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11 Liquid Phase
11.1 Wonders of H 2 O Molecular Undercoordination
and Salt Hydration
As independent degrees of freedom, water molecular undercoordination and electrostatic polarization by ions in salt solutions or an parallel field in a capacitor make the
mysterious water ice even more fascinating [1]. Undercoordinated water molecules
are referred to those with fewer than four nearest neighbors (CN < 4) as they occur
in the bulk interior of water and ice. Molecular undercoordination takes place in the
terminated hydrogen bonded network, in the skin of a large or small volume of water
and ice, molecular clusters, ultrathin films, snowflakes, clouds, fogs, nanodroplets,
nanobubbles, and water in the vapor phase.
Such a kind of water molecules shows the extraordinary features of hydrophobic,
less dense, elastoviscous, melting point (T m ) elevation, freezing temperature (T N )
and evaporating temperature (T V ) depression, and superfluidity when travelling in
microchannels [2–6]. A few molecular-layer of ice can form at the room temperature on SiO 2 substrate [7]. Nanobubbles are long lived, mechanically stronger and
thermally more stable. The T N drops from 258 K to even 150 K when the droplet
size is reduced—called supercooling or “no man’s land” [4, 8]. Likewise, nuclear
magnetic resonance and differential scanning calorimetry measurements revealed
that the melting of ice in porous glass having different pores sizes proceeds inhomogeneously. There exists a 0.5 nm thick interface liquid layer between the pores
surface and the ice crystal [9]. The superfluidity occurs at most six-layer molecular
thickness sandwiched between two graphene sheets [10].
Excessive properties also include the longer O–O distance, stiffer H–O phonons
and softer O:H phonons, deeper O 1s core level, and longer photoelectron and H–O
phonon lifetimes [11–16]. The dynamics of water molecules in the confined geometries and near different types of surfaces are substantially slower than in the bulk
(about one order of magnitude) [8, 17]. These features become more pronounced as
the molecular coordination number decreases.
Salt solvation differs the local physical–chemical properties of solutions in the
hydration shells from those of the ordinary bulk water. Intensive pump–probe spectroscopic investigations have been conducted to pursue the mechanism behind molecular
performance in the spatial and temporal domains. For instance, the sum frequency
generation (SFG) spectroscopy resolves information on the molecular dipole orientation or the skin dielectrics, at the air–solution interface [18], while the ultrafast
two–dimensional infrared absorption (2DIR) probes the solute or water molecular
diffusion dynamics in terms of phonon lifetime and the viscosity of the solutions
[19].
Salt solutions demonstrate the Hofmeister effect [20] on regulating the solution
surface stress and the solubility of proteins with debating mechanisms of structural
maker and breaker [21], ionic specification [22], quantum dispersion [23], skin induction [24], quantum fluctuation [25], and solute–solvent interactions [26]. Increasing
the chloride, bromide and iodide solute concentration stiffens the H–O stretching
vibration mode to higher frequencies. The H–O phonon blueshifts are usually referred
11 Liquid Phase
11.1 Wonders of H 2 O Molecular Undercoordination
and Salt Hydration
As independent degrees of freedom, water molecular undercoordination and electrostatic polarization by ions in salt solutions or an parallel field in a capacitor make the
mysterious water ice even more fascinating [1]. Undercoordinated water molecules
are referred to those with fewer than four nearest neighbors (CN < 4) as they occur
in the bulk interior of water and ice. Molecular undercoordination takes place in the
terminated hydrogen bonded network, in the skin of a large or small volume of water
and ice, molecular clusters, ultrathin films, snowflakes, clouds, fogs, nanodroplets,
nanobubbles, and water in the vapor phase.
Such a kind of water molecules shows the extraordinary features of hydrophobic,
less dense, elastoviscous, melting point (T m ) elevation, freezing temperature (T N )
and evaporating temperature (T V ) depression, and superfluidity when travelling in
microchannels [2–6]. A few molecular-layer of ice can form at the room temperature on SiO 2 substrate [7]. Nanobubbles are long lived, mechanically stronger and
thermally more stable. The T N drops from 258 K to even 150 K when the droplet
size is reduced—called supercooling or “no man’s land” [4, 8]. Likewise, nuclear
magnetic resonance and differential scanning calorimetry measurements revealed
that the melting of ice in porous glass having different pores sizes proceeds inhomogeneously. There exists a 0.5 nm thick interface liquid layer between the pores
surface and the ice crystal [9]. The superfluidity occurs at most six-layer molecular
thickness sandwiched between two graphene sheets [10].
Excessive properties also include the longer O–O distance, stiffer H–O phonons
and softer O:H phonons, deeper O 1s core level, and longer photoelectron and H–O
phonon lifetimes [11–16]. The dynamics of water molecules in the confined geometries and near different types of surfaces are substantially slower than in the bulk
(about one order of magnitude) [8, 17]. These features become more pronounced as
the molecular coordination number decreases.
Salt solvation differs the local physical–chemical properties of solutions in the
hydration shells from those of the ordinary bulk water. Intensive pump–probe spectroscopic investigations have been conducted to pursue the mechanism behind molecular
performance in the spatial and temporal domains. For instance, the sum frequency
generation (SFG) spectroscopy resolves information on the molecular dipole orientation or the skin dielectrics, at the air–solution interface [18], while the ultrafast
two–dimensional infrared absorption (2DIR) probes the solute or water molecular
diffusion dynamics in terms of phonon lifetime and the viscosity of the solutions
[19].
Salt solutions demonstrate the Hofmeister effect [20] on regulating the solution
surface stress and the solubility of proteins with debating mechanisms of structural
maker and breaker [21], ionic specification [22], quantum dispersion [23], skin induction [24], quantum fluctuation [25], and solute–solvent interactions [26]. Increasing
the chloride, bromide and iodide solute concentration stiffens the H–O stretching
vibration mode to higher frequencies. The H–O phonon blueshifts are usually referred