modeling green engineering of dispersed nanoparticles: measuring and modeling nanoparticle forces...
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Green Engineering of Dispersed Nanoparticles: Measuring andModelingModeling Nanoparticle Forces
Kristen A. Fichthorn and Darrell VelegolDepartment of Chemical Engineering
The Pennsylvania State UniversityUniversity Park, PA 16802
Students: Yong Qin Gretchen Holtzer
R-8290501
Nanoparticles: Potential Building BlocksFor New and Existing Materials
• Catalysts
• Optical Materials
• Structural Materials
• Electronic Materials
Nano-Electronics
Difficult to Disperse or Assemble
“Bare” Nanoparticles
1 m5 nm
Conventional Colloids:Dispersant: ~1% volume
Nanoparticles:Dispersant: ~ 90% volume
A LOT OF WASTE! ! ! ! !
Nanoparticle Forces are POORLY UNDERSTOOD!
Colloidal Forcesfrom MolecularDynamics Simulations
• van der Waals and Electrostatic Forces:DLVO theory
• Solvation Forces:Solvent Ordering
• Depletion Forces:Entropic
How Do These Work forColloidal Nanoparticles?
Particle force light scattering (PFLS) for nanoparticle forces
Ofoli & Prieve, Langmuir,13, 4837 (1997)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 500 1000 1500 2000 2500
time (ms)
(V-V
o)/
Vo
Fcrit = 0.3 pN 0.08
I N x mass2
800 nm particles
custom differentialelectrophoresis cell
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
Model Nanoparticles
Small Sphered = 4.9σ64 atoms
Large Sphered = 17.6σ
2048 atoms
Cubed = 13.2σ
2744 atoms
Icosahedrond = 4.0σ
55 atoms
Solvation Forces: Thermodynamic Integration
BSASAB
solvsolv
FFr
d
dUF
)(
j
i d
dUdAij
)(
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
-15.0
-10.0
-5.0
0.0
5.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0
δ/σ
F·σ
/kT
Solvophilic
Solvophobic
van der Waals-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
5.0
10.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0
δ/σ
F·σ
/kT
Solvophilic
Solvophobic
van der Waals
Small Sphere
Large Sphere
-600.0
-400.0
-200.0
0.0
200.0
400.0
600.0
1.5 2.0 2.5 3.0 3.5 4.0 4.5
δ/σ
F·σ
/kT
van der Waals
Solvophilic
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
222 S
Solv
CC D
F
A
A
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.5 1.5 2.5 3.5 4.5 5.5 6.5
δ/σ
ΔA
/(2A
cρc2
) or
FS
olv/(
πD
ρs2
)
(kT
σ4)
Small SphereLarge SphereCube
-0.04
-0.03
-0.02
-0.01
0.00
0.01
0.5 1.5 2.5 3.5 4.5 5.5 6.5
δ/σ
ΔA
/(2
Acρ
c2 ) o
r F
Sol
v /(π
Dρ
s2 )
(kT
σ4 )
Small SphereLarge SphereCube
Solvophilic Solvophobic
Derjaguin ApproximationDescribes the Envelope
Derjaguin Approximation Works
Influence of Surface Roughness on Solvation Forces
Particle orientation significantly affects the force profile: Particles will Rotate in Solution
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Separation (σ)
Fo
rce
(kT
/σ)
Left
Center
Right
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
0.5 1.5 2.5 3.5 4.5
δ/σ
F·σ
/kT
fixed
rotation
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.5 1.5 2.5 3.5 4.5
δ/σF
·σ/k
T
fixed
rotation
Solvophilic
Solvophobic
Rotation Reduces SolvophilicSolvation Forces
Nanocrystals Have PreferredOrientations
The Influence of Solvent Structure:n-Decane – Small Spheres
n-Decane Length Comparableto Nanoparticle Diameter
-4.0
-3.0
-2.0
-1.0
0.0
1.0
0.5 2.5 4.5 6.5 8.5 10.5δ/σ
F·σ
/kT
Solvophilic
Solvophobic
van der WaalsWeak SolvophilicForces
Step-LikeSolvophobicForces
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……