THE solubility of soaps at room temperature is usually greatly overestimated. The only soaps that do not require elevated temperature for dissolution are potassium laurate, myristate and oleate and sodium oleate, and some less saturated soaps. Sodium stearate is no more soluble in water at room temperature than limestone. Frequently in the literature various mixtures of water and soaps are indiscriminately referred to as solutions, whereas some are pastes, some are plastic liquid-crystalline phases, and some are, of course, actually solutions.
The Solubility of Sodium and Potassium Soaps. Figure 1 assembles data for the five commonest sodium soaps. From a study of Figure 1 it is at once noted that even sodium laurate does not become freely soluble until well above room temperature. It is also apparent that all the sodium salts of the pure fatty acids require much higher temperatures to produce concentrated solutions than do the potassium salts (cf. Figure 2).
The solubility of three potassium soaps is shown in Figure 2, where 4 equivalents per cent of potassium hydroxide was added to suppress hydrolysis. Comparing Figure 2 with Figure 1, a great contrast is seen between the solubility relations of sodium and potassium soaps. First, of course, is the fact that the potassium soaps are much more soluble than the sodium soaps so that concentrated solutions (21%) of potassium myristate are obtainable at 10~ whereas for sodium myristate a temperature approaching 58~ is required. Potassium stearate dissolves freely at 50~ but sodium stearate needs 75-80~ .
The sodium soaps dissolve progressively as the temperature is raised, forming saturated solutions of higher and higher concentration over a wide range of 20 ~ , whereas the potassium soaps almost abruptly become freely soluble up to high concentrations within 1 or 2 ~ of temperature.
The so-called "Krafft Point" is the temperature at which soaps are supposed to become soluble. Demareq (1) has taken the following as Krafft Points for the alkali stearates: lithium, 145-150~ sodium, 79~ potassium, 48~ rubidium, 52.5~ and cesium, 49~ . Other Krafft Points mentioned by Demarcq are sodium laurate, 38~ potassium palmitate, 30.50; lithium oleate, 72~ .
Figure 3 shows the effect upon the apparent solubility or Tc values for several concentrations of potassium palmitate and potassium stearate of progressive additions of excess alkali. Without excess alkali the value is not sharp and the true solubility point Tr is obscured but it is represented rather by a gradual decrease of turbidity over a wide range of temperature. Furthermore, since hydrolysis is more prominent in dilute solutions, Tr apparently decreases for increasing concentration of potassium soap, which would be an impossibility for any pure substance not forming a highly stable stoichiomctric hydrate consisting almost wholly of water. Upon subsequent addition of alkali Tc becomes sharp and it increases steadily, although slightly, with increasing concentration of potassium soap. After 5-8 moles equivalents per cent alkali has been added, further addition of alkali has practically no effect. In more concentrated potassium soap solution, such as 1 m, only about one equivalent per cent alkali is necessary to avoid any effects of hydrolysis, presumably because the concentration of the simple hydrolyzing fatty ions is less, and any acid soap formed would be incorporated in the micelles.
Figure 3 includes one curve in which potassium chloride is substituted for potassium hydroxide in 0.1 m solution of potassium stearate. The effect is somewhat similar to that of potassium hydroxide, but in less degree.
The data derived from the phase diagram for potassium laurate published in 1926 (3) indicated that--potassium laurate dissolved over a range up to 10~ We now find that solutions without excess alkali are indeed turbid up to 10-15~ but with excess of alkali form clear solutions up to the high concentration of 2.3 m (35.4%) at 0~ Thereupon the solution becomes a clear, plastic, liquid-crystalline mass. At about 3.6 m (46%) some curd remains at 0~ but dissolves between 5 and 9~ (Figure 4 is corrected accordingly. )
Summary
Solubility data are provided and collected for the pure sodium and potassium soaps. Hydrolysis obscures the temperatures of solution but is obviated by the presence of a small excess of alkali. Each sodium soap has a large range of temperature between fair and high solubility, whereas the potassium soaps go abruptly into solution, at almost the same temperature and concentration of each soap.
The only soaps that are even moderately soluble at room temperature are potassium laurate, myristate, and oleate, the potassium salt of acids from coconut oil, and the sodium oleate. The other sodium and potassium soaps of the saturated fatty acids require elevated temperatures for solution.
JAMES W. McBAIN and WILLIAM C. SIERICHS, Department of Chemistry, Stanford University, California
THE JOURNAL OF THE AMERICAN OIL CHEMISTS' SOCIETY, ,JUNE, 1948
