5.4 INTERMOLECULAR FORCES

An intermolecular force is the attractive force that exists between molecules.

Dipole-Dipole Forces

Dipole-dipole interactions occur between polar molecules. The slightly negative end of one polar molecule is weakly attracted to the slightly positive end of another molecule. Such attracting forces are called dipole-dipole interactions.

You know that paints and dyes are used to protect solid surfaces from atmospheric effects and to provide visual appeal. Resins are used to coat materials, offering toughness, flexibility, adhesion, and chemical resistance. For instance, dams, bridges, floors, trains, buses, and cars are painted with resins. Synthetic resins are used where water resistance is required. Chemically, resins are either adhesive or form bond linkages with the material being bonded. The nature of these linkages involves dipole-dipole interactions.

Hydrogen Bonding

Molecules in which hydrogen is covalently bonded to a very electronegative atom such as oxygen, nitrogen, or fluorine are weakly bonded to a lone pair of electrons of another electronegative atom. This other atom may occur in the same molecule or in a nearby molecule. This intermolecular interaction is called hydrogen bonding. Oxygen, nitrogen, or fluorine makes hydrogen very electron-deficient. The interaction between such a highly electron-deficient hydrogen and a lone pair on a nearby electronegative atom compensates for the deficiency.

The interaction of a highly electron-deficient hydrogen and a lone pair on a nearby highly electronegative atom such as N, O, or F is called a hydrogen bond. This phenomenon is known as hydrogen bonding.

These intermolecular forces are crucial in determining the properties of water, biological molecules such as proteins and DNA, and synthetic materials like glue, paints, and resins. The adhesive action of paints and dyes is developed due to hydrogen bonding. Synthetic resins bind two surfaces together by hydrogen bonding or dipole-dipole interactions.

Society, Technology, and Science

Epoxy adhesives exhibit excellent chemical resistance, good adhesion properties, and heat resistance, forming strong and tough coatings. They are used in propellers, aircraft parts, boats, cars, and trucks. Epoxy adhesives contain partially positively charged H-atoms and oxygen atoms with lone pairs in their molecules, making them sticky and capable of forming H-bonds with other substances. Modern aircraft, boats, automobiles, and even spacecraft use epoxy adhesives for assembly, saving money and reducing weight. Hence, glues and adhesives have become essential in daily life.