An investigation into the role of electrostatics in non-aqueous systems

Abstract

Electrostatics provide a rich array of behaviours in solid-in-liquid colloids, whether that be controlled self-assembly, coagulation, interfacial stability etc. Aqueous dispersions provide the easiest route to alter the electrostatics of a system, but due to the short-range of interactions, often add unnecessary complexity. Further, it can be preferred to move away from aqueous dispersions for a variety of other reasons. To this end, three investigations into the electrostatics of solid-in-liquid colloids in non-aqueous environments were undertaken.

The first examines the swelling behaviour of an industrially relevant microgel, Carbopol 974p NF, as a function of its degree of ionisation in three separate solvents, water, glycerol and ethylene glycol. The swelling is attributed to two competing effects, a solvency effect, and an electrostatic effect. Literature states swelling of microgels is strongly dependant on their cross-link density. An analysis of this is shown through a Monte Carlo self-avoiding walk simulation of microgels with varying cross-link concentration. Finally, macroscopic properties of Carbopol systems are examined, primarily through rheology, and results put in context of the microscopic properties and state of Carbopol particles.

The second investigation looks into the role of electrostatics in controlling the rigidity of chains of colloidal PMMA in dodecane. Electrostatics were adjusted by addition of TDAT, an organic salt, or by addition of AOT, an anionic surfactant. The rigidity was probed by bending the chains using holographic optical tweezers, with added AOT producing smooth bends, and added TDAT resulting in kinks. This is explained by surface charge mobility on the PMMA microspheres. An examination into the surface charge of the PMMA microspheres by zetapotential and electrophoresis provides a greater understanding to the cause of these different behaviours.

Finally, the third investigation examined the stability and aggregation of PMMA microspheres at the oil/water interface. Development of the cell design to enable imaging is discussed, with advantages and disadvantages of each stage shown. Due to COVID, results beyond the initial images were unable to be attained, but these initial results show the mobility of PMMA at the interface, with the interface location determined by addition of the dye Nile Red. Finally, the Gibbs excess of Nile Red, found to be surface active, is determined, and shown to be comparable to surfactant. Combined, these investigations provide a broad look at a variety of electrostatic behaviours in non-aqueous environments.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul Bartlett (Supervisor) & Wuge H Briscoe (Supervisor)