Cooperative Surface Adsorption and Emergence of Antiferromagnetic-like Order in p-Methyl Benzoic Acid

A remarkable cooperative enhancement of surface concentration at the 2D air-water interface is observed as a function of pH for p-methyl benzoic acid (pMBA) and its conjugate base (pMBA^-). Polarization-dependent vibrational sum-frequency generation (SFG) spectroscopy¹ of carbonyl and carboxylate stretching modes show that the anomalous enhancement is due to an increase in the number density of both molecular species. This SFG enhancement is observed for a narrow pH range (~0.5) centered at pH 6.3, close to the putative surface pK, and is attributed to a cooperative attractive quantum interaction between pMBA and pMBA^-. A 3-state ‘spin’ lattice model successfully reproduces the sharp peak seen in the experiments. A detailed study of mean-field theory and lattice simulations shows the emergence of a highly ordered state analogous to antiferromagnetism. With the appropriate choice of system parameters, surface concentrations as a function of pH undergo either a first or second order transition. This work lays the basis for direct detection of order caused by novel acid-anion interactions that are of considerable recent interest for understanding biological surfactants.

1. Andino et al, J. Phys. Chem A, 124, 3064-3076 (2020).