Use of NMR imaging to measure interfacial properties of asphalts

An NMR imaging method that allows for direct calculation of interfacial surface tensions of asphalts is described. The method is based upon acquiring NMR images of water drops on the surface of asphalt as a function of time. By expressing the contact angle for the water drop either in terms of Young's equation, or the liquid lens equation for the initial placement of the water drop on the asphalt surface, and for a later time when the water drop has penetrated below the asphalt surface, two equations that incorporate the asphalt-air surface and asphalt-water interfacial tensions are obtained which can be solved analytically. The NMR imaging method was used to determine the surface tensions of the eight Strategic Highway Research Program (SHRP) core asphalts at 25 °C. Asphalt-water interfacial tension values ranged from 25 to 40 dynes/cm, and asphalt-air surface tension values ranged from about 38-50 dynes/cm. These results are in general agreement with other asphalt surface tension measurements that have been made using the du Noüy ring tensiometer, or the Wilhelmy Plate method. Spreading coefficients and capillary numbers derived from the surface tension calculations are also reported.