Effects of Poly(ethylene oxide) on the Foam Properties of Anionic Surfactants: Experiment and Molecular Dynamics Simulation

Water-soluble polymers are often used as additives to adjust the foam properties of surfactant. In this study, the effects of water-soluble polymer poly(ethylene oxide) (PEO) on foam properties of two anionic surfactants, i.e., ammonium lauryl ether sulfate (ALES) and sodium dodecyl sulfate (SDS), were investigated by experimental and molecular dynamics simulation methods. Experimental results show that the addition of PEO can reduce the foaming ability of the two surfactants, but the inhibitory effect of PEO on the foaming ability is weakened at high surfactant concentration. Compared with ALES, PEO has a more significant inhibitory effect on the foaming ability of SDS. With the increase in PEO concentration, the half-life time of foam drainage in surfactant/water-soluble polymer composite systems gradually increases. The synergistic effect between PEO and ALES is stronger than that between PEO and SDS, resulting in a longer half-life time of foam drainage in ALES/PEO composite system. Molecular dynamics simulation results indicate that the addition of PEO can decline the air-water interface thickness of bubble films and the tail tilt angle of surfactant molecules at the air-water interface. The reduction in tail tilt angle means that the surfactant molecules are more vertical to the air-water interface and the hydrophobic interaction between adjacent tail chains of surfactants is weakened, which is unfavorable to the formation of bubble films, thus decreasing the foaming ability of surfactants. Because the ALES/PEO system has larger air-water interface thickness and surfactant tail tilt angle than the SDS/PEO system, the inhibitory effect of PEO on the foaming ability of ALES is weaker than that of SDS. Adding PEO can lower the peak position of the first hydration layer of surfactant head groups, increase the number of hydrogen bonds, and reduce the diffusion coefficient of water molecules, so that the surfactant/water-soluble polymer system has longer half-life time of foam drainage than the pure surfactant system. Due to the synergistic effect between ALES and PEO, the ALES/PEO system has a higher peak value of the first hydration layer of surfactant head groups, more hydrogen bonds, and lower diffusion coefficient of water molecules than the SDS/PEO system. Therefore, the half-life time of foam drainage in the ALES/PEO system is longer than that in the SDS/PEO system.