Interfacial behavior and water solubility of various asphaltenes at high temperature

Asphaltenes from six different crude oils were isolated and tested with respect to their water solubility and interfacial behavior at temperatures up to 190 °C and 125 °C, respectively, in (1) water with 18 MΩ cm resistance, (2) a 8 wt.% NaCl–2 wt.% CaCl2 salt solution, and (3) artificial seawater. None of the asphaltenes had any water solubility except for possibly very slight solubility in two cases. A higher interfacial activity was found to be loosely correlated with asphaltene molecular weight as measured by freezing point depression measurements; a lower molecular weight asphaltene had a higher interfacial activity. Chemical measurements of the asphaltenes were performed and only hydrogen content appeared to possibly correlate with interfacial activity; the interfacial activity decreased as the hydrogen content increased. Overall, the interfacial activity was highest for artificial seawater, and lowest for pure water and brine. Interfacial tensions (IFTs) were also measured from room temperature to 125 °C at a fixed concentration of 5 g/l in toluene for the artificial seawater and pure water. With a couple of exceptions, decreases of IFT with temperature were very similar for all asphaltenes and smaller than that of the pure water/toluene system. Higher temperature did lead to a slightly larger decrease of IFT at long times where rearrangement of the interfacial layer has been postulated to occur.