The Impact Of Salt Type And Concentration On The Self-assembly Of Zwitterionic Surfactants: A Molecular Dynamics Simulation Study

Surfactants can self-assemble into ordered aggregates in solutions. The rheologicalproperties of fluids are related to the self-assembled structures of surfactants. Zwitterionicsurfactants generally refer to the surfactants that have both positive and negative charges in itspolar head groups. The experimental results showed that the self-assembled structures ofzwitterionic surfactants are insensitive to monovalent inorganic cations (for example, Na+), butare dependent on the high-concentrations divalent inorganic cations (for example, Ca2+) andorganic salts (for example, NaSal). In this paper, we investigated the self-assembly processesof the zwitterionic surfactant oleamidopropyl dimethyl betaine (ODB) by molecular dynamics(MD) simulation method, and studied the impact of salt type and concentration on itsself-assembled structures.Firstly, the impact of the concentrations of inorganic salts NaCl and CaCl2on theself-assembly process and aggregation structures were investigated. It was found that the ODBsurfactants only formed spherical micelles in NaCl solutions, showing that the effect ofmonovalent Na+on the self-assembled structures of ODB molecules can be neglected. TheODB molecules still formed spherical micelles in low-concentration CaCl2solutions, whilethey formed worm-like micelles in high-concentration CaCl2solutions. Secondly, we studiedthe impact of organic salt NaSal on the self-assembly behavior and self-assembled structuresof ODB molecules. We found that the ODB molecules can form worm-like micelles even atlow NaSal concentrations. While the NaSal concentration is increased, the ODB moleculesmay form longer worm-like micelles and even entangled worm-like micelle network.By calculating the radical distribution functions of different ions and comparing theresults with experimental findings, the impact of salt type and concentration on the formationprocess and mechanism of worm-like micelles by zwitterionic surfactants was discussed indetails. For inorganic salts, the results show that both Na+and Ca2+form a cationic layer onthe surface of ODB micelles, while anionic Cl-layer is outside of the cationic layer. The effectof Na+on the distribution of water molecular layer around ODB aggregates is very weak,indicating that Na+does not reduce the effective area of polar head of ODB molecules, and thus only spherical micelles were formed. However, the high-concentration Ca2+ions reducethe effective area of the polar head of ODB molecules by significantly altering the distributionof water molecular layer around the ODB aggregate surface, resulting in the formation ofworm-like micelles. In addition, the binding energy barrier and dissociation energy barrierbetween Ca2+and ODB molecules are much higher than that between Na+and ODB.Therefore, the Ca2+layer outside the ODB micelles are more stable. For organic salt NaSal, byinserting into the ODB micelles, the Sal-ions increase the effective area of hydrophobic tail ofODB surfactants as well as inducing steric-hindrance effect, which greatly favors theformation of worm-like micelles. Our simulation results shed light on the understanding of theself-assembly mechanisms of zwitterionic surfactants in different salt solutions.