Variation of droplet acidity during evaporation.

Variation of acidity and associated chemical changes of macromolecules in evaporating droplets is of central importance in electrosprayed aerosols. We study changes in acidity during evolution of a droplet that is composed of solvent and a charge binding macromolecule. We analyze the acidity of the droplet using analytical theory and stochastic modeling. We derive a universal relation for the minimum pH of a droplet in the presence of a protein and the results are confirmed by the stochastic modeling of ubiquitin and lysozyme at varying values of pH. We establish that in acidic droplets, once the number of solvated charges reaches the macroion charge, the further droplet evaporation, counter-intuitively, reduces the number of charges on the macromolecule and increases the droplet pH.