Anionic Tensides in Powdered Laundry Detergent

Demonstration Experiment on Video

Anionic Tensides in Powdered Laundry Detergent

Objective: Surface active Agents

Peter Keusch

German version

Supermarket products:>
powdered detergent ("rei F3", Firm: fit GmbH);
ingredients: > 5 -15 % anionic tensides, polycarboxylate, enzyme (amylase, cellulase), citrate, perfume

Chemicals: methylene blue solution 0.1 %
ethylacetate
sodium carbonate

Apparatus and glass wares:
2 conical measures, graduated, 350 mL
Erlenmeyer flask 50 mL
2 beakers 100 mL
2 beakers 50 mL
2 snap-cap vials 20 mL
2 glass stirring rods

Hazards and safety precautions:

Ethyl acetate is highly flammable. Harmful if swallowed in quantity. Vapours may cause drowsiness.

Safety glasses and protective gloves must be worn. Good ventilation required.

Experimental procedure:

150 mL of dist. water are placed in each of two conical measures. 30 mL of saturated sodium carbonate solution, 70 drops of an aqueous 0.1% methylene blue solution and 90 mL of ethylacetate are added to the conical measures while stirring. The mixtures are allowed to separate into upper and lower phases. Now a spatula full of the powdered detergent is added to one of the two mixtures while stirring.

Results:

In the sample without washing powder the lower aqueous phase has a blue appearance. In the sample containing washing powder also the upper organic phase is blue.

Video clip (Download RealPlayer .rm file)

Discussion and background:

In the sample without tenside methylene blue remains in the aqueous phase.

Anionic tensides react with cationic dyes such as methylene blue under formation of an ion pair which is transfered into the organic phase (ethylacetate). The tenside is able to partition methylene blue between the two phases.

An application of continuous extraction is the determination of anionic surfactants in natural waters, in which methylene blue ion-pairs are extracted into chloroform. The anionic detergent complexes with methylene blue in alkaline solution to produce a blue complex which is extracted into chloroform. The extract is then washed with an acid solution of methylene blue before being measured at 660 nm using a colorimeter.

Almost all cleaning agents and washing powders contain tensides. Tensides are wash-active substances. They promote that substances insoluble in water can be emulgated or dispersed in it. The detergent property is based on the molecular configuration of these compounds. Anionic tensides consist of two functional groups: a polar, hydrophilic "head" (carboxylate group) and a non-polar, hydrophobic "tail" (hydrocarbon chain).

Tensides fulfill two tasks:

· Tensides reduce the surface energy of water. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules. In contrast to the molecules within a liquid whose cohesive forces cancel out, the molecules at the surface of a liquid only net inward forces. At the air/liquid interface the tensides orient the hydrophobic chains out in the air, while the hydrophilic headgroups are anchored in the aqueous phase. The tensides push themselves between the water molecules and hence reduce surface tension.

· A tenside can dissolve grease in water. The lipohilic part clings to the grease forming aggregates (micelles) with the dirt in the center and the tenside molecules around. The hydrophilic part adheres to the water. In this way the whole aggregate became water soluble, although there is a lot of dirt inside. On the other hand the hydrophilic part ot the tensides molecules inhibits the aggregation of the micelles.

Reference:
Demonstration Experiments on Video Phase Transfer Catalysis (PTC) - Host-Guest Chemistry

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