AbstractColloidal science is vital in many industrial processes due to its outstanding properties, viability and versatility to tailor numerous industrial applications such as formulations in food and beauty products, decorative layering and painting as well as ink and electronic printing. Hence, the stability of a colloidal system is an important key to determine the final properties of the colloidal-based products. The stability of colloids by electrostatic means in a fluid is outlined by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and the behaviour of colloids in the non-aqueous system is still uncertain despite its' importance in industrial applications. The main objective of this thesis is primarily to investigate the electrostatic interactions, mainly between colloidal particles in non-aqueous systems which can be applied to industrial needs such as printing and coating.
We generate controllable micrometer-range electrostatic interactions in a suspension by using a charge control additive (the surfactant, dioctyl sodium sulfosuccinate [AOT]), and an organic salt, tetradodecylammonium tetrakis (3,5-bis (trifluoromethyl)phenyl)borate (TDAT) in non-aqueous solvents. Both systems adapted different mechanisms of altering the electrostatic interaction between PMMA colloids. For AOT system, the particle surface charge is modified by adsorption of charged and neutral AOT micelles on the surface, hence affecting the interactions between particles. For the electrolytic TDAT non-aqueous system, the screening length of the solution was altered due to the presence of free ions in the solution. This is confirmed by the conductivity and theoretical Debye length values. However, from force measurement using the blinking optical tweezers (BOT), we found out that the measured screening length shows a non-monotonic dependence on the TDAT concentration. The deviation between these values revealed that at high TDAT concentrations, the classical DLVO theory is no longer valid for the system. We postulate that the formation of Janus-like particles due to charge instability resulted from the non-uniform charge distribution on the colloidal surfaces. This is preliminarily confirmed by the rotation of a PMMA colloids chain when an electric field is applied.
In a later series, the electrostatic interactions between charged PMMA colloids were investigated in an ionic liquid solution. The correlation between the number of ions in solution and the screening length was reported and the usual trend observed. The particle attraction was observed in all systems, which leads to the formation of colloidal clusters and aggregates. At high ionic liquid concentrations, we discovered that the colloidal stabilization is achieved and this may be due to the decaying particle aggregation rate, driven by the enhanced viscosity of the solution.
We relate the formation of clusters and aggregates formed in the bulk system with the deposition patterns of a colloidal droplet on a hydrophobically-coated glass substrate. The drying of droplets containing monodisperse PMMA particles was studied by confocal light scanning microscopy. Image analysis was used to quantify the spatial deposition of the particles. We explore the competition between the long-range colloidal force with the evaporation kinetics of the solvent. In highly-volatile solvents, such as hexane, hydrodynamic flows dominate particle deposition. With weak electrostatic interactions, a well-defined coffee-ring is produced. Increasing the strength of repulsions, the intensity of the ‘ring’ is observed to first reduce and then in highly-charged systems to be totally suppressed. In less volatile solvents, such as dodecane where evaporation is diffusion-controlled, the role of colloidal forces is more complex with evidence for modification of both particle-particle and particle-substrate interactions. Finally, high TDAT concentrations induce particles attraction and the formation of clusters in bulk solution which inhibit the formation of a ring-like deposit.
|Date of Award||1 Oct 2019|
|Sponsors||Ministry of Higher Education Malaysia & Universiti Sains Malaysia|
|Supervisor||Paul Bartlett (Supervisor)|