This article reports a calibration procedure for the commercial Anton Paar DMA 512 densimeter usable at pressures up to 40 MPa. The method is based on water as a reference substance and requires knowledge of the characteristics of the cell as functions of temperature when subjected to a vacuum lower than 100 Pa. Comparis on with data from the literature indicates a relative accuracy of 0.1% where the classical calibration method (i.e. assuming no dependence of the densimeter performance upon pressure) led to a relative accuracy of 0.8% for the same substances.
Asphaltene in crude oil and in other apolar solvents has intriguing solubility properties which indicate colloidal aggregation. However, none of the various models proposed so far is capable of accounting consistently for all properties. In the present article, we reconsider the current literature and list all robust experimental features redundantly reported on asphaltene solubilization. Then we build a new description, demanding that it should not be contradicted by any one of these listed facts. In our description, asphaltene aggregation and precipitation are distinct steps in a completely reversible process. Aggregation proceeds from specific strong interaction sites located at the periphery of the asphaltene molecules: they drive the reversible association in two-dimensional sheets, a morphology which is consistent with reported scattering and viscosity data. Precipitation eventually occurs, determined by van der Waals attractions between aggregates, when the solubility parameter of the solvent is shifted. One major interest of our description is that it accounts for the remarkable solubilizing properties of the "resins" within a completely reversible scheme.
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