The study of self-assembly dynamics that lead to ultra-thin films at an interface remains a very active research topic because of the ability to generate interesting and useful structures that can perform specific tasks such as aiding in magnetic separations, acting as two-dimensional biosensors and the modification of surfaces without changing their morphology. In some cases this is critical to surface functionality. This area is also important because it may ultimately yield simple screening techniques for novel surfactants and nanoparticle species before they are introduced onto the market, as is already happening. In the work presented here we studied the assembly dynamics of the insoluble lipid DPPC, common to most mammalian cell lines, in the presence of soluble surfactant and nanoparticle species. Preliminarily, we investigated the anomalous behavior of soluble surfactants at the air/water interface as an extension to work done previously and then used these results in an attempt to explain the behavior of the combined surfactant/lipid monolayer under dynamic and static conditions. Separately, we performed nanoparticle dynamics studies for a commercially available product and for particles created in-house before performing similar statics/dynamics work on the combined nanoparticle/lipid monolayer. Additionally, we constructed a novel molecular dynamics (MD) simulation code in an effort to understand this type of system at the micro-scale as our data represent averaging at the macroscopic level only.
What we found is that these types of systems are significantly more complex than they would appear otherwise. Soluble surfactant diffusion kinetics toward and away from the interface differ by at least an order of magnitude. What is even more unusual is that nanoparticle diffusion kinetics, although the particles differ in weight and in chemical nature from the surfactants, take the same form of and, in some cases, even the same magnitude of the soluble species. While merely coincidence, it is nonetheless an interesting finding. The combined soluble/lipid and nanoparticle/lipid monolayers exhibited unusual self-assembly dynamics than for either species by itself and both systems showed temporal instability upon compression to moderate or elevated pressures, indicating that there is rearrangement and exclusion from the monolayer of the non-dominant species. The computer simulation work showed that generating extended lipid structures such as stripes and islands requires much larger systems with higher lipid densities than it is possible to handle using our limited resources. However, it is still possible even with a small system containing a limited number of lipid molecules to gain useful insights into the micro-scale behavior of the system. In this case it is clear that having long-range repulsive forces and short range dipole interactions is critical to structure formation. We also determined that, due to the nanoparticles having a mass two orders of magnitude greater than the lipid, the amount of simulation time required to see nanoparticle agglomeration is far beyond our ability to study without the use of a massively parallel computing system. Nonetheless, we are able to draw conclusions by studying lipid diffusion into the particles and compare the structures that form there rather than in free solution.