The influence of the polarity of fractionated asphaltenes on their Langmuir-film properties

Abstract

The amphiphilic and aggregation properties of fractionated asphaltenes have been studied using Langmuir trough experiments and Brewster angle microscopy (BAM). In particular, two fractions from three different crude oils have been extracted by using two n-pentane:crude oil volume ratios (3:1 and 18:1) in a two-step precipitation procedure. The concentration dependence of the surface properties and the variations in their ability to form true monolayers at the air–water and liquid–liquid interfaces are investigated. The shape of the resulting isotherms are decidedly different. The greater amphiphilic character of the second fraction provides more expanded isotherms and well-behaved hysteresis cycles with more homogeneous domains in comparison with the first one. These results inferred from interface experiments corroborated previous experiments carried out with the pendant drop method. Finally, high contrast BAM images revealed a different aggregation state of the two fractions. The first fraction presents lower surface activity as derived from the air–liquid and liquid–liquid interface experiments and forms aggregates separated by empty spaces at low surfaces pressures according to BAM images. The second fraction show a more homogeneous film structure even at higher surface pressures.