Water, hydration and adsorption

Liquid and supercritical water

A tetrahedral network of water molecules is the main structural trend, due to the existence of four hydrogen-bonds per molecule (on average). The lifetime of such hydrogen-bonds is around 1 ps. Hydrogen-bonding is the main responsible of the amazing properties of liquid water. Water beyond its critical point (T > 647.13 K, d > 0.322 g/cm³, P > 220.55 bar) shows surprising new properties, like the capacity of mixing with oil. In supercritical water there is no longer distinction between the liquid and vapor phase. We have observed the breakdown of the tetrahedral structure and its substitution by cavities and water clusters. The lifetime of hydrogen-bonds is now about 0.3-0.5 ps.

Aqueous ionic solutions at finite concentration

The effect of ion concentration is relevant to the local organization of water around ionic species and it has strong influence on dielectric properties and on dynamics, such as conductivity and diffusion coefficients or on hydrogen-bond structure and lifetimes. Our main contribution in this field has been the use and development of polarizable models for water and ions, which have greatly improved the quality of simulation results at interfaces.

The water-solid interface

As another example of confinement, we have simulated liquid water embedded in between two flat graphite layers. This is a "classical" system in simulation studies which we will use to learn further about modifications in the structure and dynamics of water under extreme confinement. In the limit, we are able to speak about 2D water.The influence of the tubes is especially relevant concerning hydrogen stretching vibrations and diffussive behavior.

The most recent studies in this field are those concerning the behaviour of liquid water near a single flat graphene sheet and adsorbed at boron-nitride single sheets.

Proton transfer in supercritical water

A study of structure and dynamics of water plus an excess proton at the supercritical isotherm of 673 K was reported in 2004, revealing new features. For instance, the diffusion behaviour of the lone charge does not follow the Grotthus mechanism, differently of what happens for water at ambient conditions. In supercritical water, the spectral features of the excess proton reveal a tendency to form Eigen-like complexes, instead of the typical behaviour of Zundel pairs at 298 K.

Excess protons in reverse micelles

More recently, the analysis of the characteristics of the excess proton in a water droplet located inside a reverse non-ionic micelle Diethylene Glycol Monodecyl Ether has been performed. Is such system, we observe a quick diffusion of proton to the external surface of the micelle, in a similar fashion as it happens when larger ionic charges are dilutes near solid or gas-like interfaces. In this particular problem, we observed how the most common configuration is that of the excess proton forming Eigen-like complexes, which have long lifetimes of the order of nanoseconds, prior to tits eventual transfer to other water molecules.

Proton transfer in 2D-confined water

A study about the physical and chemical properties of the excess proton in constrained geometries around 2D surfaces has been performed. The study has analyzed structure and dynamics of water plus an excess proton when constrained near hydrophobic and hydrophilic surfaces. We found new features in the transfer mechanism and identified vibrational modes related to proton transfer, in agreement with recent works which indicate that the type and topology of the confining device will likely alter the diffusion and hydrogen-bond structure of the bulk liquid.