Krafft

Soap Bars. In soap bars the primary surfactant is predominantly sodium salts of fatty acids. These products typically contain between 70 and 85% soap. Occasionally, potassium soap ( 5-30%) is included in the formulation to increase the solubiUty of the soap and, hence, the bar s lathering properties. The low Krafft temperatures for potassium soap are the basis for the lather enhancement, but also limits their content in bars. 

Early in the 17th century, there was still vigorous disagreement as to the feasibility of empty space Descartes denied the possibility of a vacuum. The matter was put to the test for the first time by Otto von Guericke (1602-1686), a German politician who devoted his brief leisure to scientific experimentation (Krafft 1970-1980). He designed a crude suction pump using a cylinder and piston and two flap valves, and... 

Krafft, F. (1970-1980) Article on Otto von Guericke, in Dictionary of Scientific Biography, vol. 5, ed. Gillispie, C.C. (Charles Scribner s Sons, New York) p. 574. 

The Krafft point has practical implications for the solubility of surfactants. Only above the Krafft temperature can concentrated surfactant solutions be prepared. Otherwise, on cooling a hot surfactant solution a sudden precipitation may occur. A linear correlation between the Krafft temperature TK (°C) and the carbon number nc of sodium alkanesulfonates C10-C22 is given by the following equation ... 

The curve shown in Fig. 6 for sodium dodecyl sulfate is characteristic of ionic surfactants, which present a discontinuous and sharp increase of solubility at a particular temperature [80]. This temperature is known as the Krafft temperature. The Krafft temperature is defined by ISO as the temperature [in practice, a narrow range of temperatures] at which the solubility of ionic surface active agents rises sharply. At this temperature the solubility becomes equal to the critical micelle concentration (cmc). The curve of solubility vs. temperature intersects with the curve of the CMC vs. temperature at the Krafft temperature. 

The Krafft temperature for a homologous series rises as the chain length increases but with independent and different curves for even or odd members [80]. Figure 7 shows the Krafft temperatures of sodium alcohol sulfates ranging from 11 to 18 carbon atoms where the two different curves for odd- and even-numbered series and the alternancy of values can be clearly observed. Lange and Schwuger explain this effect as caused by the different crystal structures of the even and odd numbers. 

Krafft temperatures depend not only on chain length but on the cation. Eth-oxylation of the base alcohol reduces the Krafft temperature due to the higher solubility of the sulfate. Calcium and other earth alkaline metals produce an increase of the Krafft temperature that is significantly reduced by ethoxylation of the alcohol. The decrease is more significant for alkaline earth metals than for alkaline cations as shown in Table 6 [81,82], although it should be noted that, according to other workers, sodium dodecyl sulfate has a Krafft temperature of 16°C. 

Different authors sometimes report different values, as commonly happens with many other physical properties, which may be explained by the purity of the product. Mixtures of isomeric surfactants generally have Krafft temperatures... 

FIG. 6 Solubility of sodium dodecyl sulfate vs. temperature. Krafft temperature [80]. 

TABLE 6 Krafft Temperatures (°C) of Dodecyl Ether Sulfates... 

TABLE 7 Krafft Temperatures of Sodium Alcohol and Alcohol Ether Sulfates... 

Table 6 shows the Krafft temperature of C18 AOS (about 70% alkenesulfonate and 30% hydroxyalkanesulfonate) and reference compounds in water. Although C18 AOS is a mixture, its Krafft temperature is clearly defined [41] and found to be 23-24°C. The Krafft temperatures of the pure main components, alkene-and hydroxyalkanesulfonates are significantly higher at 54 and 51 °C the value for the alkenesulfonate is in line with a Krafft temperature of 57 °C for octade-canesulfonate. 

The low Krafft temperature of C18 AOS may then be explained by the presence in the mixture of small amounts of highly water-soluble isomers and disulfonates [41]. 

AOS is a useful surfactant system for the formulation of soap bars. The effect of AOS on the Krafft temperature of soap is shown in Table 27 AOS lowers the Krafft temperature of soap. AOS can also be used to cosolubilize soap in water thereby reducing the waste of insoluble soap as shown in Table 28. 

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