A Gallery of Benzoic Acid Photomicrographs

(using phase-contrast illumination)

by Brian Johnston   (Canada)

One of my favourite places is the Museum of Modern Art (MOMA) in New York.  I have spent many pleasurable afternoons wandering around the building, searching in vain for art that matches the beauty of the architecture.  For some strange reason, a red dot on a plain white canvas, or a pile of plastic excrement on the floor, doesn’t inspire me to exclaim “brilliant, insightful – a joy forever”!  I have been told many times by art connoisseurs that the fault is entirely mine.  Bearing in mind that such a beautiful building has been erected to house them, and the immense cost of such “works of art”, there is obviously something that I am missing! 

While taking the benzoic acid photomicrographs of melt specimens for this article, it struck me that the compositions and colours visible under the microscope, rivaled, or even exceeded those in modern art creations.  Of course, the structures seen on the microscope slide are not produced by a human artist, but by the laws of chemistry and physics.  As the molten state cools to the solid state, molecules with positive and negative ends attract one another, and tend to arrange themselves in three-dimensional lattices called crystals.  If the cooling process occurs slowly, there is time for large, perfect crystals to form, whereas rapid cooling results in small, disorganized groups of crystals. 

I leave it to the reader to be the “art critic” in this situation.  Is it physics and chemistry, or artists that produce the best “modern art”.  You be the judge!

Benzoic acid is a white crystalline solid with a melting temperature of about 122 degrees Celsius.  This low melting point makes it easy to produce a melt specimen by placing a few crystals on a slide, covering with a cover-glass, and heating gently over an alcohol lamp until the solid melts.  Slides prepared in this way cool to room temperature in about a minute.  It should be kept in mind that the MSDS safety document for the compound states: “May be harmful if swallowed. May act as an eye or respiratory irritant. May cause allergic respiratory or skin reaction.”

Benzoic acid C6H5COOH is the simplest aromatic (based upon a benzene ring) carboxylic acid (containing the COOH group).  The structural formula and molecular shape, (produced using HyperChem Pro), can be seen below.

This compound is often used as an anti-microbial agent in products like cosmetics, toothpastes, mouthwashes and deodorants.  Fruit products, beverages and condiments may use benzoic acid as a preservative.  In such applications the quantity used, is of course very small, in order to reduce the harmful effects mentioned above.

Benzoic acid melt specimens can be examined under the microscope using polarized light.  In this article, however, phase-contrast illumination was used exclusively.  A Leitz 402a phase-contrast condenser and Leitz 25X NPL Fluotar PHACO objective were used to form the images with a Leitz SM-Pol microscope.  (Since only one objective was used, it should be noted that all of the images have exactly the same magnification.)  An article concerning the same compound illuminated by polarized light will be published in a future issue of Micscape.

Notice the unusual mottled pattern in the image below.

Post-processing of an image is possible in Adobe Photoshop; I use the CS version.  One technique is to use the “Inversion” tool to alter the colours displayed in the original image.  The diagram below shows a colour spectrum before and after inversion.

In each of the following pairs of photomicrographs, the “normal” phase-contrast image is on the left, and the “inverted” is on the right.

Another post-processing possibility is the use of the “Desaturation” tool to remove all colour in the photograph. This of course results in a black and white image.

Since many photomicrographers consider the use of such tools to make dramatic alterations in the image to be “cheating”, no such changes have been made to the next group of photographs.

The turquoise rectangles, mostly at right angles to one another, make a rather artistic arrangement.

Large areas of the field under the cover-glass are rather amorphous, but even here there are interesting details.  Note the hair-like X’s between the darker parallel lines in the image on the right.

After the slides used in this article were prepared, the edge of each cover-glass was ringed with a very thin bead of fingernail polish.  The strange lobed pattern below is the result, and was found at the very edge of the cover-glass.  Note the tiny benzoic acid crystals re-crystallizing from the fingernail polish solvent as it evaporates.

The following seven photomicrographs show some of the more interesting fields that I observed.

If you have used phase-contrast illumination on biological specimens, you know that the visual (apparent) colour saturation of the background is controlled by the intensity of the light source.  In post processing, the use of the “Levels” tool can mimic this, and increase the contrast between foreground and background.

The background can be slightly darker than normal.

It can be darker still.

It can be almost black as in the two images below.

Benzoic acid fields may contain very structured forms,

areas with mixtures of structured and unstructured forms,

and areas which are more random.

The occasional field may even look like the one below.  Even if it is the photomicrographic equivalent of the pile of plastic excrement mentioned earlier, both share the same advantage- neither smells!

 All comments to the author Brian Johnston are welcomed.

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