Our research focuses on theoretical studies of soft condensed matter. While it is clear that the short-range structure of complex fluids plays an important role in determining the physical and chemical propreties, the complexity of these systems precludes modeling them on an atomistic level. A judicious choice of coarse-grained models that hopefully capture the essential features without incorporating much of the detail is therefore a crucial step in the theoretical study of these systems. We are interested in constructing such models, and then employing theory and computer simulation to investigate their properties, with the final aim of predicting experimental observables. Our research has two components: the development of methods, and the application of these methods to understand the structure and dynamics of condensed phases. Some areas of current interest are:
Polymers in ionic liquids
Ionic liquids (ILs) are usually composed of a large organic cation and a small organic or inorganic anion and are liquid at room temperature, They possess a number of interesting and important physical properties such as low volatility, non-flammability, high conductivity, and thermal and chemical stability. They have varied potential applications, as solvents for synthesis and catalysis, as electrolytes, as media for separations, as sorption media for gas absorption, and as lubricants. The viability of ionic liquids in materials applications is limited by their lack of mechanical integrity, which may be provided by mixing them with a polymeric material. Possible applications of composites of polymers and ionic liquids include membranes for fuel cells, separations, and batteries, gels for artificial muscles, and dielectrics for energy storage.
We are interested in the conformational properties and phase behavior of solutions of polymers, especially polyethylene oxide (PEO), in ionic liquids. The phase separation of polymers in ionic liquids is important in the fabrication of materials, e.g., actuators, drug delivery systems, optical devices, but is also of fundamental interest.
Polymer solutions can display a lower critical solution temperature (LCST), i.e., the solution is mixed at low temperatures but phase separates upon heating, or an upper critical solution temperature (UCST), i.e., the solution is mixed at high temperatures but phase separates upon cooling. It has been found that the phase behavior of polymers in ionic liquids is dramatically different from that in water or other solvents. For example, poly (N-isopropylacrylamide) (PNIPAm) exhibits an LCST in water, but a UCST in some ionic liquids. On the other hand poly (ethylene oxide) (PEO) displays both an LCST and UCST in water, but is soluble in many ionic liquids and displays an LCST in some. Interestingly, the LCST of PEO in ionic liquids is very sensitive to the identity of the anion, but the UCST of PNIMPAm in ionic liquids is very sensitive to the identity of the cation.
A delicate balance of non-covalent interactions drives the assembly of hydrated small molecule amphiphiles into materials with periodic, long-range nanoscale order. Understanding the factors that govern amphiphile self-assembly is of fundamental importance. These materials are also potentially important in a number of applications including the synthesis of mesoporous materials, protein crystallization media, and new membranes.
Lyotropic liquid crystal phases are interesting because of the variety of morphologies they exhibit, e.g. lamellar, bicontinuous cubic, and cylinders. These phases contain distinct nanoscale hydrophobic and hydrophilic domains, with periodic translational order, where the interfaces between the domains are decorated with the hydrophilic head group of the small molecule surfactant.
We are studying the self-assembly of Gemini surfactants into LLC phases, and the properties of the water in the nano-domains created by these phases. Gemini surfactants are comprised of two single-tail surfactants dimerized through a flexible hydrophobic linker near the headgroups. In addition to the propensity to form network morphologies, the stability and ordered state symmetries of LLCs derived from gemini surfactants depend quite sensitively upon the counterion as well as the linker and alkyl tail lengths.
We are using a combination of mixed resolutions simulations (united atom models for the surfactants and atomistic models for the water) to study the effect of chemical structure on the self-assembly of these surfactants and the dynamic properties of water in the LLC phases.
Coarse-grained force fields for complex fluids