Alkali treatment of cellulose II fibres and effect on
Ján Širokýa,b, Richard S. Blackburn*a, Thomas Bechtoldb, Jim Taylorc, Patrick Whitec
aGreen Chemistry Group, Centre for Technical Textiles, University of Leeds, Leeds, LS2 9JT, UK;
bChristian Doppler Laboratory for Textile and Fibre Chemistry in Cellulosics, Research Institute of
Textile Chemistry and Textile Physics, University of Innsbruck, Höchsterstraße 73, A-6850,
Dornbirn, Austria; cLenzing Fibers, Lenzing Aktiengesellschaft, 4860 Lenzing, Austria.
*Tel: +44 (0)113 343 3757; Fax: +44 (0)113 343 3704; E-mail: email@example.com.
To understand the effect of alkali treatment on sorption behaviour of cellulose II fibres, samples
were continuously pre-treated using NaOH over a concentration range of 0.0-7.15 mol dm-3, with
varying tension; treated substrates were dyed with hydrolysed C. I. Reactive Red 120. Greatest
adsorption of dye occurs for cellulose II fibres treated with 2.53 and 3.33 mol dm-3 aqueous NaOH
solution. Correlation to sorption isotherms is most closely associated with a Langmuir type isotherm,
but correlation to the Freundlich isotherm is still significant, indicating sorption via a combination of
Langmuir and Freundlich isotherms. Adsorption energy (ΔG0) increases with increasing NaOH
concentration to a maxima between 2.53 to 3.33 mol dm-3 NaOH and then decreases with further
increase in NaOH concentration. Equilibrium dye sorption shows good correlation with water
sorption as assessed by the reactive structural fraction (RSF) theory. Theoretical monolayer capacity
(q0) increases with increasing NaOH concentration to a maxima at 3.33 mol dm-3 NaOH and then
decreases with further increase in NaOH concentration; q0 is significantly in excess of the number of
available specific sites (–COO–Na+) in the substrate, indicating non-site-specific interactions, more
typical of a Freundlich isotherm. Pores in the fibre significantly affected by alkali treatment (< 20 Å
diameter) and accessibility of dye (14 Å) sorption into those pores accounts the differences observed
herein; maximum qe, q0 and ΔG0 are observed for cellulose II fibre treated with 2.53 to 3.33 mol dm-3
NaOH as this concentration range affects the greatest increase in accessible pore volume in the
Keywords: Freundlich isotherm; Langmuir isotherm; adsorption; polysaccharides; lyocell; sodium
hydroxide; hydrolyzed reactive dye; reactive structural fraction (RSF).
Cellulose is the most abundant biopolymer (Klemm et al. 2005) and of particular interest in
providing renewable, sustainable, biodegradable biopolymers for industrial applications. Cellulose is
a linear 1,4-β-glucan polymer where the units are able to form highly ordered structures, this occurs
as a result of extensive interaction through intra- and intermolecular hydrogen bonding of the three
hydroxyl groups in each cellulose unit. Treatment of cellulose with aqueous sodium hydroxide
(NaOH) solutions has a substantial influence on morphological, molecular and supramolecular
properties of cellulose, causing changes in crystallinity, pore structure, accessibility, stiffness, unit
cell structure and orientation of fibrils in cellulosic fibres (Goswami et al. 2009). Treatment with
alkali can improve mechanical and chemical properties of cellulose fibres, such as dimensional
stability, fibrillation tendency, tensile strength, dyeability, reactivity, lustre and fabric smoothness.
Lyocell fibres are produced by regenerating cellulose I (typically sourced from eucalyptus trees)
into fibre from a solution in an organic solvent (Albrecht et al. 1997). Solvent-spun lyocell fibres
consist of crystalline cellulose II and amorphous cellulose, and have a higher degree of crystallinity
(80%) in comparison with other regenerated cellulosic fibres, such as modal (49%) and viscose
(41%) (Heinze & Wagenknecht 1998). The high degree of crystallinity in lyocell is a consequence of
higher orientation during stretching and formation of fibres; lyocell fibres have the thinnest and
longest crystallites of all cellulosic fibres, even the amorphous regions are oriented along the fibre
axis (Kreze & Malej 2003). Lyocell fibres spun from cellulose solution in N-methylmorpholine-N-
oxide (NMMO) hydrate have proven commercially successful in textile products due to their
excellent mechanical properties in the wet state; the fibres also have excellent environmental
credentials (White et al. 2005; Goswami et al. 2009). Lyocell fibres have a microfibrillar structure
because a portion of the molecular chains aggregate to form microcrystals while recrystallizing along
the chains, whereas the remaining chains exist in the amorphous phase as links between these two
phases (Okano & Sarko 1984). In the crystalline regions of cellulose II polymers, the layered
structure is very regular with an antiparallel arrangement of cellulose chains with some inter-sheet
hydrogen bonding, generally leading to a perfectly distributed symmetrical structure; however, the
hydrogen-bonded network is complicated and debate is ongoing as to the absolute structure (Kolpak
& Blackwell 1976; Gessler et al. 1995; Raymond et al. 1995; Langan et al. 1999; Kono & Numata
2004). During the formation of lyocell fibres, a so-called ‘quasi-crystalline’ phase readily forms,
where polymer orientation is relatively low to moderate, compared with the crystalline phase; this
quasi-crystalline phase is attributed to the presence of imperfect microcrystals and co-exists with the
amorphous phases (Langan et al. 1999). Based on this three-phase structure of lyocell fibres, it has
been proposed (Zhu et al. 2004) that alkali can only permeate into the quasi-crystalline and
amorphous phases, leading to disassociation of the quasi-crystallites. However, based on the work of
Okano and Sarko (1984; 1985), at higher concentrations the fully crystalline phases must convert to
phases of lower crystallinity; it is unclear as to whether crystalline regions have a transitional phase
through a quasi-crystalline phase, but this is certainly worthy of investigation. Hence, changes in
lyocell crystallinity depend upon transformation of the crystalline and quasi-crystalline phases, and
whether these phases transform into less oriented states as crystallinity decreases. Recently,
Goswami et al. (2009) observed that sodium hydroxide treatment causes the density, orientation and
crystallinity of lyocell fibre to decrease with increasing sodium hydroxide concentration, and that the
greatest change in fibre properties occurs between 3.0 and 5.0 mol dm-3 NaOH. This was attributed
to the onset of formation of sodium (Na)-cellulose II at 3.0 mol dm-3 NaOH; a fully formed Na-
cellulose II structure was observed above 6.8 mol dm-3 NaOH. The work concludes that formation of
Na-cellulose II causes plasticization of the lyocell fibres as both inter and intra-molecular hydrogen
bonds are broken by these high sodium hydroxide concentrations.
Regenerated cellulosic fibres absorb significant quantities of water by the expansion of void spaces
within the semi-crystalline morphology, forming a water-cellulose two-phase structure. Dyestuffs for
cellulosic fibres are highly water soluble, with molecular structures designed to interact at the
internal interface between cellulose and water. The uptake of dyes is often used to monitor changes
in fibre properties brought about by variations in processing, hence, dyes can be considered as
coloured probe molecules that provide information on the detailed internal pore structure of fibres
(Ibbett et al. 2006a). Despite several classes of dyes being suitable for application to cellulosic fibres,
around 50% of all cellulosic fibres globally are dyed with reactive dyes (Roessler & Jin 2003).
Researchers have concluded that when cellulose I is converted to cellulose II the crystallinity index
decreases, therefore, it is interesting to understand the change in crystallinity of cellulose II polymers
when treated in alkali, particularly considering the increasing importance of lyocell fibres in the
textile industry. There is significantly limited research concerning the effect that alkali treatment of
cellulose II polymers has on fibre sorption, particularly adsorption of dye molecules from an aqueous
This paper examines the adsorption of reactive dyes on lyocell fibres to understand the effect that
alkali treatment has on the thermodynamics of the sorption system; the use of dyes as probes can
also provide information on changes in the fibre structure.
2. Experimental section
Plain woven, desized, scoured lyocell fabrics, 1/1 weave, 140 g m-2 with 35 ends and 28 picks cm-1,
comprised of 30/1 Nm yarn, was used in this research and was supplied by Lenzing AG, Austria.
Lyogen MC was supplied by Clariant, Switzerland. C. I. Reactive Red 120 (RR120; 1) was kindly
supplied by DyStar, Germany. All other chemicals were laboratory grade supplied by Aldrich.
Structures 1 & 2
2.2. Continuous alkali pre-treatment
Fabric samples (5.0 × 0.2 m) were conditioned at 65% ± 4% relative humidity (RH) and 20 °C ± 2
°C for at least 48 h prior to treatment. The continuous process of alkali pre-treatment was conducted
using apparatus as depicted schematically in Figure 1. The apparatus was divided into four
compartments (A, B, C, D), each with two sub-compartments (1 and 2) that could be heated
independently; the four stages were alkali treatment (A), stabilization treatment (B), washing (C),
and neutralization (D). The fabric was passed through the apparatus over a series of rollers including
tension compensators (T) and pressurized squeeze rollers (P). The solid line represents the usual
route of fabric passage through the machine, while the dashed line represents the route employed in
some instances. The fabric after passing through the last compartment (D) was wound on a take-up
Sodium hydroxide concentration in the alkali treatment bath was varied, using concentrations of
0.0, 2.25, 3.0, 3.75, 5.0, and 7.0 mol dm-3; in the alkali treatment bath 1 g dm-3 Lyogen MC was
added, which is an alkali-stable surfactant and wetting agent. Sodium hydroxide in the stabilization
stage was 20% of the concentration in the alkali treatment bath, i.e. 0, 0.45, 0.6, 0.75, 1.0, and 1.4
mol dm-3, respectively. Due to changes in the concentration in the first treatment stage, the
“effective” concentration of alkali was established at 0.0, 2.53, 3.33, 4.48, 4.65 and 7.15 mol dm-3
for treatment bath and 0.0, 0.73, 1.08, 1.18, 1.48 and 2.15 mol dm-3 for stabilization bath, which is
described in detail in our previous work (Široký et al. 2009). Washing was performed in water alone
at 80 °C; neutralization was carried out using 2 g dm-3 acetic acid (80% v/v) at 80°C. After the
treatment process, samples were dried continuously in a stenter at 130 °C, but not as part of the
Tension in compartment A was applied at either 49 N m-1 or 147 N m-1, tension in compartment B
was applied at 147 N m-1, and tension in compartments C and D was applied at 49 N m-1. During the
alkali treatment stage the temperature of the solution was set at 40°C, and in the stabilization stage
the temperature was 60°C. In each compartment, the volume of the liquor was 20 dm3. The speed of
passage of fabric through the system was set at 2 m min-1. Further details about the conditions and
adjustments of continuous alkali pre-treatments are available in our previous work (Široký et al.
Prior to dyeing, a 1% aqueous solution of RR120 was hydrolyzed at pH 11.5 (NaOH) at 80 °C for
120 min to ensure full hydrolysis, and then cooled and the solution neutralized (HCl), forming compound
2. Hydrolyzed RR120 (2; 1431.9 g mol-1) was used for sorption experiments rather than the parent
dye to ensure that covalent reaction between the dye and fibre did not interfere with sorption; the
hydrolyzed reactive dye behaves essentially like a direct dye. This provides particular advantages: the
substantivity (physical adsorption) for the cellulose substrate is high, so relatively short dyeing times
can be employed; these dyes can be used as molecular sensors to characterize cellulose substrate
properties, e.g. pore structure (Inglesby & Zeronian, 2002; Luo et al., 2003).
Subsequently, alkali pre-treated lyocell fabrics (2.5 g) were dyed with the hydrolyzed reactive dye
at pH 7 at varying concentrations (0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0% omf) in a Roaches
Pyrotec S Rotodyer dyeing machine, using a liquor-fibre ratio of 20:1. Alkali pre-treated lyocell
fabrics were introduced to the dyebath at the dyeing temperature (70 °C), which contained 40 g dm-3
NaCl, and held for 120 minutes. Previously, these dyeing conditions have been employed on such
substrates with this hydrolyzed dye for a total dyeing time of 24 hours; aliquots of the dyeing liquor
were analysed periodically using UV/visible spectrophotometry, and it was determined that
equilibrium in this system is achieved after 90 minutes. Hence for expediency, all dyeings herein were
held for 120 minutes, this time being sufficient to achieve sorption equilibrium based on this previous
experience, and is in agreement with other researchers (Zhou et al., 2003; Ibbett et al., 2006a; Ibbett
et al., 2006b; Wang & Hauser, 2009), for sorption studies on cellulosic substrates under very similar
conditions; Carrillo et al. (2002) demonstrated that the time to achieve equilibrium for C. I. Direct
Blue 1 on lyocell fibres was 55 minutes at 120 °C.
2.4. UV/visible spectrophotometry
From the exhausted application baths, the equilibrium dye concentration in solution (Ce; mg dm-3),
was calculated directly from absorption of the dye solution after dyeing, according to The Beer-
Lambert Law. Absorption of residual dyebaths at the end of dyeing was measured using a Jasco V-
530 UV/Visible/NIR spectrophotometer at 512 nm, the wavelength of maximum absorption (λmax)
for the dye, at 2 nm intervals. Concentrations were calculated from calibration graphs. Equilibrium
concentration of dye on the sorbent (cellulose II) (qe; mg g-1) was calculated by subtracting Ce from
the initial concentration of dye at the start of the dyeing process (C0) and accounting for the
liquor:fibre ratio. Blank dyebaths without fibre were taken through the dyeing process and it was
demonstrated that the concentration of the dye did not change over the process in the absence of
2.5. Computational chemistry
The geometry and the electronic properties of hydrolyzed RR120 (2) were explored using quantum
mechanics methods (DFT/BPW91/6-31+G(d)) employing Gaussian 03 software (Figure 2a); the
electrostatic potential of hydrolyzed RR120 was plotted on top of a total electron density isosurface
In previous research, Blackburn et al. (2006) modelled a partially oxidized cellulose I (cotton)
surface by cleaving a layer of glucose chains from the crystal structure, adding an anionic carboxylate
group (COO–), and introducing a Na+ counterion; it was demonstrated that the Na+ counterion is
preferentially located to have interactions with the COO– group. Herein similar principles are applied
to estimate geometry and electronic properties in a lyocell (cellulose II) surface by calculating the
Connolly surface (Connolly 1983) (at the van der Waals radii) for a periodic cellulose II system. The
geometry and the electronic properties of a glucose unit and a glucose unit where the C6 position is
oxidized to a COO– group and a Na+ counterion introduced were explored and the electrostatic
potential of these two moieties was plotted on top of a total electron density isosurface (Figure 3).
3. Theoretical Basis
3.1. Equilibrium Model
In order to fully understand the sorption system involved between the dye and cellulose II, it is
important to establish the most appropriate correlation for the equilibrium curves, which can be
obtained by measuring the sorption isotherm of the dye onto cellulose II using experimental data.
3.2. Langmuir Isotherm
The Langmuir isotherm describes sorption onto specific homogenous sites within an adsorbent
(Langmuir 1916; Langmuir 1918). Langmuir’s model of adsorption depends on the assumption that
intermolecular forces decrease rapidly with distance and consequently predicts the existence of
monolayer coverage of the adsorbate (the dye) at the outer surface of the adsorbent (cellulose II). It
is then assumed that once a sorbate molecule occupies a site, no further adsorption can take place at
that site. Moreover, the Langmuir equation is based on the assumption of a structurally
homogeneous adsorbent where all sorption sites are identical and energetically equivalent and there
is no interaction between molecules adsorbed on neighbouring sites. Theoretically, the sorbent has a
finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further
sorption can take place. The saturated or monolayer (as Ct → ∞) capacity can be represented by the
expression represented in equation 1:
where qe is the equilibrium concentration of sorbate on the sorbent (solid-phase) (mg g-1), Ce is the
equilibrium sorbate concentration in solution (mg dm-3), KL (dm3 g-1) and aL (dm3 mg-1) are Langmuir
constants. The constants KL and aL are evaluated through linearization of equation 1 (equation 2).
Therefore, a plot of Ce/qe versus Ce should yield a straight line of intercept value 1/KL and slope
aL/KL if the isotherm obtained through experimental observes the Langmuir expression. The
theoretical monolayer capacity is q0 (mg g-1) and is numerically equal to KL/aL. However, the linearity
of equation 1 is only respected at low solution concentrations, where the model follows Henry’s law:
as Ce becomes lower, aLCe is much less than unity and qe = KLCe.
3.3. Freundlich isotherm
The Freundlich isotherm (Freundlich 1906) suggests that sorption energy exponentially decreases on
completion of the sorptional centres of an adsorbent and describes heterogeneous systems, which are
characterized by the heterogeneity factor 1/nF. When n = 1/n, the Freundlich equation reduces to
Henry’s law. Hence, the empirical equation (eqn. 3) can be written:
where qe is the equilibrium concentration of sorbate on the sorbent (solid-phase) (mg g-1), Ce is the
equilibrium sorbate concentration in solution (mg dm-3), KF is the Freundlich constant (dm3 g-1), and
1/nF is the heterogeneity factor. The capacity constant KF and the affinity constant nF are empirical
constants dependant on several environmental factors. A linear form of the Freundlich isotherm can
be obtained by taking logarithms of equation 3 (equation 4).
Therefore, a plot of lnqe versus lnCe should yield a straight line of intercept value lnKF and slope
1/nF if the isotherm obtained experimentally observes the Freundlich expression; if n > 1, then the
adsorption is favourable. The Freundlich isotherm is another form of the Langmuir approach for
adsorption on an “amorphous” surface where the amount of adsorbed material is the summation of
adsorption on all sites. The Freundlich isotherm is derived by assuming an exponential decay energy
distribution function inserted into the Langmuir equation. It describes reversible adsorption and is
not restricted to the formation of the monolayer.
Thermodynamic data such as adsorption energy can be obtained from the Langmuir and Freundlich
equations using equation 5, where K is constant in terms of dm3 mol-1 (Kim et al. 2004).
4. Results and Discussion
Results from UV/Visible spectrophotometric analysis of residual dye solution in the exhausted
application bath (Figure 4) demonstrated that as the concentration of dye applied increases the
concentration of dye adsorbed also increases. It is observed that greatest adsorption of dye onto the
substrate at equilibrium (qe) occurs for cellulose II fibres treated with 3.33 and 2.53 mol dm-3
aqueous NaOH solution; there was little observed difference between applied tension in the
Thermodynamics of dyeing is a complex issue and many parameters have to be considered. Shore
described the importance of the relative proportion, size and the shape of crystalline and amorphous
regions in cellulose I (cotton) fibres (Shore 1995), but the added factor of quasi-crystalline regions in
cellulose II polymers only adds complication; additionally, changes in pore structure (shape and size)
caused by alkali treatment require consideration (Bredereck & Hermanutz 2005). Diffusion and
adsorption of dyes are presumed to depend upon the extent of the accessible disordered regions in
fibres, in some instances absorption may occur on the accessible surfaces of the crystallites, however,
physical chemistry of dyeing theory suggests that dye molecules are not generally absorbed within
fibre crystallites (Peters & Ingamells 1973; Holme 1976). However, examination of the fine structure
of cellulose polymers reveals a range of degrees of order in the packing of chain molecules, where
there is a continuous transition from perfectly crystalline to completely amorphous material through
paracrystalline and quasi-crystalline regions (Nevell 1995), this theory being particularly applied to
cellulose II polymers (Langan et al. 1999). In paracrystalline regions, the three-dimensional lattice is
defined by short and medium range ordering with a lack of long range order giving several sub-
regions from quasi-crystalline to a more diffuse type with partially crystalline features to totally
diffuse amorphous structure (Hosemann 1950; Hosemann et al. 1972; Voyles et al. 2001). Another
interesting concept is that of reacting structural fraction (RSF), developed and established by Fink
et al. (1986), which can provide a more detailed understanding of NaOH and water sorption at low
alkali concentrations onto cellulose, changes in crystalline and non-crystalline regions that occur due
to alkali treatment, and the application of NaOH to activate disordered regions of cellulose (Fink et
al., 1986; Heinze & Pfeiffer, 1999). The RSF is composed of: AGUs located in the amorphous
regions of the cellulose polymer; crystalline regions cellulose converted to Na-cellulose I at the
NaOH concentration in question; surfaces of the remaining cellulose I crystallites (Fink et al., 1986).
The sorption isotherms were linearized by application of the theory from Langmuir and Freundlich
isotherms (Figure 5) and sorption constants and correlation coefficient (R2) for Freundlich and
Langmuir isotherms applied to this experimental data are shown in Table 1. Summarily, it is
observed that correlation of the experimental data is most closely associated with a Langmuir type
isotherm (R2 ≥ 0.9930), indicating limited, site-specific adsorption (initial exponential rise and then
very slow increase). However, although the correlation coefficient for the Freundlich isotherm
achieved lower values, correlation is still significant (R2 0.9875 to 0.9730), which is typical of
unlimited adsorption at non-specific sites.
It is interesting to relate the observations for sorption of dye from aqueous solution onto a
cellulose II substrate that has been treated with increasing concentrations of NaOH to the RSF
concept. Using cotton linters, Fink et al. (1986) demonstrated that at low concentrations of NaOH,
the specific water sorption (H2O-sorption per AGU for the RSF) exceeds integral values (H2O-
sorption per AGU for the whole sample), with a maximum at the commencement of lattice transition
from cellulose I; this then decreases within the transition range merging in the integral sorption curve
at 3.25-3.50 mol dm-3 NaOH; and finally rises to a second maximum at 3.50-3.75 mol dm-3 NaOH,
well known from the integral water sorption curve. Fink et al. related this second maximum to alkali
cellulose modification from Na-cellulose I to Na-cellulose II associated with an increase of the
When data from Fink et al. (1986), is compared with equilibrium dye sorption (qe) of hydrolyzed
RR120 by cellulose II fibres treated with varying concentrations of NaOH, it is observed that the
trends are very similar (Figure 6a). It is noted that the variations occur at different concentrations of
NaOH, but the shape of the plots are comparable; concentration differences can be explained relative
to the adsorbing substrate – Fink et al. used cellulose I fibre, herein cellulose II fibre is used. The
first maximum occurs at ca. 2.75 mol dm-3 NaOH for cellulose I, and at ca. 3.50 mol dm-3 NaOH for
cellulose II; this represents the first transition phase for each fibre to their respective Na-cellulose
form. Subsequent decrease within the respective transition ranges then occurs to ca. 3.25 mol dm-3
NaOH for cellulose I, and to ca. 4.50 mol dm-3 NaOH for cellulose II. A second maximum occurs at
ca. 3.75 mol dm-3 NaOH for cellulose I, and at ca. 4.75 mol dm-3 NaOH for cellulose II. As
described above, for cellulose I this is associated with transition from Na-cellulose I to Na-cellulose
II, however, for cellulose II, the fibre at such NaOH concentrations is already in the Na-cellulose II
form, so no transition to a different cellulose form can occur. Goswami et al., 2009 demonstrated
that, when treated with aqueous NaOH solution, the main changes in cellulose II crystallinity occur
between 3.0 to 5.0 mol dm-3; it is proposed that the first maximum observed herein relates to the
transition from cellulose II to Na-cellulose II wherein intermolecular hydrogen bonds are broken, and
the second maximum is caused by an additional increase in the RSF of the polymer, relative to
disruption of the intramolecular hydrogen bonds in the Na-cellulose II form. This is evidenced from
our previous work (Široký et al. 2010), where we have demonstrated using ATR-FTIR of cellulose
II fibres treated with aqueous sodium hydroxide solution that minimum values in hydrogen-bond
intensity (HBI; closely related to the well ordered crystalline phase) also occur in the region 4.50-
4.75 mol dm-3 NaOH.
For the Langmuir expression, the theoretical monolayer capacity (q0) is calculated from isotherm
data and is a representation of the capacity of the sites in the substrate for adsorption of discrete
sorbate molecules without any sorbate aggregation or micellar formation. Figure 6b shows q0 of
cellulose II fibres treated with varying concentrations of NaOH, where it can be seen that q0
increases with increasing NaOH concentration to a maxima at 3.33 mol dm-3 NaOH and then
decreases with further increase in NaOH concentration to a plateau above 5.0 mol dm-3 NaOH; it can
be concluded that the maximum number of available sites for adsorption are observed for cellulose II
fibres treated with 3.33 mol dm-3 NaOH. This maximum may have been due to formation of the new
available sites, and the plateau represents the fully swollen material. Figure 6b demonstrates that the
RSF trends for water sorption (Fink et al., 1986) also show similarity to theoretical monolayer
capacity (q0) of cellulose II fibres treated with varying concentrations of NaOH, concurring that
accessibility to the AGU for sorption is concentration dependent.
Adsorption of dyes onto cellulosic fibres is typically associated with a Freundlich isotherm;
sorption theory teaches that the interaction between dye molecules and cellulosic fibres is based on
hydrogen bonding and van der Waals interactions (Shore 1995). Isotherms plotted from the
experimental data (Figure 4) do not indicate limited adsorption (Langmuir isotherm); hence, it is
possible that sorption observed herein may occur via a combination of Langmuir and Freundlich
isotherms. The most intriguing aspect is the possibility of some site specificity, as indicated by the
close correlation to the Langmuir isotherm, and the question arises as to how this occurs in a system
theoretically based on non-site-specific interactions (Freundlich isotherm).
Carboxylic acid (COOH) groups are formed in cellulosic fibres through oxidation of glucose rings
during processing operations such as bleaching or mercerizing (Nevell 1995), and Stana-Kleinschek
et al. (2002) suggested that these provide anionic sites for adsorption. Previous experimental studies
have found that bleached, mercerized cotton fibre contains 1.9 × 10-5 mol carboxylic acid groups per
g fibre (Blackburn et al. 2007); bleached lyocell fibre has a COOH content of 1.9 × 10-5 mol g-1,
decreasing to a minimum of 1.35 × 10-5 mol g-1 COOH when treated with 3.0 mol dm-3 NaOH
obtained by 9H-fluoren-2-yl-diazomethane (FDAM) methods (Öztürk et al. 2009). Figure 2b shows
that hydrolyzed RR120 has several regions of strongly negative electrostatic potential, primarily due
to the presence of six sulfonate groups (–SO3–) on the molecule. When electrolyte is added to the
system (40 g dm-3 NaCl herein), Figure 3b demonstrates that the carboxylate group (–COO–) of the
partially oxidized cellulose substrate associates with a Na+ counterion, providing an intermediate or
bridge function between substrate and sorbate, effectively creating a site with strongly positive
electrostatic potential; negatively charged dye molecules can then adsorb via electrostatic interaction
onto these positively charged sites in the fibre. However, if we consider hydrolyzed RR120 binding
through an electrostatic mechanism in 1:1 stoicheiometry with one carboxylate group (–COO–) and a
bridging Na+ ion, it is clear that q0 herein is significantly in excess of the number of available sites in
the substrate, being 5.46 × 10-5 mol g-1 for lyocell treated with 0.0 mol dm-3 NaOH (ca. 3× [–COO–
Na+] sites available), and 6.91 × 10-5 mol g-1 at 3.33 mol dm-3 NaOH (ca. 5.2× [–COO–Na+] sites
Clearly, sorption occurs in excess of experimentally determined q0 as a result of non-site-specific
interactions, more typical of a Freundlich isotherm. Hydrogen bonding and other short-range
attractive forces can occur between the dye molecule and the glucose units of the heterogeneous
polymer; Figure 3a shows positive electrostatic potential in a non-oxidized glucose moiety, offering
the opportunity for hydrogen bonding with electronegative moieties in the dye molecule. In addition,
hydrogen bonds can form directly between hydroxyl groups in the cellulose II substrate and the
extended π-electron system of the dye molecule (so-called Yoshida forces) (Yoshida et al. 1964); as
Lewis (1998) observes, if π-π interactions are considered in the wider context of electrostatic
interactions then the existence of π-facial hydrogen bonds between planar dye molecules (e.g.
RR120) and cellulose substrates appears very likely. Moreover, aromatic residues in the dye are
more likely to hydrogen bond with the cellulose –OH groups than hydrogen bond with themselves;
this does not, of course, preclude π-π interactions as a strong association force.
Another parameter which was calculated from obtained results is adsorption energy (ΔG0), also
known as a standard free energy of dyeing or standard affinity of the dye for the substrate. ΔG0 in
systems where dye adsorbs onto cellulose II fibres has been related to the internal surface character
of the substrate (Ibbett et al. 2006b), and a higher value corresponds to more thermodynamically
favourable dye sorption. For the Langmuir isotherm (which displays higher correlation), ΔG0
increases with increasing NaOH concentration to a maxima between 2.53 to 3.33 mol dm-3 NaOH
and then decreases with further increase in NaOH concentration to a plateau above 5.0 mol dm-3
NaOH (Figure 7); it can be concluded that sorption of hydrolyzed reactive dyes onto cellulose II
fibres is most thermodynamically favourable for substrates treated with 2.53 to 3.33 mol dm-3 NaOH.
However, when fitting the data to the Freundlich isotherm there is not such an obvious increase in
ΔG0 with increasing NaOH concentration and values are relatively consistent across the
concentration ranges applied. This observation does not devalue those for the Langmuir isotherm
and actually raises the issue worthy of investigation that some site-specific sorption is occurring at
some level within the fibre, hence the unexpected correlation to the Langmuir isotherm.
Blackburn et al. (2007) demonstrated that in cellulosic fibres the fibre surface exists as a layer of
fibrils and microfibrils through which liquid can flow and as their state in water changes from solid
phase, through a gel phase, to a solution phase as fibrils have heterogeneity all the way down to
molecular dimensions. Below the fibrillar layer exists a pore network within the main bulk of the
fibre through which liquid flow also occurs. For cellulose I fibres, fibrillar layer void spaces as large
as 100 µm may be expected, and in the bulk of the fibre pore sizes range from 10 to 300 Å in
diameter, hence, molecular size of the sorbate is an important factor in adsorption. Current
knowledge of alkali treatment suggests that increasing treatment concentration of alkali causes
strong lateral fibre swelling and subsequently increases accessible internal volume within the
structure of the fibre (Ibbett & Hsieh 2001; Colom & Carrillo 2002; Ibbett et al. 2006b; Široký et al.
2010). The observed differences in sorption could be explained by the accessible pore volume (APV)
of lyocell fibres to probes as a function of NaOH concentration. Öztürk et al. (2009) used inversion
size exclusion chromatography (ISEC) in combination with molecular probes up to 35 Å in diameter
to assess the APV of lyocell fibres treated with aqueous NaOH solutions (up to 8 mol dm-3 NaOH).
It was demonstrated that the upper limit of pore diameter in untreated and alkali treated lyocell was
27–32 Å, and it was additionally shown that for pore diameters of 19 Å and below that APV
increased with increasing NaOH concentration to a maxima around 2.5 mol dm-3 NaOH and then
decreased with further increase in NaOH concentration to a plateau above 4.0 to 5.0 mol dm-3
NaOH; in general APV at 2.5 mol dm-3 NaOH was ca. 10% higher than for untreated lyocell.
From molecular dynamics calculations (Figure 2a), it is observed that the dimensions of hydrolyzed
RR120 are about 30 Å (length) × 14 Å (wide) × 3.5 Å (depth) in one of the minimum energy states,
providing a molecular diameter of 14 Å (end-on). As the diameter of hydrolyzed RR120 is less than
19 Å, it seems likely that the maximum APV at around 2.5 mol dm-3 NaOH observed by Öztürk et
al. (2009) influences the accompanying maxima in q0 and ΔG0, which was indeed observed from the
experimental results obtained herein. Hence, maximum qe, q0 and ΔG0 are observed for cellulose II
fibre treated with 2.53 to 3.33 mol dm-3 NaOH as this concentration range affects the greatest
increase in APV.
It is observed that greatest adsorption of dye onto the substrate at equilibrium (qe) occurs for
cellulose II fibres treated with 2.53 and 3.33 mol dm-3 aqueous NaOH solution. Correlation to
sorption isotherms is most closely associated with a Langmuir type isotherm, indicating limited, site-
specific adsorption, however, correlation to the Freundlich isotherm is still significant, typical of
unlimited adsorption at non-specific sites, hence, sorption observed potentially occurs via a
combination of Langmuir and Freundlich isotherms.
For the Langmuir expression, the theoretical monolayer capacity (q0) increases with increasing
NaOH concentration to a maxima at 3.33 mol dm-3 NaOH and then decreases with further increase in
NaOH concentration to a plateau above 5.0 mol dm-3 NaOH; it can be concluded that the maximum
number of available sites for adsorption are observed for cellulose II fibres treated with 3.33 mol dm-
It is suggested that when electrolyte is added to the system carboxylate groups (–COO–) of the
partially oxidized cellulose substrate associates with a Na+ counterion effectively creating a site with
strongly positive electrostatic potential for sorption of negatively charged dye molecules via
electrostatic interaction. However, it is clear that q0 herein is significantly in excess of the number of
available sites in the substrate, hence, sorption occurs in excess of experimentally determined q0 as a
result of non-site-specific interactions, more typical of a Freundlich isotherm.
Adsorption energy (ΔG0), as calculated from the Langmuir isotherm, increases with increasing
NaOH concentration to a maxima between 2.53 to 3.33 mol dm-3 NaOH and then decreases with
further increase in NaOH concentration to a plateau above 5.0 mol dm-3 NaOH and it is
demonstrated that sorption of hydrolyzed reactive dyes onto cellulose II fibres is most
thermodynamically favourable for substrates treated with 2.53 to 3.33 mol dm-3 NaOH. Although the
Langmuir isotherm correlates most significantly with the data obtained, very close correlation with
the Freundlich isotherm is also observed. It is proposed that sorption occurs via a combination of
both theoretical models: sorption displays features expected for a cellulosic substrate, occurring
through hydrogen-bonding and van der Waals interactions (typical for Freundlich isotherms);
however, significant correlation with the Langmuir isotherm indicates that some site-specific sorption
is occurring within these alkali treated cellulose II substrates, and it is proposed that carboxylate
functions in the polymer may provide these sites.
Applying the work of Öztürk et al. (2009) concerning accessible pore volume (APV) of lyocell
fibres, it is concluded that pores in the fibre significantly affected by alkali treatment (< 20 Å
diameter) and the accessibility of dye (14 Å) sorption into those pores accounts the differences
observed herein; maximum qe, q0 and ΔG0 are observed for cellulose II fibre treated with 2.53 to 3.33
mol dm-3 NaOH as this concentration range affects the greatest increase in APV.
The authors thank The University of Leeds and Lenzing AG for the provision of a scholarship to Mr.
Široký. The authors would also like to acknowledge the assistance of Dr. Parikshit Goswami, Dr.
Barbora Široká and Dr. Avinash Manian for help with the experimental work and also for fruitful
discussion of the results.
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Table 1. Summary of the isotherm constants and correlations for Freundlich and Langmuir isotherms applied experimental data.
A (N m-2)
q0 × 10-5
492.53 5241.290.9781-17.92976.72 0.9986-16.2
494.485641.35 0.9757 -18.12855.820.9985-16.1
49 4.65 5101.320.9750-17.8270 6.070.9988-16.0
1474.485381.330.9775-17.9 2806.13 0.9975-16.1
147 4.65546 1.340.9749-18.0 2815.87 0.9998-16.1
1477.155531.34 0.9826 -18.0284 5.870.9992-16.1
Figure 1. Schematic illustration of apparatus used in continuous alkali pre-treatment process.
Figure 2. (a) Molecular structure of hydrolysed RR120 (2) as calculated by molecular dynamics; (b)
Electrostatic potential plotted onto a total electron density isosurface for hydrolyzed RR120 (half
molecule shown, dissected down centre). Blue is used to represent the strongest negative electrostatic
potential on the molecule and red the strongest positive electrostatic potential. The other colours are the
values in between, white is neutral.
Figure 3. Electrostatic potential plotted onto a total electron density isosurface for (a) glucose; and (b)
glucose with COO– group at C6 position with Na+ counter-ion. Blue is used to represent the strongest
negative electrostatic potential on the molecule and red the strongest positive electrostatic potential. The
other colours are the values in between, white is neutral.
Figure 4. Adsorption isotherm of hydrolysed RR120 sorbed onto cellulose II treated with varying
concentrations of NaOH, under applied tension of (a) 49 N m-2; and (b) 147 N m-2.
Figure 5. Linearized isotherms of hydrolysed RR120 sorbed onto cellulose II treated with varying
concentrations of NaOH, according to (a) Langmuir and (b) Freundlich isotherms (only samples treated
at 49 N m-2 shown).
Figure 6. (a) Equilibrium sorption (qe) of hydrolyzed RR120 (5% omf shown) by cellulose II fibres
treated with varying concentrations of NaOH; (b) Theoretical monolayer capacity (q0) of cellulose II
fibres treated with varying concentrations of NaOH, under applied tension of 49 N m-2 and 147 N m-2.
Both (a) and (b) are compared with specific water sorption (molar ratio, H2O/AGU) for cellulose I fibres
treated with varying concentrations of NaOH, after Fink et al. (1986).
Figure 7. Adsorption energy (ΔG0) of cellulose II fibres treated with varying concentrations of NaOH,
under applied tension of 49 N m-2 and 147 N m-2, as observed when applied to Langmuir isotherm.