Interfacial properties and salt tolerance of carboxylated nonylphenol ethoxylate surfactants

Ionic alkoxy-based surfactants recently attract more attentions because they possess advantages of both nonionic and anionic surfactants, and surfactant structure is the key factor affecting the interfacial property and salt tolerance of ionic alkoxy-based surfactant. In the present study, carboxylated nonylphenol ethoxylate surfactant (NPEOnC, n = 2, 4, and 7) at n-decane/water interface has been studied using molecular dynamics simulation in the absence and in the presence of NaCl with different concentrations. The interfacial activity in reducing interfacial tension and salt tolerance of NPEOnC were investigated and the effect of numbers of ethoxy unit were quantified. The interfacial activities of NPEOnC were demonstrated by the arrangement of surfactant monolayer, the structures of n-decane/water interface, and the interfacial tensions. Salt tolerance of NPEOnC were estimated by the bound pairs between surfactant and water as well as those between surfactant and Na+, especially the structural relaxation dynamics of those bound pairs. Results showed that NPEO7C exhibited the highest activity in lowering the interfacial tension of n-decane/water, and kept the interfacial activity in the presence of NaCl. Carboxyl groups were the predominant groups in binding with Na+, and ethoxy segment within NPEO4C and NPEO7C were the governing groups in being solvated with water. Moreover, the structural relaxations of bound pairs between oxygen within ethoxy segment and water/Na+ were much slower than those between oxygen within carboxyl group and water/Na+, which resulted in the higher salt tolerance of NPEOnC with more ethoxy units.