The effect of ions on solid–liquid phase transition in small water clusters. A molecular dynamics simulation study

Abstract

Small water clusters, containing ions, have been studied using molecular dynamics simulations at temperatures ranging from 0 to 250 K. The simulations are carried out systematically by varying the ion size, shape, and charge as well as the cluster size and the initial configuration. Transitions between solid and liquid phases are followed to study the effects of the ions on the cluster melting temperature, compared to pure water clusters of the same size. The effect of the ion on the ice-cluster melting appears to be a complicated process which depends simultaneously on a variety of factors, such as the initial cluster configuration and the ion position inside the cluster as well as the ion mass, size and its charge. In the case of monovalent cations the most important characteristics for the cluster evolution is the ion mass, while for divalent cations the ion charge is the most dominant factor. In the case of negatively charged ions the main factor of the system evolution is the ion size. Two principally different types of cluster structures can be observed from the simulations: The peripheral structure where the ion takes up a position, preferably on the cluster surface and the interior structure where the ion prefers the center of the system. The peripheral structure is typical for clusters containing the small monovalent Li+ cation but also for those containing the large Cl− anion, while divalent cations, large monovalent Na+ cation and small F− anions gave rise to the interior type of structure. Generally, an increase of the ion size changes the cluster structure making the peripheral variant more preferable.

Biasing the Center of Charge in Molecular Dynamics Simulations with Empirical Valence Bond Models: Free Energetics of an Excess Proton in a Water Droplet

“…This proclivity of the proton to occur near interfaces, which has been previously observed in clusters [22,23], at planar liquid-vapor interfaces [44], and near hydrophobic membranes [25], is unexpected from continuum electrostatics and is most likely due to the strain exerted by the excess charge on the surrounding hydrogen bond network. Analogous behavior has also been observed for other ions in clusters [47,48,49].…”

A global optimization perspective on molecular clusters

“…[75] and references therein), and this certainly deserves further investigation. Even though the cation prefers to stay inside the cluster (in agreement with MD simulations [27]), thus working as a structure-breaking species, the formation of clathrates with the lowest energy are not observed up to n = 27. In fact, it was shown [65] that the formation of a clathrate for Ca 2+ -(H 2 O) 20 is unfavourable, because the water-ion energy contribution rather stabilizes the GM.…”

“…Several empirical potentials have been employed to study molecular clusters. Some of the systems in our work employ standard potentials, like the TIP4P [24,25] used to describe water clusters or the OPLS-AA model [26] available for describing the interactions within Li + -water, Li + -methanol, Ca 2+ -water and Ca 2+ -methanol (see also [27] and references therein). For the remaining systems, however, we have adopted more sophisticated models.…”

The Hydrated Proton at the Water Liquid/Vapor Interface

“…[17][18][19][20][21][22][23] Ionic charge and size appear to have a striking influence on the behavior of solvated ions near the interface. For example, positive ions such as Na + are seen to prefer the interior, 18,23 while negative ions may prefer the surface or the interior, apparently based primarily on their size.…”