In the Laboratory
Viscosity of Common Seed and Vegetable Oils C. Wes Fountain, Jeanne Jennings, Cheryl K. McKie, Patrice Oakman, and Monty L. Fetterolf* Department of Chemistry, University of South Carolina Aiken, Aiken, SC 29801 Viscosity experiments using Ostwald-type gravity flow viscometers are not new to the physical chemistry laboratory. Several physical chemistry laboratory texts (1–3) contain at least one experiment studying polymer solutions or other well-defined systems. Several recently published articles (4–8) indicated the continued interest in using viscosity measurements in the teaching lab to illustrate molecular interpretation of bulk phenomena. Most of these discussions and teaching experiments are designed around an extensive theory of viscous flow and models of molecular shape that allow a full data interpretation to be attempted. This approach to viscosity experiments may not be appropriate for all teaching situations (e.g., high schools, general chemistry labs, and nonmajor physical chemistry labs). A viscosity experiment is presented here that is designed around common seed and vegetable oils. With the importance of viscosity to foodstuffs (9) and the importance of fatty acids to nutrition (10), an experiment using these common, recognizable oils has broad appeal. An empirical trend between the apparent viscosity, ηapp, and the percentage of linoleic acid or oleic acid in the oil is demonstrated for six oils: olive, peanut, sesame, wheat-germ, sunflower, and safflower. Other oils tested were corn, canola, and castor. The observed trends are discussed in terms of the molecular structure of the fatty acid components and their relative amounts in each oil. This experiment demonstrates how the basic chemical concept of molecular structure can be used to help understand the physical behavior of common materials. This experiment also opens discussions that address the problems in using simple structural interpretations. Materials and Procedures The oils used—olive, peanut, sesame, corn, canola (rapeseed), wheat-germ, sunflower, safflower, and castor—can be purchased in large quantities at grocery stores, health-food stores, and pharmacies, which keeps the cost to a minimum. Major name-brand oils were used where possible for the sake of name recognition, but this is not necessary. Viscosity data were obtained under ambient temperature conditions, approximately 24 °C. The viscosities of several oils were recorded at different temperatures and the temperature responses were sufficient to warrant monitoring the ambient temperature. Individual viscosity experiments performed at temperatures that differ by more than a few degrees may not reveal consistent data trends. The viscometer used in this work was a CannonFenske Routine Type Viscometer manufactured by the Cannon Instrument Company, State College, PA. The instrument is calibrated by the manufacturer and the calibration constant is supplied with the instrument. A diagram of this specific type of Ostwald viscometer can be found on page 372 of Experiments in Physical Chemistry by Shoemaker et al. (1). The viscometer is a U-shaped *Corresponding author.
224
piece of glass consisting of a measurement side and a filling side. The filling side is a straight piece of glass approximately 1 cm in diameter and a large reservoir (approximately 3 cm in diameter) at the bottom of that side. The measurement side consists of two in-line reservoirs 2-cm in diameter, above a capillary tube approximately 9 cm long that ends at the bottom of that side. The two sides are connected by a curved piece of glass tubing. Each viscometer has a capillary of fixed diameter. Two reference marks appear on the measurement side of the viscometer, one above and one below the second reservoir just before the capillary. The flow time for liquid to move from one reference mark to the other is directly proportional to the viscosity. The apparent viscosity, ηapp, in centipoise is obtained from ηapp = t × ρ × Co
(1)
where t is the flow time, r is the density of the oil at room temperature, and Co is the calibration constant for a particular viscometer. The viscometer used for most of this work has a capillary diameter of approximately one millimeter (Size 200, Co = 0.989 cSt/s), which allowed flow times to range from 567 s for corn oil to 855 s for olive oil. These flow times are within the recommended range set by the manufacturer. A second viscometer (Size 450, C o = 2.732 cSt/s) with a capillary diameter of approximately 3 mm was used for castor oil because of its relatively high viscosity. Three millimeters should be the upper limit on the capillary diameter for any of the oils used in this study. Larger-diameter capillaries increase the relative error in start/stop times and introduce other errors resulting from decreased flow times. Recently, several reports have appeared in this Journal (6, 11) of simple and inexpensive viscometers that would be ideal for this work. The experimental procedure is as follows. The viscometer is cleaned with detergent and warm water, rinsed thoroughly with distilled water, and allowed to dry normally under room conditions if time permits or rinsed with acetone and aspirated dry. Approximately ten minutes is allowed for an acetone-rinsed glass viscometer to come to room temperature. The viscometer is then mounted vertically on a ring stand with a clamp. The oil (7 mL) is introduced in the fill side of the viscometer and drawn up into the first reservoir on the measurement side using a pipet bulb. The pipet bulb is removed and the oil flows down through the second reservoir and the capillary. An electronic timer is started as the oil level passes the first reference mark and is stopped as the oil level passes the second reference mark. Each oil is tested at least three times and the average viscosity is reported. Since this procedure requires thirty to forty minutes before oils can be changed, the work can be divided among students. One student is given a viscometer and assigned a certain number of oils for which to gather data. At the end of a lab period, the data for all oils are shared with all of the participating students. This addition to the procedure stresses individual work and its importance in a team effort; this is a concept not usually approached in a teaching lab situation.
Journal of Chemical Education • Vol. 74 No. 2 February 1997
In the Laboratory
Results
Table 1. Experimental Data and Calculated Apparent Viscosity of Various Seed and Vegetable Oils
Table 1 shows the data for these experiments. It lists the density of each oil, obtained from the Merck Index (12); the average flow time for each oil, obtained experimentally; and the apparent viscosity, calculated using eq 1. Standard errors are also reported. For the error analysis, the range of experimental flow times for each oil is used as an estimate of flow time error, and the reported density range from the literature source (12) is used as an estimate of the density error. For canola oil, density was measured as the slope of a simple linear regression of weight versus volume. A similar experiment was performed for corn oil using the literature value for corn oil density as a standard. The canola oil density reported is the value obtained using this corn oil standard. The viscometer constants were used as absolute values. The oils are presented in two groups. The first group contains six oils that demonstrate a linear trend in ηapp when plotted versus the percentage of either linoleic or oleic acid. The second group contains the remaining three oils that do not follow this trend. The least viscous oil is corn oil, with ηapp of 51.6 cP, and the most viscous oil is castor oil, with ηapp of 749 cP. The anomalous flow time for castor oil reflects the use of the large-bore capillary viscometer discussed previously. The values listed for ηapp for olive and castor oils are consistent with values reported elsewhere (13). Eight oils are plotted as ηapp versus percent linoleic acid (Fig. 1) and percent oleic acid (Fig. 2). Both figures clearly show the linear trend in ηapp established by olive, peanut, sesame, wheat-germ, sunflower, and safflower oils as functions of both fatty acid components. The three oils shown in Table 1 behave differently. Corn oil and canola oil are both less viscous than would be expected from the trends shown in Figures 1 and 2. Castor oil is extremely viscous relative to the other eight oils and for this reason cannot be conveniently shown in either figure.
Density (g/mL)a
Oil
ηapp (cP)
Flow Time (s)
Oils showing linear relationship between ηapp and % linoleic or oleic acid
Olive
0.912
85.53
77.1 ± 0.2
Peanut
0.913
74.01
66.8 ± 0.8
Sesame
0.918
66.50
60.4 ± 0.3
Wheat-germ
0.928
66.23
60.9 ± 0.3
Sunflower
0.917
61.96
56
Safflower
0.921
58.53
53.3 ± 0.3
±1
Oils not showing linear relationship between ηapp and % linoleic or oleic acid
Corn
0.919
56.72
51.6 ± 0.2
Canola
0.913
69.55
63
Castor
0.961
28.53
749
±2
a In every case except castor oil and canola oil, the midpoint of the range of density values at 25 °C was used for calculations. For castor oil, the range was obtained at 15.5 °C and the lowest value was used for calculations because density should decrease as temperature increases. For canola oil, the density was determined as described in the text.
Analysis and Discussion As a liquid moves down a glass capillary tube, the molecules closest to the wall will encounter the greatest resistance to flow. Progressively less flow resistance will be encountered by molecules whose positions are closer to the center of the capillary tube. The interactions of molecular layers adjacent to one another in the flowing liquid determine the time it will take for a given volume of liquid to move through the capillary under the influence of gravity. The two primary factors that affect the viscosity of a liquid or solution at a given temperature are the molecular structure of the liquid or solution components and the intermolecular forces operating within the liquid or solution. Molecules with long chains and structures that allow entanglement with adjacent molecules will slow the progress or flow of liquid through the capillary tube. Strong attractive intermolecular interactions such as those seen in hydrogen bonding and strong dipole–dipole forces will result in the same transfer of flow resistance from the walls of the capillary to the interior liquid. To apply this simple interpretation for ηapp to these common vegetable and seed oils, the physical characteristics of the molecules that make up each oil will be discussed. Table 2 lists the component fatty acids of each oil studied. They are presented as weight percent total fatty acids as found in the Handbook of Chemistry and Physics (13). The data for canola oil were obtained elsewhere (10). Once again the oils are
Figure 1. Apparent viscosities of the oils plotted vs. the percent linoleic acid of each oil. Castor oil is omitted from the figure. The following abbreviations are used for the oils: olive, OL; canola, CN; peanut, PN; sesame, SS; corn, CR; wheat-germ, WG; sunflower, SN; safflower, SF. The data used in the regression analysis are shown as darkened squares.
Figure 2. Apparent viscosities of common oils plotted vs. the percent oleic acid of each oil. Castor oil is omitted from the figure. Abbreviations are the same as in Fig. 1. The data used in the regression analysis are shown as darkened squares.
Vol. 74 No. 2 February 1997 • Journal of Chemical Education
225
In the Laboratory
grouped as in Table 1. Oils in the first group are fairly homogeneous in their components: at least 80% of the fatty acid weight comes from oleic and linoleic acids and each contains three to six different saturated fatty acids. Of the remaining three oils, castor contains a substantial percentage of ricinoleic acid. This acid is substantially different structurally from oleic and linoleic, the major component fatty acids from the first group of oils. Canola and corn oils have fatty acid compositions similar to the first group of oils. Corn oil does contain a small fraction of palmitoleic acid and canola oil contains substantially more linolenic acid than any of the first group oils. Since the physical structure of the fatty acids will be important for data interpretation, Table 3 provides further information regarding the fatty acids found in the oils used in this work. The common name for the fatty acid is shown first, followed by the shorthand notation explained in Table 2, the systematic name as obtained from the Handbook of Chemistry and Physics (13), and the structural formula. All double bonds in the unsaturated fatty acids are cis isomers and only ricinoleic acid (with a hydroxyl group at C-12) contains a functionality other than carboxylic acid and double bonds. Clearly, even the simplest oils contain at least five component fatty acids. To complicate matters even more, most of the fatty acids are bound as esters formed with glycerol (4, 7, 10). The possible number of triglycerides that can form from five fatty acids, not including optical isomers but including positional isomers along the glycerol backbone, is 75 as calculated from the formula N = Table 2. Major Component Fatty Acidsa of Various Seed and Vegetable Oils Saturatedb
Oil
Oleic 18:1c
Linoleic 18:2
Linolenic 18:3
Other
Oils showing linear relationship between ηapp and % linoleic or oleic acid
Olive
9.3
84.4
4.6
—
—
Peanut
18.0
56.0
26.0
—
—
Sesame
14.2
45.4
40.4
—
—
Wheat-germ
16.0
28.1
52.3
3.6
—
Sunflower
8.7
25.1
66.2
—
—
Safflower
6.8
18.6
70.1
3.4
—
Oils not showing linear relationship between ηapp and % linoleic or oleic acid
Corn
1.5d
14.6
49.6
34.3
—
Canola
6.0
62.0
22.0
10.0
—
Castor
2.4
7.4
3.1
—
87.0e
a
Data are listed as weight percent. Both literature sources note that these data should be viewed as typical, not average, values. b The component saturated fatty acids for each oil are as follows, in order of decreasing weight percent; an arrow indicates the presence of all even-numbered saturated fatty acids between and including the two listed, but does not imply decreasing quantities: olive (16:0, 18:0, 20:0); peanut (16:0, 18:0, 22:0, 20:0, 24:0); sesame (16:0, 18:0, 20:0); sunflower (16:0, 18:0, 20:0); safflower (12:0 → 22:0); corn (16:0, 18:0, 14:0); canola (16:0, 18:0); wheat-germ (12:0 → 22:0); castor (12:0 → 18:0). c In this common shorthand notation for fatty acids, the number of carbons in the fatty acid chain is given first, followed by a colon and the number of double bonds in the chain. Other pertinent information may also be included. Thus oleic acid, an 18-carbon chain with one double bond, is designated 18:1. Ricinoleic acid (18:1:OH), the major component of castor oil, has an 18-carbon chain containing one double bond and one hydroxyl group. d Palmitoleic acid, 16:1. e Ricinoleic acid, 18:1:OH.
226
(n3 + n2)/2, where n is the number of component fatty acids (4). That a trend in viscosity is observed at all is interesting. This clearer structural picture of the fatty acids and the possible triglycerides found in each oil will aid in the discussion of the data trends. As stated earlier, a simple picture of viscosity involves influences from structure and intermolecular forces. It has been noted (10) that there is a slight dependence of viscosity on the molecular weight of triglycerides. However, the fatty acid composition of the oils used in this study should lead to a very narrow molecular weight distribution of triglycerides. Therefore, it is assumed that the molecular weight dependence will not play a role in this simple picture of viscosity. The extreme viscosity of castor oil relative to the other eight oils is attributed entirely to attractive intermolecular forces. As seen in Table 2, ricinoleic acid accounts for 87% of the fatty acids of castor oil. Ricinoleic and oleic acids differ in the presence of a hydroxyl group at C-12 in ricinoleic acid. The strong dipole–dipole attractions that result from the presence of the hydroxyl group should lead to long-range interactions between triglyceride molecules in castor oil and consequently to dramatically greater viscosity for castor oil than for the other eight oils in light of the previous discussion. This is clearly the case, as seen in Table 1. Since the other eight oils contain only saturated and unsaturated fatty acids with no other functionalities, it is assumed that the intermolecular forces in these oils should be similar and weak. Therefore, any explanation for trends among these oils will be found in their molecular structure. The explanation offered for the linear trend exhibited by the six oils in Figures 1 and 2 lies in the homogeneity of their compositions. None of the six contains more than a few weight percent of any fatty acid longer than 18 carbons, saturated or unsaturated, and oleic and linoleic acids account for at least 80% of the fatty acid composition of each oil. The distance from the glycerol backbone to the end of a fatty acid chain should be less in linoleic acid (18:2) than in oleic acid (18:1) because the presence of two double bonds in linoleic acid reduces extension of the fatty acid chain (7). Therefore oils rich in linoleic acid should contain on average smaller triglyceride molecules than oils rich in oleic acid. Smaller triglycerides have less ability to form long-range structural interactions than larger triglycerides, which should result in a less viscous oil. As the percentage of linoleic acid increases, the number of smaller triglycerides increases, leading to the observed decrease in ηapp. For these same oils, an increase in linoleic acid content is accompanied by a decrease in oleic acid content. Oils with a low percentage of linoleic acid will have a high percentage of oleic acid and vice versa. Since oleic acid triglycerides are larger than linoleic acid triglycerides, viscosities should be high for oils rich in oleic acid. This accounts for the complementary trend in the data of Figures 1 and 2. The viscosities of these six oils may also have been affected slightly by the presence of a measurable fraction of saturated fats with carbon chains longer than 18. The longer chains would tend to raise the viscosity owing to increased possibility of entanglement with adjacent triglycerides. Although canola oil and corn oils have fatty acid compositions well within the limits for the first group of oils, their observed viscosities are below the values that would be predicted based on the trends in Figures 1 and 2. A proposed explanation for this is found once again in the structures of their component fatty acids and trig-
Journal of Chemical Education • Vol. 74 No. 2 February 1997
In the Laboratory
TABLE 3. Structures of Unsaturated Fatty Acids Found in Common Vegetable and Seed Oils Common Name Oleic acid
Notation Systematic Name 18:1
Structural Formula
cis -9-octadecenoic acid
HO2C(CH2)7CH:CH(CH2)7CH3
cis-cis -9,12-octadecadienoic HO2C(CH2)7CH:CHCH2CH:CH(CH2)4CH3 acid all cis -9,12,15-octadecatrienoic HO2C(CH2)7CH:CHCH2CH:CHCH2CH:CHCH2CH3 acid
Linoleic acid
18:2
Linolenic acid
18:3
Palmitoleic acid
16:1
cis -9-hexadecenoic acid
HO2C(CH2)7CH:CH(CH2)5CH3
Ricinoleic acid
18:1
12-hydroxy-cis -9-octadecenoic acid
HO2C(CH2)7CH:CHCH2CH(OH)(CH2)5CH3
lycerides. In corn oil, none of the fatty acid chains is longer than 18 carbons or shorter than 14. This narrow range of lengths could lead to a folding pattern in the chains to form small, uniform triglycerides. With no chains longer than 18 carbons, entanglement of adjacent triglycerides is not promoted. Both of these ideas are in line with the reduced viscosity observed for corn oil. The decrease in observed viscosity for canola oil is not as great as for corn oil even though canola oil’s fatty acid chains also fall into a narrow range of lengths, either 16 or 18 carbons. Whatever decrease in viscosity results from this narrow chain-length range is moderated by the presence of linolenic acid. Since linolenic acid contains three cis double bonds, its overall structure is curved (7), which should lead to a greater possibility of entanglement and an increase in viscosity. This hypothesis suggests further experiments in which the viscosity of a set of oils with a small range of chain lengths is measured to establish a trend. But given the limited number of available seed and vegetable oils, this investigation could not be done. Conclusions The linear trend between viscosity and composition established for a set of six seed and vegetable oils can be explained by basic intermolecular forces and structural interactions. Several routine experiments are suggested based on this work. First, the viscosity of all nine oils can be obtained experimentally, as suggested earlier, by sharing the work load and data. Trends can be observed and a full discussion can follow, as in this paper. Second, viscosity measurements on four or five of the oils can be done to establish linear trends based on the percentages of linoleic or oleic acid and the data can be discussed in terms of structure, intermolecular forces, and fatty acid composition. Other oils from this experi-
ment, possibly castor or corn, can be discussed in terms of their physical characteristics and their apparent viscosity can be predicted. These predictions can then be tested. This would open the door for discussion of how and why inferences and predictions can be incorrect. Acknowledgments We would like to thank the University of South Carolina Aiken for support of this project. We would like to thank several Introduction to Physical Chemistry classes at USCA that served as unknowing guinea pigs in the early stages of this work. Thank you to Cathy Cobb and her 1994 Physical Chemistry Lab class at Augusta College, Augusta, GA, for trying the experiment and offering further insight into the procedure and discussion. Finally, we would like to thank Ann Willbrand of USCA for her help in preparing this manuscript. Literature Cited 1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1989. 2. Sime, R. J. Physical Chemistry: Methods, Techniques, and Experiments; Saunders: Philadelphia, 1990. 3. Halpern, A. M.; Reeves, J. H. Experimental Physical Chemistry: A Laboratory Textbook; Scott, Foresman/Little, Brown: Glenview, IL, 1988. 4. Farines, M.; Soulier, R.; Soulier, J. J. Chem. Educ. 1988, 65, 464–466. 5. Rosenthal, L. C. J. Chem. Educ. 1990, 67, 78–80. 6. Diagnault, L. G.; Jackman, D. C.; Rillema, D. P. J. Chem. Educ. 1990, 67, 81–82. 7. Quigley, M. N. J. Chem. Educ. 1992, 69, 332–335. 8. Richards, J. L. J. Chem. Educ. 1993, 70, 685–689. 9. Sutterby, J. L. Chemtech 1985, July, 416–419. 10. Lawson, H. Food Oils and Fats: Technology, Utilization, and Nutrition; Chapman & Hall: New York, 1995. 11. Giguere, J.; Arseneault, E.; Dumont, H. J. Chem. Educ. 1994, 71, 121–124. 12. Budavari, S., Ed. The Merck Index, 11th ed.; Merck: Rahway, NJ, 1989. 13. Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 59th ed.; CRC: Boca Raton, FL, 1978.
Vol. 74 No. 2 February 1997 • Journal of Chemical Education
227
