Soaps and detergent are both molecules that have a polar head and a long organic tail. In Soaps, the head is often a carboxylate ion $\ce{COO-}$ where as a detergent has a benzosulphonate $\ce{C6H11SO3-}$(or other) head:

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I have often seen that detergents are 'stronger' than soaps. Why is this? It is given as the reason why we don't wash our skin with detergents. The only answer I could find is that detergents don't form insoluble salts with Mg and Ca (scum) but doesn't seem like it would have a large impact. So why are detergents stronger than soaps?

  • $\begingroup$ Actually one of the advantage of detergents is right that they are less aggressive (pH) to the skin than soaps. Bath shower and hairs formulations are detergents. Still, all the pros that comes to my mind are not due to cleansing power. If there are regular differences (the process has multiple steps or facets) likely someone will answer. $\endgroup$ – Alchimista Jul 23 '19 at 10:45
  • $\begingroup$ hm, if you wouldn't mind, could you describe the pros? I haven't heard of many except being 'stronger' which apparently isn't even true! $\endgroup$ – John Hon Jul 23 '19 at 13:25
  • $\begingroup$ The pH of anionic detergents is less alkaline than that of soap, so they are better for skin and especially hairs. This is because benzensulphonic ac. is stronger than fatty acids. Those detergents should be in standard body shower formulation. Tough we are speaking of formulations that might contain more or have changed. The others are as you and especially the answer so mentions are linked to water hardness and mass production. $\endgroup$ – Alchimista Jul 24 '19 at 9:03

Soap is normally a product of natural ingredients like fatty acids and alkali salts. It is an anionic detergents. It can form insoluable salts with Ca and Mg which exist in almost every water. So the cleaning effect is reduced.

Detergents are in general products which can be either anionic, cationic or non-ionic. You will find synthetic detergents more often because the mass production is easier than soap. Non-ionic detergents do not depend on the pH of the water, so they are more "mild" for your skin.

The cleaning effect depends on the temperature and the concentration of your detergent. It also depends on what kind of dirt you want to dissolve (hydrophilic or lipophilic). Dirt can be a heterogenous mixture so there is not an ideal detergent. The solubility of dirt can change with pH that is why you mix it with additives. You need an detergent which is appropriate for the pH of your additives to maintain the cleaning effect.

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