Vibrational sum-frequency generation (SFG) spectroscopy is used to determine the surface pK a of p-methyl benzoic acid (pMBA) at the air–water interface by monitoring the carbonyl and carboxylate stretching modes over the pH range of 2 to 12. The SFG intensities of pMBA and its conjugate base, p-methyl benzoate (pMBA−), exhibit an anomalously large enhancement over a narrow pH range (∼0.5) centered at pH 6.3 near the SFG-determined surface pK a, 5.9 ± 0.1. The increase in the surface pK a relative to the bulk value of 4.34 is consistent with the trend previously observed for long chain carboxylic acids in which the surface pK a is higher than the bulk solution pK a. SFG polarization studies help distinguish the orientation and number density contributions to this observed anomalous surface phenomenon. The large SFG intensity increase is attributed to an increase in the pMBA and pMBA− surface concentrations in this narrow pH range due to a cooperative adsorption effect between pMBA and pMBA−. This cooperativity is manifested only on the 2D air–water interface, where the interactions between the acid and base are not as dielectrically screened as in the aqueous bulk phase. Surface effects are critical to understanding and controlling the reactivity, solubility, and behavior of organic acids at interfaces and can have an impact on biomedical applications.

A lattice model is described to explain a recent striking Sum Frequency Generation (SFG) observation of a cooperative surface adsorption effect for an organic acid system at an air−water interface. The reported anomalous pH-dependent enhancement in p-methylbenzoic acid (pmBA) arises from an interaction between the acid (HA) and its conjugate base anion (A − ), which competes with strong Coulombic repulsion between the conjugate bases (A − −A − ). Using a statistical mechanical approach, this lattice gas model reveals an analogy to well-studied magnetic systems in which the attraction between the two different molecular species leads to a phase transition to a two-dimensional checkerboard phase consisting of a network of anion−acid complexes formed at the low-dielectric air−water interface. Cooperative acid− anion interactions that control partitioning at solution and aerosol interfaces are of interest to fields ranging from oceanic and atmospheric chemistry, pharmacology, and chemical engineering.