Predicting apparent surface pK_a values of alkyl acids at the air-water interface: decoupling partitioning from...

Understanding both the partitioning of protonatable molecules to the air-water interface and their subsequent acid-base behavior at the surface is important for accurately modeling the impact of organic aerosol on climate and describing accelerated chemistry in microdroplets. Generally, the uncharged state of a surface-active, protonatable molecule preferentially adsorbs to the air-water interface, leading to apparent surface pK_a values that can differ significantly from those in bulk solution. However, it is unclear how apparent surface pK_a values arising from differences in partitioning behavior compare to 'true' surface pK_a values describing acid-base chemistry at the air-water interface. Here, surface tension titrations are used to measure the concentration dependence of apparent surface pK_a values of various medium-chain alkyl acids (ranging from C₆ to C₁₀). Experimental apparent surface pK_a values for each alkyl acid increase as total acid concentration increases, with surface pK_a values ranging from ∼5 (i.e., close to the pK_a in bulk solution) to ∼7. A simple Langmuir competitive adsorption model that separately describes the partitioning of protonated (uncharged) and deprotonated (charged) acid molecules to the air-water interface reproduces the observed concentration dependence in apparent surface pK_a values. Because this model successfully reproduces the data, it suggests that the total interfacial concentration is independent of surface acid-base chemistry. This is further supported by a microkinetic model describing both the partitioning of protonated and deprotonated molecules to the surface and acid-base chemistry at the surface. Based on these insights, we derive a simple algebraic expression that predicts the concentration dependence of the apparent surface pK_a of surface-active protonatable species in terms of its bulk solution concentration, the bulk pK_a value, and the Langmuir adsorption constants for the protonated and deprotonated species. Overall, this work demonstrates that the total concentration of protonatable species at the air-water interface is well-described by partitioning equilibrium, but the speciation at the surface is predicted to be controlled almost exclusively by the 'true' surface pK_a value.