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154 Ralf Steudel ported this method [1]. Hydrophilic sulfur sols are obtained by decomposi- tion of aqueous thiosulfate solutions with strong mineral acids (Raffo sols [2]) or by reaction of aqueous sulfite ions with sulfide ions at acidic pH val- ues (Selmi sols [3]). Similar sols are obtained by hydrolysis of S2Cl2 with cold water. Collectively, the hydrophilic sulfur sols are sometimes termed OdØn sols [4]. Weimarn sols are metastable but exist only at rather low sulfur concentra- tions (<0.1 g l1) while stable Raffo sols with up to 600 g l1 have been pre- pared. Thermodynamically, all sulfur sols are unstable with respect to trans- formation into orthorhombic sulfur S8 which is water-insoluble and precipi- tates from the solution. Sulfur sols play an important role in certain industrial desulfurization processes where elemental sulfur is formed [5]. This sulfur sometimes stays in the liquid state for several days creating separation and clogging prob- lems. Furthermore, certain sulfur bacteria produce so-called “globules” of liquid sulfur or sulfur-rich compounds by oxidation of sulfide HS, thiosul- fate S2O32 or polythionate ions SnO32 (see later). The characterization of these globules has considerably stimulated the interest in sulfur sols in re- cent years. Historically, aqueous sulfur sols are related to the so-called “Wacken- roders solution” which is obtained by reaction of H2S with SO2 in water at 0 C and which contains polythionic acids as well as elemental sulfur besides other sulfur compounds (e.g., sulfate). For a detailed early investigation and vivid description of this mixture, see ref. [6]; for more recent studies, see later. A compilation of 23 patents for the preparation of colloidal sulfur mixtures from the years 1931–1948 has been published by Meyer [7]. 2 Hydrophobic Sulfur Sols Weimarn sols can be obtained by dilution of an S8 solution in a solvent which is indefinitely miscible with water [4, 8]. Most often ethanol or ace- tone is used to dissolve the sulfur and this mixture is then poured into a large excess of water (>35 times the volume of the S8 solution). Since the sol- ubility of S8 in water at 25 C is only 5106 g l1 [9] the sulfur precipitates practically quantitatively, but not necessarily in the solid form. At sulfur concentrations of <100 mg l1 the freshly prepared sol shows initially a blu- ish-white opalescence which after ca. 10 min changes to a reddish-violet hue. One hour after preparation the sol is of white, milk-like appearance. Under a microscope spherical particles with diameters of up to 0.8 mm can be recognized (magnification 1000). Using polarizers the particles exhibit the typical Malteser cross pattern which is known from lamellar phases of amphiphilic substances [10]. In other words, the particles are composed of liquid rather than crystalline sulfur. Such a sol is metastable at 20 C for a few days but since liquid sulfur is thermodynamically unstable at 20 C, the sulfur eventually crystallizes and the small crystals of a-S8 form aggregates of up to 50 mm diameter which settle to the bottom of the flask. This processPDF Image | Topics in Current Chemistry
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