FLOW MICROCALORIMETRY AND SALI STUDIES OF THE ASPHALT/AGGREGATE INTERFACE

ABSTRACT

Sorption flow microcalorimetry methods were developed for measuring the amounts and heats of adsorption and desorption at the asphalt-aggregate interface. Model studies of the amounts and heats of phenylsulfoxide sorption on SHRP aggregates RH-greywacke and RC-McAdams limestone indicated strong and irreversible bonding of a large fraction of adsorbed phenylsulfoxide that resisted displacement by cyclohexane and water. In addition, the microcalorimetry results indicated that intermolccular interactions between adsorbed phenylsulfoxide molecules were very strong at submonolayer coverages. This suggests that phenylsulfoxide adsorbs with the sulfoxide directly attached and oriented perpendicular to the surface with strong face-to-face interactions of the phenyl groups. Similar experiments conducted on the sorption of SHRP asphalts AAD-1 and AAM-1 from solution on the RC and RH aggregates indicated significant differences in the sorption of these four asphalt-aggregate pairs. Similar to phenylsulfoxide, a large fraction of both asphalts adsorbed irreversibly with respect to cyclohexane and water-saturated cylohexane desorption. Surface analysis by laser ionization (SALI) was used to measure surface compositions of the aggregates and to monitor temperature programmed desorption (TPD) of adsorbed phenylsulfoxide. SALI analysis indicated that a significant fraction of die RC aggregate is covered with an aluminosilicate layer although bulk analysis indicates that this aggregate is predominantly limestone. TPD studies confirmed the microcalorimetry results that a large fraction of phenylsulfoxide irreversibly binds to silicate and aggregate surfaces and moreover that this irreversible binding leads to deoxygenation of the sulfoxide to sulfide at relatively low temperatures