Characterizing facilitated proton-transfer reaction pathways. Facilitated PT rates were measured at the water|trifluorotoluene (TFT) interface with the working electrode immersed in the aqueous phase, such that positive current could be assigned to either cation transfer from the aqueous to organic phase or anion transfer in the reverse direction. Because the polarization resistance of the cell is dominated by the micrometer-scale liquid|liquid interface supported at the pipette tip, the potential drop between the aqueous and nonaqueous electrodes is fully assigned to the potential drop at the water|TFT interface, Δ 𝑜𝑜 w 𝜙𝜙. To distinguish between pre-extraction and interfacial pathways, cyclic voltammograms of DPP and 9MA were collected in micropipette (radii ~4 𝜇𝜇m) electrochemical cells at increasing scan rates. Initially, the organic phase, and by extension the phase in which the base is originally present, was positioned inside the pipette (Figure ) and the corresponding CVs are shown in Figure . The CV for DPP facilitated proton-transfer exhibited a positive and negative peak positioned at 0.5 and 0.4 V in the cell used for that experiment, respectively, both of which grew with increasing scan rates from 50 -400 mV/s. This is qualitatively different from the case of 9MA facilitated proton-transfer, where a positive limiting-current plateau was observed on the positive sweep and a negative peak was observed around 0.1 V (that also grew with increasing scan rates). The above behavior can be rationalized by considering the asymmetric geometry of the pipette and the different diffusion profiles that would arise from species transport inside or outside the pipette. Given the confined space within the pipette relative to the pipette orifice, an inner diffusing species should follow roughly semi-infinite linear diffusion giving peaked-current behavior in the cyclic voltammetry analysis (as is typically observed for a freely diffusing redox couple in an unstirred solution at a planar electrode). In contrast, the diffusion boundary layer outside of the pipette is allowed to expand radially in all directions and has dimensions comparable to the dimensions of the micropipette, leading to a hemispherical diffusion profile and a concomitant steady-state current (as is typically observed for redox at an analogous solid nanoor micro-electrode). Thus, observation of positive and negative peaks in the DPP-facilitated proton-transfer voltammograms suggests that DPP is diffusing to and from the interface only in the organic phase. This interpretation is further supported by the data in Figure , where the organic phase containing DPP is positioned on the outside of the pipette. Only steady-state current was observed in the scan-rate range tested because DPP is now diffusing to and from the interface from the outside of the pipette, where a hemispherical diffusion profile is formed.