Abstract
In this article, we report the generation of alternating current by the application of constant and ramping DC voltages across oil–water interfaces. The work reported here can be broadly divided into two parts depending on the shapes of oil–water interfaces, i.e., flattened and curved. In the first part, an alternating current of ∼100 nA (amplitude) was generated by applying a constant DC voltage of −3 V and above across a freestanding and flattened oil–water interface. In another part, an alternating current of ∼150 nA (amplitude) was generated by applying a ramping up DC voltage starting from −5 V to 5 V, then again ramping back down to −5 V for the freestanding and curved interface. The suggested qualitative mechanism that engenders such a phenomenon includes the oil–water interface acting like a membrane. This membrane oscillates due to the electrophoretic movement of ions present in the aqueous phase by the application of a DC voltage across the interface. This electrophoretic movement of ions across oil–water interfaces causes Faraday instabilities leading to oscillations of the said interface. This method could also be used to study the stress levels in the interfacial films between two immiscible liquids. It explores the more-than-Moore’s paradigm by finding a substitute to a conventional alternator/inverter that generates alternating current upon applying a DC voltage input. This work would be of substantial interest to researchers exploring alternatives to conventional AC generators that can be used in liquid environments and in the design of novel integrated circuits that could be used for unconventional computing applications.
Highlights
Oil–water (OW) interfaces display very interesting properties at different pH values
Since the mobile charge carrier in this case is OH− that is present in the aqueous phase, its electrophoretic movement away from the negative terminal would cause a bending of the OW interfacial membrane toward the oil phase
The bending of the membrane toward the oil phase would generate a positive electrochemical current. While this electrophoretic movement of OH− ion induced bending takes place, the fatty acid molecules in the oil would get completely ionized due to interactions with the impinging OH− ions at extremely alkaline conditions, since the primary constituents of cooking oil are long chain unsaturated fatty acids with their pKa values ranging from 8 to 11.13 Such bending would continue until the point when it could go no further and the direction of movement of the membrane would reverse, i.e., toward the aqueous phase
Summary
Oil–water (OW) interfaces display very interesting properties at different pH values. Flattening of the OW interface was done by increasing the pH of the aqueous phase to 12.1 This happens due to the ionization of entire fatty acid constituents of mustard oil, short and long chains as well as mono-unsaturated and polyunsaturated. One of the major mono-unsaturated fatty acid constituents of mustard oil is erucic acid, which has an estimated pKa value of 4.7 This means that even at neutral pH (when the interface is curved), some of the fatty acids (i.e., erucic acid and the ones whose pKa values are less than 7) are ionized and form a molecularly thick membrane. The resultant products of the reaction of fatty acids and NaOH (base) are sodium salts of the corresponding fatty acids and water This sodium salt becomes a membranous interface (of molecular thickness) between the oil and aqueous phases.
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