In Defense of Vapor Pressure Osmometry for Measuring Molecular Weight

The value of the average molecular weight of petroleum asphaltenes continues to be controversial. Usually the disagreement is about the measurement of a single asphaltene fraction when the need is for a consistent picture that comprises all fractions of petroleum and includes chemical changes during processing. By using one of the better solvents for carbonaceous materials, o‐dichlorobenzene, at the maximum instrument temperature, vapor pressure osmometry provides a consistent measurement of number average molecular weight not achieved by any other technique. This average molecular weight is consistent among petroleum fractions and gives predictable changes in asphaltene molecular weight with thermal processing.