Why does Acetonitrile have higher elution strength than methanol?

Today, we delve into reverse-phase liquid chromatography (RPLC) and its two trusted solvents: methanol and acetonitrile.

As you may know, acetonitrile exhibits higher elution strength compared to methanol, making it a go-to choice for many chromatographers. But why does acetonitrile possess this edge over methanol in RPLC? Let's explore the science behind it.

First, consider the chemical structures of these two solvents. Acetonitrile boasts a polarized functional group – the Carbon triple bond to Nitrogen , rendering it aprotic. On the other hand, methanol features a polar hydroxyl group, classifying it as protic. This distinction is crucial.

Protic solvents like methanol have hydrogen atoms connected to electronegative atoms(O), allowing them to form hydrogen bonds with solutes. Examples of polar protic solvents include water, methanol, and acetic acid.

In contrast, aprotic solvents, such as acetonitrile, lack these hydrogen bonds, making them unable to form hydrogen bonds with solutes (if the solute does not have Hydrogen connected to an electronegative atom). Typical aprotic solvents include acetonitrile, acetone, and ether.

Now, let's look at electronegativity values, which play a significant role in polarity. Oxygen has an electronegativity of 3.44, nitrogen has 3.04, carbon scores 2.55, and hydrogen registers 2.2. The difference in electronegativity creates polarity. Greater differences result in more polarized compounds.

Comparing the electronegativity difference between carbon and nitrogen (0.49) with that of oxygen and hydrogen (1.24), we observe that methanol is more polar than acetonitrile. This brings us to an essential point: the reason behind acetonitrile's higher elution strength in RPLC.

In RPLC, the stationary phase is typically nonpolar (hydrophobic), while the mobile phase is slightly polar. The "like attracts like" principle applies: nonpolar compounds are attracted to the nonpolar stationary phase, and polar compounds favor polar phases.

Both acetonitrile and methanol interact with the stationary phase, but acetonitrile, being less polar, forms stronger interactions with the stationary phase. Hence, the no-polar stationary phase prefers acetonitrile, as compared to methanol.

For nonpolar analytes, acetonitrile's presence creates more competition and might elute analytes earlier. In contrast, methanol creates less competition and allows analytes to stay in the stationary phase longer, resulting in longer elution.

However, this doesn't hold in every situation. Interaction modes also play a role. Acetonitrile relies on dipole-dipole interactions, while methanol forms hydrogen bonding. This diversity can impact selectivity in certain scenarios.

For instance, in elution order, phenol and benzoic acid differ when using methanol or acetonitrile as the mobile phase. The type of interaction may affect selectivity.

Additionally, when using a phenyl column that relies on π-π interactions, acetonitrile can restrict these interactions, thus affecting the elution order.

In the below example, o-cresol, m-cresol, and p-cresol were found well separated when analyzed on a phenyl column, using a mobile phase containing methanol. However, the o-cresol and m-cresol were coeluting while using acetonitrile containing mobile phase.

So, the choice between acetonitrile and methanol depends on various factors, including the analyte's nature and the chromatographic conditions.

Share your thoughts on this topic in the comments below. Thank you for joining us in this exploration.