Drying paths of phase-separating solution coatings exposed to humidity

Drying process paths, i.e., time-dependent trajectories of solution compositions on a phase diagram, were examined for coatings of a water-immiscible acrylic monomer dissolved in ethanol and exposed to humidity. The absorption of water vapor quenched the solution into a thermodynamic nonequilibrium state and promoted a spontaneous phase separation when the water content in the solution exceeded a critical value. A simultaneous mass-loss and heat-flux measurement technique was proposed and adapted for phase-separating coatings to determine the evaporation rates of two volatile components with different latent heats of vaporization. The measured drying paths agreed with those predicted from a drying model that takes into account the gas-diffusion-limited evaporation and composition-dependent activities. The numerical predictions also revealed that the pseudo-azeotrope determined the critical drying conditions, under which any drying paths on the phase diagram did not cross the two-phase region, and thus the drying solutions remained stable.