All living things face the fundamental problem of separating the inside from the outside. The inside of a cell is filled with water and floating around in that water are the various bits,—mitochondria, ribosomes, and, of course, the nucleus—which make the cell go. There's water outside the cell as well—fresh water, sea water, blood, or lymph—water, water everywhere. What separates the inside from the outside is the cell membrane, which had better be insoluble in water or else the inside will be outside in no time flat. But the building blocks of the cell must be water-soluble or else there would be no way to move them around. This solubility dilemma demands a water-soluble material which can be rendered insoluble once it becomes part of the cell membrane. And believe it or not, every living cell on Earth employs the same solution to this fundamental problem; cell membranes, whether in bacteria or yeasts, redwoods or maples, Democrats or Republicans, are composed of fats. Before we can understand the chemistry of fats, we need to have a look at the general class of compounds to which they belong, the esters.

Equation 16-1(c) introduced the ester of ethanol and acetic acid, ethyl acetate, as a by-product of alcoholic fermentation. Normally a fermentation vessel is capped, protecting the mead or wine from atmospheric oxygen so that yeasts are forced convert sugar to ethanol rather than carbon dioxide and water. When the fermentation vessel is un-capped, however, bacteria oxidize the ethanol to acetic acid. If all of the ethanol is converted to acetic acid, the mead "goes sour," turning into vinegar, but if only some of the ethanol is converted to acetic acid the two compounds may react under the acidic conditions normally found in a fermenting mead or wine. Under such conditions, ethanol and acetic acid undergo a condensation reaction, producing water and ethyl acetate.

Whereas acidic conditions promote the condensation of ethanol and acetic acid to produce ethyl acetate, alkaline conditions promote the hydrolysis of ethyl acetate to produce ethanol and acetic acid. The acetic acid produced further reacts with available alkali to produce a salt, sodium acetate or potassium acetate, depending on the alkali used.

While alcohols, acids, and their salts are generally soluble, esters are generally insoluble in water. Ethanol, for example, is soluble in water because its OH group is essentially half a water molecule and the general rule for solubility is "like dissolves like." Similarly, acetic acid is soluble in water because the organic acid group, -COOH, contains an OH group. But when ethanol and acetic acid condense to form ethyl acetate, a water molecule is eliminated, leaving ethyl acetate with no OH groups at all. Consequently, ethyl acetate is not soluble in water, or, to be more precise, its solubility is very low. In esters, then, we have a class of insoluble compounds which can be either synthesized from soluble compounds or decomposed into them, depending on the conditions. This is precisely the behavior required by plant and animal cells for building cell membranes.

Figure 19-1. Glycerol

Cell membranes consist primarily of esters, not of ethanol and acetic acid, but of the alcohol glycerol and a class of acids called fatty acids. Whereas ethanol has only one OH group, glycerol has three and consequently glycerol can form ester linkages with up to three fatty acids; such a tri-ester is called a tri-glyceride. Whereas acetic acid has only two carbon atoms, a fatty acid has a long chain of them. A tri-glyceride may be called an oil or a fat, depending on whether it is a liquid or a solid at room temperature. Oils tend to have a relatively low ratio of hydrogen to carbon. Olive oil, for example, is derived primarily from oleic acid, CH3(CH2)7(CH)2(CH2)7COOH, whose ratio of hydrogen to carbon is 34/18, or 1.88. Lard and tallow also contain oleic acid, but with significant amounts of palmitic acid, CH3(CH2)14COOH, and stearic acid, CH3(CH2)16COOH, with hydrogen-to-carbon ratios of 32/16 and 36/18, respectively. Fatty acids with hydrogen-to-carbon ratios of 2/1 are deemed saturated, those with lower ratios, unsaturated. No matter whether they are fats or oils, saturated or unsaturated, these tri-glycerides are devoid of OH groups and so are insoluble in water, a property which makes them useful for separating the inside from the outside.

Just as the alkaline hydrolysis of ethyl acetate produces ethanol and, for example, sodium acetate, the hydrolysis of an oil or a fat produces glycerol and a mixture of fatty acid salts, for example, sodium oleate, sodium palmitate and/or sodium stearate. Insoluble fats and oils are thereby converted into soluble compounds. While living things in general hydrolyze fats and oils to move them around the cell, human beings desire to remove fats and oils from clothing, pots, pans, and hands. This is the fundamental problem of soap and the hydrolysis of a fat or oil is given the particular name, saponification.

Any alkali will saponify fats and oils and by now we have several at our disposal; potash (potassium carbonate) leached from the ashes of inland plants, soda ash (sodium carbonate) leached from the ashes of marine plants, ammonia from stale urine, lime (calcium oxide) from the calcination of limestone, caustic potash (potassium hydroxide) from the reaction of potash and lime, and lye, or caustic soda (sodium hydroxide) from the reaction of soda and lime. The choice of alkali depends to some extent on which one you can obtain most easily and inexpensively, but given this latitude, some alkalis are better than others for making soap. Strong alkalis will work faster than weak ones, which puts ammonia, potash, and soda behind lime, caustic potash and caustic soda. The choice among these three alkalis determines the relative solubility of the resulting soap.

Recall from Table 7-2 that calcium carbonate is insoluble in water and from Table 8-2 that the solubility of sodium carbonate is less than that of potassium carbonate. The solubilities of fatty acid salts follow the same pattern. Calcium soaps are insoluble; in fact, soap scum results when soluble soaps are used in hard (calcium rich) water, precipitating calcium salts of fatty acids. While sodium soaps are soluble, they are easily dried into solid cakes. In contrast, potassium soaps are so soluble that they will even absorb water from the air and so they exist primarily as solutions. Thus the choice of alkali—either caustic potash or caustic soda—is responsible for Pliny's observation that soap comes in two varieties, solid and liquid. This distinction has been central to soap-making ever since.

Figure 19-2. Sodium Palmitate

While the strong alkalis, caustic potash or caustic soda, will saponify fats and oils on clothing and cook-ware, they will also saponify those in skin. If saponification were the only means for removing fat and oil we would be left choosing between an alkali so mild that it does not work and one so harsh that it turns the skin to soap. Fortunately, the materials responsible for the cleansing action of soap are not the alkalis themselves, but rather the products of saponification—the fatty acid salts. The structure of such a salt, sodium palmitate, is shown in Figure 19-2. One end of the molecule, the end with the polar oxygen atoms, looks very much like any other ionic substance—sodium chloride or sodium sulfate or sodium acetate. Salts such as these dissociate into positive cations and negative anions when they dissolve in water. The other end of the molecule contains carbon and hydrogen but not oxygen, and so it is non-polar.

Figure 19-3. The Emulsification of Fats

Recalling the tail-sniffing analogy of Chapter 16, a fatty acid salt is like a normal dog sewn to a very long headless tail-less dog; polar water molecules will cluster around the ionic end of the molecule, with its positive sodium or potassium ion and negative oxygen atoms; water molecules will find little of interest in the long, non-polar hydrocarbon chain. In fact, this long non-polar chain looks, to a water molecule, very much like an un-saponified fat. Following the maxim "like dissolves like," the ionic end of the fatty acid salt will be soluble in water, while the non-polar end will be soluble in fats and oils. So in a tub of water containing fat and soap, the fat breaks up into tiny droplets; the surface of these droplets are covered with fatty acid salts; the non-polar ends of the fatty acid salts are dissolved in the fat, leaving the ionic end at the surface. As far as the water is concerned, the droplet looks like a giant ion. We can't really say that the fat has dissolved in the water. We say that it has emulsified and we refer to the resulting liquid as an emulsion or suspension. Whereas a solution is transparent, like apple juice, a suspension is merely translucent, like milk.

Equation 19-1. Saponification

If the fatty acid salts are responsible for the soapiness of soap, you might well wonder how we can get the most soap from a given quantity of fat, a classic stoichiometric question. According to our theory of soap, three moles of soap can be produced from each mole of fat. The fat, tri-stearine, for example, is a tri-ester of glycerol and stearic acid. Equation 19-1 gives the balanced equations for the saponification of tri-oleine, tri-palmitine, and tri-stearine. The "R" in the equation stands for the hydrocarbon chain of any of the fatty acids. If you paid any attention at all to Chapter 15, you ought to be able to answer the following stoichiometric questions:

Q: How many grams of sodium hydroxide are needed to react with 1 gram of tri-stearine?
Q: How many grams of sodium hydroxide are needed to react with 1 gram of tri-palmitine?
Q: How many grams of sodium hydroxide are needed to react with 1 gram of tri-oleine?
Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-stearine?
Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-palmitine?
Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-oleine?

The answer to such a problem, the ratio of alkali to fat, is called a saponification value and it is important to get this number right. If you use less than the stoichiometric amount of alkali, not all of your fat will be saponified. Not only will you get less soap than you might have, but that soap will have to dissolve the un-saponified fat in addition to that on your pots and pans. If you use more than the stoichiometric amount of alkali, not only will you have wasted alkali, but the leftover alkali will saponify your hands. If you are to get the most soap for your money, if you are to get the mildest soap for your delicate skin, it is important to get the saponification value, as Goldilocks might have said, just right.

The problem is that you are unlikely to find tri-oleine, tri-palmitine, or tri-stearine in the grocery store; fats and oils generally contain a variety of fatty acids, chiefly oleic, palmitic, and stearic acids but including a dozen or so other less abundant acids as well. How could you determine the saponification value for a fat of unknown composition? You could make a series of soaps, varying the ratio of alkali to fat, and then testing the finished soaps for excess alkali using pH test paper. The best ratio of alkali to fat would be the one which completely saponified the fat without leaving excess alkali. This optimum alkali/fat ratio would be the saponification value for the unknown fat.

Table 19-1. Saponification Values for Common Oils and Fats

Table 19-1 lists saponification values for a variety of common fats and oils. Compare your calculated saponification values for tri-oleine, tri-palmitine, and tri-stearine to those in the table. In practice, fats and oils vary in composition and it is better to risk having excess fat rather than excess alkali. The saponification values are generally discounted by 5% or so; simply multiply each saponification value by a factor of 0.95. You'll get a little less soap than you might have, but that's a small price to pay for soap that doesn't eat your skin off.

WarningMaterial Safety

Locate an MSDS for sodium hydroxide (CAS 1310-73-2). Summarize the hazardous properties, including the identity of the company which produced the MSDS and the NFPA diamond for this material.[1]

Your most likely exposure will be eye or skin contact. In case of eye contact flush them with cold water and call for an ambulance. In case of skin contact wash the affected area with plenty of water until your skin no longer feels slippery.

You should wear safety glasses and gloves while working on this project. Leftover sodium hydroxide can be flushed down the drain with plenty of water. Leftover fat or oil can be thrown in the trash.

NoteResearch and Development

So there you are, studying for a test, and you wonder what will be on it.



The NFPA diamond was introduced Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.