Interaction of two different samples of graphene with DNA nucleobases and nucleosides is investigated by isothermal titration calorimetry. The relative interaction energies of the nucleobases decrease in the order guanine (G)>adenine (A)>cytosine (C)>thymine (T) in aqueous solutions, although the positions of C and T seem to be interchangeable. The same trend is found with the nucleosides. Interaction energies of the A-T and G-C pairs are somewhere between those of the constituent bases. Theoretical calculations including van der Waals interaction and solvation energies give the trend G>A approximately T>C. The magnitudes of the interaction energies of the nucleobases with graphene are similar to those found with single-walled carbon nanotubes.

The nonlinear flow behaviour of a viscoelastic gel formed due to entangled, cylindrical micelles in aqueous solutions of the surfactant CTAT has been studied. On subjecting the system to a step shear rate lying above a certain value, the shear and normal stresses show interesting time dependent behaviour. The analysis of the measured time series shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics can be described by that of a dynamical system with a strange attractor whose dimension increases with the increase in shear rate.PACS numbers: 83.50. By, 83.50.Gd, 83.50.Ws, 82.70.Gg. Systems of giant wormlike micelles formed in certain surfactant solutions are known to show very unusual nonlinear rheology. In steady shear, the shear stress σ saturates to a constant value while the first normal stress difference increases roughly linearly with shear ratė γ 1,2 . The constitutive model of viscoelastic behaviour of wormlike micellar systems which incorporates reptation and reaction dynamics (breakage and recombination of micelles), predicts a mechanical instability of the shear banding type 3 where bands supporting high shear rates (low viscosity) coexist with regions of lower shear rates (higher viscosity). Flow birefringence 4 and nuclear magnetic resonance velocity imaging 5 have revealed the existence of banded flow in the shear stress plateau. An alternative explanation for the non-monotonicity of the flow curve has also been proposed 6,7 in terms of the coexistence of two thermodynamically stable phases -isotropic and nematic, present in the sheared solution. Berret 6 observed damped, periodic oscillations in the stress relaxation of CPyCl-NaSal solution at a surfactant volume fraction φ of 12% on the application of controlled shear ratesγ. Grand et al 8 have shown the existence of a metastable branch in the flow curve of dilute CPyClNaSal solution supporting stresses higher than that observed in the stress plateau. They have explained their results in terms of shear banding. Previous observations of shear stress fluctuations in CTAB-NaSal solutions have been explained in terms of shear thickening induced by the growth and retraction of shear induced structures 9 .The rheology of CTAT (cetyl trimethylammonium tosilate) has been examined extensively in the linear viscoelastic regime 10 . Above the Kraft temperature of 23 • C and at low concentrations (C < 0.04 wt.%), spherical micellar solutions are formed which exhibit Newtonian flow behaviour. At higher surfactant concentrations (0.04 < C < 0.9 wt.%), cylindrical wormlike micelles are formed which get entangled at C > 0.9 wt.% to form clear viscoelastic gels 10 . The purpose of this Letter is to report interesting time-dependence of the shear and normal stresses after subjecting the system to a step shear rate lying in the plateau region. Our detailed analysis shows unambiguously that the observed dynamics can be described as that of a low dimensional, dynamical system with a strange a...

No abstract

We have used atomistic molecular dynamics (MD) simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules.The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solid-like ordering at room temperature, which we quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not sub-diffusive. We also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. We find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal-mode diffusion inside the nanotube. We attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length.

Experiments and computer simulations demonstrate that water spontaneously fills the hydrophobic cavity of a carbon nanotube. To gain a quantitative thermodynamic understanding of this phenomenon, we use the recently developed two phase thermodynamics method to compute translational and rotational entropies of confined water molecules inside single-walled carbon nanotubes and show that the increase in energy of a water molecule inside the nanotube is compensated by the gain in its rotational entropy. The confined water is in equilibrium with the bulk water and the Helmholtz free energy per water molecule of confined water is the same as that in the bulk within the accuracy of the simulation results. A comparison of translational and rotational spectra of water molecules confined in carbon nanotubes with that of bulk water shows significant shifts in the positions of the spectral peaks that are directly related to the tube radius.