Art of Crystallization

As I mentioned in the introductory part of my blog, I am a chemist. To be more precise I am a synthetic chemist. What the synthetic chemist does? He makes molecules. How he makes them? Well, he puts on the reaction of two or more molecules (actually sometimes only one molecule is more than enough for the reaction to occur), these molecules react and in theory they should give the third molecule. If that occurs we can say that the reaction was quantitative, unfortunately, quantitative reactions in the real world are quite rare. In most of the cases when you put molecule A and molecule B to react they give the molecule C, but the problem is that they can give also molecule D and E together with unreacted molecule A and B. So in order to obtain molecule C as pure product you have to purify it. There are many ways how one can purify his product, distillation, column chromatography and trust me most of these techniques are pain in the ass and nobody really enjoys doing them, it is something that has to be done, period.

However, there is one technique which gives always pure product, it is not time consuming and to me, after five years of working in the lab, it gives enormous amounts of satisfaction when it is successful, recrystallization.
During my first year of PhD I stumbled at one book on laboratory techniques, the opening phrase in the chapter on crystallization was: “Crystallization is where boys become men and where chemistry becomes art”.

It is true, in the moment of crystallization more than science chemistry resembles art, there is no specific rules and there is no guarantee of success, but in that moment you establish a very special connection with your molecules, you feel what they like, which solvents they prefer, which environment suits them, you see how they behave and if you treat them right they will form beautiful crystals. And depending which path of crystallization you choose, molecules can form crystals of different shapes, a phenomenon called polymorphism.

Of course, there is an actual science behind it. By definition, crystallization is two event process: first molecules dissolved in supersaturated media start to gather in very small clusters, if conditions are right these clusters will reach critical size and become stable nuclei, otherwise clusters will dissolve and reenter solution. At the stage of nucleation molecules are arranged in highly ordered manner into the crystal lattice. Subsequent growth of nuclei that achieved critical size is called crystal growth and it is the second stage of crystallization process. As long as supersaturation of the media exists, which is the driving force of the process, nucleation and crystal growth will continue to occur simultaneously. Once when supersaturation is exhausted, equilibrium will be reached and crystallization is complete.
Besides being a very neat purification method that makes chemists around the world happy, the art of crystallization had an important impact on a world history.

I will mention just two products on which production the crystallization had a profound effect.

The salt. Man has used salt since early-recorded history, its ability to preserve food was fundamental for the development of human civilization. Shortages of salt often caused wars and the collapse of the nations, while salt itself was sometimes used as a currency.

A great deal of salt is obtained from natural deposits, however very large deal was obtained, even in the early history, by the process of the crystallization, which makes salt production the oldest chemical industry. The evaporation of the sea water is a method of choice in the case of the regions with warm climate and low precipitation, however in cold climates such as the region of the White Sea in Siberia, common salt is manufactured by concentrating sea water by the partial removal by freezing. After the removal of two crops of ice, resulting brine is evaporated over a fire until the salt crystals start to separate from the liquid.

While in the case of the salt production the nature of the crystals and the process of crystallization is almost irrelevant, it is of great relevance in the production of chocolate.

Much as the salt, chocolate had a big role throughout world history, the first record of drinking what was at the beginning bitter chocolate drink goes all the way to 1900 B.C to the pre-Olmec people. A thousand years later in Maya civilization chocolate beans were used as currency, just like the salt was. So, during Maya civilization 400 beans of chocolate would have bought you a brand new slave.

Chocolate is produced from seed of the tropical Theobroma cacao tree. Seeds were separate from the pulp and leave to ferment for a couple of days, after fermentation seeds are left to dry under the sun (humidity is very bad for chocolate beads, the reason why shipping them to Europe was not an easy task) and subsequently roasted. Next, the shell of each bead is removed and the nib is extracted. Finally, the nibs are ground giving the chocolate liquor.

Chocolate liquor has two major components; chocolate powder which gives the taste and cocoa butter.

So what crystallization has to do with chocolate? Well, it has everything to do with chocolate. After cocoa butter and chocolate powder are separated, they are first blended with various proportions together with sugar, milk and aromas (depending on which type of chocolate one wants to make). Then, whole material is ground for a couple of hours (even days) in order to get a nice smooth texture.

Finally, we come to the fun part. In order to obtain the solid form of chocolate that we know and love we have to crystallize it, and we have to crystallize it the right way. The fat in cocoa butter can give six different types of crystals, six different polymorphs. Crystal polymorphs have different physical properties, and in the case of cocoa butter they melt on six different temperatures.

Now, since nobody wants to eat chocolate which melts at 63° as well as nobody wants to produce chocolate which melts at 23°, which are the two random taken melting points of two different polymorphs of cocoa butter. There is a process of crystallization which gives just one form of crystal polymers, crystal polymer that melts at 34°, just a few degrees lower of our body temperature. Thanks to this process we have chocolate that is solid at room temperature but it readily melts in our mouths. The name of this process is tempering and its inventor is Rudolphe Lindt. Chocolate manufacturer and apparently one hell of a crystallographer.