Maleic acid is an industrial chemical used mainly in the production of synthetic resins, and as an intermediate in the production of other chemicals.  It is sometimes called “Toxilic acid”, probably due to the unpleasant results of exposure.  The MSDS safety data for this compound compose a litany of nasty consequences for the unwary user.  It is harmful if ingested, inhaled or absorbed through the skin.  The chemical readily destroys tissues of the mucous membranes, skin and eyes.

As can be seen in the structural formula below, maleic acid contains two COOH carboxylic acid groups, and is therefore referred to as a dibasic  or diprotic acid.

The following image shows the molecular shape.

Since the melting temperature of the white crystalline solid is quite low, about 135 degrees Celsius, it is possible to prepare a melt specimen by gently heating a very small quantity between slide and cover-glass.  Note that I do not recommend doing so!  (See above.)  My slides were prepared in the lab using a fume hood.

The first image in the article gives a good idea of what a typical maleic acid field looks like.  Many crystal structures are surrounded by a rather random matrix of amorphous material.  Under dark-ground illumination, the two types of material can be seen clearly.

An ordinary transmitted light image of a field is shown below on the left, and to its right, the identical field between crossed polars.

If the melt is photographed during the process of crystallization, one can see the tell-tale lines between liquid and the bubbles that invariably occur.

The image on the left uses polarized light, (with crossed polars), while that to the right uses in addition, two lambda/4 compensators in order to produce the white background.

If a small section of the field  below is observed under much higher magnification, interesting detail can be seen.  (bottom two images)

The unusual field below left was photographed between crossed polars, with one lambda/4 compensator.  That on the right used, in addition, a lambda compensator.

Although phase-contrast illumination is usually used to view biological specimens, it can sometimes provide an interesting perspective on the hidden detail in crystal structures.  Note that the three images below are at a higher magnification than the other examples in the article.

I noticed in a couple of the prepared slides, that strange, very thin straight and curved structures had formed.  These can be seen in the image that follows.

When these structures are viewed at higher magnification, using phase-contrast illumination, striking details are resolved.

Many of the large crystal structures that form seem to be based upon a series of imperfect diamond shapes that have grown together.  Compensators were used to change the colouring of the two images.

A similar example is shown below, again with the use of compensators to alter the appearance.

The final image shows the characteristic that I most dislike about maleic acid melt crystal structures.  Randomness!  The fields also tend to be filled with unsightly garbage, and this makes finding ‘artistic’ images difficult!
 

Photomicrographic Equipment

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.