Membrane Diffusion Measurements Do Not Detect Exchange between Asphaltene Aggregates and Solution Phase
Bibliographic record
Abstract
Although the aggregation of petroleum asphaltenes has been measured by a variety of methods, little is known about the rate of exchange between aggregated and dissolved components. This work studied the diffusion of asphaltenes from several heavy oils and bitumens in dilute toluene solutions using a stirred diffusion cell equipped with ultrafiltration membranes (Ultracel YM and Anopore). The pore sizes were varied between 3 and 20 nm to retain aggregates while allowing free molecules to diffuse. The permeate was continuously monitored using in situ UV−vis spectroscopy. These experiments demonstrated that the sizes of the asphaltene aggregates at a concentration of 1 g/L in toluene at 25 °C were between 5 and 9 nm and that rates of exchange of material between the aggregates and free solution were extremely low. An increase in the temperature results in an increase in asphaltene mobility but does not reduce the size of the asphaltene structures below 5 nm. Likewise, a decrease in the concentration to 0.1 g/L did not result in a decrease in size. The origin of the asphaltenes (Athabasca, Safaniya, and Venezuela) did not have a significant impact on the observed behavior; therefore, the above observations are widely applicable to C 7 asphaltenes. The diffusion of vanadyl porphyrins (vanadyl meso -tetraphenylporphyrin, vanadyl octaethylporphyrin, and native petroporphyrins) in the presence of asphaltenes showed that the native petroporphyrins were larger than the model vanadyl porphyrins based on the appearance of hindered diffusion within smaller pores. An increase in the temperature resulted in an increase in petroporphyrin mobility as per the Stokes−Einstein relationship, although decreasing the asphaltene concentration did not. The mobility of the vanadyl petroporphyrins varied with the origin of the sample (Safaniya, Venezuela, and Athabasca) and is therefore not universal.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".