Molecular Dynamics Study of Silica Nanoparticles and CO-Switchable Surfactants at an Oil-Water Interface.

Adsorbing CO-sensitive surfactants on the surface of nanoparticles is an important strategy for preparing stimuli-responsive Pickering emulsions. However, the microscopic mechanisms are still limited, owing to a lack of intuitive understanding at the molecular level on the interactions between nanoparticle and switchable surfactants at the oil-water interface. We employed the molecular dynamics (MD) simulations to explore the mechanism behind the reversible emulsification/demulsification of a Pickering emulsion stabilized by silica nanoparticles (NPs) and CO-switchable surfactants, named -(3-(dimethylamino)propyl)alkyl amide (CPMA). MD results show that the protonated surfactant CPMAH has strong hydrophilicity, forming an adsorption layer at the oil-water interface. The ionic surfactants can be tightly adsorbed on NP surface through electrostatic interactions. Thus, the formed colloid particle has both hydrophobic and hydrophilic properties, which is a key factor in stabilizing emulsion. When CPMAH molecules were deprotonated to CPMA, the hydration activity of the headgroups reduced greatly, inducing a mixture with oil molecules. There are still a certain number of CPMA molecules residing at the oil-water interface due to the hydrophilic amine groups. The results from repeated simulations show that NP can either stay in the water phase or locate at the interface. Even NP was finally adsorbed on the interface and combined with CPMA or oil molecules, the adsorption configuration of CPMA on the NP surface was essentially different from that of CPMAH. The potential of mean force confirmed that the combination between NP and CPMA is quite unstable due to the disappearance of electrostatic attraction. Different binding configurations and stability between NP and CPMA or CPMAH were the fundamental reason for the reversible emulsification/demulsification of Pickering emulsion.