K. Shinoda and T. Hirai
$842
Ionic Surfactants Applicable in the Presence of Multivalent Cations. Physicochemical Properties Kbzb Shinoda" and Tsuyoshi Hlral Department of Chemistry, faculty of Engineering, Yokohama National University, Ooka-2, Minamiku, Yokohama, Japan 233 (Received March 21, 1977) Publication costs assisted by Yokohama National University
Ordinary ionic surfactants are salted out and cannot be used in water in the presence of multivalent cations, because they do not dissolve in hard water more than their saturation concentrations of multivalent salts of surfactants, Le., neither micellization nor solubilization occurs. Calcium and magnesium salts of surface active anions, ClzHZ50CH2CHzSO4 0.5Ca (or Mg), which can dissolve well in water in the presence of bivalent cations at room temperature were prepared and the physicochemical properties of their aqueous solutions have been studied. The critical micelle concentration (crnc),the solubilizing power for cyclohexane, and the surface tension of aqueous solution above the cmc of ClzH250CHzCH2S04 0.5Ca were respectively 1/9th,4.7 times larger, and 8 dyn/cm lower than those of sodium dodecyl sulfate. A liquid surfactant phase was observed above the Krafft point in the presence of a large amount of bivalent cations or a small amount of trivalent cations. This phenomenon manifests the continuous change from micelle (pseudophase)to liquid surfactant phase (true phase), Le., from finite to infinite aggregation supporting the pseudophase separation model of micellar solution.
Introduction Ordinary ionic surfactants are salted out and cannot be used in hard water, because they are not soluble in the presence of multivalent cations. The surface tension of the aqueous surfactant solution is not depressed well, because a hydrated solid (multivalent cation salt of) surfactant precipitates before the solution reaches the critical micelle concentration (cmc). Micelles are not formed, nor does solubilization of oily substances occur in the solution. It is known that ionic surfactants dissolve well in water forming micelles above their Krafft points1 and the Krafft point is the melting point of hydrated solid surfactant' regardless the number of valencies of gegeni~ns.~The Krafft point of fatty acid soap is enhanced remarkably in the presence of very small amount of multivalent cations, such as calcium ions. In other words, the Krafft points of calcium alkane carboxylates are so high that they cannot dissolve in hard water. In the case of alkyl sulfate, however, the Krafft point is increased by replacing sodium by bivalent cations but as not r n ~ c h . ~The - ~ Krafft point of calcium dodecyl sulfate is about 50 OC. If we could depress the Krafft point of a surfactant below room temperature by some devices, it can be used in the presence of calcium ions and will find many useful applications. The present investigation was undertaken to study the basic pbysicochemical properties of such compounds, C12H25(OCH2CH2)nS04 0.5Me, in water. Experimental Section Materials. Pure mono-, di-, tri-, and hexaoxyethylene dodecyl ethers were obtained from the Nikko Chemicals Co. The purity of these materials were more than 98% according t o gas chromatographic analysis. Dodecyl polyoxyethylene sulfuric acids, ClzH25(OCHzCHz).S04H, were synthesized by introducing SO3 vapor into the respective polyoxyethylene dodecyl ethers a t about 30-35 "C, Alkali salts were obtained by neutralizing these acids with the respective alkali hydroxide solution. Calcium salts were obtained by neutralizing these acids with suspended calcium oxide in a 2-propanol solution of the respective acids. Excess CaO was neutralized to CaS04and separated The Journal of Physical Chemistry, Vol. 81, No. 19, 1977
TABLE I: Krafft Points and Cmc Values of
C,,H,,(OCH,CH,),SO, (l/z)Me Type Surfactantsu Cmc, Krafft equiv/L point, Ionic surf actants at 2 5 ° C 'C -----C,,H,,SO, 0.5Ca 0.0024b 50b (at 55 " C ) C,,H,,OCH,CH,SO, 0.5Ca 0.00092b 15b C,,H,,(OCH,CH,),SO, 0.5Ca 0.00071
