When water-ice grows into salt solutions ion species are excluded by the ice differentially due to non-identical solubility in the ice lattice. This causes an electrical potential across the interface during the ice growth process, initially named the Workman Reynolds Freezing Potential, and may be one of the causes for lightning. However, by measuring the voltage between the ice and water, we have found that when tetrahydrofuran hydrate crystals are grown into salt solutions all ion species are excluded equally and the potential does not manifest. When considered together, this marked difference in ion exclusion scenarios may have ramifications for hydrate exploration because of the chlorine anomaly, which is often used as an indicator of the presence of hydrate reserves.
It is estimated that about 90% of natural gas hydrate on Earth is methane hydrate, with potential quantities well above the level of carbon stored as fossil fuel. In some permafrost regions water-ice might also be expected to exist alongside, or perhaps not far above, the hydrates [
Hydrate exploration technologies are complicated and have many uncertainties. These Cl− assumptions and rationale rely on the total exclusion of ions from the cages of water making up the hydrate structures during hydrate formation and this does appear to be the case. While in general there seems to be good agreement between Cl− values changing and the actual presence of hydrates it must be noted now that the ion exclusion from hydrates and from water-ice are not the same. Full and equal ion exclusion during gas hydrate formation has always been (correctly) assumed [
In 1948, Workman and Reynolds [
Our results, typical for a single growth rate of ice, grown as polycrystalline ice, are shown in
the growth of the ice is complete [
Here, we report the markedly different effect which is seen when hydrates of tetrahydrofuran are grown. Clearly THF hydrates are not methane and are not fully representative of hydrates formed at high pressure, but they are often used as a model system. A 17-water fully enclosed cage, for example, may not mimic the THF structure but nonetheless this effect, and its comparison to water-ice, is of interest and in need of further investigation (
In 2008 Wilson and Haymet made mention of the difference between water-ice and THF hydrate grown as single crystals and measured in the method described here [
What we find here is that the voltage formed is consistently less than 100 mV over the range of concentrations measured. The value of the freezing potential
can thus be used as a direct measure of the level of differential ion exclusion, and we see that the hydrate excludes both species equally.
By measuring the Workman Reynolds Freezing Potential formed by the differential ion exclusion occurring as water-ice grows into salt solutions, we have previously calculated that at maximum potential as many as 1 in 3000 ions in the solution are involved [
As far as we are aware this is the first report of the comparison of ion exclusion properties using polycrystalline solids of both water ice and THF hydrate and represents a line of investigation in need of further consideration, particularly for those involved in using Cl− anomalies for hydrate exploration in areas where water ice may form, or may have formed either during the hydrate formation or during the test measurements. Our results will be useful for the confirmation of molecular models of the ice/water interface, such as the recent work of Nagata et al. [
Wilson, P.W. and Haymet, A.D.J. (2017) Ion Exclusion at the Ice-Water Interface Differs from That at the Hydrate-Water Interface: Consequences for Methane Hydrate Exploration. International Journal of Geosciences, 8, 1225-1230. https://doi.org/10.4236/ijg.2017.810070