MOISTURE IN TEXTILES 239 



calculated under Section 3 is between 0.7 per cent and 0.8 per cent, 

 suggesting that there is additional internal surface within the hair 

 structure upon which moisture may be held without manifesting itself 

 by an increase in diameter. This suggestion appears to be confirmed 

 by quite different considerations. 



Brunauer and Emmett^^, i3 consider it likely that the linear portion 

 of van der Waals adsorption isotherms for nitrogen on the surface of 

 ammonia catalysts indicates the building up of additional layers of 

 adsorbed molecules. They state that extrapolation of this linear 

 portion to zero pressure indicates the amount of gas needed to form 

 a monomolecular layer upon this surface. Between 3 per cent and 

 50 per cent relative humidity the adsorption isotherm for cotton in 

 Fig. 7 is very nearly linear. Applying this method to cotton the 

 intercept (o) has values between 1.4 per cent and 0.35 per cent, 

 depending on temperature. The average value is about 1 per cent 

 being of the same order as that estimated from swelling data. 



Since the estimated moisture content equivalent to a monomolecular 

 layer on the internal surface of the cotton hair is so nearly the same 

 when determined by two independent methods, it seems reasonable to 

 postulate an additional internal surface in the cotton hair, amounting 

 in extent to somewhat more than that corresponding to the fibril 

 surfaces. 



Slip spirals along the hair, crossing the pits at approximately right 

 angles (see {b) of Fig. 4) suggest that there are discontinuities in the 

 length of the fibril structure, hence further internal surface. Since 

 the additional internal surface suggested by the preceding calculations 

 and estimates does not appear to be involved in the transverse swelling 

 of the cotton hair, it is suggested that this may be held upon the ends 

 of crystallites or micellae which compose the fibril structure. 



It is considered of much less importance to pursue the detailed 

 calculations of this possibility than it is to point out that some such 

 distribution of surface within the body of the fibril structure may be 

 involved in adsorption of a small amount of moisture, and this picture 

 is of material value in accounting for some of the properties of cotton. 



5. MULTIMOLECULAR LAYERS 



It is further assumed that above 1.5 per cent moisture content, 

 addition of moisture simply increases the thickness of the moisture 

 layer upon the surfaces of the fibrils. The thickness («) of the moisture 

 layer on each fibril, expressed in number of water molecules, and the 

 percentage moisture content at 50 per cent R. H. may readily be 

 obtained. At 50 per cent R. H. the hair diameter increavSe is 5 per cent 



