Evaporation of liquids from structured and non-structured mixtures
The work presented in this thesis describes the evaporation rates of a wide variety of samples obtained under a controlled gas flow using a gravimetric technique. Evaporation rates for pure liquids with vapour pressures ranging from 0.1 to 500 Torr, water contained in silica particles, alkane/squalane mixtures, hexane gelled with silica particles, surfactant/water mixtures, immiscible layered liquid mixtures and emulsions have been determined. For pure liquids and simple unstructured liquid mixtures, the evaporation rate is limited by diffusion through a stagnant vapour layer at the liquid surface. As the degree of structure within the liquid mixture increases, the time taken for concentration gradients developing in the evaporation process to relax becomes longer relative to the time taken for diffusion through the stagnant vapour layer. For highly structured liquid mixtures, the rate limiting process switches to diffusion and convection within the liquid mixture. In the case of creamed oil-in-water emulsions, evaporation of the continuous water phase is limited by diffusion through the stagnant vapour layer, whilst the evaporation rate of the emulsified oil is consistent with a mechanism in which the oil drops remain separated from the vapour phase by a thin water film. Oil transport from the drops to the vapour occurs by diffusion of dissolved oil across this film.