Soap in salt (hard) water test

Soap in salt water test

Just on a whim, I decided to test my soap in salt water and see how it performed compared to water from my faucet. For each test, I swiped the bar over my hand 10 times, then used my other hand to swipe over my hand 10 more times (hope that makes sense). The first picture shows the salt water test, and the second shows my normal water. I figured maybe there were others out there that wanted to see how salt water affects a "normal" bar of soap's ability to lather.

Interesting! How much coconut was in the bar? What concentration salt did you use in the water?

Just to be clear, the chemistry of soap in salt water is different than the chemistry of soap in hard water.

Soap is not as soluble in salt water as it is in fresh water, all other things being equal. Less soap is able to physically dissolve off the bar in salty water, so there's less soap on your hand to make lather. The more lauric and myristic and shorter chain fatty acids in the soap, the more the soap will be able to dissolve. That means coconut/PKO/babassu/etc soaps are better at lathering and cleansing in salt water than soaps made with palm/lard/tallow and the liquid fats.

Soap dissolves perfectly fine in hard water ... but the dissolved sodium (or potassium) soap quickly reacts with the calcium and magnesium ions (aka the "hard water" chemicals) to form calcium and magnesium soaps. These soaps are completely insoluble in water, so they obviously don't make any lather. Instead, these soaps leave a greasy, sticky soap scum on the skin and bathtub. It doesn't matter what fats were used to make the soap -- hard water is an equal opportunity destroyer of lather regardless of the type of soap. The only way to deal with this is to use soft water or to use a chelator in your soap such as sodium citrate or tetrasodium EDTA. A chelator is a chemical that can bind with the calcium and magnesium ions faster than the soap can, so the soap can do its job of lathering and cleaning.

So ... yes ... less lather in salt water and less lather in hard water ... but for quite different reasons.

DeeAnna said:

Just to be clear, the chemistry of soap in salt water is different than the chemistry of soap in hard water.

Soap is not as soluble in salt water as it is in fresh water, all other things being equal. Less soap is able to physically dissolve off the bar in salty water, so there's less soap on your hand to make lather. The more lauric and myristic and shorter chain fatty acids in the soap, the more the soap will be able to dissolve. That means coconut/PKO/babassu/etc soaps are better at lathering and cleansing in salt water than soaps made with palm/lard/tallow and the liquid fats.

Soap dissolves perfectly fine in hard water ... but the dissolved sodium (or potassium) soap quickly reacts with the calcium and magnesium ions (aka the "hard water" chemicals) to form calcium and magnesium soaps. These soaps are completely insoluble in water, so they obviously don't make any lather. Instead, these soaps leave a greasy, sticky soap scum on the skin and bathtub. It doesn't matter what fats were used to make the soap -- hard water is an equal opportunity destroyer of lather regardless of the type of soap. The only way to deal with this is to use soft water or to use a chelator in your soap such as sodium citrate or tetrasodium EDTA. A chelator is a chemical that can bind with the calcium and magnesium ions faster than the soap can, so the soap can do its job of lathering and cleaning.

So ... yes ... less lather in salt water and less lather in hard water ... but for quite different reasons.

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Thanks for clearing that up DeeAnna! I thought hard water was caused by salts. I've edited my post to correct my mistake.

Seawolfe said:

Interesting! How much coconut was in the bar? What concentration salt did you use in the water?

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I only use 20% coconut, no PKO or babassu. I have no idea the concentration of salt as it was water from my bath with Epsom salts. I dumped about two cups into a tub full of water. The idea came to me when I was trying to lather up my luffa ha.

Hard water is caused by salts -- you're perfectly right to think that. The key thing though is that there are many kinds of salts. We everyday folks usually think of "salt" as just one thing -- table salt or the salt dissolved in sea water. This particular salt, sodium chloride (NaCl), is just one of many salts in our universe.

To a chemist, a "salt" by strict definition is what you get when an acid and a base are able to chemically react with each other. The offspring of that get-together is a salt.

Soap is a salt -- a sodium soap is a salt of any fatty acid (there are many) and sodium hydroxide (a base). Table salt is the reaction of hydrochloric acid (HCl) and sodium hydroxide (NaOH). Hard water ions -- the calcium (Ca) and magnesium (Mg) ions I talked about earlier -- come from naturally occurring salts in the earth. Some salts dissolve easily in water, some don't, so it can be a trial-and-error learning process to know what any particular salt will do in a given situation.

A sodium or potassium soap is fairly soluble in plain water, but a calcium or magnesium soap is completely insoluble. A calcium or magnesium soap acts just like a blob of grease in a pan of plain water. In fact certain types of these insoluble soaps are actually used as lubricants.

Hard water + soap does this:

Sodium soap (soluble in water) + Calcium (Ca) ion => Calcium soap (insoluble) + Sodium (Na) ion
Sodium soap (soluble in water) + Magnesium (Mg) ion => Magnesium soap (insoluble) + Na ion

When a sodium soap is used in salty water, the problem is less a chemical reaction and more of a physical change. The sodium ions in the water prevent the soap molecules from dissolving in the water. The longer the soap molecules are (soap made with lard, tallow, palm, or liquid oils), the less likely they will dissolve off the soap bar. The littler the soap molecules, the more likely they are to dissolve.

Think of the sodium ions in the salty water as big, burly guards trying to prevent criminals from escaping their bar-soap prison. The little criminals are faster and more agile. They are able to sneak past the guards much easier than the bigger, slower moving guys. :shifty:

I hope this helps!

DeeAnna thank you so much for your explanations, I love learning the facts. Thanks to you I understood finally why my soap bar didn't work in hard water London, soap stuck to my hair and when I used citric acid it turn back into oil! very bizarre.
If you don't mind me asking I would like you to look at this formula of a liquid SLS free shampoo I bought, if this is soap too (but liquid) why doesn't it do the same in hard water? It works well for my hair in London's hard water and I don't know why. Is this not saponification with potassium hidroxide? Sorry I know very little of liquid soaps.
Here is the
Ingredients:
Aqua (Water), Capryl/caprylyl glucoside, Cocomidopropyl betaine, Glycerine (Veg), Sodium benzoate & Potassium sorbate, Hydrolysed wheat protein, Xanthan gum, Citric acid, Phenoxyethanol, Aloe barbadensis (Aloe vera) Leaf Powder

Or is the fact that Potassium Hydroxide is ok with hard water and Is just sodium hydroxide the one that reacts with the hard water? I am confused sorry.

soapylondon said:

DeeAnna thank you so much for your explanations, I love learning the facts. Thanks to you I understood finally why my soap bar didn't work in hard water London, soap stuck to my hair and when I used citric acid it turn back into oil! very bizarre.
If you don't mind me asking I would like you to look at this formula of a liquid SLS free shampoo I bought, if this is soap too (but liquid) why doesn't it do the same in hard water? It works well for my hair in London's hard water and I don't know why. Is this not saponification with potassium hidroxide? Sorry I know very little of liquid soaps.
Here is the
Ingredients:
Aqua (Water), Capryl/caprylyl glucoside, Cocomidopropyl betaine, Glycerine (Veg), Sodium benzoate & Potassium sorbate, Hydrolysed wheat protein, Xanthan gum, Citric acid, Phenoxyethanol, Aloe barbadensis (Aloe vera) Leaf Powder

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(Not DeeAnna) I could be completely wrong, but from the ingredients, that doesn't appear to be "soap" per say. I think it is a detergent based shampoo.

Looks like syndet to me as well. I don't see anything saponified in that list.

I agree with the others that the cleanser is a synthetic detergent blend. The two main non-water ingredients are mild cleansers --

Capryl/caprylyl glucoside -- mild surfactant (cleanser)
http://swiftcraftymonkey.blogspot.com/2012/06/solubilizers-caprylylcapryl-glucoside.html

Cocomidopropyl betaine -- mild surfactant
http://swiftcraftymonkey.blogspot.com/2010/04/surfactants-betaines.html

Glycerine (Veg) -- humectant
Sodium benzoate & Potassium sorbate -- preservatives
Hydrolysed wheat protein -- skin conditioning agent
Xanthan gum -- thickener
Citric acid -- pH adjuster
Phenoxyethanol -- preservative
Aloe barbadensis (Aloe vera) Leaf Powder -- skin conditioning agent

"...Potassium Hydroxide is ok with hard water and Is just sodium hydroxide the one that reacts with the hard water..."

No, potassium soaps will react with hard water ions similar to sodium soaps. Many more people make solid (sodium) soap than liquid (potassium) soap. I think that's the reason why this the question comes up a lot more often with the bar soap makers.

Sorry I have just realised I didn't reply the your kind post explaining all the ingredients of my sindet shampoo. Thank you so much. I bought it without reading the ingredients properly so I didn't know I was buying surfactants after all. In any case it seems less aggressive than other shampoos in the high street. As already discussed in here I wouldn't be able to use natural shampoos, neither solid nor liquid as both would react badly with the hard water in London. When I move house I am going to install a water softener because I would like to use my shampoo bars! And also make my own laundry soap.

DeeAnna said:

... The only way to deal with this is to use soft water or to use a chelator in your soap such as sodium citrate or tetrasodium EDTA. A chelator is a chemical that can bind with the calcium and magnesium ions faster than the soap can, so the soap can do its job of lathering and cleaning.

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Hello DeeAnna,

I have read the above thread with interest since I also live in the London area and have, therefore, the same problems - particularly with shampoo bars. You mention using a chelator such as sodium citrate. Do you think it would work if I melted the shampoo bars that I have made and then rebatched them with the addition of sodium citrate?

It is probably a long shot but I would be interested in your views. Thanks.

Nicole

Yes, I do think you could add sodium citrate in a rebatch. If you're using actual sodium citrate powder rather than making it from citric acid and NaOH, I'd dissolve the powder in some of the liquid you plan to add to the rebatch, rather than add it directly to the soap.

Edit: Here's a related thread where we're talking about using citric acid or sodium citrate in soap: http://www.soapmakingforum.com/showthread.php?t=51785

DeeAnna said:

Yes, I do think you could add sodium citrate in a rebatch. If you're using actual sodium citrate powder rather than making it from citric acid and NaOH, I'd dissolve the powder in some of the liquid you plan to add to the rebatch, rather than add it directly to the soap.

Edit: Here's a related thread where we're talking about using citric acid or sodium citrate in soap: http://www.soapmakingforum.com/showthread.php?t=51785

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Thanks a lot. I will try it.

Sorry for necrobumping, but I found it disturbing that a major detail was missing in the discussion here:

hmlove1218 said:

Soap in salt water test

Just on a whim, I decided to test my soap in salt water and see how it performed compared to water from my faucet.

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hmlove1218 said:

salt as it was water from my bath with Epsom salts.

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DeeAnna said:

Sodium soap (soluble in water) + Magnesium (Mg) ion => Magnesium soap (insoluble) + Na ion

When a sodium soap is used in salty water, the problem is less a chemical reaction and more of a physical change. The sodium ions in the water prevent the soap molecules from dissolving in the water.

Click to expand...

Yes, yes, and yes. The thing missed here is that Epsom salt (MgSO₄) is not a sodium, but a magnesium salt! That means that the discussion wasn't wrong by itself, but was only remotely related (if at all) to the actual situation.
Original poster's “salt water” is actually closer to ultra hard water, than salt water (in the sense of sea water/brine). Knowing what happens if you mix soap with soluble Ca/Mg salts, I'm highly surprised that OP's “salt” test gave off any lather at all.