Preparation and Properties of A Soap: Objective
Preparation and Properties of A Soap: Objective
Preparation and Properties of A Soap: Objective
OBJECTIVE:
To understand the process of soap production and study the properties of soap.
BACKGROUND:
Soap- and detergent-containing products are found in many colors, fragrances,
and textures. Years of research have led to the incredible selection of soaps that we
encounter in the market today, but the basics of soap formation have not changed over the
years; the synthesis begins with fats and oils.
On the chemical level, fats and oils are referred to as triglycerides. Triglycerides
contain the ester functional group, which comes from the reaction of the alcohol,
glycerol, with three long-chain carboxylic acids, the fatty acids.
When the alkyl groups (R groups) of a triglyceride come from mostly saturated
fatty acids such as stearic acid, the fat is a solid at room temperature; this is usually a
characteristic of animal fats. If the alkyl groups are mainly unsaturated fatty acids, the fat
is a liquid at room temperature. Example of fatty acids found in these oils include oleic
and linoleic acid.
When a triglyceride is hydrolyzed in a basic medium, the products of the reaction
are glycerol and the salts of the respective fatty acids. This type of reaction is often
termed saponification. The fatty acid salts that form make up the substance soap. More
often than not, the soap consists of a mixture of fatty acid salts. Because soaps are salts
of strong bases (usually NaOH, lye, or KOH, potash) and weak acids, they should be
weakly alkaline in aqueous solution. However, soap with excess free alkali can cause
damage to skin, silk, or wool.
One way to help ensure that soap has a pH close to neutral is to use a
stoichiometric amount of lye in the preparation. Excess sodium hydroxide will speed the
reaction rate and ensure that no oil remains unconverted to its free fatty acids but results
in a finished product too basic to use safely. Home soap makers typically take advantage
of tables of saponification values, which estimate the amount of lye needed for a
stoichiometric ratio, and slowly bake their reactions for several hours in a warm oven to
ensure complete conversion of the oils to soap. In order to speed up the process, we will
use higher temperatures but still make an effort to make soaps with close to neutral pH by
calculating appropriate amounts of sodium hydroxide to use for near stoichiometric ratios
of oil to lye. The table below provides alkali ratios, which are similar to tradition
saponification values, for a number of possible common oils. Saponification values for
soaps made from a mixture of oils must be calculated based on the ratio of each of the
oils used in the preparation.
A major drawback associated with the use of soap is the soap scum that can form
in hard water. Calcium, iron, and magnesium ions are abundant in hard water. These ions
form an insoluble precipitate with soap. A second disadvantage is that, in acidic solution,
soap is converted to free fatty acids and therefore loses its cleansing action.
Consequently, soaps have been largely replaced by synthetic detergents.
Soaps and detergents possess a type of structure that allows for dual cleaning
action. One end of the compound is a long hydrocarbon chain that is water insoluble. The
other end of the compound is a very water-soluble salt structure. Dirt and oils are similar
in structure to the hydrocarbon portion of the soap molecule and are dissolved. At the
same time, soap is water-soluble because of its hydrophilic portion. So, once the soap
dissolves the dirt and oils, it can then be washed off with water.
In the process described above, the formation of micelles takes place. Micelles are
very small droplets that form by the action of soaps. The hydrophobic part of the soap
surrounds the dirt, which the polar head of the soap remains on the outside in the water.
Since soap can form micelles, it has the ability to form semi-stable emulsions. Emulsions
are mixtures of two immiscible solvents; the greasy layer of the dirt is dispersed in the
water layer via the soap micelles.
PROCEDURE:
A. SYNTHESIS OF SOAP:
B. PROPERTIES OF SOAP
Hard water reactions and alkalinity test - Place some soap in a 50-mL beaker containing
25 mL of water. Warm the beaker to dissolve the soap. Pour 5 mL of the soap solution
into each of five test tubes.