Mapping Hydration Water around Alcohol Chains by THz Calorimetry

Abstract

THz spectroscopy was used to probe changes that occur in the dynamics of the hydrogen bond network upon solvation of alcohol chains. The THz spectra can be decomposed into the spectrum of bulk water, tetrahedral hydration water, and more disordered (or interstitial) hydration water. The tetrahedrally ordered hydration water exhibits a band at 195 cm and is localized around the hydrophobic moiety of the alcohol. The interstitial component yields a band at 164 cm which is associated with hydration water in the first hydration shell. These temperature-dependent changes in the low-frequency spectrum of solvated alcohol chains can be correlated with changes of heat capacity, entropy, and free energy upon solvation. Surprisingly, not the tetrahedrally ordered component but the interstitial hydration water is found to be mainly responsible for the temperature-dependent change in ΔC and ΔG. The solute-specific offset in free energy is attributed to void formation and scales linearly with the chain length.

Wrapping Up Hydrophobic Hydration: Locality Matters

“…We present in Figure 1 A the absorption difference spectrum α difference . α difference describes the THz spectrum of water in the hydration layer and is obtained by subtracting the volume-scaled absorption spectrum of bulk water α bulk from the absorption spectrum of the solution α solution (see ref ( 16 ) for details). The measurements were carried out for a 0.5 M alcohol concentration at two temperatures (290 and 310 K) between 100 and 600 cm –1 , thereby extending the frequency range of earlier measurements.…”

“…The measurements were carried out for a 0.5 M alcohol concentration at two temperatures (290 and 310 K) between 100 and 600 cm –1 , thereby extending the frequency range of earlier measurements. 16 The absorption of aqueous solutions in the 100–210 cm –1 region can be attributed to contributions from the translational and intermolecular hydrogen bond stretch modes of water, while the intramolecular solute modes are restricted to higher frequencies (see Figure S7 and ref ( 17 )). Previously, we could show that the absorption spectrum of hydration water can be dissected into a sum of two hydration water intermolecular stretching modes centered at 164 and 195 cm –1 , both of which differ from those of bulk water, which has a broader absorption band centered around 200 cm –1 .…”

“…Previously, we could show that the absorption spectrum of hydration water can be dissected into a sum of two hydration water intermolecular stretching modes centered at 164 and 195 cm –1 , both of which differ from those of bulk water, which has a broader absorption band centered around 200 cm –1 . 16 , 17 While the amplitude of each of these two hydration water bands, called ν 164 and ν 195 , is temperature-, concentration-, and solute-dependent, the center frequency and the line width are almost constant (see ref ( 16 ) and Table S1 ). The ν 195 component displays the most pronounced temperature dependence with a rapid decrease in intensity for an increase in temperature, while the ν 164 component is less sensitive to an increase in temperature.…”

Interactions of Glycerol, Diglycerol, and Water Studied Using Attenuated Total Reflection Infrared Spectroscopy

“…Therefore, the redshifts of CH 2 in the present study can be better explained by its interaction with CHOH of surrounding polyols rather than with H 2 O molecules. On the other hand, an alternative explanation can also be possible by “blueshifting” of CH bands due to “anomalous” or “improper” hydrogen bonding interactions with increasing water molecules, which have been reported for aliphatic alcohols 23–25 through complex mixtures of multiple interactions but leading to contraction of C–H bonds. 26–29…”

The water molecule arrangement over the side chain of some aliphatic amino acids: A quantum chemical and bottom‐up investigation

“…These studies reported some estimations of the number of water molecules in the nearest neighboring hydration shell, that for methane is about 20 . Another important piece of experimentally proven information about amino acids and peptides is the slow dynamics of the water molecules composing the hydration shell with respect to the bulk water . The slow‐down reorientation has been attributed to the heterogeneity of the peptide surface and in particular to the strong peptide‐water hydrogen bonds (HBs) and to the cage around the hydrophobic side chain that largely disrupts the water‐water synergistic reorientation mechanisms …”