Lubricants crystal forms

Boron nitride is an important 13-15 binary compound that exists in a number of different crystal forms. The hexagonal form is similar in structure to graphite (as shown in Figure 3.3) and is thermodynamically the most stable. It s soft and used as a lubricant and as a sheen-producing additive in cosmetics. It s synthesized by heating boron trioxide (BjOg) or boric acid with ammonia at 900 °C ... 

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Barium acetate [543-80-6] Ba(C2H202)2, crystallines from an aqueous solution of acetic acid and barium carbonate or barium hydroxide. The level of hydration depends on crystallization temperature. At <24.7°C the trihydrate, density 2.02 g/mL is formed from 24.7 to 41 °C barium acetate monohydrate [5908-64-5] density 2.19 g/mL precipitates and above 41 °C the anhydrous salt, density 2.47 g/mL results. The monohydrate becomes anhydrous at 110°C. At 20°C, 76 g of the monohydrate dissolves in 100 g of water. Barium acetate is used in printing fabrics, lubricating grease, and as a catalyst for organic reactions. 

Graphite is another solid form of carbon. In contrast to the three-dimensional lattice structure of diamond, graphite has a layered structure. Each layer is strongly bound together but only weak forces exist between adjacent layers. These weak forces make the graphite crystal easy to cleave, and explain its softness and lubricating qualities. 

In contrast to the case of the 8CB liquid crystal, no contact potential differences between first and second layers were observed with these lubricants. This indicates that there is no special orientation of the dipole active end groups, or perhaps that the end groups form hydrogen-bonded pairs with neighboring molecules so as to give no net dipole moment. 

Before 1925, there were a few compounded oils made for special purposes, such as lubrication of marine engines and steam cylinders, but additives were not used in automotive crankcase oils. In the 1930 s, chemical compounds made by condensation of chlorinated paraffin wax with naphthalene were found to lower the pour points of oils. Pour depressants (9) apparently are adsorbed on small wax crystals which separate from oils when they are chilled. The protective adsorbed layer of additive prevents the normal interlacing of larger wax crystals which forms a gel. In 1934 polymerized unsaturated hydrocarbons first came into large scale commercial use to lower the temperature coefficient of viscosity of oils. Other compounds for increasing the viscosity index of oils have since become common. 

One curious observation is that high activities for direct sulfur extraction from thiophene derivatives are only exhibited by metal sulfides that form stacked lamellar crystallites, similar to graphite structures (1-3). MoS2 is classic in this regard and has found applications as a high-temperature lubricant with properties very similar to those of graphite. The other widely used metal sulfide in HDS is WS2, which also forms lamellar crystal struc-... 

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