Alkane Crystallization

One thing we study is the crystallization of normal alkanes - paraffin wax. Although this might seem mundane at first glance, there are several reasons to study this system. Medium length alkanes (say from 16 to 40 carbon atoms) exhibit a surprisingly rich phase diagram and correspondingly complicated crystallization behaviour. Since many biological molecules contain hydrocarbon chains, this behaviour can play an important role in biology (e.g., cell membranes) as well as in the food sciences (e.g., milk fats and chocolate).

Kinetic Inhibitors:

Our interest in alkane crystallization is motivated by problems of the petroleum industry: the precipitation of waxes from diesel fuels and fuel oils in cold climates. Traditionally, this problem — which can result in plugged fuel filters, leading to, e.g., engine failure — can be solved by dilution with lighter grades of fuel. However, since such lighter distillates can be used at a much greater profit margin in jet fuels, it is desirable to find an alternate solution.

It has long been known that trace amounts of specific impurities can greatly affect crystallization. These impurities affect neither the crystal structure nor melting point, but do influence crystallization kinetics; hence, they are termed kinetic inhibitors. Such additives play a role in several industrial processes as well as in biology. For instance, the inhibition of ice crystallization by proteins found in certain arctic fish allows them to spend their entire lives in a supercooled state, while proteins in the human body are responsible for preventing the growth of kidney stones. Petroleum companies have developed specific polymeric additives to both enhance wax nucleation (to produce many small crystals rather than a few big ones) and slow crystal growth.

Based on macroscopic evidence, it is generally believed that such impurities bond preferentially to specific faces on the growing crystal, inhibiting growth on those faces. In cases where the activity of the impurity is different on different crystalline faces, the crystal morphology is altered. However, there is very little direct evidence for this picture, and recent results - both experimental and theoretical - have shown the situation to be more complex. A better molecular-level understanding of how these additives affect growth will benefit many fields, including materials science, biocrystallization, and the petroleum industry.

 

A "blocker" molecule preventing further crystal growth.

We are investigating the effects of kinetic inhibitors on alkane crystallization rates and morphologies using a combination of optical and atomic-force microscopy. Optical imaging provides information on the crystal faces inhibited and the effective change in supercooling. Recently, the atomic-force microscope (AFM) has been used to study solidification at the molecular level; however, direct studies of additive effects have been hampered by the difficulty in achieving true molecular resolution. We are studying these effects in model systems involving macromolecular growth modifiers, such as polymers and proteins. The large size of these additives will allow them to be more easily identified.

Alkane crystals growth from solution.

  

In the presence of kinetic inhibitors, wax crystallizes as a mesh.

Pattern Formation in Crystallization

During our studies of alkane crystallization in the presence of kinetic inhibitors, we found that solidification can occur in the form of macroscopic bands (several hundred micrometers across) parallel to the front. In this growth mode, the front periodically stops growing, allowing a a new front to nucleate and spread laterally along the arrested front. This growth mode is most obvious during directional solidification, in which the sample is driven through a temperature gradient at a constant velocity.

These bands are apparently controlled by the thermodynamics of the system: As crystallization procedes, solute is depleted near the front. As the sample moves through the gradient, a region ahead of this depletion zone becomes supersaturated (a situation known as constitutional supercooling). However, due to the presence of kinetic inhibitors, the crystals do not grow appreciably. Instead, nucleation procedes via a metastable phase of wax (known as a rotator phase and which is known to have a low nucleation barrier) the instant it is stable relative to the solvated, but not the crystalline, phase, leading to a very regular structure.

We have duplicated this behaviour with numerical modeling based on a coupled set of differential equations representing solute diffusion and crystal growth.

 

Banded growth of wax in the presence of kinetic inhibitors. The slow drift is the result of sample motion through a temperature gradient.
 

Simulation of banded growth, showing crystallite density (blue). Nucleation of a new band occurs when the solute concentration (green) reaches the saturation limit of the rotator phase (red). Subsequent growth proceeds via the crystal phase.

 
We also see intriguing patterns at the sub-micrometer scale: thin layers of alkanes undergo a martensitic-like transitions and exhibit a regular series of twin boundaries over a narrow temperature range.
 

AFM height (left) and deflection (right) images of twinning in tricosane. The images span 50 µm, with a vertical range of a few 100 nm.

Last updated August 17, 2005