3502
H.NAKAYAMA, K. SHINODA, AND E. HUTCHINSON
The Effect of Added Alcohols on the Solubility and the Krafft Point of Sodium Dodecyl Sulfate
by H. Nakayama, Kozo Shinoda, Department of Chemistry, Yokohama National University, Ookamachi, Minumilcu, Yokohama, Japan
and E. Hutchinson Department of Chemistry, Stanford University, Stanford, California (Received April 96, 1966)
The solubility of sodium dodecyl sulfate in water in the presence of a small amount of hexanol, heptanol, and octanol has been measured as a function of temperature. The solubility increase caused by the addition of a small amount of these alcohols was remarkabIe due to the effective depression of the Kr&t point. The KrafTt point depression caused by added alcohol is explained quantitatively by a model in which it is proposed that the melting point of hydrated solid agent is depressed owing to the formation of a mixed micelle of surfactant and alcohol, just as the freezing point of a solvent is depressed by added solute. It is possible to calculate the amount of alcohol required to control the Krafft point of ionic surfactants.
Introduction
It is well known that the solubility of an ionic surfactant increases very rapidly a t a few degrees above a certain temperature called the Krafft p0int.l Solutions of almost any composition become homogeneous at temperatures several degrees above this temperature. Recently, a theory has been advancedal4which treats the micelle formation as a pseudo-phase (in liquid state) separationS-* and regards the Krafft point as a melting point of the hydrated surfactant, above which surfactant disperses in solution as micelles. The partial molal volume measurements of micellar and hydrated solid surfactantg and the calorimetric measurements above and below the Krafft pointlo clearly support the model. On the other hand, it is known that the added alcohol penetrates in the palisade layer of micelles” and depresses the e m ~ . ~ ~The ~ ~ cmc - 1 ~decrease has been interpreted as the lowering of the thermodynamic activity of micelle-forming molecules (or ions) due to the entropy of mixing and the decreased electrical repulsion among the ionic groups of the micelle-forming ions.la It is then readily supposed that the freezing point of the micelle, ie., the melting point of the hyThe Journal of Physical Chemistry
drated surfactant, is depressed by the presence of alcohols in it. The present investigation was undertaken in order (1) to test the proposed model with the experiments, and (2) to control the solubility and
(1) F.K r d t and H. Wiglow, Ber., 28, 2566 (1895). (2) R. C. Murray and G. S. Hartley, Trans. Faraday SOC.,31, 183 (1935). (3) K. Shinoda, T. Nakagawa, B. Tamamushi, and T. Isemura, “Colloidal Surfactants,” Academic Press Inc., New York, N. Y., 1963,pp 7,s. (4) K.Shinoda and E. Hutchinson, J . Phys. Chem., 66, 577 (1962). (5) A. E.Alexander, Trans. Faraday SOC.,38, 54 (1942). (6) K.Shinoda, Bull. Chem. SOC.Japan, 26, 101 (1953). (7) E. Hutchinson, A. Inaba, and L. G . Bailey, 2. Physik. Chem. (Frankfurt), 5, 344 (1956). (8) E. Matijevic and B. A. Pethica, Trans. Faraday SOC.,54, 589 (1958). (9) K.Shinoda and T. Soda, J . Phys. Chem., 67, 2072 (1963). (10) K.ahinoda, 9.Hiruta, and K. Amaya, J . Collodd Interface Sci., 21, 102 (1966). (11) W. D. Harkins, R. W. Mattoon, and R. Mittelman, J . Chem. Phys., 15, 763 (1947). (12) P . F. Grieger and C. A. Krauss, J . Am. Chem. SOC.,70, 3803 (1948). (13) K.Shinoda, J . Phys. Chem., 58, 1136 (1954). (14) M. J. Shick and F. M. Fowkes, ibid., 61, 1062 (1957).
EFFECTOF ADDEDALCOHOLS ON SOLUBILITY OF SODIUM DODECYL SULFATE
3503
Krafft point of ionic surfactants efficiently by small amounts of added paraffin chain alcohols. Experimental Section Materials. Sodium dodecyl sulfate which was prepared by sulfation of chromatographically pure dodecanol with sulfuric anhydride was obtained through the kindness of A h , Arai from Kao-soap Co. It was further purified by repeated recrystallization from water and extraction of possible impurities by ethyl ether.15 Extra pure grade heptanol was distilled; bp 177-178’ a t 760 mm. Hexanol and octanol were pure reagents. Water was distilled twice shortly before use and had a specific conductance less than 2 X 10+ (ohm em) -l. Procedures. The saturated solutions of surfactant were prepared by vigorous stirring of the two-phase mixture for about 1 hr in a thermostat controlled to within lt0.02. Further stirring caused no increase in solubility. The stirring was interrupted for about 4 hr before drawing off the upper portion of the solutions for analysis in order to allow any fine particles to settle. The solubility was determined by electrical conductance below the cmc and by drying a known volume of solution at 33-60’ under reduced pressure. The solubility recorded for a given temperature is the mean value of two or three measurements which did not deviate more than 1%. Results and Discussion The solubility of sodium dodecyl sulfate in water in the presence of a small amount of hexanol, heptanol, and octanol is shown in Figure 1. The Krafft point determined from the departure of the solubility curve from straight line plot was 10.2O.I6 It is obvious from Figure 1 that the solubility of the ionic surfactant is increased considerably by the addition of a small amount of alcohol. For instance, the solubility increases about 7.5 times in the presence of 0.0392 mole/ 1. of n-hexanol in solution a t 12’. If the ratio of alcohol to surfactant in solution (in the micelle) is constant, the solubility curve shifts to the lower temperature to the same extent, as shown by the dotted line in Figure 1. The heat of solution of R12S04Nawas calculated to be 11.7 kcal/mole from the slope of the logarithm of the solubility X 2 us. reciprocal temperature by the aid of the equation AH8 = 2 4 1
+
(1)
The activity coefficient of the surfactant, yz, was calculated by the Debye t,heory of 1 : l type strong electrolyte (below cmc).
I
3.4
I
I
I
35 vT x io3
I
I
3.6
Figure 1. The solubility of sodium dodecyl sulfate in water, 0 ;in 0.0392 M hexanol solution, 0; in 0.00861 and 0.00431 M heptanol solution, A and A; in 0.00204 M octanol solution, 0 . Temperatures in O K .
As a Krafft point is a temperature above which the liquid (micellar) state is more stable than the solid state, the solid-solute curve becomes higher than the liquid (micelle)-solute curve above the Krafit point; i.e., the solid state is thermodynamically less stable. The cmc curve is lowered by the addition of a small amount of alcohol, as shown by the dotted line cmc’ curve in Figure 2, but the solid-solute equilibrium curve stays unchanged. The intersection of the solid-solute curve with the cmc’ curve is the depressed Krafft point in the presence of alcohol. The phenomenon resembles the freezing point depression of water produced by the addition of solute. First, the cmc of R12S04Nain the presence of 0.0012 mole/l. of heptanol is 0.0071;5 i.e., the cmc is 12.5% depressed (at 25’) at the mole ratio of 1:6. The cmc’ curve which is 12.5% lower than that of pure Rlz(15) L. H. Princen, Ph.D. Dissertation, University of Utrecht, Netherlands, 1959. (16) E. Hutchinson, K. E. Manchester, and L. Winslow, J . Phys.
Chem., 5 8 , 1124 (1954).
Volume 70, Number 11
November 1966
H. NAKAYAMA, K. SHINODA, AND E. HUTCHINSON
3504
0012
-
0000.
L
AT
--d Single dlspersion
0.004i
8
IO
12
I 14
Temperot ur e
Figure 2. Schematic diagramlof the cmc and the Krafft point depression and solubility increase of sodium dodecyl sulfate containing 14.7 mole yo heptanol. Temperatures in "C.
S04Na intersects the solid-solute curve at a temperature about 2.4' below the Krafft point. Secondly, the Krafft point depression, dT, may be approximately given by the equation dT - RT2 dX2 mi where dT is the Kraff t point depression, ~ XisZ the mole fraction of the second component (alcohol) in the micelle, T is the absolute temperature, and AHf is the heat of fusion of the hydrated surfactant. The heat of fusion of the hydrated surfactant is obtained either from the calorimetric measurements of the heat of solution (dry agent + micellar agent) and the heat of wetting (dry + hydrated solid) or from the temperature dependences of the cmc and the solubility above and below the Krafft point. The calorimetric heat of fusion of sodium dodecyl sulfate is 12.0 kcal/mole,'O and that from solubility data is 13.5 kcal/mole.'O Substituting T = 283.4'K, AHf = 12.0 kcal/mole into eq 2, the mole fraction of alcohol in the micelle can be calculated from the Krafft point depression. Suppose the mole fraction of heptanol in the micelle is 0.147, then the KrafTt point depression is calculated to be about 1.9' (or 2.5' considering the change of charge density on the micelle surface. See eq 4 and 5 of ref 13).
The Journal of Physieal Chemistry
Thirdly, as revealed from the selective adsorption study of alcohol against R12S04Na by radiotracer techniques,?' the adsorbabilities of heptanol and R12S04Na at the air-water surface or at the micelle surface are about the same. Hence, the ratio of R70H against RlzS04Na in the micelle and in single dispersion may not differ appreciably. Under the condition of a definite composition of &OH and R12S04Na, the mole fraction of R70H in the micelle is kept almost constant regardless of the total concentration of the solution. Thus the freezing point of the micelle may be depressed to the same extent. The dotted curve is the solubility of R12S04Na whose solution contains 14.7 mole % heptanol. The solubility curve shifts by 1.9' from that in pure water and intersects with the solid-soiute solubility curve at a temperature about 2.4' below the Krafft point. All these results coincide with each other, assuring the basic plausibility of the theory and experiments. Accordingly, we can conclude that the Krafft point depression can be estimated either from eq 2 or from the cmc depression in the presence of alcohol. As the longer chain alcohol is much more adsorbable, it is an effective substance to depress the cmc and the Krafft point and therefore to enhance the solubility of ionic surfactants. Although the long chain alcohol is so effective in depressing the cmc, larger amounts of alcohol cannot be added, because the solubility of the alcohol itself is so small, on the one hand, and an alcohol-surfactant gel separates from the solution, on the other hand. The solubility of surfactant in more concentrated solution is also enhanced by the presence of a small amount of alcohol, but the effect of longchain alcohol is not so effective as in the cmc or Krafft point depression because the added alcohol is consumed by comicellization, due to the large adsorbability, and the ratio of alcohol to surfactant in single molecular dispersion decreases. For example, the intersection of the dotted line in Figure 1 tells us that the solubility curve shifts 1.9' in the presence of 26.3 mole % ' RsOH, 14.7 mole % &OH, and 13 mole % RBOH. Acknowledgment. The authors wish to thank Mr. H. Arai of the Kao-soap Co. for the very pure sodium dodecyl sulfate used in these studies.
(17) K. Shinoda and J. Nakanishi, J. Phys. Chem., 67, 2547 (1963).
