Chemicals

Maaike van Roosmalen, a chemical engineer at the Delft University of Technology, has optimised a fabric cleaning method that uses liquid carbon dioxide instead of solvents.

The process uses an amide derived from an amino acid as a cleansing agent and results are already close to those of conventional dry-cleaning.

The dry-cleaning process does not involve water. Instead, various chemicals are used, most of them highly noxious. When heated, one of these solvents – perchloroethylene – produces hydrochloric acid and phosgene, a poison gas used during the First World War. Indications are that it is carcinogenic and may induce genetic defects.

As in normal laundering, the conventional dry-cleaning process uses a drum containing the garments to be cleaned, to which the solvent is added at room temperature and pressure. Cleaning takes about 10 minutes, removing the perchloroethylene from the clothing a further hour. The fabric emerges from the machine dry and without creases, but retains a chemical smell. Dirt remains in the solvent, from which it is removed by distillation. Van Roosmalen’s method uses CO2 combined with detergents developed for the purpose.

CO2 has a low viscosity and high diffusion constant – ideal properties for moving through fabrics. The molecule’s structure makes it hydrophobic, which helps it dissolve oil and grease.

To enhance the solvent properties of the CO2, 25g of water and 250g of isopropyl alcohol are added, and the surfactant – 10g of an amide derived from an amino acid, Amihope LL.

Amines are organic nitrous compounds, in this case long molecules containing an NH2 group. Amines are neutral, but in a reaction with CO2 they form zwitterions. These dipolar ions carry both a positive and a negative charge (NH2+ and COO-). A zwitterion is also referred to as an internal salt, a molecule containing both an acid and a base group.

The effectiveness of the cleaning process are assessed using a spectrophotometer to determine the colour of the cloths in ‘UV-less daylight’.

The results of CO2 washing with added Amihope LL are comparable to those of dry-cleaning using perchloroethylene.

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