Excess entropy scaling of transport properties of Lennard-Jones chains

Excess-entropy scaling relationships for diffusivity and viscosity of Lennard-Jones chain fluids are tested using molecular dynamics simulations for chain sizes that are sufficiently small that chain entanglement effects are insignificant. The thermodynamic excess entropy S<SUB>e</SUB> is estimated using self-associating fluid theory (SAFT). A structural measure of the entropy S<SUB>2</SUB> is also computed from the monomer-monomer pair correlation function, g<SUB>m</SUB>(r). The thermodynamic and structural estimators for the excess entropy are shown to be very strongly correlated. The dimensionless center-of-mass diffusivities, D<SUB>cm</SUB><SUP></SUP>, obtained by dividing the diffusivities by suitable macroscopic reduction parameters, are shown to conform to the excess entropy scaling relationship, D<SUB>cm</SUB><SUP></SUP> = A<SUB>n</SUB>exp(a<SUB>n</SUB>S<SUB>e</SUB>), where the scaling parameters depend on the chain length n. The exponential parameter &#945; <SUB>n</SUB> varies as -(1/n) while A<SUB>n</SUB> varies approximately as n<SUP>-0.5</SUP>. The scaled viscosities obey a similar relationship with scaling parameters B<SUB>n</SUB> and &#946;<SUB>n</SUB> where &#946;<SUB>n</SUB> varies as 1/n and B<SUB>n</SUB> shows an approximate n<SUP>0.6</SUP> dependence. In accordance with the Stokes-Einstein law, for a given chain length, &#945; <SUB>n</SUB> = -&#946;<SUB>n</SUB> within statistical error. The excess entropy scaling parameters associated with the transport properties therefore display a simple dependence on chain length

A self-consistent density-functional approach to the structure of electric double layer: charge-asymmetric electrolytes

A self-consistent density-functional approach has been employed to study the structure of an electric double layer formed from a charge-asymmetric (2:l) electrolyte within the restricted primitive model which corresponds to charged hard sphere ions and a continuum solvent. The particle correlation due to hard-core exclusions is evaluated by making use of the universality of the density functionals and the correlation function of the uniform hard sphere fluid obtained through the integral equation theory with an accurate closure relation whereas mean spherical approximation is employed for the electrical contribution. Numerical results on the diffuse layer potential drop, ionic density profile, and the mean electrostatic potential near the electrode surface at several surface charge densities are found to be in quantitative agreement with the available simulation data

Fluorescence dynamics of double- and single-stranded DNA bound to histone and micellar surfaces

The study of structure and dynamics of bound DNA has special implications in the context of its biological as well as material functions. It is of fundamental importance to understand how a binding surface affects different positions of DNA with respect to its open ends. Because double-stranded (ds) and single-stranded (ss) DNA are the predominant functional forms, we studied the site-specific dynamics of these DNA forms, bound to the oppositely charged surface of histones, and compared the effects with that of DNA bound to cetyltrimethyl ammonium bromide micelles. We utilized a time-resolved fluorescence technique using fluorescent base analogue 2-aminopurine located at specific positions of synthetic poly-A DNA strands to obtain fluorescence lifetime and anisotropy information. It is observed that the binding leads to overall rigidification of the DNA backbone, and the highly flexible ends show drastic dampening of their internal dynamics as well as the fraying motions. In the case of ds-DNA, we find that the binding not only decreases the flexibility but also leads to significant weakening of base-stacking interactions. An important revelation that strong binding between DNA and the binding agents (histones as well as micelles) does not dampen the internal dynamics of the bases completely suggests that the DNA in its bound form stays in some semiactive state, retaining its full biological activity. Considering that the two binding agents (histones and micelles) are chemically very different, an interesting comparison is made between DNA−histones and DNA−micelle interactions