Conductances and Surface Tensions of Aqueous Solutions of Sodium Decanoate, Sodium Laurate, and Sodium Myristate, at 25° and 35°

Abstract

The equivalent conductances of aqueous solutions of sodium decanoate, sodium laurate, and sodium myristate have been determined from the very dilute region up to saturation concentration, at 25O and 35". The limiting conductances have been obtained from the measurements in the usual way. In addition and for the sake of completeness, the limiting equivalent conductances of sodium formate, sodium acetate, sodium propionate, and sodium butyrate have also been determined a t 35O, since they were not known a t this temperature.The effect of increasing chain length on mobility is discussed. From a consideration of the experimental and the theoretical values of the slopes of the plots of equivalent conductance against the square root of the concentration, it is concluded that some sort of interaction, possibly a reversible dimerization of anions, occurs, even in the dilute region, and this could account for the lower experimental values of the slope. This phenomenon has been observed with other long chain electrolytes, however, and the hypothesis of dimerization is not the only possible one.The critical micelle concentrations have been determined, and recent concepts of micelle formation are applied to interpret their structure. The surface tensions of the aqueous solutions have been measured a t 25O and 50".The object of this work has been described in a previous publication (I). We have now extended our work to include the decanoate, laurate, and myristate ions (14 carbons, including the carboxyl group) and this is about as far as there is any point in going. In addition, however, to our interest in the limiting conductance of the anion, it was found that the slope of the conductance curve, in the very dilute region, is not that of the Onsager theory, when the number of carbon atoms exceeds six. This phenomenon has been observed by others and it points to peculiarities in the structure of solutions of these salts. Moreover, since the soap-like character of the solutions of these sodium salts is very marked, we were obliged t o investigate other physical properties, notably the surface tension. EXPERIMENTAL Our method of determining conductances has been described previously (1). Densities were determined with Ostwald-Sprengel pycnometers, cap. 2.5 ml, and the usual precautions with respect to bouyancy, etc., were taken. The densities are correct to f 0.0001. Viscosities were determined with the Cannon and Fenske modification of the Ostwald viscometer. The stalagmometer method was used to determine surface tension. T o obtain a sufficient degree of accuracy, it was necessary to count from 100 t o 200 drops but, because these soap-like solutions are sometimes very viscous, this laborious operation could occupy a s much as 3 h. An automatic system was therefore desired. Each drop as it fell from the stalagmometer fell into a glass cone, having platinum plates on opposite sides: these plates were connected to a n electronic glow counter. The glow counter was enclosed in a Faraday cage to eliminate signal pickup.T...

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