Research Article: Large-scale structure formation in ionic solution and its role in electrolysis and conductivity

Date Published: March 26, 2019

Publisher: Public Library of Science

Author(s): Chut-Ngeow Yee, C. H. Raymond Ooi, Luck-Pheng Tan, Misni Misran, Nyiak-Tao Tang, Nikolai Lebedev.

http://doi.org/10.1371/journal.pone.0213697

Abstract

That water may not be an inert medium was indicated by the presence at water’s interfaces a negatively charged solute free zone of several hundred microns in thickness called the exclusion zone (EZ). Further evidence was demonstrated by Ovchinnikova’s experiments (2009) showing that water can store and release substantial amount of charge. We demonstrate that the charge storage capacity of water arises from highly stable large-scale ionic structures with measurable charge imbalances and discrete levels of charge density. We also show evidence that the charge zones formation requires ionic solutes, and their formation correlate to large change in conductivity, by as much as 250%. Our experiments indicate that large-scale structuring plays a pivotal role in electrolysis and conductivity in ionic solution. We propose that water is an electrochemically active medium and present a new model of electrolysis and conductivity in ionic solution.

Partial Text

The presence of solute-free zones, called the exclusion zones (EZ) near water’s interfaces is a phenomenon that is being extensively studied experimentally and theoretically in recent years [1] [2] [3] [4] [5] [6] [7]. This solute-free zone typically spans several hundred microns and is observed in a variety of hydrophilic surfaces; including natural and artificial hydrogels, the water-air interface, biological surfaces, hydrophilic polymers and metallic surfaces. Electrical measurements show large negative potential within the zone, of the order of 100 mV, and pH measurement show a large concentration of H+ beyond the zone [2]. EZ has also shown to absorb radiant energy to build and maintain its structure, with peak absorption at the 270 nm wavelength [8]. NMR spectroscopy shows that it is highly restricted and dynamically more stable [9]. It can also exert mechanical forces measurable using a laser tweezers system [10]. Refractive index measurement show a 10% increase in the EZ [11]. Cryogenic scanning electron microscopy suggests that EZ consists of high density water with a cell-like wall structure [12].

Fig 3 shows probe voltages vs. time graphs of charging 20 mM Na2SO4 solution over 6 hours at 10 V. We can see the upwards trend of the top 9 probes over time, while the bottom 6 probes are showing a downward trend, leaving a relatively large voltage gap between probes P5 and P6.

We have performed over 200 experiments on various solutes over a wide range of ion concentration, from deionized water to 320 mM solutions. We observed stable large-scale charge zone formation on all our experiments when the ion concentration is above 1 mM. But even with deionized water where large-scale charge zone is not observed, we have evidence of the formation and rapid migration of small charged zone fragments. We will dedicate a section on the surprising and revealing behaviour of deionized water later. We can conclude with high certainty that charge zone formation is inseparable from electrical conductivity in ionic solution.

Fig 3 and Fig 4 shows the gradual formation of charge zones with an external voltage source. How do the charge zones behave when the voltage source is removed? Fig 6 tracks the probe voltages for 10 hours after 6 hours of charging with a 10 V power source. Measurements were done relative to electrode B (the cathode during charging).

Gradual but significant change in charging current, by as much as 250% over a span of hours, is a prominent effect we observed in our experiments. In this section we present evidence that such large changes in conductivity is related to charge zone formation in the solution.

In this section we investigate the electrochemical properties of water with extremely low amount of ionic solutes and explore the role ionic solutes play in conductivity. We use deionized water in this experiment, which can be regarded as a solution containing minute amount of contaminants, including dissolved gas (e.g. CO2).

In this paper, we demonstrated that ionic solution form large-scale charge zones in an electrolysis setup (Figs 3 and 4). Zone bearing positive voltage form on the anode while zone bearing negative voltage form on the cathode. The two charge zones expand and grow towards each other in a slow process measured in hours. We observed charge zones formation on more than 200 experiments involving a wide range of solutes, ions concentration and charging voltage and presented some representative examples in Fig 5A–5D. In Figs 6 and 7 we demonstrate that the charge zones are highly stable and have discrete voltage levels.

 

Source:

http://doi.org/10.1371/journal.pone.0213697

 

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