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Confined water

Confined water is generally liquid water held within nanometer-sized vessels.

 

V Hydrophobic confinement
V Hydrophilic confinement

V Water molecules confined in beryl nanovoids
V Capillary rise
V Venturi effect

V Interfacial water and water-gas interfaces
V Nanobubbles

 

Confined water is found widespread in nature in granular and porous material and around and within cells, macromolecules, supramolecular structures and gels. It has recently been reviewed [1592]. When water is confined there is conflict between the energetic minimization of the hydrogen-bonded network, interactions with the relatively large cavity surface, the possibility of quantum coherence and the fit within the space available. The physical properties and state of the contained water may vary widely depending on the molecular characteristics of the cavity surface and the confinement dimensions, as well as temperature and pressure. The properties of the confined water are difficult to predict and may be very different from those of bulk water. Unsurprisingly, the effect of charged surfaces, such as negatively charged reverse micelles, are much more extensive than those of hydrophobic surfaces, such as droplets in oil [2783],. This is particularly true when the confinement is on the nanoscale. When the confinement is about or less than 2 nm the water cannot form hexagonal or cubic ice crystals such that cooled water forms a supercooled water rather than ice even through the 'no mans land'. The properties of this deeply supercooled liquid is often used as indicative of the properties of liquid water in 'no mans land' although it is best to take care over such conclusions. This scenario has been carefully examined [2613], but without concluding that such confined supercooled water is likely to be identical to a continuation of bulk supercooled water.This is unsurprising as the space available for the molecular clusters (=< 2 nm) is insufficient for the expected structure of the low-density form of water to be formed, particularly if the further space reduction due to surface interactions are taken into account. [Back to Top to top of page]

Hydrophobic confinement

Water at hydrophobic surfaces is similar to that at the liquid gas interface but confinement prevents movement of the water molecules towards the surface and van der Waals dispersion interactions loosely hold the outer molecules in preferred positions and orientations relative to the surface atoms.

Single chain of water molecules in a (6; 6) carbon nanotube

 

Much theoretical and experimental work has concerned water in carbon nanotubes; made up of a hexagonal lattice of sp2-hybridized carbon atoms rolled up into cylinders with diameters of a nanometer or so. Such structures are often thought of as hydrophobic cylinders but that can underestimate the importance of the surface. The carbon atoms provide a high number of van der Waals interactions in an orderly hexagonal lattice arrangement that varies between the different types of nanotube and with their diameters. Although uncharged, these surfaces provide extensive pi-electron orbitals that can interact with and be polarized by the water molecules to cause effects significantly different from that expected of a flat and uniform hydrophobic cylinder. The preferred arrangement of the water molecules within these nanotubes is dependent on their diameter and the temperature and pressure.

 

Opposite is shown a one-dimensional strand of water molecules within a (6; 6) carbon nanotube, diameter 0.81 nm. The water self-aligns to give an almost all-trans zigzag arrangement possessing similar entropy to that found in bulk water [1397]. Each water molecule has two cooperative hydrogen bonds plus multiple van der Waals interactions from the carbon nanotube. Such organized water chains are determined by the confinement rather than interaction with the pore surface [1398] and make rapid proton conductors due to the Grotthuss effect [1041].

Square ice formed inside a (14; 0) carbon nanotube

 

 

 

In modeling studies, penetration of water into nanotubes is modulated by small changes in the polarity of the wall and the dimensions of the cavity [1396]. The cohesive nature of the hydrogen-bonded network may allow water to travel easily through such pores, with the molecules leaving the pore pulling through a following stream of water. Thus, sub micrometer-thick membranes made from graphene oxide (a mixed hydrophilic/hydrophobic material) , which are completely impermeable to all other liquids, vapors, and gases, allow unimpeded permeation of water [1778]. Graphene sheets with nanoscale pores also function well as desalination membranes [1900].

 

A phase diagram of water within carbon nanotubes has been proposed for tube diameters up to 1.7 nm. This suggests that there may be at least nine phases possible within the cylindrical space, including those found by x-ray diffraction and by simulation. The global maximum freezing point is found to be for ‘square’ ice at 290 K [1400].

 

Shown opposite.is such a square ice nanotube formed within a (14; 0) carbon nanotube of diameter 1.11 nm. a Each water molecule has four cooperative hydrogen bonds, which are more bent than in hexagonal ice. They also have multiple van der Waals interactions from the carbon atoms.

Other ices (for example, pentagonal, hexagonal and heptagonal nanotubes of water molecules, all with two donor and two acceptor hydrogen bonds) are formed within carbon nanotubes of diameters increasing up to 1.9 nm diameter [1401]. Conversion between these ices can be achieved by increasing the pressure; such that the square ice converts to the pentagonal ice at about 200 MPa at about 275 K [1402]. The density and self-diffusion coefficients of liquid water in narrow confining nanotubes generally increases with their diameter but exhibits anomalous behavior in (8; 8) and (9; 9) carbon nanotubes [2292].

 

Interestingly, ambient water flow through carbon nanotubes with diameters of less than 2 nm exceed predictions by more than three orders of magnitude and are substantially higher than using larger diameter pores [2345]. This is probably due to the presence of a nearly frictionless surface together with the confined space preferring water chains rather than 3-D clusters, so reducing the drag. Single-file water flows down 0.8 nm diameter carbon nanotubes an order of magnitude faster than within biological water transporters [3015]. Molecular dynamics calculations have shown that the static dielectric constant of water in carbon nanotubes may increase 10-fold to 700 due to the surface excluded volume allowing an increase in dipolar fluctuations [2372]. The presence of this expected layer of “free” OH bonds, facing the nanotube inner wall, has been found experimentally for liquid water in larger diameter (1.4 - 2.1 nm) nanotubes [2777].

 

The phase boundaries of water confined within carbon nanotubes showed substantial sensitivity to the diameter [2875]. Ice was formed, melting between 105-151 °C (pore diameter 1.05 nm), 87-117 °C (1.06 nm), -35 and 10 °C (1.15 nm), < -80 °C (1.24) 15–49 °C(1.44) and 3–30 °C (1.52 nm). Both low- and high-density liquid water states are detected near ambient temperature and above ambient pressure within 1.25 nm carbon nanotubes. In the temperature–pressure
phase diagram, the low- and high-density liquid phases are separated by hexagonal ice nanotube phase [2925].

square ice formed between between graphene sheets, [2312]

The graphene -water interface has been probed by surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy that gave a strong SFG peak associated with highly coordinated, ordered water next to the graphene [2935]. A slightly irregular two dimensional square ice ([2312], see left) forms between sheets of graphene 0.65 nm apart under ambient conditions. It has a high packing density and is thought to exist under nano-confinement by many hydrophobic surfaces. This square ice is not the only two-dimensional ice possible as water simulated as confined under pressure between flat plates, 0.85 nm apart, forms a lower density twelve-fold symmetrical quasicrystal made up of mostly five-membered rings in two molecular layers [1705]. Three different ices were found between 0.8 and 1.0 nm spaced hydrophobic walls. One of these was an interlocked pentagonal bilayer ice (IPBI), with (per unit cell) six pentagons (including those sharing vertices) along the x-direction and two pentagons (without shared vertices) along the y-direction [2691]. On the basis of first principle calculations, a phase diagram of double-layer ice as a function of pressure (0.1 MPa -5 GPa) and confinement (0.85 - 1.1 nm width) at 0 K has been predicted [2907].

 

Water confined between hydrophobic molybdenum disulfide (MoS2) and graphene gives a flat two-layer crystalline ice phase, shows O atoms only

GrapheneWater confined between hydrophobic molybdenum disulfide (MoS2; a flat hexagonal sheet-like structure with sulfur-covered faces) and graphene (a flat hexagonal sheet-like sp2-hybridized carbon structure, see left) gives a flat two-layer crystalline ice phase consisting of two planar hexagonal layers with water molecules directly on top of, and hydrogen bonded to, each other (three 120° and three 90° angles between the hydrogen bonds; see right where only the oxygen atoms are shown). The intra-hydrogen bonding between the layers gives rise to four membered square rings [2952]. The lack of any surface dangling bonds gives rise to this ice's hydrophobic character. The surface of clean young graphene (see above left) is thought to be hydrophilic with any found hydrophobicity due to the accumulation of surface hydrocarbon impurities. [2958]. Approaching water molecules prefer to sit with their oxygen atoms pointing at the center of the carbon hexagons so maximizing the van der Waals contacts between the electronic charge density of the O atom and the delocalized p-electron density of graphene (13.5 kJ ˣ mol-1, [2978]).

 

There is evidence that the protons of confined liquid water exist in a novel quantum state when significant quantum fluctuations in charge take place along the hydrogen bonds [1699].

 

If water is forced into crevices in hydrophobic/superhydrophobic surface, the high surface tension of water-hydrophobic solid interfaces stores energy in the form of interfacial free energy. The spontaneous extrusion of such water out of the nanopores, related to pressure changes, allows the partial or full recovery of the stored energy [2701]. The transport of water molecules, in these hydrophobic confined carbon nanotube systems under pressure, proceeds as a wave motion with frequencies in the terahertz region which is caused by the hydrogen bonding and determined by the carbon nanotube diameter [2835].

 

Volumes of water confined within hydrophobic clefts and pores (see elsewhere) have higher activity than bulk water whereby the water has a tendency to leave theses volumes creating 'closing' pressures. This evaporation of water induced by confinement between hydrophobic surfaces has a functional role in numerous biophysical phenomena including hydrophobic self-assembly. It is extremely sensitive to the flexibility of the confining material and if the material is more flexible, the pore preferably empties of the liquid phase [2927].

 

Heavy water (D2O) binds more strongly within hydrophobic pores than light water (H2O) and this may, in future, lead to a heavy water concentration system [3000].

 

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Hydrophilic confinement

At hydrophilic surfaces, polar and hydrogen bonding interactions with the surface may vary between those that are very much weaker than water-water hydrogen bonds to those that are very much stronger. Water will condense from the gas phase onto glass to give at least four layers of absorbed water [1403], with the water next to the silica highly structured, dependent on the surface and distinguishable from more distant water by infrared [1404] X-ray [1405] spectroscopy and molecular dynamics [2854]. The negatively-charged silica surface has many isolated, vicinal and geminal silanol groups, dependent on its history [2883], that strongly hydrogen bond to water. The pKa of the silanol groups vary between 2.3 to 10.7 as determined along capillary wall, due to surface heterogeneities [3016]. Mineral oxides, such as quartz (SiO2), sapphire (Al2O3) and hematite (Fe2O3), are generally thought to be hydrophilic and well wetted by water. However, fresh faces have a modest hydrophobicity depending on how the surfaces have been prepared. Given sufficient time and an appropriate pH, surface hydroxylation occurs and the surfaces become hydrophilic [2858]. Even the second neighbors to the surface are considerably affected due to the competition between the surface water and the effects of confinement [1406]. When the confinement is less than about 2 nm wide, the viscosity may increase considerably to over a million times that of bulk water, but this viscosity increase is not noticed if the confining surfaces are hydrophobic [1304]. The surface hydroxyls on the pore walls hydrogen bond to the water and control the distortion of the hydrogen bond network and its surface dynamics [1408].

 

hysteresis in the adsorption/desorption of water on hydrophilic surfaces

Variation of the spacing between any interaction sites on polar cavity surfaces determine the strength of binding, distribution and organization of several layers of water molecules. Sawtooth-like oscillatory forces may be generated as hydrophilic surfaces approach with about 1 nm periodicity, corresponding to units reducing by clusters with a linear width of five H-bonded water molecules at a time (rather than single water molecule losses) [2105].

 

The adsorption of water to a bare surface involves continuous changes in structuring due to relaxation of the hydrogen bonding and molecular orientations as the surface becomes covered and further water layers are added. This can give rise to substantial hysteresis (shown opposite) in structure and properties on its removal [1409]. The adsorption of liquids into hydrophilic nanopores is due to both capillary flow and surface hydration followed by growth of the surface films. This latter mechanism can force out compressed gas from within the pores [2977].

 

Confinement may cause raised or lowered melting points, dependent on the surface and the space available. Generally, it might be expected that the raised boiling point and depressed freezing point are inversely related to the capillary diameter due to the Gibbs-Thomson effect [1420]. As examples, cubic ice may form in 4.2 nm diameter cylindrical silica pores at 232 K [1410] and water confined in the well-ordered, if narrow (diameter 1.5 nm), pore structure of the silica MCM-41-S-15 does not freeze above 130 K [2180]. However, surface interactions and the confinement diameter may cause opposite effects, such as the ice that forms at 400 K in very narrow pores (0.6-1.0 nm diameter) in porous glass [1411].

 

Water may freeze when confined between flat surfaces such that it can form the optimally ordered hydrogen bonds required. The confined water undergoes significant changes in density and viscosity as the surface separation is varied. For example in a model, water goes from liquid to solid and back again as the space decreases from 0.58 nm to 0.53 nm to 0.47 nm between interacting but non-hydrogen bonding surfaces [1412]. If unable to form the required ordered structures, it may still undergo wide fluctuations in density as it attempts to optimize its structure within the restrictions of the confinement.

 

When ice does form at strongly binding or atomically rough confining surfaces, there may be no room for expansion and a high-density amorphous ice may result [1413]. Different ices may form in interacting confined space, with a high-density confined ice (1.2 g cm-3) formed from two helices and crystallizing at ambient temperature in AlPO4-5 zeolite 0.73 nm diameter micropores [1407]. Both cubic and amorphous ices can form at strongly binding silica surfaces at low temperatures [1406]. However rather than forming ice within confinement, liquid water in confined spaces may leave the confinement on freezing, to crystallize freely external to the cavity, so long as they can easily be replaced by the gas phase under the ambient pressure [1399]. [Back to Top  to top of page]

emerald

Water molecules confined in beryl nanovoids

Beryl is a colorless hexagonal mineral composed of beryllium aluminum cyclosilicate with the chemical formula (BeO)3(Al2O3)(SiO2)6 that can be colored by trace amounts of other ions such as chromium (emerald) and ferrous (aquamarine); for structure see below right. Beryl contains SiO4 tetrahedra-lined hexagonal voids just 0.51 nm wide by 0.9 nm long, separated by narrow (0.28 nm) “bottlenecks” (Si6O6 rings) that hold single isolated water molecules.

The structure of beryl, showng the hexagonal voids

These water molecules exhibit quantum tunneling behavior whereby the water molecule hops into 6 × 2 equivalent positions above and below the oxygen atom within the channel, that spread out the positions of the hydrogen atoms (delocalization) into a pair of corrugated rings (see below with the calculated hydrogen charge density, going from blue (lowest) to yellow (highest) [2547].

Hydrogen charge density of water molecules within the beryl voids [2547]

Thus, the tunneling causes water's dipole to disappear and its center of mass to shift to the central axis (the crystalline 'c' axis) of the double ring structure [2547].

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Footnotes

a The octameric cubic structure (H2O)8 has also been found by crystallography, trapped within supramolecular metal–
organic networks [2093]. Its structure in the gas phase has been explored through terahertz spectroscopy [2595]. [Back]

 

 

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