Ion dipole

What is ion-dipole interaction?

An ion-dipole interaction is the intermolecular force of attraction between a charge ion (cation or anion) and a molecule. It is found commonly in the solution where ionic compounds dissolve in polar solvents. Ion-dipole attractions become stronger as the charge on the ion increases or as the magnitude of the dipole of the polar molecule increases.

Cation is the +ve charged species which can attract the partially negative end of a neutral polar molecule. Anion is the -ve charge species which can attract the partially positive end of a neutral polar molecule. A dipole molecule has a slight charge on either side and that ions have charges, either negative or positive, you can understand how these would bond together. Cations would be attracted to the negative side of a polar molecule, and anions.

Table of contents

• Example of ion- dipole interaction
• Calculation of the potential energy of ion-dipole interaction
• Strength of ion- dipole interaction depends
• Frequently Asked Questions- FAQs

Example of ion- dipole interaction

If a charge particle remains close to a polar molecule an induced dipole is produced. The ability of a substance to have a dipole induced in it is often referred to as its polarizability. The more polarizable the substance the easier it is to induce a dipole in the substance. We can represent the dipole by Simply put an ion can be attracted to the end of the dipole that has a charge opposite the charge of the ion.

Ion-Dipole Forces are involved in solutions where an ionic compound is dissolved into a polar solvent, like that of a solution of table salt (NaCl) in water. When sodium chloride dissolves in water. Sodium chloride undergoes dissociation to form Na + and Cl^– ions. Water is a weak polar solvent so it produces a dipole.The positive sodium ion will be attracted to the slightly negative oxygen atoms in the water molecule, while the negative chloride ion is attracted to the slightly positive hydrogen atoms.

Calculation of the potential energy of ion-dipole interaction:

For ion-dipole interactions the interaction is between a dipole moment (μ). If the two equal and opposite charges (q+) and (q-) are separated by distance r’. Then the dipole moment μ is

μ = qr’

The cation repels the positive end of the dipole and attracts the negative end, so the negative end is closer to the cation than the positive end. This means the attractive energies are greater than the repulsive.

If the field is an ion, the dipole will orient itself so that the attractive end will be directed towards the ion and the other end will be directed away. The charge of the ion is much larger than the charge of the dipole. So, the ionic charge is the determining factor.

The potential energy of an ion- dipole interaction is given as

E ∝ |Z^±|μ / r²

Where Z^± = charge on the ion,

r= distance between the ion and the molecular dipole.

Strength of ion- dipole interaction depends

The strength of this interaction depends on:

• The magnitude of the dipole moment
• Size of the polar molecule
• The distance between ion and molecular dipole

The strength of ion-dipole interactions is dependent on the charge and the distance. Ionic charge increases as we move across a period on the periodic table. For example, magnesium has a larger ion-dipole interaction than sodium. The distance between the ion and the dipole is dependent on the size of the molecule. A polar molecule that is only a couple of atoms will easily approach the ion. A polar molecule with hundreds of atoms will have a more difficult time coming close to the ion.