MOISTURE IN TEXTILES 241 



increasingly attacked by the acetylating mixture. Moisture adsorp- 

 tion isotherms for cotton containing different percentages of CH3CO 

 have been obtained by two investigators, New, and Storks.* Figure 8 

 shows portions of these adsorption isotherms at two humidities 

 (20 per cent and 40 per cent), sufficient to give on extrapolation, 

 monomolecular layer intercepts similar to that of Fig. 7. With 

 increasing acetyl content, the intercepts indicate progressively lower 

 moisture contents, until at 21 per cent or greater CH3CO content, the 

 extrapolated values are zero. 



Considering the structure postulated in Fig. 4, it might be expected 

 that a larger proportion of hydroxyls on fibril surfaces would be 

 acetylated than the average throughout the body of the material, 

 at any given acetyl content. The fact that the intercept is zero for 

 acetyl contents in excess of 21 per cent suggests that all surface 

 hydroxyls have been converted to acetate and no longer adsorb 

 moisture. A consequence of this hypothesis is that the slope of the 

 curve should approach zero with increasing acetyl content and reach 

 zero above 21 per cent. Obviously this is not the case. If such 

 partially acetylated cotton has all cellulose molecules in the fibril 

 surfaces converted to the triacetate, and the remaining cellulose 

 structure to the monoacetate, it is conceivable that the cohesive forces 

 originally binding the fibril surface cellulose molecules to those directly 

 beneath them have been so diminished in strength that water molecules 

 can now adsorb upon the available hydroxyls in this second layer. 

 Furthermore, it is assumed that conversion to monoacetate of all 

 cellulose molecules within the fibril structure involves acetylation of 

 all crystallite end hydroxyls, so that adsorption of moisture on cotton 

 having more than 20 per cent acetyl content is upon available hydroxyls 

 in the second layer of the fibril surface only. 



Based on these considerations, calculations have been made of the 

 moisture content of acetylated cotton containing 21.87 per cent 

 acetyl content at 20 per cent R. H. and 40 per cent R. H. The details 

 of these calculations and discussion are given elsewhere as previously 

 indicated, so that only the essential results are stated below: 



At 20 per cent R. H. the estimated moisture content held on available 

 second layer hydroxyls on fibril surfaces of the cotton hair is 1.0 per 

 cent. The actual value, show^n on Fig. S-a, is 0.9 per cent. At 40 

 per cent R. H., the estimated value is 1.85 per cent; the observed 

 value is 1.80 per cent. These agreements are considered as excellent, 

 particularly since the untreated cotton adsorbs 2.83 per cent and 

 4.50 per cent moisture content, respectively, at these two humidities. 



* Unpublished data obtained by K. H. Storks, at the Bell Telephone Laboratories. 



