60th Annual Report on Research 2015

During the report period we have performed extensive theoretical study of the morphology of Janus-like nanoparticles, i.e. nanoparticles covered with 2 types of incompatible ligands (A and B) which undergo microphase separation under certain conditions, thereby producing a Janus-like structure, where one half of the nanoparticle surface is covered with ligand A and the other half is covered with ligand B (assuming equimolar coverage). If ligand A is taken to be hydrophobic and ligand B is taken to be hydrophilic (thereby ensuring their incompatibility), then the resulting Janus nanoparticle would adsorb strongly at an oil-water interface, making it possible to modify and control the interfacial properties by varying the amount of adsorbed nanoparticles.

Our work has been performed in close collaboration with the experimental group of Professor David Green at the Department of Chemical Engineering of the University of Virginia. Green's lab utilizes matrix-assisted laser desorption/ionization mass spectroscopy (MALDI) measurements to analyze the morphology of silver nanoparticles coated with binary mixture of hydrophobic and hydrophilic ligands, while our lab develops and applies mean-field theoretical techniques, such as self-consistent field (SCF) theory, to predict and analyze ligand distribution and microphase separation on nanoparticle surfaces. The experimental and theoretical techniques complement each other by enabling quantification of the nearest-neighbor distribution of a ligand mixture in a monolayer shell. By tracking a characteristic metallic fragment family, analysis of a MALDI spectrum produces a frequency distribution corresponding to specific ligand patterning. Inherent to the SCF calculations, especially the lattice-based formulation, is the enumeration of local interactions that dictate ligand assembly. The interweaving of MALDI with SCF facilitates a comparison between the experimentally- and theoretically-derived frequency distributions as well as their residual, or deviation from a well-mixed state. Hence, we combine these techniques to detect and predict phase separation in monolayers that mix uniformly or experience varying degrees of de-mixing, including micro-phase separation and stripe formation. These various surface patterns are of primary importance for controlling the adsorption behavior of Janus nanoparticles at an oil-water interface.

Utilizing SCF techniques requires both extensive knowledge and deep understanding of statistical mechanics, as well as proficiency with cutting-edge numerical methods and optimization techniques. Accordingly, Steven Merz (a graduate student in my lab working on the theoretical aspect of this project while being supported by the present grant from PRF), is receiving solid training both in fundamentals of statistical mechanics and in numerical methods.

Turning now to specific results originating from this work, in the course of the past year we have demonstrated the ability to quantify and predict the phase separation of alkanethiols on silver nanoparticles via SCF theory as a function of both ligand concentration and length difference. These results indicate that when pairing dodecanethiol with deuterated dodecanethiol or butanethiol the ligand self-assembled monolayer morphology progresses from a random ligand distribution to an intermediate degree of phase separation. In terms of potential impact of our work, we anticipate that theoretical methods developed in our lab will find utility in the design of nanomaterials with properties arising from phase-separated ligand monolayers as they offer an efficient method for screening and prediction of ligand phase separation in nanoparticle systems.

Looking to the next year, our next step will be to utilize the knowledge we have generated regarding the formation of Janus nanoparticles with the goal of studying and analyzing their behavior at an oil-water interface. Specifically, Steven Merz will work on developing an efficient SCF-based approach that would allow him to predict structural and thermodynamic characteristics of an oil-water interface (such as surface tension) as a function of its coverage with Janus nanoparticles. Furthermore, we plan to develop a dynamical generalization of the SCF method, that would allow us to study the dynamics of Janus nanoparticles both at and across the oil-water interface. Throughout this work, close contact with the experimental group of Professor Green will be maintained. This would enable us, on the one hand, to test our theory against the available experimental data, and, on the other hand, to make predictions and suggestions for future experiments.