Random charge variations at the bitumen–water interface: Characterization by autocorrelation

Atomic force microscopy was used to measure local surface potentials (or equivalently, surface charges) at the bitumen–water interface. When plotted as a function of position along the bitumen surface, the potential appeared as a random “noise.” Autocorrelation of the random noise revealed that the surface charges were correlated over distances much larger than molecular dimensions. Further, this correlation length appeared to be pH-dependent, exhibiting an almost threefold decrease as the pH was increased from 6 to 10. The correlation length, which is a measure of surface charge heterogeneity, is of practical importance as it affects strongly the probability of coalescence between emulsified bitumen droplets.