The double layer on transition metals, i.e., platinum,
features chemical metal–solvent interactions and partially
charged chemisorbed ions. Chemically adsorbed solvent molecules and
ions are situated closer to the metal surface than electrostatically
adsorbed ions. This effect is described tersely by the concept of
an inner Helmholtz plane (IHP) in classical double layer models. The
IHP concept is extended here in three aspects. First, a refined statistical
treatment of solvent (water) molecules considers a continuous spectrum
of orientational polarizable states, rather than a few representative
states, and non-electrostatic, chemical metal–solvent interactions.
Second, chemisorbed ions are partially charged, rather than being
electroneutral or having integral charges as in the solution bulk,
with the coverage determined by a generalized, energetically distributed
adsorption isotherm. The surface dipole moment induced by partially
charged, chemisorbed ions is considered. Third, considering different
locations and properties of chemisorbed ions and solvent molecules,
the IHP is divided into two planes, namely, an AIP (adsorbed ion plane)
and ASP (adsorbed solvent plane). The model is used to study how the
partially charged AIP and polarizable ASP lead to intriguing double-layer
capacitance curves that are different from what the conventional Gouy–Chapman–Stern
model describes. The model provides an alternative interpretation
for recent capacitance data of Pt(111)–aqueous solution interfaces
calculated from cyclic voltammetry. This revisit brings forth questions
regarding the existence of a pure double-layer region at realistic
Pt(111). The implications, limitations, and possible experimental
confirmation of the present model are discussed.