Plate forme d'enseignement à distance

Preparation of a colloid: synthesis of a surfactant

1- Objectives of the practical work:

The first problem that arises is to decide exactly what the meaning of the term synthetic detergent is. A detergent is a formulation comprising the essential constituents (surface active agent). A surfactant, or surfactant, expresses its main characteristic which is to control the surface tension in colloidal suspensions. These are amphiphilic molecules or ions (which express two distinct affinities), of elongated shape which have a long aliphatic chain (tail) having a hydrophobic character (which leaks water) and a hydrophilic polar end (head) (which loves water). They have the property of being soluble in water and of aggregating, particularly at interfaces between water and other substances that are poorly soluble in water. They are present in many products: motor oils, pharmaceutical products, soaps and detergents.

2- Introduction:

The addition of a surfactant substance substantially reduces the surface tension of water by the arrangement that these molecules adopt in the liquid. The polar part immersed in water and the hydrophobic part leaks water. It is this arrangement which is at the origin of the wetting, foaming and emulsifying powers. Furthermore, beyond a certain concentration of surfactants, called critical micellar concentration, the surface of the liquid becomes saturated. The detergency power originates in particular from the formation of micelles. The representation below perfectly illustrates this power that surfactant molecules have. (A) Formation of the micellar structure. (B) attack on grease (greasy dirt) by the hydrophobic part of the surfactants. (C) Grease is drained away with the wash water.

Figure 1: The detergent and foaming power of a surfactant

The foaming power is manifested by the reinforcement, through surfactant molecules, of the thin film of water which forms the bubbles. We will speak of "liquid foam" when a gas is dispersed in a liquid, like shaving foam or shampoo foam. A foam is called "solid" when a gas is dispersed in a solid, such as, for example, the metal foams used in the automobile industry in order to reduce the weight of vehicles.

3- Experimental part:

Reflux heating assembly: heating mantle, flask (250 mL), riser, condenser, tongs, test tube (25 mL), stemmed glass (250 mL), Erlenmeyer flask (100 mL), watch glass, spatula, accessories filtration, glasses and gloves. Reagents: distilled water, edible oil (60 mL), saturated sodium chloride solution (60mL), (60mL) 95% ethanol (10 mol/L), saturated sodium hydroxide solution (60 mL), pumice stone .

3.1. Operating mode:

- Step 1: Start by making the assembly below. Using a graduated cylinder, take 40mL of a 10 mol/L sodium hydroxide solution, 30mL of edible oil (olive or sunflower) and 30mL of ethanol. Pour all of these reagents into the flask (see diagram below). Add a few grains of pumice and start heating at reflux. Maintain heating for 30 min with careful and regular monitoring of the boiling which must be gentle. At the end of the reaction, stop heating and allow the reaction mixture to cool. Pour the contents of the flask into a large stemmed glass containing salt water (30 mL of NaCl solution) and remember to mix well using a glass stirrer. You will notice that the soap becomes solid and separates from the solution because it is very poorly soluble in salt water.

- Step 2: Carry out filtration to collect the soap and dry it between two sheets of paper. Weigh the product obtained.

- Step 3: To test the foaming power of the surfactant, put a little of it in a small glass jar, add distilled water and shake vigorously using a glass stirrer.

Questions:

1- Give the definition and properties of a surfactant.

2- Why do we use salt water?

3- Discuss the result relating to the foaming test

4- Using the illustration on detergent power, explain why it is impossible to remove an oily stain with only water.

5- Describe the mechanism of formation of micelles.