Another way researchers have sought to understand U PZC is through the work function reduction (∆φ) which occurs between a surface in water and the same surface in vacuum (φ vac ). Trasatti showed that U PZFC can be directly derived from the work function of a material in ion-free water φ . Because U PZC is measured in V [here vs. the standard hydrogen electrode (SHE)], the following equation can be used to relate φ and U PZC :