Iondipól

Iondipól, or ion-dipole interaction, is the electrostatic interaction between a point charge (an ion) and a polar molecule that possesses a permanent dipole moment. It plays a significant role in solvation of ions in polar solvents, as well as in reaction kinetics and spectroscopic properties of ionic species.

The interaction depends on the distance r between the ion and the dipole and on the orientation of the dipole relative to the ion. If the dipole moment μ forms an angle θ with the line from the ion to the dipole center, the potential energy is U(r, θ) = - (1/(4π ε0)) (q μ cos θ) / r^2, where q is the ion charge and ε0 is the vacuum permittivity. The electric field of the ion exerts a torque that tends to align the dipole (favoring θ = 0) and a radial force given by F_r = -∂U/∂r, which scales as 1/r^3 in magnitude. Consequently, the interaction is strongest at short range and decreases rapidly with distance. Temperature and solvent effects influence the average orientation, often reducing the net attraction in solution.

In context, ion-dipole interactions contribute to ion solvation shells, modulate association/dissociation equilibria, and affect rates and spectra in polar media. They contrast with ion-induced dipole interactions, which arise for nonpolar molecules and depend on polarizability α, with energy scaling roughly as -α q^2 / r^4. Typical interaction energies for common ions and polar molecules at separations of a few angstroms are on the order of tens of kilojoules per mole, varying with q, μ, r, and θ.