Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations

Structural changes in water layers on a CO-covered Pt electrode triggered by rapid pulsed laser heating of the interface are studied using potential transient measurements. When the laser energy density is below 20 mJ cm^{− 2}, the water layers undergo a change in orientation, which causes a negative shift in the rest potential that recovers within 20 μs with the cooling of the interface. In contrast, when the laser intensity exceeds 20 mJ cm^{− 2}, the CO is desorbed and the rest potential first experiences a positive shift, which is followed by a negative shift. This positive shift is caused by replacement of the CO by water and subsequent restructuring of the water layer. The restructuring rate depends strongly on the electrolyte cation: the rest potential reaches a maximum value within ~ 100 μs for hydrophilic cations such as H⁺ and Li⁺, whereas it takes > 10 ms for hydrophobic cations such as Et₄N⁺ and Bu₄N⁺. Surface-enhanced IR absorption measurements suggest that the water molecules around the hydrophobic cations are more strongly hydrogen-bonded than those around the hydrophilic cations. Because the restructuring involves a reforming of the hydrogen-bonding network at the electric double layer, Et₄N⁺ and Bu₄N⁺, which have more strongly hydrogen-bonded hydration shells, require more time than H⁺ and Li⁺.