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Investigation of Surfactant Aggregation in Solutions and at the Calcite/Water Interface
Surfactants self-assemble into diverse aggregate structures like micelles and bilayers. These aggregates play a vital role in applications ranging from wastewater treatment to soil remediation to carbon utilization. Therefore, an understanding of aggregate morphologies in solutions and at interfaces allows us to tailor surfactants for specific applications. To that end, we have used molecular dynamics (MD) simulations to glean atomic-level insights into surfactant aggregation.
First, we focus on the self-assembly of Aerosol-OT (AOT), a double-tailed anionic surfactant in aqueous media. Through extensive classical MD simulations, AOT morphologies are generated that are consistent with experimental phase diagrams. Aggregates range from spherical micelles at low concentrations (1 wt.%) to bilayers at higher concentrations (20 wt.%). A transitional biphasic regime is identified at an intermediate concentration (7.2 wt.%); this regime comprises prolate spheroidal and long rod-like micelles. Metrics of micelle shape and size are computed from the moments of inertia tensor. The polydispersity in these metrics are also quantified. The bilayer thickness and area per AOT head group agree with experimental measurements. The simulations also reveal atomic-level mechanistic insights into the early stages of surfactant aggregation. Taken together, these simulations elucidate the structural diversity of AOT aggregates as a function of concentration and temperature thus being complementary to mean-field experiments.
Second, we attempt to understand the role of AOT in the valorization of carbon dioxide to calcium carbon ultrafine particles. Through an enhanced sampling MD technique called umbrella sampling, the interactions between a single molecule of AOT and the (10m14) crystal plane of CaCO3 are investigated. These simulations are complemented by a first principles Density Functional Theory (DFT) analysis which is performed through collaboration. DFT identifies the most stable adsorption configurations for AOT-like surrogate molecules and unravels the nature of chemical bonds between these molecules and the (10m14) crystal plane.
Finally, preliminary experimental studies pertaining to the synthesis of CaCO3 particles using CO2 and surfactants is discussed. The CaCO3 particles are characterized using X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The influence of the nature of the surfactant; anionic AOT, or cationic cetyl trimethylammonium bromide (CTAB), on the morphology of CaCO3 particles is discussed.