Ion-dipole interactions are the strongest type of intermolecular forces of attraction, followed by hydrogen bond, dipole-dipole interactions, and then London dispersion forces.
The reason for the greater strength is the charge on the ion (by transfer of electrons), as compared to the partial charges on dipoles (due to electronegativity difference) in other types of intermolecular forces.
As the name suggests, ion-dipole force exists between an ion and a polar molecule.
Ion-dipole interactions occur when an ionic compound is dissolved in a polar solvent. Since these are electrostatic forces, a greater charge on the ion and a higher magnitude of dipole on the polar molecule increases the strength.
Other factors that influence the strength of ion-dipole interactions are the sizes of the species as well as the distance between them.
A common example is the dissolution of table salt (NaCl) in water (H2O). Upon dissolving in water, the ionic NaCl lattice is broken into its constituent ions, Na+ and Cl–. The Na+ and Cl– ions are surrounded by water molecules. This process is known as solvation, or hydration when the solvent is water.
In solvation, the negative poles of the polar solvent molecules are directed towards the cations and the positive poles are directed towards the anions, as opposite charges attract.
Likewise, hydration occurs in such a way that the partial positive ends of the water molecules i.e. the H atoms, face the negative ions (anions), and the partial negative end of the water molecules i.e. the O atom, faces the positive ions (cations).
Since the cation is positively charged, it will attract the partial negative end and repel the partial positive end of the polar molecule. As a result, the partial negative end will be nearer to the cation, making the attractive energy greater than the repulsive.
The potential energy of an ion-dipole interaction is directly proportional to the charge and the dipole moment, and inversely proportional to the square of distance between the two,
E = -k q μ / r2
where q is the charge on the ion, μ is the dipole moment on the polar molecule and r is the distance between the centers of the two species.
Higher the charge on the ion (e.g., Mg2+ instead of Na+), stronger will be the ion-dipole interaction. Smaller distance between the ion and dipole will result in a stronger ion-dipole interaction as well. Moreover, a higher dipole moment also corresponds to more strength in the ion-dipole interaction.
The dipole moment, μ is given by
μ = ẟ r’
where ẟ is the partial charge at either end of the dipole and r’ is the distance separating the two charges. It is the measure of net molecular polarity.
Higher the electronegativity difference between the atoms bonded in a polar molecule, stronger will be the partial charges, ẟ, and greater will be the magnitude of the dipole moment.
Similarly, a greater partial charge separation, r’, will also result in a larger magnitude of dipole moment.
It is to be noted that symmetry cancels out the dipole moment.
Concepts Berg
How do you identify ion-dipole interactions?
Ion-dipole interactions are electrostatic interactions present between ions and dipoles of polar molecules. So whenever we have an ionic solid dissolved in a polar solvent, or simply ions surrounded by polar molecules, the interactions between them are ion-dipole.
Are hydrogen bonds or ion-dipole forces stronger?
Ion-dipole forces are stronger than hydrogen bonding. This is due to the fact that ion-dipole interactions involve a complete charge i.e. an ion formed by the loss or gain of electrons. This complete charge is stronger than the partial charges arising due to electronegativity difference in hydrogen bonding. A higher charge corresponds to a stronger force of attraction.
What makes a strong ion-dipole interaction?
The strength of ion-dipole interaction depends on the magnitude of charge on the ion and that of dipole moment of the polar molecule. A higher magnitude of these charges will result in a stronger interaction between them. Another factor increasing the strength is an appropriate distance between the ion and the dipole.
How do you know if a molecule is ion-dipole?
If a molecule is polar, it will have a dipole moment and will be able to form ion-dipole interactions provided the presence of an ion.
For a molecule to be polar, there should be some electronegativity difference between the atoms bonded together and the absence of symmetry.
Is NaCl an ion-dipole?
NaCl, when dissolved in a polar solvent such as water, will dissociate into its constituent Na+ and Cl– ions. These ions will be solvated by the polar solvent molecules, forming ion-dipole interactions with them.
What is an example of ion-dipole interaction?
The most common example of ion dipole interaction would be table salt, NaCl dissolved in water. Upon dissolution, the ions will have ion-dipole interactions with the polar water molecules. Other salts such as MgCl2 will also have the same intermolecular forces when dissolved in a polar solvent.
What are the 4 types of intermolecular forces?
The 4 types of intermolecular forces, from strongest to weakest are ion-dipole interactions, hydrogen bonding, permanent dipole-dipole interaction and London dispersion forces.
What molecules have ion-dipole interactions?
Polar molecules can form ion dipole interactions, together with ions. Any polar solvent can have such molecules.
What are the applications of dipole-dipole interaction?
Dipole-dipole interactions determine various properties of substances, such as melting and boiling point. Since these interactions are forces of attraction, adhesives make use of these forces to stick various objects to surfaces e.g., electronic components.
Strength of the dipole-dipole interactions can also be utilized in materials where it is required, as in construction materials.
What is a permanent dipole-dipole interaction?
Permanent dipole-dipole interaction is intermolecular force present between two polar molecules. The ‘permanent’ refers to the dipole being produced by electronegativity difference between the atoms bonded together, and not due to instantaneous fluctuation of the electron cloud, in temporary dipole-induced dipole in otherwise non-polar molecules.
What are the intermolecular forces from the weakest to the strongest?
The weakest type of intermolecular force is the London dispersion force, followed by dipole-dipole interaction. Hydrogen bonding is a stronger type of dipole-dipole interaction, and finally ion-dipole interaction is the strongest type of intermolecular force of attraction.
Reference Books
- Intermolecular and Surface Forces| Third Edition, by Jacob N. Israelachvili (University of California, Santa Barbara, California, USA)
Reference links
- Ion-Dipole Forces (chem.libretexts.org)
- Ion-Dipole (byjus.com)
