Grantee Research Project Results
2003 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, 2003 through January 31, 2004
Project Amount: $370,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Nanotechnology
Objective:
Laboratory work with nanoparticles has demonstrated their magnificent electrical, magnetic, mechanical, and optical properties, but a key barrier preventing the commercial use of nanoparticles is that they tend to aggregate. This research project involves measuring and modeling some of the fundamental forces between nanoparticles: van der Waals forces, solvation forces, and depletion forces. The expected engineering breakthrough of the proposed research project is to identify whether solvation or depletion forces can be manipulated to produce dispersed suspensions of “bare” nanoparticles (i.e., without adsorbed additives). The specific objectives of this research project are to: (1) develop particle force light scattering (PFLS) to measure nanoparticle forces; and (2) conduct molecular-dynamics simulations to predict van der Waals, solvation, and depletion forces between nanoparticles. A positive result will avert a huge waste stream of additives that otherwise would be necessary to stabilize nanoparticle systems.
Progress Summary:
The nanoparticle force measurements have been extended to triplets (Holtzer and Velegol, 2003), as shown in Figure 1. This was a vital first step to measuring nanoparticle forces because it shows that we can measure forces well between aggregates. We also have solved the electrokinetic equations for triplets, obtaining a result very similar to that for doublets.
This year, we have constructed the PFLS apparatus, which we currently are calibrating to measure forces between nanoparticles. In this apparatus, we raise the applied electric field to the critical value at which we break apart nanoparticle clusters. The critical force is a “visualized” sudden change in the light scattering signal (see Figure 2), and this force measurement enables the Velegol group to test the modeling done by the Fichthorn group.
Large-scale, parallel, molecular-dynamics simulations were used to simulate colloidal nanoparticles in Lennard-Jones liquid and to quantify their van der Waals and solvation forces. We use a variant of the established thermodynamic integration method to obtain the potential of mean force resulting from solvation and the free energy of solvation for the various nanoparticle systems. Colloidal nanoparticles that are solvophobic experience primarily attractive interactions because of the depletion of solvent in the region between two particles. The solvation forces for solvophilic (solvent-loving) nanoparticles oscillate between attraction and repulsion as a function of particle separation, resulting from oscillations in the solvent density and packing structure between the two nanoparticles.
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 μm sulfated and carboxylated polystyrene latex particles, 10 mM KCl, 10 μM NaPSS, and pH = 2.5. These particles are super-micron, but nanoparticles require a different method of "visualization."
Figure 2. The PFLS Apparatus. Top: photo of the apparatus. Right: a blowup of the differential electrophoresis cell, the key component of the apparatus. Lower left: an image showing the sudden change in light scattering when the applied force is about 0.3 pN. The sudden change in scattering signal measures the force holding the particles together. This measurement is for 800 nm particles and currently is being extended down to 100 nm particles.
The modeling findings indicate that solvation forces could impart stability on dispersions of nanoparticles, and soon we will be able to test this with PFLS. 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 reduce considerably the waste associated with the common practice of adsorbing dispersant molecules on nanoparticle surfaces to prevent them from aggregating or to achieve their selective assembly.
Figure 3. Van der Waals and Solvation Forces for Cubic, 5 nm, Solvophilic Nanoparticles. We find that surface roughness influences the phase of the oscillatory interactions. Comparing forces between rough, spherical nanoparticles and cubic nanoparticles with flat fcc (111) contacting surfaces, we find 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, indicating that solvation forces could indeed be used to stabilize colloidal nanoparticles.
Future Activities:
We will use PFLS to measure forces and evaluate schemes of stabilizing particles using solvation and depletion forces. 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 currently are modifying our molecular-dynamics code to handle these molecules as solvents.
Journal Articles on this Report : 4 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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Calbi MM, Gatica SM, Velegol D, Cole MW. Retarded and nonretarded van der Waals interactions between a cluster and a second cluster or a conducting surface. Physical Review A 2003;67:033201-1 – 033201-5. |
R829605 (2003) R829605 (Final) |
not available |
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Gatica SM, Calbi MM, Cole MW, Velegol D. Three-body interactions involving clusters and films. Physical Review B 2003;68(20):205409 (8 pp.) |
R829605 (2003) 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 |
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Qin Y, Fichthorn KA. Solvation forces between colloidal nanoparticles: Directed alignment. Physical Review e 2006;73(2 Pt 1):Art. No. 020401 |
R829605 (2003) R829605 (Final) |
not available |
Supplemental Keywords:
green chemistry, clean technologies, waste reduction, waste minimization, chemical engineering, physics measurement methods, modeling, nanoparticle forces, nanoparticle dispersion, nanoparticle stability, molecular dynamics, particle force light scattering, PFLS, differential electrophoresis,, 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.