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When I performed a saponification experiment in school, I added some ethanol to the solution (glycerol ester fatty acid + sodium hydroxide) to change the equilibrium of the reaction. (Molecules of salt of fatty acid made some chunks so they don't react efficiently.) But accidentally I spilled a lot of ethanol and the solution became hard, like a hair gel. And after I added some water to it, it became liquid again.

What I want to know is why the solution became a gel. I know that ethanol is used for an efficient reaction because ethanol can be a common solvent of grease and water and it can help sodium hydroxide and glycerol ester meet, but I don't know why they became harder when ethanol was overused.

Of course, the solution was in the water bath.

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Sodium stearate is not very soluble in water (above about 2%, it becomes viscous but not a firm gel). Ethanol can reduce the viscosity at first, but sodium stearate is not soluble in ethanol either, so an excess of ethanol causes a precipitation of solid that firms up the aqueous gel. More water dissolves that precipitate and restores the fluidity. The solubility and rheology of fatty acid soap solutions depends on temperature, concentration, solvents and the structure and purity of the fatty acid group. Stearate is very different from oleate, and both are different from laurate.

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  • $\begingroup$ Thanks for the answer. Then the reason of gelation is because of only sodium stearate? Can sodium oleate or sodium laurate cause gelation either? $\endgroup$ – 손현서 Jan 22 '18 at 1:25
  • $\begingroup$ Sodium stearate is a firm solid (Ivory Soap); sodium oleate is softer (esp. when wet, like Soft Soap - which I think is the potassium salt in solution). Sodium laureate has a shorter chain, and while it is a firm solid, it needs a higher concentration in water to firm up. (The laureate is more water soluble.) $\endgroup$ – James Gaidis Jan 22 '18 at 18:39

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