Abstract
The solubilities of ionic surfactants, sodium dodecylsulfate and dodecylamine hydrobromide, have been determined measuring the electric conductivity at pressures up to 3000 atm and at several temperatures in the range 7 to 35°C. Influences of pressure and temperature on the solubility were similar on these ionic surfactants. With increase in pressure, the solubility decreases abruptly until the “critical solution pressure” P c , but decreases gradually above this pressure. The critical micelle concentration (CMC) has also been determined under pressure. The relation between the solubility, the CMC, and the pressure is given by a phase diagram. The CMC vs pressure curve and the solubility vs pressure curve cross at a point Q. (The pressure at which the point Q exists is designated P c .) The point Q is found to be the Krafft point at the pressure P c . Micelles above this pressure are in a state of superpressing and metastable. The pressure dependences of the solubility and of the CMC are considered thermodynamically. Comparisons are made among the partial molal volumes of surfactant in micellar state ( V m ), in singly dispersed state ( V s ) and in hydrated solid state ( V c ).
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