Surfactant and Colloid Self-Assembly Simulations

Abstract

Surfactants and colloids are ubiquitous in biology, the environment, home and personal care products, and industrial applications.

The self-assembly of surfactants into micellar aggregates is crucial to their use in many industrial applications, including: detergency, cosmetics, oil-spill remediation and nanoparticle dispersion.

Because of their ubiquity, surfactant and colloid self-assembly are canonical subjects of soft matter study, yet there are concentration effects, for example the free surfactant concentration, that remain difficult to measure and understand.

We present our findings on the driving forces behind aspects of self-assembly and methodologies for characterizing the behavior of three self-assembling systems: colloids with short-range attraction long-range repulsion, nonionic surfactants and ionic surfactants.

To study molecular details of these systems, we use Monte Carlo and molecular dynamics simulations with implicit-solvent models.

We directly compare different methods for calculating the critical micelle concentration in all self-assembling systems. We establish that excluded volume and counterion condensation are the main driving forces for the decrease of free surfactant concentration in nonionic and ionic surfactants, respectively.

We compare counterion condensation and mean ionic activity measurements directly to experiments, and address experimental disagreements.

For short-range attraction long-range repulsion colloids, conditions are found where large preferred aggregates have no net effect on the pressure, which is strikingly different behavior from surfactant self-assembly.

The results in this thesis offer a deeper understanding of the phenomena of low-concentration self-assembly, leading to improved methods for estimating micellar properties over wider concentration ranges.

These methods will benefit the development of more accurate molecular models and speed up development of new industrial formulations.