A Gallery of Salicylic Acid Photomicrographs

(using a variety of illumination techniques)

by Brian Johnston   (Canada)

Salicylic acid and its chemical derivatives have long been useful in the medical drug arsenal.  Birch bark and wintergreen leaves both contain salicylic acid esters that were valuable in an earlier age as medicines for pain relief and as antiseptics.  The acid itself is still used today in ointments for skin diseases.

If salicylic acid is reacted with methanol (wood alcohol), the product formed is methyl salicylate, commonly known as “oil of wintergreen”.  (The preparation of this ester is a common lab in secondary school organic chemistry.)  The aromatic product is a common flavouring, and is also used in ointments for sore muscles (giving them the distinctive “wintergreen” smell as an additional benefit).

When salicylic acid is reacted with acetic acid, (from which vinegar is produced), the product is the most widely used synthetically produced drug in the world, acetylsalicylic acid (ASA), commonly called “aspirin”.

Salicylic acid itself is a benzene ring with alcohol (-OH), and carboxylic acid (-COOH) functional groups replacing two adjacent hydrogens.  The structural formula and molecular shape can be seen below.  (The illustrations were produced using HyperChem.)

For comparison, here is the molecular shape of aspirin, its most important derivative.

Salicylic acid is usually provided as white, powdery crystals, which have a melting temperature of 161 degrees Celsius.  The relatively low melting point means that a melt specimen can easily be prepared by heating with an alcohol lamp, a small quantity sandwiched between slide and coverglass.

Note that the MSDS safely information for the compound warns that the solid is harmful by inhalation, ingestion and skin absorption.  Care should therefore be taken when handling the crystals.

The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope.  Images were produced using several illumination techniques: phase contrast, dark-ground, and polarized light.  Crossed polars were used in all polarized light images.  Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases.  A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.

Phase Contrast Illumination

In an earlier article, I showed the results obtained by employing phase contrast illumination to investigate crystal melt specimens.  As expected, most chemicals gave disappointing results.  A few however proved the exception to the rule, and of these, salicylic acid proved to be the most photogenic.  Phase contrast works best on biological materials because they have the required prerequisites, parts with differences in refractive index and thickness.  One would expect that a melt specimen using pure compound would have a constant refractive index, and would therefore show only a slight contrast difference due to subtle changes in the thickness of the trapped crystals.  What is surprising is that many images show some colour, probably due to interference phenomena.

The first image in the article, and the three below give an indication of the striking architectural structures that this method reveals.

Many of the larger structures are surrounded by a granular looking matrix.

Notice the distinctive pink and blue colours in the two images below.  One might guess that these are produced by crossed polarizers, but they are not!

I experimented by ringing each coverglass with a small bead of fingernail polish.  Since this acts as a solvent for the salicylic acid, a one to two millimetre wide ring of the crystals dissolved.  As the solvent evaporated, some of the acid recrystallized forming the strange amoeba like structures that can be seen in the following two images.

In some locations away from the coverglass edge, the smooth gray-brown of the phase contrast field seemed to contain small individual crystals.  These can be seen in the image on the right.

Polarized Light Illumination

The next section of the article demonstrates the results of using crossed polars and a combination of two lambda/4 compensators, or a combination of lambda/4 and lambda compensators to illuminate the specimen.

In many areas, large randomly oriented needle-like structures formed, like the ones in the image below.

The first image on the top left shows a typical field.  The other three images (from the same field) were taken using higher magnification objectives.

The blue background in the following photomicrographs was produced with the lambda/4 + lambda combination.  The stage of a polarizing microscope rotates, and an angle was chosen which gave this particular hue.

These unusual crystal structures formed only in one very small location on a slide.

Dark-ground Illumination

Instead of using a dark-ground condenser in this instance, I chose to use one of the phase contrast condenser “phase” settings which produced a black background with a non-phase objective.  This accounts for the slight colouration in the images.

For this chemical, I prefer the phase contrast images to those produced by the other methods.  It’s unfortunate that this type of illumination works with so few compounds!

 All comments to the author Brian Johnston are welcomed.

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