Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle Forces

1. Green Engineering of Dispersed Nanoparticles: Measuring andModeling Nanoparticle Forces Kristen A. Fichthorn and Darrell Velegol Department of Chemical Engineering The Pennsylvania State University University Park, PA 16802 Students: Yong Qin Gretchen Holtzer R-8290501

2. Nanoparticles: Potential Building Blocks For New and Existing Materials • Catalysts • Optical Materials • Structural Materials • Electronic Materials Nano-Electronics

3. 1 mm 5 nm A LOT OF WASTE ! ! ! ! ! Difficult to Disperse or Assemble “Bare” Nanoparticles Nanoparticles: Dispersant: ~ 90% volume Conventional Colloids: Dispersant: ~1% volume Nanoparticle Forces are POORLY UNDERSTOOD!

4. Colloidal Forcesfrom MolecularDynamics Simulations • van der Waals and Electrostatic Forces:DLVO theory • Solvation Forces:Solvent Ordering • Depletion Forces:Entropic How Do These Work for Colloidal Nanoparticles?

5. Particle force light scattering (PFLS) for nanoparticle forces Ofoli & Prieve, Langmuir, 13, 4837 (1997) I  N x mass2 Fcrit = 0.3 pN  0.08 custom differential electrophoresis cell 800 nm particles

6. Large-Scale Parallel MD Simulation • Solvent: Lennard-Jones Liquid, n-Decane ( >105 Atoms) • Nanoparticles: Solid Clusters of Atoms • Solvophilic Nanoparticles: (εsf = 5.0εff) • Solvophobic Nanoparticles: (εsf = 0.2εff) Beowulf Cluster: Cruncher

7. Model Nanoparticles Small Sphered = 4.9σ64 atoms Cubed = 13.2σ2744 atoms Icosahedrond = 4.0σ55 atoms Large Sphered = 17.6σ2048 atoms

8. Solvation Forces: Thermodynamic Integration Free Energy Change Mezei and Beveridge, Ann. N. Y. Acad. Sci.482, 1 (1986). Solvation Force

9. Interactions for Spheres, Cubes Small Sphere Large Sphere • Solvophilic solvation forces are • oscillatory and comparable to van • der Waals forces • Solvophobic solvation forces • are attractive Cube

10. Fluid Ordering: Origin of Solvation Forces • Solvent ordering around nanoparticles can be observed in all solvophilic simulations (Movie)

11. Solvophobic Nanoparticles: The Drying Transition (Movie)

12. Solvophilic Solvophobic Derjaguin Approximation Describes the Envelope Derjaguin Approximation Works Derjaguin Approximation

13. Influence of Surface Roughness on Solvation Forces Particle orientation significantly affects the force profile: Particles will Rotate in Solution

14. Rotation Reduces Solvophilic Solvation Forces Solvophilic Solvophobic

15. Nanocrystals Have Preferred Orientations

16. The Influence of Solvent Structure:n-Decane – Small Spheres Step-Like Solvophobic Forces Weak Solvophilic Forces n-Decane Length Comparable to Nanoparticle Diameter

17. Conclusions • Current theories do not accurately describe forces for small nanoparticles • Solvation forces can be important for colloidal nanoparticles • Solvation forces are strongly dependent on particle size, shape, surface roughness, particle-solvent interactions, and solvent structure • Solvent-nanoparticle suspensions can be engineered for stability, assembly, environmental impact……