Unsaturated Fatty Acids in Alkane Solution:  Adsorption to Steel Surfaces

Abstract

The adsorption of the unsaturated fatty acids oleic, linoleic, and linolenic acid on steel surfaces has been investigated by means of a quartz crystal microbalance (QCM). Two different solvents were used, n-hexadecane and its highly branched isomer, viz., 2,2,4,4,6,8,8-heptamethylnonane. The area occupied per molecule of oleic acid at 1 wt % corresponds to what is needed for adsorption parallel to the surface. At the same concentration, the adsorbed amount of linoleic acid and linolenic acid indicates that they adsorb in multilayers. The chemisorbed amount estimated from static secondary ion mass spectroscopy (SIMS) measurements was found to be similar for the three unsaturated fatty acids. In the case of linolenic acid, it was found that the presence of water significantly alters the adsorption, most likely because of the precipitation of fatty acid/water aggregates. Furthermore, static SIMS results indicate that the amount of water used here inhibits the chemisorption of linolenic acid.

Effects of unsaturation on film structure and friction of fatty acids in a model base oil

“…The higher excess enthalpy implies a lack of hexadecane molecules in the vicinity of the diene parts of the linoleic acid, i.e., between the layers. Here, it is also interesting to note that recent measurements with a quartz crystal microbalance [4] indicated the formation of a close-packed, mainly physisorbed layer of linoleic acid on steel without coadsorbed n-hexadecane molecules. Although the layered structure in our confined film is stabilized by interactions between the exposed diene groups on the monolayer-covered surfaces and the ones on the intervening bilayer, this attraction is overcome by the compression discussed above, which amounted to an applied energy of 1-5 × 10 −22 J/molecule.…”

“…Solutions of 0.05 wt% stearic acid, 0.1 wt% dehydroabietic acid, and 1.0 wt% oleic and linoleic acid in n-hexadecane were prepared for the SFA experiments and dried over molecular sieves (3 Å, 8-12 mesh, Tamro Med-Lab) for 24 h. The concentrations were chosen based on previous observations of plateaus in the adsorption of fatty acids from nonaqueous solutions onto various types of surfaces [4,6,16,[21][22][23]. The solutions were filtered through an Acrodisc CR syringe filter with a PTFE membrane (pore size 0.2 μm, Pall (LifeScience)) when injected between the mica surfaces in the SFA.…”

“…A few systematic investigations have been published, revealing that the amount adsorbed on steel surfaces increases with increasing unsaturation [4] and increasing chain length [5]. Adsorption from nonaqueous solutions typically results in adsorbed amounts only one-half to one-third of those in a close-packed monolayer [4,6], with the exception of linoleic acid, which has recently been shown [4] adsorb on steel from n-hexadecane with a molecular area similar to that of its maximum close packing in Langmuir-Blodgett films. Fatty acids may form chemical bonds with metal oxide surfaces [5][6][7].…”

“…However, later investigations revealed that a more likely reason for the transition was changes in orientation of physisorbed molecules [9]. It has been shown that the amount chemisorbed on steel from alkane solution is similar for oleic, linoleic, and linolenic acid [4]. The different total adsorbed amounts [4] therefore arise from additional physisorption due to interactions between the hydrocarbon parts of the molecules, and only a fraction of the molecules on the surface are chemisorbed.…”

“…It has been shown that the amount chemisorbed on steel from alkane solution is similar for oleic, linoleic, and linolenic acid [4]. The different total adsorbed amounts [4] therefore arise from additional physisorption due to interactions between the hydrocarbon parts of the molecules, and only a fraction of the molecules on the surface are chemisorbed. In alkane solutions, coadsorption of the solvent is also expected [10,11], and "chain matching" is important for both the tribological performance and the bulk properties of mixtures of fatty acids and alkanes [12,13].…”