Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

Abstract

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 ∼ 4 (i.e., its pK w ∼ 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.

Capturing Reactive Carbanions by Microdroplets

“…The study noted that protons affect the water surface at a few millimolar bulk concentration; on the contrary, the surface remains unperturbed until a several hundred millimolar hydroxide ion concentration. While the above report can be supported by some studies and contradicted by others, − it appears to be a trade-off between two important factors, e.g., droplet charge density and charge partitioning at the interface that guides the ion ejection in our study. The high surface affinity of protons (positive charges) generates a high electric field at the charged water droplet surface, effectively desorbing the carbocation species from the air–water interface .…”

Fraction of Free-Base Nicotine in Simulated Vaping Aerosol Particles Determined by X-ray Spectroscopies

“…For example, interfacial reactions on aqueous microdroplets leading to protonation and uptake of gas phase molecules at the outermost layer have been used as probes to demonstrate that droplet surface conditions were close to Brønsted-neutral despite more acidic (i.e., pH 3 or 4) core conditions. This effect was attributed to reduced hydration of ionic species at the interface, forcing acids and bases toward their undissociated forms. − The presence of PG and VG in our experiments has likely amplified this effect, leading to the complete absence of protonated nicotine at the aerosol surface …”

Chemical transformations and transport phenomena at interfaces

“…The charge state and related acidity or basicity of the air–water interface is still not settled 20 ; electrokinetic mobility experiments 20,21 indicate that the interfaces takes on a negative charge for pH > 2–3, and thereby suggesting that the surface may be rich in OH − . While hydroxide ions can be found at the interface, more recent experimental techniques that directly probe surface concentrations find the surface to be richer in H 3 O + than bulk water 22,23 …”