Introduction
Viscosity experiments using Ostwald-type gravity flow viscometers are not new to the physical chemistry laboratory. Several physical chemistry laboratory texts (1, 2, 3) each 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 which allow a ft~ll data interpretation to be attempted. This approach to viscosity experiments may not be appropriate for all teaching situations such as high schools, general chemistry labs, and non-major 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, happ, and the percent 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 were olive, peanut, sesame seed, corn, canola (rapeseed), wheat-germ, sunflower, safflower, and castor. They 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.
The viscosity data were obtained under ambient temperature conditions: approximately 240C. The viscosities of several of these oils were recorded at different temperatures and the 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 reflect consistent data trends.
The viscometer used in this work was a Cannon-Fenske 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 piece of glass consisting of a measurement side and a filling side. The filling side is a straight piece of glass of 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, 2-cm diameter reservoirs above a capillary tube approximately 9 cm in length 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, happ, in centipoise is obtained from
where t is the flow time, r is the density of the oil at room temperature, and C0 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, C0 = 0.989 centistokes/second) which allowed flow times to range from 567 seconds for corn oil to 855 seconds for olive oil. These flow times are within the recommended range set by the manufacturer. A second viscometer (Size 450, C0 = 2.732 centistokes / second) with a capillary diameter of approximately 3 millimeters was used for castor oil due to 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 in the vertical position 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 pipette bulb. The pipette 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 repeated 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 is 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.
Results
Table 1 shows the data for these experiments. The table 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. A standard error analysis was performed and these results 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, the density was measured as the slope from a simple linear regression of weight versus volume. A similar experiment was performed for corn oil and the literature value for corn oil density was used 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 happ when plotted versus either Percent Linoleic or Percent Oleic Acid The second group listed contains the remaining three oils that do not follow these linear trends. The least viscous oil is corn oil with happ of 51.6 cP and the most viscous oil is castor oil with happ 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 happ for olive and castor oils are consistent with values reported elsewhere (13).
Eight oils are plotted in both Figures 1 and 2 as happ versus Percent Linoleic Acid and Percent Oleic Acid, respectively. In both figures, the data points are marked with an abbreviation for each oil: olive (OL), canola (CN), peanut (PN), sesame (S S), corn (CR), wheat-germ (WG), sunflower (SN), and safflower (SF). The data that give rise to the straight line shown in both figures are shown as darkened squares. Both figures clearly show the linear trend in happ established by olive, peanut, sesame, wheat-germ, sunflower, and safflower oils as functions of both fatty acid components. The group of three oils shown in Table 1 behave differently in terms of their viscosities. Corn oil and canola oil are both less viscous than either trend shown in Figures 1 or 2. Castor oil is extremely viscous relative to the other eight oils and for this reason can not be conveniently shown in either figure.
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 with progressively less flow resistance 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 liapp 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 that comprise each oil used in this study. They are presented as Weight Percent Total Fatty Acids as found in the Handbook of Chemistry and Physics (13). The data for canola oil was obtained elsewhere (10). Once again the oils are grouped as they were for Table 1. A common structural short-hand notation is used in this table for convenience. In this notation, the number of carbons in the fatty acid chain is listed followed by a colon and the number of double bonds contained in the chain. Other pertinent information may also be included.2 The first group of oils are fairly homogeneous in their components; at least 80% of the fatty acid weight comes from oleic and linoleic 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 a similar fatty acid composition 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 short-hand notation previously discussed, 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 = (n3 + n2 )/2 where n is the number of component fatty acids (4). That a trend in the viscosity is observed at all is somewhat 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 two sources, 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 all of the oils used in this study should lead to a very narrow molecular weight distribution of the triglycerides involved. Therefore, it is assumed that the molecular weight dependence will not play a role in this simple picture of viscosity. For castor oil, the extreme viscosity observed relative to the other eight oils is attributed entirely to attractive intermolecular forces. As seen in Table 2, eighty-seven percent of the fatty acid composition of castor oil is ricinoleic acid. The difference between ricinoleic and oleic acids is 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 a sample of castor oil. The viscosity should be dramatically greater for castor oil compared to the other eight oils in light of the previous discussion because of these long-range forces. This is clearly the case for castor oil as seen in the data of Table 1 Since the other eight oils contain only saturated and unsaturated fatty acids with no other flinctionalities, 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 the molecular structure.
The explanation offered for the linear trend exhibited by the six oils in both Figures 1 and 2 lies in the homogeneity of their compositions None of these six oils contain more than a few weight percent of any fatty acid longer than eighteen carbons, saturated or unsaturated, and at least 80% of the fatty acid composition of each oil is accounted for by oleic and linoleic acids only. 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) since the presence of the two double bonds in linoleic acid prevents ftiller 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 will have less ability to form long-range structural interactions than larger triglycerides which should lead to a less viscous oil. As the Percent Linoleic Acid increases, the number of smaller triglycerides increases leading to the decrease in happ that is observed. For these same oils, an increase in linoleic acid content corresponds to a decrease in oleic acid content due to their homogeneous compositions. 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 Figure 2 when compared to the data in Figure 1. The viscosity for these six oils may have also been affected slightly by the presence in each of a measurable fraction of saturated fats with carbon chains longer than eighteen. These longer chains would tend to raise the viscosity due to the increased possibility of entanglement with adjacent triglycerides.
Although canola oil and corn oil have a fatty acid composition that falls well within the limits described for the first group of oils, they both remain outside the trends seen in Figures 1 and 2. The observed viscosity for both oils is below the value that would have been predicted based on the trends in both figures. A proposed explanation for this is found once again in comparing structures of component fatty acids and the resulting triglycerides. In corn oil, none of the fatty acid chains is longer than eighteen carbons or shorter than fourteen carbons. This narrow range of lengths could lead to a folding pattern in the chains that form small, uniform triglycerides. With no chains longer than eighteen carbons, entanglement of adjacent triglycerides is not enhanced. Both of these ideas lead to conclusions in line with the reduced observed viscosity for corn oil. The decrease in the observed viscosity for canola oil is not as great as for corn oil even though the canola oil fatty acid chains fall into a narrow range of lengths, either sixteen or eighteen carbons. Whatever decrease in viscosity that 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 ft~rther 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
A linear trend between viscosity and composition has been established for a set of six seed and vegetable oils that uses basic intermolecular forces and structural interactions as an explanation. Several routine experiments can be suggested based on this work. First, the viscosity of all nine oilS could be obtained experimentally, as suggested earlier, by sharing the work load and data. The trends could be observed and a full discussion could be done as in this paper. Second, viscosity measurements on four or five of the oils could be done to establish the Percent Linoleic Acid and the Percent Oleic Acid linear trends and the data discussed in terms of structure, intermolecular forces, and fatty acid composition. other oils from this experiment, possibly castor oil or corn oil, could be discussed in terms of their physical characteristics and predictions made as to their apparent viscosity. These predictions could 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 State University, Augusta, Georgia for trying the experiment and offering flirther 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; McGraw-Hill: New York, 1989, 5th ed.
2. Sime, R. 3. 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, Ii., 1988.
4. Farines, M.; Soulier, R.; Soulier, J. J. Chem. Ed. 1988, 65, 464-466.
5. Rosenthal, L. C. J. Chem. Ed. 1990, 67, 78-80.
6. Diagnault, L. G.; Jackman, D.C.; Rillema, D. P. J. Chem. Ed. 1990, 67, 81-82.
7. Quigley, M. N. J. Chem. Ed. 1992, 69, 332-335.
8. Richards, J. L. J. Chem. Ed. 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. Ed. 1994, 71, 121-124.
12. Budavari, S., Ed. The Merck Index; Merck: Rahway, NJ, 1989, 11th ed.
13. Weast, R. C., Ed. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1978, 59th ed.
FOOTNOTES
1. Author to whom correspondence should be addressed.
2. As an example, oleic acid is an eighteen carbon-chain molecule that has one double bond. Its short hand notation is therefore 18:1. Ricinoleic acid, short hand notation of 18:1:OH, is an eighteen carbon-chain molecule that contains one double bond and one hydroxyl group.
| Name of Oil | Density (g/mL)a | Flow Time (s) | happ (cP) |
| 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 +/- 1 |
| Safflower | 0.921 | 58.53 | 53.3 +/- 0.3 |
| Corn | 0.919 | 56.72 | 51.6 +/- 0.2 |
| Canola | 0.913 | 69.55 | 63 +/- 2 |
| Castor | 0.961 | 28.53 | 749 |
a) In every case except castor oil and canola oil, the mid-point of the range of the density values at 25oC was used for calculations. For castor oil, the range was obtained at 15.5oC and the lowest value in the range was used for calculations since density should decrease with temperature increase. For canola oil, the density was determined as described in the text.
| Name of Oil | Saturatedb | Oleic (18:1) | Linoleic (18:2) | Linoleic (18:3) | Other |
| 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 | ----- |
| Corn | 14.6 | 49.6 | 34.3 | ----- | 1.5 (Palmitoleic; 16:1) |
| Canola | 6.0 | 62.0 | 22.0 | 10.0 | ----- |
| Castor | 2.4 | 7.4 | 3.1 | ----- | 87.0 (Ricinoleic; 18:1:OH) |
a) Both literature sources note that this data should be viewed as typical and not average values.
b) The component saturated fatty acids for each oil are listed in short-hand form and are given in order of decreasing Weight Percent from left to right. Those listed with an arrow indicate the presence of all even-numbered saturated fatty acids between and including the two listed but do not indicated 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 to 22:0); Corn (16:0, 18:0, 14:0); Canola (16:0, 18:0); Wheat-germ (12:0 to 22:0); Castor (12:0 to 18:0)
| Oleic (18:1) cis-9-octadecenoic acid | HO2C(CH2)7CH:CH(CH2)7CH3 |
| Linoleic (18:2) cis-cis-9,12-octadecadienoic acid | HO2C(CH2)7CH:CHCH2CH:CH(CH2)4CH3 |
| Linolenic (18:3) all cis-9, 12, 15-octadecatrienoic acid | HO2C(CH2)7CH:CHCH2CH:CHCH2CH:CHCH2CH3 |
| Palmitoleic (16:1) cis-9-hexadecenoic acid | HO2C(CH2)7CH:CH(CH2)5CH3 |
| Ricinoleic (18:1:OH) 12-hydroxy-cis-9-octadecenoic acid | HO2C(CH2)7CH:CHCH2CH(OH)(CH2)5CH3 |
Figure 1: The apparent viscosities of the common oils used in this work are plotted versus the Percent Linoleic Acid of each oil. The line is a regression fit to only the oils shown with darkened squares. Castor oil is omitted from the figure.
Figure 2: The apparent viscosities of the common oils used in this work are plotted versus the Percent Oleic Acid of each oil. The line is a regression fit to only the oils shown with darkened squares. Castor oil is omitted from the figure.
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