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In situ Raman spectroscopy revealed that transiting the concentrated NaI/H2O solutions to an ice VI phase and then into an ice VII phase at 298 K proceeds in a way different from that activated by the solute type. Unlike the solute type that raises both the critical pressures PC1 and PC2, for the liquid-VI, the VI-VII transition simultaneously occurs in the Hofmeister series order: I > Br > Cl > F ∼ 0; concentration increase raises the PC1 faster than the PC2 that remains almost constant at higher NaI/H2O molecular number ratios. Concentration increase moves the PC1 along the liquid-VI phase boundary and it finally merges with PC2 at the triple-phase junction featured at 350 K and 3.05 GPa. The highly-deformed H-O bond is less sensitive to the concentration because of the involvement of anion-anion repulsion that weakens the electric field in the hydration shells. Observations confirm that the salt solvation lengthens the O:H nonbond and softens its phonon but relaxes the H-O bond contrastingly. Compression, however, has the opposite effect from that of salt solvation. Therefore, compression recovers the polarization-deformed O:H-O bond first and then proceeds to the phase transitions. The anion-anion interaction discriminates the effect of NaI/H2O concentration from that of the solute type at an identical concentration on the phase transitions.

A combination of the differential Raman spectrometrics and contact-angle measurements has enabled resolution of the hydrogen bond (HB, O:H O) relaxation and molecular dynamics in the NaX hydration shells. Observations suggest that the aqueous ions create each an electric field to form the hydration shells through O:H O bond elongation and polarization. The ionic polarization shares the same, and even stronger, effect of molecular undercoordination that shortens the H O bond and stiffen its phonon, and meanwhile, the O O Coulomb repulsivity lengthen and soften the O:H nonbond with an association of polarization. O:H O bond electrification raises not only the structure order of molecules and the viscosity in the hydration shells but also the skin stress and thermal stability of the solution. Exercises not only deepen our insight into the salt solute-solvent interaction involved in the Hofmeister series in terms of O:H O bond electrification but also provide a powerful means quantifying the dynamic, local, molecular-site-resolved information regarding the O:H O bond cooperativity in aqueous solutions.

The solute–solvent interaction of salts has a striking impact on various biological and industrial processes but its mechanism remains yet mysterious despite intensive studies since 1888 when Franz Hofmeister established the salt series. A combination of confocal Raman spectroscopy and contact angle measurements has enabled us to resolve the hydrogen bond relaxation (O:H―O, HB) and the associated charge polarization dynamics at different molecular site because of alkali halides hydration. Results show consistently that salt hydration softens the O:H phonon but stiffens H―O phonon cooperatively. The extent of HB relaxation and polarization is proportional to the electronegativity difference and ionic radius, following the order of Hofmeister series: X (R/η) = I (2.2/2.5) > Br (1.96/2.8) > Cl (1.81/3.0) > F (1.33/4.0) ≈ 0 for anions, and Y(R/η) = Na (0.98/0.9) > K (1.33/0.8) > Rb (1.49/0.8) > Cs (1.65/0.8) for cations. Observations suggest that ions create each an electric field that aligns, stretches, and polarizes water molecules, which relaxes the O:H―O bond cooperatively, depresses the molecular dynamics, and enhances the hydration shell viscosity and the skin stress. Exercises also demonstrate that Raman spectroscopy performs as a powerful tool for probing the molecular-site-resolved HB network relaxation dynamics in terms of phonon stiffness, molecular fluctuation dynamics, and phonon abundance transition under external stimulus. Copyright © 2016 John Wiley & Sons, Ltd.

In situ Raman spectroscopy revealed that transiting H2O/NaX (∼64) solutions into an ice VI phase and then into an ice VII phase at a temperature of 298 K requires excessive pressures with respect to pure water. The increase of the critical pressures varies with the solute type in the Hofmeister series order: X = I > Br > Cl > F ∼ 0. The results suggest that the solute hydration creates electric fields that lengthen and soften the O:H nonbond and meanwhile shorten and stiffen the H–O bond through O–O Coulomb repulsion. Compression, however, does the opposite to solute electrification upon the O:H–O bond relaxation. Therefore, compression of the aqueous solutions recovers the electrification-deformed O:H–O bond first and then proceeds to the phase transitions, which requires excessive energy for the same sequence of phase transitions. Ice exclusion of solute disperses the frequencies of characteristic phonons and the critical pressures with unlikely new bond formation.

Aqueous charge injection in forms of electrons, protons, lone pairs, ions, and molecular dipoles by solvation is ubiquitously important to our health and life. Pursuing fine-resolution detection and consistent insight into solvation dynamics and solute capabilities has become an increasingly active subject. This treatise shows that charge injection by solvation mediates the O:H–O bonding network and properties of a solution through O:H formation, H↔H frag-ilization, O:⇐⇒:O compression, electrostatic polarization, H 2 O dipo-lar shielding, solute–solute interaction, and undercoordinated H–O bond contraction. A combination of the hydrogen bond (O:H–O or HB with ‘:’ being the electron lone pairs of oxygen) cooperativity notion and the differential phonon spectrometrics (DPS) has enabled quantitative information on the following: (i) the number fraction and phonon stiffness of HBs transiting from the mode of ordinary water to hydration; (ii) solute–solvent and solute–solute molecular nonbond interactions; and (iii) interdependence of skin stress, solution viscosity, molecular diffusivity, solvation thermodynamics, and critical pressures and temperatures for phase transitions. An examination of solvation dynamics has clarified the following: (i) the excessive protons create the H↔H or anti-HB point breaker to disrupt the acidic solution network and surface stress. (ii) The excessive lone pairs generate the O:⇐⇒:O or super–HB point compressor to shorten the O:H nonbond but lengthen the H–O bond in H 2 O 2 and basic solutions; yet, bond-order-deficiency shortens and stiffens the H–O bond due H 2 O 2 and OH − solutes. (iii) Ions serve each as a charge center that aligns, clusters, stretches, and polarizes their neighboring HBs to form hydration shells. (iv) Solvation of alcohols, aldehydes, complex salts, carboxylic and formic acids, glycine, and sugars distorts the solute–solvent interface structures with the involvement of the anti-HB or the super-HB. Extending the knowledge and strategies to catalysis, solution–protein, drug–cell, liquid–solid, colloid–matrix interactions and molecular crystals would be even more fascinating and rewarding.

Phonon spectrometrics and contact-angle measurements revealed the essentiality of anti-HB (hydrogen bond), super-HB, and electrified-HB representing molecular interactions in the Lewis acid, base, and adduct (salt) solutions, respectively. Hydronium creation (H3O tetrahedron with one lone pair) in acid solution results in the H↔H anti-HB that breaks the O:H–O bond network, diluting blood flow for instance; hydroxide (OH tetrahedron with three lone pairs) leads to the O:↔:O super-HB that serves as a point compressor to the hydration network, releasing heat by softening the H–O bond at hydrating. Salt ions create each an electric field that aligns, clusters, polarizes, and stretches water molecular dipoles in a supersolid hydration-shell manner. The electrification-induced phonon relaxation disperses the quasisolid phase boundary outwardly and hence lowers the freezing temperature and raises the melting point. O:H–O bond electrification also raise the viscosity, skin stress, H–O phonon lifetime, but depresses the order of fluctuation and the coefficient of molecular rotation and self-diffusion in the hydration shells. The extent of electrification is molecular site, solute concentration and type dependent, following the Hofmeister series.

Solute ionic electrification of the O:H–O bond modulates significantly the critical pressures, temperatures, and gelation times for transiting aqueous solution into solid by dispersing the boundaries of the quasisolid phase. High-pressure in situ Raman spectrometrics revealed that transiting NaX solutions into ice VI and then into ice VII phase requires higher excessive pressures at 298 K temperature. The ΔPC varies in the order of Hofmeister series: X = I > Br > Cl > F ~ 0. Meanwhile, salting st​iffens the ωH and elongates the dOO throughout the course of compressure when transiting phase VII to phase X at even higher pressure. Recovering the electrification-shortened H–O bond needs excessive energy for the same sequence of phase transitions. Concentration dependence of the NaI solution indicates a different mechanism from that of solution type but it is similar to heating on the Liquid-VI-VII phase transition dynamics.