Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.7 Dry cleaning 887
the pigments, binding them to fabric. In order to remove this spot, first the oil has to be dissolved
then pigments can be removed.
The removal of solvent soluble “soiling” in dry cleaning is very simple. It will be just
dissolved by physical action. Polymers can be removed by the combined activity of detergents,
water and mechanical action. The efficiency of this process depends on the quality
and concentration of detergent, the amount of water added into the system, and the operating
time. Higher water additions and longer operating time increase the risk of shrinkage
and felting of the textiles.
Water soluble “soiling” can be removed by water, emulsified in the solvent. The efficiency
here depends on the emulsifying character of the detergent and the amount of water
addition. The more water is emulsified in the solvent, the higher is the efficiency of the process
and the higher is the risk of shrinkage and felting of the textiles.
Pigments can be removed by mechanical action and by dispersing activity of detergents.
The higher the intensity of mechanical action, influenced by cage diameter, rotation,
gravity of the solvent and operating time and the better the dispersing activity of the detergent,
the better is the removal of pigments. The same parameters influence the care of the
textiles. The better the cleaning efficiency, the higher the risk for textile damage.
14.7.1.5 Activity of detergents in dry cleaning
The main component of detergents is surfactant. The eldest known surfactant is soap.
Chemically soap is an alkaline salt of fatty acid. Characteristic of soap (and surfactant) is the
molecular structure consisting of apolar - hydrophobic - part (fatty acid) and a polar - hydrophilic
- part (-COONa), causing surface activity in aqueous solution.
Surface activity has its function in the insolubility of the hydrophobic part of molecule
in water and the hydrophilic part of molecule influences water solubility. This gives a tension
within soap molecules in water forming layers on every available surface and forming
micelles if there is a surplus of soap molecules compared to the available surface. This soap
behavior stands as an example of mechanism of action of surfactants in general.
Micelles of surfactants in water are formed by molecular aggregates of surfactants oriented
in such a way that the hydrophobic parts are directed internally, so that the hydrophilic
parts are directed outwards. In this way the aggregates form spheres, cylinders, or laminar
layers, dependent on its concentration.
Because of this behavior, it is possible to remove oil, fat or grease from substrates in
aqueous solutions, if surfactants are present. The surfactants act to disperse the oil into
small particles and build up micelles around these particles, so that oil, fat or grease incorporated
inside the micelle (Figure 14.7.1). If aggregates are small,
the solution is clear. If aggregates are larger,
the solution (emulsion - type oil in water) becomes
milky.
In the same way, but in the opposite direction,
surfactants form micelles in solvents
(Figure 14.7.2). In this case, not the hydrophobic,
but the hydrophilic part is directed
Figure 14.7.1. Schematic
diagram of surfactants aggregated
in micelle in
aqueous solution.
internally and the hydrophobic outwards.
Emulsions in this case are not formed by oil
in water, but by water in oil (solvent).
Figure 14.7.2. Schematic
diagram of surfactants aggregated
in micelle in solvent
solution.