Electrocapillarity
= Electrocapillarity
<Electrocapillarity> is the dependence of interfacial <surface tension> on an electrical potential difference. Charging an electrode–electrolyte interface changes its interfacial free energy.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Electrocapillarity E01941 and electrocapillary equation E01942]{ref}
At fixed <temperature>, <pressure> and chemical potentials, the electrocapillary relation reduces to
$$
d\gamma=-\sigma\,dE,
$$
where $\gamma$ is interfacial <surface tension>, $E$ the appropriately defined <electrode potential> and $\sigma$ the electrode's free <surface charge density>. Thus an experimental curve of $\gamma$ versus $E$ measures charge through its slope. If $C=d\sigma/dE>0$, then $d^2\gamma/dE^2=-C<0$; zero charge gives a local maximum under these conditions. Adsorption or changing solution composition adds chemical-potential terms, so a measured maximum is not an unconditional universal charge marker.