Kitchen chemistry: soap in action

Our previous kitchen chemistry post discussed soap. Soap consists of sodium salts (or potassium salts) of fatty acids. For example, sodium stearate soap consists of sodium (Na+) and stearate C17H35COO) ions:

These ions do their soapy job because the charged oxygen end of the molecule is attracted to water (since the hydrogen side of a water molecule has a slight positive charge). The long hydrocarbon tail, on the other hand, is attracted to oil and grease. Mixing soap and water with oil or grease therefore produces little grease droplets surrounded by many, many stearate (or other fatty acid) ions, with their tails embedded in the grease droplet, and their oxygen heads poking out into the water. I’ve only had the patience to draw eight for this droplet:

Because these little droplets are surrounded by negative charges (on the oxygen atoms), the droplets repel each other. This means the droplets stay separate from each other, and cannot combine into larger oily blobs. Because the negatively charged oxygen atoms are attracted to water molecules, the droplets also remain dispersed within the water (so that they can later be rinsed away). In fact, what we have here is an emulsion (or sol) of oil or grease in water, stabilised by the soap. Recall the image with which we began this post series:

Other kinds of emulsion will likewise need some kind of molecule that keeps the droplets separate – usually also a molecule with a distinct head and tail. In mayonnaise, for example, the emulsion of oil in water is stabilised by phospholipid molecules from egg yolks. These molecules also have a “head” attracted to water and a “tail” attracted to oil.

This post brings our kitchen chemistry series to a close, at least for now.

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Kitchen chemistry: soap

Our previous kitchen chemistry post discussed fats and oils, which are “triple esters” of glycerol:

Apart from their role in diet, fats are also used to produce soap:

  

The soap-making process involves reacting fats with strongly alkaline substances, such as lye (sodium hydroxide, NaOH). This can be done at home, but since lye is dangerous, soap-making is not appropriate for children (see these precautions: 1, 2, 3).

In solution, the lye exists as sodium (Na+) and hydroxide (OH) ions (indeed, the presence of hydroxide ions is what “alkaline” means). The hydroxide ions react with the fat to free the glycerol:

Saponification reaction

Fatty acid ions (such as stearate ions, C17H35COO) are also produced:

Since the sodium ions from the lye still exist, soap is basically sodium stearate, sodium palmitate, or something similar. Because it is the result of reacting a very strongly alkaline substance (sodium hydroxide) with very weak acids (fatty acids), soap itself is also alkaline. This alkaline nature can be harsh on the skin, and especially on the hair. Shampoos are therefore usually made from synthetic detergents, and formulated to be mildly acidic (with a pH between 5 and 7).

Another problem with soap is that it reacts with dissolved calcium, iron, or magnesium ions in hard water, giving an insoluble soap scum of compounds such as magnesium stearate. This can be demonstrated at home by mixing soap solution with a solution of epsom salts (see here or here).

Kitchen chemistry: fats and oils

Our previous kitchen chemistry post discussed esters. Fats and oils (triglycerides) are an important special case of esters. The alcohol in triglycerides is glycerol, a “triple alcohol” with three OH groups:

The glycerol combines with “fatty acids” (like the one on the right) which resemble acetic acid (left), but with a much longer hydrocarbon chain hanging off the COOH group:

      

The resulting triglyceride esters have three COO groups:

Fatty acids have important dietary implications, and they can be classified in dietary terms, but the most common classifications are chemical. The three main chemical classifications all refer to the presence of carbon-carbon double bonds:

One important classification is in terms of the number of carbon-carbon double bonds:

  • Saturated fatty acids have no carbon-carbon double bonds (they are “saturated” in the sense of containing as much hydrogen as possible). Fats made from saturated fatty acids (“saturated fats”) tend to be solid at room temperature, because the straight-line molecules stick to each other. Saturated fats are usually of animal origin (although coconut oil and palm oil are also mostly saturated).
  • Monounsaturated fatty acids have exactly one carbon-carbon double bond per molecule. Oleic acid (in e.g. olive oil) is an example.
  • Polyunsaturated fatty acids have two or more carbon-carbon double bonds per molecule. Linoleic acid (in e.g. sunflower oil) is an example.
Saturated fatty acid
Monounsaturated fatty acid   Polyounsaturated fatty acid

The position of carbon-carbon double bonds is also significant. A common classification counts the position of the first double bond, starting from the “omega” end of the molecule (the end furthest from the oxygen atoms). For example, there are omega-3, omega-6, and omega-9 fatty acids:

Omega-3 fatty acid  Omega-6 fatty acid  Omega-9 fatty acid

Finally, the orientation of double bonds is very important. In cis fatty acids, there are two hydrogen atoms on the same side of the double bond, giving a molecule with a “kink.” In trans fatty acids, the two hydrogen atoms are on the opposite sides of the double bond, giving a straight-line molecule (trans fats are usually synthetic, resulting from the partial hydrogenation of vegetable oils). Since the straight-line molecules tend to stick together, “trans fats” (made from trans fatty acids) tend to be solid at room temperature, while “cis fats” (made from cis fatty acids) tend to be liquid – that is, oils (such as olive oil) rather than fats:

Cis fatty acid   Trans fatty acid

Fatty acids can also be classified in dietary terms. The body needs fatty acids, but can manufacture most of them itself. Essential fatty acids are fatty acids that must be included in the diet. Both ALA and linoleic acid (found in vegetable oils) are essential. Adult men need about 13 grams of linoleic acid and 1.3 grams of ALA per day. Some omega-3 fatty acids from oily fish should also be included in the diet.


Fish oil capsules contain the omega-3 fatty acids EPA and DHA.

In contrast, trans fats are particularly unhealthy, and should be eliminated from the diet completely. This can be difficult in the USA, since pre-packaged foods there often contain trans fats (because of their long shelf life). Monounsaturated and polyunsaturated oils are also preferable to solid saturated fats.


Butter contains a large amount of saturated fat.