Terahertz pulsed spectroscopy as a new tool for measuring the structuring effect of solutes on water

Terahertz spectroscopy of thick and diluted water solutions

While bright terahertz sources are used to perform nonlinear experiments, they can be advantageous for high-precision linear measurements of opaque samples. By placing the sample away from the focus, nonlinearities can be suppressed, and sizeable amounts of transmitted radiation detected. Here, this approach is demonstrated for a 0.5 mm thick layer of liquid water in a static sample holder. Variations of the index of refraction as small as (7 ± 2) · 10-4 were detected at 0.58 THz for an aqueous salt solution containing ten millimoles of sodium chloride. To my knowledge, this precision is unprecedented in time-domain spectroscopy studies of diluted aqueous systems or other optically thick and opaque materials.

Insights into ion specificity in water-methanol mixtures via the reentrant behavior of polymer

In the present work, we have for the first time systematically investigated the ion specific reentrant behavior of poly(N-isopropylacryamide) (PNIPAM) in water-methanol mixtures. Turbidity measurements demonstrate that SCN(-) and ClO(4)(-) depress the reentrant transition, whereas other anions enhance the transition. As the anion changes from chaotropic to kosmotropic, the minimum critical phase transition temperature (T(min)) decreases and the corresponding volume fraction of methanol (X(M)) shifts to a larger value. Our results demonstrate that anion specificity is due to the anionic structure making/breaking effect on water/methanol complexes. Cations are found to have a lesser but still significant effect on the reentrant transition, and as T(min) decreases the corresponding X(M) also shifts to larger values as with the anions. Our studies show that cation specificity is induced by specific interactions between cations and PNIPAM chains. Furthermore, both anion and cation specificities are amplified as X(M) is increased due to the formation of additional water/methanol complexes. Calorimetry measurements demonstrate that the ion specificity is dominated by changes in entropy.

Probing Intermolecular Interactions in Water/Ionic Liquid Mixtures by Far-infrared Spectroscopy

Far-infrared spectra in the range from 600 to 20 cm-1 of two hydrophilic (1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate) and one hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate) ionic liquids and their mixtures with water at different concentrations are reported. Shifts of the librational water bands depending on the nature of the anion are found to be related to the strength of the interaction between the water molecules and the anions. For both hydrophilic ionic liquids, the librational band is centered around 460 cm-1, whereas for the hydrophobic ionic liquid, it is shifted to 388 cm-1, indicating less hindered rotation of single water molecules. Multivariate curve resolution, paying special attention to the spectral range from 50 to 350 cm-1, was used to investigate the presence of different species with increasing water concentration. For both hydrophilic ionic liquids, a band located at 153 cm-1 was resolved into two different contributions. A small contribution at 202 cm-1 can be attributed to intermolecular interactions between water molecules forming dimers. The major contribution (centered at 148 cm-1) corresponds to water molecules that do not bond to each other via H-bonding. It is therefore assigned to a hindered translation arising from the stretching of the hydrogen bond between BF4- anions and water molecules. Formation of water dimers in the hydrophobic ionic liquid does not occur. Furthermore, the spectral contribution of the stretching of H-bonds between water molecules and PF6- cannot be unambiguously detected, which indicates an extremely weak interaction between water molecules and this anion.