Spatial localization of xylan on cellulose model films by AFM using

Spatial localization of xylan
on cellulose model films by
AFM using functionalized tips
2
2.1
C. Czibula1, 2, C. Ganser1, 2, A. Miletzky1,3, S.
Spirk4, W. Bauer1,3, R. Schennach1,5, and C.
Teichert1, 2
1
Christian Doppler Laboratory for Surface
Chemical and Physical Fundamentals of Paper
Strength, Graz, Austria
2
Institute of Physics, Montanuniversität of Leoben,
Austria
[email protected]
3
Institute for Paper, Pulp and Fibre Technology,
Graz University of Technology, Austria
[email protected]
4
Institute for Chemistry and Technology of
Materials, Graz University of Technology, Austria
5
Institute of Solid State Physics, Graz University of
Technology, Austria
1
Introduction
Xylan is one of the predominant biopolymers in
higher plants and wood. Wood is the main resource
for papermaking. During chemical pulping, part of
the xylan is getting dissolved and degraded and
accumulates as a byproduct in the cooking liquor.
Therefore, it is of interest to find possible
applications to add value to xylan. A proposed area
is the influence of (additional) xylan on pulp fibers
to affect the physical properties [1,2,3,4,5].
It is suggested that the interaction of regenerated
cellulose and xylan is dependent on the origin,
surface area, porosity, crystal plane, and degree of
order of cellulose as primary factors for the
adsorption behavior of xylan [6]. Linder et al. [7]
have detected a formation of colloidal structures on
bacterial cellulose surfaces and a covering of the
cellulose microfibrils during xylan treatment with
respect to time and temperature. Assembled xylan
on pulp fibers is often observed in particle like
aggregates and heterogeneously distributed [8,9].
In this study, quartz crystal microbalance with
dissipation monitoring (QCM-D) was used for
controlled xylan adsorption onto cellulose thin
films. The films were than scanned by atomic force
microscopy (AFM) using hydrophilically and
hydrophobically tips to investigate the spatial
distribution of adsorbed xylan as well as the surface
chemistry and the adhesive forces between the OHand CH3-functionalized AFM tips and the surface
via force mapping.
Materials and methods
To study the distribution of adsorbed birch xylan
on cellulose model thin films via AFM, QCM-D
was performed for controlled xylan adsorption.
Cellulose model film
Cellulose thin films were prepared, according to
Kontturi et al., Mohan et al., and Rohm et al.
[10,11,12,13,14]. Trimethylsilyl cellulose was
dissolved (1 wt%) in toluene, and then deposited by
spin coating (υ = 4000 rpm, a = 2500 rpm/s, t = 60 s)
onto quartz crystal microbalance substrates. The
coated substrates were placed in a petri-dish
containing 3 mL of 10 wt% of HCl. After closing
the dish with its cap, the films was converted to
amorphous cellulose II by exposing it to the HCl
vapor for 15 min. Several substrates have been
prepared for xylan adsorption
2.2
Xylan adsorption on cellulose thin
films using QCM-D
The cellulose coated QCM sensors were mounted
in the QCM chambers and equilibrated to MQ
water and then to a NaCl solution (1 or 100 mM) at
pH 8 for 60 min. Then, a solution of a birch xylan
(0.5 g/L) at pH 8 and varying ionic strengths (1 or
100 mN NaCl) was pumped over the sensors at a
flow rate of 0.1 mL/min for 60 min. Afterwards, the
sensors were rinsed again with NaCl solution and
MQ water for 60 min. The Δf and ΔD values were
recorded to determine the adsorbed masses via
viscoelastic modeling (Voigt model) [15,16].
2.3
Detection of xylan by AFM
Atomic force microscopy (AFM) was employed to
determine the morphological structure and
chemistry of surfaces in the nanoscale range. An
MFP-3D AFM (Asylum Research) was utilized
which was equipped with a planar closed-loop
scanner and operated in tapping mode. The AFM
tips have been functionalized with OH and CH3
groups (Nanocraft) and had an estimated tip radius
of 20–40 nm. The spring constants of the
cantilevers were 2–3 N/m. To obtain information on
the
adhesive
interaction
between
the
hydrophilically and hydrophobically tip and the
sample. The surfaces were scanned in the repulsive
regime.
3
3.1
Results and discussion
QCM-D
The dissipation for all samples was larger than
1 × 10-6 Hz indication the formation of a
viscoelastic xylan layer on a cellulose film, which
is in accordance with [17,18,19]. The adsorption
take place rather fast. The results of viscoelastic
modeling (Voigt) are presented in Table 1 which
gives information about the average thickness of
adsorbed xylan layer (d), adsorbed mass (ΓQCM),
shear modulus (μ), and its viscosity (η). According
to the Voigt model, the layer thicknesses refer to
swollen layers which contain water and electrolyte
as well.
Table 1. Voigt type modeling of xylan layer
properties (thickness d, adsorbed mass ΓQCM, shear
modulus μ, and viscosity η, frequency value Δf3)
obtained from different electrolyte concentrations
after rinsing with MQ water. The density of the
xylan was assumed 1.2 g/cm³.
pH 8,
pH 8,
1 mM NaCl
100 mM NaCl
d, nm
3.6
6.4
ΓQCM, mg/m²
4.3
7.7
η, 10-3 kg/m s
1.5
1.5
μ, 104 Pa
6.8
4.8
Δf3,
-11.7 ± 3.7
-13.0 ± 2.4
3.2
Spatial localization of xylan
The topography and the phase contrast of an
amorphous cellulose film before the treatment with
the xylan solution are presented in Figure 1 (p. 4)
which looks similar to topography images depicted
by [12]. The surface exhibits large areas of the
same height with a homogeneous and uniform
phase contrast, meaning the surface of the
regenerated cellulose film is relatively smooth.
After the treatment with the xylan solution, the
cellulose film still appears smooth, but particles of
different sizes with diameters between 10–20 nm
can be detected at irregular distances. It was also
seen that the shape of the particles is dependent on
the salt concentration and can be either globular
(1 mM NaCl) or elongated (100 mM NaCl). These
areas are marked in blue in the topography image in
Figure 2a (p. 4). Further, these particles exhibit a
lower attraction to the OH and CH3 groups at the
functionalized AFM tip, as depicted as darker areas
in the phase contrast image (Figure 2b, p. 4).
Besides hydroxyl groups, xylan carries carboxyl
groups as well, which feature a higher negative
charge density than hydroxyl groups. This implies
that a stronger repulsion and lower adhesion
between these groups is expected. In fact, lower
adhesion forces were determined in the darker areas
(6 ± 2 nN, OH groups) than for regenerated
cellulose (12 ± 2 nN, OH groups) via force
mapping measurements. A similar trend was
detected with the CH3 AFM tips. In force mapping,
force-distance (F-x) curves are recorded as a
function of the lateral coordinates. From these F-x
curves, the adhesion force is extracted. The
adhesion force is the force that is needed to separate
the tip from the surface after contact [20].
Therefore, it is suggested that the darker areas in
the phase contrast image (Figure 2b, p. 4) are
related to precipitated xylan.
Nevertheless, further questions have to be
answered, like the stability of such functionalized
AFM tips. In a next step, this method will be
applied to pulp fibers.
However, with this technique it is possible to
investigate not only the distribution of xylan on
cellulosic surfaces, but also to study the adhesion
forces of cellulose and xylan from various species.
Acknowledgement
The authors gratefully thank for the financial
support of the Lenzing AG, the Austrian Federal
Ministry of Economy, Family and Youth, and the
National Foundation for Research, Technology and
Development.
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(a) Topography of a pure cellulose thin film.
(a) Topography of a cellulose thin film after xylan
adsorption. The blue areas represent adsorbed
xylan.
(b) Phase contrast of a pure cellulose thin film.
(b) Phase contrast of a cellulose thin film after
xylan adsorption.
Figure 1. Topography (a) and phase contrast (b) of
a pure cellulose model. The images were scanned
with an OH-functionalized AFM tip.
Figure 2. Phase contrast images of a pure cellulose
model film before (a) and after (b) the treatment
with xylan. The images were scanned with an OHfunctionalized AFM tip.

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