In
colloidal chemistry
Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with ''colloids'', heterogeneous systems consisting of a mechanical mixture of particles between 1 nm ...
, the Krafft temperature (or Krafft point, after German chemist
Friedrich Krafft) is defined as the minimum
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
at which the formation of
micelle
A micelle () or micella () ( 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 colloidal system). ...
s occurs in a
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Solu ...
of dissolved
surfactant
Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word ''surfactant'' is a Blend word, blend of "surface-active agent",
coined in ...
. It has been found that
solubility
In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...
at the Krafft point is nearly equal to
critical micelle concentration
In colloidal chemistry, 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). 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 Phase change may refer to:
* Phase transition
In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term ...
below which the surfactant remains in
crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...
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 as ...
, with the solution becoming cloudy or opaque due to the surfactant molecules undergoing
flocculation
In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...
.
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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