Salt sorption on regenerated cellulosic fibers: electrokinetic measurements
Cellulose
Salt sorption on regenerated cellulosic fibers: electrokinetic measurements
Avinash P. Manian 0 1 2
. Adisak Jaturapiree . Thomas Bechtold 0 1 2
0 A. Jaturapiree Faculty of Industrial Technology, Phranakorn Rajabhat University , Bangkok 10220 , Thailand
1 A. P. Manian (&) T. Bechtold Research Institute of Textile Chemistry/Physics, University of Innsbruck , Hoechsterstrasse 73, 6850 Dornbirn , Austria
2 Lyocell Viscose Salt sorption
Streaming potential measurements were conducted on lyocell and viscose fibers, to determine the relative order in sorption extents of salt cations and anions. The sorption of K? was greater than Na? ions, and the sorption extents of the anions, Cl- and Br-, were similar. Previously, we had examined accessibility of the same ions in the fibers, and found them to follow the order: K? \ Na? and Cl- \ Br-. From these two contrasting results, we find that the mode of salt interaction with cellulose, from aqueous solutions, changes with the salt concentration. At low concentrations, the interaction is governed by ion-exchange processes with the cellulose carboxyl groups and the Donnan equilibrium; but at higher concentrations, the interaction is a function of the mobility (or diffusivity) of the ions. Thus, sorption and accessibility of cellulose fibers as measured with salts may not apply for other solutes, and conversely, similar studies with other probe molecules may not be relevant for salts.
Accessibility
Introduction
The accessibility of fibers to reagents and catalysts
influences the uniformity and extent of chemical
reactions, in treatments intended to improve their
performance. Nearly all textile treatments of cellulosic
fibers are performed with aqueous solutions, and
cellulose swells in water. Thus, it becomes useful to
measure fiber accessibility in aqueous media, where
the effect of fiber swelling is accounted for; rather than
in non-swelling media, such as for example when
accessibility is determined by vapor or gas sorption.
With this in mind, we previously investigated the
accessibility of two regenerated cellulosic fibers,
viscose (CV) and lyocell (CLY), by measuring the
sorption of salts from aqueous solutions of salt–alkali
mixtures. Salts were chosen as probe molecules since
they do not exhibit chemical affinity for the polymer,
and alkalis were added because fiber treatments are
often performed in alkaline solutions, and the
alkalinity influences fiber swelling.
Both CV and CLY are composed of cellulose, and
derived from wood pulp through dissolution and
reprecipitation (coagulation). In the viscose process, the
pulp is first derivatized to sodium cellulose xanthate,
dissolved in NaOH, and then re-precipitated in an acid
solution. In the lyocell process, the pulp is directly
dissolved in N-methylmorpholine N-oxide (NMMO),
and re-precipitated in a water bath
(Woodings 2001)
.
Generally, CV fibers have a lower degree of
polymerization, lower degree of crystallinity, higher
swelling degree in water, and exhibit higher moisture
regain as compared to CLY fibers
(Bredereck and
Hermanutz 2005)
.
In the previous work, 0.5 g fiber specimens were
immersed in 50 ml solutions of the following
combinations: NaOH with NaCl or NaBr, and KOH with
KCl or KBr
(Jaturapiree et al. 2006)
. The salt
concentrations were always 0.02 mol/l, and the alkali
concentration ranged from 0 to 8 mol/l. There were no
differences of salt accessibility between the fibers.
However, in both fibers, there were differences of
accessibility between the salts. Regardless of the alkali
content in solutions, the salt accessibility increased in
the order: KCl \ NaCl \ KBr \ NaBr; i.e. the
Brsalts exhibited greater accessibility than the
corresponding Cl- salts, and the Na? salts exhibited greater
accessibility as compared to the corresponding K?
salts.
Among the ions that were studied, the charge
density increases in the order: Br- \ Cl-, and
K? \ Na?. Cellulosic fibers develop a negative
surface charge in aqueous solutions
(Stana-Kleinschek
et al. 2002; Trotman 1984)
, and electrostatic forces
will influence fiber-salt interactions. As the strength of
such forces is proportional to the charge densities on
the ions, it was concluded that the observed results
could be explained by the greater attraction between
the negatively charged cellulose and Na? as compared
to K?, and a greater repulsion of the Cl- as compared
to Br-. The order also suggested that the salt
accessibility was dictated more by the repulsion of
anions than attraction of the cations. We explored
these findings further with electrokinetic
measurements, and the results are presented here.
Materials and methods
Materials
Two fibers were used in this investigation: lyocell
(CLY) and viscose (CV), both of 38 mm length and
1.3 dtex linear density. They were kindly donated by
Lenzing AG, Austria, and used as supplied. The salts
and alkalis were of analytical grade and purchased
from Sigma-Aldrich; and deionized water w (...truncated)