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Effects of unsaturation on film structure and friction of fatty acids in a model base oil
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.
Department of Chemistry, University of Massachusetts Lowell.
YKI, Institute for Surface Chemistry.
YKI, Institute for Surface Chemistry.
2008 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 326, no 2, 530-536 p.Article in journal (Refereed) Published
Abstract [en]

The normal and friction forces between layers of three fatty acids (stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid) have been measured in n-hexadecane with a surface forces apparatus. Stearic, oleic, and dehydroabietic acid form loose-packed monolayers on mica surfaces when adsorbed from dry n-hexadecane. Linoleic acid forms an additional dimer layer between monolayer-covered surfaces, where it is stabilized by interactions between the double-bond-rich regions of the molecules. The monolayers formed by linoleic and dehydroabietic acid are thinner than the ones formed by stearic and oleic acid, but are not as easily removed from between the mica surfaces when the load or pressure is increased. The friction force increased linearly with load in all systems, and the friction coefficient increased with increasing unsaturation. Linoleic acid showed two regimes of linear friction with increasing load, corresponding to two different film thicknesses. its friction was sensitive to sliding speed and adsorption time, and the thinner film observed at higher load had a lower friction coefficient. Such features were not observed for stearic and oleic acid, where the monolayers were removed and the friction coefficient changed to that of pure n-hexadecane at a pressure of 3.5 MPa.

Place, publisher, year, edition, pages
2008. Vol. 326, no 2, 530-536 p.
Keyword [en]
friction, surface forces, adsorption, fatty acid, rosin acid, surface forces apparatus (SFA), mica
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-24200DOI: 10.1016/j.jcis.2008.05.068ISI: 000259243600034Scopus ID: 2-s2.0-50649106199OAI: oai:DiVA.org:kth-24200DiVA: diva2:345241
Note
QC 20100824Available from: 2010-08-24 Created: 2010-08-24 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Unsaturated Fatty Acids in Alkane Solution: Adsorption and Tribological Properties
Open this publication in new window or tab >>Unsaturated Fatty Acids in Alkane Solution: Adsorption and Tribological Properties
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Fatty acids and blends of fatty acids are used as friction modifiers and anti-wear additives in diesel fuels. A large part of some fatty acid blends are unsaturated fatty acids. Even though fatty acids have been thoroughly investigated, the adsorption and tribological behaviour of unsaturated fatty acids are no fully understood. The aim of this thesis was to investigate the influence of fatty acid unsaturation on the adsorption from alkane solution and the tribological properties, such as friction and wear scar characterization, of such systems.

To increase the fundamental understanding of the influence of the degree of fatty acid unsaturation the three fatty acids, oleic, linoleic and linolenic acid, have been studied. In boundary lubrication the amount of material and how well the material is bound to the surface can affect the friction and wear of the system. Thus, as a first step the adsorption of fatty acids onto steel surfaces was investigated with the quartz crystal microbalance (QCM). This technique is straight forward to used and is ideal for these samples that are difficult to investigate with optical techniques due to small differences in refractive index between the alkane solvent and fatty acid. To determine the chemical binding to steel surfaces static secondary ion mass spectroscopy (SIMS) has been used.

Next, the influence of fatty acid unsaturation on the friction and wear was determined. Two different studies were made, one on a molecular level with the tribological surfaces forces apparatus (T-SFA) and the other one with a technique used for determining friction and wear of diesel fuel additives. The T-SFA technique provides data from which the packing of the fatty acids at mica surfaces can be estimated as well as how the friction is altered due to the packing. For determining the friction and wear of steel surfaces the high frequency reciprocating rig (HFRR) and pin on disc apparatus (POD) were used. The HFRR is a standardized technique, where, if the settings specified in the ISO12156 protocol are used, a diesel fuel additive must provide an average wear scar radius less than 0.230 mm to pass. The wear scars received from the two techniques were further analysed with Raman spectroscopy and also a visual rating of the wear scars was performed.

As the project progressed, there was an increasing interest in the influence of water on the adsorption and lubricating properties of the fatty acids. Moisture uptake of the fatty acids was therefore determined gravimetrically as well as with nuclear magnetic resonance spectroscopy (NMR). The adsorption of the moist fatty acids onto steel surfaces was then investigated with QCM and static SIMS, while HFRR and POD were used for investigate the friction and wear.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. iv, 44 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:5
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-4616 (URN)978-91-7178-854-2 (ISBN)
Public defence
2008-02-15, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100824Available from: 2008-01-28 Created: 2008-01-28 Last updated: 2010-08-24Bibliographically approved

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