Swelling of cotton fibre in Water
Cellulose is hydrophilic and swells in the presence of water. Normally cellulose–water interactions
are considered to occur either in intercrystalline regions or on the surfaces of the
crystallites and the gross structures.
Water vapor adsorption isotherms have been obtained on cotton from room temperature
up to 1508C. Theoretical models for explaining the water vapor sorption isotherms
of cellulose have been reviewed . Only adsorption theories will be discussed here at
ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs
and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer
(GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor
pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 . The GAB
equation, which was not discussed in the chapter in the book Cellulose Chemistry and
Its Applications , can be applied up to RVPs above 0.9. Initially as the RVP
increases, a monomolecular layer of water forms in the cellulose. By a RVP of 0.19–0.22 the
monomolecular layer is complete , and the moisture regain, when a monomolecular layer
has just formed, for cotton and mercerized cotton is 3.27 and 4.56%, respectively . By
a RVP of 0.83–0.86, about three layers of water molecules are formed, and at higher RVPs it
is thought that condensation occurs in the permanent capillary structure of the sample .
It is well known that at low moisture uptakes, the water associated with the cellulose exhibits
properties that differ from those of liquid water and it has been called by such terms as ‘‘bound
water,’’ ‘‘nonsolvent water,’’ ‘‘hydrate water,’’ and ‘‘nonfreezing water.’’ From a review of the
literature,which included determinations by such techniques asNMRand calorimetry,Zeronian
[303] concluded that between 0.10 and 0.20 g=g of the water present in the fiber cell wall appeared
to be bound. Such regains are obtained at RVPs between 0.85 and 0.98.
The fiber saturation point (FSP) of cotton is the total amount of water present within the
cell wall expressed as a ratio of water to solid content. It is equivalent to the water of
imbibition of the fiber, also called its water retention value. The FSP has been measured
using solute exclusion, centrifugation, porous plate, and hydrostatic tension techniques. It
occurs at RVP greater than 0.997 and from the review of the papers, it has been concluded
that the studies have yielded a value for FSP in the range of 0.43 to 0.52 g=g.
At equilibrium and at a particular RVP, the amount of adsorbed water held by a cellulose
generally will be greater if it has been obtained following desorption from a higher RVP and
not by adsorption from a lower RVP. The cause of this hysteresis is not fully established.
One explanation is based on the internal forces generated when dry cellulose swells, limiting
the amount of moisture adsorbed whereas when swollen cellulose shrinks, stress relaxation
occurs since the cellulose is plastic and permits a higher uptake of moisture
