Physical properties and structure
Physical properties depend on the ingredients. Typical simple organic acids are liquids that have a strong smell (pungent), eat at or wear away other substances (corrosive), and dissolve in water (water-soluble). Organic acids with higher molecular weights are generally odorless, waxlike solids. Lower molecular weight acids boil at relatively high temperatures because their molecules are linked together by relatively weak hydrogen bonds.
Carboxylic acids can react to form salts, esters (discussed in the next article), anhydrides (oxygen compounds that unite with water to form an acid or base), acid halides (acids containing one of the halogens—fluorine, chlorine, bromine, iodine, or astatine), and amides (a type of nitrogen-oxygen compound). Carboxylic acids can also be reduced to form aldehydes and alcohols (discussed in the two previous articles).
Soaps are salts of one of the alkali metals (lithium, sodium, potassium, rubidium, cesium, and francium) combined with an organic (fatty) acid. The alkali metal salt is usually sodium hydroxide, called lye or caustic soda. The fatty acid comes from animal fat or a vegetable oil, such as palm oil or olive oil.
In water, the soap molecule has a hydrophobic ("water-fearing") end and a hydrophilic ("water-loving") end. Soaps work by surrounding dirt or grease particles with many soap molecules. The hydrophobic ends dissolve in the grease or dirt particles. The hydrophilic ends point outward, dissolving in the water. Thus, the soap molecules form an emulsion. This is a suspension of the oil or fat in the water. The molecules are "suspended" rather than dissolved. These soap molecules enclose the dirt or grease in a water-soluble envelope called a micelle. In other words, soap molecules package dirt in droplets that can be taken into solution and washed away.
| C More soap anions embed themselves in the grease, forcing it away from the fabric
| A Soap molecule
w H H
\ I I
H—C — C— C-
./ I I
I IH H
Hydrophilic anionic "head"-
Hydrophobic hydrocarbon "tail".
Date: 2015-12-11; view: 817