Evaluating soap efficacy?

[Sapo]

Is there a scientific way of evaluating soap efficacy? I.e. 100% olive soap versus 100% coconut soap in terms of how much oils each one can emulsify and subsequently wash off?

The expected answer is that the coconut one is more efficient, but by how much?

The answer, as I see it, could be theoretical and explained in chemical formulas, or experimental, obtained by an experiment.

Any takers? I'm at a loss.

 

[The Efficacious Gentleman]

I think that you're trying too much to be able to quantify things about soaping which may or may not be quantifiable and even when it is, does it mean what we think it means? It might be because you're looking to sell, or just curious, I don't know.

Back to the question - yes, there is. Think of someone holding on to a cliff-face, one hand on the cliff and the other holding a weight. That person is "dirt" and the weight is the soap. A lighter weight might not be able to break a strong grip, but a heavier one could. Likewise, some soaps form stronger bonds with the water and the dirt than the dirt does with the surface to which it clings - the result is that the dirt and soap molecules are carried off in the water.

As for testing or having a figure of how much more one soap cleans than another, have you looked up the emollient properties for sodium cocoate and sodium olivate?

 

[BattleGnome]

A non scientific way to evaluate: first thing you do in the morning is use one soap and write down every bit of analysis you can think of. The next morning use a different soap. The theory working that you hands will have time to average out during the night to whatever your personal balance is.

Another thought is to stick your hand in a bowl of oil and see how long it takes to wash it off.

Some non-scientific options but someone could probably build up from there.

 

[Millie]

I think the efficacy of soap has to do with the length of the hydrocarbon chain of various oils. A short chain (as found in coconut oil) is more soluble than a long chain oil. Some oils also have more unsaponifiables... "other stuff". So keep all that in mind before the next part.

A triglyceride (oil molecule in natural state) has three ends to attach to three naoh, to make three soap molecules. One way to compare the efficacy of one soap to another is to look at the amount of lye needed to saponify each oil, and therefore how much actual soap is created (I can only compare one to another, I do not know how many unsaps are in each oil, or what they might be)

Example: one pound olive oil takes 61.5 grams lye (naoh, zero superfat). One pound coconut oil takes 83 grams lye.

61.5 / 83 x 100% =74%. So olive soap is 74% as efficient as CO soap (if the comparative percentage of soap molecules can be used to judge efficacy)

p.s. if there are any organic chemistry experts around, please correct me if I'm totally off base with this

Edit: the more I think about it, just skip all that and look at the cleansing number in soap calc there are just too many variables to account for and SO MUCH I don't know!

 

[DeeAnna]

"...A short chain (as found in coconut oil) is more soluble than a long chain oil...."

While that is true, you also have to consider the effect of double bonds on solubility. Stearic acid (no double bonds), oleic acid (one double bond), and linoleic acid (two double bonds) all have the same number of carbon atoms in their structure, but stearic soap is much less soluble than oleic soap, all other things being equal.

 

[Millie]

DeeAnna, I love your posts! I always feel like teacher is in the room and we'll finally get the real answer how do you address efficacy, do you make calculations or go by skin feel and experience?

 

[Scooter]

People have different reasons for wanting natural soaps. Here is an old thread that asked about "efficacy" but in measuring something completely different: http://www.soapmakingforum.com/showthread.php?t=29222

 

[Sapo]

Aye, basically what I'm wondering is where the cleansing value offered by various soap calculators comes from (and possibly how to verify it), if anyone happens to know.

Using 100% olive right now and experiencing a squeaky clean feel after showering - usually associated with the more cleansing CO soap - which is what prompted me to asking the question. But this is beyond subjective, so I wanted to learn if anyone knows of a way to be more objective about it. Will attempt a basic experiment in the following days:

-stain my hands with known quantity of oil
-use 100% OO soap, rub well, rinse
-document
-keep increasing initial oil quantity until the OO soap can no longer effectively clean it (if it ever happens)
-use 'threshold' quantity of oil and attempt to wash off with 100% CO soap - theoretically, given it's superior cleansing, the hands should come out clean, as opposed to unclean with OO soap.

So far I'm liking Millie's explanation and the number of soap molecules. Could the efficacy of the soap also be affected by the solubility of the soap? Numbers are fake, but for the sake of argument (and ignoring the soap molecule calculation):

1. If 10ml of coconut oil soap could wash off 100g of oil by using 1000ml of washing water...
2. could 10ml of olive oil soap wash off 100g of oil by using 1500ml (to account for it's lower solubility) of washing water?

 

[DeeAnna]

Thanks, Millie! Glad I could help.

As far as what I do to measure cleansing efficacy ... the short answer is I don't. I know that every fatty acid commonly used to make soap will cleanse skin sufficiently well, so the issue is moot for me. I do pay attention to the fatty acid profile as a starting guide, but I also know additives including chelators, the superfat, the household water hardness, water temperature, the levels of agitation, abrasion and mixing, etc. also affect soap performance as much as the fatty acids in the soap. This is true for household and laundry soaps as it is for bath soap.

What I do to test a bath soap -- I look at how fast the soap begins to make lather, the amount and texture of the lather, the feeling of my skin after using the soap, and the longevity and appearance of the soap bar as it is used in the shower. If all those qualities are in line with my expectations, that's good enough for me -- I'm sure the soap will also clean my skin as well.

If a person wanted to research cleansing efficacy, I'd suggest an study of actual soaps with strict control of variables -- for example, each soap should be made from pure fatty acid with a true zero superfat and a consistent simple recipe, rather than a mixed-fat soap with varying superfat and additives like we usually make. Then design a test to see how many grams of soap dissolved in distilled water will emulsify a given number of grams of a specific fat. Or something along those lines. One could then use the data from such experiments to create a mathematical model. The next step would be to use the model to predict the cleansing of a mixed fat soap and see how well the model predicted the actual performance of a mixed fat soap.

Edit: A high oleic soap (olive oil soap) is actually about as soluble in water as a high lauric-myristic soap (coconut oil soap.) I think the perception that an olive oil soap is less soluble in water is because of the distinctive goopy gel that forms when an oleic soap is wetted. A lauric-myristic soap doesn't make this gel. Or maybe it's because the lauric-myristic soap has a fluffier lather.

 

[earlene]

But why limit the experiment or question to just oil? Although I know that a mechanic may have a lot of oil in the dirt that needs to be washed away, what about the just plain dirt from digging a ditch? Or the mud when kids make mud pies? It's still dirt and I'm thinking not much oil except their own body oil. And I know I don't want to strip my (almost 70-year-old) skin of all of its natural oils.

Just wondering.

 

[dixiedragon]

Earlene - that's another good question. The best soap for washing off dirt might be very different from the best soap for washing off oil. I give away a lot of soap to my co-workers at christmas time, and one lady said her husband just loved my soap and it was better for getting rid of grease (petroleum based, not kitchen grease) than the powerful orange soap with the grit. This is just my simple lard soap with 5% super fat, well-aged.

I do wonder of the fact that the soap is pretty and smells nice makes people use it longer, so they are getting more soap on their hands and washing longer to enjoy it?

 

[DeeAnna]

But why limit the experiment or question to just oil?

I really don't know what the OP has in mind. The things you mentioned came to my mind too, Earlene. Like you, I think context is really important when asking and answering questions like this, and we weren't given much context for this inquiry.

 

[lenarenee]

Maybe its because I'm in the child care profession (or perhaps because we're all sick in bed with yet another fun virus), when I think of soap cleaning, I associate it more with sanitizing, not actually removing dirt. If I could stop coughing long enough to think - I'd wonder if formulations affect the ability to remove and suspend bacteria/viruses.

 

[Steve85569]

The "cleansing value" is the sum of Lauric and Myristic fatty acids in the soap profile. It assumes that those are going to be fully saponified in the process and not left as superfat.

 

[DeeAnna]

Well, actually "cleansing" is a poor name for the lauric-myristic content of a soap. Any soap from any fat will cleanse the skin, so using the word "cleansing" for ONLY the lauric-myristic content in a soap is unfair to soaps made with other fatty acids.

A more accurate name for lauric + myristic content might be something more like "stripping." Myristic acid (and other shorter chain fatty acids such as butyric, capric, caprylic, etc.) does more than emulsify excess oils -- it can remove natural body oils and proteins from the non-living surface layer of the skin. Some people are very sensitive to the irritation from a soap high in these fatty acids, and many more people find such soap overly drying.

 

[TokraDog]

From a scientific perspective the efficacy of soap refers to 'cleaning efficiency' of the soap. There are other aspects to consider like for example; lather, size of bubbles, hardness of the finished soap, feel of the skin after being washed etc. Of course the qualities of the base largely determine these qualities. You could develop your own scoring system, but make it simple. Scoring systems are always subjectivity, and that’s fine, it’s your system. Perhaps create simple star system, say minimum of one star to maximum of three stars. So for example, you could say 'Blue soap' (my alias for your specific soap) has a score of 1,2,2 and 2, meaning has a 'light lather, medium bubbles and hardness of bar etc.'. The marketing industry creates these score all the time. Even Environmental Impact Assessments are almost completely 'subjective opinions' of the evaluator. Hope this makes sense.

 

[Millie]

Found an interesting article in the course of researching little tidbits found in this thread. 'Microbes of the skin' http://mobile.the-scientist.com/article/40228/microbes-of-the-skin

I'm also trying to learn more about fatty acids in relation to skin and found this http://immunology.sciencemag.org/content/1/4/eaah4609 but I can't view the full content. Wondering if any scientists or students have acces to the whole thing and can tell us if it is specific short chain fatty acids only or if it is based on the size and shape of the molecule.

Sapo, thanks for asking the original question, my mind is so boggled but I'm having fun

 

[Millie]

DeeAnna, are you saying shorter chain fatty acids are more penetrative? Also, are the polar regions of soap molecules made from different fatty acids similar? I read that they are all negatively charged and repel eachother, keeping the soap / oil suspended in water, but I am also thinking back to the Gent's cliff face analogy and wondering if some carry a stronger charge, or if the stronger bonds he was talking about are happening in the nonpolar region (okay, after typing that out something clicked and I think he was talking about the latter). That brings me to the lower solubility of long and double bonded chains, I have some questions but my brain is struggling to even figure out how to ask what is going on there. My current understanding is that soap molecules form micelles in water, with the hydrophilic ends sticking out and the hydrophobic tails sticking in. When it is less soluble, does that mean that the attraction between the nonpolar ends is keeping the molecules in a micelle formation rather than forming a bridge between oils on the skin and the water? Or does low solubility mean the molecules are less likely to migrate into water in the first place? I'm still very confused about the solubility of different fatty acids.

 

[DeeAnna]

What I'm saying is exactly what I said -- shorter fatty acids when turned into soap can bind with and remove fats and proteins from the surface of the skin (stratum corneum). That can lead to the skin feeling dry and tight all the way to becoming red and irritated, depending on the person's sensitivity to the effects and how often the soap is used. I haven't said anything about "penetrative."

The polar end of a soap molecule is a sodium ion (in a bar soap). Sodium ions have a positive charge, not negative. The polar (sodium) end of a soap molecule will thus have the same positive charge.

"...That brings me to the lower solubility of long and double bonded chains..."

I'm not quite sure where this info is coming from -- if you look carefully at solubility charts, this is not true. As I wrote in Post 9, a high oleic soap (olive oil soap) is actually about as soluble in water as a high lauric-myristic soap (coconut oil soap.)

If you look at the most common fatty acids used in soap, they run in order from smallest to largest:

FAs with single carbon bonds: myristic -> lauric -> palmitic -> stearic
Solubility decreases as chain length goes up, so myristic and lauric are highly soluble soaps. Palmitic soap is less soluble. Stearic soap is highly insoluble. Palmitic and stearic in a soap will add longevity and some myristic and lauric in the soap will add fast developing, big bubbles.

The FAs with one or more double bonds in the carbon chain are all variations of stearic. These FAs include oleic, linoleic and linolenic. Add one double bond to stearic -> turns into oleic. Add a second double bond to oleic -> linoleic. Add a third double bond to linoleic -> linolenic. The double bonds in an oleic soap dramatically increase the solubility of the soap compared to its straight chain cousin stearic soap.

 

[Millie]

Sorry DeeAnna, I think I am going in circles here trying to understand everything - I should probably get a 'Chemistry 101' book for lay people. I need charts and pictures (lots) and info all in one place. Bouncing from one article to the next and trying to piece the info together is confusing.