Solid-amorphous transition is related to the waterlike anomalies in a fluid without liquid-liquid phase transition

The most accepted origin for the water anomalous behavior is the phase transition between two liquids (LLPT) in the supercooled regime connected to the glassy first order phase transition at lower temperatures. Two length scales potentials are an effective approach that have long being employed to understand the properties of fluids with waterlike anomalies and, more recently, the behavior of colloids and nanoparticles. These potentials can be parameterized to have distinct shapes, as a pure repulsive ramp, such as the model proposed by de Oliveira et al. [J. Chem. Phys. 124, 64901 (2006)]. This model has waterlike anomalies despite the absence of LLPT. To unravel how the waterlike anomalies are connected to the solid phases we employ Molecular Dynamics simulations. We have analyzed the fluid-solid transition under cooling, with two solid crystalline phases, BCC and HCP, and two amorphous regions being observed. We show how the competition between the scales creates an amorphous cluster in the BCC crystal that leads to the amorphization at low temperatures. A similar mechanism is found in the fluid phase, with the system changing from a BCC-like to an amorphous-like structure in the point where a maxima in $k_T$ is observed. With this, we can relate the competition between two fluid structures with the amorphous clusterization in the BCC phase.Those findings help to understand the origins of waterlike behavior in systems without liquid-liquid critical point

Ion fluxes through nano-pores and transmembrane channels

We introduce an implicit solvent Molecular Dynamics approach for calculating ionic fluxes through narrow nano-pores and transmembrane channels. The method relies on a dual-control- volume grand-canonical molecular dynamics (DCV-GCMD) simulation and the analytical solution for the electrostatic potential inside a cylindrical nano-pore recently obtained by Levin [Europhys. Lett., 76, 163 (2006)]. The theory is used to calculate the ionic fluxes through an artificial trans-membrane c hannel which mimics the antibacterial gramicidin A channel. Both current-voltage and current-concentration relations are calculated under various experimental conditions. We show that our results are comparable to the characteristics associated to the gramicidin A pore, specially the existence of two binding sites inside the pore and the observed saturation in the current-concentration profiles.Comment: 15 pages, 8 figures, accepted for publication in Physical Review