Molecular dynamics simulation of self-assembly and viscosity behavior of PAM and CTAC in salt-added solutions

In this study, the self-assembly of cationic surfactant cetyltrimethyl ammonium chloride (CTAC) and nonionic polymer polyacrylamide (PAM) in aqueous solution with added sodium salicylate (NaSal) as counter-ion salt is studied using a coarse-grained molecular dynamic simulation (CGMD). The microstructure evolution shows that in the mixed solutions, surfactant micelles are readily to associate with polymer and can form a new complex of CTAC and PAM with interconnected network structures, which can be attributed to the hydrophobic interactions, the attraction between acylamino and the head of CTAC, and the attraction between Sal⁻ and acylamino. The calculation of radial distribution function for different sites also proved the aggregate of the surfactant and polymer. Then a reverse non-equilibrium molecular dynamics (RNEMD) simulation is used to investigate the rheological property of the mixed solutions, the CTAC/NaSal solution and the PAM solution. The molecular dynamics simulation demonstrates that the viscosity of the CTAC/PAM/NaSal solution is significantly higher than that of the CTAC/NaSal solution, mainly because the surfactant micelles interacted with the polymer and formed rod-like micelles.