Forces operating in molten ionic solids

Question

What type of forces exist between ions in molten ionic solids?

If this force is electrostatic attraction only, then how is it different from electrostatic interaction when ionic compound exist as solid?

Answer 1

The forces operating in liquids and solids are the same, it is just the kinetic energy of the ions or molecules that differ.

A variety of forces operate between ions or molecules in solids and liquids. In ionic solids, the electrostatic forces are often strong and dominant, but that doesn't mean that weaker dispersion forces are not present (or that the strong forces preventing atoms close to each other from getting even closer are missing).

The reason why things become liquids is simply that, when the temperature is high enough, the average molecule or ion has enough kinetic energy to break free from the crystal lattice of the solid. At some level of kinetic energy the attractive forces are no longer sufficiently strong to prevent the vibration of the surface molecules from shaking them from the lattice.

But even in ionic solids or liquids, the other forces also exist and make some contribution. The strength of the forces don't change with temperature, just the amount of vibration or the speed of the individual components. Ionic forces are particularly strong for small, compact ions (think Cl^- and Na⁺ in table salt. After all the electrostatic force strength decays with the square of the distance between the ions.

But, if you make a salt using large floppy ions like 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) (see this question), the irregular shape and large size of the ion can weaken the strength of the ionic bond enough that the compound is a liquid at room temperature. in these molecules the forces are no longer strong enough to make the compound crystalline even at low temperatures. The specific balance of forces and the kinetic energy of the ion movement in these liquids has been altered from the more typical situation in more familiar ionic compounds so they can be liquids at lower temperatures.

In short, there is nothing different about the forces acting in an ionic liquid and an ionic solid. The difference in the liquid is just that the kinetic energy of the molecules are large enough to overcome the forces.