ANALYSIS OF SULFUR COMPOUND TYPES IN ASPHALT

Sulfur compound types were determined in asphalts from the SHRP Materials Reference Library. Sulfoxides were determined by infrared spectrophotometry (IR); sulfides were measured via oxidation to sulfoxides with tetrabutylammonium periodate followed by IR; thiophenes were determined by difference (total sulfur minus other types). Compound types were determined in whole asphalts as well as for fractions from each asphalt obtained using nonaqueous ion exchange liquid chromatography (NIELC). Results from the individual NIELC fractions were compared against those for the corresponding whole asphalt. In addition, results from this analysis were compared against those obtained by other workers utilizing X-ray (XANES) techniques. The neutral fraction from NIELC was further fractionated into saturated hydrocarbon, neutral-aromatic (contains thiophenic sulfur types), and sulfide classes using liquid chromatographic (LC) techniques. The distribution of sulfur types obtained from the LC separation of neutrals was compared with that obtained from the above instrumental methods. Generally, results from the IR-oxidation techniques were in good agreement with those from LC separations and XANES analysis, except that the latter include aromatic or other sulfide types in with total sulfides, whereas the oxidation-IR approach appears to be specific for aliphatic sulfides, per se. Considerable variation of sulfur types within NIELC fractions was observed. Compounds with one or more acidic groups plus a sulfoxide group constitute a significant proportion of the total amphoteric compounds present in most asphalts.