Interfacial Rheology and the Controlled Fabrication and Disruption of Stabilized Emulsions

Jerome Jamal Nash, Purdue University

Abstract

Fluid interfaces stabilized by surface-active species (e.g., surfactants, polymers, and particles) have rheological properties that are vital to the kinetic stability of emulsions. Many practical applications of emulsions necessitate superb stability during storage, such as in emulsionbased therapeutic delivery systems. While in other cases, stabilized systems are entirely unwanted (e.g., separating oil and aqueous phases in enhanced oil recovery and bilge water applications). Techniques for modulating emulsion phase separation processes are highly desired and are largely determined by the mechanics of interfacially trapped species which preserve the overall stability of bulk emulsions. However, the utility of these techniques is often limited by difficulties in measuring and interpreting the rheological properties of complex fluid interfaces. Lack of control over interface formation during emulsification magnifies this problem, further obscuring relationships between interfacial rheology and bulk emulsion stability. Therefore, the objectives of this thesis were to (1) elucidate fundamental relationships between the mechanics of complex fluid interfaces and the anticipated stability of the bulk emulsions they comprise through interfacial rheological measurements, and (2) present innovative methodologies for modulating the kinetic stability of model oil-in-water emulsions using physical chemistry principles. The introductory chapter of this thesis will provide a detailed overview of contemporary experimental tools used to probe the rheology of single- and multi-component complex fluid interfaces. Additionally, this chapter will include a discussion on the role of interfacial rheology in an emulsion’s susceptibility to the key destabilization mechanisms that dominate its performance (e.g., coalescence, flocculation, and gravitational phase separation). The second and third chapters of this thesis focus on experimental methods that can be utilized to characterize an oil-in-water emulsion’s stability to droplet coalescence based on the interfacial adsorption and dilatational rheological characteristics of the stabilizing emulsifiers. In Chapter 2, a criterion for inhibiting the susceptibility to droplet coalescence of dilute oil-in-water emulsions based on the rheological properties of the surfactant-stabilized interfaces is presented. Chapter 3 details how droplet coalescence in concentrated oil-in-water emulsions can be inhibited by the interfacial steric hinderance of SiO2-surfactant complexes. The fourth and fifth chapters of this work focus on how physical chemistry principles can be used to control emulsion droplet destabilization and produce desirable physical outcomes within bulk emulsions. Chapter 4 details an investigation of how the gravitational phase separation of dilute, electrostatically stabilized oil-in-water emulsions can be induced by a complex coacervation mechanism. Here, attractive electrostatic interactions between the interface-stabilizing anionic surfactant sodium lauryl ether sulfate (SLES) and positively charged silicon dioxide (SiO2) nanoparticles can be used to encourage droplet flocculation in model oil-in-water emulsions. Chapter 5 outlines an innovative methodology for encouraging tightly controlled internal mixing between coalescing water droplets via interfacial rheological methods. The methods established in this study have direct application in the development of micro-droplet reactors, which necessitate controlled mixing in microliter volumes. The knowledge garnered from this body of work is highly relevant to academic and industrial emulsion formulators who seek inexpensive, yet robust methods for predicting, characterizing and tailoring the extended kinetic stability of oil-in-water emulsions.

Degree

Ph.D.

Advisors

Erk, Purdue University.

Subject Area

Physical chemistry|Mechanics|Chemistry|Materials science|Nanotechnology|Petroleum engineering|Physics

COinS