Date on Master's Thesis/Doctoral Dissertation

8-2014

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Willing, Gerold A.

Committee Co-Chair (if applicable)

Fu, Xiaoan

Committee Member

Fu, Xiaoan

Committee Member

Berson, Eric

Committee Member

Harnett, Cindy K.

Subject

Suspensions (Chemistry); Nanoparticles; Colloids

Abstract

Over the past decade, charged nanoparticles have been found to enhance the stability of colloidal suspensions. One promising explanation of this stabilization mechanism is “Nanoparticle Haloing”- the formation of a non-adsorbing nanoparticle layer surrounding neutral colloids that would induce an effective electrostatic repulsion between them. The objective of this work was to investigate the effect of additional charged nanoparticles on the interaction between neural colloids in nanoparticle-regulated complex fluids. Firstly, colloidal probe Atomic Force Microscopy (CP-AFM) is used to directly measure the interaction force in such system with nanoparticle volume fractions varying from 10-6 to 10-2. It is found that at a critical low volume fraction (10-5), the colloidal system is stabilized due to the domination of an electrostatic repulsion between colloidal surfaces induced by the enhanced concentration of nanoparticles in the charged layer. As the nanoparticle concentration is increased, the effective repulsion increases due to a rising charge density built up by the surrounded nanoparticle layer. A Debye length fitting model (DLFM) was subsequently developed to theoretically estimate the interaction between colloids in the nanoparticle suspensions. The DLFM suggests: 1) the interaction between microspheres in the presence of nanoparticles is mainly composed of a van der Waals attraction and an electrostatic repulsion; 2) there is a non-zero distance between the nanoparticle layer and the colloidal surface, and the effect of nanoparticle adsorption on the interaction force between colloidal surfaces is negligible at low volume fractions (10-6 to 10-4). The follow-up adsorption test and force modeling confirmed that the degree of nanoparticle adsorption is negligible at volume fraction < 0.5× 10-3, but becomes evident as the volume fraction increased to 10-3, indicating charged nanoparticles are strongly adsorbed onto silica surfaces at relatively high concentrations rather than haloing around them. Thus, we propose that 1) the fundamental mechanism of nanoparticle-regulated stabilization is “nanoparticle haloing” at low nanoparticle concentrations, and becomes “adsorption” at high concentrations; 2) there is a transition region within which the stabilization can be influenced by both nanoparticle haloing and adsorption. This transition was observed around a nanoparticle volume fraction of 10-3 in our experiments. Nanoparticle haloing and adsorption two stabilization mechanisms are not mutually exclusive when using charged nanoparticles to regulate the stability of colloidal suspensions; they work continuously over the increasing nanoparticle concentrations. Our study suggests that, when using highly charged nanoparticle to stabilize weakly charged colloidal suspension, the reversibility of stabilization and accessibility of colloidal surfaces can be controlled by simply tuning the nanoparticle concentration.

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