[Readers Insight] Why Do Substances with Similar Polarity Dissolve in Each Other?

Author: Chromatography Mound

When we talk about “like dissolves like,” the first thing that comes to mind is polarity — the more similar, the better one substance dissolve in another.

Polarity comes from the existence of a dipole moment — the distance between the centers of positive and negative charges. Polar molecules usually have a permanent dipole moment, while nonpolar molecules can also develop an induced dipole moment under the influence of the electric field of a polar molecule.

However, for substances with higher polarity, hydrogen bonding also comes into play. Since the strength of a hydrogen bond is greater than that of a dipole-dipole interaction, it adds another layer of complexity to understanding why “like dissolves like”.

Let’s exclude the influence of hydrogen bonding for now. Then what we first need to understand is how dipole-dipole interactions arise.

When two polar molecules come close to each other, and their dipole moments are similar, their dipoles interact to create electrostatic attraction: the positive charge center of one molecule is drawn to the negative charge center of another. This interaction causes polar molecules to align in a way that minimizes their potential energy, with the positive and negative ends facing each other.

If the dipole moments are not similar, then while one end of the molecules may attract each other, the other end won’t align. As a result, the overall energy cannot be minimized. Since the two substances fail to achieve an optimal molecular arrangement, they do not dissolve well in each other.

We can understand this through the energy changes involved in the dissolution process, which includes three main components:

1. Solvent-solvent interaction energy, ΔH1: the interaction energy between solvent molecules.
2. Solute-solute interaction energy, ΔH2: the interaction energy between solute molecules.
3. Solvent-solute interaction energy, ΔH3: the interaction energy between solvent and solute molecules.

The total energy change during dissolution can be expressed as:

ΔH = ΔH3 - (ΔH1 + ΔH2).

When two polar substances with similar polarity mix, they have an energetic advantage. In this case, ΔH3 is usually large, while ΔH1 and ΔH2 are relatively small — and since ΔH3 is close in magnitude to ΔH1 plus ΔH2, the overall ΔH becomes small. That is, the total energy change is small, meaning the dissolution process occurs more easily.

This happens because molecules with similar polarity form stronger dipole-dipole interactions, effectively lowering the total energy of the system.

In addition to thermodynamics, the entropy effect and mixing free energy also play a crucial role.Whether the dissolution process occurs spontaneously depends on the change in Gibbs free energy. According to the Gibbs equation: ΔG = ΔH - T × ΔS, where ΔH is the enthalpy change, T is the temperature, and ΔS is the entropy change.

We further explain this phenomenon: solvents with similar polarity feature a clear entropy advantage. When the solute and solvent have similar polarity, ΔH tends to be small, while ΔS is relatively large. This happens because mixing similar polar molecules significantly increases the disorder of the system, resulting in a higher entropy value. As a result, the change in free energy becomes negative, meaning the dissolution process occurs spontaneously.