Unique Solubility of Switchable Alkyl‐Amine Surfactants in Water

Because many amine surfactants are soluble in both water and CO2 phases, they attract interest with regard to stabilizing CO2‐in‐water dispersion systems. In our recent research, we find that the solubility of alkyl‐amine surfactant in water can be significantly enhanced by salts, even though the salts are usually “salting‐out” to other surfactants. The influence of various anions (NO3−, Br−, Cl−, and SO42−) and cations (Na+, Ca2+, and Mg2+) on the alkyl‐amine surfactants is investigated. The results are contrary to Hofmeister series and show that all the anions can enhance the solubility (salting‐in) in the order of: NO3− > Br− > Cl− > SO42−, while the impact of the cations is insignificant. A physical–chemistry model based on the switchable property of the surfactant is proposed and well explains the experimental results. Therefore, the switchable alkyl‐amine surfactants have potential to be applied under high‐salinity and high‐temperature conditions, for example, in enhanced oil recovery processes for a hot and salty carbonate reservoir.