A Gallery of n-Butyl-p-Aminobenzoate Photomicrographs

(using a variety of illumination techniques)

by Brian Johnston   (Canada)

N-butyl-p-aminobenzoate is used as an intermediate in the manufacture of other organic compounds.  The substance itself is used in some suntan lotions to block sunburn producing ultra-violet radiation.  It has as well, some small application as an anesthetic in medicine.

The structural formula and molecular shape can be seen below.  (HyperChem was used to produce both illustrations.)

Since the pale yellow-white crystals have an exceptionally low melting temperature, 58 degrees Celsius,  it is easy to produce a melt specimen by gently heating a few crystals sandwiched between microscope slide and coverglass.  Note however that the MSDS safety information about the compound warns that a fume hood should be used, and that skin contact with the substance be avoided.

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: dark-ground, phase contrast 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.

The first image in the article shows a typical field at low magnification.  It  uses crossed polars, and two lambda/4 compensators, one above and one below the slide, to alter the colour.  Exactly the same field without the use of compensators can be seen below left.  To the right is a higher magnification image of the centre of the upper star-burst shape.

Many of the fields on the typical slide contain complex right-angled shapes.  These forms can be clearly seen in the dark-ground image below.

The compensators mentioned above allow almost limitless control over the colouration of the final image.  To the left below is an image formed by crossed polars that uses two lambda/4 compensators.  Since one can rotate, additional colour control is possible.  Compare this image with the one to the right, and the first image in the article.  A small rotational adjustment in each case is the only difference.

The colour intensity of any particular location on the slide depends on the thickness of the crystal layer between the slide and coverglass.  If the layer is too thick or too thin, the colours appear muted.  The trick is to obtain the ‘perfect’ thickness.  Unfortunately, trial and error seems to be the only way to the ‘ideal’ image.  (It’s for this reason that about three out of every five melt specimen attempts are useless garbage!  The problem is that, unlike other garbage, this contains potentially hazardous chemical material and cannot simply be thrown out in the trash.  Since there is no inexpensive way of disposing of old slides, I keep a large screw-capped jar in which all of my failures over the last thirty years reside!)

After a successful slide has been prepared, one must choose an area to photograph, and a magnification that will reveal the detail that is important.  The image below shows a medium magnification photomicrograph of the radial arms of a star-burst.  The apparent structure of the image is produced by the red colour of the arms contrasted against the blue background.

By contrast, it is the crystal shape in the three images below that determine the apparent structure in the image.  Notice that although compensators have been used to alter the colour dramatically, the viewer easily discerns that the shapes have remained constant.

All of the images in the article so far have been from the central area of melt specimens.  For the remainder of the article,  it is at the edges of the sample, near the coverglass margins, that the images have been obtained.  As an experiment, I ringed the edge of each coverglass with a bead of nail polish.  This action turned out to be serendipitous, for the solvent in the nail polish acted as a solvent for n-butyl-p-aminobenzoate.  The melt crystals near the coverglass edge first dissolved, and then recrystallized to form fascinating new fields of view.

Some of the crystals that recrystallized from the solvent had the perfectly rectangular shape that can be seen in the left (dark-ground) and right (crossed polars) images.

Other locations displayed branching feather-like structures as well.  (Both images use crossed polars with both lambda/4 and lambda compensators.)

Under dark-ground illumination, the edges of the feather-like structures stand out clearly.

At the very edge of the coverglass, some very unusual structures formed that only phase-contrast illumination reveals clearly.

I remember that the study of optical crystallography at university was an onerous task, with complex math and terminology.  It is certainly more fun to simply wonder at the beauty produced when molecules attract to form the regular three dimensional lattices that we call crystals!

 All comments to the author Brian Johnston are welcomed.

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