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M. Hat6 and K. Shinoda
Calcium and Sodium Dodecylpoly(oxyethy1ene) Sulfates and Their Mixtures ~
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Resderch institute for Polymers and Textiles, Sawatari-4, Kanagawa-ku, Yokohama, Japan
Cepar!roent of Chemistry, faculty of Engineering, Yokohama Nationai Universjty, Ooka-2, Minami-ku, Yokohama, Japan (Received June 23, 1972)
The Krafft points of pure calcium and sodium dodecylpoly(oxyethy1ene) sulfates (C12H25fOCH2CH2jnS04M, n 0, 1, 2, and 3) have been determined. The Krafft points were depressed with an increase in oxyethylene chain length. It is noteworthy that the Krafft points of the calcium salts can be depressed effectively by an increase in oxyethylene chain length, and therefore such surfactants can be used in hard water. In order to gain an insight into the solubility and the Krafft point of C ~ ~ H Z ~ ( O C H ~ C HinZhard ) ~ Swater, ~ ~ Nthe ~ Krafft points of binary surfactant mixtures of different gegenions or ethylene oxide chain length were studied over the entire range of composition. The Krafft points of binary surfactant mixtures can be explained semiquantitatively based on the model that the Krafft point is the melting point of the hydrated solid agent.
1ntroductio:n The Krafft points of calcium salts of ionic surfactants are generally higher than room temperature, and therefore they are usua1l:y far less soluble than corresponding sodium salts and do not form micelles around room temperature. It is clear from OUT recent work on fluorinated surfactants1 that the Krafft point is affected by the gegenion(s), structure of the lipophilic group, mixing of two kinds of surfactants, etc., ,just as the melting points of corresponding organic compcunds are. These phenomena can be explained by the theory2 that the Krafft point is the melting point of the hydrated solid agent. If the Krafft point of the calcium s a k of an ionic surfactant is lowered by introducing the oxyethylene group into the molecule, such a surfactant may be used in hard water as well as soft. In the present paper, the relation between Krafft points and the structures of ionic surfactants has been studied in order to find ionic surfactants useful in hard water, Le., the Krafft paints of sod.ium and calcium dodecylpoly( oxyethy1ene)sulfates (ClzH25((3CH2CI.lz)nS04Naor %Ca, n = 0, 1, 2, 3) and the solubilities of sodium dodecylpoly( oxyethylene) sulfates in the presence of a bivalent salt have been studied. xperiment:al Section
C H Z ) ~ S O ~ ~n / =~ C 1,~2,, 3) were obtained by adding a 2propanol solution of calcium chloride to 2-propanol solutions of the respective acids. The compounds were washed with 2-propanol to remove the hydrogen chloride produced and purified in the same way as the sodiurn salts. The compounds were identified by infrared spectroscopy4 and elementary analysis (C,H). Sodium dodecyl and tetradecyl sulfates were obtained from the Kao-Soap Co. through the kindness of Dr. Arai. These salts were recrystallized three times from water after extraction of possible impurities with petroleum ether. Calcium dodecyl and tetradecyl sulfates have been obtained by metathesis in water between calcium chloride and the sodium salts, and were purified in the same way as above. Procedures. Provided composition of a mixed micelle is constant, a solubility curve above the rafft point shifts to lower temperature to the same extent (compared with the solubility curve of a pure compound).5 It is also time consuming to determine the Krafft points of surfactant mixtures from the phase diagram. Thus, for the systems C12H25S04~hCa + C12H25(OCH2CHz)n5041~Ca, C12H25C I ~ H ~ ~ O C H ~ C H ~and S O C14~N~, OCH2CH2S041/2Ca H~sS04~/2Ca -t- C14H29S04Na, the Krafft points of the mixtures were estimated from the solutioq temperatures of the surfactants at 1 wt % (0.031-0.035 equiv/lO3 g of H2O) the~ N ~ , or, for the mixture C12H25SO4~/2Ca + C I ~ H ~ ~ S O Krafft point was estimated from the solubilities of 3 wt % (0.11 equiv/103 g of HzO) solutions. The Mrafft point estimated by this efficient method agrees within 1" with that determined by the phase diagram. The critical micelle concentrations (cmc) a t 25" have been determined from electrical conductivity measurements. The electrical con-
+
Materials. Pure poly( oxyethylene) dodecyl ethers were obtained from the Nikko Chemicals Co. The purity of these material:; was more than 99% according to gas chromatograph:.^ analysis. A series of dodecylpoly(oxyethylene) sulfuric acids were synthesized by introducing SO3 vapor into the poly(oxyethy1ene) dodecyl ethers a t 30-35O.3 Sodium salts were obtained by neutralizing these acids with sodium hydroxide arid repeatedly recrystallizing from a (1) K. Shinoda, M. tiato, and T. Hayashi, J . Phys. Chem., 76, 909 (1972). (2) K. Shinoda, et a/., "Colloidal Surfactants," Academic Press, New mixture of 2-propanol ancl ethanol ( 3 : l by volume) after York, N.Y., 1963, pp 7-8. extracting the uiireacted ethers with petroleum ether.3 (3) F. Tokiwa and K. Ohki, J . Phys. Chem., 71, 1343 (1967). (4) D. Hummel, "identification and Analysis of Surface-Active Agents," C12Hzj;(OCW.2CX12)3~04~a~ (average oxyethylene chain SpectraVol. No. 46, Interscience, New York, N.Y., 1962 I'ength given) was obtained through the kindness of Dr. (5) H . Nakayama. K. Shinoda, and E, Hutchinson, J . Phys. Chem., 70, Tokiwa of th'e 14;a~Soa.pCo. Calcium salts ( C I ~ H ~ ~ ( . O C H ~ - 3502 (1966). The Journal ofP,bysical Chemistry, Vol. 77, No. 3, 1973
Calcium and Sodibm Dodecylpaly (oxyethylene)Sulfates ductivity of water was 1.1 pmholcm a t 25". The detailed procedures are described in the preceding paper dealing with fluorinated surfactants.l The calcium salts as well as the sodium salts behave as strong electrolytes below the Cmc.
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40
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Results and l ~ ~ ~ c ~ ~ ~ i ~ ~ +c .( I ) Effect o f Oxyethylene Chain Length of Surfactants 0 a on the cmc Values anti Krafft Points. The cmc values at 20 c 25" and the Kiafft points have been listed in Table I.6 +The Krafft ploint of 6121125(OCHzCHZ)zS04Na is below 0" ? Y and was estimated from the solubility in a sodium chloride solution, whereas no precipitation was observed for ClzCWzCHz)&Odi$Za even in a 0.5 M calcium chloride 0 solution a t 0-1". It i s clear from Table I that the cmc values I of calcium and sodium dodecylpoly(oxyethy1ene) sulfates 0 I 2 3 4 decrease with the increase in oxyethylene chain length. This results from the decrease in electrical repulsion due to oxyethylene chain length, n the decrease in charge density on the micelle surface7 with Figure 1. Effect of oxyethylene and hydrocarbon chain length on an increase in oxyethyiene chain length. Figure 1 shows the Krafft points of C m H 2 m ~ (OCH~CHZ)~SO~M. l the change in Eirafft point with oxyethylene chain length. The data for e161-133(OC€-I~CHz)nS04Naand C18H37I ' I I I I I ~ ~ C ~ 2 C ~obtained ~ ) ~by ~Weil, ~ ~et 4aL8~(1awt. %), are included., It is clear from Table I that the Krafft points of calcium a.nd sodium dodecylpoly( oxyethylene) sulfates are depressed with an increase in oxyethylene chain length. It i s noteworthy that the Krafft points of the calcium salts are effectively depresc;ed by introducing one oxyethylene group. The Krafft points of CrzHz5(OCHzCHz)zS04MCa and C1zW25(0CH2CIXa)3S041/zCa are below O", thus these surfactants di.ssolve well in the presence of bivalent cations such as Ca2+ and can be ulsed in hard water. (2) Effect of Mixing Two Surfactant&of Different Gegenions on the Krafft Point. The Krafft points of binary sura factant mixtures ( G m H ~ ~ + l S 0 4 ~ h+CCmHzm+lSO4Na, m = 12, 14, and C l ~ M ~ 5 0 C H ~ C H ~ S 0 4 1 h+C C12H25a IDGH2CH2SO4Na) are shown in Figure 2 as a function of the fraction of calcium ions in the system, i.e., XCaO = 2 N ~ a / (NNa ZNC,,). I t is evident from Figure 2 that there exists a minimum hi the Krafft point at a certain composition. This phenomenon i s usually observed in the melting point us. connpositioin curve. (3) Effect of Added Salts on the Krafft Point. It is well 0 0.2 0.4 0.6 0.8 1.0 known that the Krafft points as well as solubilities of ionic 2Nca /("a+ 2Nca) surfactants arc affected by the addition of salts.g In order to find the effect of added salts on the Krafft points and the Figure 2. Krafft points of surfactant mixtures of sodium and calsolubilities of sodium dodecylpoly(oxyethy1ene) sulfates, cium salts as a function of the fraction of calcium ions of t h e system. th,e temperatures above which 1 wt % (0.024-0.035 M ) of sodium doclecylFoly(oxyethylene) sulfates dissolves have been measured and are shown in Figure 3 as a function of the ratio of bivalent cation to surface active anion TABLE 1: Cmc at 25" and Krafft Point of C12H~~(OCH&H~)nS04M -(2M2+/R-). The solubilities of calcium dodecylpoly(oxyethylene) sulfates as a function of the ratio 2M2+/RKrafik point, Cmc, m C Compound are also shown by the filled circles. It is found from Figure 3 that the Krafft point is initially depressed by the addition 0.0012a of different gegenions, just as in the case of mixing of two 0.00046 surfactants (Figure 2), then increases rapidly up to the 0.00037 Krafft point of the added bivalent salt, until the ratio 0.00034 2M2+/R- reaches nearly 1. Above this concentration, at 0.0081 which the sodium ion may be almost completely replaced 0.0042 Y
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0.0031 0.0028d a i h e value at 5S6, &The extrapolated value from the solubility in aqueous NaCi soltilion. Average oxyethylene chain length of this comuound was 3. Rejerence 8.
(6) F. Tokiwa, J. Phys. Chem., 72, 1214 (1968). (7) K. Shinoda, "Colloidal Surfactants," Academic Press, New York, N . Y . , 1963, Chapter 1, pp 41 and 51. (8) J. K. Weil, R. G. Bistiine, Jr., and A. J. Stirton, J. F'hys. Chem., 62, 1083 (1958). (9) H. Nakayama and K, Shinoda, Bull. Chem. SOC. Jap., 40, 1797 (1967). The Journal of Physical Chemistry, Vol. 77, No. 3, 1973
M. Hat6 and K. Shinoda
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is akin to phase separationlo and the Krafft point is considered to be the melting point of a hydrated solid surfactant,2J0 the change in the Krafft points of binary surfactant mixtures is considered to correspond to the freezing point depression of binary mixtures of ordinary substances. It is interesting to examine whether the Krafft point of surfactant mixture can be explained by this model2 or not. Just as for the phase diagrams of solid-liquid equilibria of binary mixtures, the curves in Figures 2 and 4 illustrate the temperatures at which a solid phase appears from the liquid phase (mixed micelle). In order to find whether the solid phase is composed of a pure compound or a solid solution, the composition of the solid phase was analyzed for the mixtures C12H25S041/2Ca -t Cl2Hd304Na and ~ Saqueous O ~ ~ / ~ C ~ . C12H25S04Y2Ca + C ~ Z H ~ ~ ~ C H ~ C HThe solutions of surfactant mixtures of given mole ratio ( X I 0 = 0.7, 0.5, 0.3) were kept at 15 f 0.2" foa 2-3 days, and the composition of the solid precipitate was estimated from the Krafft point depression, which was less than 0.3" (compared with the Krafft point of a pure compound) for both systems. The purity of solid phase was more than 98%. If the mixing in the micelle behaves ideally and the solid phase in equilibrium with the mixed micelle is pure, the change in Krafft point of a binary surfactant mixture may be expressed by the familiar equation for the freezing point depression of a binary mixturell
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" 0
IO 20 Ratio of bivalent ration to surface octlve anion ( 2 M++/ R-
Figure 3. Effect of added salts on the dissolution temperature (-- Krafft point) of 1 wt ?An C12H25(0CH2CH2),S04Na. 60
t 1) d In alm = (AHlf/RT2)dT where alm is the activity (first component) in the mixed micelle, AHlf is the heat of fusion (from the hydrated solid to the micelle) of the first component, R is the gas constant, and T is the absolute temperature. Supposing that ideal mixing is allowed between two types of cations (or anions) whereas cations and anions do not mix with each other, the activity a l m is expressed as a1m
= (Xl+m)"+(Xl-mP-
(2)
where
XI+^ = ~ I + N I + / ( Z I + Ni-Iz+~ + N 2 + ) Xi-m = Zl-Nl-/(z1-N1- -t Z Z - N Z - )
v 40 v
c
where Xlim and Xlpm are the fractions of cations and anions (surface active ions), respectively, of the first component in the mixed micelle; and V + and V - are the numbers of cations and anions, respectively, in the molecule. Nl+ and Nz+ are the numbers in the micelle and zli and z2* are the valence of cations (or anions) of the first and the second components, respectively. Assuming AHlf is independent of temperature, eq 1is integrated to give
c ._ 0 a LL c .I-
20
Ej
Y
C
(3)
-__i-L_i
02
I
1
1
04
I
06
mole fraction
CIZHZ~(O~HZCHZ)~SO~'/~~O
I
0.8
I I
C1zH25S04hCa
Figure 4. Krafft points of surfactant mixtures of different sxyethylene chain length as a function of the mole fraction of t h e system
by the bivalent ion, the Krafft point is raised only a little.g The change in Krafft point with the change in concentration of an added sal: can be semiquantitatively explained from the results in Figure 2. (4) Effect of Mixing Two Surfactants of Different Oxyethylene Chain on the Krafft Point. Figure 4 shows the Krafft points of the mixtures C12H25S04%Ca + C12H25(OCH2CH2)nS041/26a, n =: 1, 2, 3, as a function of the mole fraction of the components. As the micelle formation The Journalof Physical Chemistry, Vol. 77, No. 3, 1973
where 7'1O is the Krafft point of the pure first component and AH1f is obtained by calorimetry or calculated from the slope of the cmc and the solubility curve as a function of temperature.12 By introducing Tl0, AHlf, and Xlm into eq 4, the change in Krafft point can be calculated. Tl0 and AH1f are summarized in Table 11. The calculated values are shown by dotted lines in Figures 2 and 4 as a function of XIo. X1O (the composition of the system) was calculated from X l m (the composition of the mixed micelle) by making a correction for selective adsorptivity. The selective (10) K. Shinoda and E. Hutchinson, J. Phys. Chem., 66, 577 (1962). ( 11) W. J. Moore, "Physical Chemistry," Prentice-Hall, Englewood Cliffs, N.J.9 1962, p 134. (12) K. Shinoda, S. Hiruta, and K. Amaya, J. Coiioid interface Sci., 21, 102 (1966).
Viscosities of Nimelectrolvtes in Water
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the micellar state at 1 wt 70 (0.030-0.035 equiv/l.), X 1 - m and XI-O agreed within experimental error. The initial slope of the calculated Krafft point us. composition curve !
