Structural Insights into Self-Assembled Aerosol-OT Aggregates in Aqueous Media Using Atomistic Molecular Dynamics

Area of Science:

• Physical Chemistry
• Materials Science
• Soft Matter Physics

Background:

• Surfactant dioctyl sulfosuccinate (Aerosol-OT) forms various aggregates in water, including micelles and lamellae.
• A detailed atomic-level understanding of AOT aggregate formation and structure is currently lacking.

Purpose of the Study:

• To investigate the self-assembly of AOT in water across different concentrations and temperatures using atomistic molecular dynamics.
• To elucidate the mechanisms of aggregate formation, including molecular association, dissociation, fusion, and fission.
• To characterize the structural properties of the formed aggregates and their dependence on concentration and temperature.

Main Methods:

• Atomistic molecular dynamics (MD) simulations were performed for AOT in water at 1, 7.2, and 20 wt % at 293 K, and at 7.2 wt % at 353 K.
• Simulations spanned microsecond timescales to capture the dynamic self-assembly process.
• Graph theory was employed to identify and analyze individual aggregates.

Main Results:

• At 293 K, AOT self-assembled into spherical micelles (1 wt %), a biphasic system of rod-like and prolate spheroidal micelles (7.2 wt %), and bilayers (20 wt % at 293 K).
• Increasing temperature to 353 K at 7.2 wt % resulted in prolate micelles, but no rod-like structures were observed.
• The simulated phase behavior aligns with experimental observations, with rod-like structures hypothesized as precursors to lamellar microdomains.
• Structural metrics such as radius of gyration, shape anisotropy, and prolateness quantitatively described aggregate size and shape variations.

Conclusions:

• Atomistic MD simulations provide unprecedented atomic-level insights into AOT self-assembly in water.
• The study successfully reproduces experimentally observed phase behavior and identifies key structural transitions.
• The findings offer a physics-based approach to generate micelle structures, aiding in the rationalization of experimental data and advancing the understanding of surfactant systems.