Molecular dynamics simulation of the influence of polyacrylamide on the stability of sodium dodecyl sulfate foam

- The foam stability of SDS-PAM mixtures is investigated by MD simulations.
- The elasticity of foam liquid film increases with increasing of PAM concentration.
- The denser SDS monolayer is observed at HPAM 20% in the four foam systems.
- The foam stability of SDS-PAM systems is the best in the range of HPAM 20-30%.

Molecular dynamics simulations were performed to investigate the effect of the concentration of polyacrylamide (PAM) and the hydrolysis degree of partially hydrolyzed polyacrylamide (HPAM) on the stability of sodium dodecyl sulfate (SDS) foams. The effect of polymer on the complex systems (PAM-SDS and HPAM-SDS) was investigated by analyzing the density distribution of SDS headgroups, the radial distribution function (RDF) of hydration of SDS headgroups, the mean square displacement (MSD) of hydrated water around the amide group. The results show that an increase of PAM concentration can enhance the interaction between PAM and SDS, increase the number of hydrated water molecules around SDS headgroups and weaken the mobility of the hydrated water molecules around PAM, which will be beneficial to improve the foam stability. It has been observed that the favorable structure of SDS monolayer for stable foam appears when the hydrolysis degree of HPAM is 20% in the four foam systems of our simulations. In all four foams, the numbers of water molecules around headgroups increase first with the increase of HPAM hydrolysis degree, reaching a maximum value at HPAM 20%, followed by a decrease with larger hydrolysis degrees. These observations indicate that the foam is the most stable when the hydrolysis degree of HPAM is within the range of 20-30%.