PLASTIC and ALLOTROPIC FORMS of SULFUR HERBERT F. SCHAEFER AND GEORGE D. PALMER University of Alabama, University, Alabama
S
ULFUR appears in many allotropic forms, which are characterized by differences in solubility, specific gravity, crystalline form, etc. The varieties now called alpha-sulfur and beta-sulfur were discovered by Mitscherlich (1) in 1823. The former is the ordinary variety furnishing octahedral crystals belonging to the rhombic system. At 95.4'C. the rhombic form is in equilibrium with a monoclinic modification, beta-sulfur. With the absorption of 105 calories of heat for each 32 grams, rhombic sulfur is converted into the monoclinic (2). Gernez (3) recognized another form of sulfur which he called gamma-sulfur. , He obtained gamma-sulfur, mixed with some alpha- and beta-sulfur, when sulfur that had been heated to 150°C.was cooled to QO°C. and crystallization was initiated by rubbing the walls of the glass container with a glass rod. Gamma-sulfur crystallizes in the form of pale yellow-white, needle-like crystals with a mother-of-pearl luster. The crystals are monoclinic but they have different axial ratios from those of beta-sulfur. A fourth modification of sulfur, also monoclinic, is called delta-sulfur. It is obtained in thim tabular crystals, mixed with some gamma-sulfur, when a solution of sulfur in alcoholic ammonium sulfide is cooled to 5'C. This form is so unstable that measurements of the crystal faces, etc., have not been determined. Linck and Korinth (4) have fgund three other forms of sulfur-theta-, zeta- and nu-sulfur. Theta-sulfur is formed by the evaporation of a c a r b b bisulfide solution of sulfur thickened with Canada Balsam or rubber. It forms pale yellow tetragonal crystals. Zeta-sulfur forms colorless, rhombic plates which have a weak double refraction. The other variety, nu-sulfur, bas colorless, doubly refracting, hexagonal plates. These varying forms of sulfur have diierent stabilities, and a t ordinary temperatures and pressures they change from one form to the other. Korinth (5) believes tbat the following transformations take place, with increasing stability from theta- through alphasulfnr: Theta'Delta'Nu'Zet~~Gamm-BeteAlpha
tance of 2.12Angstrom units. The ring is in the shape of a puckered octagon. Sulfur melts a t approximately 115'C., giving a pale yellow mobile liquid. Between 160°C. and 170°C.i t hecomeg deep brown in color with a sudden and large increase of viscosity. The maximum viscosity is reached in the neighborhood of 200°C. Smith and Holmes (7) state that the peculiar behavior of molten sulfnr may be accounted for on the assumption that liquid sulfur can exist in two s t a t e s o n e a pale yellow liquid called lambda-sulfur which predominates from the melting point to 160°C., and the other a brown viscous liquid, called mu-sulfur, which prevails above 160°C. Warren and Burwell (6) suggest that the formation of this highly viscous sulfur depends upon the Ss rings breaking open and forming irregular chains which tangle with one another, thereby giving rise to the marked increase in viscosity. The work of Kellas (8) on the measurement of the viscosity of molten sulfur indicates that the surface of molten sulfur is composed of inactive Ss molecules and active S, molecules. He alsq found that between 117°C. and 155'C. the sulfur molecules existed as 8 and between 160°C.and 444OC.the sulfnr was in the form of Sls molecules. From this work i t would seem that the sulfur undergoes an endothermal polymerization a t about 160°C.: 3%
* (Sdr
';
The formation of this polymer of high molecular weight might account for the increase in viscosity. Amorphous sulfur (mu) is insoluble in carbon bisulfide. It was first isolated and described by Deville (9). Work on the freezing point of sulfur has shown that amorphous sulfnr may exist dissolved like a foreign body in soluble sulfur, indicating that amorphous sulfur is a distinct variety and is not solid monoclinic or melted soluble snlfur. Amorphous sulfur is formed as soon as sulfnr melts and increases in amount as the temperature rises. The progressive increase of this amorphous snlfur, formed with rising temperature, indicates that the amorphous sulfur is generated by an endothermal reaction from the original soluble sulfur (7). Therefore, a t ordinary temperatures amorphous sulfur is less stable than crystalline sulfur. The actual proportion that is present is determined by the reversible reaction (10):
Different methods of determining the molecular weight of rhombic sulfnr show that the molecule of the element in the solid state may be represented by Ss. Warren and Burwell (6) have shown by X-ray analysis that the crystal cell of rhombic sulfur contains sixteen lambda-sulfuremu-sulfur molecules of 5%.The sulfur atoms in the Ss molecule are linked together in a ring structure with an S S dis- Above 160°C.only mu-sulfur is stable. Rapid heating 473
and the presence of certain foreign substances postpone the transition from lambda-sulfur to mu-sulfur. Conversely, the rapid cooling from temperatures above 160°C. postpones the transition from mu-sulfur to lambda-sulfur, and when conditions are favorable a large proportion of mu-sulfur may be found in the chilled product. The product obtained on rapid cooling is then a more or less plastic mass, which in time partly reverts to soluble sulfur and partly to a hardened, difficultly soluble form. The latter, like all amorphous substances, is a supercooled state of a liquid, namely mu-sulfur (7). Berthelot (11) has assumed that sulfur present in diierent sulfur compounds is in differentphysical forms. as an electronegative element is That is, sulfur soluble: when nresent as an electronositive element it is insoluble. He considers this to be shown by the experimental evidence that the electrolysis of an aqueous solution of hydrogen sulfide will give only soluble sulfur, wbich appears a t the positive pole, while the electrolysis of sulfurous acid will give almost insoluble sulfur a t the negative pole. Lange and Cousins (12) have shown that as the temperature rises the complex Ss molecules, in the molten state, dissociate into simpler molecules. They assumed, from the work of Aten (13) and Beckmann ( l a ) , that the dissociation followed the reversible reactions: The molecule SPwas isolated by Aten (13) who called i t pi-sulfur. At low temperatures Sz is not stable and probably immediately polymerizes to Sq and Se molecules. It has been suggested by some investigators that the S8 molecules represent mu-sulfur, the variety insoluble in carbon bisulfide (15). The form of sulfur isolated by Engel (16) bas been given the formula Se by other investigators; however, it is soluble in carbon bisulfide. At temperatures above 1800°C. the vaporized Sa molecules dissociate into singl: atoms. Above 160°C. sulfur is stable in an amorphous form. Sudden cooling yields a rubbery, elastic material which is unstable a t ordinary temperatures. This plastic mass normally retains its elasticity for only a few hours after cooling below the transition temperature, for it gradually changes into the crystalline variety of the element. Crystallization begins in the center of the mass; that is, a t the part where the cooling has been slowest (17). Kastle and Kelly (18) have shown that the rate of change of plastic sulfur into crystalline sulfur varies according to the temperature a t wbich the sulfur is poured. It diminishes with the degree of supercooling and also with the temperature a t which it is preserved. If the plastic sulfur is kept a t a snfficiently low temperature it will not crystallize. Mondain-Monval (19) found that the temperature below which plastic sulfur would not crystallize was -299C. The work of Smith and Holmes ( 7 )points out that the proportion of the different polymers in freshly prepared plastic sulfur cannot be accurately determined. They pointed out that the soluble crystalline sulfur, un-
doubtedly present, appears to be covered with the other forms of the element that are present, and that this entire particle tends to dissolve in carbon bisulfide. Alone, the crystalline variety cannot be obtained in a plastic condition. If the plastic mass is allowed to harden before analysis, a change in its composition is certain to take place. At normal temperatures plastic sulfur is in a metastable condition; therefore, on hardening, an undeterminable portion will pass into the soluble crystalline form. After the plastic mass has become hard and opaque the proportion of insoluble sulfur is easily determined by extraction. Once this stage has been reached the further transformation of insoluble sulfur into soluble sulfur proceeds very slowly. There is little reduction in the proportion of the former even when the sample is kept for months (20). Plastic sulfur may also be obtained by the decomposition of certain sulfur compounds, such as the thiosulfates; by the interaction of hydrogen sulfide and sulfur dioxide; or by the decomposition of xanthates. Iredale (21) prepared plastic sulfur by treating powdered sodium thiosulfate with concentrated nitric acid. A yellow, transparent, elastic mass was obtained, which, after distention to four times its length, regained its original form. After twenty-four hours it passed completely into the crystalline form. Since the sudden separation of sulfur from a very concentrated solution is favorable to gel formation, Iredale regarded his elastic sulfur as the gel form of colloidal sulfur. Von Weimarn (22) found that when sulfur heated above 400°C. was poured in thin streams into liquid air, the sulfur was obtained in-the form of thin threads with a diameter of 0.5-1.O'inm. When these threads were warmed they became very elastic. The maximum extension of a thread about one mm. in diameter was approximately 5.5 times the original length, and if the extension was less than the maximum, the thread was able to return to its original length. The elasticity was lost in about thirty minutes after the thread was removed from the liquid air. Apparently one condition necessary for the formation of plastic sulfur is that the Ss rings be broken. Whether these broken rings are by themselves plastic or whether they undergo f u e e r polymerization has not been definitely settled. From the nature of the preparation of plastic sulfur i t would seem that the plastic property is a function of the amorphous sulfur content. Hammick and Zvegintzov (23) suggested that amorphous sulfur is itself a gel, and confirmed their view by observing the Tyndall effect in pure molten sulfur. Some writers describe plastic sulfur as a distinct soluble variety of amorphous sulfur (24, 25). Erdmann (26) believed the formation of amorphous sulfur could be accounted for by the polymerization of thiozone Sa. From the X-ray analysis of Meyer and Go (27) i t has been shown that plastic sulfur has a fiber structure. The sulfur atoms are arranged in long chains linked by covalencies arranged parallel to the direction of the fiber. On stretching, the fiber can be made to crystallize, though normally it is amorphous;
that is, the sulfur chains are tangled with each other. Sulfur has been preserved in the plastic condition by the use of other chemicals. Dietzenbacher (28) found that iodine would confer unusually persistent plasticity on supercooled sulfur. Plastic sulfur of a relatively stable character was prepared by Hamor and Duecker (29) by heating a mixture of elementary sulfur and phosphorus sulfide to a temperature above 200°C. and rapidly cooling the resulting fluid to a temperature below the melting point of sulfur. Arsenic and thallium, or their oxides or sulfides, also act as stabilizers (30). The products prepared by treating phenol or its homologs with sulfur chloride also act as stabilizers for plastic sulfur (31). Whatever the nature of plastic sulfur, it holds promise
of great usefulness in the future. Trillat and Forestier (32)have shown that plastic sulfur, prepared by heating sulfur to a temperature above 250°C. and then pouring into cold water, stretches upon rapid drawing to a permanent lengthening of 800-1000 per cent. The sulfur was amorphous before drawing but the stretched fibers showed temporarily a characteristic orientation of the crystals in the direction of the axis of the fiber (33). Determination of the stress-strain curves of raw rubber and of plastic sulfur, in which data were recorded practically independent of plastic flow, showed that there is a marked similarity between the curves of these two substances (34). With a means of stabilizing this elasticity, plastic sulfur should in many instances be a satisfactory substitute for rubber.
LITERATURE CITED
MITSCHERLICH, Siteber. Akad. Berlin, p. 43, 1823.
LEWISAND RANDALL, J. A m . Chem.Soc.. 36,2468 (1914). GERNEZ,Compt. rend., 97, 1477 (1883). LINCKAND K O R ~ HZ., anorg. allgem. Chenz., 171, 312 114'7R>
KORINTH, a i d . , 174,57 (1928).
WARRENAND BURWELL.J.. Chem. Phvs.. 3. SMITHAND HOLMES, J. A m . Chem. So;., 27; KELLAS, J. C h m . Soc., 113,903 (1915).
D e v r ~ ~Compt. e, rend., 26, 117 (1848). KDSTER. Z. m o w . Chem.. 18. 365 (1898).
BERTHE~OT,~ n chim. ; jhys.', (3) 49,481 (1857). LANCE AND COUSINS. Z . Physik. Chem., 143,135 (1939j.
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34)
DAGUIN, Compt. rend.. 20, 1665 (1845). K s n AND ~ KELLY.Am. C h . J.,32,483 (1904). MONDAIN-MONVAL, Compt. rend., 198, 1431 (1934). SMITHAND HOLNBS, Z. physik. C h . , 42,473 (1903). IREDALE, Kolloid-Z.. 28, 126 (1921). VONWEIMAEN, ibid., 6, 250 (1910). HAMMICK AND ZVEGINTZOV, J. Chem. SOC.,Part2,273 (1930). DEVILLE, Ann. chim. phys., (3) 47, 94 (1855). MAG NU^, Pogg. Ann., 94,308 (1854). ERDMANN, Ann., 362, 133 (1908). MEYERAND GO, Helu. Chim. Acla., 17, 1081 (1934). DIETzENBACHER, Comfit. rend., 56.39 (1863). HAMOR AND DUEcKER, U. S . P a t . 1,959,026 (1934). HAMOR AND DUECKBR, U.S . Pat. 1,981,232(1934). ELLIS,U. S . Pad. 1,676,604 (1928). TRILLAT AND FORESTIER, Compt. rend.. 192,559 (1931). TRILLATAND FORESTIER, Bull. soc. ckim., 51, 248 (1932). STRONG, J. Phys. Chem.. 32, 1225 (1928).
