(50g) A Molecular Dynamics Study to Investigate the Structural Features of Surfactant Aggregates Adsorbed at Mineral/Water Interfaces

Surfactants have long been employed to stabilize colloidal nanoparticles in suspensions through a combination of electrostatic and non-covalent interactions. Colloidal nanoparticles remain dispersed because of favorable interactions between surfactant assemblies such as micelles and bilayers, and the surface of these nanoparticles. Prior studies have discussed the structural and energetic features of these surfactant assemblies using experiments and molecular simulations. Despite recent advances, the formation mechanism for surfactant aggregates beginning with individual surfactant molecules at mineral/water interfaces (MWI) remains relatively unexplored. Herein, with the CaCO₃/water interface as a model system for the MWI, we use molecular dynamics simulations to rigorously investigate the mechanism by which surfactant aggregates like micelles and lamellar bilayers are formed. We also discuss the morphological features of these aggregates. We equilibrate our system through successive MD simulations in the NVT and NPT ensembles, following which we perform a production run in the NPT ensemble. Free energies from our MD simulations reveal the relative stabilities of micelles versus lamellar bilayers as a function of the surfactant concentration and environmental variables like temperature and counter-ion identity.