Crystal structure urea-hydrocarbon complexes

Smith, A.E. The crystal structure of the urea-hydrocarbon complexes. Acta Crystallogr. 1952, 5, 224. 

In Sects. 5 and 6, a few investigations of urea-polyethylene complexes (UPEC) were discussed. The UPEC is an interesting material because a single polyethylene chain is located in an hexagonal canal of urea molecules and it must be expected that the polyethylene chain can behave differently from the bulk systems like solution-grown crystals or materials recrystallized from the melt. The inclusion complex system composed of short hydrocarbon molecules and urea molecules was studied more than 30 years ago. The crystalline structures of urea-hydrocarbon complexes are known The urea-polyethylene complex system was prepared rather recently by Monobe et al. , replacing the hydrocarbon molecules in the urea-hydrocarbon complex by... 

A. E. Smith, The Crystal Structure of Urea-Hydrocarbon and Thiourea-Hydrocarbon Complexes, Journal of Chemical Physics 18 150-152 (1950). 

The strangest type of co-crystal is formed from urea and fatty acids or hydrocarbons in aqueous solutions. Nonbranched fatty acids and hydrocarbons form clathrates in water. The water molecules are ordered around the long alkyl chains mqre than in pure water, and at saturation and low temperatures water-hydrocarbon complexes may form ice structures with entrapped hydrocarbons. The hydrocarbon chains just fill the hollow space in tetrahedral ice and thereby stabilize the ice crystals and raise their melting point. however, urea is added, the urea replaces the water molecules on the hydrocarbon surface. The hydrophobic chain is now entrapped in tubular urea channels. Pure urea crystals obtained from water solutions do not contain such channels. Somehow urea, the most polar of all naturally occurring compounds (dipole moment 3.8 daltons ) develops an affinity to irregular apolar chains in the aqeous medium and entropy is overcome (Fig. 2.5.17). 

Qathrate compounds are of this type molecules of one substance are trapped in the open structure of molecules of another. Hydroquinone forms clathrate compounds with SO2 and methanol, for example. Urea and thiourea also have the property of forming complexes, known as adducts, with certain types of hydrocarbons. In these cases molecules of the hydrocarbons fit into holes or channels in the crystals of urea or thiourea the shape and size of the molecules determine whether they will be adducted or not. 

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