Topics - Water
Water is probably one of the most polar substances on the planet surface and its unique character has made life on this earth, as we know it, possible. Water consists of two hydrogen atoms covalently bonded to oxygen by two of the oxygenís four outer orbit electrons. Water has a molecular weight of 18.01 and yet is a liquid having a density of unity and with a boiling point of 100C compared with n-heptane which is also a liquid having a density of about 0.75 and a boiling point of 98.4C. The high density and high boiling point of water is caused by its extremely strong polarity. In turn, this strong polarity is caused by its strong dipole that causes it to interact strongly with itself by hydrogen bonding. Hydrogen bonds are not true chemical bonds but result from strong dipole interactions that have energies approaching that of a weak chemical bond. This hydrogen bonding gives water a much higher effective molecular weight than its true molecular weight which causes it to have properties similar to substances having intrinsically much higher molecular weights (cf n-heptane). In chromatography water can exhibit very strong polar (hydrophilic) interactions with another molecule but only very weak dispersive (hydrophobic) interactions. Consequently, when used as a mobile phase, water is often used in conjunction with a more dispersive solvent such as acetoniltrile or methanol. In common practice, the acetonitrile or methanol content is increased continuously during chromatographic development (a process called gradient elution), thus, increasing the dispersive interactivity of the mobile phase and causing the more dispersive components of a mixture to be eluted more quickly.