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Reply to Comment on Can Water Store Charge?
In a recent paper entitled Can Water Store Charge? (Langmuir 2009, 25, 542-547), we obtained a seemingly unexpected result: charge imparted to water could be stored and subsequently released as current. Most physical chemists do not anticipate that water can store charge, but such storage is well-known among atmospheric scientists: the charge stored in neighboring clouds creates intercloud potential differences on the order of millions of volts. The subsequent discharge (lightning) is evidently more intense and of shorter duration that that demonstrated in our paper, but the two phenomena rest on the same principle: charge separation between juxtaposed bodies of water. Corti and Colussi (C&C) (Langmuir 2009, 25, 6587-6589) challenge the conclusions drawn in our article. We had demonstrated long-term charge separation between two halves of a water-filled chamber and subsequent release of the stored charge as current. Their argument suggests an alternative mechanism by which current could flow as observed, but without actual charge separation and storage. If valid, their argument would preserve the long-held principle of electroneutrality. Although the reasoning of C&C is straightforward, it rests on the assumed presence of salt in the water. Indeed, 10 mM NaCl was used in the lion’s share of the reported experimental runs. However, the same charge-recovery phenomenon occurs in the absence of salt. This fact was brought forth in both the Methods and Results sections and particularly in Table 1, where the ratio of recovered charge to input charge, or the recovery ratio, was shown to be independent of whether salt was added to the water. Hence, for the results obtained, salt was not necessary. It was used merely for convenience;for increasing the conductivity and thereby shortening the duration of charging and recovery. Because the analysis of C&C rests on the presence of salt, their argument is not on track. In fact, the water that was used was extremely pure; it came from a top-grade laboratory source, Diamond Nanopure, with resistivity of up to 18.2 MΩ cm. The numerous preliminary experiments carried out with pure water showing results similar to those reported led us to formalize and systematize the experiments that were eventually published. Similar charge separation was likewise demonstrated in a prior paper on this subject (Klimov and Pollack, Langmuir 2007, 23, 11890-11895). Ultrapure water was used throughout, mainly without salt; the few additional experiments carried out with salt showed largely similar results. In those and the present experiments, the water that was used was as ion-free as is reasonably attainable in a laboratory setting. And in the present study, the pH-sensitive dye was omitted in control experiments (Figure 2) to be absolutely certain that even minor “contaminants” in the water could not be responsible for the observed charge recovery. Hence,
11202 DOI: 10.1021/la901533c
the evidence shows that charge storage in and release from water are genuine features that occur independently of the addition of salt, similar to what occurs in clouds. The prevailing sense that charge storage in water is unlikely stems largely from the principle of electroneutrality invoked by C&C as though axiomatic. Exactly where this principle originated is unclear, but violations can be found at levels ranging from the atomic to the cosmic. At the atomic level, electrons remain physically separated from the nucleus. At laboratory-scale levels, regions of water near hydrophilic surfaces contain long-lasting zones of separated charge (Zheng et al., Adv. Colloid Interface Sci. 2006, 127, 19-27). The intercloud charge separations mentioned above (which give rise to 80% of lightning discharges) occur at larger length scales. And at still larger scales, as well known to geophysicists and atmospheric scientists, the earth itself has a net negative charge whereas the enveloping atmosphere compensates with a net positive charge. Thus, persisting charge separations occur throughout nature. Although varied interpretations have been put forth to account for all of these separations, they still violate the principle of electroneutrality. Perhaps the most dramatic violation is the classical waterdropper experiment, first devised by Lord Kelvin and now demonstrated amply on the web. Two closely juxtaposed metallic chambers progressively fill with water derived from the same source, which is split to create two falling streams. Each stream descends through a metal ring that is electrically cross-connected to the opposite metal chamber. As the chambers fill with water, the potential difference between them increases to more than 50 kV until the chambers discharge onto one another with a loud coronal flash. Prior to discharge, one chamber of water is positively charged, and the other is negatively charged. This experiment demonstrates in the laboratory setting what is seen naturally in clouds: that bodies of water can sustain huge amounts of positive or negative charge. Although electroneutrality is a logical end point, this end point is by no means universally observed; potential differences between bodies of water are commonly seen, as they were in the experiments in question. These multiple violations of the principle of electroneutrality raise the question of whether this principle can really be the foundational platform on which all else must rest. The results in our article lend additional doubt, although they do beg the question of the energy needed for maintaining the persisting charge separations, which is an issue beyond the scope of this brief response but treated in detail elsewhere (http://uwtv.org/ programs/displayevent.aspx?rID=22222). Kate Ovchinnikova and Gerald H. Pollack*, Department of Bioengineering, Box 355061, University of Washington, Seattle, Washington 98195
Published on Web 08/20/2009
Received April 29, 2009
Langmuir 2009, 25(18), 11202–11202
