Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Chemical and Biological Engineering

First Advisor

Balaji Narasimhan

Second Advisor

Monica H. Lamm

Abstract

Nanoparticles form the fundamental building blocks for many exciting applications in various scientific disciplines. However, the problem of the large-scale synthesis of nanoparticles remains challenging. It is necessary to understand the nanoparticle aggregation for the rational design of reactors for high-throughput synthesis of nanoparticles with well-controlled properties. Often, nanoparticle aggregation is modeled using stochastic methods based on scaling arguments and assumptions about the nanoparticle interaction potential. Therefore, a more rigorous approach is desired for understanding nanoparticle aggregation. In this dissertation, a novel framework integrating experiments and multi-scale simulations for studying nanoparticle aggregation is presented.

Atomic force microscopy (AFM) was employed to measure the force between polystyrene (PS) micro- and nanoparticles. Specifically, AFM was used to directly measure the force (in air) between a 300 nm PS nanoparticle and a PS film, which was compared with the force measured between a 2 ym PS particle and a PS film. A novel approach based on layer-by-layer assembly to functionalize an AFM probe was developed and applied to the measurement of the force between nanoparticles. The nanoparticle force was deduced from the variation of force between a silica colloidal probe (5-30 ym) functionalized with a monolayer of 300 nm PS particles and a PS film as a function of the diameter of the silica particle. It was shown that continuum models are inadequate to explain the measured forces, which underlines the need for a more rigorous multiscale modeling methodology to understand nanoparticle interaction potential.

In principle, nanoparticle interaction potentials can be derived from electronic structure calculations for a molecule using a multiscale modeling approach. To this end, a systematic method of coarse-graining based on force matching was implemented and applied to coarse-grain three common solvent molecules (carbon tetrachloride, benzene and water) to their center of mass. The coarse-grained potentials derived from first principles based effective fragment potential (EFP) were able to reproduce the structural properties that were in reasonable agreement with those obtained using EFP molecular dynamics while achieving a computational speed-up of four orders of magnitude.

The nanoparticle interaction potential determines the morphology of corresponding aggregates. On the other hand, the aggregation kinetics are governed by the diffusivity of the aggregates. Therefore, it is essential to relate the aggregate morphology to its mobility in order to study aggregation kinetics. The diffusion of nanoparticle aggregates in the limit of infinite dilution was studied as a function of their mass (N) and fractal dimension (df) using molecular dynamics simulations in the presence of explicit solvent molecules. The diffusion coefficient (Do) for aggregates was found to scale as Do ∼ N-1/df. The ratio of the hydrodynamic radius to the radius of gyration was found to be independent of mass for aggregates of a given fractal dimension, thus enabling an estimate of the diffusion coefficient for a fractal aggregate based on its radius of gyration.

The research presented in this work provides a robust framework integrating experiments and multiscale simulations for studying nanoparticle aggregation.

Copyright Owner

Gaurav Pranami

Language

en

Date Available

2012-04-30

File Format

application/pdf

File Size

139 pages

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