A Gallery of Palmitic Acid Photomicrographs

(using a variety of illumination techniques)

by Brian Johnston   (Canada)

Palmitic acid is an extremely common substance.  The name correctly implies that it is a major constituent of palm oil (from palm trees), but it is also present in many of the foods we eat like meat, milk, butter and cheese.

Industry makes use of this chemical in the manufacture of products as diverse as soap and food additives.  Many shaving creams use palmitic acid as an emulsifier to prevent separation of the oil and water constituents.

The structural formula and molecular shape are shown below.  (HyperChem software was used to prepare both illustrations.)

The white crystals have a very low melting temperature of about 63 degrees Celsius and thus make an ideal candidate for a melt specimen.  Only very gentle heating is required to melt a thin layer between slide and cover-glass.  Note that palmitic acid is considered a skin, eye, and respiratory irritant.  (One wonders then, why it is present in so many of the products that we use in everyday life!)  If the acid is heated to decomposition, carbon dioxide and poisonous carbon monoxide are produced.

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:  transmitted light, dark-ground illumination, 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.

Most melt specimens contain double fan-shaped structures.  Two examples are shown below, with the left image using dark-ground illumination, and the right polarized light.

Higher magnification gives a different view of the base of the fans.

If the layer of crystals being studied is thinner than in the two examples above, the colours are more muted and tend towards gray.

When the top edge of a fan is highly magnified, and phase-contrast illumination is utilized, details are revealed.  Notice that the fan seems to be composed of long crystal ‘fibers’ that project out from the growth front during crystallization.

Many double fan structures have complex detail at the joining point, where growth started.  Notice the coloured radial spikes in the high magnification image to the right.

Two other examples follow.  The low magnification image on the left uses polarized light, while the much higher magnification image on the right uses phase-contrast illumination.

The three images below of a melt specimen are in order of increasing magnification.  Notice the large number of randomly oriented needle-like structures in the last image.

Both of the low magnification images of specimen fields that are shown below use crossed polars, with the addition of two lambda/4 compensators, (one below and one above the specimen).  This combination produces white ‘bubble’ areas.  The first image in the article utilizes the same illumination.

When these ‘bubble’ areas are studied with dark-ground illumination, they may contain no crystal material (like that on the left) or an amorphous crystal mush (like that on the right).

Palmitic acid would make a good starting substance for the investigation of melt specimens under the microscope.  It is relatively harmless if treated carefully, and has a melting temperature lower than that of boiling water!  The distinctive, colourful fan shapes make for a rewarding viewing experience.

 All comments to the author Brian Johnston are welcomed.

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Published in the November 2005 edition of Micscape.
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