Influence of hydrophobic chain structure and headgroup ionization of amino acid surfactants on emulsion stability and recyclable liquid-liquid interfacial catalysis

Abstract

Emulsion catalysis, a subset of ’on-water’ catalysis, is influenced by surfactants that act as emulsion stabilizers. The hydrophobicity of these surfactants, closely tied to the molecular structure of their chains and their headgroup ionization, plays a crucial role in controlling emulsion reactions and subsequent processes of surfactant recovery and product purification. However, the influence of the specific chain structure of surfactants and its cooperation with the headgroup in different solution environments on the emulsion reaction and demulsification processes remains uncertain. In this study, the stability of emulsions stabilized by amino acid surfactants featuring various hydrophobic chains, specifically side chains, was studied across a range of pH conditions. Amino acid surfactants with long chain are pivotal in enhancing the stability of emulsions. The combined effect of hydrogen bonding and electrostatic interactions between the amino acid headgroups boosts the stability when the pH of the solution aligns closely with the pKa2 of the surfactants. Under these conditions, a relatively stable oil–water interface significantly increases the yield of the Knoevenagel reaction to 94 %. As the pH of the system increases, the surfactants become more hydrophilic, enabling the phase separation of the emulsion, which facilitates the collection of the product and the recycling of the surfactants. This study provides important insights on regulating the structure and function of the oil–water interface based on surfactants and proposes a potentially effective approach for environmental-friendly and convenient chemical synthesis.