Hard water

Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates. Hard drinking water may have moderate health benefits. It can pose critical problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of foam formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects. Origins Natural rainwater, snow and other forms of precipitation typically have low concentrations of divalent cations such as calcium and magnesium. They may have small concentrations of ions such a

Molecular ordering effects in bulk and at lipid/water nanoscopic interfaces: A story of water, light, and order

Nathan Dupertuis

Water is ubiquitous on Earth, playing a critical role for a plethora of structures, processes and chemical reactions in nature. Its unique physicochemical properties originate mainly from its hydrogen bond network. Despite its recognized importance, many aspects of the structure of water as a liquid remain elusive. It is especially difficult to understand the complex nature of fluctuations and ordering in pure water, or to get a complete picture of aqueous interfaces at the molecular level. In this thesis, our aim is to improve our understanding of the underlying interactions between water and ions, macromolecules, and interfaces, especially the long range interactions. To this aim, we use two nonlinear optical techniques, second harmonic scattering (SHS) and vibrational sum frequency scattering (SFS), on liquid water as well as on model lipid membranes mimicking the cell membranes.We investigate with SHS how the orientational ordering of water molecules is affected by temperature changes and nuclear quantum effects. Scattering measurements of pure water reveal that the intermolecular correlations undergo a symmetry transformation with increasing temperature. On the other hand, with increasing temperature, aqueous electrolyte solutions exhibit a diminished influence of the combined electrostatic field on the water- water correlations. This trend is predicted qualitatively by a Debye-Hückel model, but not quantitatively due to its non-inclusion of hydrogen bonding and nuclear quantum effects. These insights facilitate the elaboration of future nuclear quantum mechanical models of water.The same interactions are further studied by a comparison of pure liquid water and pure liquid carbon tetrachloride. SHS measurements and simulations reveal that transient nanoscale voids exist in water and carbon tetrachloride. The coherent SHS emissions observed in water are generated by charge density fluctuations around the cavities. These measurements strongly hint towards non-uniformity in the structure of water, on femtosecond timescale and nanometric range.Next, we turn towards the structure of water around model lipid membranes. We explore first the three-dimensional confinement of liquid water inside liposomes of various size. By analyzing SHS patterns of zwitterionic and anionic liposomes, we observe that the effect of confinement is visible on a range of a hundred nanometers, larger compared to the previously reported ranges on the order of a nanometer or about ten nanometers. The water orientational ordering at the interface as well as the surface potential of the liposomes are retrieved with a nonlinear light scattering modeling, showing that the confinement effect is mainly attributable to the hydrogen bond network. The effect is in particular 8 times larger for light water than for heavy water.Finally, we investigate how the water structure is modified around lipid monolayers and bilayers. The hydration of lipid membranes is asymmetric with respect to charge. SHS and vibrational SFS reveal that the charge hydration asymmetry is drastically different depending on the nature of the lipid layer. It depends on a delicate balance between electrostatic and hydrogen bonding interactions. The charge hydration asymmetry is further modified by the presence of charges on the lipid headgroups on both sides of the bilayers.

EPFL2021

Polyelectrolytes induce water-water correlations that result in dramatic viscosity changes and nuclear quantum effects

Jan Dedic, Halil Ibrahim Okur, Sylvie Roke

Ions interact with water via short-ranged ion-dipole interactions. Recently, an additional unexpected long-ranged interaction was found: The total electric field of ions influences water-water correlations over tens of hydration shells, leading to the Jones Ray effect, a 0.3% surface tension depression. Here, we report such long-range interactions contributing substantially to both molecular and macroscopic properties. Femtosecond elastic second harmonic scattering (fs-ESHS) shows that long-range electrostatic interactions are remarkably strong in aqueous polyelectrolyte solutions, leading to an increase in water-water correlations. This increase plays a role in the reduced viscosity, which changes more than two orders of magnitude with polyelectrolyte concentration. Using D2O instead of H2O shifts both the fs-ESHS and the viscosity curve by a factor of similar to 10 and reduces the maximum viscosity value by 20 to 300%, depending on the polyelectrolyte. These phenomena cannot be explained using a mean-field approximation of the solvent and point to nuclear quantum effects.

American Association for the Advancement of Science (AAAS)2019

Molecular ordering effects in bulk and at lipid/water nanoscopic interfaces: A story of water, light, and order

Nathan Dupertuis

EPFL2021

Molecular ordering effects in bulk and at lipid/water nanoscopic interfaces: A story of water, light, and order

Polyelectrolytes induce water-water correlations that result in dramatic viscosity changes and nuclear quantum effects

Comment on "Water- water correlations in electrolyte solutions probed by hyper-Rayleigh scattering" [J. Chem. Phys. 147, 214505 (2017)]

Molecular ordering effects in bulk and at lipid/water nanoscopic interfaces: A story of water, light, and order

Comment on "Water- water correlations in electrolyte solutions probed by hyper-Rayleigh scattering" [J. Chem. Phys. 147, 214505 (2017)]

Polyelectrolytes induce water-water correlations that result in dramatic viscosity changes and nuclear quantum effects