Ind. Eng. Chem. Res. 2006, 45, 3735-3740
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Friction Behavior of Some Seed Oils: Biobased Lubricant Applications Atanu Adhvaryu,†,‡ Girma Biresaw,§ Brajendra K. Sharma,†,‡ and Sevim Z. Erhan*,‡ Department of Chemical Engineering, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, USDA/NCAUR/ARS, Food and Industrial Oil Research, 1815 North UniVersity Street, Peoria, Illinois 61604, and USDA/NCAUR/ARS, Cereal Products and Food Science, 1815 North UniVersity Street, Peoria, Illinois 61604
Seed oils are renewable and an environmentally friendly alternative to mineral based oils in lubrication and other important industrial applications. They are generally triesters having a complex distribution of fatty acid (FA) chains. All the physical and chemical properties of seed oils are dependent on their FA distribution, composition, and additional functional groups in the chain structure. In this work, various seed oils are studied for their boundary lubrication properties using the Langmuir adsorption model for their possible use as industrial lubricants. This information can be used to design suitable lubricant molecules that will have optimum structure for effective metal adsorption as well as exhibit excellent boundary lubrication properties. The free energy of adsorption (∆Gads) of cottonseed, canola, olive, and meadowfoam oil is investigated in boundary lubrication regime using steel ball-on-disk geometry. Adsorption values were compared with monoesters with varying chain lengths. It was observed, based on computed ∆Gads, that molecular polarity, hydrocarbon chain length, and relative distribution of unsaturation in the FA chain can affect adsorption on the metal surface. Statistical analysis on FA distribution and ∆Gads was helpful in making a generalized assumption on adsorption behavior. The results are consistent with theoretical assumptions on surface adsorption as a function of molecular structure. Introduction Seed oils are derived from renewable agricultural sources.1,2 They are also nontoxic and ecofriendly with excellent environmental and safety characteristics. These properties make seed oils attractive alternatives to petroleum-based oils for use as lubricants, industrial fluids, and other applications.3-5 Most seed oils are triglycerides (TG) constituting a complex mixture of fatty acids (FA) with different chain length and unsaturation content. Some seed oils are also found as monoesters of long chain FA and fatty alcohols of varying chain length and degree of unsaturation. Seed oils are amphiphilic in nature as they constitute a polar head that is hydrophilic in character and a nonpolar hydrophobic chain. These oils are primarily water insoluble due to the presence of a long hydrocarbon chain in the molecule. Depending on the type of seed oil, functional groups such as epoxies and hydroxides might be present in the hydrocarbon portion of the molecule. Tribological and other important properties such as oxidation, low-temperature stability, and rheology6 of seed oils are highly dependent on the exact chemical composition of its polar and nonpolar groups. Amphiphilic properties affect the boundary lubrication or additive properties while fluid or rheological properties affect the hydrodynamic properties of seed oils. Together, they affect the performance of seed oils in lubricant applications that occur in boundary, hydrodynamic, and mixed film lubrication regimes.7 Boundary lubrication phenomena are often associated with adsorption and tribochemical reaction occurring on the metal surface.8-13 Adsorption refers to the ability of lubricant molecules to attach to the friction surface and therefore prevent their contact during a tribological process. This attachment * Corresponding author. Tel.: (309)681-6532. Fax: (309)681-6340. E-mail address:
[email protected]. † Pennsylvania State University. ‡ USDA/NCAUR/ARS, Food and Industrial Oil Research. § USDA/NCAUR/ARS, Cereal Products and Food Science.
occurs mainly through the polar groups of the molecules and can be quantified using free energy of adsorption (∆Gads) terms. Tribochemical reactions on the other hand lead to the formation of tribo-film resulting out of chemical reactions of the lubricants themselves or with other materials (e.g., oxygen, moisture, and metal) in the interface or friction zone. Such reactions occur owing to the high temperature, pressure, and shear of the lubrication process. Since this is a complex phenomenon, thus far, tribochemical reactions are not fully understood and are often responsible for mechanical failures resulting from oil degradation by oxidation and generation of friction polymers. Our group is engaged in the development of biobased lubricants including that from seed oils.4,6,7,14,15 Such development requires a thorough understanding of the tribological properties of biobased ingredients. We are particularly interested in understanding the relationship between the chemical and physical properties of biobased ingredients with their tribological properties. In the past,14,15 we have conducted investigations looking into the effects of seed oil chemistry on their boundary properties. In these studies, we looked at the effects of degree of unsaturation of the fatty acid residues14 as well as the degree of functionalization15 of the seed oils on free energy of adsorption. These studies indicated that the degree of functionalization has a big effect on adsorption properties of seed oils, while the degree of unsaturation has minimal effect.14,15 Recently, we extended our investigation into the effect of seed oil chemistry on its boundary lubrication properties by looking into the effect of fatty acid chain length. It is reported that additive chain length affects adsorption,10 and we want to know if this will be the case with seed oils. To do this, we investigated the adsorption properties of cottonseed, canola, olive, and meadowfoam oils, which have different average chain lengths. Free energy of adsorption (∆Gads) was estimated using the Langmuir model as a function of average chain length of TG structure. The results are discussed in terms of fatty acid (FA) composition, average chain length, and relative distribution of unsaturation in the chain structures. Statistical analysis of
10.1021/ie051259z CCC: $33.50 © 2006 American Chemical Society Published on Web 04/05/2006
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Figure 1. Schematic of ball-on-disk friction measurement configuration.
the FA composition was attempted to predict ∆Gads with some degree of certainty. Materials and Methods Lubricants. Cottonseed, canola, and olive oils were obtained from commercial sources (Liberty Vegetable Oil Co., Santa Fe Springs, CA). Meadowfoam oil (USDA, Peoria) was used without any further purification. Hexadecane (99%+, anhydrous) was obtained from Aldrich Chemical Co. (Milwaukee, WI). The lubricant formulation consisted of 0.001-0.6 M of each seed oil in hexadecane at room temperature with stirring. The resulting lubricant formulation was homogeneous and stable during the entire duration of the experiment. Friction Measurement Setup. This study, designed to understand the effect of seed oil structure on friction and wear, was done using a ball-on-disk configuration (Figure 1) using a friction and wear test apparatus from Falex (Sugar Grove, IL). The test zone was a ball moving with a specified speed making a point contact with a stationary disk. The resistance to the motion of the ball (i.e., friction force) was measured by a load cell connected to the stationary disk. The coefficient of friction (COF) is obtained by dividing the friction force by the normal force pressing the ball against the disk. The balls and the disks were obtained from Falex and were thoroughly degreased by sonication with fresh reagent grade isopropyl alcohol and hexane (Aldrich Chemical Co.) prior to each experiment. After being cleaned, the disks and balls were stored in a moisture-free container until used. The balls were as follows: 52100 steel, 12.7 mm diameter, 64-66 Rc hardness, and extreme polish. The disks were 1018 steel, 25.4 mm outer diameter, 15-25 Rc hardness, and 0.36-0.46 µm roughness. The experiments were performed at room temperature with the test fluid covering the ball and the disk completely in the sample chamber. The disk and the sample chamber are fitted with a thermocouple to record any change in temperature during the test period. The instrument is equipped with a PC and software that allows for automatic acquisition and display of the following data at selected rate: torque on the disk (friction force), vertical height change (wear), load, speed, chamber temperature (test oil), specimen temperature (stationary disk), etc. During this experiment, the coefficient of friction was calculated automatically and displayed in real time. Friction Measurements. The ball was held by the upper specimen holder to make a point contact radius of 11.9 mm on the disk. The disk was attached on the bottom specimen holder and enclosed in a fluid-tight cup. The two-component test fluid consisted of base oil (hexadecane) and one of the seed oils (additive in this case) dissolved in it. The concentration of each additive in the base oil ranged from 0.00 to 0.60 M. Fifty milliliters of the test fluid was poured in the cup to totally immerse the ball and disk. The disk assembly was then raised and allowed to touch the ball attached to the shaft. The shaft holding the ball was then rotated to the set speed and immediately after that, a preset load was applied to the disk
Figure 2. Friction derived adsorption isotherms of cottonseed oil. Fractional surface coverage (θ) is computed from COF data using eq 1.
assembly which then rose to touch the ball attached to the shaft. Friction and other data were recorded until the set time elapsed. A duplicate test was conducted with the same test fluid and a new set of ball and disk. Data reported are the average of the two tests and generally were within (5% of the measured value. The duration of friction test was 15 min at a sliding speed of 6.22 mm/s (5 rpm) and 1778 N load (400 lb) at room temperature. The temperature of specimen and test fluid was 25 ( 2 °C, which increased by 2-3 °C at the end of the 15 min test duration. COF for pure hexadecane was determined as a function of load from 444.5 to 1333.5 N (100 to 350 lb). It was observed that the COF increased with increasing load and leveled off to an average COF of 0.5 beyond a load of 1111.3 N (250 lb). Statistical Methods. A professional statistical software package, Minitab Release 12.0 (MINITAB, State College, PA), was used in the current study to see if ∆Gads values correlate with fatty acid composition, average chain length, and unsaturation number of the seed oils tested. This package performs statistical analysis of the numerical data including linear regression. A multiple-regression was also performed using ∆Gads values as the response variable. The different predictor variables used in multiple-regression were chain length, unsaturation, and fatty acid composition, for example, palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and linolenic acid (C18:3) percentages. Different regression equations with different predictor variables were compared for best fit, coefficient of determination (R2), T-test, and P-values. The selected model for predicting ∆Gads values was based on highest R2 and the attained significance level, or P-value. Results and Discussion Effect of Run Time on COF. During the 15 min test, the COF data from each concentration increases with time and reached a steady-state value typically after 5 min into the run. The COF data in the steady-state region were averaged to obtain the COF for each concentration. COF from duplicate runs were within 5% of each other, which was then averaged to obtain the final COF for the selected concentration. This value was used in subsequent analysis. Effect of Additive Concentration on COF. Additives refer to different seed oils used in this study. They play a significant role in wear and surface protection of a dynamic mechanical system. Their effect is observed in lowering the COF between the two metal surfaces in relative motion. The effect of additive concentrations on COF using a ball-on-disk geometry is shown in Figure 2 for cottonseed oil. It was observed that all the additives follow similar trend with maximum COF at low concentration (approaching that of pure hexadecane) and
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Figure 3. Schematic of monomolecular adsorption of seed oil on the metal surface. Table 1. Fatty Acid Distribution (%, by gas chromatography: AACC method, 58-18, 1993) of Seed Oils seed oils meadowfoama cottonseeda canolaa olivea safflowerb soybeanb
C16:0 C18:0 C18:1 C18:2 C18:3 other UNd ACLe 22 4 13 7 11
3 2 3 2 4
2.0 19 62 72 13 24
0.5 55 22 11 78 54
1 10 1 0 7
97.5c 1.19 1.32 1.36 0.97 1.69 1.53
20.62 17.56 17.92 17.74 17.86 17.78
a This work. b Previous work.14,15 c Other fatty acids are C20:1 (64%); C22:1 (15%); C22:2 (18.5%). d Unsaturation number obtained using eq 9. e Average chain length obtained using eq 10.
decreasing sharply with increasing additive concentration, reaching a steady state. Beyond this point (additive concentration of typically 0.1 M), no further lowering of COF was observed (i.e., the COF was independent of additive concentration). The COF values observed are a function of surface coverage of additive molecules, which is a function of their concentrations in hexadecane. At low additive concentration, the steel surfaces are almost exclusively covered with hexadecane and approach an environment similar to pure hexadecane (no dissolved additive). With an increase in additive concentration, the nonpolar hexadecane molecules are increasingly displaced from the metal surface with additive molecules, competing for active site for attachment to the metal surface. The polar heads of additives (triglycerides, TG ester groups) are adsorbed (through adhesive interaction) to the metal surface making a unimolecular barrier, resulting in a dramatic decrease in COF (Figure 3). At very high additive concentration, the surface is completely covered by the additive molecules, and the COF becomes independent of additive concentration in hexadecane. The corresponding steady-state COF values observed for the additives studied are as follows: cotton ) 0.095; canola ) 0.092; olive ) 0.112; and meadowfoam ) 0.095. It was observed that olive oil reached steady state at a much lower additive concentration (0.03 M) as compared to other oils (0.1 M). From the fatty acid composition of the oils (Table 1), it is observed that the oleic content (C18:1) of olive oil (71%) is significantly higher than the rest followed by canola oil (62%) [C18:x; numbers indicate 18 carbon chain with x ) 0, 1, 2, or 3 unsaturated sites, respectively]. At this point, it may appear that lower unsaturation in the fatty acid chain may have a positive effect on the adsorption profile. Friction-Derived Adsorption Isotherm. An adsorption isotherm shows the relationship between the concentration of an additive in solution (hexadecane) and its concentration on the rubbing surfaces. The exact relationship between these two
concentrations depends on the adsorption model being considered. Analysis of adsorption isotherms using appropriate adsorption models yields ∆Gads values of the additives on the rubbing surfaces. Such analysis also provides insight into additive/ friction surface and additive/additive interactions. In this work, adsorption isotherms were derived from friction measurements using different additive concentrations (in hexadecane) and steel ball-on-disk geometry to simulate boundary lubrication condition. Whenever a solute in solution is in contact with a solid, there will be an equilibrium established between the molecules in the liquid phase and the corresponding adsorbed species (molecules or atoms) which are bound to the surface of the solid. The condition of equilibrium will depend on factors such as the following: (i) The duration of friction test; the equilibrium represents a state in which the rate of adsorption of molecules onto the surface is exactly counterbalanced by the rate of desorption of molecules back into the solution phase. (ii) non-dissociative (molecular) adsorption process (i.e., molecules should not undergo chemical transformation during the adsorption process). (iii) There are a fixed number of localized surface sites present on the metal surface and adsorption takes place only at specific localized sites on the surface and the saturation coverage corresponds to complete occupancy of these sites. (iv) Temperature and pressure of the system should remain constant. Friction-derived adsorption isotherms can be obtained from boundary COF versus concentration data for systems that follow the stringent conditions discussed above as follows:8-10
θ ) θa ) ( fb - f )/( fb - fa)
(1)
where fb is the COF of pure solvent (in this case hexadecane); fa is the COF at full surface coverage of the additive (at steady state in Figure 2); f is the COF at a given additive concentration; and θ is the fractional surface coverage at the given concentration. θ has values ranging from 0.0 at no surface coverage (i.e., at f ) fb) to 1.0 at full surface coverage (i.e., at f ) fa). Detailed derivation of eq 1 is given elsewhere.8 When θ is plotted against additive concentration, it shows a profile that is mirror image of the COF data (Figure 2). Figure 2 depicts the data for the cottonseed oil studied in this work; similar results were observed for canola, olive, and meadowfoam oils used in this study. The concentration versus θ data shown in Figure 2 is important because they allow for the determination of the ∆Gads of the seed oils on steel by analyzing these friction-derived adsorption isotherms using appropriate adsorption models. There are several adsorption models for analyzing adsorption isotherms (Langmuir, Temkin, and Freundlich)16 that differ in one or more of the assumptions made in deriving the expression for the surface coverage. In particular, they differ on how they treat the surface coverage dependence of the enthalpy of adsorption. ∆Gads is the sum of the free energy of adsorption of the additive molecules on the metal surface (∆Go) and the free energy of lateral interaction (Rθ) between the molecules:
∆Gads ) ∆Go + Rθ
(2)
In the Timken model, a net repulsive lateral interaction is assumed (i.e., R > 0). However, from our previous work14,15 on a series of triglycerides and long chain fatty esters, it was observed that in such systems attractive and repulsive lateral interactions cancel out, resulting in very small or zero net lateral
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Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 Table 2. Free Energies of Adsorption, ∆Gads, of Seed Oils, Computed from Friction-Derived Adsorption Isotherms Using the Langmuir Adsorption Modelsa
Figure 4. Langmuir analysis of friction derived adsorption isotherm for cotton seed oil.
interactions. It can be assumed that R = 0. Therefore, eq 2 reduces to eq 3:
∆Gads ) ∆Go
(3)
While the Langmuir isotherm is one of the simplest, it still provides a useful insight into the concentration dependence of the extent of surface adsorption. Hence, the Langmuir model will be used to analyze the adsorption isotherms of all the additive samples discussed in this work. For solutes that meet the stringent conditions of equilibrium of solute adsorption on metal surfaces, we can consider nondissociative adsorption to be represented by the following chemical equation:
S0 + A ) S A
(4)
where S0 represents a vacant surface site, A represents an additive in solvent (hexadecane), and SA represents the surface site occupied by an additive. The equilibrium constant (K) can be defined as
K ) [SA]/[S0][A]
(5)
where [SA] is concentration of additive on surface, which is proportional to the surface coverage of adsorbed molecules (i.e., proportional to θ); [S0] is the concentration of vacant sites, which is proportional to (1 - θ); and [A] is the concentration of solute (additive, mol/L) in solvent and is proportional to the initial concentration of solute in solvent (i.e., C). Hence, eq 5 can be expressed in terms of new constant, Ko, which is equilibrium constant for the displacement of a solvent molecule from the surface by a solute molecule in solution: or
Ko ) θ/(1 - θ)C
(6)
1/θ ) [(1/Ko) × (1/C)] + 1
(7)
It is observed that the Langmuir adsorption model predicts a linear relationship between θ-1 and C-1 (Figure 4), with a correlation coefficient (R2) > 0.95 for cottonseed, canola, and meadowfoam with the exception for olive oil (R2 ) 0.86). As an example, the slope of the linear plot (Ko-1) and Y-intercept for cottonseed oil is shown in Figure 4. ∆Gads values for individual oils can be calculated from the slope (Ko-1) of the linear plot of 1/θ versus 1/C plot. The equilibrium constant (Ko) is related to the Gibbs free energy and hence to the enthalpy change for the adsorption process as follows:
∆Gads ) ∆Go ) -RT ln(Ko)
(8)
Values of Ko from the plots were used in eq 8 to obtain ∆Gads
seed oils
∆Gads (kcal/mol), Langmuir model
canolab cottonseedb oliveb meadowfoamb Safflowerc Soybeand methyl oleatec methyl palmitatec methyl laurated
-3.81 -3.71 -3.98 -3.57 -3.65 -3.60 -2.91 -2.70 -1.90
a Data obtained from friction measurement on steel/steel friction surface using ball-on-disk test geometry. b This work. c Previous work.15 d Previous work.14
values for all the oils. The resulting ∆Gads values are summarized in Table 2. Effect of Triglyceride Chemistry on ∆Gads. The purpose of this work is to study the different aspects of triglyceride chemistry on boundary lubrication properties of seed oils. Boundary lubrication properties are associated with adsorption of additive molecules on the metal surface. Adsorption can be quantified using free energy of adsorption (i.e., ∆Gads values shown in Table 2). The following aspects of triglyceride chemistry were investigated. Degree of Functionality. This refers to the number of polar groups present in a molecule. Comparison of the ∆Gads data for TG used in this study with monoesters14,15 indicate that triglycerides consistently have lower ∆Gads values (Table 2). Lower ∆Gads corresponds to stronger adsorption on the metal surface. This has been attributed to the availability of multiple polar groups per molecule in TG. According to the assumption in Langmuir adsorption model, each surface site can be singly occupied. The TG are capable of occupying up to three sites versus only one site for monoesters. The presence and availability of polar functional groups of the adsorbing molecule thus significantly affect the ∆Gads. Degree of Unsaturation. This refers to the average number of double bonds per molecule of triglyceride. The unsaturation number (UN) was calculated from the fatty acid distribution using eq 9 and is summarized in Table 1:
UN ) [{1 × (C18:1 + C20:1 + C22:1)} + {2 × (C18:2 + C22:2)} + {3 × (C18:3)}]/100 (9) Where Cx:y is the percentage of fatty acid residue with carbon chain lengths as x and number of double bonds as y. A single-component statistical analysis was performed to study the correlation between UN and ∆Gads value. In this analysis ∆Gads value of seed oils were used as the response variable and UN as predictor variable. On an average as the unsaturation number increases, the ∆Gads value also increases, which means weaker adsorption. Olive oil with least unsaturation number of 0.97 has the lowest ∆Gads value of -3.98, corresponding to stronger adsorption on the metal surface as compared to other seed oils. Similar results were obtained when the predictor variables used were different components of unsaturation number (i.e., polyunsaturation (C18:2 + C22:2 + C18:3), diunsaturation (C18:2 + C22:2), and monounsaturation (C18:1 + C20:1 + C22:1)). ∆Gads value increases with an increase in polyunsaturation and diunsaturation, while it increases with a decrease in monounsaturation. The unsaturation parameter UN along with its components has some effect on adsorption properties of seed oils.
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Chain Length. This refers to average carbon chain lengths of fatty acids present in triglyceride molecule. The effect on ∆Gads as a function of fatty acids chain length can be studied using average chain length (ACL) of fatty acids in triglyceride structures. ACL was calculated using percent fractions ( fn) of the corresponding fatty acid carbon chain length (Cn), as in eq 10 and is tabulated in Table 1:
ACL )
∑ fnCn
(10)
On using ACL as a predictor variable for the response variable ∆Gads in single-component regression analysis, the ACL values do not show any trend on adsorption properties of seed oils with R2 value of 0.22. The percent values of C16 and C18 fatty acids were also tried to see if there is any correlation between these and adsorption properties of the oils. Although, there is no clear trend, but the meadowfoam oil with highest ACL value of 20.62 has the highest ∆Gads value of -3.57, while all other oils having an ACL value ranging between 17.56 and 17.92 have ∆Gads values lower than meadowfoam oil. With the present data, it appears that there might be an optimum chain length of 17-18 that favors strong adsorption of seed oils on metal surface. Fatty Acid Distribution. The effect of fatty acids percentages were tried to examine their correlation with ∆Gads values. The fatty acid percentages were used as predictor variables for predicting ∆Gads values in single-component statistical analysis. It was found that except for oleic acid content, other fatty acids do not show a trend with increasing ∆Gads values. The ∆Gads values showed a very good correlation with oleic acid with coefficient of determination, R2 value of 0.85. As the oleic acid percentage increases in seed oils, the ∆Gads value decreases. The olive oil having higher oleic acid percentage (72%) also have a lower Gads value of -3.98 compared to other seed oils. On the other hand, the meadowfoam oil have higher value of Gads (-3.57) value as compared to other oils. One of the factors for this higher value may be its lower oleic acid content (2%). Thus, it can be inferred that higher oleic acid content does have some effect on the adsorption properties of seed oils. Higher degree of mono-unsaturation in the TG can lead to a lower ∆Gads. Multicomponent Statistical Analysis for ∆Gads of Triglycerides. There are limitations of using a single structural parameter (e.g., UN, ACL) of TG to explain the variations in ∆Gads observed. We have earlier established that degree of unsaturation and degree of functionality in the molecule play important role in determining ∆Gads values.14,15 Seed oils from different sources (native and genetically modified) are usually a complex mixture of triglyceride molecules with diverse fatty acid constituent. It may be useful to know if a combination of some of these parameters (UN, ACL, fatty acid distribution) can explain the variation in ∆Gads of seed oils. Multi-component statistical analysis was attempted on several seed oils using experimental ∆Gads values as the response variable and other parameters such as UN, ACL, fatty acid composition (C16:0, C18:0, C18:1, C18:2, and C18:3 percentages) as predictor variables. Different regression equations with different predictor variables were compared for best fit, coefficient of determination (R2) values. The selected model for predicting ∆Gads values was based on highest R2 and the attained significance level, or P-value. The best model provides an equation, which can be used to calculate the predicted values of ∆Gads. If we use UN and ACL separately as predictor variable, the R2 values are 0.37 and 0.22, respectively. On using UN and ACL together as predictor variable in multi-component
Figure 5. Correlation plot of ∆Gads (predicted) vs ∆Gads (experimental). R2 (adj) ) 0.95 and error of Y-estimate (σY) ) 0.05. Table 3. Multicomponent Statistical Analysis of Triglycerides Fatty Acid Chain as a Function of ∆Gads predictor
coefficient
SD
T-test
P-value
constant C16:0 C18:0 C18:1
-3.5637 -0.00918 0.0462 -0.00553
0.05 0.00 0.03 0.00
-75.07 -2.03 1.70 -6.01
0.00 0.18 0.23 0.03
analysis, the R2 value increases to 0.81, thereby improving the prediction of ∆Gads value. It was found that if we use the components of UN (monounsaturation, diunsaturation, polyunsaturation) as predictor variable along with ACL, it gives better prediction. The best one in these variables was obtained using ACL and monounsaturation as predictor variable giving an R2 value of 0.89. Another set of predictor variables used were fatty acid composition (C16:0, C18:0, C18:1, C18:2, and C18:3 percentages). The correlation was developed on six different seed oils with varying FA composition. The best fit was obtained using C16:0, C18:0, and C18:1 percentage as predictor variables and experimental ∆Gads as response variable in a multicomponent analysis with R2 value of 0.95. Equation 11 was obtained for this prediction model:
∆Gads ) -3.56 - (0.00918 × C16:0) + (0.0462 × C18:0) - (0.00553 × C18:1) (11) The equation yielded an R2 value of 0.95 with an error of Y-estimate (σY) ) 0.05. The data from multi-component regression analysis including T-test and P-values for individual components is presented in Table 3. Based on the attained significance level or P-value, the probability of obtaining more extreme value of the test statistic by chance if the hypothesis was true, is 0.05. This value indicates that the statistical significance for C18:1 in eq 11 is high since it has a P-value of 0.03. This equation is then used to check the validity of multicomponent regression model by predicting the ∆Gads values, ∆Gads (predicted) using the percentages of palmitic, stearic, and oleic acids in different seed oils. A correlation plot for ∆Gads (predicted) using eq 11 versus experimental ∆Gads is shown in Figure 5. The residuals obtained for different seed oils by subtracting ∆Gads (predicted) from ∆Gads (experimental) is no more than 0.04, which shows that predicted values are with in (0.04 of experimental ∆Gads values. Although, the prediction is not based on an extensive data set, the approach is extremely useful in determining the relative importance of different FAs on ∆Gads. In conclusion, it can be stated that seed oils are a potential replacement for mineral oil-based lubricants and industrial fluids. The amphiphilic character of seed oil molecule allows for adsorption on metal surfaces and the separation between the dynamic mechanical surfaces. The Langmuir adsorption model
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can be applied to friction derived isotherm to compute free energy of adsorption (∆Gads) for a given additive system. The triglyceride chemistry affects the ∆Gads values of seed oils in the following way: • Degree of functionality has a strong inverse effect on ∆Gads value. • Degree of unsaturation has a strong direct effect on ∆Gads value. • Average chain length of 17-18 favors lower ∆Gads values (i.e., strong adsorption). • From various fatty acids, oleic acid content has a strong inverse effect on ∆Gads value. As we can see, ∆Gads values are largely dependent on the availability of polar heads for adsorption on metal surface. Chain lengths and unsaturation in the fatty acids also contribute to the free energy of adsorption in these molecules. Based on this information, suitable lubricant molecules can be designed that will have optimum structure for effective metal adsorption as well as exhibit excellent boundary lubrication properties. Acknowledgment Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product. The use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. Literature Cited (1) Salunkhe, D. K.; Chavan, J. K.; Adsule, R. N.; Kadam, S. S. World Oil Seed Chemistry, Technology and Utilization; Van Nostrand Reinhold: New York, 1992; pp 1-8. (2) Bockish, M. Fats and Oils Handbook; AOCS Press: Champaign, IL, 1998. (3) Lawate, S. S.; Lal, K.; Huang, C. Vegetable oilssstructure and performance. In Tribology Data Handbook; Booser, E. R., Ed.; CRC Press: Boca Raton, FL, 1997; pp 103-116.
(4) Erhan, S. Z.; Asadauskas, S.; Dunn, R. O.; Knothe, G. Vegetable oils for environmentally-friendly applications. In Proceedings of the 48th Oilseed Conference: Competing in World Markets in the New Millenium; 1999; F1-F10. (5) Rieger, M. Use of natural fats and oils in cosmetics. In Baily’s Industrial Oil and Fat Products, 5th ed.; Hui, Y. H., Ed.; WileyInterscience: New York, 1996; pp 349-380. (6) Adhvaryu, A.; Erhan, S. Z.; Liu, Z. S.; Perez, J. M. Oxidation kinetics of oils derived from unmodified and genetically modified vegetables using PDSC and NMR spectroscopy. Thermochim. Acta 2000, 364, 87-97. (7) Adhvaryu, A.; Erhan, S. Z.; Perez, J. M. Tribological studies of thermally and chemically modified vegetable oils for use as environmentally friendly lubricants. WEAR 2004, 257, 359-367. (8) Jahanmir, S.; Beltzer, M. An adsorption model for friction in boundary lubrication, ASLE Trans. 1986, 29, 423-430. (9) Jahanmir, S.; Beltzer, M. Effect of additive molecular structure on friction coefficient and adsorption. J. Tribol. 1986, 108, 109-116. (10) Jahanmir, S. Chain length effects in boundary lubrication, WEAR 1985, 102, 331-349. (11) Beltzer, M.; Jahanmir, S. Role of dispersion interactions between hydrocarbon chains in boundary lubrication, ASLE Trans. 1987, 30, 4754. (12) Beltzer, M.; Jahanmir, S. Effect of additive molecular structure on friction. Lubr. Sci. 1988, 1, 3-26. (13) Schey, J. A. Tribology in Metalworking Friction, Lubrication and Wear; American Society of Metals: Metals Park, OH, 1983; pp 27-130. (14) Biresaw, G.; Adhvaryu, A.; Erhan, S. Z.; Carriere, C. J. Friction and adsorption properties of normal and high oleic soybean oils. J. Am. Oil Chem. Soc. 2002, 79 (1), 53-58. (15) Biresaw, G.; Adhvaryu, A.; Erhan, S. Z. Friction properties of vegetable oils. J. Am. Oil Chem. Soc. 2003, 80 (7), 697-704. (16) Chaudhary, D. S.; Vigneswaran, S.; Ngo, H. H.; Kim, S. H.; Moon, S. H. Comparison of association theory and Freundlich isotherm for describing granular activated carbon adsorption of secondary sewage effluent. J. EnViron. Eng. Sci. 2003, 2, 111-118.
ReceiVed for reView November 14, 2005 ReVised manuscript receiVed February 14, 2006 Accepted March 1, 2006 IE051259Z
