5.5 Water in a Strong Electric Field
201
Important results were recently obtained by ultrafast pump–probe spectroscopy [62]
and neutron diffraction [66], which showed that the supramolecular structure inside
the bridge is different than that observed in undisturbed water. Although there is no
reason to expect anything else than just ordinary electrostatics, one should analyze
the dynamic structure of water at high voltage in order to obtain more information
on the microscopic level.
Fuchs et al. reported [67] a unidirectional transfer of both mass and charge through
the bridge from anode to cathode. The intrinsic charges of water, being involved by
the electric field, drag the neutral H 2 O dipoles and carry the mass from the positively
charged electrode to the negatively charged electrode. This direction corresponds
to the direction of the excess-proton flow and can be understood in terms of electrophoresis.
10 Excess protons (H 3 O
+ ) and proton holes (OH
− ) as charges move in
opposite directions, i.e., from anode to cathode, and from cathode to anode, respectively, but the mass (the hydrogen nuclei) always flows from anode to cathode,
because both ions transfer the proton via Grotthuss mechanism (see Sect. 1.3.1). For
instance, the dynamics of OH
− ions by the Grotthuss schematic assume that charge
and the mass (of the proton) are moving in opposite directions, while in case of H 3 O
+
the mass and the charge move in the same direction. Thus, the movement from anode
to cathode of excess protons physically translate the mass from one beaker to another,
while the charges move in the opposite directions. The unidirectional flow of protons
through the bridge is confirmed [67] by the pH gradient between the beakers, which
increases with time.
Namin et al. found [63] that the proton current is proportional to the applied
voltage. But, when the voltage reaches 20–25 kV, the current saturates. This effect
is similar to that observed in electrolytes, and known as Wien effect,
11 explained by
Onsager and Kim [68]. In the ionic model of water (see Sect. 4.4), excess protons
and proton holes have the potential to mutually interact. The external electric field
tilts the potential, allowing them to drift along and against the field respectively,
increasing the current density proportionally to the electric field. However, when the
electric field strength becomes very high, the current saturates, as there is a limited
amount of the ionic species in the potentials. The height of the potential determines
the maximal current that can be passed through water without electrical breakdown.
12
Interestingly, the voltage of the saturation of the protonic current in the water bridge
coincides with the threshold of electrostriction, observed in aqueous solutions [70],
thus indirectly confirming that ionic species in water have the potential to mutually
interact.
13
10 Electrophoresis is the motion of dispersed particles of a fluid under the influence of a spatially
uniform electric field.
11 The Wien effect is the experimentally observed increase in ionic mobility or conductivity of
electrolytes at very high gradients of electrical potential [69].
12 Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material (such as water), subjected to a high enough voltage, suddenly becomes an electrical
conductor and a current flows through it.
13 This interaction is missing in the standard Bernal–Fowler water model as discussed in Sect. 1.3.
201
Important results were recently obtained by ultrafast pump–probe spectroscopy [62]
and neutron diffraction [66], which showed that the supramolecular structure inside
the bridge is different than that observed in undisturbed water. Although there is no
reason to expect anything else than just ordinary electrostatics, one should analyze
the dynamic structure of water at high voltage in order to obtain more information
on the microscopic level.
Fuchs et al. reported [67] a unidirectional transfer of both mass and charge through
the bridge from anode to cathode. The intrinsic charges of water, being involved by
the electric field, drag the neutral H 2 O dipoles and carry the mass from the positively
charged electrode to the negatively charged electrode. This direction corresponds
to the direction of the excess-proton flow and can be understood in terms of electrophoresis.
10 Excess protons (H 3 O
+ ) and proton holes (OH
− ) as charges move in
opposite directions, i.e., from anode to cathode, and from cathode to anode, respectively, but the mass (the hydrogen nuclei) always flows from anode to cathode,
because both ions transfer the proton via Grotthuss mechanism (see Sect. 1.3.1). For
instance, the dynamics of OH
− ions by the Grotthuss schematic assume that charge
and the mass (of the proton) are moving in opposite directions, while in case of H 3 O
+
the mass and the charge move in the same direction. Thus, the movement from anode
to cathode of excess protons physically translate the mass from one beaker to another,
while the charges move in the opposite directions. The unidirectional flow of protons
through the bridge is confirmed [67] by the pH gradient between the beakers, which
increases with time.
Namin et al. found [63] that the proton current is proportional to the applied
voltage. But, when the voltage reaches 20–25 kV, the current saturates. This effect
is similar to that observed in electrolytes, and known as Wien effect,
11 explained by
Onsager and Kim [68]. In the ionic model of water (see Sect. 4.4), excess protons
and proton holes have the potential to mutually interact. The external electric field
tilts the potential, allowing them to drift along and against the field respectively,
increasing the current density proportionally to the electric field. However, when the
electric field strength becomes very high, the current saturates, as there is a limited
amount of the ionic species in the potentials. The height of the potential determines
the maximal current that can be passed through water without electrical breakdown.
12
Interestingly, the voltage of the saturation of the protonic current in the water bridge
coincides with the threshold of electrostriction, observed in aqueous solutions [70],
thus indirectly confirming that ionic species in water have the potential to mutually
interact.
13
10 Electrophoresis is the motion of dispersed particles of a fluid under the influence of a spatially
uniform electric field.
11 The Wien effect is the experimentally observed increase in ionic mobility or conductivity of
electrolytes at very high gradients of electrical potential [69].
12 Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material (such as water), subjected to a high enough voltage, suddenly becomes an electrical
conductor and a current flows through it.
13 This interaction is missing in the standard Bernal–Fowler water model as discussed in Sect. 1.3.