Evaporation rates of structured and non-structured liquid mixtures

We have used a gravimetric technique to measure the rate of evaporation of a volatile liquid in mixtures with a second, involatile component under conditions of controlled gas flow. A range of non-structured and structured mixtures were investigated in order to examine whether the rate limiting step for evaporation may switch from vapour diffusion across the stagnant gas layer above the liquid to mass transfer within the liquid mixture. Evaporation rates of pentane and hexane from mixtures with squalane (involatile) show excellent agreement with rates calculated on the basis that vapour diffusion across a stagnant gas layer is rate limiting and that mass transfer within the liquid mixture is fast. Hexane gelled by the addition of silica particles is found to evaporate at a rate very similar to that for un-gelled hexane because the equilibrium vapour pressure of hexane is unaffected by silica particle addition. Water evaporation rates from mixtures with the non-ionic surfactant n-dodecyl hexaoxyethylene glycol ether (C12E6) were found to be up to 10 times slower than calculated vapour space diffusion controlled rates owing to the slow development of concentration gradients within these highly structured liquid mixtures.