supercritical fluids: liquid-like density and solubility gas-like diffusivity and viscosity an...
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Supercritical fluids: Liquid-like density and solubility Gas-like diffusivity and viscosity An ideal medium for the purpose of deagglomerating
nanoparticles, because it can penetrate the pores within the nano-agglomerates, and upon rapid depressurization, can cause separation of the nanoparticles
Supercritical CO2 Tc = 31.1 °C Pc = 7.38 MPa Low toxicity High stability
One-center effective Lennard-Jones particle = 3.68 Å
/kB = 286.2 K
Dumbbell with point quadruple(Moller & Fischer, 1994 and 1997)
3.033 Å
/kB = 125.57 K
l = 0.699 Å
Q2/5 = 3.0255
The fluid model must exactly reproduce the phase diagram and thermodynamic properties of the bulk fluid in the given range of temperature and pressure.
Spherical shell model(GCMC)
FCC structured model
(MD)– 276 SiO2 units
– D = 2.2 nm
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hRhRhRhRh
RhRU ssf
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. Morishige - MCM41 36A
Bakaev -hydroxylated glassglass
Bakaev - dehydroxylated glass
CO2 on MCM at 195K -- Morishige vs Bakaev
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FSM12
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CO2 on FSM - Katoh: different samples at 273K
Sorption isotherms at 195K at different amorphous silicas
Sorption isotherms at 273 K on amorphous silicas that differ only by hydroxylation level
Here sf and sf parameters are chosen to get the best fit of GCMC and MD isotherms with the Steele potential to experiment
Experiment vs GCMC 195K 303K ~ Critical temperature
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LJ model: one set of parameters. LJ model does not fit experimental isotherm at low temperature. Dumbbell model: different sf to account for hydroxylation
Interaction potential (with spherical shell nanoparticle)
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hRhRhRhRh
RhRU ssf
This potential reduces to the 10-4 form of Steele potential when R approaching infinity, which represents a flat surface.
T = 318K, pbulk = 68atm
The disjoining force is repulsive when nanoparticles are close, then becomes attractive, and finally diminishes to zero when the separation is sufficiently large.
particles at contact
large separation
Both Monte Carlo and Molecular Dynamics approaches were successfully applied to study the forces between two spherical silica nanoparticles in a supercritical carbon dioxide environment at realistic pressures.
The two models considered (dumbbell and one-center LJ) with validated parameters, accurately reproduce experimental data on bulk CO2 and CO2 sorption on silica.
Particles effectively attract at the lower pressures ranging 68-100 atm and they experience repulsive forces for pressures above 100 atm.
These conclusions do not depend on the molecular model considered.
Energetic inhomogeneities do not significantly affect the value of the force between the particles.
iii. dependence on surface roughness
Disjoining pressure for dumbbell model with smooth walls (10-4-3 potential) and inhomogeneous walls at pbulk =102atm
Here, the influence of inhomogeneities is negligible
ii. dependence on fluid model
Disjoining pressure for LJ and dumbbell models with smooth walls (10-4-3 potential only) at pbulk =68atm
Nature of force oscillations on the width differs in narrow pores. Pronounced oscillation periodicity when LJ model is used. However, in general the results are consistent
Solid-fluid parameters: fitting sf and sf for a surface in the presence of inhomogeneities
GCMC and experimental isotherms of CO2 (Dumbbell model) at 195K
GCMC and experimental isotherms of CO2 (Dumbbell and LJ models) at 303K
Pdisjoining 1
AFw1i Fw2
i i1
N
Pbulk
RESS: Rapid Expansion in Supercritical Solvents The drug is dissolved in the supercritical fluid The drug containing supercritical fluid is passed through an
expansion valve The nanoparticles are collected when the particles settle on a
collection plate
Reduction of pressure causes evaporation of the supercritical solvent --> supersaturation of drug and subsequent precipitation
Disjoining pressure for LJ model at T=318K and different pbulk. Long-range repulsion at pbulk =102 and 200 atm, not observed at 68 atm
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68atm102atm200atm68 atm (bulk)102 atm (bulk)200 atm (bulk)
LJ model - influence of bulk pressure
i. dependence on bulk pressure
Solid-fluid potential: Steele’s potential + point inhomogeneities
Attractive: blue; repulsive: red
Point inhomogeneity: distance between the solute molecule and inhomogeneity vs. extra energy added to the “base” 10-4-3
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Usf Usf(1)(z1) Usf
(2)(z2) U inh
Uinh 4ff for r < 0.75
= 8ff r -10ff for 0.75 < r < 1.25
Interactions strongly depend on surface hydroxylation: Surface hydroxylation increases => more energetic adsorption sites => adsorption is intensified
Interaction potential (with FCC structured nanoparticle)
The attraction is most prominent for strongly hydroxylated particles
The attraction is substantially weaker for dehydroxylated particles
T = 77.4K, pbulk = 1atm
bulkex NNN
derived form the Derjaguen approximation
Bakaev, V. A., W. A. Steele, et al. (1999). Journal of Chem. Phys. 111(21): 9813-9821. Katoh, M., K. Sakamoto, et al. (2000). PCCP 2(19): 4471-4474. Morishige, K., H. Fujii, et al. (1997). Langmuir 13(13): 3494-3498. Möller, D. and J. Fischer (1994). Fluid Phase Equilibria 100: 35-61. Span, R. and W. Wagner (1996). J. of Phys. and Chem. Refer. Data 25(6): 1509-1596.
References
Motivation
Fluid Models (CO2)
Background
Experimental Sorption Isotherms
Solid-Fluid Interaction
Interparticle Forces
Larger Particles andSurface Inhomogeneity
Conclusions
Disjoining Pressure Nanoparticle Models (SiO2)
Nanoparticles (NP) and nanocomposites have great potential to improve performance of drugs, biomaterials, catalysts and other high-value-added materials. They offer unique properties that arise from their small size and large surface area.
A major problem in utilizing nanoparticles is that they often lose their high surface area due to grain growth or unavailability of the high surface area where it matters. It is difficult to produce forces required to deagglomerate the nanoparticles at a sufficiently small length scale.
The addition of nanoparticles to polymer composites has been shown to significantly influence the mechanical, optical, and electrical properties. However when nanoparticles aggregate, they lose their nanoscale size and corresponding properties.
The breakup of nanoagglomerates, driven by the tensile stresses generated by depressurization, has not been studied previously for nanoparticles and there is a paucity of published analysis on this subject.
Environmentally Benign Deagglomeration and Mixing of Nanoparticles in Supercritical CO2
Yangyang Shen4, Aleksey Vishnyakov4, M. Silvina Tomassone4 Program NIRT; Award: DMI: 0506722; PI: Dr. Rajesh Davé1
Co-PIs: R. Gupta2, R. Pfeffer, S. Sundaresan3, M. S. Tomassone4
1New Jersey Institute of Technology, Newark, NJ, 2Auburn University, Auburn, AL, 3Princeton University, Princeton, NJ, 4Rutgers University, New Brunswick, NJ
Contact: Tel.: +1-732-445-2972 Fax: +1-732-445-2421 Email: [email protected] Funding from NSF – NIRT (Award # 0506722) and IGERT (Award # 050497) is acknowledged.
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GCMC - strong fieldGCMC - medium fieldGCMC - weak fieldexpt, MCM41 (Morishige)reference isotherm
Peaks correspond to the pore width when a new layer is formed and the separation distance is small
Minima correspond to pore width with large distance between adjacent layers