# Presentation: 416668

## Green Engineering of Dispersed Nanoparticles:  Measuring andModeling 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**

## Nanoparticles: Potential Building Blocks

**For New and Existing Materials**

-  Catalysts
-  Optical Materials
-  Structural Materials
-  Electronic Materials

- Nano-Electronics

## Difficult to Disperse or Assemble
“Bare” Nanoparticles

**“****Bare” Nanoparticles**

**Conventional Colloids:**
- Dispersant: ~1% volume

**Nanoparticles:**
- Dispersant: ~ **90% volume**

- Nanoparticle Forces are **POORLY UNDERSTOOD!**

## Colloidal Forcesfrom MolecularDynamics Simulations

- van der Waals and Electrostatic Forces:DLVO theory
- Solvation Forces:Solvent Ordering
- Depletion Forces:Entropic

- How Do These Work for
- Colloidal Nanoparticles?

## Particle force light scattering (PFLS) for nanoparticle forces

- Ofoli & Prieve, *Langmuir,*
**13**, 4837 (1997)

- Fcrit = 0.3 pN  0.08

- I  N x mass2

- 800 nm particles

- custom differential
- electrophoresis cell

## Large-Scale Parallel MD Simulation

- Solvent: Lennard-Jones Liquid, n-Decane ( >105 Atoms)
- Nanoparticles: Solid Clusters of Atoms
- Solvophilic Nanoparticles: (*ε**sf **= *5.0*ε**ff*)
- Solvophobic Nanoparticles: (*ε**sf **= *0.2*ε**ff*)

- Beowulf Cluster:  Cruncher

## Model Nanoparticles

- Small Sphered = 4.9σ64 atoms

- Large Sphered = 17.6σ2048 atoms

- Cubed = 13.2σ2744 atoms

- Icosahedrond = 4.0σ55 atoms

## Solvation Forces: Thermodynamic Integration

- Free Energy Change

- Solvation Force

- Mezei and Beveridge,
*Ann. N. Y. Acad. Sci.* **482**, 1 (1986).

## Interactions for Spheres, Cubes

- Solvophilic solvation forces are
-     oscillatory and comparable to van
-     der Waals forces
- Solvophobic solvation forces
-     are attractive

- Small Sphere

- Large Sphere

- Cube

## Fluid Ordering: Origin of Solvation Forces

- Solvent ordering around nanoparticles can be observed in all solvophilic simulations

**(Movie)**

## Solvophobic Nanoparticles:  The Drying Transition

**(Movie)**

## Derjaguin Approximation

## Influence of Surface Roughness on Solvation Forces

**Influence of Surface Roughness on Solvation Forces**

-    Particle orientation significantly affects the force profile: Particles will Rotate in Solution

## Rotation Reduces Solvophilic

**Solvophilic**

**Solvophobic**

- Solvation Forces

## Nanocrystals Have Preferred

**Nanocrystals Have Preferred**
**Orientations**

## The Influence of Solvent Structure:n-Decane – Small Spheres

- n-Decane Length Comparable
- to Nanoparticle Diameter

- Weak
- Solvophilic
- Forces

- Step-Like
- Solvophobic
- Forces

## 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......