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.

The Great Lakes, unfrozen

The NASA image above (click to zoom) shows the Great Lakes in Autumn 2011. The image is again from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite. Lake Michigan and Lake Huron are coloured light blue from sediment brought to the surface by winds, while Lake Erie is coloured green by a severe algal bloom. Autumn colours are also visible in some of the forests surrounding the lakes.

Given that it’s autumn here in Australia right now, the image seems vaguely appropriate.

Mars gets closer and brighter…

On April 8, the planet Mars was in opposition. That is, Mars was on the opposite side of the sky to the Sun. Also, on April 14, Mars will be at the closest point to Earth in its orbit (92 million km). A great time to observe the “red planet”!

This video from Slooh explains various aspects of the planet at length:

For the current distance to Mars, see Wolfram’s calculator or the live diagram of the solar system at Fourmilab, which includes images (green lines show orbits below the plane of the ecliptic):

Live Solar System image

The Heartbleed bug

Just a reminder of the Heartbleed bug, which potentially compromises sensitive user data on affected systems. This means some passwords need to be changed now; others later. Some sites, such as LinkedIn and eBay, were apparently not affected.

A number of organisations, such as online banks, have produced somewhat confusing responses to the problem. In general, if your bank claims to have “patched” the problem, you should probably change your password, even if they say not to.

Update: Count on XKCD to see the lighter side of the problem – data stored on clay tablets is perfectly safe:

XKCD also has a good explanation of what the actual problem is.

Accurate to one second in 300 million years

The US National Institute of Standards and Technology has announced a new caesium-based atomic clock, NIST-F2 (see photo above). The clock is accurate to one second in 300 million years. Together with the older NIST-F1 clock, it will serve as the US civilian time standard – underlying time services such as the one at time.nist.gov. The diagram below gives a conceptual view of how network time servers interconnect. Accurate clocks like NIST-F1 and NIST-F2 underpin the whole enterprise.

See also this story in Wired.

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).