MOISTURE IN TEXTILES 245 



by volume (about 0.3 per cent by weight). This increase occurs in 

 the range consistent with the completion of the monomolecular layer 

 of moisture on the internal surface of the material, particularly since 

 the internal surface of paper and cotton appears to be of the same 

 order of magnitude. 



9. General Discussion of Theory 

 Peirce ^^ proposed a two-phase theory for the adsorption of moisture 

 by cotton, based upon the facts that a small quantity of water is 

 adsorbed by dry cotton very much more rapidly than the same amount 

 added to cotton with a moderate water content, that it has much 

 greater effect on the elastic properties, evolves more heat, and is more 

 difficult to remove. He regards the moisture attached to the hydroxyl 

 points as in phase (a) while that in phase (b) consists of an indefinite 

 number of molecules adsorbed in a looser fashion over all available 

 surfaces, limited only by the conditions of space and of equilibrium 

 with the external concentration of aqueous vapor. 



The differentiation between fibril and other internal surfaces is con- 

 sidered as giving a more definite explanation of such two-phase 

 adsorption than that proposed by Peirce. The (a) phase may be 

 pictured as moisture added to the active hydroxyls which lie on the 

 fibril surfaces where such surfaces are readily available for moisture 

 adsorption, while the (b) phase is associated with a definite number 

 of active hydroxyls but within the body of the fibrils and therefore 

 less accessible than those on the surface. As has been pointed out, 

 it is quantitatively reasonable to suppose these available internal 

 hydroxyls to be located at the ends of the crystallites. 



According to the cotton hair structure presented in this paper, 

 there is actually more than twice as much internal surface available 

 for moisture adsorption from the dry state as there is in cotton already 

 in equilibrium with a 1 per cent moisture content. The slow diffusion 

 of moisture to those surfaces most deeply buried in the fibrils may ac- 

 count for the fact that while cotton very nearly reaches equilibrium with 

 any given humidity in a relatively few minutes, the final establishment 

 of equilibrium conditions, involving a change of less than 1 per cent 

 in moisture content, may take more than 24 hours. The first water 

 molecule to attach itself to an active hydroxyl might be expected to 

 evolve more heat than subsequent molecules in the chain since the 

 interaction is between water and cellulose hydroxyls, not between 

 water and water. Also, water held within the fibril structure no 

 doubt is the most difficult to remove since it must diffuse out through 

 this fibril structure. 



