Ionic liquids made up of organic cations and organic or inorganic anions have very low vapor pressure, which sets them apart from commonly used organic solvents. This makes them attractive for use as solvents for synthesis and catalysis and other industrial applications.

Atomistic molecular dynamics (MD) studies have been performed to study aggregation in these systems. Aqueous solutions of [C10mim][Br] spontaneously form cation aggregates. Simulations of the vapor-liquid interface of the same system shows that the alkyl tails of the cations aggregate at the surface to minimize the unfavorable interactions with water. In higher concentration, the cations also form aggregates in the aqueous phase with polar head groups (imidazolium ring) pointing outwards and the alkyl tails inwards. The determined aggregate sizes agree with experimental observations. The simulated systems appear to be in a metastable state with reference to the aggregation numbers. To extend the accessible observation times, a coarse grained model has been developed for this system.

Coarse grain MD simulations of aqueous [C10mim][Br] solutions at several concentrations provide insight into their microscopic structure. At 37% (w/w) water concentration there is a separation of hydrophilic (head groups, anions and water, yellow) and hydrophobic regions (tail groups, magenta) forming a hexagonal columnar phase (see figure on the right: (a) side view of columns, (b) top view of columns).

In dilute aqueous solutions of [C10mim][Br] quasi-spherical micelles form spontaneously. Initially, a large number of monomers and few small aggregates are observed. Yet over time the aggregates grow in size by incorporating those monomers.

For the small aggregates it is difficult to grow by merging with other micelles due to repulsion of their charged surfaces. The observed aggregates are poly-disperse and typically contain between 40 and 60 cations. The bulk region of the vapor/liquid interface of aqueous ionic liquids shows similar aggregation behavior.

References:
B.L. Bhargava, M. L. Klein, J. Phys. Chem. A., 113, 1888 (2009)
B.L. Bhargava, M. L. Klein, Mol. Phys., 107, 393 (2009)
B.L. Bhargava, M. L. Klein, Soft Matter, 5, 3475 (2009)
B.L. Bhargava, M. L. Klein, J. Phys. Chem. B, 113, 9499 (2009)

Contact: Bhargava B. L. Go back to research index.

Meeting the demands of the rapidly advancing nanotechnological frontier requires novel, multifunctional nanoscale materials. Among the most promising nanomaterials to fulfill this need are biopolymer-carbon nanotube hybrids (Bio-CNT). Bio-CNT consists of a single-walled carbon nanotube (CNT) coated with a self-assembled layer of biopolymers such as DNA or protein. Experiments have demonstrated that these nanomaterials possess a wide range of technologically useful properties with applications in nanoelectronics, medicine, homeland security, environmental safety and microbiology. We use all-atom molecular dynamics (MD), parallel tempering replica exchange (REMD) and free energy methods gain insight into properties and behavior of these unique nanomaterials.

Simulation of Nano-Biosensors: Carbon Nanotube Functionalized with the Coxsackie-Adenovirus Receptor

Detection of viruses and other harmful biological agents has important applications in homeland security and medicine. Recently, the experimental nanoscience group of Dr. Charlie Johnson at the University of Pennsylvania has developed sensors capable of detecting proteins associated with the adenovirus (one of the viruses responsible for the common cold). These sensors are depicted below. They consist of the Coxsackie-adenovirus receptor protein (green) attached to a carbon nanotube (gray cylinder). Knob proteins (orange) that are located on the surface of the adenovirus bind to this receptor protein. This binding event changes the electrical properties of the carbon nanotube. Thus, by monitoring the electrical properties of the carbon nanotube, one can detect the presence of the adenovirus. ICMS uses a variety of computational techniques to help rationalize the fabrication and operation of these sensing devices.

References
R.R. Johnson, A.T.C. Johnson, M. L. Klein, Small, 6, 31, (2010).
R.R. Johnson, B.J. Rego, A.T.C. Johnson, M.L. Klein, J. Phys. Chem. B, 113, 11589 (2009)
R.R. Johnson, A. Kohlmeyer, A.T.C. Johnson, M. L. Klein. Nano Lett., 9, 537 (2009)

Contact: Robert Johnson Go back to research index.

A worm-like micelle is a classic example of a molecular self-assembly that possesses a unique elongated shape. The polymer hydrophobic tails self assemble in the core of the micelle, while the hydrophilic groups maximize contacts with water by self-assembling in the corona of the micelle. Using mesoscopic simulation techniques, dissipative particle dynamics (DPD), we can test the stability of worm-like micelles at different hydrophilic block fractions. Worm-like micelles exist in a narrow region of the phase diagram. If the hydrophobic portion of the diblock copolymer is degradable, such as PCL (poly(caprolactone) (PCL) or poly(lactic acid) (PLA), this leads to shortening of the relative molecular weight of the tail, and an increase in the polydispersity, over time. The leads to instabilities, and ultimately, a breakup of the worm-like micelle morphology. We emulate this process in simulation by mixing in diblocks of differing hydrophobic length. At a critical concentration, this leads to a breakup of the previously stable worm. This instability begins with radial undulations along the core, bud formation at the end of the micelle, and finally expulsion of a spherical micelle from the end-cap. Analysis of the mean curvature of the worm micelle during break-up proves a simple linear relationship with the mean interfacial concentration of each component diblock copolymer relative to each equilibrium unmixed micellar phase.

Other examples of self-assembly structures are bilayers. We use Coarse-Grained Molecular Dynamics (CGMD) to gain insight about the strong lateral segregation observed in polymersomes composed by neutral and charged diblock-copolymers (Christian et al, 2009) in the presence of calcium. Our approximation involved toy models in which the charged groups were represented with LJ interaction sites. These simple models are able to successfully reproduce the morphology phase diagram for charged diblock copolymers as well as the spontaneous segregation observed in bilayers composed by charged and neutral copolymers. The shape of the aggregates appeared correlated to its state: crystalline patches are irregular, liquids are regular. As suggested by experiments, after several nanoseconds of simulation, we found strong correlation between the local composition of the two lealets (registration). Simulations suggest that differentiated interlealet interaction between ordered and disordered phases would contribute to the registration.

References:
D. A. Christian, A. Tian, W. G. Ellenbroek, I. Levental, K. Rajagopal, P. A. Janmey, A. J. Liu, T. Baumgart & D. E. Discher, Nature Materials 8, 843 - 849 (2009)
Y. Geng and D. E. Discher.Journal of the American Chemical Society, 127, 12780-12781 (2005)
S.M. Loverde, V. Ortiz, R.D. Kamien, M.L. Klein, and D.E. Discher, Soft Matter (2010) DOI:10.1039/b919581e

Contact: Diego Pantano, Sharon Loverde Go back to research index.