Molecular Dynamics Study of Silica Nanoparticles and CO2-Switchable Surfactants at an Oil-Water Interface

Adsorbing CO2-sensitive surfactants on thesurface ofnanoparticles is an important strategy for preparing stimuli-responsivePickering emulsions. However, the microscopic mechanisms are stilllimited, owing to a lack of intuitive understanding at the molecularlevel on the interactions between nanoparticle and switchable surfactantsat the oil-water interface. We employed the molecular dynamics(MD) simulations to explore the mechanism behind the reversible emulsification/demulsificationof a Pickering emulsion stabilized by silica nanoparticles (NPs) andCO(2)-switchable surfactants, named N-(3-(dimethyl-amino)propyl)alkylamide (CPMA). MD results show that the protonated surfactant CPMAH(+) has strong hydrophilicity, forming an adsorption layer atthe oil-water interface. The ionic surfactants can be tightlyadsorbed on NP surface through electrostatic interactions. Thus, theformed 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 theheadgroups reduced greatly, inducing a mixture with oil molecules.There are still a certain number of CPMA molecules residing at theoil-water interface due to the hydrophilic amine groups. Theresults from repeated simulations show that NP can either stay inthe water phase or locate at the interface. Even NP was finally adsorbedon the interface and combined with CPMA or oil molecules, the adsorptionconfiguration of CPMA on the NP surface was essentially differentfrom that of CPMAH(+). The potential of mean force confirmedthat the combination between NP and CPMA is quite unstable due tothe disappearance of electrostatic attraction. Different binding configurationsand stability between NP and CPMA or CPMAH(+) were the fundamentalreason for the reversible emulsification/demulsification of Pickeringemulsion.