Soaps and their Cleansing Action
Saponification, Micelles, and the Hard Water problem explained.
For thousands of years, humans have used chemical compounds to remove dirt, grease, and oil from surfaces. The oldest and most famous of these cleansing agents are Soaps. In chemistry, soaps are not just arbitrary cleaning bars; they have a very specific molecular structure that allows them to interact with both water and oil simultaneously.
1. What are Soaps Chemically?
Chemically, soaps are sodium or potassium salts of higher fatty acids such as stearic acid, oleic acid, and palmitic acid.
- Sodium Stearate: $C_{17}H_{35}COONa$ (The most common soap)
- Sodium Palmitate: $C_{15}H_{31}COONa$
Saponification Reaction
Soaps are prepared by heating animal fat or vegetable oil (which are naturally occurring esters known as glyceryl esters of fatty acids) with an aqueous solution of sodium hydroxide ($NaOH$) or potassium hydroxide ($KOH$). This alkaline hydrolysis of esters is called Saponification.
$\begin{matrix} CH_2-O-CO-C_{17}H_{35} \\ | \\ CH-O-CO-C_{17}H_{35} \\ | \\ CH_2-O-CO-C_{17}H_{35} \end{matrix} \quad + \quad 3NaOH \quad \xrightarrow{\Delta} \quad \begin{matrix} 3C_{17}H_{35}COONa \\ (\text{Sodium Stearate / Soap}) \end{matrix} \quad + \quad \begin{matrix} CH_2-OH \\ | \\ CH-OH \\ | \\ CH_2-OH \end{matrix} $
(Glyceryl ester of stearic acid) $\qquad\quad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad$ (Glycerol)
2. Types of Commercial Soaps
Essentially all soaps are made by boiling fats or oils with a suitable soluble hydroxide. Variations are made by adding different ingredients.
Prepared using better grades of fats and oils, and care is taken to remove excess alkali. Color and perfumes are added. Potassium soaps are often used as they are softer on the skin.
Made by dissolving the soap in ethanol and then evaporating the excess solvent.
Substances of medicinal value are added. For example, Bithionol is added to impart antibacterial properties, or deodorants are added.
Contain Glycerol to prevent rapid drying of the lather. A gum called Rosin is added, which forms sodium rosinate that lathers exceptionally well.
3. The Cleansing Action of Soap
Dirt is usually oily in nature, and oil does not mix with water. So, water alone cannot wash away dirt. A soap molecule is a master of two worlds because it has two distinct parts:
- The Hydrophobic Tail: The long hydrocarbon chain (e.g., $C_{17}H_{35}-$) is non-polar. It hates water (water-repelling) but loves oil and grease (lipophilic).
- The Hydrophilic Head: The carboxylate ion ($-COO^-Na^+$) is polar. It loves water (water-attracting) but hates oil.
Figure 1: The Micelle. Tails dissolve in the oil, heads remain in the water. The whole droplet becomes surrounded by negative charges, preventing it from coalescing with other droplets.
When soap is added to water, the hydrophobic tails embed themselves into the oil/dirt droplet, while the hydrophilic heads stick out into the water. This forms a spherical structure called a Micelle. The micelle surrounds the dirt, pulls it off the surface, and suspends it in the water as an emulsion. When you rinse with water, the micelle-trapped dirt washes away!
4. Why do Soaps fail in Hard Water?
Hard water contains dissolved Calcium ($Ca^{2+}$) and Magnesium ($Mg^{2+}$) ions. When you use soap in hard water, these ions react with the soap to form insoluble calcium and magnesium salts of fatty acids.
(Soap) $\qquad\qquad\qquad\qquad\qquad\qquad$ (Insoluble Scum)
This insoluble precipitate separates out as sticky, gummy mass known as Scum.
- The scum hinders good washing because the precipitate adheres to fabric fibers like a gummy substance.
- Hair washed with hard water looks dull because of this sticky precipitate.
- Dyes do not absorb evenly onto cloth washed with soap using hard water.
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