Question

In flushing and cleaning columns used in liquid chromatography to remove adsorbed contaminants, a series of solvents is used. Hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right)\), chloroform \(\left(\mathrm{CHCl}_{3}\right)\), methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\), and water are passed through the column in that order. Rationalize the order in terms of intermolecular forces and the mutual solu- bility (miscibility) of the solvents.

Short answer

The order of solvents used for cleaning a chromatography column – hexane, chloroform, methanol, and water – is based on the gradual increase in polarity and strength of intermolecular forces. Hexane is nonpolar with London Dispersion Forces, chloroform is polar with Dipole-Dipole Interactions, methanol and water are polar with Hydrogen Bonding interactions. This order allows for the efficient removal of contaminants with varying polarities and is facilitated by the miscibility between consecutive solvents due to their similarity in intermolecular forces.

Step-by-step solution

1. Identifying Intermolecular Forces

For each solvent, we need to identify the main type of intermolecular forces involved. Intermolecular forces can be divided into three main categories: London Dispersion Forces (LDF), Dipole-Dipole Interactions, and Hydrogen Bonding. Hexane \(\left(\mathrm{C}_{6}\mathrm{H}_{14}\right)\): LDF (nonpolar) Chloroform \(\left(\mathrm{CHCl}_{3}\right)\): Dipole-Dipole Interactions (polar) Methanol \(\left(\mathrm{CH}_{3}\mathrm{OH}\right)\): Hydrogen Bonding (polar) Water: Hydrogen Bonding (polar)

2. Explaining Miscibility and Order of Solvents

Now, we will discuss the miscibility of each solvent with the previous one in the sequence, to rationalize the order. - Hexane \(\left(\mathrm{C}_{6}\mathrm{H}_{14}\right)\) is a nonpolar solvent with LDF. It is used first to remove hydrophobic (nonpolar) contaminants. - Chloroform \(\left(\mathrm{CHCl}_{3}\right)\) is a polar solvent with Dipole-Dipole Interactions. It is miscible with hexane due to the relatively weak polarity of chloroform. This allows for a gradual transition from nonpolar to polar and helps dissolve moderately polar contaminants. - Methanol \(\left(\mathrm{CH}_{3}\mathrm{OH}\right)\) is a polar solvent with Hydrogen Bonding interactions. It is miscible with chloroform due to both being polar solvents and will dissolve more polar contaminants. - Lastly, water is used to dissolve highly polar contaminants because it is a polar solvent with strong hydrogen bonding interactions. Methanol and water are miscible due to both having hydrogen bonding. The order of the solvents allows for a gradual increase in polarity and strength of intermolecular forces, which enables the efficient removal of contaminants with a range of polarities from the chromatography column.

Key concepts

Intermolecular Forces

Intermolecular forces play a vital role in determining how substances interact with each other, especially in liquid chromatography. These forces can be categorized into three main types:

• London Dispersion Forces (LDF)
• Dipole-Dipole Interactions
• Hydrogen Bonding

Understanding these forces helps us predict the behavior of different solvents used in chromatography. For instance, hexane exhibits London dispersion forces because it is nonpolar. These are generally the weakest intermolecular forces and are related to the presence of temporary dipoles in molecules.
On the other hand, chloroform has moderate dipole-dipole interactions due to its polar nature. Dipole-dipole interactions occur between molecules that have permanent dipoles, resulting in a stronger attraction than LDF.
Lastly, methanol and water are capable of hydrogen bonding. This type of force is quite strong due to the attraction between a hydrogen atom bonded to an electronegative atom (like oxygen) and another electronegative atom. It's these variations in intermolecular forces that guide the order in which solvents are used to flush chromatography columns, ensuring effective cleaning.

Miscibility of Solvents

Miscibility refers to the ability of two substances to mix and form a homogeneous solution. In the context of liquid chromatography, understanding the miscibility of solvents is critical for systematically cleaning columns.
Hexane, being nonpolar, can efficiently remove nonpolar contaminants first. As hexane is miscible with chloroform, it allows a smooth transition from a nonpolar to a polar environment within the column.
Then, chloroform's partial miscibility with methanol permits the gradual increase in polarity needed to address moderate polar contaminants. Methanol can effectively mix with water, facilitating the removal of polar contaminants last.
Each step in solvent miscibility helps create optimal conditions for tackling contaminants of increasing polarity, ensuring that each type of impurity is effectively stripped from the column.

Polar and Nonpolar Solvents

Solvents can be broadly divided into polar and nonpolar categories. This distinction is based on the distribution of electrical charges across their molecules.
Nonpolar solvents, like hexane, have a uniform charge distribution, resulting in an absence of positive or negative poles. This makes them ideal for dissolving nonpolar substances, as "like dissolves like."

• Example: Hexane, used first for nonpolar residues

Polar solvents, such as chloroform, methanol, and water, have an uneven charge distribution, leading to positive and negative poles. These solvents are proficient in dissolving polar contaminants.

• Chloroform and Methanol: Polar solvents with varying strengths, transitioning from moderate to strong polarity
• Water: Highly polar, used last for its exceptional ability to dissolve polar contaminants

This classification underscores the systematic approach in liquid chromatography where the increase in solvent polarity aligns with the molecular nature of the contaminants being targeted.