Diffusion-limited Conditions Research Articles

Dynamics of colloidal aggregation in microgravity by critical Casimir forces

By combining static and dynamic structure factor measurements under microgravity conditions, we obtain for the first time direct insight into the internal structure of colloidal aggregates formed over a wide range of particle attractions under ideal diffusion-limited conditions. By means of near-field scattering we measure the time-dependent density-density correlation function as the aggregation process evolves, and we determine the ratio of the hydrodynamic and gyration radius to elucidate the aggregate's internal structure as a function of its fractal dimension. Surprisingly, we find that despite the large variation of particle interactions, the mass is always evenly distributed in all objects with fractal dimension ranging from 2.55 for shallow potentials to 1.78 for deep ones.

Effects of Chloride on the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) represents perhaps the best studied redox process in electrocatalysis because of its key role in fuel cells1 and, more recently, metal-air batteries2. As is well known, the problems associated with improving the performance of low and intermediate temperature fuel cells are associated with the lack of suitable catalysts that can reduce the overpotential for the ORR. This multi-electron transfer process has been found to be highly sensitive not only to the detailed electrocatalyst microstructure, but also to the presence of adsorbed species on the surface. Particularly significant are the deleterious effects derived from the adsorption of trace amounts of ionic3,4 and molecular species5. In particular, trace amounts of halides can poison the four-electron reduction of dioxygen, promoting instead, the two-electron reduction to peroxide. This work describes a new approach toward investigating the effect of adsorbed chloride on the ORR on polycrystalline Pt in 0.1 M HClO4 using a rotating ring-disk electrode coupled with transient techniques. EXPERIMENTALMeasurements were performed in a conventional all-glass, three-compartment electrochemical cell with a rotating Pt ring- Pt disk electrode, Pt|Pt RRDE, (Pine Instruments, (A = 0.164 cm2) using a bipotentiostat (Pine Instruments). A Pt foil and a reversible hydrogen electrode in the same solution (RHE) were used as counter and reference electrodes, respectively. Oxygen reduction polarization curves were recorded in oxygen saturated 0.1 M HClO4.RESULTS AND DISCUSSIONShown in the top panel, Fig.1 are polarization curves recorded at a scan rate, ν = 20 mV/s for a Pt disk electrode in oxygen saturated 0.1 M HClO4in the absence (black curve) and in the presence of 20 μM (red curve) and 100 μM NaCl (blue curve). The corresponding currents, measured with the ring polarized at a potential positive enough to detect hydrogen peroxide oxidation under diffusion limited conditions (ca. 1.2 V) are shown in the bottom panel in this figure. As evident from these results, the presence of chloride in the electrolyte leads not only to a significant shift in the onset of oxygen reduction, but also to the generation of hydrogen peroxide. Particularly interesting is the peak-like feature observed in the ring current as a function of the disk potential. Correlations between the chloride coverage and the ORR rate were sought by implementing a method in which the potential was stepped from 0.05 V, a value at which there is no chloride adsorption, to various more positive potentials where chloride adsorption is known to ensue in solutions devoid of oxygen. The results of these experiments, shown in Panel A, Fig. 2, indicate that for certain potentials, the current attains a limiting value associated with the diffusion controlled adsorption of chloride and later decays down to zero signaling that the equilibrium coverage at that potential was reached. Additional insight into this phenomenon was obtained from theoretical simulations performed within the COMSOL platform by coupling the convective diffusion equation with the surface dynamics for the adsorption|desorption process. This strategy yielded transient curves (see Panel B, Fig. 2) which strongly resemble those found experimentally. Efforts are now underway to explore whether a more detailed analysis of the curves will afford reliable values of the kinetic parameters involved.ACKNOWLEDGEMENTSThis work was supported by a grant from NSF.REFERENCES1. J. Zhang, Frontiers in Energy, 5, 137, 2011.2. Wang, Z.-L.; Xu, D.; Xu, J.-J.; Zhang, X.-B., Chem. Soc. Rev. 2014. 3. N. M. Markovic, H. A. Gasteiger, B. N. Grgur, and P. N. Ross, J. Electroanal.Chem., 467, 157, 1999.4. T. J. Schmidt, U. A. Paulus, H. A. Gasteiger, and R. J. Behm, J. Electroanal. Chem., 508, 41, 2001.5. N. M. Markovic, R. R. Adzic, B. D. Cahan, and E. B. Yeager, J. Electroanal.Chem., 377, 249, 1994. Figure Captions: Fig. 1. Polarization curves recorded at a scan rate, ν = 20 mV/s for a Pt disk electrode in oxygen saturated 0.1 M HClO4 in the absence (black) and in the presence of 20 μM (red) or 100 μM (blue) NaCl. The corresponding ring currents recorded when the ring was polarized at Ering= 1.2 V are shown in the lower panel. Fig. 2. Panel A: Chronoamperometric curves recorded in 0.1 M HClO4 with 20μM NaCl at a rotation rate of ɷ= 400 rpm starting from Eini=0.05 V to the various final potentials as indicated in the legend. Panel B : Simulated currents for kads/kdes =106/M, 900 rpm, C0=.1mM, Γmax=2x10-9mol/cm2 and for various values for the adsorption rate (legend). Limiting current is consistent with Levich equation.

Effect of diffusion limitations on multianalyte determination from biased biosensor response.

The optimization-based quantitative determination of multianalyte concentrations from biased biosensor responses is investigated under internal and external diffusion-limited conditions. A computational model of a biocatalytic amperometric biosensor utilizing a mono-enzyme-catalyzed (nonspecific) competitive conversion of two substrates was used to generate pseudo-experimental responses to mixtures of compounds. The influence of possible perturbations of the biosensor signal, due to a white noise- and temperature-induced trend, on the precision of the concentration determination has been investigated for different configurations of the biosensor operation. The optimization method was found to be suitable and accurate enough for the quantitative determination of the concentrations of the compounds from a given biosensor transient response. The computational experiments showed a complex dependence of the precision of the concentration estimation on the relative thickness of the outer diffusion layer, as well as on whether the biosensor operates under diffusion- or kinetics-limited conditions. When the biosensor response is affected by the induced exponential trend, the duration of the biosensor action can be optimized for increasing the accuracy of the quantitative analysis.

Unravelling the Impact of Obstacles in Diffusion and Kinetics of an Enzyme Catalysed Reaction

Lattice gas automata simulations of diffusion-limited reactions in heterogeneous media exhibit fractal-like kinetics, which is a generalised mass action kinetics with time-dependent rate constants.1,2,3 We develop a two dimensional lattice gas automata simulation of the Michaelis-Menten mechanism in diffusion-limited conditions to investigate the effect of density and size of obstacles on reactant diffusion and rate coefficients. In order to simulate more physicochemical realistic conditions, reactants rotate and interact according to their specific orientation. We also model weak interaction forces between reactants and obstacles. Our results show that obstacle density and size affect diffusion, first- and second- order rates. We also find that particle rotations and weak force interactions among particles lead to a significant decay in the fractal-like kinetic exponent h. These results suggest that the effects of fractal-like kinetics disappear under less restricted conditions than previously believed in lattice based simulations. [1] S. Schnell and T. E. Turner, Prog Biophys Mol Biol, 2004, 85, 235-260. [2] R. Kopelman, J Stat Phys, 1986, 42, 185-200. [3] R. Kopelman, Science, 1988, 241, 1620-1626.

Unravelling the impact of obstacles in diffusion and kinetics of an enzyme catalysed reaction

Lattice gas automata simulations of diffusion-limited reactions in heterogeneous media exhibit fractal-like kinetics, which is a generalised mass action kinetics with time-dependent rate constants. We develop a two dimensional lattice gas automata simulation of the Michaelis-Menten mechanism in diffusion-limited conditions to investigate the effect of density and size of obstacles on reactant diffusion and rate coefficients. In order to simulate more physicochemical realistic conditions, reactants rotate and interact according to their specific orientation. We also model weak interaction forces between reactants and obstacles. Our results show that obstacle density and size affect diffusion, first- and second-order rates. We also find that particle rotations and weak force interactions among particles lead to a significant decay in the fractal-like kinetic exponent h. These results suggest that the effects of fractal-like kinetics disappear under less restricted conditions than previously believed in lattice based simulations.

Electrodeposition of silver on stepped platinum electrode surfaces with (1 0 0)-oriented terraces: Generation of confined reaction sites

Ag monolayers and sub-monolayers were deposited onto Pt(s)[n(100)×(111)] electrodes both in the over potential deposition (OPD) region under diffusion limited conditions and in the under potential deposition (UPD) region. Diffusion limited deposition from Ag+ concentrations in the micromolar range was monitored by the suppression of the hydrogen adsorption peaks; selective suppression of the peaks for hydrogen adsorption at (100) terraces sites as opposed to that at (111) step sites demonstrates that Ag is preferentially deposited at terrace sites. Underpotential deposition gives rise to three peaks: Two large peaks correspond to Ag deposition in the first and the second layer. An additional small peak increases linearly with step density and is due to Ag deposition at the (111) step sites. Its peak potential, which is lower than that of Ag deposition at terraces, again demonstrates the lower stability of Ag at step sites as compared to terrace sites. By controlling the upper potential limit for the oxidative removal of deposited Ag, Ag at steps can be selectively removed and reaction sites consisting of one dimensional rows of Pt atoms can be generated. At these, formic acid is oxidised at a high rate, whereas methanol cannot be oxidised at all. Poison formation (adsorption of CO) is inhibited completely for both fuels.

Spontaneous Formation of Two-Dimensional Micropatterns with Straight and/or Curving Dendrites through Crystal Growth of Ba(NO3)2 in Polymer Matrix

Microscale patterns consisting of two-dimensional (2D) dendrites with trunks and branches 1–3 μm wide were precisely controlled through crystal growth in a thin polymer matrix by the use of a dipping technique. A variety of 2D micropatterns, such as orthogonal lattices, bull’s horns, randomly curving weaves, and aligned dots, were homogeneously formed with an aqueous solution of Ba(NO3)2 and poly(vinyl alcohol) in a wide area ranging over several centimeters on flat and rounded substrates. The micrometric dendritic growth that produced these several patterns was tuned by changing the withdrawal rate and the polymer concentration. The crystallographic orientation of the micropatterns was characterized to discuss on the formation mechanism of the specific morphologies. The curving branches were found to be induced by gradual change in the growth direction through low-angle grain boundaries under a highly diffusion-limited condition. This simple, bottom-up patterning process is applicable for various crystalline materials, including inorganic and organic substances.

Electrochemical deposition of platinum within nanopores on silicon: Drastic acceleration originating from surface-induced phase transition

An electrochemical reaction within nanopores is remarkably decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. Here, we report a powerful method of overcoming this problem for electrochemical deposition of platinum within nanopores formed on silicon. We made the pore wall surface of the silicon electrode hydrophobic by covering it with organic molecules and adopted platinum complex ions with sufficiently large sizes. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface and rather hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using our statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition: The space within nanopores is abruptly filled with the second phase within which the ion concentration is orders of magnitude higher. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed, which can also be elucidated by our theory. The utilization of the surface-induced phase transition sheds new light on the design and control of a chemical reaction in nanospace.

Photocatalytic generation of hydrogen from water using a cobalt pentapyridine complex in combination with molecular and semiconductor nanowire photosensitizers

Recently, a family of cobalt pentapyridine complexes of the type [(R-PY5Me2)Co(H2O)])(CF3SO3)2, (R = CF3, H, or NMe2; PY5Me2 = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine) were shown to catalyze the electrochemical generation of hydrogen from neutral aqueous solutions using a mercury electrode. We now report that the CF3 derivative of this series, [(CF3PY5Me2)Co(H2O)](CF3SO3)2 (1), can also operate in neutral water as an electrocatalyst for hydrogen generation under soluble, diffusion-limited conditions on a glassy carbon electrode, as well as a photocatalyst for hydrogen production using either molecular or semiconductor nanowire photosensitizers. Owing to its relatively low overpotential compared to other members of the PY5 family, complex 1 exhibits multiple redox features on glassy carbon, including a one-proton, one-electron coupled oxidative wave. Further, rotating disk electrode voltammetry measurements reveal the efficacy of 1 as a competent hydrogen evolution catalyst under soluble, diffusion-limited conditions. In addition, we establish that 1 can also generate hydrogen from neutral water under photocatalytic conditions with visible light irradiation (λirr ≥ 455 nm), using [Ru(bpy)3]2+ as a molecular inorganic chromophore and ascorbic acid as a sacrificial donor. Dynamic light scattering measurements show no evidence for nanoparticle formation for the duration of the photolytic hydrogen evolution experiments. Finally, we demonstrate that 1 is also able to enhance the hydrogen photolysis yield of GaP nanowires in water, showing that this catalyst is compatible with solid-state photosensitizers. Taken together, these data establish that the well-defined cobalt pentapyridine complex [(CF3PY5Me2)Co(H2O)]2+ is a versatile catalyst for hydrogen production from pure aqueous solutions using either solar or electrical input, providing a starting point for integrating molecular systems into sustainable energy generation devices.

High Resolution Protein Crystals Using an Efficient Convection-Free Geometry

Macromolecular crystallography is the most direct and accurate approach to determine the three-dimensional structure of biological macromolecules. The growth of high quality single crystals, yielding diffraction to the highest X-ray resolution, remains a bottleneck in this methodology. Here we show that through a modification of the batch crystallization method, an entirely convection-free crystallization environment is achieved, which enhances the purity and crystallinity of protein crystals. This is accomplished by using an upside-down geometry, where crystals grow at the “ceiling” of a growth-cell completely filled with the crystallization solution. The “ceiling crystals” experience the same diffusion-limited conditions as in space microgravity experiments. The new method was tested on bovine insulin and two hen egg-white lysozyme polymorphs. In all cases, ceiling crystals diffracted X-rays to resolution limits beyond that for other methods using similar crystallization conditions without further optimization. In addition, we demonstrate that the ceiling crystallization method leads to crystals with much lower impurity incorporation.

The development of the depletion zone during ceiling crystallization: phase shifting interferometry and simulation results

The growth of high quality protein crystals is essential for the determination of their structure. This process is governed by many physical factors such as mass transport and solution flow. The quality of the crystals is usually better under diffusion-limited growth conditions, where a depleted zone of the solution encapsulates the crystal. We developed a Mach–Zehnder-based phase shifting interferometer coupled to image processing software to study the concentration gradients which develop around a crystal during its growth or dissolution. The depletion zones and the diffusion boundary layers around growing and dissolving KH2PO4 crystals are monitored and processed by a MATLAB based algorithm. Our main emphasis was to analyze the ceiling crystallization conditions in which the crystal is placed at the very top of the growth cell and therefore the solute transport is largely diffusion-limited. The experimental results are compared with simulations using finite element-based numerical calculations. The combined results clearly demonstrate the positive effect of the ceiling crystallization approach on crystal growth.

Spontaneous Formation of Sinuous and Wavy Micropatterns with Helical Growth of CuSO4·5H2O in Polymer Matrix

Various 2-D patterns of copper sulfate pentahydrate (CuSO4·5H2O) were spontaneously formed through crystal growth in a thin gelatin matrix on a glass plate. The form of the grown crystal changes from polyhedrons to irregularly branching morphologies with increasing the polymer density in the matrix. Characteristic patterns, such as sinuous dendrites and wavy lattices, are obtained under specific intermediate conditions. The formation of periodically curving sinuous trunks is based on the helical growth with a pile of tilted units of CuSO4·5H2O. The wavy microlattices are produced with periodical branching of the sinuous trunks. The characteristic 2-D patterns with the curving trunks and branches are achieved with the controlled growth of the triclinic crystal under a diffusion-limited condition in the polymer matrix.

Kinetics of n-hexane cracking over ZSM-5 zeolites – Effect of crystal size on effectiveness factor and catalyst lifetime

The catalytic cracking of n-hexane over ZSM-5 zeolite (MFI-type zeolite, Si/Al=150 and 240) catalysts was examined at reaction temperatures ranging from 823 to 923K under atmospheric pressure. The reaction rate constants and activation energies of n-hexane cracking over ZSM-5 zeolites with various crystal sizes and Si/Al ratios were evaluated. The catalytic cracking of n-hexane was first order with respect to the n-hexane concentration, and the activation energies of n-hexane cracking over ZSM-5 zeolites were found to be approximately 123–128kJmol−1. Compared with the macro-sized zeolite, the nano-sized zeolites exhibited high n-hexane conversion with stable activity for 50h. This is because the cracking reaction with nano-sized zeolite proceeded under reaction-limiting conditions, whereas the reaction with macro-sized zeolite proceeded under transition conditions between reaction- and diffusion-limiting conditions. As a result, the application of nano-zeolite to the catalytic cracking of n-hexane was effective and gave light olefins with high yield and excellent stable activity.

Platinum electrodeposition in porous silicon: The influence of surface solvation effects on a chemical reaction in a nanospace

A chemical reaction in a nanospace is decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. We illustrate how to overcome this problem for platinum electrodeposition within nanoporous silicon electrodes. The surface-induced hydration structure of reactants is essential. We make nanopore surfaces hydrophobic by covering them with organic molecules and adopt platinum complex ions with sufficiently large sizes as reactants. Due to the resulting fact that the ions are strongly excluded from the bulk solution to the surface, the ion concentration is greatly enriched within nanopores. This enrichment accelerates the reaction.

How Chain Plasmons Govern the Optical Response in Strongly Interacting Self-Assembled Metallic Clusters of Nanoparticles

Self-assembled clusters of metallic nanoparticles separated by nanometric gaps generate strong plasmonic modes that support both intense and localized near fields. These find use in many ultrasensitive chemical and biological sensing applications through surface enhanced Raman scattering (SERS). The inability to control at the nanoscale the structure of the clusters on which the optical response crucially depends, has led to the development of general descriptions to model the various morphologies fabricated. Here, we use rigorous electrodynamic calculations to study clusters formed by a hundred nanospheres that are separated by ∼1 nm distance, set by the dimensions of the macrocyclic molecular linker employed experimentally. Three-dimensional (3D) cluster structures of moderate compactness are of special interest since they resemble self-assembled clusters grown under typical diffusion-limited aggregation conditions. We find very good agreement between the simulated and measured far-field extinction spectra, supporting the equivalence of the assumed and experimental morphologies. From these results we argue that the main features of the optical response of two- and three-dimensional clusters can be understood in terms of the excitation of simple units composed of different length resonant chains. Notably, we observe a qualitative difference between short- and long-chain modes in both spectral response and spatial distribution: dimer and short-chain modes are observed in the periphery of the cluster at higher energies, whereas inside the structure longer chain excitation occurs at lower energies. We study in detail different configurations of isolated one-dimensional chains as prototypical building blocks for large clusters, showing that the optical response of the chains is robust to disorder. This study provides an intuitive understanding of the behavior of very complex aggregates and may be generalized to other types of aggregates and systems formed by large numbers of strongly interacting particles.

Unsteady-State Mathematical Modeling of a Fungal Biofilter Treating Hexane Vapor at Different Operating Temperatures

Fungal biofiltration is an efficient way to treat hydrophobic compounds. Although temperature is a key factor in the biofiltration, the process modeling based on temperature effect in the kinetic parameters has been rarely considered due to its complexity. Therefore, in this contribution, a dynamics mathematical model which takes into account dispersion in gas phase, diffusion in biofilm, and temperature effect in the kinetic constants and other physical properties has been presented. The model was calibrated and validated by using previously reported experimental research in which the effect of temperature and intermittent and continuous loading on the biodegradation of n-hexane vapor had been investigated. The model prediction results revealed that a zero-order kinetic reaction fitted intermittent loading in the temperature range of 30–35 °C and continuous loading in the temperature range of 35–45 °C. Sensitivity analysis showed that at high inlet loads, approaching optimum bed temperature could change the rate limited biodegradation to a diffusion limited condition, and a part of the biofilm became inactive.

Twisted growth of organic crystal in a polymer matrix: sigmoidal and helical morphologies of pyrene

The crystal morphology of pyrene formed in a solution changes from polyhedral shapes surrounded with flat planes into characteristic curving forms, such as sigmoidal and helical architectures, in the presence of polyvinyl pyrrolidone. Dendritic patterns and linear alignments of lozenge units are produced under the intermediate conditions. The formation of hierarchical structures with increasing the polymer concentration is based on a pile of pyrene units. The curving forms are inferred to be constructed through a twisted assembly of asymmetric units under diffusion-limited conditions. Because similar morphological variation was observed in other systems, such as chrysazin–polyvinyl pyrrolidone and pyrene–polystyrene, the controlled growth of organic crystals using the polymer matrix would provide a novel type of functional materials having various curving morphologies.

Electrochemical generation of hydrogen from acetic acid using a molecular molybdenum–oxo catalyst

We recently reported the catalytic generation of hydrogen from water mediated through the in situ reduction of the molybdenum(IV)–oxo complex [(PY5Me2)MoO]2+ (1; PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) at a mercury electrode. To gain further insight into this unique molecular motif for hydrogen production, we have now examined the competence of this complex for the catalytic reduction of protons on an alternative electrode material. Herein, we demonstrate the ability of the molybdenum–oxo complex 1 to reduce protons at a glassy carbon electrode in acidic organic media, where the active catalyst is shown to be diffusing freely in solution. Cyclic and rotating disk voltammetry experiments reveal that three reductive electrochemical processes precede the catalytic generation of hydrogen, which occurs at potentials more negative than −1.25 V vs. SHE. Gas chromatographic analysis of the bulk electrolysis cell headspace confirms that hydrogen is generated at a Faradaic efficiency of 99%. Under pseudo-first order conditions with an acid-to-catalyst ratio of >290, a rate constant of 385 s−1 is calculated for the reduction of acetic acid in acetonitrile. Taken together, these data show that metal–oxo complex 1 is a competent molecular motif for catalytic generation of hydrogen from protons under soluble and diffusion-limited conditions.

Predictive Model for Diffusion-Limited Aggregation Kinetics of Nanocolloids under High Concentration

Smoluchowski's equation for the rate of aggregation of colloidal particles under diffusion-limited conditions has set the basis for the interpretation of kinetics of aggregation phenomena. Nevertheless, its use is limited to sufficiently dilute conditions. In this work we propose a correction to Smoluchowski's equation by using a result derived by Richards ( J. Phys. Chem. 1986 , 85 , 3520 ) within the framework of trapping theory. This corrected aggregation kernel, which accounts for concentration dependence effects, has been implemented in a population-balance equations scheme and used to model the aggregation kinetics of colloidal particles undergoing diffusion-limited aggregation under concentrated conditions (up to a particle volume fraction of 30%). The predictions of population balance calculations have been validated by means of Brownian dynamic simulations. It was found that the corrected kernel can very well reproduce the results from Brownian dynamic simulations for all concentration values investigated, and is also able to accurately predict the time required by a suspension to reach the gel point. On the other hand, classical Smoluchowski's theory substantially underpredicts the rate of aggregation as well as the onset of gelation, with deviations becoming progressively more severe as the particle volume fraction increases.

Aggregation Kinetics and Dissolution of Coated Silver Nanoparticles

Determining the fate of manufactured nanomaterials in the environment is contingent upon understanding how stabilizing agents influence the stability of nanoparticles in aqueous systems. In this study, the aggregation and dissolution tendencies of uncoated silver nanoparticles and the same particles coated with three common coating agents, trisodium citrate, sodium dodecyl sulfate (SDS), and Tween 80 (Tween), were evaluated. Early stage aggregation kinetics of the uncoated and coated silver nanoparticles were assessed by dynamic light scattering over a range of electrolyte types (NaCl, NaNO(3), and CaCl(2)) and concentrations that span those observed in natural waters. Although particle dissolution was observed, aggregation of all particle types was still consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of citrate-coated particles and SDS-coated particles were very similar to that for the uncoated particles, as the critical coagulation concentrations (CCC) of the particles in different electrolytes were all approximately the same (40 mM NaCl, 30 mM NaNO(3), and 2 mM CaCl(2)). The Tween-stabilized particles were significantly more stable than the other particles, however, and in NaNO(3) aggregation was not observed up to an electrolyte concentration of 1 M. Differences in the rate of aggregation under diffusion-limited aggregation conditions at high electrolyte concentrations for the SDS and Tween-coated particles, in combination with the moderation of their electrophoretic mobilities, suggest SDS and Tween imparted steric interactions to the particles. The dissolution of the silver nanoparticles was inhibited by the SDS and Tween coatings, but not by the citrate coating, and in chloride-containing electrolytes a secondary precipitate of AgCl was observed bridging the individual particles. These results indicate that coating agents could significant influence the fate of silver nanoparticles in aquatic systems, and in some cases these stabilizers may completely prevent particle aggregation.