Krafft effect

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. 

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. 

Sodium a-sulfonated fatty acid esters of long-chain alcohols have a structural effect on the Krafft point different from that of amphiphiles with short alkyl chains [60]. In a series of homologs with the same total carbon number the Krafft points are highest when the hydrophilic alkyl chain lengths in the a-sulfonated fatty acid and the alcohol are fairly long and equal. In this case the packing of the molecules becomes close and tight. 

Addition of as little as 5% soap to an amphoteric LSDA of limited water solubility (high Krafft point) brought about a substantial lowering of the Krafft point and thus markedly improved water solubility. On the other hand, addition of 10% of amphoteric LSDA to sodium palmitate lowered the Krafft point of the soap by 10-14°C. Addition of anionic LSDA to sodium palmitate showed smaller Krafft point depression. Addition of a builder-type salt, such as sodium metasilicate, had essentially no effect on the Krafft points of soap-LSDA mixtures. 

Dietrich KN, Krafft KM, Bier M, et al. 1986. Early effects of fetal lead exposure Neurobehavioral findings at 6 months. International Journal of Biosocial and Medical Record 8 151-168. 

Dietrich KN, Krafft KM, Bomschein RL, et al. 1987a. Low-level fetal lead exposure effect on neurobehavioral development in early infancy. Pediatrics 80 721-730. 

Dietrich KN, Krafft KM, Shukla R, et al. 1987b. The neurobehavioral effects of early lead exposure. Monogr Am Assoc Ment Defic 8 71-95. 

An especially effective reduction of the Krafft Point results from the insertion of ether groups into the molecule of the anionic surfactant. In table I this is examplified with Na dodecyl sulfate and Na-tetra-decyl sulfate in comparison to various n-alkyl ether sulfates of the same chain length (10). As a measure of the Krafft Point, a temperature is deTined at which a 1 7o solution dissolves clearly. By the incorporation of oxyalkylene groups into the molecule, the Krafft -Point and the melting point are greatly depressed. This depression is especially effective if there is branching in the oxyalkylene groups. 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

S. Marie Bertilla, J.L. Thomas, P. Marie, M.P. Krafft, Co-surfactant effect of a semi-fluorinated alkane at a fluorocarbon/water interface. Impact on the stabilization of fluorocarbon-in-water emulsions, Langmuir 20 (2004) 3920-3924. 

For ionic surfactants micellization is surprisingly little affected by temperature considering that it is an aggregation process later we see that salt has a much stronger influence. Only if the solution is cooled below a certain temperature does the surfactant precipitate as hydrated crystals or a liquid crystalline phase (Fig. 12.4). This leads us to the Krafft temperature1 also called Krafft point [526]. The Krafft temperature is the point at which surfactant solubility equals the critical micelle concentration. Below the Krafft temperature the solubility is quite low and the solution appears to contain no micelles. Surfactants are usually significantly less effective in most applications below the Krafft temperature. Above the Krafft temperature, micelle formation becomes possible and the solubility increases rapidly. 

Nonionic surfactants tend to show the opposite temperature effect As the temperature is raised, a point may be reached at which large aggregates precipitate out into a distinct phase. The temperature at which this happens is referred to as the cloud point. It is usually less sharp than the Krafft temperature.2 The phenomenon that nonionic surfactants become less soluble at elevated temperature will be important when we discuss the phase behavior of emulsions. 

Han, S. K., S. M. Lee, M. Kim, and H. Schott. 1989. Effect of protein denaturants on cloud point and Krafft point of nonionic surfactants . Colloid Interface Sci132 444-450. 

The hydrophilic part of the most effective soluble surfactants (e.g. soaps, synthetic detergents and dyestuffs) is often an ionic group. Ions have a strong affinity for water owing to their electrostatic attraction to the water dipoles and are capable of pulling fairly long hydrocarbon chains into solution with them for example, palmitic acid, which is virtually un-ionised, is insoluble in water, whereas sodium palmitate, which is almost completely ionised, is soluble (especially above its Krafft temperature - see page 93). 

In comparison to aqueous soap solutions there exist relatively few determinations of Krafft points in nonpolar surfactant solutions. An interesting example is due to Addison and Furmidge21 who determined the solubilities of four alkylpyri-dinium iodides (dodecyl-, tetradecyl-, hexadecyl-, and octadecyl-) in xylene (mixture of isomers, b.p. 138 °C). The solubilities were plotted against 1/T (see Fig. 22). The breaks in the curves and the chain length effect (which is much less pronounced compared to aqueous solutions) are clearly shown. Similar experiments have been reported by Mehrotra et al.146 on copper soaps in various organic solvents and by Malik et al.140 on cobalt hexamine soaps. 

Whilst studying the effects of additives in the PK reaction, Krafft and coworkers were able to isolate an internally stabilised pentacarbonyl intermediate (75 - Scheme 27).86... 

To explain this observation, it was suggested that electron density differences at the acetylenic carbons can combine with steric effects to determine the regiochemical outcome of the reaction. Krafft concludes electronic effects seem to play a contributing role in determining the regiochemical outcome of the Pauson-Khand reaction, but, steric influences over the transition state apparently exert a more powerful directing effect. ... 

The first reason lies in the fact that the interaction between solvent molecules (usually water) is stronger than the interaction between the solvent and the solute. This effect alone would lead to a precipitation of the solute. In the case of amphiphiles which form micelles, however, the head groups are strongly hydrated and repulse each other. The hydration forces and steric forces which are made responsible for this repulsion effect prevent crystallization above the Krafft point and also above the cmc. Where the formation of 3D crystals is impeded, the smallest possible droplet is formed, removing the alkyl chains from the solvent. The interactions between solvent molecules are therefore disturbed to a minimal extent, allowing the head groups to be solvated with a minimal entropy loss. It is irrelevant whether the solvent contains clusters or not. Micelle formation only occurs as a result of a solvation of head groups and non-solvation of a solvophobic core. ... 

Fig. 9 is a schematic phase diagram of a dilute aqueous cationic surfactant solution showing temperature and concentration effects on its microstructures. When the temperature is lower than the Krafft point [the temperature at which the solubility equals the critical micelle concentration (CMC)], the surfactant is partially in crystal or in gel form in the solution. At temperatures above the Krafft point and concentrations higher than the CMC, spherical micelles form in the surfactant solution. With further increase in concentration and/or on addition of counterions, the micelles form cylindrical rods or threads or worms with entangled thread-like and sometimes branched threadlike structures. 

By treating the Krafft point as the melting point of the hydrated solid surfactant, the partition coefficient of the solute can be calculated from its effect on the Krafft point. Simple thermodynamic considerations lead to the following relationship at low mole fractions of solute in the micellar phase ... 

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