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Page 1: Supercritical fluids:  Liquid-like density and solubility  Gas-like diffusivity and viscosity  An ideal medium for the purpose of deagglomerating

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p, kPa

mo

l/m2

exp (FSM10)exp (FSM12)GCMC (d) - strong fieldmGCMC (d) - medium fieldGCMC (d) - weak field (d)GCMC (LJ)

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

4410102

5

2

5

22,

hRhRhRhRh

RhRU ssf

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0 0.2 0.4 0.6 0.8 1

p /p 0

mic

rom

ole

/sq

ua

re m

ete

r

. Morishige - MCM41 36A

Bakaev -hydroxylated glassglass

Bakaev - dehydroxylated glass

CO2 on MCM at 195K -- Morishige vs Bakaev

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p, kPam

ol/m

2 ...

FSM10

FSM12

FSM14

FSM16

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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4410

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61.035

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zzzzU sfsfsf

sfsfssf

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)

4410102

5

2

5

22,

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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Hin,

p,

Gp

a

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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4410

2

61.035

22

zzzzU sfsfsf

sfsfssf

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