The Effect of Added Salts on the Solubilities and Krafft Points of Sodium Dodecyl Sulfate and Potassium Perfluoro-octanoate

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Abstract
The solubilities of sodium dodecyl sulfate and potassium perfluoro-octanoate in the presence of added salts have been measured as a function of the temperature. The solubility decrease caused by the added salt was remarkable at temperatures a few degrees higher than the Krafft point of a system which contained no salt. The solubility of the solid agent below the Krafft point decreases, approximately inversely proportionally to the concentration of the gegenion as was expected from the solubility product principle, whereas in these experiments the critical micelle concentration (the single-dispersion solubility of a liquid agent), the CMC, decreases inversely proportionally to about the 0.6th power of the gegenion concentration. Hence, the point of the intersection of the solute-solid and solute-liquid equilibrium curves, i. e., the melting point of a hydrated solid agent, shifts to a higher temperature upon the addition of salt. The Krafft-point elevation caused by added salt is explained by the model; the Krafft point is the melting point of a hydrated solid agent, and the formation of the micelle is regarded as a pseudo-phase separation. It is easy to evaluate graphically the Krafft-point elevation in the presence of a 1-1 type salt by applying the above experimental relations.