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#1 2008-12-03 09:42:32


Why are potassium salts soft soaps and sodium salts hard soaps?

Yahoo Answers wrote:

Q: Why do potassium salts yield soft soaps while sodium salts yield hard soaps?
i have been searching relentlessly for weeks trying to find an answer to this. it was a part of my organic lab assignment from a while back and i never found the answer to it.

A: This is an interesting question. I had to do a little research to learn what this meant. From searching the internet for hard and soft soaps. Hard soaps are physically hard, in this case meaning they contain little water. Sodium chloride may be added to help salt out the soaps and make them hard. The problem with hard soaps is that they are less effective. Sodium salts are more easily isolated as hard soaps.

Potassium soaps are soft soaps, meaning they are softer due to more water being present in their isolation.

From what I can gather, the key difference is due to the solubility of the salts. Potassium salts are more soluble in water than their equivalent sodium counterparts. I found a journal reference in the www here. There are several chemistry experiments that essentially repeat this experiment on a practical scale and compare the temperature at which different salts dissolve, for example.

This is my opinion of the chemistry involved. As one goes across and down the periodic table, more and more protons are introduced into the nucleus of each atom. That increases the Coulombic attraction for electrons. Additional electrons will encounter a repulsion by the inner and outer shell electrons as well. Since the differences are in the metal cations, we need only look at how sodium, potassium, lithium, magnesium, and calcium might differ. Lithium might be predicted to be less soluble as lithium only has a single pair of electrons to interfere with any attraction to the nucleus of 3 protons. Sodium has more protons and an additional completed shell of electrons to interfere with an attraction to the nucleus. That completed shell of electrons will diminish the force of attraction for another electron at it will be at a much greater distance than those electrons of a halogen. The distance of an electron that completes a shell will be much closer to the nucleus than an electron that will start the next outer shell.

It is difficult to predict the effect a priori. It is easier to look at data and rationalize what has happened. Since the reactivity decreases K>Na>Li, potassium is the most reactive and the lowest affinity for its outermost electron. Logically, it should also have a low affinity for the electrons of the carboxylate (the more basic component present). However, by entropic arguments, in solution, no charge localization will exist. Thus the soap anion and potassium cation will become surrounded by water molecules.

If sodium (calcium or magnesium) has a higher affinity for the carboxylate, that increased force will decrease the ion separation and solvent insertion. Presumably, this increases the affinity between the hydrocarbon ends of soap for each other and exclude water. This will lead to a harder soap.


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