| Abstract
| - Asphaltenes are known to be interfacially active in many circumstances such as at toluene−waterinterfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenesin good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the criticalmicelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate forasphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration(CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q,ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shownto exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods,we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with theliterature. Using density determinations, we obtain micelle compressibilities consistent with previousliterature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocityversus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L,although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, usingasphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar tomicellar compressibilities obtained with standard nonionic surfactants in toluene. These results stronglysupport the contention that asphaltenes in toluene can be treated roughly within the micelle framework,although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore,our extensive results on known surfactants agree with the literature while the asphaltene CNACs reportedhere are one to two orders of magnitude lower than most previously published results. (Previous workutilized the terminology “micelle” and “CMC” for asphaltenes.) We believe that the previously reportedhigh concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they referto higher levels of aggregation or perhaps to a particular surface structure.
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