Ion‐Transfer Voltammetry at the Interface between Organic and Salt‐Doped Ice Phases

Abstract

Ions play critical roles in reactions of biological, environmental, and industrial importance under frozen conditions. Transfer voltammetry of tetramethylammonium (TMA) and K + ions is measured at the interface between frozen aqueous NaCl and dichloroethane to give an insight into chemistry in frozen aqueous phases. The ion-transfer current of TMA initially increasesa st he temperature becomes lower from the freezing point, but the further decrease in temperature reduces the transfer current. The half-wave potentialo ft he TMA transfer shows anegative shiftwhen the liquid phase volumeiss maller than 10 %o ft he total volume of the frozen phase. Similarly, the half-wave potential of the facilitated transfer of K + by crown ether complexation also becomes negative with decreasingt emperature, due to the freeze concentration of K + in the liquid phase.Freezingi sa ni mportant process widely utilized in agriculture, fishery,t he food industry,m edicine, biological technologies, and so forth. [1,2] Although chemical reactions are usually retarded at lower temperatures,s ome examples have been reported for freeze enhancements of reactionk inetics.[3] Frozen aqueous solutions are therefore systems of scientific and practical interest. Salts are often involved in aqueous frozen mediaa nd play important roles in chemical processes occurring therein. In frozen aqueous solutions, salt crystals and ice are separately formed at the temperature lower than the eutectic point of the system (T eu ). When the temperature of this solid mixture increasesp ast T eu ,t he salt is dissolved in the liquid phase (LP) that coexists with ice below the melting point. [4] One possible reason for the accelerationso fr eactions in the frozen systems is the freeze concentrationo fs olutes. This aspect has been wells tudied from ak inetic viewpoint;t he reactions of order two or highera re accelerated by freeze concentration.[5] Another explanation of the kinetic acceleration is ap Hc hange in the LP upon freezing. As stated above, ions are mostly expelled from the ice phase as salts but are partly entrapped by ice. If an anion is better distributedt ot he ice phase than ac ation,t he negative charge shortage occursi n the LP,w hich is relaxed by the transfer of OH À to the LP.[6] The LP becomes more basic and the ice is acidified as ar esult. The pH shift by freezing explains the acceleration of some reactions quite well. [7] However,an umber of reactions occurring in the LP cannot be explained by theseorigins. [3,4,8] Unusual reactions in the LP possibly come from the properties of water therein,w hicha re different from those in unfrozen aqueous solution. Ar ecent molecular dynamics study has indicated that protont ransfer,a cids olvation, and ionization in aq uasi-liquid layer,w hich is formed on the surface of ice below the melting point, are different from that of bulk water.[9] The structure of liquid water in contact with ice is affected by ice, and as ar esult its properties are also influenced. However, no experimentali nformation is a...

Voltammetry of Viologens Revealing Reduction of Hydrophobic Interaction in Frozen Aqueous Electrolyte Solutions

“…The CVs obtained in frozen solution have different features from those recorded from unfrozen solution. Voltammetric currents decrease with increasing α because the electrode surface is partially covered by ice crystals, which reduces the effective surface area . Changes in peak current with increasing α are summarized in Figure .…”

“…Voltammetric currents decrease with increasing a because the electrode surface is partially covered by ice crystals, which reduces the effective surfacea rea. [27,28] Changes in peak current with increasing a are summarized in Figure 2. The peak currents of P 1 Red ,P 2 Red , and P 2 Ox decrease similarly with increasing a;t he current at a = 100 is 30 %o ft he corresponding current fora nu nfrozen solution.I nc ontrast, the peak current forP 1 Ox with a = 100 is one-twelfth of that measured for an unfrozen solution.…”