Stripping away ion hydration shells in electrical double-layer formation: Water networks matter.

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl- hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

Aqueous TMAO solution under high hydrostatic pressure.

Trimethylamine N-oxide (TMAO) is a well known osmolyte in nature, which is used by deep sea fish to stabilize proteins against High Hydrostatic Pressure (HHP). We present a combined ab initio molecular dynamics, force field molecular dynamics, and THz absorption study of TMAO in water up to 12 kbar to decipher its solvation properties upon extreme compression. On the hydrophilic oxygen side of TMAO, AIMD simulations at 1 bar and 10 kbar predict a change of the coordination number from a dominating TMAO·(H2O)3 complex at ambient conditions towards an increased population of a TMAO·(H2O)4 complex at HHP conditions. This increase of the TMAO-oxygen coordination number goes in line with a weakening of the local hydrogen bond network, spectroscopic shifts and intensity changes of the corresponding intermolecular THz bands. Using a pressure-dependent HHP force field, FFMD simulations predict a significant increase of hydrophobic hydration from 1 bar up to 4-5 kbar, which levels off at higher pressures up to 10 kbar. THz spectroscopic data reveal two important pressure regimes with spectroscopic inflection points of the dominant intermolecular modes: The first regime (1.5-2 kbar) is barely recognizable in the simulation data. However, it relates well with the observation that the apparent molar volume of solvated TMAO is nearly constant in the biologically relevant pressure range up to 1 kbar as found in the deepest habitats on Earth in the ocean. The second inflection point around 4-5 kbar is related to the amount of hydrophobic hydration as predicted by the FFMD simulations. In particular, the blueshift of the intramolecular CNC bending mode of TMAO at about 390 cm-1 is the spectroscopic signature of increasingly pronounced pressure-induced changes in the solvation shell of TMAO. Thus, the CNC bend can serve as local pressure sensor in the multi-kbar pressure regime.

Urea's match in the hydrogen-bond network? A high pressure THz study.

We present results of the measurement of the low frequency spectrum of solvated urea. The study revealed a blue shift of the intramolecular mode of urea centered at 150 cm-1 of Δν= 17 cm-1 upon increasing the pressure up to 10 kbar. The blue shift scaled linearly with the increase in density and was attributed to a stiffening of the water-urea intermolecular potential. We deduced an increase in the number of affected water molecules from 1 to 2 up to 5-7, which corresponds to the sterical coordination number of urea. The increase in hydration number can be explained by an suppression of the NH2 inversion and the hydrogen bond switching around the NH2 group. Pressure induced sterical constraints are proposed to hinder the rapid switching of hydrogen bond partners and make the water around urea less bulk-like than under ambient conditions.

Does hydrated glycine act as solidification nucleus at multi-kilobar conditions?

The investigation of aqueous solutions containing biomolecules as a function of thermodynamic parameters, such as the pressure, is crucial for understanding biological processes. Here we report the first low frequency spectra of 1.5 M aqueous glycine from ambient pressure up to 8 kbar, i.e. in the pressure range which is crucial for understanding biological processes under extreme conditions. We observe a linear pressure dependent blue shift of the specific N-C-C-O open/close mode at ∼320 cm-1 indicating an increasing compression of the solvated glycine. In contrast, the characteristic peak of the hydrogen bond hydration water network centered, at ambient conditions, at ∼184 cm-1 non-linearly blue shifts with increasing pressure, as well, but with a slower rate than the intramolecular peak. This indicates that the macroscopic liquid-solid phase transition observed above 8 kbar pressure is driven by hydrated glycine as solidification nucleus.