Water confined between sheets of mackinawite FeS minerals

Abstract

Wet iron-sulfur minerals have been shown to be ideal environments to allow for simple chemical reactions to occur in nature, for instance, in the framework of prebiotic chemistry. Yet, not much is known about such water/mineral interfaces beyond those involving pyrite, FeS(2), which is, however, chemically rather inert. In contrast, mackinawite is chemically reactive and consists of a layered crystal structure comprising FeS sheets that can be easily cleaved. Here, the properties of water confined between such sheets in lamella-like setups is investigated in the spirit of surface science model systems. The properties of this intercalated water are found to depend significantly on the interlayer distance and change from "arrested water" (in the limit of small interlayer distances) to liquid-like behavior.

Nanoconfinement effects on hydrated excess protons in layered materials

“…2b) is readily visible. These total density profiles in both systems are virtually indistinguishable from those determined earlier for neutral water 35 , that is, the excess proton does not affect the overall structure of the water lamellae.…”

“…All these data show that there is essentially no charge transfer from FeS into the protonated water layers; note that the mineral slabs are internally polarized owing to the presence of protonated water, which results in negative charge accumulation inside the mineral and close to the interface with water. The interactions at this liquid À solid interface are limited to (time-and space-) local induced polarization of the mineral surface due to the water and/or hydronium dipoles in the fluctuating hydrogen bond network much in the same way as found earlier for neutral water 35 ; in particular, there is no excess correlation between the position of the cationic defect at the contact plane and accumulation or depletion of negative charge at the surface.…”

“…The methods and system set-up are the same as those that have been carefully tested and validated in our previous investigation of pure water confined between mackinawite sheets 35 . Thus, the Perdew À Burke À Ernzerhof density functional 39,40 , together with ultrasoft pseudopotentials 41 (containing d-projectors in the case of sulphur and semicore states as well as scalar relativistic corrections for iron), at a plane wave cutoff of 25 Ry is used.…”

“…1, respectively. At the simulation temperature of 500 K, this yields a pressure of roughly 20 MPa according to the experimental data, chosen as to mimick typical hydrothermal vent conditions 35 . As a consistent reference, a protonated bulk water system containing 32 H 2 O molecules and an excess proton in a cubic supercell with a ¼ 10.415 Å was simulated with the same computational approach and parameter settings.…”

“…Within this scenario, structures consisting of mackinawite layers with intercalated water at elevated temperature and pressure conditions, about 500 K and 20 MPa, are ideally suitable nanoreactors for such prebiotic processes. Indeed, it has been demonstrated that the structural and dynamical properties of the confined water lamella are dramatically dependent on the interlayer distance 35 , thus offering rich confinement effects on subsequent processes. Although sufficiently small intersheet distances lead to 'arrested water', such larger distances support liquid-like dynamics, where a stratified structure consisting of two water layers parallel to the FeS mineral surfaces is adopted with a low-density region separating them.…”