Langmuir 1996,11,1980-1983
1980
Formulation of Lyotropic Lamellar Phases of Surfactants as Novel Lubricants Stuart Fuller, Yan Li, Gordon J. T. Tiddy, and Evan Wyn-Jones" Division of Chemical Sciences, Science Research Institute, University of Salford, Salford M5 4WT, U.K.
R. Derek Arne11 Research Institute for Design, Manufacture and Marketing, University of Salford, Salford M5 4WT, U.K. Received December 30, 1994@ A number of lyotropic lamellar liquid crystalline phases have been formulated using the surfactant cetyltrimethylammonium chloride in the solvents ethylene glycol and glycerol, using decanol, dodecanol, oleic acid, and oleyl alcohol as cosurfactants. The lamellar phases and their existence range were characterized using optical microscopy, X-ray diffractionand differential scanningcalorimetry. The purpose of the exercise was to test these lamellar phases as potential lubricants by measuring their coefficients of friction in metal to metal contact. The results indicate that some of the samples have coefficients of friction similar to or better than selected commercial oils.
Introduction In recent years there has been a n intensive effort focusing on the colloidal properties of dilute and concentrated solutions of surfactants in nonaqueous solvents. As a result, the existence of m i ~ e l l e s l -and ~ a range of liquid crystalline phases me so phase^)^-^ have now been observed and characterized. The purpose of this communication is to explore the potential application of the lamellar liquid crystalline phase of surfactants in ethylene glycol and glycerol as novel lubricants. The structural characteristics of the lamellar phase which should make it function as a lubricant can be appreciated by reference to graphite, in which carbon exists as a n array of thick sheet lamellae which are strong in compression but can slip easily over one another (Figure 1).The high resistance to compression gives a small area of contact while the ease of slipping gives a low value for the interfacial shear strength. Although this is an oversimplification of friction, it does nevertheless illustrate the principles of lubrication via layered structures. Typically a lamellar liquid crystal is illustrated in Figure 2. The lamellar phase has a bilayer structure, which has directional order giving load-carrying capacity, and localized layer disorder giving low resistance to shear. The bilayers of the lamellar phase can extend over large distances (commonly ofthe order of micrometers or more), and their properties of being strong in compression and Abstract published in Advance ACS Abstracts, May 15, 1995. (1)Gharibi, H.; Palepu, R.; Tiddy, G. J. T.; Hall, D. G.; Wyn-Jones, E. J . Chem. Soc., Chem. Commun. 1990,115. (2)Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992,8, 782. (3)Palepu, R.; Gharibi, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1993.9.110. (4jRico, I.; Lattes, A,; Belmejdoub, A,;Marchal, J. P.;Canet, D. Nouu. J . Chzm. 1987,11, 415. (5) Warnheim, T.; Jonsson, A. J . Colloid Interface Sei. 1988,125, 627 ._ ( 6 )Jonstromer, M.; Sjoberg, M.; Warnheim, T. J . Phys. Chem. 1990, 94,7549. (7)Bleasdale, T. A.; Tiddy, G. J. T.; Wyn-Jones, E. J . Phys. Chem. 1991,95,5385. (8) Auvrey, X.; Perche, T.; Petipas, C.; Anthore, R.; Marti, M. J.; Rico, I.; Lattes, A.Langmuir 1992,8, 2671. (9)Bleasdale, T. A.;Tiddy, G . J. T. In Organised Solutions; Friberg, S.E., Lindman, B., Eds.; Marcel Decker: New York, 1992;p 128. @
0743-7463/95/2411-1980$09.00/0
Figure 1. Structure of graphite. The bonding between the lamellae is weak compared with the bonding between the atoms in the sheets themselves. able to slide easily over each over make the lamellar phase a n ideal prospect as a lubricant.1° Lamellar phases are the most common lyotropic mesophase and consist of surfactant bilayers separated by solvent layers. The surfactant head groups are restricted to the chaidsolvent interface. The solvent layer thickness of a typical lamellar liqyid crystal can vary from around 8 A to greater than 100 A depending on the composition, while the surfactant bilayer thickness is generally about 10-50% less than twice the all-trans chain length. Normally a surfactant tail is terminated by a methyl group. The importance of methyl group layers as a sliding zone in lubrication has been suggested to be a key molecular function in lubrication by lamellar species.l0 In many systems, a t temperatures below that a t which the lamellar phase forms, there exists a gel phase (LB). Such phases consist of similar bilayer structures to those in lamellar phases: however, the alkyl chains are frozen in a n all-trans configuration (see Figure 3). At present we are not aware of any two-component surfactantlnonaqueous polar solvent systems where the lamellar phase exists over a temperature range suitable for lubrication purposes (5-150 "C). The Krafft boundary (10)Lee, H. S.; Winoto, S. H.; Winer, W. 0.;Chiu, M.; Friberg, S. E. In Tribology and the Liquid Crystalline State;ACS Symposium Series No. 441;Beresaw, G., Ed.; American Chemical Society: Washington, DC, 1990;Chapter 7.
1995 American Chemical Society
Lamellar Phases of Surfactants as Lubricants
Langmuir, Vol. 11, No. 6, 1995 1981 Strain Gauge I
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Figure 4. Pin on disk test arrangement. -
SURFACTAH1
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aqueous solvents as lubricants are the original work by Friberg.13-16
Experimental Section
Figure 2. Structure of lamellar phase showing bilayers.
(a) Normal bllayer
(b) Tilted bllryer
(c) interdlgitmtod bllryer
Figure 3. Typical gel phase structures.
for the various lamellar phases of binary systems that have been reported occurs a t high temperature ( ' 6 0 "C). Therefore, the component mixtures have to be manipulated to lower the Krafft boundary to a n ambient temperature. This can be achieved by the addition of cosurfactants . The surfactant used in this work was hexadecyltrimethylammonium chloride (CTAC) with ethylene glycol (EG) and glycerol (GL) as solvents. The cosurfactants used were decanol (DE), dodecanol (DO), oleyl alcohol ( O K ) ,and oleic acid (OAC). Although there are reports of aqueous based mixtures having been used as lubricants,11J2as far as we are aware the only reports of the use of lamellar phases in non(11) Biersaw, G.; Festsa, R. P. US.US 4781849 A, 1 Nov 1988.
Materials. Ethylene glycol (99+%)and glycerol (99.5%)were used as received from Aldrich Chemical Co. with water contents of
