Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 47

50

0

10

Sodium Alkyl Sulphates
(number of C atoms)

FIGure 3.7 Variation of KP with chain length of sodium alkyl sulfates.

temperature). It is high at low temperature, but decreases abruptly at a specific tem-
perature called the cloud point (CP) (Figure 3.8). (This term refers to the temperature
at which the solution becomes cloudy.) This means that nonionic detergents will not
be suitable if used above the CP. The solubility of such detergent molecules in water
is due to hydrogen bond formation between the hydroxyl (-OH) and ethoxy groups
(-CH 2 CH 2 O-) and water molecules. At high temperatures, the degree of hydrogen
bonding gets weaker (due to high molecular vibrations), and thus nonionic deter-
gents become insoluble at CP. The solution separates into two phases: a rich water
phase and one of low concentration of nonionic surfactant. The rich nonionic deter-
gent phase is found to have low water content. Experiments have shown that there
are roughly four molecules of bound water per ethylene oxide group (-CH 2 CH 2 O-)
(Birdi, 2008).

Two Phase

CP

LC

0 1

Water - Nonionic (weight fraction)

FIGure 3.8 Solubility of a nonionic surfactant in water (cloud point: CP) (schematic)
dependent on temperature.