Molecular characteristics of the mixed cationic/anionic collector interface probed by sum-frequency vibrational spectroscopy and molecular dynamics simulations

•The interfacial water in the NaOL system is more strongly hydrogen bonded than for the DDA and mixed collector systems.•The alignment of interfacial water molecules in the DDA system was more disrupted.•The COO– group stretches deeper into water than NH4+ groups.•It is hard for DDA to form a stable compact and DDA has a tendency to form a bilayer. The molecular structure and orientation of collectors and water molecules on bubble surfaces in ore pulp is a fundamental phenomenon in the process of froth flotation. Considering the adsorption behavior and orientation of collectors and water molecules at the air/solution interface is of particular significance. In this study, the molecular structure and orientation of an aqueous solution of mixed dodecylamine hydrochloride (DDA) and sodium oleate (NaOL) with different molar ratios, and water molecules were examined at the air/solution interface and characterized using a combination of surface pressure measurements, sum frequency vibrational spectroscopy (SFVS) and molecular dynamics (MD) simulations. Both individual NaOL and DDA collectors were observed to be well ordered at the interface, while gauche defects appeared in some mixed collector systems. The ordering of interfacial water molecules in the presence of collectors with increasing molar ratios of NaOL to DDA is manifested by an enhancement of OH symmetric stretch from tetrahedrally coordinated water and a decreased bond disorder in the molecular arrangement or a “liquid-like” structure. In these mixed collector systems, the COO– group from NaOL stretches deeper into water than NH3+ groups from DDA, and Na cations originating from NaOL distribute closer to COO– groups compared with the Cl originating from DDA to NH3+ groups. The interfacial water in the single NaOL system is more strongly hydrogen bonded than those in the DDA and mixed collector systems. Conversely, the alignment of interfacial water molecules in the DDA system was more disrupted. These results provide a better understanding of the ordering of mixed NaOL/DDA collectors, as well as the orientation of water molecules at the air/water interface. This might therefore provide valuable insight into the relationship between the flotation phenomena and molecular characteristics of collectors at the air/water interface and should help to improve the froth flotation technology.