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A surface active agent (= surfactant) is a substance which lowers the surface tension of the medium in which it is dissolved, and/or the interfacial tension with other phases, and, accordingly, is positively adsorbed at the liquid/vapour and/or at other interfaces. The term surfactant is also applied correctly to sparingly soluble substances, which lower the surface tension of a liquid by spreading spontaneously over its surface.

A soap is a salt of a fatty acid, saturated or unsaturated, containing at least eight carbon atoms or a mixture of such salts.

A detergent is a surfactant (or a mixture containing one or more surfactants) having cleaning properties in dilute solution (soaps are surfactants and detergents).

A syndet is a synthetic detergent; a detergent other than soap.

An emulsifier is a surfactant which when present in small amounts facilitates the formation of an emulsion, or enhances its colloidal stability by decreasing either or both of the rates of aggregation and coalescence.

A foaming agent is a surfactant which when present in small amounts facilitates the formation of a foam, or enhances its colloidal stability by inhibiting the coalescence of bubbles.

The property of surface activity is usually due to the fact that the molecules of the substance are amphipathic or amphiphilic, meaning that each contains both a hydrophilic and a hydrophobic (lipophilic) group22.

Surfactants in solution are often association colloids, that is, they tend to form micelles, meaning aggregates of colloidal dimensions existing in equilibrium with the molecules or ions from which they are formed.

If the surfactant ionizes, it is important to indicate whether the micelle is supposed to include none, some, or all of the counterions. For example, degree of association refers to the number of surfactant ions in the micelle and does not say anything about the location of the counterions: charge of the micelle is usually understood to include the net charge of the surfactant ions and the counterions bound to the micelle: micellar mass and micellar weight usually refer to a neutral micelle and therefore include an equivalent amount of counterions with the surfactant ions.

The relative molecular mass ($ M_r$) of a micelle is called the relative micellar mass or micellar weight and is defined as the mass of a mole of micelles divided by the mass of $ \frac{1}{12}$ mole of $ ^{12}$C.

There is a relatively small range of concentrations separating the limit below which virtually no micelles are detected and the limit above which virtually all-additional surfactant forms micelles. Many properties of surfactant solutions, if plotted against the concentration appear to change at a different rate above and below this range. By extrapolating the loci of such a property above and below this range until they intersect, a value may be obtained known as the critical micellization concentration (critical micelle concentration), symbol $ c_M$, abbreviation c.m.c. As values obtained using different properties are not quite identical, the method by which the c.m.c. is determined should be clearly stated23.

Solubilization. In a system formed by a solvent, an association colloid and at least one other component (the solubilizate), the incorporation of this other component into or on the micelles is called micellar solubilization, or, briefly solubilization. If this other component is sparingly soluble in the solvent alone, solubilization can lead to a marked increase in its solubility due to the presence of the association colloid. More generally, the term solubilization has been applied to any case in which the activity of one solute is materially decreased by the presence of another solute.

Concentrated systems of surfactants often form liquid crystalline phases, or mesomorphic phases. Mesomorphic phases are states of matter in which anisometric molecules (or particles) are regularly arranged in one (nematic state) or two (smectic state) directions, but randomly arranged in the remaining direction(s).

Examples of mesomorphic phases are: neat soap, a lamellar structure containing much (e.g. 0.75%) soap and little (e.g. 0.25%) water; middle soap, containing a hexagonal array of cylinders, less concentrated (e.g. 0.50%), but also less fluid than neat soap.

A soap curd is not a mesomorphic phase, but a gel-like mixture of fibrous soap-crystals (`curd-fibers') and their saturated solution.

Myelin cylinders are birefringent cylinders which form spontaneously from lipoid-containing material in contact with water.

Krafft point, symbol $ t_K$ (Celsius or other customary temperature), $ T_K$, (thermodynamic temperature) is the temperature (more precisely, narrow temperature range) above which the solubility of a surfactant rises sharply. At this temperature the solubility of the surfactant becomes equal to the c.m.c. It is best determined by locating the abrupt change in slope of a graph of the logarithm of the solubility against $ t$ or $ 1/T$.

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