germany). Zinc sulfide microspheres were synthesized using a modified version of the method reported by Wilhelmy and Matijevic´.19 First, two 50 mL aliquots of water (pH 2) were purged with argon gas for 1 h. In the first aliquot, 2.25 g of thioacetamide was added and dissolved via brief sonication, while 1.79 g Zn(NO3)2‚6H2O was dissolved in the second. Both solutions were then poured into in a 100 mL volumetric flask that was then sealed and left to react without stirring or agitation for 90 min at 75 °C. The resultant polydisperse zinc sulfide spheres were separated via centrifugation, rinsed in ethanol, and stored under vacuum. Resultant diameters varied from 0.5 to 10 µm, with the majority of microspheres possessing a diameter between 2 and 4 µm. Additionally, ZnSmounted cantilevers were kept under vacuum whenever possible and never reused. Scanning electron micrographs of the ZnS microspheres were taken using a LEO 1530 Gemini SEM that does not require a conductive coating of the surfaces. Sample Cleaning. All glassware and relevant surfaces were cleaned immediately prior to use with a 2 M HNO3 solution followed by a saturated KOH solution then extensive rinsing with water. Such a process is likely to roughen silica surfaces; thus, interacting silica surfaces were exposed to KOH solutions for only a minimal duration. The cleaning of mounted ZnS microspheres was particularly challenging. When microspheres were plasma-cleaned with argon, force data closely resembled that determined for a hydrophilic silica sphere approaching a bubble, i.e., coalescence was not observed, or only at large forces, indicating a zero or near zero contact angle, respectively. The method used in this investigation involved leaving ZnS-mounted cantilevers under vacuum for at least 24 h then rinsing with analytical grade ethanol filtered through a 0.22 µm membrane filter, followed by rinsing in pH 3.5 HNO3. This procedure gave the qualitatively similar results as ZnS microspheres etched in strong acid solution (pH less than 1). Determination of Interaction Forces. Interaction forces were determined using an Asylum standalone Molecular Force Puller 1-D. All interactions were determined in polystyrene Petri dishes. Equilibration times of 1 h were used unless otherwise stated. Such long interaction times were necessary in particular when decreasing the pH from alkaline conditions. Additionally, the system was pre-equilibrated at pH 5.8 at the beginning of every measurement series. Particle-plate interactions were determined by pressing the colloid probe, either silica or zinc sulfide, against silica substrates and monitoring the deflection and piezo displacement. Particle-bubble interactions were determined in the same fashion after first aligning the particle approximately a few micrometers above the crest of the bubble with micrometer screws along x, y, and z axes in combination with the built-in video microscope. Loading (extend) and unloading (retract) force cycles were determined at 2.0 µm‚s-1 over trajectories of 12 µm in each direction. To quantify the force data, the optical sensor was calibrated from the constant compliance region of force curves. For particle-bubble interactions compliance slopes were determined against the solid substrate prior to and after determining particle bubble interactions. The force (F) acting between the probe and the bubble or surface was determined from the cantilevers deflection using Hooke’s Law, F ) kcx, where kc represents the spring constant and x the deflection of the cantilever. Determination of Contact Angles. It is well established now that the receding contact angle of a microsphere can be determined from particle-bubble force data. In this article, we calculate the receding contact angles using the analysis proposed by Preuss and Butt9 (eq 5)

cos Θsphere ) r

R-D R

(5)

is the apparent receding contact angle, R is the Where Θsphere r radius of the microsphere, and D is the breadth of the force (19) Wilhelmy, D. M.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 563.

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Figure 1. Normalized interaction force between a ZnS microsphere and a silica surface in a 10-3 M KNO3/10-5 Zn(NO3)2 M solution at pH 6.4. Lines plots show theoretical electrical double-layer forces according to the linear approximation (solid line, middle), constant charge21 (dashed upper line), and constant potential calculations21 (lower dotted line). Theoretical fits in this figure were calculated with potentials of 32 mV on each surface. The inset shows a logarithmic plot of the interaction curves and the linear approximation for electrical double-layer forces.

Figure 2. Scanning electron micrograph of a ZnS microsphere mounted near the apex of an AFM cantilever. The size of the scale bar is 1 µm. minimum at zero force. Advancing contact angles are not discussed within this article in depth, as they must be calculated from the magnitude of adhesion,20 which in turn is dependent on the applied load. In general, it was found that the advancing contact angle was between 10° and 25° larger than determined receding contact angles.

Results and Discussion Zinc Sulfide-Silica Interactions. Typical interactions between a silica surface and a zinc sulfide microsphere in a 10-3 M KNO3/10-5 M Zn(NO3)2 solution at pH 6.4 (Figure 1) reveal that electrical double-layer forces dominate interactions. Both the symmetric silica-silica and asymmetric ZnS-silica interactions were always monotonically repulsive, consistent with other studies in the absence of zinc ions.2,4,5 It was observed that, at separations greater than one Debye length, interaction forces were well described by the linear Poisson-Boltzmann theory. However, within separations of one Debye length, interactions were underestimated by the linear Poisson-Boltzmann theory yet overestimated by the Hogg Healy Fuerstenau constant charge analysis.21 Atkins and Pashley attribute a lack of observable van der Waals attraction to surface roughness effects. Scanning electron microscopy (Figure 2) of our attached microspheres reveals rough surfaces; however, hydration effects masking the (20) Scheludko, A.; Toshev, B. V.; Bojadjiev, D. T. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2815-28. (21) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638.

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Figure 3. Normalized interaction force between a ZnS microsphere and an air bubble in a 10-3 M KNO3/10-5 M Zn(NO3)2 electrolyte solution at pH 4 (9), 5.8 ([), 7.5 (O), and 9 (4). For pH < 9, a small repulsive barrier (shown in the inset) is observed prior to the snap-in.

van der Waals interaction are also plausible. That is, determined contact angles, discussed below, indicate that the surfaces are hydrophilic, which may result in a shortranged (∼1 nm) repulsive force associated with the removal of water molecules from the surfaces.22 The surface potential of the zinc sulfide microspheres was determined from an asymmetric interaction with a silica surface, in which the surface potential of the silica was determined from a symmetric interaction between two silica surfaces. Calculating the potential of colloids on the basis of symmetric force experiments has been shown to give large variations in determined potential,23,24 most likely due to uncertainties in cantilever spring constants. To further quantify the potential and the sign of the charge of the silica surface, the zeta potential of silica microspheres in zinc solution was determined using a Malvern Mastersizer. In solutions of 10-3 M KNO3/10-5 M Zn(NO3)2, calculated surface potentials of zinc sulfide varied between 40 and -40 mV with an isoelectric point between 6.4 and 7.4. This isoelectric point is marginally lower than we have previously reported25 and most likely corresponds to a refinement in synthesis and handling of the microspheres. Zinc sulfide dispersions are often characterized with isoelectric points between pH 3 and 6; larger isolectric points are generally attributed to partially oxidized surfaces.26,27 However, in this instance, the isoelectric point is largely dominated by the concentration of zinc ions in solution, which was chosen such that the isoelectric point reflects the data of Muster et al.,28 as discussed in ref 25. Particle-Bubble Interactions. The interactions between a ZnS microsphere and a bubble in 10-3 M KNO3/ 10-5 M Zn(NO3)2 between pH 4 and 8.5 (Figure 3) show a repulsive force barrier prior to an attractive jump-in. The magnitude of this repulsive force barrier was found to vary between 0.02 and 0.12 mN/m with no statistically significant variation over a pH range of 4-8.5. This value is similar in magnitude to that determined by Fielden et al.14 for a hydrophobized silica sphere approaching an air bubble. However, this repulsive force barrier increased (22) Israelachvili, J. N.; Wennerstro¨m, H. Nature 1996, 379, 219. (23) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Langmuir 1997, 13, 2109. (24) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (25) Gillies, G.; Bu¨scher, K.; Preuss, M.; Kappl, M.; Butt, H.-J.; Graf, K. J. Phys.: Condens. Matter 2005, 17, S445. (26) Mirnezami, M.; Restrepo, L.; Finch, J. A. J. Colloid Interface Sci. 2003, 259, 36. (27) Dura´n, J. D. G.; Guindo, M. C.; Delgado, A. V. J. Colloid Interface Sci. 1995, 173, 436. (28) Muster, T. H.; Toikka, G.; Hayes, R. A.; Prestidge, C. A.; Ralston, J. Colloids Surf., A 1996, 106, 203.

Surface Forces Encountered by ZnS Microspheres

Figure 4. Dependence of the observed receding contact angle on the radius of the ZnS microspheres measured in a 10-3 M KNO3/10-5 M Zn(NO3)2 solution at pH 4.2. Here, data was determined from four separate force investigations, indicated by varying symbols. Filled diamonds and empty circles correspond to contact angles determined using the same cantilever array against the same bubble. Where no error bars are shown, variation in data was smaller than the size of the symbol.

by an order of magnitude once the pH was increased above pH 9, furthermore, if microspheres were sufficiently small, typically R < 2 µm, the net interaction force remained repulsive. At pH 9, the majority of aqueous zinc ions form Zn(OH)2; this results in increased (surface) precipitation and hydrophilic surface chemistry precluding attachment. Upon decreasing the pH from 9 to neutral or acidic conditions, long equilibration times, typically 1 h, were required to reproduce the behavior determined at the same pH prior to subjecting microspheres to alkaline conditions. The sudden attractive force upon further approach is a well-known effect of particle-bubble attachment, upon where the capillary forces suddenly dominate the interaction, resulting in rapid movement (jump-in) of the cantilever.25 The initial repulsive force prior to the jumpin has not been resolved in most other studies and is attributed to the stability of wetting films, a direct result of repulsive van der Waals and electrical double-layer forces and possibly a hydration layer. Typically, van der Waals forces and hydration effects span over a several nanometers, while electrical double-layer forces are observed within interfacial separations of four to five Debye lengths (in this case 40-50 nm). Thus, the observation of repulsive forces prior to attachment indicates attachment occurs within separations of 50 nm. Therefore attachment via an attractive short-ranged hydrophobic force, rather than the long-range hydrophobic force. Contact Angles of ZnS Microspheres. Determined receding contact angles were found to be dependent on the pH, radius, and sample history. All reported receding contact angles discussed within this article were determined from the first five contacts between microspheres and bubbles. Care was taken not to engulf the microsphere in the bubble prior to these measurements, as the contact angle decreased by approximately 3° over 30 measurements in approximately half of the microspheres investigated. It was found that this decrease in contact angle was a result of successive engulfments and not effects due to time. The reason for this change in contact angle is not entirely clear, although it is speculated that the air-water interface is either cleaning or coating the microsphere with each attachment-detachment interaction. The benefit of determining contact angles with an array of consecutively mounted cantilevers is that subtle variations such as bubble size or exact value of the pH are reduced. The apparent contact angle was found to increase with decreasing radius, (Figure 4). Previously an increase

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Figure 5. Receding contact angles of four ZnS microspheres (R ≈ 2µm) in a 10-3 M KNO3/10-5 M Zn(NO3)2 solution plotted against pH. Empty symbols correspond to microspheres that were first aged in solution (pH 7) for 24 h.

in contact angle with decreasing radius has been observed by Yakubov at al.11 for silica and polystyrene microspheres and was attributed to a negative line tension effect. Unfortunately a unique value of the line tension could not be calculated from our results but was determined to lie between -0.1 and -0.4 µN, which is of similar magnitude as that observed by Yakubov et al. The apparent contact angle of microspheres was also found to vary moderately with pH (Figure 5) with a maximum contact angle less than 45° for R ) 2 µm. Such small contact angles indicate a significant portion of the surface is adsorbed zinc hydroxide. That is, sulfur groups are hydrophobic and the formation of zinc oxide is not anticipated over the time frame of this experiment.29 Previously Subrahmanyam et al.30 reported ZnS advancing contact angles of 44° and 53° by using the wetting rate and the equilibrium capillary pressure techniques, respectively. By comparison, an advancing contact angle of 51.1° ( 0.6° (R ) 2.4 µm) was determined from the adhesion data at pH 6. Care must be taken in interpreting this value, as pinning effects arising from surface roughness may result in larger values, and one must assume that the air-water interface subtends the advancing contact angle. Previously, Muster et al.28 reported that concentrated zinc sulfide dispersions aggregate around pH 7. Both the distinct lack of long-range hydrophobic forces in particle-bubble interactions and the small contact angle values suggest that the aggregation is not a result of hydrophobic forces. However, it should be noted that this analysis is intended where the isoelectric point is relatively large, e.g., dispersions with alkaline equilibration conditions or large volume fractions, and do not necessarily describe the aggregation behavior of dilute suspensions with relatively low isoelectric points, e.g., ref 31. Leaving microspheres in the zinc solution at pH 7 for 24 h resulted in contact angles ∼10° larger than for microspheres without such pretreatment. The contact angle of these aged microspheres showed a pH dependency with larger contact angles under alkaline conditions. It is believed that the increase in contact angle is a result of the slow formation of zinc oxide from some of the adsorbed hydroxide species which takes place over tens of hours at pH 7 but is accelerated under more alkaline conditions (eq 6). That is, zinc oxide is more hydrophobic than zinc hydroxide due to a reduction in the number of (29) Buckley, A. N.; Woods, R.; Wouterlood, H. J. Int. J. Miner. Process. 1989, 26, 29. (30) Subrahmanyam, T. V.; C. A., P.; Ralston, J. Miner. Eng. 1996, 9, 727. (31) Vergouw, J. M.; Anson, J.; Dahlke, R.; Xu, Z.; Gomez, C. O.; Finch, J. A. J. Miner. Eng. 1997, 10, 1095-1105.

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Figure 6. Filled symbols: interaction between a ZnS (R ) 2.4 µm) and an air bubble in a 10-3 M KNO3, 10-5 M Zn(NO3)2 electrolyte solution at pH 6. Empty symbols: interaction between the same particle and an air bubble in a 10-3 M KNO3/ 10-5 M Zn(NO3)2 10-5 M KED solution after ‘activating’ the particle in a 10-3 M KNO3/10-3 M Cu(NO3)2/10-5 M Zn(NO3)2 solution for 2 h.

hydrogen bonding sites. Prestidge and Ralston32 observed that the contact angle of galena (PbS) increases when left in air while Muster et al.33,34 showed that zinc oxidehydroxide particles formed by the precipitation of zinc ions at pH 8 possessed contact angles of 88.6°. Alternatively, Dura´n et al.27 suggest that oxidation results in the presence of elemental sulfur at the interface, thus making the surface more hydrophobic. However, this mechanism is not anticipated here due to the presence of zinc ions in solution driving eqs 2 and 3 to the right.

Zn(OH)2 + OH- T ZnO + H2O + OH-

(6)

Despite differences observed in contact angles, the aging did not lead to a statistically significant change in the magnitude of the repulsive force present prior to engulfment. Influence of KED. The surface chemistry of the microspheres was altered via the adsorption of KED. To increase the adsorption of KED, copper ions are adsorbed by pre-equilibration of mounted cantilevers in 10-3 M KNO3/10-3 M Cu(NO3)2/10-5 M Zn(NO3)2 solution (pH 5.3) for 2 h, after which the cantilevers where quickly rinsed in water then placed in a 10-3 M KNO3/10-5 M KED/10-5 M Zn(NO3)2 pH 6 solution. The interactions of an activated zinc sulfide sphere in the KED solution approaching a bubble showed qualitatively the same behavior as microspheres in the absence of adsorbed species (Figure 6). However, there was a marked decrease in the magnitude of the repulsive force prior to the snap-in and an increase in the determined contact angle. That is, the repulsive force barrier prior to attachment was found to be 0.03 ( 0.01 m‚Nm-1 and the determined contact angle for R ) (32) Prestidge, C. A.; Ralston, J. Miner. Eng. 1996, 9, 85. (33) Muster, T. H.; Neufeld, A. K.; Cole, I. S. Corros. Sci. 2004, 46, 2337. (34) Muster, T. H.; Cole, I. S. Corros. Sci. 2004, 46, 2319.

2.4 µm was 77°. The decrease in the repulsive force prior to attachment is thought to be a result of the short-ranged hydrophobic force (arising from a deficiency of water molecules near a hydrophobic interface35) becoming more long-ranged. The adsorption of copper and KED results in a surface partially functionalized with hydrophobic alkyl groups. This in turn causes an additional deficiency of water molecules at the solid-liquid interface. Consequently, the short-ranged hydrophobic force dominates at larger separations where DLVO forces are smaller. Furthermore, the capillary force upon attachment was sufficiently large such that capillary forces saturated the detection limits of the cantilevers used in this system. This increase in contact angle is consistent with observations of greater yields from flotation when ZnS (and sphalerite mineral samples) is modified in the same manner. In activating ZnS, there was dependence on pH and the concentration of copper ions; however, such effects are outside the scope of the current article. Conclusions The interactions between ZnS microspheres and an air bubble in aqueous solution have been determined using the colloid probe technique. When interactions are determined in the presence of 10-5 M Zn2+, the isoelectric point lies between 6.4 and 7.4 and microspheres with radii larger than 4 µm possess a contact angle (receding) of less than 30°. The apparent contact angle was found to increase with decreasing radii, especially for radii smaller than 4 µm. The behavior of water near the solid-liquid interface greatly influences the particle-bubble interaction. Hydration layers and/or DLVO forces result in repulsive forces prior to attachment; hydrophobizing the solidliquid interface results in attachment from greater separations where these forces are smaller. Under the same conditions, the interactions between ZnS microspheres and a silica plate are repulsive and similar to those determined in the absence of zinc ions,2,4 except one instance where attraction was observed.6 Xu et al. determined interactions under conditions that would result in more zinc ions in solution than Toikka et al. and Atkins and Pashley, but less than the amount used in this study. This further indicates a concentration dependence of zinc ions in solution and warrants further investigation. The surface chemistry of ZnS microspheres was modified considerably with the adsorption of copper and KED. This treatment resulted in an increase in the contact angle much larger than effects of pH and oxidation and comparable to the most dramatic increase apparent contact angle due to line tension. Despite an increase in contact angle, attractive hydrophobic forces never exceeded the range of repulsive DLVO forces. Acknowledgment. We thank Lars Heim for his assistance with SEM imaging. LA050226L (35) Israelachvili, J. N.; Pashley, R. Nature 1982, 300, 341.