pubs.acs.org/Langmuir © 2009 American Chemical Society
Transition Force Measurement between Two Negligibly Charged Surfaces: A New Perspective on Nanoparticle Halos Xiaoting Hong and Gerold A. Willing* Department of Chemical Engineering, University of Louisville, Eastern Parkway, Louisville, Kentucky 40292 Received December 12, 2008. Revised Manuscript Received February 11, 2009 Since 2001, silica microspheres have been reported to be stabilized by highly charged hydrous ZrO2 nanoparticles which form halos around the microspheres at pH 1.5. However, the exact mechanisms behind this novel stabilization method in terms of the relevant interaction forces remain unclear. In order to gain a greater insight into this mechanism, the interaction between a silica flat and a silica sphere in different ZrO2 nanoparticle suspensions was investigated by the colloid probe technique. The interaction force between a silica flat and a 600 nm silica sphere was first investigated in a ZrO2 nanoparticle (D ≈ 8 nm) suspension with volume fractions of 10-3, 10-4, 10-5, and 10-6. When the volume fraction of ZrO2 is 10-6, only a purely attractive van der Waals force was observed between the silica surfaces. With an increase in the ZrO2 nanoparticle volume fraction, a peak was detected on the transition force curve at a ZrO2 volume fraction of 10-5 while a purely repulsion force was observed for ZrO2 volume fractions of 10-4 and 10-3. The average distance difference between the peak and the zero distance point on the transition force curve which should define the distance between the halo on the microsphere is approximately 2.3 nm. Additionally, the repulsion increases with the effective zeta potential of the binary composite sphere (BCS, the entity of the silica sphere and the surrounding zirconia particles) on an increase of the nanoparticle volume fraction while the adhesion force decreases, which indicates a denser nanoparticle halo.
Introduction Colloidal suspensions are intensively investigated due to their widespread use in ceramics preparation,1,2 drug delivery, 3,4 cosmetics,5 and paint formulation.6 The balance of the opposing forces of attraction and repulsion between particle surfaces plays a key role in colloidal stabilization.7-9 It has long been recognized that surface forces can be tailored by employing charge regulation10,11 and polymer steric stabilization.12,13 Since 2001, a new stabilization strategy, known as nanoparticle haloing, has been developed for binary colloidal mixtures by adding highly charged zirconia nanoparticles to a negligibly charged silica colloidal sphere suspension.14,15 *Corresponding author. Telephone: 502-852-7860. Fax: 502-8526355. E-mail:
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Subsequently, Kong and co-workers showed a new stabilized system composed of alumina powders and silica sol and explained the halo formation by a deduced nanoparticle halo model.16 Zhu et al. extended nanoparticle haloing from binary colloidal sphere systems to single-walled carbon nanotube (SWNT) dispersions by adding nanoparticles.17 The original developers of naoparticle haloing have further extended their methods using a system with silica microspheres and polystyrene nanoparticles.18 Whereafter, the stabilization system of the mixture of phospholipid liposomes and negatively charged polystyrene nanoparticles has further reinforced the generality of nanoparticle haloing.19 As new nanoparticle regulated colloidal dispersions are being successively developed, there is an obvious interest in the fundamental mechanisms responsible for nanoparticle haloing. Theoretical demonstration by a hypernetted chain integral equation20 and numerical Monte Carlo simulation21,22 has dramatically strengthened the fundamental understanding of halo formation by showing the role of charge and size asymmetry. The halo mechanism was elucidated as an aspect of nanoparticle weight fraction and pH effect for Al2O3-CeO2 bidispersed suspensions.23 Scheer and Schweizer systematically investigated the effect of the variation of size ratio, volume fraction, and charge on nanoparticle haloing by integral equation theory.24 Quite recently, a quantitative ultrasmall-angle X-ray scattering measurement has (16) Kong, D.; Yang, H.; Yang, Y.; Wei, S.; Wang, J.; Cheng, B. Mater. Lett. 2004, 58, 3503–3508. (17) Zhu, J.; Yudasaka, M.; Zhang, M.; Iijima, S. J. Phys. Chem. B 2004, 108, 11317–11320. (18) Chan, A. T.; Lewis, J. A. Langmuir 2005, 21, 8576–8579. (19) Zhang, L.; Granick, S. Nano Lett. 2006, 6, 694–698. (20) Karanikas, S.; Louis, A. A. Phys. Rev. Lett. 2004, 93, 248303. (21) Liu, J.; Luijten, E. Phys. Rev. Lett. 2004, 93, 247802. (22) Liu, J.; Luijten, E. Phys. Rev. E 2005, 72, 061401. (23) Karimian, H.; Babaluo, A. A. J. Eur. Ceram. Soc. 2007, 27, 19–25. (24) Scheer, E. N.; Schweizer, K. S. J. Chem. Phys. 2008, 128, 164905.
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provided an unprecedented perspective on nanoparticle haloing where the researchers verified the existence of the halo and computed its spatial distribution.25 While these experiments have strengthened our understanding of nanoparticle haloing, there has yet been no direct quantitative measurement of the colloidal interparticle forces within this stabilization system. The total interaction force is the summation of a number of different previously identified surface forces such as attractive van der Waals force, repulsive electrostatic force, structural forces,26 depletion forces,27-29 hydration forces,30,31 and hydrophobic forces.32,33 The development of colloid probe microscopy (CPM) where a particle of interest is attached to the end of the cantilever has made it possible to directly measure and quantify the total interaction force versus distance curves between a colloidal particle and a microscopic flat surface in an electrolyte solution.34 These force measurements have been successfully performed between two similarly charged surfaces35-37 as well as between two dissimilarly charged surfaces.38,39 CPM has even been extended to measure the surface interaction between two colloidal particles with diameters up to 2 μm.40 However, the majority of the CPM force measurement has involved the measurement of double layer repulsion between two charged surfaces in an electrolyte solution. Explanation of the interaction force between two surfaces in a colloidal suspension is of even more interest due to its potential importance in industrial and medical applications. Recently, the surface force measurement for a zirconia sphere/flat system in a dispersant suspension has been utilized to explain the steric stabilization of nanozirconia dispersions.41 The interaction forces between two hard surfaces in sodium dodecyl sulfate containing aqueous systems have also been studied by CPM.42 Subsequently, Drelich and co-workers have measured the colloidal surface forces between different surfaces in alumina/silica nanoparticle suspensions.43,44 However, none of these experiments were focused on the investigation of nanoparticle haloing from a (25) Zhang, F.; Long, G. G.; Jemian, P. R.; Ilavsky, J.; Milam, V. T.; Lewis, J. A. Langmuir 2008, 24, 6504–6508. (26) Mitlin, V. J. Colloid Interface Sci. 2005, 285, 879–880. (27) Gao, H.; Xiao, C.; Ke, H. Phys. Lett. A 2007, 362, 234–238. (28) Neu, B.; Meiselman, H. J. Biochim. Biophys. Acta, Gen. Subj. 2006, 1760, 1772–1779. (29) Xiao, C.; Wylie, J. Phys. Lett. A 2006, 357, 245–248. (30) Suematsu, N. J.; Nishimura, S.; Yamaguchi, T. Langmuir 2008, 24, 2960–2962. (31) Perera, L.; Essmann, U.; Berkowitz, M. L. Langmuir 1996, 12, 2625– 2629. (32) Alsteens, D.; Dague, E.; Rouxhet, P. G.; Baulard, A. R.; Dufr^ene, Y. F. Langmuir 2007, 23, 11977–11979. (33) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562–1569. (34) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239– 241. (35) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885–11890. (36) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831– 1836. (37) Gu, C.; Kirkpatrick, A.; Ray, C.; Guo, S.; Akhremitchev, B. B. J. Phys. Chem. C 2008, 112, 5085–5092. (38) Ghzaoui, A. E. J. Appl. Phys. 1999, 86, 5894–5897. (39) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114–2118. (40) Li, Y. Q.; Tao, N. J.; Pan, J.; Garcia, A. A.; Lindsay, S. M. Langmuir 1993, 9, 637–641. (41) Renger, C.; Kuschel, P.; Kristoffersson, A.; Clauss, B.; Oppermann, W.; Sigmund, W. Phys. Chem. Chem. Phys. 2004, 6, 14671–474. (42) McNamee, C. E.; Tsujii, Y.; Ohshima, H.; Matsumoto, M. Langmuir 2004, 20, 1953–1962. (43) Drelich, J.; Long, J.; Xu, Z.; Masliyah, J.; White, C. L. J. Colloid Interface Sci. 2006, 303, 627–638. (44) Drelich, J.; Long, J.; Xu, Z.; Masliyah, J.; Nalaskowski, J.; Beauchamp, R.; Liu, Y. J. Colloid Interface Sci. 2006, 301, 511–522.
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surface force measurement perspective, and the spherical particles used in these experiments typically exceed the colloidal size scale. In our present investigation, an experimental measurement of the transition force between silica surfaces has primarily been made in order to unveil the mystery surrounding the interaction forces responsible for nanoparticle haloing stabilization. Direct measurement of the interaction force between silica spherical particles is obviously more practical for investigating nanoparticle haloing mechanisms; however, it is currently difficult to align two spheres coaxially when using smaller spherical particles (600 nm), so the initial measurements discussed in this work focus on a simplified sphere on flat system. Specifically, this study focuses on the investigation of the interaction forces between a silica sphere (∼600 nm) and a silica flat surface in zirconia nanoparticle suspensions with a variable volume fraction at pH 1.5. Zirconia nanoparticle haloing around silica spheres was observed at pH 1.5 as reported in prior work.14,15 At this pH value, the repulsive interaction between silica surfaces could be neglected due to its negligible surface potential of +1 mV.15 The direct observation of interaction forces in a nanoparticle haloing system will provide a significant understanding of the phase diagram of nanoparticle/microsphere mixtures while providing a means for future development of a methodology to guide the design of stable binary mixtures.
Experimental Section The force measurement experiments were performed between silica surfaces using an XE-100 atomic force microscope (Park Systems, Santa Clara, CA) in contact mode with the zirconia nanoparticle suspensions being contained by a Petri dish with a diameter of 47 mm (see Figure 1 for a schematic description of the experimental setup). The original HNO3-containing zirconia dispersion (Nyacol Nano Technologies Inc., Ashland, MA) was diluted through the addition of HNO3 solution to obtain the various zirconia suspensions used in this work while maintaining a consistent ionic strength between them. A small silica plate (Quartz Scientific, Fairport Harbor, OH) was used as the flat surface for the experiment. After sonic cleaning for 10 min, the plate was cleaned alternatively by 18 MΩ RO water and anhydrous ethanol three times and dried in a laminar flow hood prior to each experiment. A colloidal probe (NOVASCAN, Ames, IA) with a preattached 600 nm silica sphere was used for the force measurement. The spring constant of 0.052 ( 0.007 N/m was calibrated by a DLVO force fitting method.45 All force-distance curves were obtained at a cantilever speed of 0.10 μm/s and a set point of 1.2 nN. During CPM force measurement, the asgenerated text file contains the interaction force (not deflection for XE-100 atomic force microscope) versus the scanner displacement away and toward the flat surface. By defining the zero points for both the force and the separation distance, these data can be directly converted to force versus distance interaction curves. The zero force position was defined to occur when the deflection of the silica probe cantilever maintained a constant value and the zero separation distance was chosen from the onset of the position where cantilever deflection was linear with respect to sample displacement at high force. Zirconia nanoparticle suspensions were purchased from Nyacol Nano Technologies Inc. (Ashland, MA). A 90 Plus-Zeta particle size analyzer (Brookhaven Instruments, Holtsville, NY) was used to measure the size distribution and zeta potential of the binary composite sphere (BCS) in different zirconia suspensions. The average diameter of the hydrous zirconia nanoparticles was
(45) Hong, X. T.; Willing, G. A. Rev. Sci. Instrum. 2008, 79, 123709.
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Figure 1. Schematic description of the experimental setup (ngap, nanoparticle number density in the gap between the silica sphere and flat; nbulk, bulk nanoparticle number density).
found to be ∼8 nm: 15% are 10 nm.
Results and Discussion First, the zeta potential effect on the stabilization was investigated due to its important contribution to the repulsive electrostatic force. The zeta potential of zirconia nanoparticle suspensions with different volume fractions from 10-6 to 10-3 at pH 1.5 has a consistent value of 70 mV as reported in literature.14 As shown in Figure 2, the zeta potential value of the BCS in aqueous suspensions of the zirconia nanoparticle at pH 1.5 increases with the volume fraction of the nanoparticle. The highest effective zeta potential of the BCS is close to that of pure zirconia, which is consistent with other researchers’ observations.14 Due to the lack of charge on the silica surface, the as-confirmed haloing nanoparticles are weakly attracted to the silica sphere and contribute to the effective zeta potential of the BCS. We may treat the BCS as if they have an effective diameter with contributions from the silica microsphere, Debye length, and nanoparticle halo. In fact, the zeta potential is effectively measured at the interface between the bulk suspension and the zirconia halo. The spatial distribution of charges around the BCS varies with the nanoparticle volume fraction. When the nanoparticle volume fraction is low, the charge density of the BCS is small. Under these conditions, the effective zeta potential value of BCS is smaller than the value of zirconia. At high nanoparticle volume fraction, the charge density is accordingly increased, which leads to a large effective zeta potential value. Figure 3 shows the transition force-distance curves between a silica flat and a 600 nm silica sphere in aqueous zirconia nanoparticle (D ≈ 8 nm) suspensions with volume fractions of 10-3, 10-4, 10-5, and 10-6 at pH 1.5. The most prominent feature in these force curves is that there is a metastabilization peak which represents an energy barrier ascribed to the formation of the halo at a Φzirconia value of 10-5 for the ZrO2 nanoparticles (∼8 nm diameter). The average distance difference between the peak and the zero distance point on the transition force curve is approximately 2.3 nm, which is close to a recently reported value of the gap between the microsphere and surrounding nanoparticles.25 On an increase (10-4, 10-3) or decrease (10-6) of the zirconia volume fraction, a purely repulsive or attractive force appeared, respectively. This appreciable transition of the dominant Langmuir 2009, 25(9), 4929–4933
Figure 2. Semilog plot of the effective zeta potential (ζ) of the BCS as a function of zirconia nanoparticle volume fraction in a pH 1.5 solution (Φsilica = 8.5 10-4; Φzirconia = 10-6-10-3).
Figure 3. Interaction force between silica surfaces in zirconia nanoparticle suspensions with different volume fractions at pH 1.5. interaction force from attractive to repulsive is consistent with the significant increase of the effective zeta potential. This repulsive force is mainly ascribed to the overlap of the effective charge of the BCS, and may involve a contribution from the repulsive depletion force due to the difference of the osmotic pressure between the sphere-flat gap and the bulk suspension. The nanoparticles’ motion is relatively static compared to the solvent and the solute when the probe approaches the flat surface, which leads to an increase of the nanoparticle number density in the gap (ngap). At a Φzirconia vaue of 10-6, the purely attractive force resembles that observed in pure nitric acid solution due to the low nanoparticle number density which induces a negligible repulsive double layer interaction between silica surfaces. This force-distance curve transition characteristic exactly coincides with the Monte Carlo simulation result mentioned previously,21,22 although the corresponding separation distance is different. Force measurement results are consistent with the experimental phase diagram of the binary mixtures at low silica DOI: 10.1021/la804103g
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Figure 5. Schematic description of the nanoparticle number density in the gap between the silica sphere and silica plate.
Figure 4. Statistical analysis of the interaction force curve between silica surfaces in a zirconia nanoparticle suspension with a volume fraction of 10-5 at pH 1.5. volume fraction and at nanoparticle volume fractions of 10-6, 10-5, 10-4, and 10-3 as shown elsewhere.14 Therefore, the asobserved energy barriers at higher nanoparticle volume fractions and the lack of a barrier at lower nanoparticle volume fraction may establish a relationship between surface interaction forces and the stabilization properties of nanoparticle/ microsphere suspensions. The measurement of the sphereflat interaction is by no means an exact replica for the real resultant force from the multiple sphere-sphere interactions in a binary suspension; however, this is a significant measurement of the forces between silica surfaces in nanoparticle suspensions for a haloing system which raises a possibility that these force measurements may eventually prove useful for predicting the stability of a binary mixture. The reproducibility of the force curve measurement is clearly important for quantitative analysis of surface force. To assess reproducibility, force curves were repeatedly taken at several different surface positions for 35 measurements. Figure 4 statistically compares the percentage of three different types of interaction force curves between silica surfaces with a Φzirconia value of 10-5 at pH 1.5. Fifty-four percent of the force curves have a reproducible meta-stabilized peak. Forty percent of the force curves are purely repulsive. The purely attractive force curves only account for 6% of the total curves measured. Nevertheless, there is a difference between the force curves with a majority of the curves (>90%) showing a repulsive feature. Although the time scale of force curve acquisition (∼minute) is much longer than that of convective diffusion and Brownian nanoparticle diffusion (∼microsecond),46,47 this statistical difference is possibly caused by the change of the nanoparticle number density in the gap due to dynamic nanoparticle diffusion driven by the probe motion and the collapse and rebuilding of the halo when the probe approaches and retracts from the flat surface. As the probe approaches the surface, nanoparticles begin to squeeze out of the gap. Oppositely, nanoparticles are sucked into the gap while detaching the probe from the surface. Additionally, this dynamic halo forming process may be (46) Kao, M. H.; Yodh, A. G.; Pine, D. J. Phys. Rev. Lett. 1993, 70, 242– 245. (47) Hunter, R. J. Foundations of Colloid Science, 2nd ed.; Oxford University Press: 2001; Chapter 4.
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Figure 6. Semilog plot of the adhesion force as a function of zirconia nanoparticle volume fraction in a pH 1.5 solution (Φzirconia = 10-6-10-3). affected by the time interval between successive measurements, different surface positions, variable nanoparticle size distribution in the gap, and different clearances between the sphere and the flat after each run. For qualitative analysis purposes, nanoparticle density is compared for three different zirconia suspensions under the ideal assumption of static equilibrium. Interestingly, as shown in Figure 5, only about five zirconia nanoparticles are included in the gap at a volume fraction of 10-5 based on a clearance distance of ∼300 nm. Comparatively, ∼1 and ∼46 nanoparticles are presented in the gap at volume fractions of 10-6 and 10-4, respectively. These number density differences may explain why three different types of interaction force curves evolved as a slight change in either time or clearance may lead to a variation in the number of nanoparticles in the gap. The change in the nanoparticle density in the gap will lead to variances in the zeta potential of the resulting BCS and, ultimately, to a change in the resulting force profile, as observed here. The nanoparticle numbers shown here are not the exact values for the actual dynamic processes and should only be used as a qualitative comparison. We also monitored the adhesion force (pull-off force) for each measurement. The force acting on the cantilever immediately before snapping out of contact is a measure of the adhesion force between the sphere and flat surface. As shown in Figure 6, the adhesion forces varied from 0 to 0.55 nN. The largest adhesion force was observed to be 0.94 nN in a pure HNO3 solution at pH 1.5, though this is not shown in Figure 6. Langmuir 2009, 25(9), 4929–4933
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These data are relatively stable due to the cleanliness of the silica surfaces in pure nitric acid solution. We can easily discern the systematic trend of a decreasing adhesion force with increasing nanoparticle volume fraction. The measurement in nanoparticle suspensions with the highest volume fraction of 10-3 exhibits no obvious adhesion due to a relatively dense nanoparticle layer around the silica surfaces or a possible accumulation of the nanoparticles in the gap between the sphere and flat. We found the same tendency of the adhesion force related to each measurement shown in Figure 4 where the corresponding adhesion forces of three different types of dynamic curves, that is, the purely attractive curve, meta-stabilized curve, and pure repulsive curve, are 0.46, 0.26, and 0.16 nN, respectively. The variation of the pulloff force values from measurement to measurement is an additional indication that dynamic nanoparticle diffusion occurs between successive measurements.
Conclusion The work presented herein clearly demonstrates the transition from a purely attractive to a purely repulsive force through a meta-stabilized state for a silica sphere-flat system in zirconia nanoparticle suspensions by colloid probe microscopy surface force measurement. To our knowledge, this is the first study of the nanoparticle haloing stabilization method by direct force measurement. The binary composite sphere (BCS) displays an increase in the effective zeta potential with an increase in the nanoparticle volume fraction. We have shown the markedly dominant force transition from purely
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attractive to purely repulsive with increasing nanoparticle volume fraction from 10-6 to 10-3. At high nanoparticle volume fractions, different energy barriers were observed which corroborated the nanoparticle haloing stabilization method. The most prominent feature in these force curves is that of a meta-stabilization peak in the force curve at a Φzirconia of value 10-5 which corresponds to the distance between the microsphere and the halo. The characteristic curve at this concentration was further analyzed by a simple statistical analysis method resulting in three possible force curves which are most likely caused by the dynamic nanoparticle diffusion into and out of the gap between the sphere and flat during each experimental run. Finally, adhesion force measurements further verified the role of the nanoparticle halo as a barrier for microparticle aggregation by demonstrating a decrease in the adhesion force with an increase in nanoparticle concentration. While these direct measurements do provide some insights into the nanoparticle haloing mechanism, the ability to directly engineer stable systems for widespread use will lie in determining the interaction forces responsible for this stabilization technique. Work is currently being conducted to fit these as-measured experimental force curves with a theoretical model to determine which interaction forces are operating in this system and their relative importance to the nanoparticle haloing mechanism. Acknowledgment. The authors appreciate the sponsor by a Research Initiation Grant from the University of Louisville Office of the Vice-President for Research.
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