Polymeric Coatings on Silver Nanoparticles Hinder Autoaggregation

Jan 13, 2012 - The increased affinity of the AgNPs for the porous medium in this case may be explained by a shifted contact frontier where electrical ...
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Polymeric Coatings on Silver Nanoparticles Hinder Autoaggregation but Enhance Attachment to Uncoated Surfaces Shihong Lin,†,§ Yingwen Cheng,‡,§ Jie Liu,‡,§ and Mark R. Wiesner*,†,§ †

Department of Civil and Environmental Engineering, Pratt School of Engineering, ‡Department of Chemistry, and §Center for the Environmental Implications of NanoTechnology (CEINT), Duke University, Durham, North Carolina 27708, United States S Supporting Information *

ABSTRACT: The propensity of silver nanoparticles (AgNPs) having two different polymer coatings (poly(vinylpyrrolidone), PVP, or gum arabic, GA) to aggregate, or to deposit to a reference surface (silica), was explored as a basis for differentiating the effect of surface coating on the stability of nanoparticles in aggregation and in deposition. Surface polymeric coatings stabilize nanoparticles against aggregation as shown by either an increased critical coagulation concentration as for PVP-coated AgNPs (AgPVP) or the absence of observable aggregation even at a high ionic strength as for GA-coated AgNPs (AgGA). In experiments of AgNPs deposition in a silica porous medium, dissimilar surfaces favored deposition, such as the case where polymer coatings were present on the AgNPs but were absent on the porous medium. The increased affinity of the AgNPs for the porous medium in this case may be explained by a shifted contact frontier where electrical double layer interaction is weaker. When coating polymers were introduced to the porous medium and allowed to preadsorb to the silica surfaces, the attachment efficiencies for both the AgPVP and AgGA were reduced due to steric and electrosteric stabilization, respectively. The results suggest that polymeric coatings that are usually deemed as stabilizers (as they indeed are in the case of autoaggregation) might not necessarily stabilize nanoparticles against deposition unless the collector surfaces are also coated with polymer.

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INTRODUCTION A growing number of manufactured nanomaterials have raised concerns regarding the impact of these materials and the products they inspire on human health and the environment.1−3 The environmental risks potentially associated with nanomaterials will depend on the levels of environmental exposure and the hazards produced.1,4 The affinity between nanoparticles and environmentally relevant surfaces is likely to have a profound influence in determining the fate, transport, and exposure of nanomaterials.5 For example, the affinity between nanoparticles and the surfaces they encounter will affect the transport of nanoparticles introduced intentionally to contaminated groundwater aquifers for remediation and the removal of nanoparticles in water and wastewater treatment processes. The affinity of nanoparticles for various surfaces is also pertinent to understanding heteroaggregation in surface waters (i.e., the scavenging of nanoparticles by larger flowing collectors), autoaggregation between nanoparticles (i.e., aggregation between particles of the same species), and the affinity of nanoparticles for biotic surfaces that may affect bioavailability and uptake. The surfaces of manufactured nanoparticles may be modified with polymeric surface coatings that stabilize particles via steric- and/or chargerelated phenomena.6 Steric stabilization, composed of an osmotic contribution and an elastic contribution,7 is typically more robust than charge stabilization alone in preventing aggregation over a range of pH values and ionic strength (IS).8 The osmotic contribution is rooted in entropy reduction that © 2012 American Chemical Society

occurs when hydrophilic polymer coating layers on two approaching surfaces interpenetrate. The adsorption of charged, high molecular weight species may provide electrosteric stabilization, which includes an additional repulsion due to the electrostatic interaction between the interpenetrating charged polymers. Steric and electrosteric stabilization may be engineered into nanoparticles9 or occur spontaneously when polymeric species in natural or physiological systems adsorb to particle surfaces. For example, copolymers may be used to enhance the mobility of zero-valence iron nanoparticles to deliver them to contaminated zones in groundwater remediation,10,11 while natural organic matter (e.g., humic acid) may stabilize colloidal aggregates of nanoparticles and facilitate their transport in porous media.12,13 Although manufactured nanoparticles with polymeric coatings may be efficiently stabilized with respect to autoaggregation, it does not necessarily follow that the polymers coated on a particle surface will also be found on the surfaces that nanoparticles encounter, in which case steric stabilization might be severely undermined due to the absence of the osmotic contribution. In this study, we explore the importance of the existence of polymeric coatings on both nanoparticle surfaces and the surfaces they interact with in stabilizing nanoparticles against Received: July 25, 2011 Revised: January 6, 2012 Published: January 13, 2012 4178

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hydrodynamic radius, ah, which was determined by a least-squares fit to the initial linear regime of the ah(t)−t curve up to 1.25 ah. The diffusion-limited aggregate rates are different for different types of particles due to the difference in both their hydrodynamic radii and particle concentrations. However, these differences do not invalidate the comparison of attachment efficiency between particles as the aggregation rates were normalized by that under diffusion-limited aggregation. Column Tests and Determination of Particle−Collector Affinities. Packed column experiments were conducted to assess the affinity of AgNPs for the silica surfaces of the porous medium under different solution chemistries and surface modifications. A chromatography column (C10/10, GE Healthcare, Piscataway, NJ) was packed with glass beads (Potters Industries Inc., Berwyn, PA) of an average size of 360 μm. The glass beads were washed using a protocol reported previously12 before use. During the wet packing process, the column was placed in a sonication bath where mild sonication was applied to ensure minimum and consistent porosity. The diameter of the column was 10 mm, and the length was adjusted to 4 cm using a column adaptor (GE Healthcare). The flow of carrier solution was driven by a magnetically driven gear pump (Cole-Parmer Instrument Co., Chicago, IL) to a poly(tetrafluoroethylene) (PTFE) Y-connector (Biochem Fluidics, United Kingdom) where it mixed with the nanoparticle suspension delivered by a syringe pump (Harvard Apparatus, Holliston, MA). The volume inside the Y connector is on the magnitude of microliters and did not introduce significant residence time to the system. The flow rate of the background electrolyte solution was 0.94 mL min−1, while that for the nanoparticle suspension was 0.04 mL min−1, which in total corresponds to a Darcy velocity of 0.02 cm s−1 in this column. The primary parameters for the column test are summarized in Table 1.

deposition. Silver nanoparticles (AgNPs) that are charge stabilized only, AgNPs coated with an uncharged polymer, and AgNPs coated with polyelectrolytes were tested and compared. Although a significant amount of effort has been recently spent on understanding the disinfection,14−16 dissolution,17 aggregation,18−21 and deposition22 behavior of AgNPs, the current study is concerned less with the specific properties of AgNPs and focuses more on the effect of polymeric coatings on the deposition behavior of nanoparticles. The role of differences in the presence of a surface coating on particles or/and collectors in determining the particle attachment efficiency of deposition (αdep) was investigated by comparing the αdep of nanoparticles in the three following scenarios: (1) deposition of nanoparticles in a column with uncoated silica collectors, (2) deposition of nanoparticles with the corresponding polymers used to modify AgNPs in the carrier flow so that the polymers may also adsorb to the surface of the silica collectors, and (3) deposition of nanoparticles in a column precoated with the same polymer used to coat the AgNPs.

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MATERIALS AND METHODS

Preparation and Characterization of Silver Nanoparticles. Three kinds of AgNPs were prepared: AgNPs (AgBare) that were charge stabilized by borohydride (or borate when oxidized later), AgNPs coated with 10 kDa poly(vinylpyrrolidone) (AgPVP), and AgNPs coated with gum arabic (AgGA), which has an average molecular mass greater than 200 kDa. AgBare was prepared following Creighton’s method.23 AgPVP was prepared by the polyol method24 with a slight modification. AgGA was prepared using a synthesis method described in the Supporting Information. AgBare was used as synthesized, while both AgPVP and AgGA were subjected to extensive washing using the ultracentrifuge-redispersion procedure to reduce to the greatest extent possible any free coating material that might remain in the solution containing the nanoparticles. Details of the synthesis and washing procedures are provided in the Supporting Information. The AgNPs were subjected to multiple characterizations to obtain information on different properties. A Cary 500 scan UV−vis−NIR spectrophotometer (Varian, Palo Alto, CA) was used to obtain the UV−vis spectra to confirm the distinct surface plasmon adsorption of the AgNPs. Transmission electron microscopy (Tecnai G2 Twin, FEI, Hillsboro, OR) was used to observe the morphology and to obtain the size distribution of the AgNPs. TEM samples were prepared by airdrying a droplet of AgNP suspension on a carbon/Cu grid (200 mesh, Ted Pella Inc., Redding, CA). At least 100 representative particles from the images were analyzed using an image analysis package (ImageJ). Dynamic light scattering conducted using a multiangle goniometer system (ALV/CGS-3, Germany) was applied to determine the average hydrodynamic radii of the primary particles in deionized (DI) water. A ZetaSizer (Nanosizer ZS, Malvern Instruments, Worcestershire, U.K.) was employed to determine the electrophoretic mobility (EPM) of the nanoparticles under relevant solution chemistries. Aggregation Experiments. The effect of the coating on the kinetics of autoaggregation was evaluated by time-resolved dynamic light scattering (DLS) to monitor the evolution of the average size of the particle aggregates25 over time. For all the DLS measurements, 50 μL of AgNP suspension was added into 1.0 mL of background electrolyte solution with a pH in the range between 6.9 and 7.2 as buffered by 0.1 mM NaHCO3. The IS of the electrolyte solution ranged from 1 mM to a concentration above the critical coagulation concentration (CCC; except for AgGA) and was controlled by adding the appropriate amount of NaNO3. Immediately after the addition of AgNPs into the background solution, the mixture was vortexed briefly (vortex mixer, Fisher Scientific) to ensure a homogeneous dispersion. The scattering angle (i.e., the angle between the incident light and the photodetector) was set at 90°. Absolute aggregation rates were determined from the initial change in the ensemble average of the particle

Table 1. Experimental Parameters for Column Testsa param

value

packed medium length column inner diameter medium porosity collector diameter Darcy velocity particle suspension/electrolyte volume ratio Solution Chemistry

4 cm 1 cm 0.378 360 μm 0.02 cm/s 0.04

nanoparticles

polymer coating during equilibration

polymer coating during filtration

NaNO3 concn (mM)

AgBare, AgPVP

no

no

AgGA AgPVP, AgGA AgPVP, AgGA

no yes yes

no yes no

1, 5, 10, 20, 50, 100 1, 10, 100 20, 50, 100 100

a

The concentration of free polymer added during equilibration or filtration was 0.1 g/L.

The ionic strength varied from 0.001 to 0.1 M as adjusted by NaNO3. Before the injection of the nanoparticles, at least 10 pore volumes (PVs) of background solution were passed through the column to ensure that the collector surface was equilibrated with the background solution. In some sets of experiments, 0.1 g L−1 free polymer was introduced only during the equilibration stage to precoat the glass beads but was absent in the background solution when nanoparticles were introduced. In this case, 10 additional PVs of background solution without polymers (i.e., the background solution used in the filtration stage) were passed through the column to remove the residual free polymer solution from the column before the initiation of nanoparticle injection. The concentrations of AgNPs in the effluent were monitored by measuring the absorbance at 400 nm using a spectrophotometer. The attachment efficiency αdep, which expresses the probability of successful attachment per collision between nanoparticles and the 4179

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Figure 1. TEM images of AgBare (A), AgPVP (B), and AgGA (C) nanoparticles. All the scale bars are 50 nm in length. The size distribution and UV−vis spectra of these particles are available in the Supporting Information. collector, was calculated from the fraction of particles passing through the column (C/C0) using the following equation:26

αdep =

kd ln(C /C0) = kd,fav ln(C /C fav )

(1)

where kd is the deposition rate coefficient and kd,fav that under favorable deposition conditions, C and C0 are the effluent and influent concentrations, respectively, and Cfav is the effluent concentration under favorable deposition conditions.27

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RESULTS AND DISCUSSION Characterization of the AgNPs. The number-weighted average diameters based on TEM image (Figure 1) analysis, dTEM, of AgBare, AgPVP, and AgGA were 12.6 ± 3.3, 27.7 ± 4.7, and 5.4 ± 1.7 nm, respectively. The intensity-weighted average hydrodynamic diameters dh for these three AgNPs were respectively 21.8 ± 0.6, 69.3 ± 1.8, and 174.2 ± 4.4 nm as measured with DLS in nanopure water. Even at a high NaNO3 concentration, the (initial) average hydrodynamic diameters remain similar to those measured in DI water, as can be observed in the aggregation curves (Figure 2A). The large discrepancies between dTEM and dh for AgPVP and AgGA are attributable to the coatings surrounding the core particles that were not observable by TEM due to their low electron density. A thicker coating for the GA layer compared with PVP is reasonable given the much larger mean average molecular weight (MWav) of GA. However, given the many different configurations that these polymers may assume, a reliable relationship between the MWav values of the polymer and the coating is not obtainable.7 The UV−vis spectra of these AgNPs all give a characteristic wavelength for absorbance of around 400 nm at which the absorbance was tested to be proportional to the particle concentration over the range of concentrations used in the deposition experiments (Supporting Information). Aggregation Kinetics. The stability of AgNPs against autoaggregation was studied by comparing the aggregation kinetics of different AgNPs over a range of ionic strengths. The evolution of the intensity-weighted average particle (or aggregate) radius, over time, ah(t), for AgBare and AgPVP is shown in Figure 2A. Similar experiments with AgGA show no observable aggregation even at 1 M NaNO3. The initial aggregation kinetics were quantified by the slope of the linear stage of the ah(t) curves. Higher ionic strengths favor aggregation of both the AgBare and AgPVP NPs. In both cases, aggregation rates suggested an ionic strength beyond which the aggregation rate remained constant and was considered diffusion-limited.28 The attachment efficiencies for aggregation (αagg), which were calculated by normalizing the aggregation rates to the rate

Figure 2. (A) Representative curves for temporal evolution of the hydrodynamic radius of AgBare and AgPVP suspension under different ionic concentrations (NaNO3). Note that AgGA (green circles) shows no appreciable aggregation even at 1 M NaNO3. (B) Attachment efficiency (αagg) for autoaggregation of AgBare and AgPVP at different ionic strengths as calculated using data from (A).

corresponding to the diffusion-limited condition, are shown in Figure 2B, where it was determined that the CCC for AgBare was around 0.20 M NaNO3 while that for AgPVP was around 0.53 M NaNO3. The CCC for AgPVP aggregation was significantly higher than that reported by Huynh and Chen21 probably due to the difference in particle preparations: while AgPVP in Huynh’s work was prepared by suspending premade bare AgNPs in a PVP suspension, AgPVP in our work was 4180

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that surface polymeric coatings would in general enhance the nanoparticle stability. In addition, the affinity between AgPVP and the collector surface was more sensitive to the change of ionic strength. The critical deposition concentration (CDC) for AgPVP was near 0.02 M NaNO3, while that for AgBare was around 0.1 M NaNO3, which was the maximum ionic strength tested in the deposition experiments. Although protected by a polymeric coating, the thickness of which is multiple times its core size, AgGA also had a higher affinity for the collector surface as compared to AgBare when the IS was low. The higher affinity of AgPVP for the silica collector surface may be attributable to the interaction between the polymers and the silica via a mechanism similar to bridging flocculation33 except that in this case the “bridge” was not free or added polymer in the suspension but rather the coatings themselves, which were anchored to, and thus became part of, the particles during synthesis. Assuming AgPVP to be a charged core particle with an uncharged polymer coating, interactions between AgPVP and the collector surface can be modeled using the linear superposition approximation (LSA) expression for sphere−plate interactions34,35 to calculate the potential for electrical double layer (EDL) interaction between the core particle and the collector surface and by using the approach proposed by Israelachvili to account for the effect of the PVP coating on the potential of vdW interaction. Figure 4 shows the potential energy curves of the interactions between AgBare and silica (A) and that between AgPVP and a silica plane (B) at different ionic strengths for illustrative purpose. Note that the interaction energy curves shown on Figure 4A only consider the interaction before the contact between polymer and the silica surface, while ignoring any possible postcontact interactions (e.g., elastic compression) because it is assumed that the polymeric coating would adsorb onto the collector surface upon contact. It is worthwhile to point out that successful attachment between two polymer-coated surfaces might require that the coated surfaces (i.e. the particle core) become close enough to each other because the vdW interaction between two polymer layers is usually very weak36 and the steric interaction (mainly the osmotic contribution) is significant; therefore, postcontact interaction has to be considered. However, depending on the material of the polymer and the segment density of the coating layer, the Hamaker constant for polymer−silica interaction can be 2 orders of magnitude higher than that for polymer−polymer interaction (Supporting Information). Successful attachment can possibly result by vdW interaction, if not by even stronger chemisorption, between the polymer and silica surface. With different contact frontiers, the energy barrier between an uncharged-polymer-coated particle (e.g., AgPVP) and an uncoated surface (e.g., silica) is calculated to be lower than that between an uncoated particle and an uncoated surface at the same ionic strength (Figure 4), and it is also more sensitive to the change of ionic strength. The increased affinity (for the surface) and sensitivity (to the change of IS) of AgPVP are both attributed to the fact that the location for attachment shifted away from the core surface by a distance equal to the thickness of the coating layer. Such a shift would lower the EDL interaction potential that must be overcome for a successful attachment to occur (Figre 4C). These observations suggest that, in general, a charged particle coated with an uncharged polymer layer which itself may interact with the collector surface should become less stable due to a weaker EDL repulsion at the shifted contact frontier.

prepared using the glycol reduction method at high temperature with PVP added as a stabilizer during the synthesis, which turns out to yield more stable AgPVP suspensions. Decreasing stability at higher ionic strength due to double layer compression is qualitatively consistent with the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory.28,29 The difference in CCC should be attributed to the steric stabilization given by the PVP coatings. In addition to steric stabilization, the coatings likely play another role in increasing the stability of AgPVP: the van der Waals (vdW) interactions between the coated particles are likely to be lower than those occurring between solid particles of the same size (composed of the Ag core material). On the basis of the approach of Israelachvili for calculating the Hamaker constant for vdW interaction between layered surfaces30 and the method by Gregory to calculate Hamaker constants for vdW interactions in a medium,31 the Hamaker constant for AgBare was calculated to be 25 times higher than that for the coated AgNPs. Experiments showed that AgGA was even more stable than AgPVP: no aggregation was detectable even at 1 M NaNO3. GA is superior to PVP10K in stabilizing the nanoparticles not only because it is of a much higher molecular weight and thus is able to provide stronger steric stabilization, but also because GA is a polyelectrolyte (as compared to PVP10K as an uncharged polymer) and is able to exert stabilization by both steric and electrostatic means that yield greater stability than steric effects alone.32 Deposition Kinetics. The attachment efficiencies of deposition (αdep) are shown in Figure 3 for five series of experiments:

Figure 3. Attachment efficiencies αdep of various AgNPs in different solution chemistries. The shown error bars stand for standard deviations from three replicates. Note that the error bars (representing standard deviation) are skewed because of the log scale used, which appears more apparent for smaller values.

deposition of AgBare, AgPVP, and AgGA with only NaNO3 but without free polymers in the carrier solution, of AgPVP with 0.1 g L−1 PVP10K present in the carrier solution, and of AgGA with 0.1 g L−1 GA in the carrier solution. In each of these cases, increased attachment efficiency was observed with increasing NaNO3 concentration. However, it is observed in Figure 3 that the affinity between AgPVP and the collector surface was higher than that between AgBare and the collector surface throughout the ionic strengths tested: an observation that is contrary to the conventional understanding 4181

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challenging case. The expression for EDL interaction energy between a polyelectrolyte-coated particle and an uncoated surface prior to contact was proposed by Ohshima:38 ⎛ ⎞ 1 a VEDL(H ) = 4πε0εra⎜ψpψse−κH − ψs2 e−κH ⎟ ⎝ ⎠ 4 H+a (3)

where a is the particle radius (including the coating shell), ψp is the potential of the plate, and H is the particle−plate separation. Increasing IS should have a drastic effect on the deposition rate in this case due to both charge screening as revealed in the exponential of eq 3 and a reduction of the surface potential based on eq 2. However, the Ohshima theory was built upon a perfectly idealized and hypothesized model. Its applicability to depict the realistic particle behavior has rarely been tested experimentally. However, the most striking feature of the data in Figure 3 is the decrease in attachment efficiencies when free polymer was introduced with the carrier fluid. Compared with AgBare, the αdep values of AgPVP and AgGA were at least an order of magnitude lower in the presence of the free polymer. Such a dramatic change is most likely caused by the steric stabilization when free polymer from the carrier solution adsorbed onto the silica collector surface. It is thus evident that while the presence of the organic coating on the particles alone might increase the affinity between the particles and the collector and thus enhance deposition, the presence of the same coatings on both of the interacting surfaces can drastically decrease the affinity between the AgNPs and the collector surface due to steric stabilization. AgGA had a slightly lower affinity for the polymercoated porous medium than did AgPVP, probably because (1) the GA coating is thicker than the PVP coating, which leads to stronger steric stabilization for AgGA than for AgPVP, and (2) the polyelectrolyte nature of GA leads to electrosteric stabilization, which is in general more robust than steric stabilization alone. With either steric or electrosteric stabilization, the electrolyte concentration nonetheless still influences particle−collector interactions.

Figure 4. (A, top left) Calculated interaction energy between a bare particle and a planar bare surface using the linear superposition approximation corrected for the sphere−plate system for the EDL interaction potential with the following parameters: ψs = −25.7 mV, ψp = −25.7 mV, ap = 30 nm, Ap/W/s = 1.53 × 10−20 J. (B, top right) Interaction energy between a polymer (uncharged) coated particle and a planar bare surface. The calculation method and parameters are the same as in (A) except that the coating thickness δ = 6 nm and Ap/W/s = 1 × 10−21 J. (C, bottom) Results extracted from (A) and (B) for IS = 10 mM with added EDL and vdW interaction curves to illustrate how the interaction barrier is reduced. Note that the parameters were arbitrary and the simulations were conducted for qualitative explication of the role of the PVP coatings but not for quantitative prediction for the results in Figure 3.

Modeling the interaction between AgGA and the uncoated silica surface requires quantitative knowledge of the surface coating coverage, the charge density of the polymers, and the distribution of the polymer configuration. Moreover, gum arabic itself is not a well-defined polymer. Ohshima soft particle theory37 considers the scenario when the thickness of the polyelectrolyte coating is much greater than the Debye length (which was likely the case for AgGA with a thick coating). In this case, the surface potential of the spherical particle can be approximated by ψs =

ψDON 2

=

ZeN 4ε0εr κ 2

Figure 5. αdep for AgPVP and AgGA for three different cases: presence of free coatings throughout the whole measurement, presence of free coatings only during equilibration, and absence of any free polymer. The error bars stand for standard deviations.

(2)

where ψDON is the Donnan potential, Z is the valence of ionized groups on the polyelectrolyte, N is the volumetric density of the ionized groups, e is the elementary electric chare, ε0 is the permittivity of a vacuum, εr is the relative permittivity of the solution, and κ is the Debye constant. The theoretical description of the behavior of nanoparticles coated with polyelectrolyte (i.e., charged polymer) such as AgGA presents an even more

Steric Stabilization with a Precoated Surface. To further confirm that stabilization was steric and not depletion related, we also studied the deposition of AgPVP and AgGA onto a precoated collector surface but without the presence of 4182

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free coatings in the suspension. Figure 5 shows the attachment efficiencies calculated for deposition in three scenarios: (1) with coating during the equilibration and injection (deposition) stage (equ + inj), (2) with coating only during equilibration and not the injection stage (only equ), and (3) without coatings throughout the experiment (w/o). For both AgPVP and AgGA, αdep obtained from experiments using preconditioned silica beads (scenario 2) was similar to that obtained when free polymer was added during both the preconditioning period and nanoparticle filtration (scenario 1), while significantly different from that in the third scenario. The greater degree of stabilization (lower affinity toward the coated collector surface) provided by GA as compared with PVP is consistent with observed aggregation kinetics of these AgNPs. The salient difference between αdep in scenario 3 and those in scenario 1 and scenario 2 emphasizes the importance of having the polymer coatings on both of the interacting surfaces to deliver strong steric stabilization. A similar effect of steric stabilization by natural organic matter (NOM) was also reported in the literature.13 Steric Interaction vs Bridging. Steric interaction is the major mechanism for many of the observed colloid stabilization phenomena. The steric stabilizers can be synthetic polymers introduced into the particle suspension during or after synthesis.9,39−44 On the other hand, NOM such as humic acid, alginate, and fulvic acid can also stabilize nanoparticles against aggregation and deposition and thus enhance their colloidal persistence.44−47 However, it has to be realized that the effect of NOM on colloidal stability can be far more complicated than just steric stabilization. For example, depending on the concentration, fulvic acid can neutralize or even reverse the surface charge of hematite nanoparticles.48 It was also found that the addition of the same polymer can lead to bridging destabilization at relatively low concentrations but steric stabilization at higher concentration.49,50 For engineered nanoparticles with polymer stabilizers anchored to the particle surface, steric stabilization against aggregation can be expected as the coating layers are always present in both of the interacting surfaces as depicted in Figure 6A. However, when it comes to the case of deposition, two very different scenarios are possible: In the first scenario, there are free polymer coatings that would adsorb onto the collector surface. These adsorbed polymers covering the collector surface would interact with the polymer on the particle to impart steric stabilization (Figure 6B). In the second scenario, the free polymers in the suspension are removed and the collector surface is also devoid of such a polymer. In this case, the polymers on the particle surface can serve as a bridge because the EDL interaction due to the charge on the core surface is significantly lower at the contact frontier. In other words, it takes a much lower kinetic energy for the particle to overcome the energy barrier to reach the contact frontier than to reach the core surface without any coating. This is the focused scenario of our current study and is depicted in Figure 6C. Following the above descriptions, it then seems that whether removing the free polymer or not after particle synthesis is also key to the affinity of these particles for a foreign surface. If the particles have to be purified by removing free polymer in the solution, high affinity between a particle and a surface would be expected even though the polymer coatings still effectively protect the particle from coagulation. For the effect of NOM on particle deposition, there are also two different scenarios: The first scenario depicts a deposition process with a higher concentration of NOM which suffices to cover both the particle

Figure 6. Different modes by which polymer affects the interaction between particles and between a particle and a surface: (A) steric stabilization between two polymer-anchored particles, (B) steric stabilization between a particle and a surface both coated with polymeric stabilizers, (C) interaction between a nanoparticle with an anchored polymer coating and an uncoated surface (bridging is possible if the polymer can adsorb onto the collector surface), (D) steric stabilization provided by a high concentration of adsorbing polymers in suspension, (E) bridging destabilization provided by a low concentration of adsorbing polymers in suspension.

and the collector surface (Figure 6D), in which case steric stabilization would be expected. The second scenario features a deposition process with lower polymer concentration which could likely lead to bridging destabilization (Figure 6E). Besides the mode by which the polymer (or macromolecule) interacts with the particle and surface as discussed above, the properties of the polymers (or macromolecule) are also very important. As pointed out in the earlier analysis, the approaches 4183

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both be relatively insignificant on the basis of the existing theories. In this case, perhaps those small organic molecules would have their primary effect on the colloidal interaction by altering the surface potential, which in turn affects the EDL interaction.48,52 Finally, we have to point out that while the effect of NOM on the colloidal interaction of bare particles has been widely studied and relatively well-understood, the effect of NOM on the colloidal stability of polymercoated nanoparticles has not yet been explored. It is likely that the NOM would replace the existing polymer, if its affinity for the surface is stronger, or would interact with the polymer. Depending on the mechanism of NOM−particle interaction, its effect on the colloidal stability of polymercoated particles might be very different, which is worth exploring in future research.

to calculate the EDL interaction energy prior to contact between (1) a particle coated with an uncharged polymer and an uncoated surface and (2) a particle coated with a polyelectrolyte and an uncoated surface are quite different. As for the postcontact interaction, a set of expressions quantifying the energy of steric stabilization for the interaction between two identical spherical particles coated with polymers are given as follows without considering the charge on the coating:51 Vs , mix = 0; Vs , el = 0

(4)

if H > 2δ Vs , mix =

2 4πakBT 2 ⎛ 1 ⎞⎛ H⎞ Φ p⎜ − χ ⎟⎜δ − ⎟ ; Vs , el = 0 ⎝2 ⎠⎝ υs δ⎠

(5)

■

if 2δ ≥ H > δ 4πakBT 2 ⎛ 1 ⎛ H ⎞⎞ ⎞ ⎛H 1 Φ p⎜ − χ ⎟δ 2⎜ − − ln⎜ ⎟⎟ ; Vs , mix = ⎝ ⎠ ⎝ δ ⎠⎠ ⎝ υs 2 2δ 4

CONCLUSIONS Polymeric coatings, either uncharged (as PVP) or charged (as GA), that are engineered to nanoparticle surfaces as steric or electrosteric stabilizers are shown to be able to stabilize AgNPs against aggregation effectively. The CCC of AgPVP was significantly higher than that of the uncoated AgBare, and aggregation of AgGA was not detectable even at very high IS, although the EPMs of these particles were not significantly different. However, when it comes to the deposition of these AgNPs onto an uncoated silica surface, a stabilization effect enabled by PVP and GA similar to that in aggregation was not observed. The affinity between the coated AgNPs and the uncoated surface was relatively high due to the absence of an osmotic type of steric stabilization. Steric stabilization can be recovered either by introducing free polymers in the background solution during the deposition process or by precoating the collector surface with these polymers during the equilibration process. These comparisons between aggregation and deposition, and between depositions under different conditions, emphasize the importance of having polymer coatings on both interacting surfaces for their effectiveness in functioning as steric stabilization agents.

(6)

if δ ≥ H ⎧ ⎡ ⎛ H ⎞2 ⎤ ⎪H ⎢H⎜3 − δ ⎟ ⎥ 4πakBTδ 2ρP ΦP ⎪ ⎨ ln⎢ ⎜ Vs , el = ⎟ ⎥ MwP ⎪δ ⎢δ⎜ 2 ⎟ ⎥ ⎪ ⎠ ⎥⎦ ⎢⎣ ⎝ ⎩ ⎫ H⎞ ⎛ ⎪ ⎜3 − δ ⎟ ⎛ ⎞⎪ H − 6ln⎜ ⎟ + 3⎜⎝1 − ⎟⎠⎬ δ ⎪ ⎜ 2 ⎟ ⎪ ⎝ ⎠ ⎭

(7)

if δ ≥ H Vsteric = Vs , mix + Vs , el

(8)

where a is the radius of the core, H is the separation between the core surface, δ is the thickness of the coating layer, ρp is the polymer density, MWp is the molecular weight of the adsorbed polymer, Φp is the volume density, vs is the molar volume of the solvent, which is around 55 mL for water, χ is the Flory− Huggins solvency parameter, VS,mix is the contribution to the steric interaction energy from mixing of polymers (excluded the volume effect), and VS,el is the contribution from the compression of the coating layer. The derivation of these expressions assumes that the segment density is uniform throughout the coating layer and that the interacting coating layers are identical. Clearly, these expressions consider only the entropic contributions due to mixing and compression, and they are usually used as being superimposed onto the vdW interaction and EDL interaction calculated using expressions assuming bare surfaces. They do not, and no established theory to the best of our knowledge does, account for any effect due to the additional electrostatic interaction due to the crowding of similar charged polyelectrolytes during interpenetration for the postcontact electrosteric interaction between polyelectrolytecoated particles. It should be expected that, due to such an extra electrostatic interaction, polyelectrolyte would probably be a more effective steric stabilizer than its uncharged counterpart of similar molecular weight and morphology. In addition, molecular weight is also critical. Most of our discussions above assume a polymer of large molecular weight. If small organic molecules (and thus small coating thickness δ) are instead concerned, the shifted frontier effect or bridging effect (prior to contact) and the steric effect (postcontact) should

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ASSOCIATED CONTENT

S Supporting Information *

Details on silver nanoparticle synthesis and characterizations (Figure S1−S5), breakthrough curves of column experiments (Figure S6), solution chemistry used in aggregation and deposition experiments (Table S1), parameters for column experiments (Table S2), determination of the Hamaker constant for layered materials, and interaction potential calculation. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (919) 660-5292. Fax: (919) 660-5219.

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ACKNOWLEDGMENTS This paper is based upon work supported by the National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-0830093, Center for the Environmental Implications of NanoTechnology. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review, and no official endorse4184

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ment should be inferred. We thank Rixiang Huang from Baylor University for giving useful comments.

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