Chemistry Reference
In-Depth Information
Total Surfactant
Micelles
cmc
Monomers
Monomer Solubility Curve
Krafft Temperature
Temperature (°C)
Figure 4.3.
Temperature-solubility relationship for typical ionic surfactants.
referred to as the Krafft temperature, T
K
. Below that temperature, the solubility of
the surfactant is determined by the crystal lattice energy and the heat of hydration
of the system. The concentration of the monomeric species in solution will be lim-
ited to some equilibrium value determined by those properties. Above T
K
, the solu-
bility of the surfactant monomer increases to the point at which aggregate formation
may begin, and the aggregated species (e.g., a micelle) becomes the thermodyna-
mically favored or predominant form in solution.
The micelle may be viewed, to a first approximation, as structurally resembling
the solid crystal or a crystalline hydrate, so that the energy change in going from the
crystal to the micelle will be less than the change in going to the monomeric species
in solution. Thermodynamically, then, the formation of micelles favors an overall
increase in solubility. The concentration of surfactant monomer may increase or
decrease slightly at higher concentrations (at a fixed temperature), but micelles
will be the predominant form of surfactant present above a critical surfactant con-
centration, the critical micelle concentration (cmc). The apparent solubility of the
surfactant, then, will depend on not only the solubility of the monomeric material
but also the solubility of the micelles or other aggregate structures. A schematic
representation of the temperature-solubility relationship for ionic surfactants is
shown in Figure 4.3.
The Krafft temperatures of a number of common ionic surfactants are given in
Table 4.1. It can be seen from the data that T
K
can vary as a function of both the
nature of the hydrophobic group and the character of the ionic interactions between
the surfactant and its counterion. It should be noticed that no data are listed for non-
ionic surfactants. Nonionic surfactants, because of their different mechanism of
solubilization, do not exhibit a Krafft temperature. They do, however, have a char-
acteristic temperature-solubility relationship in water that causes them to become
