Krafft point surfactant mixtures

Alkyl ether sulfates are/after alkyl benzene sulfonates(LAS),the group of technically important anionic surfactants with the largest production voluJne and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed,for example,in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application. 

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. 

A simple yet highly effective nonbleach, two-step process had been described for purifying palm Cie ig potassium methyl ester sulfonates [54]. In the first step, water is added to the impure surfactant mixture. The temperature of the system is maintained above the Krafft point of the surfactant, so that the surfactant and associated impurities are completely solubilized. Once completely dissolved, the surfactant mixture is allowed to cool, and the potassium-neutralized methyl ester sulfonate precipitates out selectively. In the second step, the purified surfactant is recovered by gravity or pressure filtration or by centrifugation, followed by drying. 

Even if the micelle is regarded as a chemical species, the melting-point model is not correct. In this case, the system is monovariant (/= ), and the Krafft point is determined automatically at 1 atm pressure. That seems reasonable for a single surfactant solution, but when the model is applied to a mixed surfactant solution, it is found to be incorrect. Figure 6.2 shows the phase diagram of a water/sodium dodecyl sulfate (SDS)/manganese (II) dodecyl sulfate [Mn (08)2] system, with temperature as the ordinate. The CMC of the surfactant mixture gives a curved surface between the... 

CMC-temperature curves of each component. By analogy with the two-component phase diagram, we can conclude from this three-component phase diagram that the rational Krafft point for a binary surfactant mixture is determined by the intersection of the mixed CMC surface with both humped surfaces of the surfactant solids. 

Fig. 6.10 Krafft point of binary surfactant mixtures (I) C8F17SO3NH4 + CgFiySOsK (II) C8F17SO3H + C8F17SO3K (III) C8F17SO3NH3C2H4OH + C8F17SO3K. (From Ref. 59. Reproduced by permission of the American Chemical Society.)...

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