Origin of theλTransition in Liquid Sulfur

Abstract

Developing a novel experimental technique, we applied photon correlation spectroscopy using infrared radiation in liquid Sulphur around T λ , i.e. in the temperature range where an abrupt increase in viscosity by four orders of magnitude is observed upon heating within few degrees. This allowed us -overcoming photo-induced and absorption effects at visible wavelengths -to reveal a chain relaxation process with characteristic time in the ms range. These results do rehabilitate the validity of the Maxwell relation in Sulphur from an apparent failure, allowing rationalizing the mechanical and thermodynamic behavior of this system within a viscoelastic scenario. PACS numbers: 62.60.+v,64.70.Ja,62.10.+s, Common wisdom holds that liquids become less viscous as the temperature is raised. On a microscopic ground, indeed, viscosity can be regarded as arising from continual brushing of molecules which are close to each other. On increasing temperatures the more vigorous thermal motion of the molecules is expected to render such mutual friction progressively less effective. This naive description provides a qualitative interpretation of the decrease in viscosity normally observed in almost all substances as the temperature increases. There exist, however, a few remarkable exceptions exhibiting complex behavior, which can be regarded as extreme examples of how critically viscosity depends upon molecular interactions. Specifically, there are ranges of temperatures in which the viscosity of Sulphur exhibits anomalous temperature dependence. In the liquid state, just above melting (T m =119 o C), eight-membered rings (S8) are the most abundant species, and the viscosity slightly decreases with temperature as normally expected, reaching values as low as 0.01 Pa·s at T = 157 o C. Further increase of temperature causes a dramatic increase of viscosity [1], accompanied by gross changes in optical [2,3,4] and thermodynamic [5,6] properties, which, at T ≈185 o C, reaches a maximum value of 100 Pa·s. Beyond this temperature, a gradual viscosity decrease is observed. Although this phenomenon, known as λ-transition (T λ = 159 o C), has been tackled for more than 150 years, its comprehension is still far from being reached: to rationalize this puzzling temperature dependence of viscosity, indeed, a liquid-liquid transition from a monomeric (i.e. a S8 rings liquid) to a polymeric phase has been invoked [7,8]. The temperature dependence of the mass fraction of S atoms participating in the polymeric component turns out to be a central parameter for extracting information on many thermodynamic aspects of the λ-transition [9,10]. This phenomenon has been classified as a living polymerization [11] transition that essentially involves two steps, the initiation (formation of diradicals through opening of S8 rings) and propagation (concatenation of species to form long polymeric chains). Thermo-chemical models were developed in the past [12,13,14] to relate the transport properties of liquid sulphur to the underlying polymerization phenomenon. T...

Sulfur and few-layer graphene interaction under thermal treatments

“…The other important change in sulfur is the so called λ transition at 159 °C (Tλ). After melting point viscosity decreases with temperature but there is a dramatic increase of it with a maximum at 159 °C, beyond this temperature viscosity decreases again [18]. For comparison, in Figure 1 b) differential scanning calorimetry of bulk sulfur (S) and sulfur immersed in a nanoporous anodic alumina template (NAA) are presented [15].…”

Correlating Melt Dynamics and Configurational Entropy Change With Topological Phases of AsxS100–x Glasses and the Crucial Role of Melt/Glass Homogenization

“…And when Sulfur is heated to a T of nearly 159 • C, also called T λ , the S 8 rings open up into chains, leading to the polymerization transition (Tobolsky and Eisenberg, 1959). The T λ transition in pure S has been more recently examined in Photon correlation spectroscopy (Scopigno et al, 2007), to reveal an underlying chain relaxation process in the millisecond range, validating the Maxwell relation for the increase of viscosity for the polymerization transition. Recent Raman scattering experiments (Bellissent et al, 1990;Kalampounias et al, 2003;Andrikopoulos et al, 2005), have permitted to establish the polymerized Sulfur fraction as a function of T as T>150 • C, and compare these to theory (Tobolsky and Eisenberg, 1959).…”

Pressure dependence of structural and dynamical properties in melt sulfur: Evidence for two successive chain breakages

“…Then upon rising temperature the molecular liquid undergoes a so-called transition at around 432 K, which is linked to the breakdown of S 8 molecular rings and polymerization into long atomic chains [19][20][21][22][23][24][25]. The length of the atomic chains reaches a maximum at about 460 K and then decreases at higher temperatures, as does the viscosity, which is strongly correlates to the length of the chains.…”