Changes in Protonation State of Atmospherically Relevant α-Hydroxyacids at the Air-Water Interface Measured by...

Characterization of the acid-base behavior and surface protonation state of atmospherically relevant organic acids is of key importance in our understanding of interfacial reactivity, as well as our ability to accurately model aerosol impact on climate. Here we investigate the protonation state of two medium-chain α-hydroxyacids, 2-hydroxyhexanoic acid (HHA) and 2-hydroxyoctanoic acid (HOA), at the air-water interface and in the bulk. The ratio of surface-deprotonated to surface-protonated species at varying pH was examined using surface tension titrations, finding an effective surface-pK_a of 4.5 ± 0.2 for HHA and 5.41 ± 0.05 for HOA, both of which are significantly higher than their bulk pK_a values of 3.9 ± 0.1 and 4.0 ± 0.1, respectively, which were determined via potentiometric titration. However, the effective surface-pK_a obtained from surface tension measurements also contains contributions from adsorption and desorption processes, which means that it does not directly probe differences in the dissociation equilibrium at the interface. We show that infrared reflection-absorption spectroscopy (IR-RAS) can be used to directly probe the surface dissociation of α-hydroxyacids in situ for the first time, demonstrating the utility of IR-RAS as a technique for these types of studies. By correcting for the relative surface activity of the anion and acid species, the surface-pK_a obtained using IR-RAS is a better measure of the actual shift in dissociation equilibrium at the interface. Through comparison to the bulk spectra obtained using attenuated total reflectance (ATR) spectroscopy, we confirmed that the protonated form of the α-hydroxyacids is favored at the water surface. However, we find that the difference between the surface-pK_a and bulk pK_a obtained spectroscopically is 0.2 ± 0.1 for HHA and 0.4 ± 0.2 for HOA. This suggests that the relative shift in the dissociation constant at the interface is modest, and that adsorption processes play an important role in the speciation at the interface and must be explicitly considered in these studies. Overall, we confirm the importance of fundamental lab studies to examine the speciation at air-water interfaces as a function of solution condition, as bulk pH alone is not sufficient to predict the distribution of species present at the interface.