A new thermodynamic approach is presented giving the relations between the size of water droplets in supersaturated moist air and the chemical affinity. Two simplified chemical mechanisms in connection with actual atmospheric situations are studied : (a) oxidation of a negligibly soluble precursor into a soluble compound with ionic dissociation; (b) oxidation in air or water of a less or moderately soluble precursor into a soluble compound with ionic dissociation. For these two cases the theory shows that the larger the size of the droplets the larger the chemical affinity, However, other rare cases can lead to the opposite results. Several examples are studied in the framework of our theory.