Specific anion effects on adsorption and packing of octadecylamine hydrochloride molecules at the air/water interface

Abstract

The spread of collector molecules on bubble surfaces in salt solution is a fundamental phenomenon in the froth flotation of soluble salts, such as KCl, and consideration of ionic effects on adsorption behavior and packing of collector molecules at the air/solution interface is appropriate for study. In this work, specific anion effects on the adsorption and packing of one common collector (octadecylamine hydrochloride, ODA) at the air/water interface were investigated by employing a combination of surface pressure measurements, sum frequency vibrational spectroscopy (SFVS) and molecular dynamics simulations (MDS). The results from experiments and MDS showed that the addition of external anions disturbs the arrangement of ODA molecules due to the penetration of these anions from the water subphase into the ODA monolayer at the air/water interface. During monolayer compression, the penetrated anions might be driven back to the water subphase by electrostatic forces and the added compression force. These anions could screen the charge of NH3 groups resulting in a reduction of the minimum area of the ODA molecule (Amin) for the solid phase state of the monolayer. With the exception of the sulfate anion, the added anions increase hydrogen bonding of interfacial water molecules at the air/water interface. These results allow for a better understanding of the anionic effect on adsorption and packing of ODA molecules at the air/water interface, and might therefore provide new insights into froth flotation of soluble salts and should help to improve soluble salt flotation technology.