Destruction of the hydration shell around tetraalkylammonium ions at the electrochemical interface

The structure and behavior of the hydration shells around tetraethyl-, -propyl-, and -butylammonium ions (Et(4)N(+), Pr(4)N(+), and Bu(4)N(+), respectively) at a CO-covered Pt electrode was studied by surface-enhanced IR absorption spectroscopy. Selective concentration of cations at the electrochemical interface by controlling the applied potential facilitates the observation of the hydration shells without interference from hydrated anions. We report for the first time that the hydration shells around Pr(4)N(+) and Bu(4)N(+) are decomposed at sufficiently negative potentials because of the strong electrostatic attraction with the surface. On the other hand, the hydration shell around Et(4)N(+), the smallest ion, is more stable and does not decompose in the potential range examined. The difference in the stability can be ascribed to the difference in the size of the central cation. On the other hand, the hydration shells are easily deconstructed on a bare Pt surface because of the stronger interaction with the surface.