Elemental Sulfur Nanoparticle Coarsening Kinetics and Changes in Raman and Voltammetric Signals

Elemental sulfur exists as a variety of forms in natural systems, from dissolved forms (noted as S8 (aq.)) to bulk elemental sulfur most stable as α-S8 (bulk) (but existing in at least 180 different allotropes and polymorphs). Elemental sulfur can form via a number of biotic and abiotic processes, many of which begin with S-S bonding of practically unlimited length, forming chains and rings of many sizes. These molecules aggregate into larger and larger forms summarized by the following pathway: S → S8 (aq.) → S8 (nano) → α-S8 (bulk) Formation of elemental sulfur nanoparticulates (S8(nano)) can be produced via three primary techniques to create emulsions of liquid sulfur in water called sulfur sols (Steudel 1999) that approximate some mechanisms of possible elemental sulfur formation in aqueous systems. These techniques produce Weimarn sols (S8 (Weimarn)) (Weimarn 1925), Raffo or LaMer sols (S8 (Raffo)) (LaMer & Barnes 1945), and S8 (polysulfide) by the decomposition of polysulfides. These emulsions begin as single S8 (aq.) molecules but quickly become nanoparticulate and coarsen into micron-sized particles via two possible pathways: Ostwald ripening (collision and collapse) and aggregation process (nanoparticle clustering). In an effort to investigate fundamental questions related to elemental sulfur particle size in natural systems, we conducted a series of experiments to study the rate of elemental sulfur particle coarsening using dynamic light scattering analysis under different physical and chemical conditions. Results showed that S8 (sol) coarsening rates are strongly temperature dependent. The addition of surfactants, to emulate biotic mechanisms to transport elemental sulfur inside the cell, shows a significant reduction in the rate of coarsening, in addition to known effects of these molecules on elemental sulfur solubility (Steudel & Holdt 1988). Dynamic light scattering, XRD, and Cryo-SEM results suggest coarsening is largely a product of ripening processes rather than particle aggregation. Unique Raman spectra with a peak located on 423 (cm) was found for elemental sulfur nanoparticles. This elemental sulfur nanoparticle peak does not overlap the Raman spectra of α-S8 (bulk) with peaks located at 215cm, 247cm, 434cm, and 470cm for similar conditions. Previous experiments and in situ observations utilizing Au-amalgam electrode voltammetry were assigned peaks at -0.85 and -1.15 (Volts vs. Ag/AgCl) to colloidal (S8 (nano)) and dissolved elemental sulfur (S8 (aq.)) respectively. No voltammetric peaks were found in these series of experiments related to elemental sulfur as a colloid or dissolved form from sulfur sols.