Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Applied Mathematics

Supervisor

Dr.Mikko Karttunen

Abstract

We performed coarse-grained molecular-dynamics (MD) simulations to study the structural and dynamical properties of surfactant micelles in equilibrium and under Poiseuille-like flow in a nano-confined geometry. We used the MARTINI force-field to model the interactions between water molecules, counter-ions, and sodium dodecyl sulfate (SDS) surfactants. SDS surfactant was chosen as the standard model because of its potential application in drug delivery systems. First, we focused on the self-assembly of SDS in equilibrium. To form stable spherical mi- celles, we ran simulations in the isothermal-isobaric ensemble (NPT) on a system of free SDS surfactants, counter-ions and water molecules. We studied the aggregation number, shape and radius of the SDS micelles in equilibrium. These results agree well with all-atom simulations of SDS. Second, we studied the spreading of a spherical micelle on a solid surface over various interaction strengths in a system consisting of a spherical SDS micelle, and counter-ions in the vicinity of a surface and water molecules. The interaction energies between walls and surfac- tants were parameterized at three distinct levels corresponding to non-, low-, and high-wetting walls surfaces. The wetting properties of the surfaces were determined by calculating the con- tact angles of the micelle on the surface in equilibrium. We calculated the contact angle from Young’s equation through measuring the surfaces tension of wall-water, wall-SDS, and water- SDS. As the micelle interacts with the surface, it either forms a cap, a bulb-shape structure, or detaches itself and floats away on high-, low-, and non-wetting surfaces respectively. Third, we explored the effect of flow, confinement, and wetting on SDS micelles when the micelle is forced through a channel slightly smaller than its equilibrium size. We performed simulations on micellar solutions confined in a die geometry in the isothermal ensemble (NVT). We show that the flowing micelle adopts different shapes to pass through the channel. Inside the channel, the micelle may fragment into smaller micelles. We demonstrate that in addition to the flow rate, the wettability of the channel surface dictates whether the micelle fragments and determines the size of daughter micelles.