Simple Estimate of the Potential Drop across an Amphiprotic Liquid–Liquid Interface

Two immiscible liquids in contact with each other can have different internal electrostatic potentials. An associated electric double layer (EDL) therefore exists within each liquid. For amphiprotic liquids, the exchange of protons between the two liquids gives rise to two EDLs, a positively charged EDL in one of the liquids and negatively charged EDL in the other. Using the pKa and pKb of one liquid dissolved in the other and the pH equivalent within each amphiprotic liquid, we can estimate the potential drop, Δφ, between the interior of the two liquids, also known as the Galvani potential or liquid–liquid junction potential. This estimation is independent of surface charge and ionic strength. By using the ionic strength to find the thickness of the EDL, we also estimate the average electric field strength across the interface. For the special case of water (H2O) in contact with an immiscible alcohol (ROH), the potential drop across the interface from the water to the alcohol is Δφ = 2.303VT (pKb + pH – pKw – pH2OR), where VT is the thermal voltage at a given temperature T.