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I was reading about why detergents do not form scum with hard water but soaps do, and according to wikepedia it has to do something with electrostatic interaction of sulfonates( most commonly) with ions present in hard water

These substances are usually alkylbenzene sulfonates, a family of compounds that are similar to soap but are more soluble in hard water, because the polar sulfonate (of detergents) is less likely than the polar carboxylate (of soap) to bind to calcium and other ions found in hard water.

https://en.wikipedia.org/wiki/Detergent

I am not able to find any reason for this anywhere?

According to me it has to do something with Lattice energy and size of sulfonate and carboxylate as sulfonate is a big ion relatively so it has less lattice interaction with ions like calcium and sodium, but i am not sure about it

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    $\begingroup$ You should really brush up on formatting... $\endgroup$
    – Mithoron
    Commented Jul 14, 2021 at 21:11
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    $\begingroup$ Use slant or bold for highlighting details, not make whole paragraphs less readable, in particular. $\endgroup$
    – Mithoron
    Commented Jul 14, 2021 at 21:16
  • $\begingroup$ Is lattice even involved in detergent action? Those things take place on micelles as far as I know. . . $\endgroup$
    – napstablook
    Commented Jul 15, 2021 at 3:39
  • $\begingroup$ He is asking why the sulfonate salts do not form precipitates in water not about detergent action. $\endgroup$
    – ACR
    Commented Jul 15, 2021 at 3:48
  • $\begingroup$ If I knew nothing about soaps, sulphonates and detergents, then, based on general calcium behaviour toward acetic and sulphuric acid, I would rather expect sulphonates to cause trouble with calcium. Perhaps the difference is in calcium complexation abilities of R-COO- vs R-SO2-O-. $\endgroup$
    – Poutnik
    Commented Jul 15, 2021 at 6:57

2 Answers 2

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Well. Solution of sodium alkylsulfonates are good detergents, because they produce calcium alkylsulfonates in hard water, and these calcium derivates are soluble in water. By contrast, usual soap produce calcium alkylcarboxylates (like oleate, palmitate, etc.) in hard water, and these compounds are not soluble in water. Soap may be a good detergent : it removes stains. But it replaces the stain by a deposit of calcium carboxylate. This is not what is expected when cleaning dirty clothes.

Anyway, the problem of the solubility is not yet perfectly understood. For example, why is $\ce{CaCl2}$ extremely soluble in water ($56$ g. $\ce{CaCl2}$ in $100$ mL water), and why is $\ce{CaF2}$ extremely insoluble in water, despite the fact that chlorine and fluor should have similar chemical properties ?

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  • $\begingroup$ Is this difference in solubility due to lattice effect? $\endgroup$
    – Samardeep singh
    Commented Jul 14, 2021 at 20:53
  • $\begingroup$ can you add some data to support your answer please? $\endgroup$
    – Samardeep singh
    Commented Jul 14, 2021 at 20:54
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    $\begingroup$ Nobody really knows how to calculate the solubility. If you find something, it is the Nobel Prize for you. There are plenty of publications which "explain" the solubility of a group of substances. They take care of lattice energy, electronegativity, geometry, ligand field, entropy, hydrogen bonds, etc. But these articles have too many exceptions. $\endgroup$
    – Maurice
    Commented Jul 15, 2021 at 7:56
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It is probably related to the micellar behavior differences of the soap and sulfonate. The carboxylate is a stronger base so the hydrophlic portion will be less ionic and less able to stabilize the micelle in the water. What data I found indicates that Ca++ and Mg++ stearates etc. are at best slightly soluble. I would think that a micelle of these could be even more insoluble. Even detergents have a limited ability to solubilize organics and fats.

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