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SEPARATIONS Removal of Decomposition Products from Sodium Oleate Edward Mularczyk ul. M. C. Sklodowskiej 15/46, 50-381 Wroclaw, Poland
Jan Drzymala* Technical University of Wroclaw, Wybrzeze Wyspianskiego 27, Wroclaw, Poland
Sodium oleate (NaOl) is air- and light-sensitive and decomposes during storing. When highpurity NaOl is needed, a fresh sample can be purchased from a chemical company or it can be purified in the laboratory. In this work a procedure of NaOl purification was worked out which consists of dissolution of NaOl in boiling ethyl alcohol (containing 4% water), removal of the insoluble impurities by hot filtration, followed by precipitation of NaOl from the solution at 4 °C. The infrared spectroscopy tests revealed that the main decomposition products of NaOl are oleic acid and sodium bicarbonate, apparently formed due to reaction with carbon dioxide and water vapor from the air. In addition to that, an alkaline environment facilitates partial decomposition of oleate leading to the formation of impurities responsible for the yellow color and a characteristic odor of the oleate. The presence of these impurities was detected by liquidgas chromatography, but their chemical formulas were not determined. The presented procedure of purification removes from the aged sodium oleate its decomposition products but does not eliminate sodium salts of other fatty acids when present in the purified sample. Introduction Oleic acid and its sodium and potassium salts are important industrial and laboratory reagents. Oleates are used as surface active agents, components of paint, and as chemicals in the textile, leather, cosmetics, pharmaceutical, agricultural, and mineral processing industries. Literature on oleates in the form of papers, reports, and patents is enormous. In 1994 alone there were nearly 3000 entries in Chemical Abstracts on oleic acid ((Z)-9-octadecenoic acid) and its derivatives. These facts indicate that oleates are very frequently used in industry and research laboratories. In many applications oleate samples should be of high purity. Unfortunately, sodium oleate (NaOl) is not very stable because it slowly decomposes under the influence of air and light. The signs of decomposition include changes of color from white to yellow, a characteristic odor, and a tendency of the oleate flakes to be sticky and aggregate. It was observed in our laboratory that the aqueous solutions prepared from aged samples of NaOl are turbid and their pH values are much lower than that of pure sodium oleate solutions. According to Han et al. (1973) a prolonged 24 h bubbling of nitrogen through a 2.5 × 10-5 M NaOl aqueous solution removes the turbidity, but our tests showed that it does not occur for more concentrated solutions of aged oleate. It is common practice in laboratory work that organic substances are purified before use and upgrading procedures are known for a great number of reagents. For instance xanthates are routinely purified by dissolution in dry acetone, removal of the impurities by filtration, and addition of ether to the filtrate to precipitate xanthate (Rao, 1971; Harris, 1988). To our knowledge a simple and direct method of purification is unavailable for NaOl. Instead, indirect methods are used in which sodium oleate is converted to oleic acid and the acid is 0888-5885/96/2635-0788$12.00/0
purified using a variety of procedures (Mularczyk and Drzymala, 1989). A need for a procedure of purification of sodium oleate samples inspired us to work out such a method as well as identify the main impurities. This paper reports the results of our investigations. Experimental Section Materials. Sodium oleate (NaOl) and oleic acid (HOl) were purchased from the J. T. Baker Chemical Co. in 1985. The chromatographic analysis indicated that the NaOl sample in 1985 contained 80% oleate, 3.5% myristate, 5.2% palmitoleate, 7% palmitate, 3% stearate, and 1.3% unspecified compounds. The content of oleic, myristic, palmitoleic, palmitic, and stearic acids in the oleic acid sample was very likely similar because NaOl is produced from HOl by neutralization with NaOH. The sample of NaOl has been stored in a closed but not sealed white semiopaque plastic bottle on a shelf in our laboratory at room temperature for 9 years. The sodium oleate sample has developed a yellow color, characteristic odor, and greasy texture indicating its partial decomposition while oleic acid turned lightly yellow. The sample of the aged NaOl was used to working out a procedure for its purification while HOl, stored in the laboratory in a dark-brown glass bottle, served as a reference material. Sodium Oleate Purification. A scheme of NaOl purification is given in Figure 1. A 50 g sample of NaOl was placed in a 1 L round-bottom Pyrex flask that contained 500 cm3 of 96% ethyl alcohol and 4% water azeotrope also called rectified alcohol. The mixture was boiled for 30 min, and then the fraction which was insoluble in the boiling alcohol was separated from the solution by vacuum filtration. The filtrate was cooled and kept for several hours at 4 °C for precipitation of NaOl. The precipitated sodium oleate was separated © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 789
Figure 1. Scheme of purification of aged impure NaOl sample.
from the supernatant by vacuum filtration and rinsed several times with cold (-10 °C) 96% ethyl alcohol. Next, the crystallization was repeated three times. The purified NaOl was obtained from combined filtrates of the third and fourth crystallizations by reducing the volume of the solution by half by evaporation of ethanol followed by a final precipitation from ethanol at 4 °C. The precipitate of the purified NaOl was separated from the supernatant under a reduced pressure, rinsed with cold (-4 °C) rectified ethanol, and finally dried in a vacuum drier. The precipitate from the fourth-crystallization was a low-grade NaOl while fractions 1-4 were the wastes of the process. During crystallization the ratio between the mass of the sample and the volume of ethanol was 1:10. The yield of the purified NaOl was 31%. Transformation of NaOl into the Acidic Form. To analyze the fractions produced during purification of sodium oleate samples by means of liquid-gas chromatography, each product was converted into the acidic form. This was accomplished in a glass column 2 cm in diameter and filled with 50 cm3 of 70% ethanol solution in water and 10 g of the activated air-dry Dowex 50WX2 cationite. A 1 g sample of each analyzed fraction was dissolved in 20 cm3 of 70% ethyl alcohol, passed through the column with the cationite, and then the cationite was rinsed twice with 20 cm3 of 70% ethyl alcohol. The solutions passing through the column were subjected to removal of the alcohol by evaporation in a vacuum evaporator. To remove traces of water from the produced sample, 20 cm3 of anhydrous ethyl alcohol was added to dissolve the sample and the alcohol in the form of water-alcohol azeotrope was again evacuated by evaporation. Analytical Methods. Sodium oleate and various fractions produced during its purification were analyzed with a Perkin-Elmer FTIR spectrophotometer and a
Figure 2. Infrared (IR) spectra of aged (curve 1), purified (curve 4) sodium oleate, and two other fractions produced during purification of sodium oleate sample.
Hewlett-Packard gas-liquid chromatograph equipped with a HP-1 cross-linked methyl silicone gum capillary column that was 12 m long and 0.2 mm in diameter. For the chromatographic analysis the samples were transformed into acidic form with the Dowex 50WX2 cationite according to the procedure given in the previous paragraph, and then analyzed at 240 °C in the form of 10% solutions in chloroform. Results and Discussion Storing sodium oleate at ambient temperature and in contact with air results in its decomposition. A comparison of the IR spectrum of the 9 year old sodium oleate sample (curve 1 in Figure 2) with that of purified NaOl sample (curve 4 in Figure 2) indicates that the main decomposition products are oleic acid and sodium bicarbonate. The presence of oleic acid in the aged sample is indicated by the peak at 1710 cm-1. The presence of sodium bicarbonate is barely seen on curve 1, but strong evidence of its existence is provided by a peak at 1624 cm-1 on the IR spectrum of the fraction that is insoluble in boiling ethyl alcohol (curve 2, Figure 2). The conclusion that the decomposition products of NaOl include HOl and NaHCO3 is based on the fact that the most characteristic adsorption band of oleic acid and sodium bicarbonate are 1710 (Keller, 1986) and 1624 cm-1 (Nyquist and Kagel, 1975), respectively. The IR spectra also indicate that the fraction insoluble in boiling alcohol (curve 2 in Figure 2) is a complex mixture
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of bicarbonate, oleic acid, and sodium oleate (peak at 1559 cm-1 (Keller, 1986)) while the solution after precipitation of NaOl (curve 3) contains oleic acid and sodium oleate. The main product which was produced by purification, referred to as the purified NaOl (curve 4), contains neither bicarbonate nor oleic acid. The presented IR spectra indicate that water vapor and carbon dioxide from the air react with sodium oleate leading to the formation of sodium bicarbonate and oleic acid according to the reaction
NaOl + H2O + CO2 ) HOl + NaHCO3
(1)
The neutralization of NaOl with CO2 reduces the alkaline character of the sample. Therefore, aqueous solutions of aged NaOl are less alkaline than those of pure or purified samples. For instance, in our tests the pH of 0.001 M aqueous solution of the aged NaOl sample was 8.88 while the pH of the purified sample was 9.85. A similar value of pH for the 0.001 M aqueous solution of pure NaOl was found by Zimmels et al. (1975). Pure sodium oleate is white and has a soapy smell while the aged sample has a yellow color and a characteristic odor caused by impurities other than HOl and sodium bicarbonate. Unfortunately, these impurities were not detected by the IR analysis. Therefore, the chromatography technique has been employed and the results are given in Figure 3. First of all, the chromatograms show that the investigated sample contains not only oleate but also stearate (STE), palmitate (PAT), palmitoleate (PAL), and myristate (MYR). These compounds are commonly present in oleate samples. Their concentrations in different fractions produced during purification are given in Table 1. The chemical analysis of the aged oleic acid sample is also given in Table 1 for comparison. A comparison of the chromatographic data given in Figure 3 and Table 1 for aged NaOl and aged HOl indicates that storing NaOl samples for 9 years reduces the content of NaOl from about 80% to about 45% due to the formation of oleic acid and sodium bicarbonate as well as various decomposition products of the oleate molecule. The additional decomposition products are visible in Figure 3 (curves 1-3) as peaks for the elution time between 4 and 5 min. For instance, in the aged NaOl the content of these impurities was about 15%. However, due to lack of adequate standards we were unable to determine the chemical formula of the impurities. These impurities are not present in the 3-fold purified NaOl sample (curve 4 in Figure 3). According to Park and Obendorf (1994) the decomposition of oleate leads to the formation of 8-, 9-, 10-, and 11-monohydroperoxides, their dimers and polymers (primary oxidation products), epoxyhydroperoxides, ketohydroperoxides, dihydroperoxides (secondary oxidation products) as well as volatile breakdown products (aldehydes, ketones, alcohols, hydrocarbons), and acids of shorter than oleate carbon length. It can also be seen from Figure 3 and Table 1 that the purification procedure increases the NaOl content in the sample from a small number (the sum of NaOl and HOl in the aged sample was 46%) to 74%, that is, to the value comparable to the oleate content in the original unaged sample. Conclusion It appears from this work that aged sodium oleate samples can be purified by dissolution and precipitation
Figure 3. Chromatograms of different fractions produced during purification of aged sodium oleate sample. Before analysis the fractions were transformed into acid form with a cationite. Identification: 1, myristic acid (MYR). 2, palmitoleic acid (PAL). 3, palmitic acid (PAT). 4, oleic acid (HOl). 5, stearic acid (STE). Table 1. Chemical Composition of Different Fractions Produced during Purification of Sodium Oleate Samplea fraction (%) aged NaOl fraction 1 (insoluble in boiling ethanol) fraction 2 (soluble in ethanol at 4 °C) fraction 3 (low-quality NaOl) purified NaOl Aged HOl
HOl
MYR
PAL
PAT
STE
45.7 69.8
5.7 6.4
3.1 5.4
8.1 9.0
3.0 0.8
34.0
1.2
4.9
1.0
17.9
65.9 73.9 80.2
5.0 4.4 6.1
0.3 4.1 5.1
13.7 6.4 7.1
2.2 2.5
