We present results of Brownian dynamics simulations of aqueous 1-1 electrolyte solutions in the 1-molar concentration range. The electrical conductivity and the self-diffusion coefficients obtained from the simulations are compared to experimental data. The interaction potential between the ions is modeled by pairwise repulsive 1/rn soft-core interactions (n=9 or n=12) and Coulomb forces. We take into account hydrodynamic interactions and integrate the stochastic equations of motion with large time steps of about 100 femtoseconds, combined with an acceptance criterion known from the Smart Monte Carlo method. In this way, details of the dynamics of particles in close contact are not considered and the short-ranged repulsive forces act effectively as constraint forces preventing overlap configurations. The lengths of the performed simulations (about 10 nanoseconds) and the number of ions (216) allow to obtain single particle as well as collective transport coefficients with sufficient precision. For this purpose we use Kubo expressions which can be applied on the mesoscopic time scale of Brownian dynamics simulations. It is shown that hydrodynamic interactions must be taken into account to obtain agreement with the experimental data. They lower the electrical conductivity, as expected, but increase the self-diffusion coefficients, confirming a recent finding for colloids.

The structure of DNA Binding Proteins enables a strong interaction with their specific target site on DNA. However, recent single molecule experiment reported that proteins can diffuse on DNA. This suggests that the interactions between proteins and DNA play a role during the target search even far from the specific site. It is unclear how these non-specific interactions optimize the search process, and how the protein structure comes into play. Each nucleotide being negatively charged, one may think that the positive surface of DNA-BPs should electrostatically collapse onto DNA. Here we show by means of Monte Carlo simulations and analytical calculations that a counter-intuitive repulsion between the two oppositely charged macromolecules exists at a nanometer range. We also show that this repulsion is due to a local increase of the osmotic pressure exerted by the ions which are trapped at the interface. For the concave shape of DNA-BPs, and for realistic protein charge densities, we find that the repulsion pushes the protein in a free energy minimum at a distance from DNA. As a consequence, a favorable path exists along which proteins can slide without interacting with the DNA bases. When a protein encounters its target, the osmotic barrier is completely counter-balanced by the H-bond interaction, thus enabling the sequence recognition.DNA stores the genetic material of all living cells and viruses. This huge amount of information is effective only if DNA binding proteins (DNA-BPs) manipulates DNA in very specific locations. When the protein finds its DNA target, the shape complementarity of DNA Binding Proteins and their specific DNA sequence enables to maximize the number of hydrogen bonds, thus leading to a strong protein-DNA association [1,2,3,4,5,6]. The rate of protein-DNA association is however not controlled by the association step itself, but by the whole searching process. It is well established now that DNA-BPs diffuse along DNA before they reach their specific site [7]. During this search, the only interactions between protein and DNA which can play a role are non sequence-specific. Those non-specific interactions between protein and DNA remain poorly documented. Altough the predominance of electrostatics is unquestionable [1,2,3,4,5,6], it remains unclear how the protein structure comes into play [5,6,7]. Does the typical concavity of DNA-BPs which favors the specific association also influence the non-specific electrostatic interaction? In DNA-protein complexes, the mean charge of the protein residues located at the interface is positive [1,2]. Nevertheless, structural studies of non-specific complexes have shown that the protein atoms and the DNA atoms are weakly packed together at the interface [1,2,3,5,6], thus suggesting that a force counterbalances the electrostatic attraction. In this letter, our purpose is to establish the general mechanisms that control the mean force between protein and DNA and that are applicable to a wide variety of DNA-BPs. That goal in mind, we design coarse-grained D...

Capillary phenomena governing the mass-transport (capillary filling, condensation/evaporation) has been experimentally investigated in around 20 different silica thin films exhibiting various porosities with pores dimension ranging from 2 to 200 nm. Films have been prepared by sol-gel chemistry combined with soft-templating approaches and controlled dip coating process. Environmental ellipsometric porosimetry combined with electronic microscopy were used to assess the porosity characteristics. Investigation of lateral capillary filling was performed by following the natural infiltration of water and ionic liquids at the edge of a sessile drop in open air or underneath a PDMS cover. The Washburn model was applied to the displacement of the liquid front within the films to deduce the kinetic constants. The role of the different capillary phenomena were discussed with respect to the porosity characteristics (porosity vol%, pore dimensions and constrictions). We show that correlation between capillary filling rate and pore dimensions is not straightforward. Generally, with a minimum of constrictions, faster filling is observed for larger pores. In the case of mesopores (<50 nm in diameter), the presence of bottle necks considerably slows down the infiltration rate. At such a small dimension, evaporation/capillary condensation dynamics, taking place at the meniscus inside the porosity, has to be considered to explain the transport mode. This fundamental study is of interest for applications involving liquids at the interface of mesoporous networks such as nanofluidics, purification, separation, water harvesting or heat transfer.