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Phase Behavior of the n-Decanoic Acid + Thallium(I) n-Decanoate System J. A. Rodrı´guez Cheda,* M. Ferna´ndez-Garcı´a, and P. Ungarelli Departamento de Quı´mica Fı´sica I, Universidad Complutense, Madrid 28040, Spain
P. Ferloni Dipartimento di Chimica Fı´sica and CSTE-CNR, Universita´ di Pavia, Pavia, Italy
F. Ferna´ndez-Martı´n Departamento de Ingenierı´a, Instituto del Frı´o, CSIC, Ciudad Universitaria, Madrid, Spain Received April 15, 1999. In Final Form: March 24, 2000 The temperature and enthalpy vs composition diagrams of the binary system [(1 - x) n-C9H21CO2H + (x) n-C9H21CO2Tl, where x ) mole fraction] were determined by means of differential scanning calorimetry, differential thermal analysis, and optical microscopy. Besides the usual regions of phase separation, miscibility was observed in isotropic liquid, liquid crystalline, and solid phases. This binary system displays the formation of: (1) a 1:1 molecular compound which melts incongruently at T ) 322.2 ( 0.1 K, ∆fusHm ) 50.7 ( 0.5 kJ‚mol-1; (2) an eutectic at x ) 0.125 and T ) 302.0 ( 0.1 K, ∆fusHm ) 28.3 ( 0.3 kJ‚mol-1; (3) a smectic A lyotropic mesophase occurring at T ) 373.5 ( 0.1 K over the composition range x > 0.770; (4) a solid solution phase occurring over the composition x > 0.913, and 315.3 K < T < 404.2 K.
1. Introduction Lyotropic mesomorphism in many aqueous systems such as those formed with detergents and lubricants is well-known and is the object of permanent applied research due to the technological interest and practical applications of these systems. On the other hand, among the nonaqueous systems formed with ionic compounds and displaying liquid crystalline behavior, only a few systems containing mixtures of organic salts and organic acids have been reported.1-4 In ref 5, in particular, a variety of binary systems with common cations or common anions formed with alkali alkanoates were reviewed and critically evaluated. The existence of mesophases has not been recognized in several of the diagrams illustrated in ref 5, and more work is needed to get a deeper understanding of the phase behavior in the pertinent systems. A better knowledge of the association, aggregation, and phase behavior in these nonaqueous systems from the viewpoint of thermodynamics may be valuable for describing the interactions occurring in different colloidal and surfactant systems, in particular in the aqueous systems, for which Krafft point, critical micellar concentration, and other structural properties have been reported by many authors. In recent literature, a paper was devoted to investigate the spectroscopic and thermal properties of some 1:2 sodium soap-fatty acid compounds,6 and in a work by (1) Cheda, J. A. R.; Ferna´ndez-Garcı´a, M.; Ferloni, P.; Ferna´ndezMartı´n, F. J. Chem. Thermodyn. 1991, 23, 495. (2) Ferna´ndez-Garcı´a, M.; Cheda, J. A. R.; Westrum, E. F.; Ferna´ndezMartı´n, F. J. Colloid Interface Sci. 1997, 185, 371. (3) Mirnaya T. A.; Polishchuk, A. P.; Bereznitski, Y. V.; Ferloni P. J. Chem. Eng. Data 1996, 42, 1337. (4) Roux, M. V.; Turrio´n, C.; Sa´nchez Arenas, A.; and Cheda, J. A. R. Langmuir 1996, 12, 2367. (5) Franzosini, P., Ferloni, P., Schiraldi, A., Spinolo, G., Eds. Molten Alkali Metal Alkanoates; Solubility Data Series; Pergamon Press: Oxford 1988; Vol. 33.
this group the molecular association of normal alkanoic acids with their thallium(I) salts was investigated.7 Actually, a general feature of the n-alkanoic acid-metal n-alkanoate systems which is well-known in the literature is the existence of molecular association, giving origin to molecular complexes (or acid soaps) with fairly strong hydrogen bonds and possible different stoichiometries.8-17 However, only a few complete phase diagrams of binary systems containing potassium and sodium acid soaps were reported by McBain and co-workers,8,9 Brouwer and Spier,13 and Lynch et al.6 More recently, two complete phase diagrams of systems with thallium(I) salts where investigated by our research group.1,2 The aim of the present paper is to present a thorough study of the phase diagram formed with n-decanoic acid and thallium(I) n-decanoate decanoate over the whole composition range from 0 to 100 mol % of thallium salt, as determined by means of different experimental techniques, pointing out in particular the problems related to preparing and handling the compounds since it may be easy to get them contaminated. A further target is to (6) Lynch, M. L.; Pan, Y.; Laughlin, R. G. J. Phys. Chem. 1996, 100, 357. (7) Ferna´ndez-Garcı´a, M.; Garcı´a, M. V.; Redondo, M. I.; Cheda, J. A. R.; Ferna´ndez-Garcı´a, Ma.; Westrum, E. F.; Ferna´ndez-Martı´n, F. J. Lipid Res. 1997, 38, 361. (8) McBain, J. W.; Stewart, A. J. Phys. Chem. 1933, 37, 924. (9) McBain, J. W.; Field, M. C. J. Phys. Chem. 1933, 37, 1920. (10) Ryer, F. V. Oil Soap 1946, 9, 310. (11) Pacor, P.; Spier, H. L. J. Am. Oil Chem. Soc. 1968, 45, 338. (12) Brouwer, H. W.; Skoda, W. Kolloid-Z. Polym. 1969, 234, 1138. (13) Brouwer, H. W.; Spier, H. L. Proc. 3rd Int. Conf. Thermal Anal. 1971, 3, 131. (14) Trzebowski, N. Wiss. Z. Friedrich-Schiller University Jena, Math.-Nat. Reihe 1976, 25, 891. (15) Goddard, E. D.; Goldwasser, S.; Golikeri, G.; Kung, H. C. Adv. Chem. Ser. 1968, No. 84, 67. (16) Kung, H. C.; Goddard, E. D. J. Colloid Interface Sci. 1969, 39, 242. (17) Andreeva, E. D.; Boyarchuk, Yu. M.; Shul’gin, E. I. Zh. Struct. Khim. 1978, 19, 168.
10.1021/la9904511 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000
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Table 1. Thermodynamic Data of Pure Components Acid ∆H (kJ‚mol-1)
T (K)
∆S (J‚mol-1‚ K-1)
typea
this work
ref 23
ref 24
this work
ref 23
ref 24
this work
fusion
304.7 ( 0.1
304.7
304.55
29.1 ( 0.3
29.40
27.80
96 ( 1
Salt ∆H (kJ‚mol-1)
T (K)
∆S (J‚mol-1‚ K-1)
typea
this work
ref 22
this work
ref 22
this work
ref 22
SV/SIV SIV/SIII SIII/SII SII/SI SI/LC LC/IL total (SVfL)
234.4 ( 0.1 287.9 ( 0.1 305.9 ( 0.1 327.7 ( 0.1 404.2 ( 0.1 483.0 ( 0.1
232.4 ( 0.5 288.6 ( 0.4 306.8 ( 0.8 327.4 ( 0.5 405.0 ( 0.5 484.0 ( 0.8
2.99 ( 0.03 0.58 ( 0.01 3.68 ( 0.04 4.40 ( 0.04 5.60 ( 0.06 2.54 ( 0.03 19.8 ( 0.2
2.41 ( 0.08 0.60 ( 0.08 4.24 ( 0.17 3.97 ( 0.33 5.67 ( 0.21 2.55 ( 0.17 19.44 ( 0.17
12.8 ( 0.1 2.02 ( 0.01 12.0 ( 0.1 13.4 ( 0.1 13.8 ( 0.2 5.27 ( 0.05 59.4 ( 0.6
10.4 ( 0.4 2.1 ( 0.2 13.9 ( 0.6 12.1 ( 1.0 14.0 ( 0.5 5.2 ( 0.4 57.7 ( 0.5
a
Roman numerals indicate polymorphic crystalline phases.
provide a good definition of the lyotropic liquid crystal region, the knowledge of which is needed to obtain a complete phase diagram, thus allowing a comparison with those studied previously.1,2 2. Experimental Section 2.1. Pure Components. The n-decanoic acid was a >99% Fluka “puriss” product, which was recrystallized from waterfree Merck >99.8% ethanol and dried in vacuo at 300 K for several hours. It was used as the first component of the phase diagram as well as for the synthesis of the salt. Thallium(I) n-decanoate was prepared by slow reaction of a solution of the acid in Fluka “puriss” >99.5% acetone-free methanol with a methanolic suspension of thallium(I) carbonate (>99% Fluka “puriss” reagent, used as supplied) in slight excess to avoid the possible coprecipitation of the acid-salt complex.18 The mixture was refluxed under stirring and gentle heating for about 3 h, then filtered to remove the salt in excess, and concentrated. The white precipitate obtained by addition of Fluka “puriss” >99.8% diethyl ether was washed with ether and Fluka “puriss” >99.5 acetone, then recrystallized several times from ethanol, and finally dried as above. The absence of water and/or acid in the salt was checked by means of IR spectroscopic analysis. The purity of the salt as determined by differential scanning calorimetry (DSC) was 99.85 mol %. 2.2. Mixture Preparation. To cover the whole composition range, 28 mixtures (about 2 g of each) were prepared by weighing the proper amounts of both components (with a precision of (10-5 g) in Pyrex test tubes, which were sealed under N2 atmosphere. After having been melted twice on a hot plate under stirring, the samples were cooled to room temperature, finely ground in order to get a homogeneous powder, and then stored in a dessiccator. The estimated error in the mole fraction x was (0.002. 2.3. Techniques. Differential thermal analysis (DTA) was performed by means of a FP-84 Mettler analyzer coupled with a FP-80 control unit in the temperature range 300-490 K using Al pans for more accurate analysis and Pyrex glass pans for visual observation, inserting the Mettler furnace in the microscope. The sample masses ranged between 2 and 4 mg ((0.0005 mg) and were analyzed at a heating rate of 5 K‚min-1 with a flux of pure N2. Calibration in temperature and energy of the instrument was obtained with the following standard materials: Appl. Sci. Lab. stearic acid >99.8%, NBS benzoic acid >99.997%, NBS indium >99.999%, and Perkin-Elmer tin >99.9%. Onset temperatures were read in all the experiments as in the calibration procedure.19 The temperature precision obtained was (0.1 K. DSC analysis was carried out by means of a Perkin-Elmer DSC-2 instrument in the temperature range 200-500 K, carefully calibrated as in ref 19. Samples (3-9 mg) of the mixtures tightly sealed in Al pans were analyzed at scan speed 5 and/or 10 K min-1 with a flux of pure He and N2 as the purge gas below and above room temperature, respectively. Onset temperatures were read as in the calibration procedure.19 The temperature precision was (0.1 K. Enthalpy values are considered to have standard
deviations within 1%. In some cases, records of the thermograms at lower heating rates of 2-3 K‚min-1 were obtained without finding any significant changes. The agreement between the DSC and DTA data above 300 K was within the experimental error. The data published correspond to measurements carried out with the DSC technique. The region of existence of the mesomorphic phases as well as their characterization were defined by polarizing microscopy, with a Zeiss Jenalab pol-30-GO527 microscope, equipped with an automatic Zeiss C-35 camera. The microscope was coupled with the above-mentioned Mettler furnace and an x-y plotter to record the thermograms. It has to be pointed out that the visual detection at the microscope was the only suitable technique one could use above about 380 K, inasmuch as the signals obtained by DTA or DSC on the small samples used were too low for detecting peaks corresponding to the transition from the lyotropic mesophase to the isotropic liquid.
3. Results and Discussion 3.1. Pure Components. The thermal behavior of thallium(I) n-decanoate in the superambient region was reported in previous works.20,21 Molar heat capacities and relevant thermodynamic functions of the pure salt as determined by equilibrium adiabatic calorimetry and DSC from 6 to 480 K have been published,22 with a thorough description of the phase transformations and of the stepwise melting process occurring in this compound. The data obtained by DSC on the sample used in the present work are in fair agreement with those provided previously.22 As for the pure acid, no solid-to-solid transition was found in the considered temperature range. Several authors have reported its melting point and enthalpy. An excellent agreement was found between the present data and those of Adriaanse et al.23 and Schaake et al.24 The experimental results obtained on the same two pure substances used in the preparation of the mixtures are listed in Table 1. The results obtained in this work for the molecular complex were the same as those published in ref 7 for the whole thallium series acid soaps. (18) Labban, A. K.; Lo´pez de la Fuente, F. L.; Cheda, J. A. R.; Westrum, E. F.; Ferna´ndez-Martı´n, F. J. Chem. Thermodyn. 1989, 21, 375. (19) Zabinska, G.; Ferloni, P.; Sanesi, M. Thermochim. Acta 1987, 122, 87. (20) Pelzl, G.; Sackmann, H. Mol. Cryst. Liq. Cryst. 1971, 15, 75. (21) Lindau, F.; Diele, S.; Kruger; Dorfler, H. D. Z. Phys. Chem. (Leipzig) 1981, 262, 775. (22) Lo´pez de la Fuente, F. L.; Cheda, J. A. R.; Ferna´ndez-Martı´n, F.; Westrum, E. F. J. Chem. Thermodyn. 1988, 20, 1137. (23) Adriaanse, N.; Dekker, H.; Coops, J. Recl. Trav. Chim. Pays-Bas 1964, 83, 557. (24) Schaake, R. C. F.; van Miltenburg, J. C.; de Kruif, C. G. J. Chem. Thermodyn. 1982, 14, 771.
Phase Behavior of Acid Soap System
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Figure 2. Phase diagram of the n-decanoic acid + thallium(I) n-decanoate system: A ) acid; IL ) liquid; M ) mesophase; MC′ and MC ) different energetic states but considered the same phase of the molecular complex; SI, II, III, IV, V ) crystalline phases of the salt; (SI) ) crystalline phase with internal conformational disorder.
Figure 1. DTA traces normalized to 1 mol of mixture (Y scale in arbitrary units): A, x < 0.5; B, x > 0.5, 290 < T < 360 K; C, x > 0.5, 360 < T < 490 K.
3.2. Binary System. The thermal behavior of the investigated mixtures was fairly reliable if the samples had been properly prepared and were really homogeneous. Figure 1 shows a tridimensional presentation of some DTA records for most of the considered mixtures. For the sake of clearness, in the following the phase relations occurring in the three regions of the binary system illustrated in Figure 1 will be discussed in the same order. In Figure 2 the complete T vs x phase diagram is illustrated. In all the defined regions, except in the isotropic liquid, IL, in the neat phase, M, and in the crystalline phase (SI), two conjugate phases, i.e., solid + solid, solid + mesophase, solid + liquid, and mesophase + liquid, are present. Eutectic Reaction. The region of compositions richer in acid (for x < 0.5) displays only a simple eutectic at x ) 0.125 and T ) 302.0 K. This eutectic has an enthalpy of 28.3 ( 0.3 kJ‚mol-1 and is formed by the reaction between the acid and a molecular complex (CM) consisting of the equimolar association of the acid and the salt. Denoting with A and S the pure acid and the pure salt, respectively, this reaction can be represented by
0.875A + 0.125S 9 8 Melt 9 8 0.857A + Tg302.0K T 0.960. In Figure 3, the small region of existence of (SI) is magnified. This phase may be considered as an internally conformational disordered solid phase (solid (25) Garcı´a, M. V.; Redondo, M. I.; Lo´pez de la Fuente, F. L.; Cheda, J. A. R.; Westrum, E. F.; Ferna´ndez-Martı´n, F. Appl. Spectrosc. 1994, 48, 338.
Phase Behavior of Acid Soap System
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Figure 9. Mixture of composition x ) 0.840 (the two pictures to the left): (a) solid (I) and isotropic liquid (IL) at 363 K (90 °C); (b) smectic A fan shape (neat phase) at 385 K (112 °C). Mixture of composition x ) 0.756, at 391 K (118 °C) (the picture to the right): coexistence (in equilibrium) of the isotropic liquid (black) and the lyotropic mesophase (fan shape). Observed by polarizing light microscopy (crossed polars).
solution), and its formation can be formulated as, referred to 1 mol of mixture: ∆H)4.69(0.04kJ/mol
MC′ + SII {\ } (SI)xg0.913 315.3eTe404.2K The solubility of this (SI) must be energetically low since is not observed by calorimetry and must decrease with increasing the temperature until the melting to the liquid crystal phase. In Figures 4 to 7, the Tammann-like plots, consisting in the representation of the enthalpy versus composition, related to the considered equilibria are illustrated. These diagrams, combined properly with the temperature evolution (constant or variable) of the peaks, contribute to the final resolution of the phase diagram. This was in particular the case of the so-called “effect 1”, which is shown in Figure 6. A separation of the total enthalpy involved in these peaks had to be made because the transitions appeared very close to each other. Lyotropic Mesophase Structure. Some interesting series of pictures taken at the hot stage polarizing microscope are presented as an example in Figures 8 and 9 for three compositions rich in salt. In Figure 8, the typical smectic A fan-shape (neat phase, or, using the lyotropic nomenclature, “planar structure” or “lamellar”), for a mixture of x ) 0.985, is shown at three specified temperatures. This texture was obtained on cooling the sample from the isotropic liquid and behaves exactly the same as the thermotropic mesophase of the pure salt, showing also a change in color with increasing temperature. A good definition of the mesophase region may be seen from the two pictures at the left of Figure 9, which corresponds to the composition x ) 0.840. Upon heating, the presence of the (SI) + isotropic liquid (IL) (darker parts of the picture) can be seen at 363 K (90 °C), and after further heating, the mesophase is clearly apparent at 385 K (112 °C). An example of the coexistence (in equilibrium) of the isotropic liquid (black color) and the lyotropic mesophase (enlighten fan-shape) is shown at the right of the same Figure 9, for a mixture of composition x ) 0.756, at 391 K (118 °C). 4. Conclusions The phase diagram of the n-decanoic acid + thallium(I) n-decanoate binary system has been carefully studied by
DSC and polarizing light microscopy coupled to a DTA, in the temperature range from 200 K up to the temperatures of existence of the isotropic liquid. The temperature vs composition and enthalpy of transition vs composition diagrams were thoroughly determined for each region of the binary system. By combining these diagrams, the domains of existence of the various phases were brought out. The main features of the binary system are as follows: a 1:1 molecular compound (acid soap), which melts incongruently at 322.2 K (peritectic point); an eutectic point between the acid and the acid soap at x ) 0.125 and T ) 302.0 K; a singular point at x ) 0.770 and 373.5 K (eutectic type), above which a lyotropic mesophase occurs. The designed MC′ f MC apparent transition taking place in the molecular complex at T ) 317.4 ( 0.1 K, associated with an enthalpy of 6.38 ( 0.06 kJ‚mol-1 needs some explanation. There is no doubt about the isotherm associated to this effect and its existence in the same range of composition than the peritectic. It was observed also in all the thallium(I) alkanoate + alkanoic acid systems.2,7 According to the phase rule, MC′ f MC cannot be considered as a thermal transition, and both MC′ and MC have to be the same phase. This effect could be explained as a “premelting” of the incongruent melting of the molecular complex. The isotherm associated to this effect has been drawn in the “temperature vs composition” diagram with a discontinuous line to distinguish it from the other ones that are really lines separating phase regions. The presence and properties of the lyotropic mesophase in the high-temperature and salt-rich region of the diagram are in fair agreement with those of the similar mesophases previously described for the systems n-tetradecanoic acid + thallium(I) n-tetradecanoate1 and n-heptanoic acid + thallium(I) n-heptanoate.2 The only feature that differentiates the studied phase diagram from those two diagrams is the presence of a new homogeneous region attributed to a solid solution phase, in the region of x > 0.913, and 315.3 K < T < 404.2 K, coinciding with the presence of a “condis phase” (conformational disordered solid phase), proved to exist in the pure thallium(I) n-decanoate in a previous work.25 This phase has been
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detected in some organic salt series and in many polymers and was considered as a new mesophase by Wunderlich et al.26 Acknowledgment. Partial support of this research by the DGICYT of the Spanish “Ministerio de Educacio´n (26) Wunderlich, B.; Moler, M.; Grebowick, J.; Baur, H. Conformational Motion and Disorder in Low and High Molecular Mass Crystals; Ho¨cker, H., Ed.; Springer-Verlag: Berlin, 1988.
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y Ciencia” (Grant in aid for Scientific Research PB960650) and by the Italian “Ministero della Universita` e della Ricerca Scientifica e Tecnologica” (Poject Cofin. MURST 97 CFSIB, Rome) is gratefully acknowledged. The authors thank Dr. C. Merino Casals for helpful discussions. An ECTS/ERASMUS grant from the European Commission, Brussels, is gratefully acknowledged by P.U. LA9904511
