On the solvent evaporation for a sessile drop: Solvent–vapour equilibrium must be achieved in open system

We examined here the conditions of liquid–vapour equilibrium, in open system, for drops of pure liquid or solution, put on a substrate. The presence of a curved liquid–vapour interface increases the value of the chemical potential of the pure liquid as a result. The addition in this solvent of a solute (molecule or electrolyte) leads to decreasing the chemical potential of the liquid (colligative properties). These two effects are of inverse direction and can thus cancel each other out. We showed in this study that when the osmolarity of the solution is equal to the pressure difference on both sides of the liquid–vapour interface (Laplace's rule), the vapour pressure of the solvent above the drop is equal to that of pure liquid in unlimited phase. For liquid drops of 1 μL, an osmolarity equal to 10−4 mol L−1 is enough so that the colligative effect compensates the effect of curvature. This implies that, in open system in which the gases of the atmosphere are soluble at such concentrations, the liquid–vapour equilibrium must be realized.