Aromaticity, Asphaltenes, Maltenes, Asphalt and Asphalt Concrete

9/11/24. At present numerous oil production platforms in the Gulf of Mexico have been evacuated because of Hurricane Francine. One of them is the “Who Dat” O&G field. The Who Dat field produces O&G with low wax content and no asphaltene flocculation. The origin of the phrase “Who Dat” comes from the Acadiana region of Louisiana. There are numerous claims to the origin of the phrase and there have been legal spats as to the trademark ownership of the phrase. The NFL in particular has thrown its ponderous weight around in the matter. The reader is encouraged dive into ‘controversy’ for themselves.

The low asphaltene-flocculation of the Who Dat field is fortuitous since asphaltenes can accumulate and choke the well bore or downstream piping, interfering with recovery. The graphic below is borrowed from a book chapter by Abdullah Hussein, Essentials of Flow Assurance Solids in Oil and Gas Operation, published by ScienceDirect in 2023. Fouling by asphaltenes can be removed by dissolution in aromatic hydrocarbons or detergents.

As asphaltene-bearing oil rises in the wellbore, it will begin to depressurize and cool causing the asphaltenes to precipitate out of solution and aggregate, resulting in flocculation and fouling of the wellbore, downstream equipment and pipelines.

The simplified cartoon of a distillation column above shows that used in petroleum refining to isolate hydrocarbon components (fractions) in broad groupings by boiling point. These columns are quite tall and can be spotted easily as you drive by a refinery. Crude oil has suspended solids that are removed by water extraction which is then vaporized in a separate furnace under pressure. This hot, crude vapor is then pumped into the bottom of the distillation tower. For the non-industrial chemists out there who are accustomed to heating a reaction or distillation vessel directly nested in a heating mantle, this offset heat exchange approach is quite common. The fuel for the furnace can be several in-house sources.

Crude oil is a highly complex mixture of hydrocarbons and NSOs (nitrogen, sulfur, oxygen and heavy metals) with a broad range of structures and boiling points. These petroleum hydrocarbons contain a bit of nitrogen, sulfur and oxygen, produced water, natural gas, and inorganics. The hydrocarbons are further divided into linear, branched, aromatic/nonaromatic and cyclic carbon skeletons in which each can be subdivided again into differing molecular formulas and degrees of unsaturation. At this point the reader may need to take a cool refreshing dip into the pool of chemical bonding and the covalent bond in particular because this has a direct bearing on degrees of unsaturation.

A swerve into the weeds of chemical bonding

Here is the long and short of how the different types of chemical bonds affect the formula and structure of a molecule. Carbon atoms make up the skeleton of a molecule and are connected through the sharing of electron pairs. The shared electrons in a bond spend some fraction of their time in orbit between the two carbon nuclei and as such continue to screen out some of the mutual repulsion of the two nuclei. But that is not all. It turns out that this is where quantum mechanics rises from the murky depths of reality. The two bonding electrons are each able to occupy more space than what is available in the individual atoms and so drop in overall energy just a bit. This energy drop is manifested as heat which diffuses into the local environment. The amount of energy lost consists of a discrete quantity of electron orbital energy and occurs in a stepwise manner. This discrete step change is a “quantum jump”. [Note: a ‘quantum jump’ is often portrayed in the popular media as some type of Disneyesque dramatic and abrupt big shift in something or other. In reality it refers to a discrete step change in energy at the sub-nanometer scale.]

The number of bonds between the carbon atoms has consequences in the 3D shape of the molecule. In the 3 ball and stick representations below, only skeletal single bonds are shown (for reasons known only to ChemSketch).

But what do you mean by ‘aromatic’?

The aromatic feature of a molecule is very special. It has a unique type of pi-bonding involving ‘special’ numbers of bonding electrons arranged in a ring and limited by the formula (4n + 2), where n is a counting integer. For n = 0, 1 2, etc., the numbers of electrons involved will be 2, 6 and 10 bonding electrons alternating in a ring. The most frequent ‘special’ number of electrons is 6, as in 3 alternating pairs of 2 electrons. Here, aromatic does not refer to fragrance, although many aromatic compounds like vanillin do have a fragrance. A cyclic group of 3 alternating bonding pairs of electrons will spontaneously ‘delocalize’ and occupy a lower energy level- a much-favored situation. That is, they will circulate around in a continuous ring and occupy a space above and below the ring atoms in a ‘sandwich’ fashion. Okay, we’re drifting a little too far into the weeds.

Knowing full well that I am presenting a highly truncated explanation of aromaticity, I have borrowed 3 representations below of the aromatic compound benzene. Given that single C-C bonds are longer than double C=C bonds, one might expect to find that with both C-C and C=C bonds present, 3 bonds would be longer than the other 3. But this isn’t what’s observed. Measurements show that all 6 CC bonds are of the same length, 1.397 Angstroms. And the CC bond angles are all 120 degrees, again different from C-C bond angles. These observations tell us that all 6 CC bonds are equivalent yet different from isolated CC bonds. Note in the upper right structure the 2 blue rings situated above and below the carbon skeleton. These rings represent delocalized electrons that are off-axis to the C-C bonds. They sandwich the carbon skeleton of C-C bonds. The blue rings show the space where the 6 C=C bonding electrons may be found. The bottom structure is a space filling model showing approximately the space occupied by the electron cloud. Think of it as the location of where another molecule will collide with it.

The imaginary large molecule shown below consists of a central region that is flat and two domains that are kinked and bristling with hydrogen atoms. The structure is shown stationary but in reality, it is vibrating vigorously, tumbling in solution, being battered by adjacent molecules and mashing its way through any liquid that may be around it. You know, the usual liquid scenario. The cyclic alkyl groups on the left are locked in space allowing only limited wagging motion, but the alkyl group on the right is free to rotate about all of the C-C bonds allowing the chain to writhe and snake around in its immediate 3D space.

Above I referred to “… the sharing of a pair of electrons.” This is only just 1 part in a story of several kinds of chemical bonding. One carbon atom can bond to another carbon atom with 1, 2 or 3 pairs of electrons, producing 1, 2 or 3 chemical bonds. Carbon can also bond by sharing electrons (covalent bonding) with other atoms like nitrogen, oxygen and sulfur to form single or multiple covalent bonds. Atoms like hydrogen, boron, fluorine, chlorine, silicon, phosphorus, bromine and iodine bond well with carbon but with only a single bond. In general, as we move to the left and down on the periodic table the sharing of electron pairs becomes more and more one sided favoring the atom nearest to the upper right of the table. [Note to the purists: we are going to ignore carbanions, carbocations and carbenes in this post.]

Back to our regularly scheduled program

So what does all this bonding jazz have to do with asphaltenes? The type of chemical bond that makes aromatic rings flat, the pi-bond, is very abundant in asphaltenes. An aromatic ring of 6 carbon atoms has 3 alternating pi-bonds. Such compounds are commonly referred to ‘unsaturated’ meaning they have multiple bonds between carbon atoms rather than bonds with hydrogen- they are unsaturated with hydrogen atoms. These molecules consisting of hexagonal arrays, sharing edges like a thin honeycomb and are able to stack on one another- sometimes called pi-stacking.

The asphaltene component of asphaltene (!?!) has been defined as that fraction which is soluble in aromatic solvents like benzene, toluene and xylene, etc., but not in alkane solvents. From this link, “Asphaltenes are referred to [as] the poly-dispersed distribution of the heaviest and most polarizable fraction of the crude oil.” This property derives from the large fraction of aromatic structures in the asphaltene.

Asphaltenes are a complex mixture of hydrocarbons containing variable amounts of nitrogen and sulfur atoms built into the structures. As a category, asphaltenes are substantially aromatic in nature with rather high molecular weights but may have cyclic and acyclic saturated hydrocarbon, or alkyl, fragments attached. These alkyl fragments lend solubility of individual asphaltene components in saturated hydrocarbon solvents (alkanes) and thus offer a means of semi-selective isolation. Maltenes are asphaltene components that are viscous liquids soluble in n-alkane solvents. Maltenes provide the adhesive qualities in asphalt. Asphaltene is divided into 2 fractions: Asphaltene and Maltene.

The words asphalt and bitumen are sometimes used interchangeably, both referring to the same material. Asphalt is the American English version. To help avoid confusion, the terms “liquid asphalt”, “asphalt binder” or “asphalt cement” are used in the U.S. to distinguish it from asphalt concrete. We recall that concrete is comprised of aggregate held together by cement.

Colloquially, various forms of bitumen are sometimes incorrectly referred to as “tar“. Asphalt or bitumen occurs naturally or can be manufactured. The roadbeds we drive on are “asphalt concrete” where stone aggregate is mixed with hot liquid asphalt as a binder that upon cooling forms a hard, durable surface. In common usage “asphalt concrete” is shortened to asphalt while the British call it tarmac.

Thus far, we have been talking about crude oil-based asphalt. Roughly similar materials like tar or coal tars are derived from sources other than petroleum. Generally, the words tar and pitch are used interchangeably but each can be considered specific to separate starting materials. Tar is a dark brown or black viscous liquid and is the result of the destructive distillation of a wide range of organic materials like wood, peat, coal. or petroleum. Pitch derives just from plant material.

In the early 1960’s, Dr. Fritz Rostler and coworkers of the Golden Bear Oil Company, now Tricor Refining LLC, discovered the cause of asphalt deterioration. He found that during the heat treatment used in asphalt processing and/or under prolonged exposure to sunlight in the presence of oxygen, the maltenes are degraded and their adhesive attribute is diminished, allowing the asphalt/aggregate components to crumble.