Fundamental interfacial properties of n-heptane insoluble asphaltenes derived from five heavy crude oils, HCO-A, HCO-B, HCO-C, HCO-D, and HCO-E, were determined at the oil-water interface. Two oils were used in the study, (a) a model oil made of 60/20/20 tridecane/toluene/cyclohexane, and (b) the corresponding deasphalted oil (DAO), from which the asphaltenes were separated. Use of the model oil has limitations in determining fundamental interfacial properties of extracted asphaltenes. The deasphalted oil is a better medium and provides more representative interfacial properties of asphaltenes in crude oil. The deasphalted oil composition and, in particular, the naphthenic acid content of the deasphalted oil influences the interfacial properties of the asphaltenes at the crude oil-water interface. We hypothesize that the basic nitrogen-containing asphaltenes and crude oil naphthenic acids interact with each other synergistically, leading to naphthenic acid-asphaltene acid-base complexes. The high propensity for aggregation of these complexes at the hydrocarbon-water interface leads to the observed high interfacial activity of asphaltenes in crude oils.