Grantee Research Project Results
2002 Progress Report: Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle Forces
EPA Grant Number: R829605Title: Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle Forces
Investigators: Velegol, Darrell , Fichthorn, Kristen
Institution: Pennsylvania State University
EPA Project Officer: Aja, Hayley
Project Period: February 1, 2002 through January 31, 2004 (Extended to January 31, 2005)
Project Period Covered by this Report: February 1, 2002 through January 31, 2003
Project Amount: $370,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Nanotechnology
Objective:
Nanotechnology promises tremendous advances in electronic circuits, superstrong ceramics, optical imaging, and gene vector materials. However, the transition from the laboratory to the storeshelf has a critical barrier: nanoparticles spontaneously aggregate, negating their beneficial properties. Various methods have been used to stabilize particles, but all have involved dispersant molecules such as surfactants or polyelectrolytes. Not only do these dispersants alter the chemistry and physics of the nanoparticle systems, they also produce a tremendous waste stream during burnoff because they occupy a significant (>50 percent) mass fraction of a nanoparticle system.
The expected engineering breakthrough of the proposed research is to identify whether solvation or depletion forces can be manipulated to produce dispersed suspensions of "bare" nanoparticles (i.e., without adsorbed additives). A positive result will avert a huge waste stream of additives that would otherwise be necessary to stabilize nanoparticle systems. The central scientific questions to be answered are: What are the magnitudes of the van der Waals, solvation, and depletion forces for nanoparticle systems? What variables can we control to alter these forces? The research will involve two primary components: (1) development and use of "particle force light scattering" (PFLS) for measuring subpiconewton nanoparticle forces; and (2) use of molecular dynamics (MD) simulations to predict the solvation forces. The specific objectives of this research project include:
· Molecular dynamics modeling to delineate the magnitude of van der Waals, solvation, and depletion forces for nanoparticles systems; the modeling will demonstrate the pertinent variables that control these forces. Large-scale, parallel molecular dynamics simulations are required for these large simulations.
· PFLS to measure the small nanoparticle forces. This involves the construction of a small-angle light scattering device coupled with an electrophoresis cell and solutions of the electrokinetic equations for multiple particles.
· Results from the MD modeling to test specific nanoparticle systems and to take measurements of nanoparticle forces between silica, titania, and barium titanate particles in water.
Synergy is essential to this research-PFLS is the first technique capable of measuring the nanoparticle forces and molecular dynamics enables the interpretation of exactly how the forces are acting.
Progress Summary:
Nanoparticle forces are subpiconewton in magnitude. This is more than two orders of magnitude below the resolution of atomic force microscopy. Of the methods available for measuring such small forces between Brownian particles with small interparticle gaps, the best is perhaps "differential electrophoresis." For two Brownian particles held together by colloidal forces, the technique can measure attractive interparticle forces with subpiconewton resolution by measuring doublet breakup in an electric field (see Figure 1). The technique previously used to measure doublet breakup (video microscopy) is inadequate for visualizing nanoparticle systems and the fluid mechanics results required to interpret the experiments will not extend from doublets to triplets. Thus, the experimental work has two components: (1) derive the force expression from the electrokinetic equations for triplets of nanoparticles; and (2) develop a Rayleigh light scattering device to measure nanoparticle triplet breakup (similar to the system used to measure colloidal stability).
Figure 1. Time Evolution of a Triplet Aggregate Breaking Under the Influence of an Electric Field. The solution conditions for this case were: 4.5 mm sulfated and carboxylated polystyrene latex sphere particles, 10 mM KCl, 10 µM NaPSS, and pH = 2.5. These particles are super-micron, but nanoparticles require a different method of "visualization."
The first of these objectives-extending to triplets the equations needed to interpret the force measurements-has been accomplished (Holtzer and Velegol, Langmuir 2003). The tension applied to triplets using differential electrophoresis is:
The theoretical result is almost identical to that for doublets, except that the coefficient is 8.76 instead of 7.94. By using 8.30 as the coefficient, we can approximate doublets and triplets with the same equation, which simplifies experimental protocol greatly (i.e., we do not need to distinguish doublets from triplets). The equation above was tested for triplets of micron-size particles to examine the interaction forces in the presence of a polyelectrolyte (sodium polystyrene sulfonate). Similar forces were found between particles in the triplets as compared to doublets, giving support to this equation. The forces for the triplets are shown in Figure 2.
Our initial work has focused on solid nanoparticles in Lennard-Jones (liquid) solvent. In this nonpolar fluid, we are interested in understanding the interplay between attractive van der Waals dispersion forces, which cause undesirable nanoparticle aggregation, and solvation forces, which could be either attractive or repulsive. Van der Waals forces are considered in current theories of colloidal forces such as Derjaguin and Landau (1941) and Vervey and Overbeek (1948) (DLVO) theory. However, it is questionable whether or not these theories, which were designed for micron-size particles, are appropriate for nanoparticles. The role of solvation forces in determining colloidal stability is not well understood, and our work provides insight into these forces.
Figure 2. Plot of the Critical Force of Each Triplet as a Function of NaPSS Concentration for 1.5-micron PSL Particles. The solution had 10 mM KCl and pH = 5. Filled circles represent triplets that broke into a doublet and a singlet, while open circles represent triplets that did not break. Note that some of the aggregates broke with forces of O(1 pN).
We used large-scale, parallel, MD simulations to simulate colloidal nanoparticles in Lennard-Jones liquid and to quantify their van der Waals and solvation forces. Solid nanoparticles were investigated with two different sizes and shapes (spherical particles with diameters of ~ 1 and 5 nm and a cubic particle with an edge length of ~ 5 nm). Additionally, the role of the nanoparticle-solvent interaction was investigated through the simulation of "solvophobic" and "solvophilic" nanoparticles. We utilized three different methods to calculate van der Waals forces between the nanoparticles: Bradley’s equation, Hammaker's formula, and direct evaluation of the pair-wise additive dispersion interactions. We found that Hammaker’s formula is accurate, even for the smallest nanoparticle, at separations as close as 3 fluid-molecule diameters.
We used a variant of the established thermodynamic integration method to obtain the potential of mean force due to solvation and the free energy of solvation for the various nanoparticle systems. Colloidal nanoparticles that are solvophobic experience primarily attractive interactions, due to the depletion of solvent in the region between two particles. For all particles, the solvation forces for solvophilic (solvent loving) nanoparticles oscillate between attraction and repulsion as a function of particle separation. These oscillatory forces can be linked to oscillations in the solvent density in the region between the two nanoparticles. We found that surface roughness influences the phase of the oscillatory interactions. Comparing forces between rough, spherical, and cubic nanoparticles, with flat fcc(111)-contacting surfaces, we found that the solvation forces between the cubic nanoparticles are significantly stronger. The solvation forces between solvophilic nanoparticles are comparable to the van der Waals forces. Figure 3 shows the van der Waals and solvation forces for cubic solvophilic nanoparticles.
Figure 3. Van der Waals and Solvation Forces for Cubic, 5-nm, Solvophilic Nanoparticles
Our findings indicate that solvation forces could play an important role in determining the stability of colloidal nanoparticle suspensions. By engineering the solvent-nanoparticle interaction and by carefully choosing the nanoparticle shape, it may be possible to achieve stable suspensions or assemblies of bare nanoparticles. This would considerably reduce the waste associated with the common practice of adsorbing molecules on nanoparticle surfaces to prevent them from aggregating or to achieve their selective assembly.
Future Activities:
Experimentally, we have now ordered all of the equipment required to construct the PFLS apparatus. This includes small-angle light scattering equipment, along with equipment required to enable the light scattering to be coupled with electrophoresis. Some of this equipment has arrived. Experimental steps currently being conducted are the: (1) examination of nanoparticles in differential electrophoresis using fluorescence techniques; and (2) construction of the PFLS device.
For the modeling component of the research, work is underway to study nanoparticle forces in n-alkanes, which represent a more complex model of solvent. We are currently modifying our MD code to handle these molecules as solvent. With n-alkane chains as solvent, it is possible for the length of a chain to become comparable to the diameter of a nanoparticle, and it is unclear what the relative roles of solvation and van der Waals forces will be in this case.
In the simulation component of the research, we used molecular dynamics simulations to characterize the interaction between two model nanoparticles (Lennard-Jones Au solids) immersed in solvent (Lennard-Jones spheres, n-alkanes, and water) in the presence and absence of polymer. We will quantify the interaction as a function of particle size, particle separation, fluid type, and polymer model. One anticipated outcome of the proposed research is increased fundamental insight into the interaction between two nonpolar nanoparticles in various fluids. A second anticipated outcome is that we will develop a new simulation method to quantify interparticle forces. Both depletion and solvation forces will be modeled, and the results will be used to develop the experiments.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 36 publications | 7 publications in selected types | All 7 journal articles |
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Type | Citation | ||
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Holtzer GL, Velegol D. Force measurements between colloidal particles of identical zeta potentials using differential electrophoresis. Langmuir 2003;19(10):4090-4095. |
R829605 (2002) R829605 (Final) |
not available |
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Qin Y, Fichthorn KA. Molecular-dynamics simulation of forces between nanoparticles in a Lennard-Jones liquid. The Journal of Chemical Physics 2003;119(18):9745-9754. |
R829605 (2002) R829605 (2003) R829605 (Final) |
not available |
Supplemental Keywords:
green chemistry, clean technologies, waste reduction, waste minimization, chemical engineering, physics, measurement methods, modeling, nanoparticle, nanoparticle forces, nanoparticle dispersion, nanoparticle stability, molecular dynamics, particle force light scattering, differential electrophoresis, molecular dynamics simulations., RFA, Scientific Discipline, Sustainable Industry/Business, Sustainable Environment, Environmental Chemistry, Physics, Chemistry, Technology for Sustainable Environment, Analytical Chemistry, New/Innovative technologies, Chemistry and Materials Science, Engineering, Environmental Engineering, molecular dynamics, particle force light scattering, green engineering, nanotechnology, environmental sustainability, environmentally applicable nanoparticles, differential electrophoresis, sustainability, innovative technology, nanoparticle forcesRelevant Websites:
http://www.personal.psu.edu/dxv9 Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.