Anomalous Viscosity Behavior of Fatty Acid Esters in Solution J.A. Ibemesi* and I.O. Igwe Department of Pure and Industrial Chemistry, Universityof Nigeria, Nsukka, Anambra State, Nigeria 147 Solution viscosity of oils of melonseed (Colocynthis vulgaris Schrad.), soyabean [Glycine max (L). Merr], and rubberseed [Hevea brasiliensis (Kunth) Muell.] were determined in different solvents at various concentra- tions. Reduced viscosity (~red) VS concentration (C) plots showed three types of behavior: ~red increased linearly with C {2-30 g/dl); ~red levelled off with C (2-1.2 g/dl); and ~red rose steeply with decrease in C (below 1.2 g/dl), the latter behavior is anomalous. Similar viscosity be- havior was observed for the methyl ester of melonseed oil, methyl palmitate and n-butyl acetate. Molecular cluster formation was believed to cause the observed anomalous viscosity behavior, the extent of which var- ied, depending on the nature of the fatty acid ester and the solvent. KEY WORDS: Anomalous, behavior, fatty acid esters, molecu- lar cluster, solution, viscosity. Fats and oils are fatty acid esters of glycerol. They are amphipathic in nature due to the presence of the polar glyceride (ester) group and the nonpolar hydrocarbon chain. Both segments play significant roles in deter- mining the physical and chemical properties of the glycerides, such as degree of unsaturation, molecular weight, density, acidity, etc., which, in turn, influence other oil properties such as melting point and viscos- ity. The viscosity of oils in the pure state has been the subject of many studies, particularly in the older lit- erature. These studies have established some of the effects of the physical and chemical properties of oils on their viscosities. For instance, oils containing fatty acids have been found to show decreasing viscosity with increasing degree of unsaturation (1). Also, the viscosity of a fatty oil was found to increase with increasing molecular weight of the oil if saturated ac- ids alone were present (1). A study of the influence of density on viscosity of oils showed that if oils were divided into three groups, namely, drying, semidrying and nondrying (i.e., on the basis of their degree of unsaturation) viscosity increased considerably with den- sity in each group (2). The study of the solution viscosity of oils has also received attention in the literature. Johnstone et al. (3) determined the viscosities of mixtures of soyabean oil and hexane, ethylenedichloride and trichloroethylene, and reported rapid increase in fluidity and the possibil- ity of molecular complex formation in solvents that can form hydrogen bonds. Luck et al. (4) determined the apparent specific volume (+v) and intrinsic viscosi- ties (~) for the oils of olive, peanut, soyabean and lin- seed in CS2, Et20, C6H14, C6H12 and C6H6, and found that the +v of the oils in solution was usually smaller *To whomcorrespondenceshouldbe addressed. than that of the pure oils, and that the (~) in C6H12 and C6H6 and tetralin were greater than those for the other solvents. The latter observation was attributed to the uncoiling of fatty acid chains in the three solvents. More recently, Zymrya et al. (5) determined the viscos- ity of corn oil-ligroine solutions at 0-100% corn oil with a capillary viscometer, and observed that viscos- ity increased rapidly and nonlinearly with increasing corn oil concentration in solution. In the course of a study of network formation in drying oils by dilute solution viscometry in our labora- tory (6), it was observed that the intrinsic viscosities of both the pure and polymerizing oil showed a steep rise in the dilute concentration range--0.170 to 0.869 g/dl, in toluene. This behavior is anomalous and, to our knowledge, has not been reported for fatty acid esters, including fats and oils. The objective of this work is to examine in detail this anomalous viscosity behavior by extending the concentration range and using other solvents in order to establish its validity and scope, and perhaps attempt to offer a molecular explanation for its occurrence. EXPERIMENTAL PROCEDURES Materials. Vegetable oils. The oils of rubberseed [Hevea brasilliensis (Kunth) Muell.], soyabean seed [Glycine max (L). Merr] and melonseed [Colocynthis vulgaris Schrad.] were solvent extracted and bleached with fuller's earth. Rubber and melonseed oils were alkali refined by Cocks and Reed's method (7) to reduce their acid numbers from 29.12 and 2.38 to 0.78 and 0.31, respectively. Soyabean oil has a low acid number of 0.27. The iodine values of the oils determined by the ASTM method (D1959-69, 1973} are rubber seed oil, 137.56; soyabean oil, 130.45; and melonseed oil, 113.70. Solvents. For viscosity measurements, the follow- ing analytical grade solvents were used without fur- ther purification: toluene, cyclohexane, xylene and tetrahydrofuran were products of Merck (Darmstadt, Germany); chloroform was purchased from BDH Chemi- cals Ltd. (Poole, England); and dichloromethane and carbon tetrachloride were products of Vickers Lab. Ltd. (Burley-In-Wharfedale, West Yorks, England). Methyl esters of melonseed and palmitic acid. Both esters were prepared by refluxing 10 g of the oil or acid with 300 cm 3 of 2% methanolic sulphuric acid for 1 hr. The resulting mixture was treated with 500 cm 3 of distilled water in a separatory funnel, and then ex- tracted twice with 250 cm 3 portions of petroleum ether (b.p. 40-60~ The combined extracts were washed until acid-free. The extract was dried with anhydrous sodium sulphate and the solvent removed under a stream of N2 on a steam bath. Butyl acetate was also from Merck, and was used as obtained. Equipment. A viscosity bath, a thermoheater and a Cannon-Fenske viscometer, size 50, were used. Preparation of ester solutions for viscosity meas- JAOCS, Vol. 68, no. 3 (March 1991)