Law and Mechanism Study on Salt Resistance of Nonionic Surfactant (Alkyl Glycoside) Foam

Abstract

In this paper, the effects of three cations, Ca2+, Mg2+, and Na+, on the stability of APG foams were investigated experimentally. The results show that cations can slow down the process of liquid drainage and coarsening of APG foam, which is beneficial to the stability of the foam. The salt resistance mechanism of nonionic surfactant (APG) was investigated by molecular dynamics simulation and compared with that of anionic surfactant (SDS) foam. Firstly, the distribution characteristics of cations in APG foam and SDS foam were explored. It was found that the cations in the APG foam were mainly distributed in the water layer away from the head groups, and the cations in the SDS foam were more likely to appear near the head groups. Then, the hydration of the head groups and the cation was investigated. The results show that cations have little effect on the number of water molecules in the hydration layer of APG head groups but will reduce the diffusion capacity of water molecules and increase the water retention capacity of the foam film, thereby enhancing the foam stability. The addition of cations will reduce the water retention capacity of the SDS foam film. In addition, the behavior of surfactant head and tail groups was also analyzed. It was found that the cations made the head groups of APG more inclined to be aligned perpendicular to the liquid interface, and the tail groups were more inclined to realize a cross-arrangement and cover the gas–liquid interface. This can not only slow down the gas phase mass transfer process of the adjacent foam and slow down the coarsening process of the foam but also increase the viscoelasticity and anti-disturbance ability of the foam film. The cations will weaken the staggered arrangement of the SDS molecular tail groups, and at the same time, will cause the SDS molecules to aggregate, which greatly reduces the stability of the foam.