Grantee Research Project Results
Final 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 Amount: $370,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Nanotechnology
Objective:
Laboratory work with nanoparticles has demonstrated 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 involves measuring and modeling some of the fundamental forces between nanoparticles—van der Waals forces, solvation forces, depletion forces—and 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). The specific objectives of this research project were to: (1) develop particle force light scattering 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 would otherwise be necessary to stabilize nanoparticle systems.
Summary/Accomplishments (Outputs/Outcomes):
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.
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 PSL 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 particle force light scattering (PFLS) apparatus has been constructed and calibrated. In this apparatus we raise the applied electric field to the critical value where we break apart particle clusters. The critical force is “visualized” sudden change in the light scattering signal (Figure 2), and this force measurement enables the Velegol group to test the modeling done by the Fichthorn group.
Figure 2. The PFLS Apparatus. Upper part: 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 is currently being extended down to 100 nm particles.
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 caused by 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 function of particle separation, a result of oscillations in the solvent density and packing structure between the two nanoparticles.
The modeling findings indicate that solvation forces could impart stability on dispersions of nanoparticles, and soon we will be able to test this with particle force light scattering. 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, Five-Nanometer, 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 experimental activities center on using ultrasound PFLS to measure forces and evaluate schemes of stabilizing particles using solvation, including in cosolvent systems. 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. Studies with cosolvent also are planned.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
| Other project views: | All 36 publications | 7 publications in selected types | All 7 journal articles |
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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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Fichthorn KA, Qin Y. Molecular-dynamics simulation of colloidal nanoparticle forces. Industrial & Engineering Chemistry Research 2006;45(16):5477-5481. |
R829605 (Final) |
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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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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) |
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Holtzer GL, Velegol D. Limitations of differential electrophoresis for measuring colloidal forces: A Brownian Dynamics Study. Langmuir 2005;21(22):10074-10081. |
R829605 (Final) |
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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) |
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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, 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 forcesProgress 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.