Table 1.

Electrochemical Stark tuning rates ($\mathrm{\Delta }\mu /d_{\mathrm{H}}$) extracted from fitting a linear model to the data in Fig. 4 and derived Helmholtz double-layer widths ($d_{\mathrm{H}}$)

Cation	$\mathrm{\Delta }\mu /d_{\mathrm{H}}$, ${\mathrm{V}}^{-1}\cdot$cm−1	$d_{\mathrm{H}}$, Å
${\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$	$41.8\pm 0.6$	$3.1\pm 0.0$
${\mathrm{E}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$	$28.8\pm 0.4$	$4.5\pm 0.1$
${\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$	$24.3\pm 0.2$	$5.4\pm 0.0$
${\mathrm{B}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$	$17.2\pm 0.6$	$7.6\pm 0.3$