為何肥皂在酸性環境下, 去油污能力會下降?

The active ingredient in soap is the sodium salt of fatty acid. The general structure is (R-COO-Na+). This molecule can act as surfactant to make the gease soluble in water. The principle of acting as the surfactant is because of its hydrophobic (water-dislike) tail,(R-group) and the hydrophilic (water-like) head, the carboxylic group, (-COONa). The R-group can dissolve in teh gease and its head is remain at the outside and make the gease soluble into water due to the presence of the carboxylic head. However, if the soap is worked in acidic media, the carboxylate will be converted to carboxylic acid. i.e.
R-COONa + H+ = R-COOH + Na+.

Since the solubility of carboxylic acid in water is not as good as its carboxylate salt, the molecule is no longer act as surfactant. Although the R-gorup still dissolve into the gease. But the carboxylic head cannot make the gease soluble in water anymore in the presence of acid.

If we use the soapless detergent such as those with sulfonic head (R-SO3-Na+), this sulfonate group is not easy to be protonated. So, it can still function well even in acidic media.

I hope the information can help.

佢個ion head會同酸無反應
但soupless detergen就可以在酸性環境下去油污能力不變

因為肥皂係屬於鹼性,
而當它在酸性環境下, 就會中和左自己的鹼度,,
令到去油污能力下降!!