Langmuir 1993,9, 2232-2236
2232
Surface Forces between ZnS and Mica in Aqueous Electrolytes David T. Atkins*pt and Richard M. Pashley Department of Chemistry, The Faculties, Australian National University, Canberra, ACT 0200, Australia Received March 23,1993 We report on a study of the interaction forces between surfaces with dissimilar electrostatic potentials but of the same sign. The theoreticalpossibility of an attractive electrostaticforce induced by the interaction was not observed. Our resulta demonstrate the need for a more complete theoretical model for the interaction of dissimilar charge-regulating surfaces. ducting nature of the substrate. The AFM has been used to give atomic resolution images which were reconciled Surface forces are important in processes as disparate with crystallographic projections, including those of as cellular recognition, colloidal stability, and soap films.l pressed powder samples of inorganic and bioinorganic An appreciation of surface interactions is central to an carbonate^.^ More recent attention has focused on the understanding of many processesin the physical,biological, role of high resolution and real time imaging in the and material sciences. Surface forces between dissimilar biological fields,with the (controversial9 real time imaging materials have been least studied because of experimental of immunoglobulin adsorption! plasmid DNA imaging limitations. However, this type of interaction is more and dissection,1° gap junction imaging and modification,ll common than the homogeneous case and includes that and the real time imaging of the underpotential electrobetween positive and negatively charged surfaces, as well chemical deposition of copper on gold(lll).12 as a more subtle heterogeneous interaction: between two The potential of the AFM as a force measuring device different surfaces of unequal electrostatic potential but of has been rapidly exploited. Examples include electrostatic the same sign. Attractive forces between colloidalparticles as well as frictional,14magforces due to capa~itance,'~ of opposite charge are utilized in the Australian Sirofloc netic,15 and adhesionls forces. The first hint of how water clarification process.2 Almost every naturally ocsuccessfully AFM could be adapted to surface force curring colloidal has a negative surface potential and will measurements was given by Burnham et al.,l7 when they be attracted to amagnetite colloid with a positive potential, were able to differentiate between the surface forces due resulting in heterocoagulation. Following magnetic septo alkyl- and fluoro-substituted surfactant monolayers. aration of the resulting aggregated material from the Despite the eleganceand sophistication of the measuring mother liquor, the magnetite is regenerated by altering technology involved, each of these experiments shared the the solution pH, inducing a negative surface potential on single flaw that because the geometry of the tip was not the magnetite particles, and thus a repulsion between the characterized, quantitative comparisons with theory were natural colloids and the magnetite. This process, which not possible. It was not until a (single) colloid particle was developed in Australia, is now used throughout the was attached to an AFM cantilever that surface forces world for water purification and sewage treatment.3 The were measured which could be compared directly with forces of attraction utilized here are relatively well theoretical predictions. In this first colloidal AFM exunderstood. However, the Poisson-Boltzmann theory periment,18 forces were measured between a flat silica predicts the possibility of an attractive electrostatic force substrate and a silica sphere as a function of both between colloidswith the same sign of surface p ~ t e n t i a l . ~ ~ ~ electrolyte concentration and pH. The results obtained This phenomenon, which is much less well understood, is were in close agreement with earlier measurements using the subject of this report. macroscopic silica surfaces. We have used a modified atomic force microscope (AFM) to measure these forces. A daughter product of the more (7) Friedbacher, G.; Hansma, P. K.; Ramli, E.;Stucky, G. D. Science famous scanning tunneling microscope (STM), the AFM 1991,253, 1261. is closely related to its parent but was designed to be used (8) Lea, A. S.; Pungor, A.; Hlady, V.; Andrade, J. D.; Nerron, J. N.; Voss, E. W., Jr. Langmuir 1992,8, 68. with any substrate. The AFM6was designed to circumvent (9) Lin, J. N.; Drake, B.; Lea, A. S.;Hansma, P. K.; Andrade, J. D. the substrate restrictions of its predecessor and, for this Langmuir 1990,6, 509. reason, did not use the conducting tip of the STM, but a (10)Hansma, H. G.;Vesenka, J.; Siegerist, C.; Kelderman, G.; Morrett, H.; Simheimer, R. L.; Elings, V.; Bustamante, C.; Hansma, P. K. Science spring as the force-sensing device. The forces between 1992, 256, 1405. this spring and the substrate surface are utilized to provide (11) Hoh, J. H.; Ratneehwar,L.; John, S. A.; Revel, J. J.-P.; h d o r f , deflection and hence an image, regardless of the nonconM. F. Science 1991,253, 1405. Introduction
+ Present address: Department of Applied Mathematics, Australian National University. (1) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1986; Vol. I. (2) Australian Technology in New Sci. 1992, December 15. (3) Dixon, D. R. Chem. A u t . 1992,112, 394. (4) Barouch, E.; Matijevic, E.; Ring, T. A.; Finlan, J. M. J. Colloid Interface Sci. 1978, 67, 1. (5) Barouch, E.; Matijevic, E. J. Chem. SOC.,Faraday Trans. 1 1986, 81, 1797. (6) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Heu. Lett. 1986,56,930.
0743-7463/93/2409-2232$04.00/0
(12) Manne, S.;Hanema, P. K.: Maeeie,. J.:. Elinus, _ .V. B.: Gewirth, A. A. Science 1991,251,183. (13) Stem, J. E.;Terris, B. D.; Mamin, H. J.; Roger, D. Appl. Phys. Lett. 1988, 53, 2717. (14) Mate, C . M.; McClelland, G.M.; Erlandeson, R.; Chiang,S.Phys. Rev. Lett. 1987, 59, 1942. (15) den Boef, A. J. Appl. Phys. Lett. 1990,56, 2045. (16) Bryant, P. J.; Kim, H. S.;Decken, R. H.; Cheng, Y. X. J. Vac. Sci. Technol. 1990, A8, 3502. (17) Burnham, N. A.; Dominquez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Reu. Lett. 1990, 64, 1931. (18)Ducker, W. A,; Senden, T. J.; Pashley, R. M. Nature 1991,353, 239; Langmuir 1992,8, 1831.
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 8, 1993 2233
Surface Forces between ZnS and Mica
Materials and Methods Water was purified in a three-stage process. Firstly, tap water was passed through a Memtec Krystal Kleen water purification unit. A 5 pm lambs wool prefilter removed particulate matter, a reverse osmosis membrane removed dissolved ions, and finally an activated charcoal postfilter removed soluble chlorine and organic contaminants.lg This water was used as the feed for an all Pyrex and Teflon distillation unit equipped with a 30-cm condensing column. The Pyrex receptacle was located in a laminar flow cabinet, to reduce the likelihood of contamination of this partially purified water. The final stage of water purification was to pass this distilled water through a Millipore Milli Q commercial water purification unit immediately prior to collection and use. Water purified in this manner was found to have a pH = 5.6 due to equilibration with dissolvedcarbon dioxide, a conductivity of approximately 1pS cm-l, and very low bubble persistence. All chemicals used in making solutions were of analytical reagent grade and were used without further purification. All solutions were prepared gravimetrically, with less concentrated solutions being prepared by successivedilution. Solutions were prepared in stoppered quickfit conical flasks which, like all glassware used in AFM experiments, had been cleaned by a half hour rinsing in concentrated NaOH solution followed by repeated washing with purified water. Any glassware which was not completely wetted by purified water was recleaned before use. Plastic vesselsused in experiments were washed with 10%NaOH solution, concentrated nitric acid, ethanol, and finally purified water to ensure cleanliness. Colloidal zinc sulfide was prepared following the method of Wilhelmy and Matijevic,20in which acid catalyzed hydrolysis of thioacetamide produced H2S in situ, in zinc solution. Water bath temperatures were used to control the kinetics of hydrolysis and, hence, the growth in size of the particles.21 The colloids were collected by vacuum filtration witha Millipore ultrafiltration system and Sartorius 0.45-pm filter pads. Technical grade thioacetamide was recrystallizedtwice from A.R. toluene, vacuum dried, and stored refrigerated in sealed glass bottles until used, and all other chemicals were analytical reagent grade. SEM examination showed the colloids to be smooth and spherical, with an average diameter of ca. 4 pm. Figure 1shows some of the particles produced. Particle crystallinity was determined by X-ray powder diffractionn as pure sphalerite zinc sulfide (cubic phase). We used commerciallyavailable cantileversa with an integral stylus at the free end of the cantilever in the AFM device for these measurements. The cantilevershad a reflectivegold coating to maximize the laser light reflected into the photodiodes. The spring constants quoted by the manufacturers were assumed to be accurate. This spring constant has since been measured and found to be accurate to within 10% for this type of cantilever (Senden, T. J.; Ducker, W. A. Submitted for publication). Under a stereomicroscope(Olympus,X200) a thin copper wire (diameter = 40 pm) attached to a micrometer xyz translator stage was used to apply a small amount (=1 fL)of molten resinU onto the stylus region of the cantilever. Great care was taken to not allow glue to coat the underside of the cantilever, as this would interfere with the reflection of the laser spot. Once glue was applied to a cantilever, a new piece of thin copper wire was honed to a sharp point with tweezers and used to manipulate an individual colloid particle onto the glued free end of the cantilever spring. When allowed to cool, the resin instantly solidified without a change in volume. The probe could be examined under a more powerfullight microscope (Olympus,400X) to determine whether or not attachment had been successful, although this was not always obvious. An example of a colloid probe examined after (19) Memtec Krystal Kleen unit specifications. (20) Wilhelmy, W. D.; Matijevic, E. J. Chem. SOC.,Faraday Trans. 1 1984,80, 563. (21) Williams, R.; Yocom, P. N.; Stofko, F. S. J. Colloid Interface Sei. 1985,106,388. (22) Compared with standards from the JCPDS-ICDD CD-ROM
powder pattern index. (23) Supplied by Digital Instrumente, Santa Barbara,CA. (24) Shell Epicote 1004.
Figure 1. Scanning electron micrograph of some of the colloidal particles of ZnS produced in this work.
I
?
Figure 2. A typical "colloid probe" examined by scanning electron microscope after an experiment. Crystallized KC1 is visible on the surface of the probe and arises due to evaporation of the dilute background electrolyte solution (aqueous KCl) in which the experiments were conducted. The surface and probe approach a t an angle of approximately loo, which corresponds approximately to the horizontal in this micrograph. an experiment is shown in Figure 2. The resin can be clearly seen and does not seem to contaminate the upper surface of the colloid. Residual crystals from evaporated electrolyte can also be seen on the colloid probe. Also, it is clear that the colloid is much higher than the pyramidal stylus on the right. Following each AFM experiment, the radius of the colloid probes was determined using scanning electron microscopy. Cantilever legs were also examined for damage and data from any damaged cantilever was discarded. Muscovite mica was obtained from Bihar, India, and was cleaved immediately prior to use. For these experiments we used a Nanoscope I1 AFM with a fluid cell made of machined glass and a silicon O-ring to ensure a liquid-tight seal between the surface and the cell. To enable flushing of the approximately 0.1 mL volume of the fluid cell, it has an inlet and outlet port located on its front side. Both the mounted flat surface and the cantilever were treated with a water plasma (10 W, 18 MHz for 30 s, PH@ = 0.04, Ph = 0.02 Torr) to remove organic contamination immediately prior to being mounted into the AFM. Once treated, the materials were left in uacuo for a short period of time to allowingfor removal of any volatile organicmaterials created by the plasma treatment. Mounting of the surfaceor cantilever was performed with tweezers in a laminar flow cabinet. Image data (topography)recorded with the Nanoscope11AFM was written into a buffer which may then be saved on computer disk. Force data collected by the AFM was captured as a screen
Atkins and Pashley
2234 Langmuir, Vol. 9, No. 8,1993
file of diode output (volts) versus sample displacement (nm). This fie WBB then digitized, and convertad to foreelradius (FIR) versus distance information using a program specificallywritten for this purpose. Constant compliance between the probe and the flat surface occurs when the two surfaces are “in contact”;here the signal differenceacrw the photodiode detector is linearly proportional to the piezotube expansion. This defies the zero separation distance and also servesto calibrate the photodiode reading with spring displacement. We were thus able to convertthe deflection data from the screen file to force data and convert sample displacement to surface separation. When the gradient in the interactive force exceeds the spring constant, the cantilever is subject to mechanical instability and will “jumpin”to the next region where the gradient is once again smaller than the spring constant. Any data recorded between these two regions is under nonequilibrium conditions and has not been further analyzed. We denote these regions by usingan arrow to indicate the direction and approximate slope of the force curve in each of the figures.
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6x10-SM repeat
Results and Analysis Electronic spectroscopy for chemical analysis (ESCA) was used to characterize the depth profile of oxidation and the Zn:S ratio of the colloidal material prepared for these experiments, since both the electrostatic and van der Waals forces are critically dependent upon surface properties. The analysis was performed at the South Australian Surface Technology Centre,University of South Australia, by Professor Roger St C. Smart. Following characterization of the surface of the colloidal material, an argon ion beam was used to etch to a depth of 10 nm and ESCA reperformed to examine the Zn:S ratio and oxygen signal at this depth to elucidate information on the depth profile of oxidation. If the Zn:S ratio was significantly different from 1, and an oxygen signal was present, this would imply that oxidation products existed, and the relative magnitudes of these two parameters would indicate the extent of oxidation present. Previous studies by ESCAB and FT-IRB determined the oxidation products were present, especially sulfate groups, but quantified neither the relative amounts nor the depth profile of this oxidation. In summary, these experiments indicate that at the surface the 2n:S ratio was 0.94 and a small yet significant oxygen signal was present, yet by a depth of 10 nm, the Zn:S ratio was 1, and the oxygen signal halved in magnitude. Thisindicated that a small degree of oxidation was present at the surface of the colloidal material which diminished rapidly with depth. The remaining oxygen signal represents oxidation in the layer of hydrocarbon contamination on the colloidal material. The crystalline phase of the colloidal material was previously determined by X-ray powder diffraction as sphalerite (cubic phase) ZnS. Force experiments on ZnS and mica substrates were conducted in solutions of increasing electrolyte concentration (6 X 1o-S M 1 X 10-1 M, pH = 5.6), with the fluid cell thoroughly washed using purified water prior to repeating the experiments. In order to evaluate the precision of the AFM force measurements, repeat data were taken for the case of mica/ZnS in dilute electrolyte. These results are presented in Figure 3, and it is clear that the precision is acceptable; even over a 100 nm distance range and 2 orders of magnitude of FIR,the two sets of experimental data coincide precisely. The accuracy of the experimental data could only be tested by comparison
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(25) Dunetan, D. E.; Hugfeldt, A.; Almgren, M.;Siegbahn, H. 0. G.; Miekhtar, E.J. Phys. Chem. 1990,94, 6797. (26) Hertl, W. Langmuir 1988,4, 594.
10 Distance (nm)
Figure 3. Data precision was examined by comparison of two seta of experimental results recorded at the same background electrolyte concentration (6 X 106 M KC1).
with theoretical calculations. This can be done using the , E fact that the scaled force (FIR)is equal to ~ T Ewhere is the equivalent interaction energy between flat surfaces.n The latter result is accurate for the high KU ( r l= Debye length, a = probe radius) values used in these experiments of 200 or more. For the analysis of the measured forcecurves,theoretical DLVO calculations were performed using an exact numerical solution of the Poisson-Boltzmann equation%and a nonretarded Hamaker force. Interaction energies were calculated at the boundary conditions of constant charge and constant potential, for comparison. Constant charge and constant potential are, respectively, the upper and lower limits upon the magnitude of interaction energy (or equivalently FIR)in the DLVO theory. From the DLVO theory we would expect that for the interaction of two dissimilar surfaces at constant but unequal surface potentials, the surface charge on the lower potential surface would decrease in magnitude as the surfaces approach, until a point at which it passed through zero (at maximum repulsive force) and then changed sign. Thiswould result in an electrostaticdouble layer attraction between the two surfaces at close separation. Physically, it is unclear what this changeover from a double layer repulsive to a double layer attraction means in terms of the osmotic pressure method%from which the interaction energy is calculated using the numerical solution. In fact, the algorithm modified to calculateinteraction at constant potential fails at the point at which the charge on the surface of lower potential approaches zero. However, in order to demonstrate the change from electrostatic repulsion to attraction in this region, the approximate . used ~ to calculate the equations of Hogg et ~ 1 were interaction energy over the last few nanometers, demonstrating clearly the “turning over” from a repulsive to an attractive interaction energy. This combined theoretical (27) Derjaguin, B. V. Kolloid Zh. 1934,69, 155. (28) Chan, D.Y.;Pashley, R. M.; White, L.R.J. ColloidZnterface Sci. 1980, 77, 283. (29)Hogg, R.;H d y , T. W.; Fuerstenau, D. W. Trans. Faraday SOC. 1966,62, 1638.
1
Surface Forces between ZnS and Mica
..
cxperimental+ata
o constantcharge A
1
constantpotential
Langmuir, Vol. 9, No.8,1993 2235 Table I. Oscillator Model Parameters Used in Calculating Aid mica water ZnS cw 1.45 0.769 4.09 ww 2.38 X loi6 1.906 X 10'" 9.13 X lois Cm 3.43 3.74 wm 5.66 x 1014 9.43 x 10'8 Gnhvaw 74.8 wnkzowaw 1.00 x 10" a w has units radians per second. constant charge constant potential constant Dotential aomximation .. I
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30 Distance (nm)
40
Figure 4. Forces measured in a background electrolyte concentration of 1 X 1W M KC1 (d = 102 A). The theoretical solution to the Poisson-Boltzmann equation was calculated as the upper and lower limits of constant surface (unequal)charges and constant surfaceunequal potentials,using surface potentials for mica and ZnS of -100 and -90 mV, respectively. model is used in Figure 4 to compare with the experimental data obtained in 1 X 103 M KC1. The interaction for constant (unequal) charge remains repulsive at all separation distances, and hence the numerical version of the algorithm could be used for the full force curve and is given in the figures. The fitted theoretical curves were calculated for surface potentials of -90 mV for ZnS and -100 mV for mice, using a Debye length of 10.2 nm and Hamaker constant of 1.97 X 10-20J. The value of the mica potential was obtained from earlier force studies on the mica-mica interaction in KC1 solution.30 The Hamaker constant was calculated using the full nonretarded Lifshitz equation extrapolated to zero separation distances. Accepted oscillator models already exist for mica31 and but a ZnS model was set up using the Cauchy plot method.33 The parameters used in the oscillator model for each material are given in Table I. Figure 5 presents the results obtained in 5 X 103 M KC1 solution, and theoretical calculationsare fitted accordingly. The experimental data even more clearly follow the constant charge interaction. Figure 5 shows that the theoretical curves calculated at constant charge and constant potential do not deviate from each other until the separation distance was less than ca. 7 nm. In this case, the constant potential curve does not rise above ca. lo00 pN m-1, while the constant charge curves reaches a maximum value of almost 4000 pN m-l, at a separation of close to 1nm. The approximate results of Hogg et aLm for constant unequal potentials is included to clarify the nature of the turning point of the constant potential curve, which in this case becomes fully attractive from a separation of ca. 5 nm. This approximation does not fit (30) Pashley, R. M. J. Colloid Interface Sci. 1981,83, 531. (31) Chan, D. Y.; Richmond, P. h o c . R. Soc. London, A 1977,353, 1163. (32) NU, S.; Rein, R.; Webs, L. J. Theor. Biol. 1972,34, 136. (33) Hough,D. B.; White, L. R. Adu. Colloid Interface Sci. 1980,14, 3.
A
A 0 0
A A
b
io
i
Distance (nm)
Figure 5. Forces measured in a 5 X 10-9M KCl solution (8= 40 A). The Poisson-Boltzmann curves were calculated with surface potentials of -80 and -30 mV, respectively, for mica and ZnS. An approximate (hear)solution of the Poisson-Boltzmann equation is included to clarify the nature of the 'turn over" in the force for constant (unequal) surface potentials. the exact solution (at constant potential) well because it is a low potential approximation, while these are clearly high potential conditions. The experimental data closely follows the constant charge calculation until the attractive van der Waals forces dominate. No clear attractive force was observed in the 1-2 nm before contact. The theoretical curves were calculated using a mica potential of -80 mV (from ref 30) and a ZnS potential of -30 mV, with a Debye length of 4.0 nm. The ZnS surface potential is, in effect, measured by this best-fit procedure. The data presented in Figure 6 were obtained in an experiment conducted at an electrolyte concentration of 10-l M KC1. At such high electrolyte concentrations, the electrostatic contribution to the interaction was heavily screened and no repulsion was evident in the experimental data. The "jump in", which is indicated by the arrow, occurswhen the force gradient exceeds the spring constant. The experimental results are compared with the van der Waals attraction for ZnS and mica separated by water. The magnitude of the surface potentials on ZnS obtained from fitted force curves indicate that KC1is an indifferent electrolyte. It has been shown previously that H+is the potential determining ion for ZnS.S4 (34) Williams,R.;Labib, M. E. J. Colloid Interface Sci. 1985,106,251.
Atkins and Pashley
2236 Langmuir, Vol. 9,No. 8,1993 10-1MKCI data A van der Waals amction
200
Distance (nm)
Figure 6. At the high concentration of background electrolyte of 1 Y. 10-l M KC1 or r1 = 9.7 A, the forces appear to be well modeled by the van der Waals attraction alone.
Discussion and Conclusions Since its original formulation, DLVO theory has often been written in terms of constant potential interactions, largely for reasons of mathematical convenience. For a surface such as ZnS, where the surface charge is due to adsorption of a potential determining ion, it is unreasonable to assume that a change in the sign of surface charge will be induced by the approach of another surface, as is predicted by the theoretical interaction model of constant (unequal) potentials. The ZnS surface is populated largely by sulfate groups, which are dissociated in solution to give a negative surface charge. It would be more reasonable to assume that the ZnS surface charge would regulate in response to the approaching second surface. However, it would be most unlikely to adsorb further to form a positively charged surface, as the energy of binding a second cation to such a site must be large. The surface of colloidal ZnS is also populated with hydroxyl gr0ups,3~ which are amphoteric. Indeed, ZnS does show a reversal in the sign of its surface potential at a pH of around 4. This fact complicates the issue, but strengthens our argument that the charge reversal of constant potential regulation is unlikely. Even though surface groups exist on this surface which could exhibit charge reversal, the interaction observedhere was moderated toward constant charge. (36) Reiche, R. A.; McCurdy, K. G.; Hepler, L. G. Can. J. Chem. 1975, 53, 3841.
The constant unequal potential condition has been used in the explanation of the mechanism of emulsion polymerization,96 without an examination of the chemical implications. This description relies upon the reversal of charge, and the subsequent double layer attraction it induces, to explain the aggregation of precursor particles in emulsion polymerization. Similar to the surface of zinc sulfide, the emulsion precursor particles are usually populated by sulfate groups, and it would be unreasonable to assume a charge reversal can occur on these surfaces, just as it was for the zinc sulfide surface. It appears to be more likely that latex particles aggregate and grow via the hydrophobic intera~tion.~' DLVO theory predicts that constant charge and potential are, respectively, the maximum and minimum boundary conditions of the interaction energy.38 For any given system, a more reasonable fit of surface force data could be obtained by allowing surface charge site-binding (law of mass action) to determine the concentration of potential determining ions in solution next to each surface, which in turn will define the equilibrium surface potential. Ninham and P a r ~ e g i a nwere ~ ~ the first to use the sitebinding model to explain the interaction of biological membranes, solvingthe full nonlinear Poisson-Boltzmann equation. Experimentally, this approach was confirmed for interacting mica surfaces with the SFA,N at several ratios of electrolyte concentration to pH (differing ratios of potential determining species). The present work and the potential use of the AFM technique demonstrate the need for a completenonlinear Poisson-Boltzmann solution for the heterogeneous interaction case with site binding/ regulation a t each surface.
Acknowledgment. We thank Professor Barry Ninham for support and the use of the AFM facility, Professor Smart of the University of South Australia for performing ESCA on our samples of ZnS, and the electron microscopy unit at the ANU for providing SEM facilities and assistance. We also thank Tim Senden for his assistance in the experimental side of this work. (36) Feeney, P. J.; Nepper, D. H.; Gilbert, R. G. Macromolecules 1987, 20, 2922. (37) Karaman, M. E.; Meagher, L.; Paehley, R. M. Langmuir 1993,9, 1220-1227. (38) Chan, D. Y.; Mitchell,D. J. J. Colloid Interface Sci. 1983,95,193. (39) Ninham, B. W.; Pmegian, V. A. J. Theor. Biol. 1971, 31, 405.
