THE DIRECT ATTACHMENT OF DYES TO TISSUES I99 



ance of basic or acidic side-groups, and the appearance under the 

 microscope would therefore necessarily be diffuse. 



The electrostatic forces between oppositely-charged ions act 

 over much greater distances than the 'short-range' forces concerned 

 in covalent and hydrogen bonds. That such short-range forces may 

 be important in textile-dyeing has been especially stressed by 

 Neale.^^^ The process we have been considering will not account 

 for the dyeing of cotton, for this is a negatively-charged substance 

 that is generally dyed by acid dyes. The energy necessary to bring 

 the similarly-charged bodies together is provided by thermal 

 agitation. It is for this reason that high temperatures are used. 



In microtechnique we are not faced with exactly this situation, 

 because we do not ordinarily dye negatively-charged objects with 

 the negatively-charged ions of ordinary acid dyes. Still, if dye-ions 

 can be attached to cotton bv forces other than those we have been 

 considering, the possibility exists that in microtechnical dyeing 

 long-range electrostatic forces merely play a part similar to that 

 played by thermal agitation in the dyeing of cotton. The close, 

 final attachment may then be achieved through hydrogen bonds. 

 A useful short summary of the various parts of dye-molecules that 

 could serve for hydrogen bonding to the hydroxyl groups of 

 cellulose is given by Evans. ^^^ Either the hydrogen or the oxygen of 

 these hydroxyl groups can participate in a hydrogen bond, the 

 former making a link (for instance) with the nitrogen atom of an 

 amino-group or one of the nitrogens of an azo-group of the dye, 

 the latter with a hydrogen of a -C=C- group. These reactions 



H H 

 only occur so long as the hydroxyl groups of the cellulose are intact. 

 Bonding of this sort presumably takes place in microtechnique 

 when we use a direct cotton dye such as Congo red to colour 

 cellulose cell- walls. 



It is not only cellulose, however, that can bind itself to dyes in 

 this way. The dyes commonly used to colour nylon (which are not 

 the ones mentioned above (p. 198) as colouring it from acid solu- 

 tion) appear to form hydrogen bonds with the peptide groups of 

 the fibre. If so, the amido-groups of the protein chain may also be 

 available for the attachment of dyes. A link could be formed 

 connecting the hydrogen of a peptide group forming part of a 

 protein chain to the nitrogen of an amino- or azo-group in a 

 dye. 



