[arnyk]

I understand that you would generally use NaOH.  This would yield a "hard" soap, while using KOH would yield a softer soap.  KOH has the lower MP.  

So those are the two commonly used bases.  Would *any* other bases such as LiOH, Mg(OH)₂, or Ca(OH)₂ produce a soap?  

Edit*

*Hard soaps are made from NaOH, soft soaps from KOH and NH4OH.  What is the difference in the base used which allows for this?  Understanding that the use of saturated/unsaturated fats also plays a part.

[Alberto_Kravina]

I won't use Ca or Mg hydroxide! Calcium and magnesium salts of fatty acids are insoluble! This precipitation reaction happens if you add hart water to a soap solution. Some types of soaps are synthetized with Na₂CO₃ instead of NaOH, btw....

LiOH would be a bit expensive...

[arnyk]

Insoluble in a polar solvent.  Does that have to do with the electronegativies for Mg and Ca which appear to be greater than 1?

Ah so it doesn't have to be a hydroxide base.  What about NH3?

[Alberto_Kravina]

Given that, could we use Rb or Cs instead which have EN of 0.8 and 0.7 respectively.  But those are much less stable, which would be practical?

Cs⁺ and Rb⁺ are pretty stable...

Cesium and Rubidium are much more expensive than other alkali metals....  :P

[arnyk]

If it doesn't have to be a hydroxide, what about Ammonia?

[Alberto_Kravina]

You mean to create an ammonium salt of the fally acid?

[HP]

Potasium and ammonium salts of fatty acids are used as pesticidal soaps because they are safer for plants than sodium soaps:
"Soap salts include the two pesticide active ingredients potassium salts of fatty acids (including potassium laurate, potassium myristate, potassium
oleate and potassium ricinoleate), and ammonium salts of fatty acids (ammonium oleate).
Potassium salts of fatty acids are used as
insecticides, acaricides,
herbicides and algaecides. They are used to control a variety of insects and
mosses, algae, lichens, liverworts and other weeds, in or on many food and
feed crops, ornamental flower beds, house plants, trees, shrubs, walks and
driveways, and on dogs, puppies and cats.
Ammonium salts of fatty acids are used as a rabbit and deer repellent
on forage and grain crops, on vegetables and field crops, in orchards, and
on nursery stock, ornamentals, flowers, lawns, turf, vines, shrubs and trees."
 Thats what i found this topic. And yes amonia pretty nice act on fats to soaps

[arnyk]

Hmm, I was leaning more towards hand soaps that we would use ourselves, possible with NH3?

[HP]

Yes you may and the soap you'll obtain with KOH or with NH4OH will be the so called "liquid soaps" which ae often used at the hotel rooms because are hard to steal

[arnyk]

And also I could use unsaturated fats to make liquid soaps.

I understand why unsaturated fats would be liquid at room temperature.  But why is it that NaOH will produce a harder soap than KOH?  Does it have to do with polarity, electronegativities, or IMFs?

[HP]

You are close friend. I cant give you exact answer why K+ make the soap liquid at r.t. I know K-athom have bigger athomic radius but i am not expert in theoretical chemistry beeing practiciant chemist...If anyone knows much this problem please write us it'll be interesting and for me.

[pantone159]

I am curious what a Li, Rb or Cs soap would be like.  I know they are too expensive (especially Rb, Cs) for any real use, though.
I have no plans to break my Cs ampule in a beaker of water to make CsOH and so find out for myself