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5 Electrodynamics of Aqueous Media
5.2 The Electrodynamics of Confined Water
In many natural and artificial systems, such as rocks, polymers, nanostructures, and
biological objects, water is confined in small cavities bounded by a substance (liquid
or solid) and exhibits properties different than those of bulk water. Such water can
be considered as one more state of water: confined water. Confined water can be
defined as water that is subject to geometric constraints on a nanoscale (typically a
few nanometers) so that molecules are close enough to an interface to sense some
difference from standard bulk conditions. Confined water is most frequently found
in porous media or in solvation shells of colloidal particles. Since confined water
is of high practical importance, it has been studied by means of different methods
and techniques, and many parameters have been found to differ from those in the
bulk water. In particular, low viscosity, a low dielectric constant, enhanced molecular mobility, surface charge, and shifted freezing and evaporation points have been
observed experimentally or studied theoretically [31–37].
The electrodynamics of confined water is a matter of separate discussion, as it
reflects the molecular mechanisms near the interfaces, and is also important for
modern electric power storage systems, such as accumulators, flow batteries, supercapacitors, and fuel cells (see Sect. 5.6). For these systems, the properties of water
near the interface are crucial and either serve as the reference parameters for the
quantitative models or are used directly for the accumulation and separation of electric charges. Typically nuclei are separated from electrons (for example near the
electrodes), but ion–ion separation also takes place (for example, in membranes and
separators). However, the electrodynamics of water at nanoscales and near the interfaces is still poorly understood, in spite of the long history of its study, starting from
the famous analysis of the electric double layer
8 by Helmholtz in 1853 [38].
The molecular mechanisms of the electrical charge separation near the aqueous
interfaces (or in confined water) are important for processes on different scales, from
the global [39] (see Sect. 5.3), to the microscopic in which the electric properties of
confined water define the communication of cells, by providing neuronal-network
pulses a propagation environment. Although the mechanism of the nerve pulse transfer has been considered from different angles, and the basic models are well established [40], the role of water in the charge-transfer mechanism in biological systems
is underestimated. The picosecond timescale processes, discussed in Chap. 4, are
very important for the electrodynamic properties of water near the interfaces and
significantly influence local chemical reactions and transport in biological systems.
There are two basic parameters of interfacial water which characterize its electrodynamic properties: the static dielectric constant (0), and the protonic conductivity σ dc . However, neither have been studied experimentally at length, because
it is extremely difficult to separate the intrinsic properties of confined water from
the properties of the confining matrix. Fumagalli et al. [32] studied the dielectric
8 An electric double layer occurs when two phases, one of which is a liquid, come into contact.
Trying to lower the surface energy, the particles at the interface form a different dynamical structure
than that in the bulk.
5 Electrodynamics of Aqueous Media
5.2 The Electrodynamics of Confined Water
In many natural and artificial systems, such as rocks, polymers, nanostructures, and
biological objects, water is confined in small cavities bounded by a substance (liquid
or solid) and exhibits properties different than those of bulk water. Such water can
be considered as one more state of water: confined water. Confined water can be
defined as water that is subject to geometric constraints on a nanoscale (typically a
few nanometers) so that molecules are close enough to an interface to sense some
difference from standard bulk conditions. Confined water is most frequently found
in porous media or in solvation shells of colloidal particles. Since confined water
is of high practical importance, it has been studied by means of different methods
and techniques, and many parameters have been found to differ from those in the
bulk water. In particular, low viscosity, a low dielectric constant, enhanced molecular mobility, surface charge, and shifted freezing and evaporation points have been
observed experimentally or studied theoretically [31–37].
The electrodynamics of confined water is a matter of separate discussion, as it
reflects the molecular mechanisms near the interfaces, and is also important for
modern electric power storage systems, such as accumulators, flow batteries, supercapacitors, and fuel cells (see Sect. 5.6). For these systems, the properties of water
near the interface are crucial and either serve as the reference parameters for the
quantitative models or are used directly for the accumulation and separation of electric charges. Typically nuclei are separated from electrons (for example near the
electrodes), but ion–ion separation also takes place (for example, in membranes and
separators). However, the electrodynamics of water at nanoscales and near the interfaces is still poorly understood, in spite of the long history of its study, starting from
the famous analysis of the electric double layer
8 by Helmholtz in 1853 [38].
The molecular mechanisms of the electrical charge separation near the aqueous
interfaces (or in confined water) are important for processes on different scales, from
the global [39] (see Sect. 5.3), to the microscopic in which the electric properties of
confined water define the communication of cells, by providing neuronal-network
pulses a propagation environment. Although the mechanism of the nerve pulse transfer has been considered from different angles, and the basic models are well established [40], the role of water in the charge-transfer mechanism in biological systems
is underestimated. The picosecond timescale processes, discussed in Chap. 4, are
very important for the electrodynamic properties of water near the interfaces and
significantly influence local chemical reactions and transport in biological systems.
There are two basic parameters of interfacial water which characterize its electrodynamic properties: the static dielectric constant (0), and the protonic conductivity σ dc . However, neither have been studied experimentally at length, because
it is extremely difficult to separate the intrinsic properties of confined water from
the properties of the confining matrix. Fumagalli et al. [32] studied the dielectric
8 An electric double layer occurs when two phases, one of which is a liquid, come into contact.
Trying to lower the surface energy, the particles at the interface form a different dynamical structure
than that in the bulk.