Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 46

Solubility
Curve
CMC Curve
Krafft
Point
Temperature/C
FIGure 3.6 Solubility (Krafft point: KP) of ionic (anionic or cationic) surfactants in water
(as a function of temperature).
of surfactants is also dependent on the charge present on the polar group. Ionic sur-
factants exhibit different solubility characteristics compared to nonionic surfactants,
with regard to dependence on temperature.
Ionic surfactants: The solubility of all ionic surfactants (both anionics, that is,
negatively charged, and cationics, that is, positively charged) is low at low tempera-
ture, but at a specific temperature it suddenly increases (Figure 3.6). For instance,
the solubility of SDS at 15°C is about 2 g/L. This temperature is called Krafft point
(KP), and can be obtained by cooling an anionic surfactant solution (ca. 0.5 M)
from a high to a lower temperature until cloudiness appears sharply. KP is not very
sharp in the case of impure surfactants that are generally found in the soap and
detergent industry.
In fact, solubility near the KP is almost equal to the critical micelle concentration
(CMC). The magnitude of KP is dependent on the chain length of the alkyl chain
(Figure 3.7).
The linear dependence of KP on alkyl chain length is very clear. The KP for C12
sulfate is 21°C, and it is 34°C for C14 sulfate. It may be concluded that KP increases
by approximately 10°C per CH 2 group. It is also interesting to note that the KP of C8
is found from extrapolation to be −3.5°C. In fact, for C8 it is not possible to measure
this KP from experiments.
Since no micelles can be formed below the KP, it is important that one keep this
information in mind when using any anionic detergent. Therefore, the effect of vari-
ous parameters on KP needs to be considered in the case of ionic surfactants. The
following are some of these:
Alkyl chain length (KP increases with alkyl chain length).
KP decreases if a lower-chain surfactant is mixed with a longer-chain surfactant.
Nonionic surfactants: The solubility of nonionic surfactants in water is completely
different from that of charged surfactants (especially concerning the effect of