Ind. Eng. Chem. Res. 2004, 43, 5347-5349
5347
GENERAL RESEARCH Aqueous Solubility, Polymorphism, and Pseudopolymorphism of meso-1,2,3,4-Butanetetracarboxylic Acid Cletus E. Morris,† Brenda J. Trask-Morrell, Nancy M. Morris,* and Sarah L. Batiste Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 19687, New Orleans, Louisiana 70179
The solubility of meso-1,2,3,4-butanetetracarboxylic acid (BTCA) in water and the nature of the solid phases in BTCA-water mixtures were studied. The form of BTCA that was isolated after crystallizing from a saturated aqueous solution and remaining in contact with the solution for over 4 years was BTCA‚H2O. At 25 °C, a saturated aqueous solution of this product contained 6.99% BTCA. Other hydrates of BTCA were isolated after equilibration of BTCA with water at 25 °C for 1 h to 36 days. Thermal analysis identified these products as BTCA‚2H2O after 1 h, BTCA‚4H2O or BTCA‚2H2O after 5 h, and BTCA‚H2O after 15 and 36 days. The BTCA concentration in the liquid phase decreased from 18.67% after 5 h to 10.32% after 36 days. A saturated solution of BTCA‚2H2O contained 18.38% BTCA. Several thermograms for BTCA‚ H2O or BTCA‚2H2O contained endothermic peaks that are attributed to transitions between forms of anhydrous BTCA. The solubility of meso-1,2,3,4-butanetetracarboxylic acid (BTCA) in water is of interest because BTCA is effective as a formaldehyde-free cross-linking agent for cellulosic textiles.1,2 If BTCA were to be used commercially as a finishing agent for fabric, concentrated aqueous solutions would be desirable at two stages: the product as supplied by BTCA’s producer and its application to fabric by a reduced liquor (reduced wet pickup) process.3 We found previously that the solubility of BTCA in water decreases with time, the decrease being attributed to solution-mediated phase transitions to less soluble crystal forms of BTCA.4 However, we obtained no thermal analysis data for solid phases in equilibrium with the saturated solutions that were analyzed. The primary goal of the present work was to determine the solubility of the form of BTCA that is most stable (hence, least soluble) in an aqueous system. We also report thermogravimetric (TG), differential scanning calorimetric (DSC), and spectroscopic data on solid phases isolated from BTCA-water mixtures after equilibration for various periods, in addition to BTCA concentrations in the liquid phases. Experimental Section BTCA was obtained from Aldrich Chemical Co. Its claimed purity of 99% was verified by acidimetric titration. Deionized water was used in all experiments, and that used in solubility determinations was further purified with a Milli-Q Plus system. (Names of compa* To whom correspondence should be addressed. Current address: 52 New Salem Road, Griffin, GA 30223-6217. Tel.: (770) 467-0129. Fax: (770) 467-0199. † Deceased. 10.1021/ie030305x
nies or commercial products are given solely for the purpose of providing specific information. Their mention does not imply recommendation or endorsement by the U.S. Department of Agriculture over others not mentioned.) A form of BTCA that is stable in an aqueous system was obtained by crystallization from an aqueous solution containing 20% BTCA as described previously4 and allowing the crystals (which formed within 3 days) to remain in contact with the solution for over 4 years. This product was collected, washed with water, airdried, and stored in a vacuum desiccator over Drierite until its weight no longer decreased. Water in the dried product was determined by Karl Fischer analysis at a commercial laboratory. Mixtures of water and an excess of BTCA or BTCA‚ H2O were equilibrated at 25 °C by stirring in stoppered flasks thermostated by circulating water. For equilibration times greater than 24 h, the mixtures were kept at room temperature initially and stirred in thermostated flasks only for the final 25 h. The liquid phases were filtered through a 0.45-µm poly(vinylidene fluoride) membrane. Weighed aliquots of the filtrates were analyzed for BTCA by titration with standard NaOH. The reported solubility values are means of data from three replications and are given as the means plus/ minus standard errors. Solutions containing 20% BTCA were filtered and seeded with crystals of BTCA‚2H2O obtained as described previously.4 The mixtures were then processed as described for mixtures of water and BTCA; the equilibration time at 25 °C was 6 h. The recovered solids were air-dried. Thermal analyses were performed with a TA Instruments 2100 thermal analyzer under a dynamic nitrogen atmosphere. TG data were obtained with the automated Hi-Res TGA 2950 system. The samples were held at 60
This article not subject to U.S. Copyright. Published 2004 by the American Chemical Society Published on Web 07/07/2004
5348 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 1. Solubility of BTCA in Water at 25 °C after Various Equilibration Times equilibration time
wt % BTCA in the liquid phase
5h 15 days 36 days
18.67 ( 0.12 12.40 ( 0.21 10.32 ( 0.18
°C for 3 min, heated at 50 °C/min with a resolution setting of 5-300 °C, and held at 300 °C for 3 min. DSC data were obtained at a heating rate of 15 °C/min, using hermetically sealed sample pans. Infrared spectra of the solids were obtained at 4 cm-1 resolution on a Nicolet Magna550 FT-IR spectrometer with a SpectraTech IRPlan research-grade microscope and a liquid nitrogen cooled, Hg/Cd/Te detector. A Linkam heated stage for the microscope replaced the normal stage. The software used to collect the data was Nicolet Omnic Series, version 2.0. The reference spectrum was that of the clean diamond window (200 scans at a resolution of 4 cm-1). The samples were heated at 5 °C/min from ambient temperature to 220 °C. Data were collected at 75 scans per data set. The conditions of data collection resulted in averaging the data over about 30 s, which was approximately equivalent to acquiring spectra at 2.5 °C intervals. The data were imported into Grams32 (Galactic Industries) for plotting. Results and Discussion Solubility of a Form of BTCA That Is Stable in an Aqueous System. The starting material for this experiment was a monohydrate of BTCA (Karl Fischer water content, 7.69%; calculated for C8H10O8‚H2O, 7.14%). After equilibration in water for 24 h at 25 °C, the saturated solution contained 6.99 ( 0.17% BTCA. Thus, aqueous solutions of BTCA more concentrated than this must be metastable unless they contain another component that increases BTCA’s solubility. An aqueous solution containing 20% BTCA and an equimolar amount of triethanolamine (to form a salt of BTCA) was stable for at least 4 years. The recovered solids were stored over Drierite at reduced pressure until their weights no longer decreased. TG analyses verified that they were BTCA‚ H2O: the weight loss was 7.175 ( 0.003%, with maximum rates at 119-122 °C. The DSC thermograms contained incompletely resolved endothermic peaks with maxima at 121-123 °C (∆H ) 126-135 J g-1) and 135-148 °C (∆H ) 73-97 J g-1). The higher-temperature endotherms are evidence of a transition between forms of anhydrous BTCA. When the analyses were repeated after 6 months, the latter peaks usually were absent. Variation with Time of BTCA’s Solubility and of the Solid Phase in BTCA-Water Mixtures. The results of solubility determinations when anhydrous BTCA was equilibrated with water are presented in Table 1. The decrease in the BTCA concentration with equilibration time was consistent with the data in our previous reports.4,5 TG data in Table 2 show that the solids obtained by filtering portions of BTCA-water mixtures after equilibration for 1 h were BTCA‚2H2O: the total weight losses below 180 °C were 12.02-13.44% (calculated for the dihydrate, 13.34%). The DSC data for these solids showed both run-to-run variation and possibly also variation with storage time before analysis. Comprehensive in-
Table 2. Thermoanalytical Data for Solid Phases Recovered after Equilibration in Water at 25 °C TG data product no.
storage time before wt rate analysis (days) loss, maxima, TG DSC % °C
DSC endotherms below 190 °C peak temp, °C
∆H, J g-1
69.1 107.6 164.7 69.9 107.7 167.0 69.2 101.6 69.2 101.5 113.7 131.9
18.2 40.4 112.7 17.2 38.1 103.3 49.0 118.4 18.7 49.0 34.6 59.8
69.2 106.4 166.3
0.4 32.7 93.1
69.0 106.7 165.4
0.9 38.6 106.2
67.6 103.4 165.2 63.6 102.2 109.7 143.8 177.6 185.5
14.3 31.9 91.3 9.6 24.0 38.3 15.9 74.4 309.3
After Equilibration for 15 days 16 6.98 120.8 133.3 184.8 16 6.98 117.0 122.1 131.1 16 7.08 122.0 129.7 183.9 1 7.45 118.3 112.5 1 7.37 118.9 113.8
81.8 163.7 143.4 84.4 78.2 157.1 197.3 205.5
After Equilibration for 36 days 23 7.15 116.0 123.4 127.8 23 7.16 118.2 127.6
166.1 64.7 241.9
After Equilibration for 1 h 6 7.68 85.0 5.31 98.4
1
2
1
2
6
7.93 5.34
84.8 98.4
2
1
1
3
1
1
6.02 6.00 7.16 6.28
85.3 108.1 80.8 107.6
4
2
4
2
5
1
6
1
7
16
8
16
9
16
10 11
1 1
12
21
13
21
After Equilibration for 5 h 6 1.79 82.1 4.92 108.4 10.36 147.2 3.25 158.6 6 1.74 80.8 4.36 104.3 4.94 137.2 13.34 154.4 1 6.49 90.4 6.98 111.4 1
terpretation of these data and some of the other DSC data in Table 2 is beyond the scope of this study. However, the endotherms with maxima at 132-167 °C, well above the dehydration temperatures, are evidence of transitions between anhydrous forms of BTCA. The composition of the solids isolated after equilibration for 5 h varied. TG data for product 4 showed weight losses of 20.32-24.38% below 175 °C (calculated for BTCA‚4H2O, 23.52%). Upon storage, this product rapidly lost water, forming BTCA‚H2O. Product 5 was BTCA‚2H2O, as shown by its two-stage weight loss of 13.46%. FT-IR spectra of products 5 and 6 on the heated stage showed two stages of water loss: hydrate bands at 3381-3398 and 3440-3451 cm-1 were replaced by bands at 3498-3505 and 3574-3586 cm-1, with the latter pair of bands last being seen at 90 °C. The DSC data for all three products showed substantial endotherms at 165-178 °C, and at least the one for product 5 is attributed to a polymorphic transition of anhydrous BTCA.
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5349 Table 3. Thermoanalytical Data for Solid Phases Recovered after Seeding BTCA Solutions with BTCA Dihydrate TG data product no.
storage time before analysis (days) TG DSC
14
25
3
15
19
21
16
19
21
DSC endotherms below 190 °C
wt loss, %
rate maxima, °C
peak temp, °C
∆H, J g-1
4.22 7.28 2.04 11.15 2.16
88.8 101.9 140.9 105.4 128.2
8.26 4.63 0.40
88.1 98.2 125.4
77.3 97.7 116.2 74.5 109.8 166.8 80.1 98.8 123.7
62.8 67.0 226.8 37.2 42.6 100.7 75.1 61.8 257.6
concentration of BTCA in the liquid phase. After 1 h, the solids were dihydrates. After 5 h, they were either a dihydrate or a tetrahydrate, and after 15 days or longer, they were monohydrates. The BTCA content of the liquid phase was 18.67% after 5 h but had decreased to 10.32% after 36 days as the solid phases in the system had changed. The solubility of a BTCA dihydrate in water at 25 °C was found to be 18.38%. DSC data for the hydrates included endotherms at 131-132 °C (two products) or 165-167 °C (three products), well above the temperatures at which the water of hydration was lost from these products. We interpret this as evidence of transitions between anhydrous forms of BTCA. Investigation of these forms was beyond the scope of this study. Acknowledgment
The solids isolated after equilibration for 15 days had weight losses of 6.98-7.45% at 117-122 °C (calculated for BTCA‚H2O, 7.144%). The DSC thermograms for products 7 and 9 were very unusual in that they contained no melting endotherm (usually at least 340 J g-1) at the usual temperature (199-205 °C) for BTCA‚ H2O. Instead, they contained smaller (193-196 J g-1) endotherms at 224-233 °C. The TG data for the solids isolated after equilibration for 36 days showed that these products also were BTCA‚ H2O. Solubility of a BTCA Dihydrate. At 25 °C, the saturated aqueous solutions of BTCA‚2H2O with which 20% BTCA solutions were seeded contained 18.38 ( 0.06% BTCA. TG data in Table 3 show that the recovered solids still were BTCA‚2H2O 19-25 days after they were collected: the mean weight loss below 175 °C was 13.38 ( 0.08%. However, when the TG analyses were repeated after an additional 3-4 months, these weight losses had decreased to 4.5-10.0%. The DSC thermogram for product 15 was quite similar to that for product 5. Conclusions The form of BTCA that is stable in an aqueous system at room temperature is a monohydrate. At 25 °C, a saturated aqueous solution of this form contains 6.99% BTCA. This accounts for the instability of more concentrated solutions that we noted previously. The solid phases in mixtures of water and an excess of anhydrous BTCA varied with time, as did the
We thank Andre Johnson for the thermal analysis data. Supporting Information Available: DSC and TGA thermograms. This material is available free of charge via the Internet at http://pubs.acs.org. Nomenclature BTCA ) meso-1,2,3,4-butanetetracarboxylic acid DSC ) differential scanning calorimetric TG ) thermogravimetric
Literature Cited (1) La¨mmermann, D. New Possibilities for Non-Formaldehyde Finishing of Cellulosic Fibres. Melliand Textilber. 1992, 73, E105E107, 274. (2) Welch, C. M. Formaldehyde-Free DP Finishing With Polycarboxylic Acids. Am. Dyest. Rep. 1994, 83 (9), 19. (3) Morris, C. E.; Harper, R. J., Jr. Effects of Wet Pickup Variation and Swelling Pretreatment on Properties of Cotton Fabrics Crosslinked with Butanetetracarboxylic Acid. Am. Dyest. Rep. 1991, 80 (9), 58. (4) Morris, C. E.; Morris, N. M.; Trask-Morrell, B. J. Variation in Solubility and Crystal Form of meso-1,2,3,4-Butanetetracarboxylic Acid. Ind. Eng. Chem. Res. 1992, 31, 1201. (5) Morris, C. E. Solubility of meso-1,2,3,4-Butanetetracarboxylic Acid and Some of Its Salts in Water. J. Chem. Eng. Data 1992, 37, 330.
Received for review April 4, 2003 Revised manuscript received April 22, 2004 Accepted April 29, 2004 IE030305X
