Conjugate Acid–Base Interaction Driven Phase Transition at a 2D Air–Water Interface

Abstract

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.

Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

“…The adsorption behaviors of these mixtures with pH have been investigated at the oil/water and air/water planar interfaces by using a variety of techniques. ,, Planar studies show that the charged carboxylate surfactant on its own preferentially adsorbs to the oil/water interface, adopting a significant degree of net orientation, while the neutral carboxylic acid surfactant on its own is present in small amounts but has only a small degree of net orientation . With a mixture of carboxylic acid and carboxylate surfactants there is thought to be the formation of acid–anion complexes at the planar air/water interface and in the bulk, which was not observed at the planar oil/water interface. , …”

Sixty Years of Surface-Specific Spectroscopy

“…Singh and co-workers used VSFG to study how DNA changes the structure of water at the surface of cationic and zwitterionic lipid monolayers. , In many of these studies, the interpretation of the experimental data is enhanced by atomistic simulations. For instance, statistical mechanics techniques were employed to aid the interpretation of nonlinear optical responses from aqueous interfaces by Rajagopal et al who used a lattice model to obtain detailed molecular insights into cooperativity in interfacial acid–base chemistry.…”

“…51,52 In many of these studies, the interpretation of the experimental data is enhanced by atomistic simulations. For instance, statistical mechanics techniques were employed to aid the interpretation of nonlinear optical responses from aqueous interfaces by Rajagopal et al 53 who used a lattice model to obtain detailed molecular insights into cooperativity in interfacial acid−base chemistry. Despite a relatively slow start in life (more than 30 years from discovery to widespread implementation), SHG and VSFG have become perhaps the most important tools available to physical chemists for investigating the properties of interfaces when the traditional methods of surface science that require ultrahigh vacuum are not applicable.…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…This observation dates back to the 1930s for carboxylic acids 27 and amines 28 and to the 1950s for polypeptides and proteins, 29 and it is still an active area of research today. [30][31][32][33][34][35][36] Accordingly, surfactants in atmospheric aerosol particles present rich systems for both fundamental and applied chemical research.…”