61st Annual Report on Research 2016

During the report period we have continued theoretical studies 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). This kind of microphase separation makes Janus nanoparticles ideal candidates for controlling oil-water interfacial tension. The graduate student Steven Merz has been continuing his theoretical work (initiated during the previous report period) based on self-consistent field theory (SCF) calculations. While SCF approach is fast and efficient, thereby allowing a comprehensive scan of the parameter space involved in the problem, it also has some important limitations. First, as a mean-field theory, SCF neglects fluctuations, which could play an important role in the vicinity of the oil-water interface. Second, it is not straightforward to incorporate specific chemical and structural details of the system into SCF model, even though some of these details may play an important role in the interfacial behavior of Janus nanoparticles and therefore must be included (e.g. faceted nanoparticle cores are difficult to model in the SCF lattice-based approach, but at the same time it is well established that the facets play an important role in the initialization of the microphase separation of incompatible ligands on the nanoparticle surface). Given the fact that our theoretical work is performed in close collaboration with the experimental group of Professor David Green at the Department of Chemical Engineering of the University of Virginia, it is important to incorporate as many chemical and physical details into our model as possible, in order to ensure that the theory can be used both for analyzing the existing experimental data and for making predictions and guiding future experiments. One viable theoretical approach to achieve this goal would be to employ detailed atomistic computer simulations. While this approach is less time-efficient than SCF theory, it does not suffer from the aforementioned drawbacks of the latter. Accordingly, Steven Merz has received extensive training in computer simulation methods and has been carrying out detailed atomistic simulations of ligand microphase separation on the nanoparticle surface. Another line of investigation dealt with adsorption and self-assembly of linear polymers at oil–water interfaces modeled by means of extensive Molecular Dynamics simulation. By varying the size, concentration, stiffness,and composition of nonionic surfactant, we examined their impact on surface tension at the phase boundary between oil and water. Our results indicate that alternating AB-copolymers are much more efficient than homopolymers or diblock copolymers in reducing the surface tension. This efficiency of the tested linear polymers is not very sensitive with respect to surfactant chain length, except for the AB-diblocks, where the shortest chains are also the most efficient ones and rival the alternating architecture in reducing surface tension. In contrast, increasing stiffness of all surfactants is found to make them significantly less efficient with regard to surface tension reduction. Stiffer chains are also found to form rafts, and at higher concentration, quasi-crystalline blocks at the oil-water interface leaving significant interfacial area devoid of surfactants. For the shorter rigid surfactants one observes gradual tilting during their self-assembly into bundles whereby the tilt angle increases substantially with increasing coverage.