Liquid Hand Soap: The Difference Between Clear and Opaque

The liquid soap that we use to wash our hands comes from the process of saponification in which oils and fats are combined together with potassium hydroxide (KOH) to obtain a final liquid formulation with the specific characteristics of all soaps, including the well-known cleaning and antibacterial actions. But why are some liquid soaps opaque, while others are transparent? The difference is given by the different chemical composition: following a traditional recipe, an opaque color is obtained due to the spontaneous growth of small soap crystalswhich will tend to spread the light. To avoid the opaque appearance, auxiliary solvents, also called “transparent agents”, such as glycerin, ethyl alcohol (ethanol) and sugar (sucrose). These substances not only prevent the growth of crystals, but can also dissolve them should they form, guaranteeing the desired transparency to the final product.

Because soap crystals make the color opaque

Following a classic recipe, once the saponification reaction has taken place, small soap crystals can remain in suspension which diffuse the light leading to a final opaque appearance.

This phenomenon, called diffusion or scatteringoccurs when light enters the substance and “bounces” off the surface of the crystals. By exiting in directions different from the one it entered, the light does not allow us to see through the substance. Similar behavior can be observed in emulsionssuch as milk or orange juice: the suspended microparticles scatter light, making the liquid non-transparent, just like soap crystals in opaque formulations. In contrast, when a compound dissolves completely, such as table salt (NaCl) in water, light passes through unhindered and allows objects to be observed through the liquid.

Auxiliary solvents in the production of transparent soaps

In the production of transparent liquid soap the main solvent remains water, useful in dissolving potassium hydroxide (KOH) before combining it with the oils to be saponified. Unlike the classic method, the so-called “transparent agents” are added, usually represented by glycerin, ethyl alcohol (ethanol) and sugar (sucrose).

These must be added after heating the mixture and before cooling it, otherwise you risk not obtaining the desired final transparency: unwanted crystals, in fact, form precisely when the temperature drops. Despite this, one of the actions of auxiliary additives is also that of prevent the formation of crystals when the soap comes into contact with the air: it is important that the solvents retain moisturepreventing the evaporation of the water and the drying of the soap, which would otherwise take on a dull appearance. Glycerin itself performs this function very well.

In addition to ethanol, glycerin and sucrose, often used together in different proportions, there are other substances exploited by industries to guarantee the transparency effect, including propylene glycolmethyl alcohol and natural substances derived from some types of resins.

Does clear liquid soap clean hands “better”?

Having established that the absence or presence of crystals and the chemical composition changes between a transparent and an opaque liquid hand soap, one might ask whether they also have a different cleaning action.

The answer is no! The transparency effect it does not provide greater cleaning action to the detergent, although the end consumer may be more attracted by a non-opaque formulation, which at first glance may appear “more effective”. Therefore, our hands will be washed well in both cases.