-2
$\begingroup$

Is the hydrophilic part of the soap both the cation ($\ce{Na+/K+}$), just the $\ce{COO-}$, or both of those sections together like the picture shown below? Do the cation and the anion dissociate from each other instead, leaving only the negative charge on the soap; therefore, intermolecular forces are due to the negative charge attracted to the partial positive charge on water's hydrogens?

diagram of soap molecule

$\endgroup$
7
  • $\begingroup$ For table salt NaCl, is hydrophilic Na+, Cl- or both? $\endgroup$
    – Poutnik
    Commented Aug 4, 2023 at 11:48
  • $\begingroup$ Both? But only when they're dissociated, I think? $\endgroup$
    – cabbagesss
    Commented Aug 4, 2023 at 11:58
  • $\begingroup$ And when they are not? There are no NaCl molecules on NaCl crystals. Similarly there are no soap molecules in classical soap but sodium and carboxylate ions. $\endgroup$
    – Poutnik
    Commented Aug 4, 2023 at 12:00
  • $\begingroup$ It's not hydrophilic? So the soap must be dissociated in the water in order to be hydrophilic? $\endgroup$
    – cabbagesss
    Commented Aug 4, 2023 at 12:01
  • $\begingroup$ In that case, the Na+ doesn't really belong to the soap molecule when you explain its hydrophilic head and hydrophobic tail? $\endgroup$
    – cabbagesss
    Commented Aug 4, 2023 at 12:08

3 Answers 3

1
$\begingroup$

Do the cation and the anion dissociate from each other instead, leaving only the negative charge on the soap

At low concentration in water, cation and anion would dissociate, and they would be solvated by water. As you remove the water (e.g. in a soap bubble on a dry day), the ions would form pairs again. If your solution contains other cations, the soap (technically not a molecule but an anion) might pair with these other cations.

The details of this are quite complicated, as glancing at the abstract and figures of e.g. this paper illustrates.

Which part of a soap molecule is the hydrophilic part?

The carboxylate is commonly called hydrophilic. Calling the counter-ions $\ce{Na+}$ or $\ce{K+}$ hydrophilic seems strange to me. They are not attached to the soap, although they will be enriched near the negatively charged carboxylate groups e.g. when the soap forms a micelle to maintain local electroneutrality.

$\endgroup$
1
$\begingroup$

Soap is made of equal amounts of cations $\ce{Na+}$ and anions $\ce{RCOO-}$, where R is usually a chain of $n$ carbon atoms generally included in $\ce{C_{n}H_{2n+1}}$. This anionic chain is hydrophobic, but the charged $\ce{COO^-}$ terminal group is hydrophilic. In rather dilute solutions, the ions are separated from one another by water molecules. But their attraction remains, so that $\ce{Na+}$ cations stay not too far away from the hydrophilic end of the $\ce{RCOO-}$ anions. If the soap concentration increases, colloidal micelles start to be formed : the long hydrophobic carbon chains of the anions are joining one another by Van der Wals attractions. This makes a small bubble called a micelle whose outer shell contains the hydrophilic $\ce{-COO^-}$ groups.

$\endgroup$
0
$\begingroup$

Soaps belong to a class of electrolytes called colloidal electrolytes. The term "colloidal electrolyte" was defined by McBain, J. W. as "salts in which an ion has been replaced by a heavily hydrated polyvalent micelle that carries an equivalent sum-total of electrical charges and conducts electricity just as well or even better than the simple ion it replaces."$^\text{1}$

The distinguishing feature of colloidal electrolytes is the combination of low osmotic or colligative effects with good electrical conductiviy.$^\text{1}$

The properties of colloidal electrolytes are different than general inorganic electrolytes: even below critical micelle concentration (CMC), solutions of colloidal electrolytes show different properties; consider, for example, surface properties.$^\text{2}$

The hydrophobic-hydrophilic nomenclature applies to the nature of soap molecule arrangement in a micelle or at the surface. For hydrocolloids, hydrophilic ends form the surface of the micelle or are submerged in the water at the surface. Nevertheless, being weak electrolytes, soap molecules do not dissociate to an appreciable extent, meaning that the hydrophilic surface of the micelle consists of both carboxylate ends and sodium carboxylate ends. Since polarity itself leads to hydrophilicity, stating that either the cationic or anionic part of the molecule is hydrophilic is incorrect.

The polar end of the molecule is hydrophilic, while the nonpolar end is hydrophobic.

References

  1. Merrill, R. C. (1950). Colloidal Electrolytes. J. Chem. Educ., 27, 6, 312. 10.1021/ed027p312
  2. McBain, J. W. (1940). Colloidal Electrolytes. Nature, 145, 702–703. 10.1038/145702a0
$\endgroup$
1
  • $\begingroup$ Depending on the pH, weak acids (such as fatty acids) will dissociate fully. Traditional ("anionic") soap is already the deprotonated salt of the fatty acid, and will remain ionized unless you add acid to lower the pH and make the solution acidic. $\endgroup$
    – Karsten
    Commented Aug 4, 2023 at 15:26

Not the answer you're looking for? Browse other questions tagged or ask your own question.