Evaporation rates from emulsions stabilised by surfactants and nanoparticles
This thesis is concerned with the evaporation rates of emulsions stabilised by either surfactant molecules or nm-size solid particles. Understanding of the mechanisms of the evaporation process plus the control of the rate limiting step of each mechanism involved in such processes is important for a number of practical applications which include cosmetics, paints and agrochemicals. The work of this thesis deals with three main aspects. Firstly the evaporation rates of pure liquids have been determined. Secondly, evaporation rates of surfactant-stabilised creamed, gelled and high internal phase emulsions have been investigated. Lastly, evaporation rates of emulsions stabilised by solid particles will be discussed. The study concerned with the pure liquids was mainly to verify the set-up of the evaporation rig, to compare our results with those obtained by K. J. Beverley (Thesis in preparation, University of Hull) and determine the diffusion coefficients of different liquids in the vapour phase. For creamed emulsions stabilised by surfactants it was found that the evaporation rate of the continuous phase is as that of bulk phase and the evaporation rate of the dispersed phase is slowed down up to 20 times. The retardation in the evaporation rate of the dispersed droplets depends mainly on the solubility of the dispersed phase in the continuous phase. In the case of gelled emulsions, the mass loss of the emulsions gelled by different gelling agents (colloidal particles and water-soluble polymers) show that the evaporation rates depend on the gelling agent. For high internal phase emulsions, evaporation rates of water from emulsions containing involatile oils have been studied. Depending on the geometrical properties of the initial conditions, the mass loss rate is controlled by either diffusion of the liquid in the vapour (large emulsion/sample tube volume ratio) or diffusion of the liquid in the nm-thin films which separate the close-packed oil drops (small emulsion/sample tube volume ratio). The nature of the colloidal forces acting between the dispersed droplets also plays an important role during the evaporation. For emulsions stabilised solely by nm-size silica particles (Pickering emulsions), the evaporation rate is slower compared with that stabilised by surfactants under the same conditions. Moreover, we have found that the solid residues left behind after the total evaporation of the volatile species (oil and water) from Pickering emulsions show macroporous structures. The type of the microstructure is related to that of the parent emulsion and the vapour pressure of the oil. These results may lead to the production of novel macroporous inorganic materials.