If a sample containing asphaltene is titrated with a precipitation agent the chemical balance is changed until asphaltenes are precipitating. Until that moment, the sample is just diluted, which can be observed by an increase of the partial translucency. This is due to the fact, that the sample becomes optically "thinner", so more light can pass through it.
As soon as the asphaltenes form flakes (colloids) these disperse the passing light. By this dispersion, less light is admitted throug the sample, which causes a decrease in the translucency1. Precipitation and cumulation of bigger flakes result in a higher decrase of translucency.
Now two effects take place in parallel:
on one hand the further dilution of the sample by the titration fluid - thus, actually a gradient increase, on the other hand the precipitation, resulting in stronger light dispersion - thus causing a decrease in translucency.
Dilution is a constant effect with a nearly linear gradient. The precipitation starts slowly, then increases fast and approaches a constant value, when the process is finished.
The overlay of the two gradients leads to a typical measuring chart:
it shows a linear increase at the beginning. Then the gradient is reduced to an angular point, after which it drops down. This dropdown designates the beginning of precipitation. The amount of asphaltenes in the original sample can be concluded from the relation between sample and precipitation agent at starting point of the precipitation. With further titration more asphaltene crystals are forming, resulting in a decrease of the partial translucency and a dropping of the gradient. The process is finished when all soluted asphaltenes in the sample are present as colloids. This event can be observed as a minimum angular point in the chart. Further titration of the sample afterwards only leads to a dilution of the sample, but not to more precipitaion. This is shown in a fresh increase of the translucency and thereby of the measuring gradient.
Asphaltenes are one fraction of crude oils and are very inhomogeneous in their structure. The molecular weight is inconsitent, depending on source and used measurement method limits of 400 to 800 g/mol as lower limit and 1,500 up to >10,000 g/mol as upper limit can be found. Asphaltenes consists mainly of carbon and hydrogen, high amounts of heteroatoms with esp. nitrogen, oxygen and sulfur. In comparison with other fractions of crude a high portion of metal atoms like vanadium and nickel is contained.
The asphaltenes are defined by their solubility. They are soluble in toluene and insoluble in n-heptane. According to DIN 51595 it is the part of crude that is at temperatures between 18 °C and 28 °C insoluble in the 30-times amount of n-heptane.
Crudes consists of four general fractions which are differed by their solubility and polarity:
- satuarated hydrocarbons (e.g. alkanes)
- aromatic compounds
Simplified can be said, that the components are hold in solution by the component above. Asphaltenes form micelles which are surrounded by a shell of resin molecules, stabilised by aromatic compunds soluted in satuarates. By this point of view there are multiple shells connected by different equations of equillibrium conditions. These equillibria can be disturbed by adding e.g. n-alkanes or polar organic substances like ethyl acetate. By this the solubility of the asphaltene colloids can be reduced. If a point of thermodynamic instability is reached it comes to asphaltene flocculation, the asphaltenes precipitate. The colloidal asphaltenes agglomerate to bigger particles. This process is accelerated by the Ostwald ripening2, where due to thermodynamic reasons bigger particles are favoured compared to smaller particles.
There is no general constitution given for asphaltene moleculs, but a polycondensed aromatic core with aliphatic side chains is typical. The aromatic cores are flat and due to van der Waals forces between these the asphaltenes agglomerate in layers.
Flocculation is a process of solids crystallise out of a solution in the form of flakes (flocs) in difference to amorphous precipitation. The term is used also if in a dispersion smaller particles aggregate to bigger ones. During flocculation is comes due to Gibbs-Thomson effect3 to Ostwald ripening2. The surface energy gets smaller if the particle grows, so the total energy of the system decreases if the particles grow in size and their number is decreasing.
(Links will open in a new window)
- Translucency: External link (Wikipedia)
- Ostwald ripening: External link (Wikipedia)
- Gibbs-Thomson effect: External link (Wikipedia)