Structure and Thermodynamics of Linear, Ring, and Catenane Polymers in Solutions and at Liquid-Liquid Interfaces.

In recent decades, advances in the syntheses of mechanically interlocked macromolecules, such as catenanes, have led to much greater interest in the applications of these complexes, from molecular motors and actuators to nanoscale computational memory and nanoswitches. Much remains to be understood, however, regarding how catenated ring compounds behave as a result of the effects of different solvents as well as the effects of solvent/solvent interfaces. In this work, we have investigated, using molecular dynamics simulations, the effects of solvation of poly(ethylene oxide) chains of different topologies─linear, ring, and [2]catenane─in two solvents both considered favorable toward PEO (water, toluene) and at the water/toluene interface. Compared to ring and [2]catenane molecules, the linear PEO chain showed the largest increase in size at the water/toluene interface compared to bulk water or bulk toluene. Perhaps surprisingly, observations indicate that the tendency of all three topologies to extend at the water/toluene interface may have more to do with screening the interaction between the two solvents than with optimizing specific solvent-polymer contacts.

Tension Amplification in Tethered Layers of Bottle-Brush Polymers.

Molecular dynamics simulations of a coarse-grained bead-spring model have been used to study the effects of molecular crowding on the accumulation of tension in the backbone of bottle-brush polymers tethered to a flat substrate. The number of bottle-brushes per unit surface area, Σ, as well as the lengths of the bottle-brush backbones Nbb (50 ≤ Nbb ≤ 200) and side chains Nsc (50 ≤ Nsc ≤ 200) were varied to determine how the dimensions and degree of crowding of bottle-brushes give rise to bond tension amplification along the backbone, especially near the substrate. From these simulations, we have identified three separate regimes of tension. For low Σ, the tension is due solely to intramolecular interactions and is dominated by the side chain repulsion that governs the lateral brush dimensions. With increasing Σ, the interactions between bottle-brush polymers induce compression of the side chains, transmitting increasing tension to the backbone. For large Σ, intermolecular side chain repulsion increases, forcing side chain extension and reorientation in the direction normal to the surface and transmitting considerable tension to the backbone.

Computation of the binding free energy of peptides to graphene in explicit water.

The characteristic properties of graphene make it useful in an assortment of applications. One particular application--the use of graphene in biosensors--requires a thorough understanding of graphene-peptide interactions. In this study, the binding of glycine (G) capped amino acid residues (termed GXG tripeptides) to trilayer graphene surfaces in aqueous solution was examined and compared to results previously obtained for peptide binding to single-layer free-standing graphene [A. N. Camden, S. A. Barr, and R. J. Berry, J. Phys. Chem. B 117, 10691-10697 (2013)]. In order to understand the interactions between the peptides and the surface, binding enthalpy and free energy values were calculated for each GXG system, where X cycled through the typical 20 amino acids. When the GXG tripeptides were bound to the surface, distinct conformations were observed, each with a different binding enthalpy. Analysis of the binding energy showed the binding of peptides to trilayer graphene was dominated by van der Waals interactions, unlike the free-standing graphene systems, where the binding was predominantly electrostatic in nature. These results demonstrate the utility of computational materials science in the mechanistic explanation of surface-biomolecule interactions which could be applied to a wide range of systems.

Characterization of CF4/CF3Br binary mixture adsorption on hydrophobic/hydrophilic surfaces via atomistic MD simulation.

Molecular dynamics simulations of multilayer adsorption of binary mixtures of two tetrasubstituted halomethanes (CF(4) and CF(3)Br) on two very different substrates (graphite vs hydroxylated SiO(2)) were performed for three different bulk compositions (40%, 50%, and 60% CF(4)) and over a range of temperatures from 80 to 200 K. The goal of these simulations was to investigate in depth how these factors affect film structure, layer composition, lateral arrangement, and molecular orientation in the first adsorbed layer on each substrate. In line with a previous study of single-component adsorption on these surfaces, mixtures adsorbed on the hydroxylated SiO(2) surface show stable number density profiles that are largely independent of temperature, up to 160 K. This level of stability is essentially absent in the case of adsorption on graphite, which show densities and surface populations that are largely dependent on overall film composition, molecular orientation, and adsorbate-substrate interactions, in addition to system temperature. Further, the composition of the first adsorbed layer at each solid surface appears to be influenced by the choice of substrate, with CF(3)Br the majority component at the graphite surface for all compositions and temperatures, while the first adsorbed layer on hydroxylated SiO(2) more clearly mirrors the overall film composition at temperatures below 160 K.

Structure and dynamics of tetrahalomethane adsorption on (001) surfaces of graphite and α-quartz.

Molecular dynamics (MD) simulations were used to study the structural and dynamic properties of multilayer adsorption of each of three halomethanes, CF(4), CF(3)Cl, and CF(3)Br, adsorbed onto the (001) surface of either of two atomically flat but chemically and structurally different substrates (graphite and hydroxylated α-quartz) at temperatures ranging from 60 to 300 K. Analysis of the data shows a strong influence on the adsorption characteristics of these halomethane films due to the surface characteristics of the chosen substrate. In particular, the nature of the hydroxylation of α-quartz shows a striking ability to alter the affinity with which species adsorb onto its surface. This effect appears to be at least partly responsible for the differences in the orientation and packing of molecules in the first film layer as well as differences in the effect of temperature variation on phase behavior and dynamics.