Partial Molar Volume of the Hydrated Electron

Area of Science:

• Physical Chemistry
• Chemical Physics
• Solution Chemistry

Background:

• The structure and properties of the hydrated electron (e^-_aq) are crucial in understanding aqueous solution chemistry.
• Previous models proposed conflicting structures for the hydrated electron, impacting its physical characteristics like molar volume.

Purpose of the Study:

• To experimentally determine the partial molar volume of the hydrated electron (e^-_aq).
• To investigate the structural implications of the hydrated electron's molar volume, specifically its existence in a solvent cavity.

Main Methods:

• Pulse radiolysis and transient absorption spectroscopy were employed to study the pressure dependence of the equilibrium constant for the reaction e^-_aq + NH₄⁺ ⇔ H + NH₃.
• Molar volumes of reactants and products were analyzed, incorporating tabulated data and molecular dynamics simulations.

Main Results:

• The equilibrium constant showed a minimal decrease (6%) at 2 kbar, indicating a small volume change.
• The partial molar volume of the hydrated electron (V̅(e^-_aq)) was determined to be 26 ± 6 cm³/mol.
• This positive molar volume confirms the hydrated electron exists within a solvent cavity, contrasting with non-cavity models.

Conclusions:

• The positive partial molar volume of the hydrated electron supports a cavity model, where the electron resides in a void within the solvent structure.
• This experimental evidence refutes recent theoretical models suggesting a non-cavity structure and negative molar volume for the hydrated electron.