by Catherine Haug, January 7, 2011
I’m working on a series of tutorial posts on fats – both dietary and for other uses such as soap, lotions, and candles/lamp oil. This is a daunting task because to understand them fully, you have to be open to learning a bit of chemistry. And I know that is off-putting, but I’ll try to keep it simple. I introduce you to a few technical terms, and provide links to learn more about them.
All articles in this series have their own Topic (in the ‘Blog Post’ pull down on our home page): “Topics: Cat’s Fats.”
Our January 2011 gathering is on Making Soap at Home. One of the topics our presenter, Kathy Mansfield, will discuss is the different fats and oils that are commonly used in soapmaking. I present here my own take on this subject.
See my recent post: Basic Organic Chemistry of Fats for an overview of fats and fatty acids, and a few molecular diagrams. And see Summer Bee Meadow (7) for much more on soapmaking.
Saponification (the soap reaction)
In soapmaking, the essential reaction happens in two stages (NOTE: Commercial soaps generally take this one step further, to remove the glycerin, which is sold for other uses. But homemade soaps retain the glycerin for a better product):
1. First is hydrolysis: the three fatty acids of each triglyceride are removed from the glycerol backbone, resulting in one molecule of glycerine and three fatty acid molecules. This hydrolysis happens because the alkali medium alters the pH of the solution (raises the pH, making it more alkaline).
triglyceride (in presence of lye) + water ==> glycerin + 3 fatty acids
2. Second is saponification: the alkali reacts with each fatty acid, replacing the H (hydrogen) of the acid with the Na (sodium) or K (potassium) of the alkali, to produce a salt. This salt is the soap. The glycerine is left unchanged by this reaction, and mixes with the saponified salts to enhance the creamy texture of the resultant product.
fatty acid + lye ==> soap + water
(see Chemistry of Saponification, below, for more detail).
Note that other alkalis can react with fatty acids in a saponification reaction, but many of these resultant soaps are not soluble in water. For example, calcium or magnesium in hard water binds with the fatty acids in soap and then precipitate out, forming a soapy deposit on the surface. This is known as “soap ring” in washtubs and bathtubs.
Note also that not all fats are saponifiable, but may still be used in soapmaking to affect the properties of the soap (see Fats for Soap section below, for more)
Fatty Acid Composition of Fats
Each fatty acid of a triglyceride can be different. For example, one can be a saturated fatty acid, another can be mono-unsaturated, and the third can be poly-unsaturated omega-3 (or omega-6) as in the following example (see Basic Organic Chemistry of Fats for explanation of these terms):
Each triglyceride in a vat of fat (or bottle of oil) can have different combinations of fatty acids, and the properties of each fatty acid affect the properties of the overall fat, for cooking and soapmaking. Generally, the overall mix of fatty acids is known and represented by percentages.
Refer to the next section for a look at some of the most common fats used in soapmaking. NOTE that different sources give sightly different percentages, depending on the sample used for the analysis.
Fats for Soap
Trivia: Did you know that the name of a popular dishwashing soap, Palmolive, is derived from the two oils originally used in its manufacture, palm and olive?
From Wikipedia on Soap (4):
“Saponifiable oils and fats include olive, coconut, palm, cocoa butter, hemp oil, and shea butter to provide different qualities. For example, olive oil provides mildness in soap. Coconut oil provides lots of lather. Coconut and palm oils provide hardness. Sometimes castor oil can also be used as an humectant. Most common, though, is a combination of coconut, palm, and olive oils. Smaller amounts of unsaponifiable oils and fats that do not yield soap are sometimes added for further benefits.”
and on Unsaponifiable Oils (5):
“Unsaponifiables can be beneficial to a soap formula, because they may have properties such as moisturization, conditioning, vitamins, texture, etc. On the other hand, if the proportion of unsaponifiables is too high, or the specific unsaponifiables present don’t provide significant benefits, a defective or inferior soap product can result”
The properties of common fatty acids for soapmaking are (7):
- Lauric Acid: Hard bar, excellent cleansing, lots of fluffy lather, can be drying to skin.
- Linoleic Acid: Conditioning, silky feel.
- Myristic acid: Hard bar, cleansing, fluffy lather
- Oleic acid: Conditioning, slippery feel, stingy lather, kind to skin.
- Palmitic acid: Hard bar, cleansing, stable lather.
- Ricinoleic acid: Softer bar, conditioning, moisturizing, lots of fluffy stable lather, kind to skin
- Stearic acid: Hard, long lasting bar, stable lather
Lets look at the fatty acid makeup of some of the most common fats used in soapmaking: olive, coconut, and palm oils, and lard (rendered pork fat). See Summer Bee Meadow’s Properties of Soapmaking Oils (7) for many more. NOTE that different sources give sightly different percentages, depending on the sample used for the analysis.
Olive Oil (7):
- Oleic: 63 – 81% (a monounsaturated C18, omega-9 fatty acid)
- Linoleic: 5 – 15% (a polyunsaturated C18, omega-6 fatty acid)
- Palmitic: 7- 14% (a saturated C16, fatty acid)
- Stearic: 3 – 5% (a saturated C18, fatty acid)
“Contributes to: soap hardness, stable lather, slippery feel, conditioning, moisturizing, Olive Oil attracts external moisture to your skin, helping to keep skin soft and supple. Traditionally “Castile” soap was made using only olive oil, but the term has loosened now to include soaps that have olive oil as a major proportion of the oils in them.”
Coconut Oil (7)
- Lauric: 39-54% (a saturated C12, fatty acid)
- Myristic 15-23% (a saturated C14, fatty acid)
- Palmitic 6-11% (a saturated C16, fatty acid)
- Capric 6% (a saturated C10, fatty acid)
- Oleic 4-11% (a monounsaturated C18, omega-9 fatty acid)
- Stearic 1-4% (a saturated C18, fatty acid)
- Linoleic 1-2% (a polyunsaturated C18, omega-6 fatty acid)
“Contributes to: soap hardness, fluffy lather, quicker trace. Coconut oil makes soaps lather beautifully but can be drying when it makes up an overly large portion of your soap’s fats. It will make a very hard, white bar of soap that lathers well even in very hard water – even in sea water. Coconut oil is light and not greasy and is resistant to spoiling. Used in skin care formulations, coconut oil is emollient, moisturizing, conditioning and protecting to the skin. This oil is solid at room temperatures under 76 degrees and liquid at higher temperatures.”
Palm Oil (7)
- Palmitic 43-45% (a saturated C16, fatty acid)
- Oleic 38-40% (a monounsaturated C18, omega-9 fatty acid)
- Linoleic 9-11% (a polyunsaturated C18, omega-6 fatty acid)
- Stearic 4-5% (a saturated C18, fatty acid)
- Myristic 1% (a saturated C14, fatty acid)
“Contributes to: soap hardness, stable lather, conditioning, silky feel, quicker trace. Palm oil makes a hard bar that cleans well and is also mild. It is a good substitute for animal tallow in all-vegetable soaps. Palm oil is processed from the flesh of the fruit of tropical oil palm plants. This oil is solid at cool temperatures, becomes slushy at warm temperatures and a golden, clear liquid at higher temperatures.”
Lard (rendered pork fat) (7)
- Oleic 44-46% (a monounsaturated C18, omega-9 fatty acid)
- Palmitic 26-28% (a saturated C16, fatty acid)
- Stearic 13-14% (a saturated C18, fatty acid)
- Linoleic 6-10% (a polyunsaturated C18, omega-6 fatty acid)
- Myristic 1-2% (a saturated C14, fatty acid)
“Contributes to: soap hardness, stable lather, conditioning, quicker trace. Its advantages are that it is cheap, easily obtainable, and makes a nice lathery, white bar of soap. This fat should be combined with vegetable oils such as coconut or palm. Without other oils it can tend to not work very well in cold water.”
See Curezone (2), Wikipedia (3), or Summer Bee Meadow: Properties of Soapmaking Oils (7) for more on fat composition of fats and oils.
See Summer Bee Meadow’s Properties of Soapmaking Oils (7) for fatty acid content of many fats for soapmaking.
How Does Soap Clean?
Soap falls into a chemical category called “surfactant,” which means that it works on the surface to remove foreign molecules. The soap molecule is negatively charged at one end (the carboxyl, or COO- group), which makes it comfortable in water (hydrophilic). The rest of the molecule, a hydrocarbon chain, is hydrophobic, meaning that it avoids water.
Soap Bubble (Micelle) Cross-Section
So when in an aqueous (water) environment, such as a wash tub, the molecules align themselves into tiny micelles or bubbles, such that (see diagram, right, source 4):
- the charged end is on the outside of the bubble (white circles in the diagram below), and
- the hydrocarbon extends into the interior of the micelle (orange stems in the diagram below).
This arrangement allows the charged end to interact with water, and keeps the hydrocarbon chain away from the water.
When the bubble encounters a water-soluble dirt particle on the surface of an object, it coaxes the unwanted particle to depart. When it encounters an oil-soluble (hydrophobic) particle, the bubble captures the particle in its hydro-phobic center effectively allowing the particle to be taken up by the soapy water.
Similar biological structures to a soap bubble
Here’s two examples of biological structures that have similar fatty acid layer to that of soap bubbles.However, in both of these examples the fatty acid has a phosphate attached at the hydrophillic end. This is called a phospholipid.
Cell Membranes: Biological cell membranes are made up primarily of fatty acids in a bi-layer arrangement, each layer with similar structure to a soap bubble. That is, referring to the diagram (from Wikipedia (8)):
-
Lipid Bilayer of Cell Membrane
On the outside of the cell facing the aqueous exterior of the cell, a layer of phospholipids have the hydrophilic end on the outside (yellow circles), with the hydrophobic hydrocarbon chain toward the interior of the cell (grey stems).
- And a second layer of phospholipids have the hydrophilic end facing the aqueous interior of the cell, with the hydrophobic hydrocarbon chain facing that of the outer layer.
There are other substances embedded in the cell membrane such as cholesterol and proteins (receptors and active transport agents). Here’s a great diagram of a cell membrane showing the phospholipid bilayer and the embedded items (from Estrella Mountain Community College (8)):
Cell Membrane
Cholesterol particles, which carry fatty acids in the blood, have a single-layer membrane of phospholipids with the hydrophilic end facing the aqueous blood, and the hydrophobic hydrocarbon chain facing the fatty interior of the particle.
Referring to the LDL cholesterol diagram (below (9)), cholesterol particles also have:
- Carrier proteins attached to the outside of the particle (Apoprotein);
- Some molecules of cholesterol (black chain) embedded in the hydrophobic part of the membrane; and
- Hydrophobic triglycerides and cholesteryl esters (cholesterol bonded to a fatty acid) in the interior of the particle.
Chemistry of the Saponification Reaction
For those chemistry buffs, here is the reaction of the second step, showing one fatty acid molecule (linoleic acid, C18) reacting with one alkali molecule. In this example,
linoleic acid (fatty acid) + sodium hydroxide (lye) ==> sodium linoleate (soap) + water
More precisely:
CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH + NaOH ==>
CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COONa + H2O
This particular fatty acid (linoleic) is an omega-6 ploy-unsaturate, with two double bonds represented by the ‘=’. One of the oxygens is also bonded to its carbon with a double bond; the other oxygen is bonded to the same carbon with a single bond and has a negative charge. The sodium has a positive charge.
See What is Soap (6) for a much more detailed description of how soap is formed, with great 3-D diagrams.
For more on soapmaking:
- The EssentiaList: The Basics of Soapmaking
- The EssentiaList: Making Your Own Soap Bars
- The EssentiaList: Making Liquid Soap from Scratch
- The EssentiaList: Working with Lye
- The EssentiaList: Homemade Cleaning Supplies
Sources:
- Triglyceride diagram: en.wikipedia.org/wiki/Triglyceride
- Fat content of olive oil: www.oliveoilsource.com/page/chemical-characteristics#Fatty and curezone.com/foods/fatspercent.asp
- Fat content of cooking oils/fats: en.wikipedia.org/wiki/Lard
- Wikipedia on Soap, and micelle diagram: en.wikipedia.org/wiki/Soap
- Wikepedia on Unsaponifiable: en.wikipedia.org/wiki/Unsaponifiable
- What is Soap: www.deancoleman.com/whatissoap.htm
- Summer Bee Meadow: www.summerbeemeadow.com and on properties of soapmaking oils: www.summerbeemeadow.com/content/properties-soapmaking-oils
- Cell Membrane: en.wikipedia.org/wiki/Cell_membrane and www2.estrellamountain.edu/faculty/farabee/biobk/biobooktransp.html
- LDL Cholesterol: www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/lipoprot.htm