Iondipole

Ion-dipole interaction refers to the electrostatic attraction or repulsion between a charged ion and a polar molecule that has a permanent dipole moment. It is a key component of solvation in polar solvents and influences processes such as ion transport, solvation energies, reaction rates, and spectral properties of solvated species.

A simple model treats the ion as a point charge q located at distance r from the center of a polar molecule with a permanent dipole moment μ. The electric field produced by the ion at the dipole is E = (1/(4π ε0)) (q / r^2) in the radial direction. The interaction energy is U = - μ · E, which in magnitude is U = - (1/(4π ε0)) (q μ cos θ) / r^2, where θ is the angle between μ and the line from the ion to the dipole. The dipole experiences a torque τ = μ × E, with magnitude τ = μ E sin θ, tending to align μ with the ion’s field.

The interaction strength scales approximately as 1/r^2 and depends on orientation through cos θ. When the dipole is aligned with the field (θ ≈ 0), the interaction is most attractive; when anti-aligned (θ ≈ π), it is repulsive. The field also tends to rotate the dipole toward alignment.

Compared with related interactions, ion-dipole forces are typically stronger than ion-induced dipole interactions (which scale as 1/r^4) at the same separation and are part of the continuum of Coulombic forces experienced in condensed phases. They play a crucial role in ion solvation, hydration shells, dielectric response, and various models of chemical reactivity in polar media.