Title: Soybean oil and methyl oleate adsorption onto a steel surface investigated using a quartz crystal microbalance with dissipation monitoring and atomic force microscopy**1 Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: May 10, 2012
Publication Date: May 10, 2012
Citation: Evans, K.O., Biresaw, G. 2012. Soybean oil and methyl oleate adsorption onto a steel surface investigated using a quartz crystal microbalance with dissipation monitoring and atomic force microscopy**1 [abstract]. Society of Tribologists and Lubrication Engineers. Paper #1245562. Technical Abstract: The United States’ 2010 annual production of soybean oil exceeded 8 million metric tons, making a significant vegetable oil surplus available for new uses, particularly as a lubricant. Investigation of soybean oil and methyl oleate adsorption onto steel using a quartz crystal microbalance with dissipation monitoring revealed that both adsorbed as rigid films with nearly the same surface concentration. Further analysis revealed that the films formed with individual molecules tilted toward the surface. Atomic force microscopy also revealed that methyl oleate adsorbed with periodicity of alignment on highly ordered pyrolytic graphite. EXTENDED ABSTRACT INTRODUCTION More and more recent research has been focused on renewable sources such as vegetable oils as lubricants. This interest is a result of the global environmental concerns about the continued use of petroleum products. Vegetable oils are abundant, biodegradable, and environmentally friendly. Typically liquid at room temperature, vegetable oils have low volatility and a narrow temperature range for viscosity changes. Therefore, vegetable oils can be treated as functional fluids and thus can be studied as lubricant base oils or additives. The amphiphilic properties of triacylglycerides (TAGs), the major component of most vegetable oils, affect the boundary lubrication of vegetable oils. However, understanding the fluid and boundary characteristics of vegetable oils is required to fully understand their tribological properties. Boundary characteristics of vegetable oils can be better understood by exploring adsorption properties of the oils. Adsorption, occurring mainly through polar groups of vegetable oil, can be monitored using the quartz crystal microbalance with dissipation (QCMD) technique. QCMD measures changes in thin film deposition on an oscillating surface by simultaneously monitoring frequency shifts (delta f) in the resonating surface and changes in the dissipation of the oscillations (delta D). Structural properties may be monitored using atomic force microscopy (AFM). Previously it was shown that the chemical properties of a vegetable oil affected its free energy of adsorption (delta Gads) on steel. These studies, however, did not provide any information about the adsorption behavior of these oils onto the steel surface. Thus, the objective of this work is to use QCMD and AFM to investigate structural and absorption behavior of two previously studied oils, soybean oil and methyl oleate, onto steel. RESULTS AND DISCUSSION Soybean oil over the concentration range of 1 to 200 mM was monitored for adsorption onto stainless steel using QCMD. As figure 1 depict, soybean oil exhibited rapid adsorption in hexadecane. The frequency shifts (delta F) exhibited a large decrease with the introduction of increasing concentration of soybean oil, followed by an increase in frequency with a subsequent rinse with hexadecane. Dissipation shifts exhibited rapid increases with increasing soybean concentrations followed by reduction after rinsing. These frequency and dissipation changes are indicative of increasing bulk viscoelastic properties of the material. After the subsequent rinsing with hexadecane, soybean oil was permanently bound to the steel (as depicted by frequency shifts lower than the pre-injection values). Dissipation values remained well within those taken for adsorbing thin, rigid films (delta D = 1e-6). Methyl oleate exhibited similar behavior. Rodahl and Kasemo(1996) detailed thin film adsorption to have a linear relationship with frequency shifts assuming that the adsorbed mass is small compared to the crystal, the adsorbed mass is rigidly bound and not slipping, and is distributed evenly over the surface. For soybean oil and methyl oleate above 100 mM, the delta F values after hexadecane rinsing remained constant. This was interpreted as full surface coverage by soybean oil and methyl oleate equal to a maximum adsorbed mass of 130 and 50 ng/cm2, respectively. Further analysis suggests that soybean oil orients with its acyl chains perpendicular to the surface and methyl oleate orients parallel to the surface. Surface properties of methyl oleate were explored using AFM on highly-ordered pyrolytic graphite (HOPG). Analysis revealed that the flat surface developed a roughness of 0.12nm and peak-to-peak features of approximately 1 nm tall. This was also the characteristic behavior on mica. Comparison with self-assembled monolayers and oils on metal surfaces indicates that soybean oil orients similar to other hydrocarbons. The angle of adsorption (gleaned from thickness analysis) suggest that soybean packs as a disordered liquid. Thickness analysis suggests that methyl oleate orients in a head-to-head/tail-to-tail orientation, similar to the orientation expected for oleic acid on a surface. Further analysis done by comparing dissipation changes versus frequency shifts (which explicitly removes time and emphasizes energy associated with adsorbed mass) showed no slope change. This indicates that structural arrangements of both soybean oil and methyl oleate occurred instantaneously.