The resulting free-standing 2D PEDOT thin film could be recovered from the L|L interface, washed with acetone and suspended in an acetone:sulfuric acid mixture (Fig. ) for storage before ex-situ characterisation or applications. Optimal conditions for thin film formation required a low concentration of oxidant (with the monomer always in excess), which is unconventional in chemical synthesis. A thermodynamic analysis of this biphasic system explains the need for an external electrochemical driving force to drive interfacial electrosynthesis with significant kinetics (Supplementary Information section 3). DPSCA cycles (Fig. ). These trends corroborated the mechanism outlined in Fig. , as detailed in Supplementary section 4.1. Evidence of interfacial ion-pairing between adsorbed PEDOT oligomers and SO4 2-anions during interfacial electrosynthesis is provided by comparison of differential capacitance measurements of the blank aqueous|TFT interface and after DPSCA cycles 2 to 6 in the presence of Ce 4+ and EDOT (Fig. and Supplementary section 4.2). In situ parallel-beam UV/vis absorbance measurements show the depletion of Ce 4+ on the aqueous side of the L|L interface with continuous DPSCA cycling and PEDOT thin film formation at the ITIES after 300 DPSCA cycles (Supplementary Fig. ). The Tyndall effect was used to explore the partition of small PEDOT oligomers from the organic to aqueous phase during interfacial electrosynthesis (Supplementary Fig. ).