Krafft temperature is defined as the minimum temperature from which the
micelle
A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
formation takes place. It is named after German chemist
Friedrich Krafft
Friedrich Krafft (February 21, 1852 in Bonn – June 3, 1923 in Heidelberg) was a German chemist. He studied with Friedrich August Kekulé von Stradonitz, Rudolf Clausius and Gerhard vom Rath.
In colloidal chemistry, the Krafft temperature ...
. It has been found that solubility at the Krafft point is nearly equal to
critical micelle concentration
In colloidal and surface chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants above which micelles form and all additional surfactants added to the system will form micelles.
The CMC is an important c ...
(CMC). Below the Krafft temperature, the maximum solubility of the surfactant will be lower than the critical micelle concentration, meaning micelles will not form. The Krafft temperature is a point of phase change below which the
surfactant remains in crystalline form, even in an aqueous solution. Visually the effect of going below the Krafft point is similar to that of going above the
cloud point
In liquids, the cloud point is the temperature below which a transparent solution undergoes either a liquid-liquid phase separation to form an emulsion or a liquid-solid phase transition to form either a stable sol or a suspension that settles a ...
, with the solution becoming cloudy or opaque due to the surfactant molecules undergoing
flocculation
Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from ...
.
Surfactants in such a crystalline state will only solubilize and form micelles if another surfactant assists it in overcoming the forces that keep it crystallized, or if the temperature increases, thus causing entropy to increase and encouraging the crystalline structure to break apart.
Structural effects
Surfactants are usually composed of a hydrocarbon chain and a polar head group.
Increasing the length of the hydrocarbon chain increases the Krafft temperature because it improves
Van der Waals forces.
Moreover, since Krafft point is related to solid-liquid transition, better-packed polar heads within surfactant crystals increase Krafft temperature.
[H.A. Van Doren, Tailor-made carbohydrate surfactants? Systematic investigations into structure-property relationships of N-Acyl N-Alkyl 1-Amino-1-Deoxy-D-Glucitols, Carbohydrates as Organic Raw Materials III, Wiley-VCH Verlag GmbH, 2007, pp. 255-272.]
References
External links
IUPAC Gold book - Krafft temperature
Colloidal chemistry
Physical chemistry
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