Research

 

We are a theoretical chemistry research group in the Department of Chemistry at the University of Wisconsin-Madison and a part of the Theoretical Chemistry Institute. Our interests lie in understanding the structure and dynamics of complex fluids in solution and in complex enviroments using the methods of equilibrium and non-equilibrium statistical mechanics. More specifically, our current research focuses on using computer simulation for statistical mechanical understanding of structure and dynamics of biophysical and nanoscale phenomena. We use two complementary approaches: a phenomenological approach where we devise simple models to obtain insight into complex behavior and a bottom-up approach where we parameterize atomistic force-fields, then coarse-grain these to obtain realistic but efficient models for the molecular simulation of complex fluids. The main computer simulation techniques we employ are molecular dynamics, Brownian dynamics, and Monte Carlo methods. Our research is interdisciplinary, and we collaborate with experimentalists in the department as well as scientists in chemical engineering, physiology, and food science.

Current areas of research in our group lnclude coarse-grained models for complex fluidspolymers in ionic liquids, and gemini surfactants.

Coarse-grained mdels for complex fluids

Computational studies play an important role in our understanding of complex fluids. They can provide physical insight and molecular information that is hard to o

btain from experiment.  The most important part of a computer simulation is the choice of molecule model and for systems where quantum effects are not deemed important classical models have played an important role.  In classical models each molecule is represented by a collections of sites; in an atomistic model each site represents a single atom.

Atomistic models are computationally demanding for systems with large molecules, such as polymers, or with a large number of molecules, such as polymers in aqueous solution.  We are interested in developing coarse-grained modelswhere several atom-sites are grouped intoa coarse-grained (CG) site.  The hope is that we will be able to retain chemical detail while still being able to investigate large systems.

 

Current work focuses on the development of CGmodels for aqueous solutions, ionic liquids, and macromolecules such as polymers, peptides, and lipids.

The BMW-MARTINI Coarse-grained force field for peptide-membrane simulation

 

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.

Current research in our group has three thrusts: 1) Developing atomistic force fields for polymers in ionic liquids from first principles, 2) Developing coarse-grained force fields, 3) Developing integral equations and density functional theories for the properties of ionic liquids.  These methods will be used to study the properties of bulk systems and systems at surfaces and interfaces.

Gemini surfactants

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 surfactantsare 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.