Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 63

oil used. They respond well to additive treatments which fortifies the grease for high
loads. They are oxidation and rust inhibited [13].
6.5 AQUEOUS LUBRICANTS AND PHASE DIAGRAM OF SOAPS IN
WATER
Emulsions of soaps are also used as metal-metal lubricants. Hollinger et al., 2000 in-
vestigated high pressure lubrication with lamellar structures in aqueous lubricant [22].
Two solutions of dispersed organic fatty particles in water were essentially compared:
a vesicle solution (called vesicles) and a lamellar crystallites solution, (called lamel-
lar). The two systems had the same chemical basis. The water phase contained primary
and secondary amines, as well as mineral phosphate esters. The solution was buffered
at pH = 8.5. The oily phase was composed of fatty phosphates and fatty acids, which
were surfactants, since they had polar heads. The lamellar solution was obtained by
adding copper and zinc complex ions to the vesicle solution and was characterized by
a change in color from a white milky emulsion to a blue one. The lubrication mecha-
nisms of water-based fluids were not yet well understood, especially when extreme
load conditions was applied. It was shown that the presence of such nanostructures
in the sliding interface could provide extreme pressure lubrication. This water-based
lubricant was particularly efficient when one of the two solid surfaces was made of
brass. In this case, a multilayer was created in the contact. It was formed by a brass
layer transferred to the counter face, the brass surface itself and a lamellar film adher-
ent to the former layers [22].
Phase Diagram of Soap Species in Water
The typical phase diagram of a soap species, in the presence of a solvent system has
the general form depicted in Figure 2. The diagram is characterized by the T k line cor-
responds to Krafft boundary, which essentially gives the temperature above which the
soap molecules exist in a molten but aggregated form [23]. The Krafft point is defined
as the point of intersection on the temperature axis between the curve of molecular
solubility for the solid soap, depicted by the dashed line extrapolation, and the curve
of critical micella concentration (CMC), depicted by the dotted line. As the soap mol-
ecules are introduced into water, at very low concentration (to the left of the dotted
line), they exist as individual molecules in solution, exhibiting preferential adsorption
at surfaces or excess concentrations in the interfacial regions. Upon an increase in con-
centration beyond CMC curve, the molecules start to form spherical micelles. The in-
dividual micelles start packing together as the charge repulsion diminishes in the pres-
ence of higher counterion concentrations. This leads the formation of liquid crystalline
phases in the molten state. Hexagonal phases (H 1 ) are composed of rodlike micelles
packed in a hexagonal pattern, whereas maximum packing of molecules is achieved
through formation of lamellar structures (L Į ), wherein flattened disc micelles come
together and form large stacks of double layered sheets, as shown in Figure 2. The
Krafft boundary the soap molecules can form chain frozen solid crystals or gel phases,
which consist of rigid lamellae of soap double layers with a disordered liquid solution
sandwiched between them. The chain frozen solid soap crystals exhibit a number of