Microscopic Examination of Cotton Fibres in Cuprammonium Hydroxide Solutions
Authors: Hock Charles W. , Harris Milton 
Published: March 2008

It has been known for nearly a century that cupram monium hydroxide solutions have a pronounced effect upon the cell walls of plants. Many investigators have ob served, for example, that when cotton fibres are treated with this reagent, the bulk of the fibre appears to be dis solved, thereby producing a viscous solution from which cellulose can be precipitated by various means. The solu bility of cellulose under these conditions has been utilized industrially in the manufacture of artificial textile fibres, and also in the laboratory as a means of evaluating the quality of the fibre, especially as regards tensile strength.

There are two different points of view concerning the behavior of the cellulose fibres when treated with cupram monium hydroxide solutions. According to one group of investigators the cellulose of the cotton fibres does not dissolve in the reagent but maintains a visible state of aggregation in the forma of diminutive particles, each of which is surrounded by cementing material which swells in cuprammonium solutions, thereby giving rise to a gel-like structure upon which many of the characteristic properties of the dispersion depend. The other group of investigators has supported the view that cellulose is dissolved in cu prammonium hydroxide, and that properties of the solu tions, such as viscosity, are functions of the chain-length of the cellulose molecule.

Since the above viewpoints lead to widely different interpretations of the behavior of cotton in this solution, it appeared advisable to undertake a new investigation of the microscopically observable changes which cotton fibres undergo in this reagent.

It was found that during the treatment of cotton with cuprammonium hydroxide solutions, the cellulose dissolves, leaving residues which vary in amount and in structure depending upon the degree of purification of the fibres. The undissolved residue from raw and from dewaxed fibres consists principally of fragmented shells which formed the outer surface of the fibres, and to a lesser extent of ma terial from the lumen. Both of these residues are isotropic and stain deeply with ruthenium red. Fibres from which both wax and pectic substance have been removed dissolve in cuprammonium reagent, leaving only a very small amount of isotropic residue which exhibits no definite cytological structure.

When a steady flow of cuprammonium hydroxide solu tion is applied to fibres mounted on a slide, the initial swelling of the fibres is followed by the appearance of small ellipsoidal particles which range from 1-2 μ in size, appear bright between crossed nicols, and become dark in color when stained with iodine and sulfuric acid. Identi cal particles appear, however, when cuprammonium hydrox ide solution is drawn under the coverglass, in the absence of fibres. These particles, which result from the exposure of the cuprammonium hydroxide, reagent to the air, are then washed under the coverglass where they become inti mately associated with the fibre residue. Formation of these particles may be prevented by using a specially con structed cell which prevents exposure of the reagent. Under these conditions, no particles were observed during the slow passage, even for prolonged periods, of the cu prammonium reagent over the cotton fibres.

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