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84 Ralf Steudel All these temperature-related changes of the physical properties are com- pletely reversible if the melt is cooled down slowly. It must be pointed out, however, that only measurements made with highly purified sulfur using the method by Bacon and Fanelli [4] produce reliable results. The melting point of b-S8 (120 C) is reversible only if the freshly pro- duced melt is cooled down immediately before the last crystal has disap- peared. If the melt is kept for some hours at temperatures above 120 C the freezing point is observed at a constant temperature of 115 C (triple point). This phenomenon is explained by the formation of novel low molecular weight molecules Sn with n61⁄48. In the older literature these molecules have been termed collectively as p-sulfur or Sp (in contrast to S8 which was termed l-sulfur [17]). In addition, polymeric sulfur S1 is a component of liquid sulfur at all temperatures. After isolation from the melt the polymer is usually called m-sulfur [17] or Sm. In the following sections we will show how the molecular nature of p-sulfur has been found out and how the physical properties of liquid sulfur can be understood on the basis of the rather com- plex but very interesting molecular composition. 2 Historical Review [18] 2.1 p-Sulfur 2.1.1 Preparation and Properties of p-Sulfur If liquid sulfur, after the chemical equilibrium has been established (12 h at 120 C or 1 h at 250 C), is rapidly quenched to low temperatures and imme- diately extracted with carbon disulfide the polymeric Sm will remain undis- solved while S8 and Sp will dissolve. After filtration, most of the S8 will crys- tallize out on cooling of the solution to 78 C while Sp together with some S8 remains in solution. It is not possible to isolate Sp completely free of S8. Therefore, in the literature the mixture S8+Sp has often been simply termed “Sp” but in this chapter we will differentiate between Sp on the one hand and mixtures of Sp with S8 on the other hand (“S8+Sp”). If a solution of S8+Sp is evaporated in a vacuum a yellow resin-like mass is obtained which solidifies at low temperatures as a glass but decomposes at room temperature within a few days to a crystalline mixture of S8 and Sm [19]. Because of the instability of Sp at room temperature the quenching of the sulfur melt has to be very rapid and efficient. Different authors have used ice water [20–22], liquid air [23], or sheets of glass [19] or metal [19, 24] onto which the melt was blown by a strong jet of cold gas to produce either a thin film or small particles. Because of the low heat conductivity of elemental sul- fur the quenching in water is insufficient and water is not even inert chemi- cally towards hot sulfur. The quenched melt has to be extracted by CS2 atPDF Image | Topics in Current Chemistry
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