Dielectric anisotropy in polar solvents under external fields

We investigate dielectric saturation and increment in polar liquids under external fields. We couple a previously introduced dipolar solvent model to a uniform electric field and derive the electrostatic kernel of interacting dipoles. This procedure allows an unambiguous definition of the liquid dielectric permittivity embodying non-linear dielectric response and correlation effects.We find that the presence of the external field results in a dielectric anisotropy characterized by a two-component dielectric permittivity tensor. The increase of the electric field amplifies the permittivity component parallel to the field direction, i.e. dielectric increment is observed along the field. However, the perpendicular component is lowered below the physiological permittivity, indicating dielectric saturation perpendicular to the field. By comparison with Molecular Dynamics simulations from the literature, we show that the mean-field level dielectric response theory underestimates dielectric saturation. The inclusion of dipolar correlations at the weak-coupling level intensify the mean-field level dielectric saturation and improves the agreement with simulation data at weak electric fields. The correlation-corrected theory predicts as well the presence of a metastable configuration corresponding to the antiparallel alignment of dipoles with the field. This prediction can be verified by solvent-explicit simulations where solvent molecules are expected to be trapped transiently in this metastable state

Electrostatic interactions in charged nanoslits within an explicit solvent theory

Within a dipolar Poisson-Boltzmann theory including electrostatic correlations, we consider the effect of explicit solvent structure on solvent and ion partition confined to charged nanopores. We develop a relaxation scheme for the solution of this highly non-linear integro-differential equation for the electrostatic potential. The scheme is an extension of the approach previously introduced for simple planes (S. Buyukdagli and Ralf Blossey, J. Chem. Phys. 140, 234903 (2014)) to nanoslit geometry. We show that the reduced dielectric response of solvent molecules at the membrane walls gives rise to an electric field significantly stronger than the field of the classical Poisson-Boltzmann equation. This peculiarity associated with non-local electrostatic interactions results in turn in an interfacial counterion adsorption layer absent in continuum theories. The observation of this enhanced counterion affinity in the very close vicinity of the interface may have important impacts on nanofludic transport through charged nanopores. Our results indicate the quantitative inaccuracy of solvent implicit nanofiltration theories in predicting the ionic selectivity of membrane nanopores

Correlation-induced DNA adsorption on like-charged membranes

The adsorption of DNA or other polyelectrolyte molecules on charged membranes is a recurrent motif in soft matter and bionanotechnological systems. Two typical situations encountered are the deposition of single DNA chains onto substrates for further analysis, e.g., by force microscopy, or the pulling of polyelectrolytes into membrane nanopores, as in sequencing applications. In this paper, we present a theoretical analysis of such scenarios based on the self-consistent field theory approach, which allows us to address the important effect of charge correlations. We calculate the grand potential of a stiff polyelectrolyte immersed in an electrolyte in contact with a negatively charged dielectric membrane. For the sake of conciseness, we neglect conformational polymer fluctuations and model the molecule as a rigid charged line. At strongly charged membranes, the adsorbed counterions enhance the screening ability of the interfacial region. In the presence of highly charged polymers such as double-stranded DNA molecules close to the membrane, this enhanced interfacial screening dominates the mean-field level DNA-membrane repulsion and results in the adsorption of the DNA molecule to the surface. This picture provides a simple explanation for the recently observed DNA binding onto similarly charged substrates [G. L.-Caballero et al., Soft Matter 10, 2805 (2014)] and points out charge correlations as a non-negligible ingredient of polymer-surface interactions

Towards more realistic dynamical models for DNA secondary structure

We propose a dynamical model for the secondary structure of DNA, which is based on the finite stacking enthalpies used in thermodynamics calculations. In this model, the two strands can separate and the bases are allowed to rotate perpendicular to the sequence axis. We show, through molecular dynamics simulations, that the model has the correct behaviour at the denaturation transition.Comment: accepted for publication in Chemical Physics Letter