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J . Phys. Chem. 1990, 94. 2546-2552
Physical Science of the Dioctadecyldimethylammonium Chloride-Water System. 1. Equilibrium Phase Behavior R. G. Laughlin,* R. L. Munyon, Y . 4 . Fu, and A. J. Fehl Miami Valley Laboratories, The Procter & Gamble Company, Cincinnati, Ohio 45239-8707 (Received: March 21, 1989; In Final Form: September 27, 1989)
The equilibrium phase diagram of the dioctadecyldimethylammonium chloride (D0DMAC)-water system was determined below 80 OC, and concentrated regions were investigated to 150 OC. Two equilibrium polymorphic forms of the dry crystal and two equilibrium crystal hydrates (a mono- and a dihydrate) were found. (A metastable dihydrate was also prepared.) The only liquid crystal phase which exists is the lamellar phase; its composition ranges from about 31.5% DODMAC to as high as 95%, depending on temperature. The Krafft discontinuity lies at 47.5 O C . The “coagel” and “gel” states, previously reported within concentrated regions of this system, were shown to consist of the mono- and dihydrate crystals, respectively. Slow phase reactions and the formation of stable colloidal structure in biphasic regions severely hampered this study.
Background DODMAC may be viewed as a prototype for di-long-chain cationic surfactant salts. This compound, the bromide, shorter and numerous structurally more complex cationic surfactant salt^^.^ have been the subject of extensive physical studies during the past few year^.^-^ The first reported phase study of the DODMAC-water system was that of Kunieda and ShinodaIo (Figure 1). Their data imply that the Krafft discontinuity lies at about 40 OC and that above this discontinuity a dilute liquid and the lamellar liquid crystal phases exist. The liquid crystal phase region was reported to extend from undefined upper limits to DODMAC compositions as low as 3.5%, and crystal hydrates of unspecified composition were suggested to exist. The solubility of crystal and liquid crystal phases was reported to be small below 100 “C, but to increase significantly at higher temperatures. This pioneering study of DODMAC provided important information and has been widely quoted. It is not to be regarded, however, as a definitive phase study. The composition of the sample used is uncertain, the data span only half the composition range, violations of the phase rule and alternation rule” are apparent in the diagram, and it extends to far above the thermal stability limit of this compound (135 “C). Besides this phase investigation, physical studies using polarized infrared spectroscopy of concentrated mixtures have been reported.12J3 During these investigations two highly ordered states of ill-defined structure and composition, termed “gel” and “coagel”, were characterized. It was suggested that only one kind of water existed in the coagel state, while two kinds existed within the gel state. Despite this earlier research, the equilibrium phase behavior ( I ) Kunitake, T.; Okahata, Y.; Tamaki, K.; Kumamaru, F.; Takayanagi. M. Chem. Lett. 1977, 387-390. (2) Kumano, A.; Kajiyama, T.;Takayanagi, M.; Kunitake, T.;Okahata,
Y. Ber. Bunsen-Ges. Phys. Chem. 1984.88, 1216-1222. (3) Kajiyama, T.; Kumano. A.; Takayanagi, M.; Okahata, Y.; Kunitake, T . Contemp. Top. Polym. Sci. 1984, 4, 829-854. (4) Kunitake, T.; Okahata, Y. Chem. Lert. 1977, 1337-1340. (5) Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.;Takarabe, K. J. Am. Chem. SOC.1981, 103, 5401-5413. (6) Kcdama, M.; Kuwabara, M.; Seki, S. Thermochim. Acta 1981, 50, 81-91. (7) Kodama, M.; Kuwabara, M.; Seki, S. Mol. Cryst. Liq. Cryst. 1981, 64, 277-282. (8) Kodama, M.; Kuwabara, M.; Seki, S. Thermal Anal., Proc. Inr. ConJ, 7th; Miller, Bernard, Ed.; Wiley: Chichester, U.K., 1982; pp 822-828. (9) Kodama, M.; Seki, S. Hyomen 1984, 22, 61-76. (IO) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710-1714. ( 1 I ) Purdon, F. F.; Slater, V. W. Aqueous Solution and the Phase Diagram; Edward Arnold: London, 1946. (12) Umemura, J.; Kawai, T.; Takenaka, T.; Kcdama, M.; Ogawa, Y.; Seki, S . Mol. Crysf.Liq. Cryst. 1984, 112, 293-309. (13) Kawai, T.; Umemura, J . ; Takenaka, T.;Kodama, M.; Ogawa, Y.; Seki, S . Langmuir 1986. 2, 739-743.
0022-3654/90/2094-2546$02.50/0
of the DODMAC-water system has remained uncertain. Two major causes for this uncertainty are apparent from our work. One stems from the fact that the attainment of phase equilibrium is often kinetically anomalous and is sometimes extremely slow. The other arises because colloidal structure is easily formed in biphasic regions, and once formed is exceptionally stable. This stability may be attributed to three factors: (1) the low solubility of the compound (estimated to be M14), which eliminates Ostwald ripening,15 (2) the fact that dispersions are positively charged, which retards flocculation, and (3) the stiffness of the phases present, which retards coalescence. In the present study the equilibrium phase diagram of the DODMAC-water system was determined over the temperature range 25-80 “C, and studies of concentrated mixtures were performed to 150 “C. Samples of very high quality were synthesized for the study. It is the first study in which the diffusive interfacial transport (DIT) phase studies methodI6 played an important role. Calorimetric, gravimetric, infrared, X-ray, and various optical methods were used as well. Besides defining equilibrium phase behavior, new information regarding the kinetics and mechanism of phase changes and the structure of colloidal dispersions within multiphase regions was obtained. The kinetic and colloidal aspects of the physical science of this system will be reported ~eparately.’~ Terminology of Phase Discontinuities. The accepted terminology for isothermal 2-3-2 phase state transitions’* is inadequate to describe some of the phenomena encountered and requires clarification. Two such transitions are widely recognized. One is the “eutectic” discontinuity, at which three phases coexist (crystal/liquid/crystal) and the phase of intermediate composition (the liquid) exists only at temperatures above the discontinuity. The less familiar “monotectic” discontinuity resembles the “eutectic”, except that the three phases involved are liquid/liquid/crystal.19 The second is the “peritectic” discontinuity, at which the three coexisting phases are liquid/crystal/crystal and the phase of intermediate composition (a crystal) exists only below the discontinuity. The “syntectic” discontinuity is similar except that the three coexisting phases are liquid/crystal/liquid.zo Besides these, binary systems containing compounds which undergo crystal polymorphic transitions display a third distinctive type wherein two of the three coexisting phases have the same (14) Evans, E.; Needham, D. J. Phys. Chem. 1987, 91, 4219-4228. (15) Kabalnov, A. S.; Pertsov, A. V.; Shchukin, E. D. J . Colloid Interface Sci. 1987. 118. 590-597. (16) Laughlin, R. G.; Munyon, R. L. J. Phys. Chem. 1987,91,3299-3305. (17) Laughlin, R . G. To be reported. ( I 8) Glasstone, S. Textbook of Physical Chemistry; D. Van Nostrand Co.:
New York, 1946. ( I 9) Haase, R.; Schoenert, H. Solid-Liquid Equilibrium; Halberstadt, E. S . , translator; Pergamon Press: Oxford, 1969. (20) Tamas, F.; Pal, I . Phase Equilibria Spatial Diagrams; Ward, L. S., translator; Illiff Books: London, 1970; p 84.
0 1990 American Chemical Society
Phase Behavior of the DODMAC-Water System
The Journal of Physical Chemistry, Vol. 94, No. 6, I990 2541
3500
3000 2500 Zoo0 1500 Wavenumber, cm-’
100 600
Figure 2. FTIR powder spectra of X, X.W, and X.2W. L”
~
0.1 H20
0.2 0.3 0.4 0.5 Weight Fraction (CIBH~~)ZN(CH~)ZCI
Figure 1. Kunieda and Shinoda phase diagram of the DODMAC-water system. Reprinted with permission from ref 10.
composition.I8 This transition is degenerate with respect to the above types and has not been given a name. In deference to its prominence in systems having polymorphic crystals, “polytectic” is suggested as an apt generic term. Surfactant systems display a variety of 2-3-2 transitions which resemble eutectic, peritectic, and polytectic transitions, except that liquid crystal or other phases replace the specific phases associated with the classical terms. Following Kekicheff et the terms “eutectoid, peritectoid, and polytectoid” are used herein to describe transitions which qualitatively resemble the classical transitions but differ in the structures of the phases involved. Experimental Section Material Used. The material for these studies was prepared from methyl octadecanoate, following procedures developed specifically for the preparation of high-purity standard compounds.” Satisfactory elemental analyses, as the monohydrate X-W, were obtained. The crystal structure of X-W has been determined22 and provides unimpeachable proof of molecular structure and composition. The assay (GC peak area) of the starting methyl octadecanoate was 99.24%. Intermediates were recrystallized after each of five stages during the synthesis, and the final assay of DODMAC (pyrolysis G C with injection port temperature of 290 “C), neglecting water, was 99.87%. The only impurity detected was 0.13% of the octadecyleicosyl (Cl&o) homologue. N o impurities were visible by TLC on silica gel using a 100:20:1:4 chloroform: methano1:water:formic acid developing solvent. Crystal Preparations. X.W. X-W was easily prepared by recrystallization from acetone containing some methanol and allowing the crystals to stand exposed to the atmosphere (35-40% relative humidity) overnight. Gravimetric assay of X-W for water gave a value of 2.96% (calculated water content of X.W, 2.98%). The assay was carried out by heating ca. 500-mg samples in open weighing bottles at 75 OC overnight in a vacuum oven at
