Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

Atmospheric cloud, fog and aerosol microdroplets are more acidic than previously assumed. The fact that interfacial reactions on microdroplets are faster than anticipated has enhanced their role in atmospheric chemistry and raised the question of whether their interfaces are more or less acidic than the bulk phase. It turns out that acidity and its pH dependence sharply change across interfacial layers. Surface-specific experiments show that the protonations of gas-phase molecules at the outermost layer (OTL) of aqueous microdroplets are very different from those dissolved in deeper layers. Trimethylamine (TMA) is protonated, whereas the weak base isoprene (ISO) is not as expected, when dissolved in pH < pK(a) (TMA) = 9.8 microdroplets. In dramatic contrast, both gas-phase TMA and ISO are protonated at the OTL of pH < 4 microdroplets. Because ISO is only protonated in concentrated acids H3O+ ions at the OTL of pH < 4 microdroplets are superacidic. Conversely, the OTL of pH > 4 microdroplets lacks the H3O+ ions that protonate TMA in deeper layers. H3O+ ions become more acidic toward the surface (i.e., the free energies of proton transfer, H3O+ + B = H2O + BH+, become more negative) because hydration losses in lower density OTL water destabilize the small H3O+ ion relative to the larger protonated bases BH+. Because the OTL behaves as neutral at pH similar to 4 (i.e., its pK(w) similar to 8) interfacial water may be more dissociated than in the bulk. In short, the acidity of aqueous microdroplets probed by gas-phase molecules at the OTL is different from the acidity experienced by solutes in deeper layers and should not be confused with pH, which represents the uniform thermodynamic activity rather than the local acidities of H3O+ ions as a function of depth. These concepts should become standard in interfacial atmospheric chemistry.