Studies on the Preparation and the Structure of Lithium Soaps
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426 Grasas y Aceites Vol. 50. Fase. 6 (1999), 426-434 Oleochemicals I: Studies on the preparation and the structure of lithium soaps By Zein E. Shoeb, Bayed M. Hammad and A.A. Yousef National Research Centre, Dokki, Cairo. Egypt. RESUMEN 1. INTRODUCTION Oleoquímica I: Estudios sobre la preparación y la es The term metallic soap is used to describe alkaline tructura de los jabones de litio. earth and heavy metal salts of fatty acids that are Se han preparado estearato, palnnitato, miristato, laurato, caprato insoluble in water and either soluble in or solvated by y caprilato de litio por fusión. Se ha realizado el análisis elemental, los organic solvents, such as aromatic hydrocarbons, espectros de absorción en el infrarrojo y las difracciones de rayos X de chlorinated or hydrogenated hydrocarbons. They are los Jabones de litio preparados, además del análisis termogravimétrico also soluble in oils, greases and fatty acids preferably de los mismos. Las sales de litio anhidra de los ácidos con 12 átomos de naphthenate, linoleate, myristate, palmitate, stéarate, carbono o menos mostraron una estabilidad térmica hasta los and cerotinate successively. Lithium stéarate is used in 300 ± 78 °C dependiendo de la longitud de la cadena. Por otro lubricating grease as gelling agent because it gives a lado las sales con 14 átomos de carbono o más se descompusie better thermal resistance property ron a 126 ± 4 °C formándose carbonato de litio. Los jabones homólogos de litio mostraron una estructura cristalina Ralston (1) prepared the lithium soaps of lauric, similar, siendo los enlaces metal-oxígeno del mismo tipo para los myristic, palmitic and stearic acids by adding the ácidos grasos de número de átomos de carbono entre 8 y 18. calculated amount of lithium acetate to ethanol Sin embargo el ángulo de inclinación que forma la molécula con el plano basal aumentó con la disminución del número de solution of the respective fatty acids and determined átomos de carbono, lo que se observó por difracción de rayos X. the solubility of those soaps in a number of solvents. Se comprobó por espectroscopia infrarroja la intensificación They found that the lithium salts of the long chain del carácter iónico del enlace metal-oxígeno al disminuir el fatty acids are sparingly soluble in water and número de átomos de carbono. somewhat were soluble in ethanol. PALABRAS-CLAVE: Análisis elemental - Análisis termogravi The same authors reported the following melting métrico - Difracción de rayos X - Espectrofotometría infrarroja - points for the lithium soaps: laurate 229.2-229.8, Hidróxido de litio - Jabón metálico. myristate 223.6-224.2, palmitate 224-225, stéarate 220.5-221.5 °C. Lottmoser and Giese (2) reported the preparation of lithium salts of higher fatty acids and also measured the SUMMARY surface tension of their solutions. Void (3) prepared Oleochemicals I: Studies on the preparation and the lithium palmitate using precipitation method. structure of lithium soaps. Many investigators (4-7) studied the lubricant character of plastics and greases using lithium Lithium stéarate, palmitate, myristate, laurate, caprate and soaps. Barbooty et al., (8) and Mysak et al., (9) caprylate were prepared by means of fusion method. Elementary analysis and the infrared absorptions spectra of the prepared lithium studied the thermogravimetric analysis of some soaps as well as their X-ray diffractions and the thermogravimetric lithium soaps as lithium stéarate, lithium 12-hydroxy analyses were carried out. stéarate and related greases. The anhydrous lithium salts of fatty acids with 12 carbon In this paper we describe the preparation of six atoms or less showed thermal stability up to ca. 300 ± 78 °C depending on the chain length. On other hand the salts with 14 lithium soaps with fatty acids having carbon chain of carbon atoms or more were decomposed at 126 ± 4 °C leading to 18 (stearic acid) till carbon chain of 8 (caprylic acid). formation of lithium carbonate. These soaps will be prepared by fusion method and The homologous lithium soaps had very similar crystal subjected to Elementary analysis, IR-analysis, structure among them and their metal -to- oxygen bonds were similar for the acyl chains between 8 and 18 carbons. However thermogravimetric analysis, and finally X-ray the angle of inclination of the molecular axes to the basal plane diffraction measurements. increased with the decreasing of the number of carbon atoms of the fatty acid chain, as determined by X-ray diffraction. In addition, the ionic character of the metal -to- oxygen bond was enhanced with the decrease of the number of carbon atoms as shown by 2. MATERIALS AND METHODS infrared spectroscopy. KEY-WORDS: Elementary analysis - Infrared spectrophotometry - • Lithium hydroxid-1-hydrate. Assay 98 %; Riedel- Lithium hydroxide - Metallic soap - Thermal gravimetric analysis - de Haën. X-ray diffraction. • Fatty acids CI 8-C8 (Merck) Assay «GC» (95-99 %). (c) Consejo Superior de Investigaciones Científicas http://grasasyaceites.revistas.csic.es Licencia Creative Commons 3.0 España (by-nc) Vol. 50. Fase. 6(1999) 427 2.1 Preparation of Lithium Soaps 3.2 Infrared Absorption Spectra of Lithium Soaps The fatty acids used for the preparation of lithium soaps under investigation are stearic acid, palmitic acid, myristic acid, lauric acid, capric acid and Infrared absorption spectra of the lithium soaps caprylic acid. The lithium soaps of the above fatty under investigation, and their corresponding fatty acids are prepared by fusion method using lithium acids, namely, stearic, palmitic, myristic, lauric, capric hydroxide-1 -hydrate. and caprylic acid were determined with a pu 9712 infrared spectrophotometric recording by the Procedure: potassium bromide pellet technique in the sodium chloride region (4000-200 cm"^) at room temperature • Equimolecular amounts of the lithium hydroxide- (ca. 25 °C). 1 -hydrate and the corresponding fatty acid are Figure 1 showes the infrared absorption spectra melted together at elevated temperature. The of the stearic and the lithium stéarate. The required reaction temperature of each of the frequencies (cm'^) of the absorption maxima in the six lithium soaps is specific. both spectra are also given with assignments for the • The calculated weight of the fatty acid is most of them in Table II. weighed in 250 ml glass beaker which heated There are marked differences between the in an oil bath at the required temperature. spectrum of the stearic acid and that of the lithium • The corresponding calculated weight of the stéarate in some spectral region (Figure 1). The lithium hydroxide-1 -hydrate is added gradually absorption maxima which are characteristic of the and slowly to the melted fatty acid during a aliphatic portion of the acid molecule are essentially period of 20 minutes with constant stirring. unchanged on going from the acid to the soap. They • The reaction mixture is agitated (1000 r.p.m.) are assigned with ease by referring to a table of for another 20 minutes to complete the characteristic group frequencies. It is well known that dehydration reaction period during which the fatty acids in the solid state exist with the dimeric considerable foaming occures. structure through hydrogen bonding. The absorption • The resulting hot lithium soap is transfered maxima in the spectrum of the stearic acid at into a petri dish and cooled. On cooling the 3200-2600 (very broad, probably consisting of two soap solidifies to the required particle size. separate broad absorption), 1702, 1431, 1296 and 943 cm"'' are thus associated with the localized -COOH group of the acid molecule in the dimeric 3. RESULTS AND DISCUSSION structure. In the spectrum of lithium stéarate the strong absorption maximum at 1702 cm'^ which has 3.1 Elementary Analysis its orgin in the C = O of the acid molecule has disappeared completely. This disappearance of the The check of purity of the lithium soaps were carbonyl frequancy in the soap spectrum indicates made by the elementary analysis as show in Table I. the fact that there is a complete resonance in the two C - O bonds of the carboxyl group of the soap Table I molecule. Duval et al., (10) have shown from the extensive Elementary Analysis of Lithium Soaps investigations of the infrared absorption spectra of many metal salts of mono- or dibasic organic acid, Soap %U %c %H whether acyclic or aromatic that the coi frequencies lie in the region 1400-1300 cm'\ the coa frequencies Lithium stéarate Found: 74 12.5 Calculated: 2.4 74.3 12.1 in the region 1610-1550 cm'^ and the m frequencies in the region 950-800 cm"\ Lithium palmitate Found: 73.3 13.7 Similar assignments were made for all the lithium Calculated: 2.6 73 13.3 soaps: Again, the absorptions due to the aliphatic Lithium myristate Found: 70.6 11.2 portion of the soap molecule are in general similar in Calculated: 3 71 11.5 intensities and in their frequencies of absorption maxima, although slight but significant spectral Lithium laurate Found: 69.8 11.3 Calculated: 3.4 69.8 11.2 differences can be observed in some restricted regions. Lithium caprate Found: 66.7 10.2 Calculated: 3.9 67.3 10.7 Of course, there exist considerable differences in the absorptions due to the aliphatic portion of soap Lithium capiryiate Found: 63.4 10.2 molecules if the spectra of soaps of different fatty Calculated: 4.6 63.9 10 acids are compared. (c) Consejo Superior de Investigaciones Científicas http://grasasyaceites.revistas.csic.es Licencia Creative Commons 3.0 España (by-nc) 428 Grasas y Aceites 60 Stearic acid Lithium stéarate OL AOOO 3000 200 Wavenumber (cm" ) Figure 1 IR Absorption Spectra of Stearic acid and Lithium Stéarate Table II The Frequencies (cm"^) of Absorption IVlaxima in the Spectra of Lithium Soaps (Fatty Acids from Cs to Cis) in the region 4000-700 cm"^ Frequency (cm"^) ^^ Aecsînnûinûnt ASbiyílcmcili Li caprylate Li caprate Li laurate Li myristate Li palmitate Li stéarate 3446 (b) 3446 (b) 3446 (b) 3446 (b) 3446 (b) 3446 (b) Broad absorption of 0-H stretching vibration 2960 (sh) 2960 (sh) 2960 (sh) 2960 (sh) 2960 (sh) 2960 (sh) CH3, C-H antisym .