Surface tailoring of cellulose aerogel-like structures with ultrathin coatings using molecular layer-by-layer assembly

Carbohydrate Polymers 282 (2022) 119098

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Surface tailoring of cellulose aerogel-like structures with ultrathin coatings
using molecular layer-by-layer assembly
Zhaleh Atoufi a, Michael S. Reid a, Per A. Larsson a, Lars Wågberg a, b, *
a
b

Division of Fibre technology, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56–58, SE-100 44 Stockholm, Sweden
KTH Royal Institute of Technology, Department of Fiber and Polymer Technology, Wallenberg Wood Science Center (WWSC), Stockholm, Sweden

A R T I C L E I N F O

A B S T R A C T

Keywords:
Cellulose nanofibril
Aerogels
Molecular layer-by-layer deposition
Surface functionality
Wet strength
Oil absorption

Cellulose nanofibril-based aerogels have promising applicability in various fields; however, developing an effi­
cient technique to functionalize and tune their surface properties is challenging. In this study, physically and
covalently crosslinked cellulose nanofibril-based aerogel-like structures were prepared and modified by a mo­

lecular layer-by-layer (m-LBL) deposition method. Following three m-LBL depositions, an ultrathin polyamide
layer was formed throughout the aerogel and its structure and chemical composition was studied in detail.
Analysis of model cellulose surfaces showed that the thickness of the deposited layer after three m-LBLs was
approximately 1 nm. Although the deposited layer was extremely thin, it led to a 2.6-fold increase in the wet
specific modulus, improved the acid-base resistance, and changed the aerogels from hydrophilic to hydrophobic
making them suitable materials for oil absorption with the absorption capacity of 16–36 g/g. Thus, demon­
strating m-LBL assembly is a powerful technique for tailoring surface properties and functionality of cellulose
substrates.

1. Introduction
Substituting petroleum-derived, non-biodegradable materials with
sustainable and biodegradable materials from renewable sources is of
great scientific and economic interest (Aalbers et al., 2019). In this re­
gard, bio-based, sustainable aerogels are considered as promising ma­
terials for a wide range of applications, including thermal and acoustic
insulations (Eskandari et al., 2017; Nguyen et al., 2020), adsorption of
liquids and gases (Jatoi et al., 2021), energy storage (Sun et al., 2021),
drug carriers (Liu et al., 2021) and catalyst supports (Gu et al., 2021).
Aerogels can be prepared from many different sources, such as carbon
nanomaterials (Khoshnevis et al., 2018; Pruna et al., 2019), cellulosic
nanomaterials such as cellulose nanofibrils (CNFs) and cellulose nano­
crystals (CNCs) (Cervin et al., 2012; De France et al., 2017), synthetic
polymers (Wu et al., 2019), and silica-based materials (He et al., 2018).
Among these, CNF-based aerogels, here defined as lightweight materials
derived from deaerated CNF gels, have attracted great attention due to
the intrinsic properties of CNFs, such as high mechanical strength, high
aspect ratio, bio-degradability, and their green preparation process.
Moreover, due to the numerous hydroxyl groups, CNFs can be func­
tionalized with various functional groups, leading to diverse properties

that can be tailored to the desired application (Tavakolian et al., 2020).
In order to achieve CNF-based aerogels with particular physical and
chemical properties, CNFs can either be pre-functionalized before
forming the aerogel, or aerogels can be post-functionalized to target a
specific application. CNFs can be modified via chemical modification of
hydroxyl groups, such as TEMPO-oxidation (Kim et al., 2021), carbox­
ymethylation and phosphorylation (Patoary et al., 2021), or via chem­
ical modifications via ring opening reactions, which typically breaks the
C–C bonds of carbons at C2 and C3 position. These reactions can further
involve chemical grafting of polymers or single molecules to the CNF
surface (Abushammala & Mao, 2019; Rol et al., 2019). Although these
modifications have become relatively common, with acceptable yields,
implementation of these reactions while maintaining dispersion stability
can be challenging. Specifically, it is difficult to avoid aggregation of the
system or to properly remove excess reagents, which can subsequently
inhibit aerogel formation. Thus, it is often favorable to perform a post
functionalization of the aerogels. Generally, CNF-based aerogels can be
modified via vapor- or liquid-based methods. Commonly, silanes,
organosilanes or other fluorine-containing chemicals are evaporated
and allowed to diffuse through the aerogel (Liao et al., 2016; Yu et al.,
2021; Zhu et al., 2020). However, these chemical vapor deposition

* Corresponding author at: Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56–58, SE-100 44 Stockholm, Sweden.
E-mail addresses: (Z. Atoufi), (M.S. Reid), (P.A. Larsson), (L. Wågberg).
/>Received 27 September 2021; Received in revised form 29 December 2021; Accepted 1 January 2022
Available online 10 January 2022
0144-8617/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( />

Z. Atoufi et al.

Carbohydrate Polymers 282 (2022) 119098

methods usually involve toxic and expensive chemicals, are diffusion
limited and can result in non-uniform modification. Another method is
Cold plasma which is more environmentally friendly, but similarly
limited and require lengthy processing and high discharge energy
(Schroeter et al., 2021). As a result, it can be more effective to use liquid
modification methods, whereby the capillary forces uniformly wet the
aerogel surfaces. Although these chemical grafting methods are usually
very efficient, they can require complex reactions and processing con­
ditions which are not favorable for large-scale production (Xu et al.,
2019). Therefore, there is a demand for an efficient and fast modifica­
tion technique which is applicable for cellulose aerogels.
Molecular layer-by-layer (m-LBL) deposition is a unique technique
which allows for the deposition of ultra-thin layers onto a surface
through sequential covalent reactions. The result is the formation of a
precise molecular-scale coating, essentially via surface oligomerization,
which cannot be achieved by bulk synthesis methods (Chan et al., 2012;
Johnson et al., 2012). With m-LBL, it is possible to precisely control the
surface chemistry, roughness and the thickness of the deposited film
(Chan et al., 2013). The presence of numerous hydroxyl groups on the
surface of CNF-based aerogels make them suitable substrates for m-LBL
modification. However, CNF-based aerogels must be prepared such that
they are mechanically robust and can maintain their structure through
the modification process.
CNF-based aerogels are typically prepared through freeze drying or
critical point drying of CNF dispersions (Mulyadi et al., 2016). However,
due to the hydrophilicity of CNFs, unmodified aerogels are generally not
wet-stable and disintegrate, (Kim et al., 2017) making them unsuitable
for m-LBL modification. Additionally, scale-up of these aerogels is

challenging due to the high energy consumption of freeze drying and
critical point drying techniques. One way to prepare wet-stable CNFbased aerogel-like materials with scale-up possibility is the so-called
freeze-linking technique (Erlandsson et al., 2016), whereby aerogellike structures are prepared through molding and freezing of CNF
dispersion followed by solvent exchange and ambient drying. During
freezing, ice crystal growth forces CNFs together to form a strong
micrometer thick interconnected network structure. By solvent
exchanging to acetone capillary forces are significantly reduced such
that the aerogel-like structure is maintained during ambient drying.
Although freeze-linked aerogel-like structures usually have larger pores
and lower specific surface area compared to conventional aerogels, the
fact that they can be prepared without using critical point drying or
freeze drying is indeed a huge advantage. Additionally, by incorporating
calcium ions or using aldehyde-containing CNFs, the aerogel-like
structure can be physically or covalently crosslinked during freezelinking to obtain wet stability (Erlandsson et al., 2018; Franỗon et al.,
2020).
We hypothesized that m-LBL modification is an efficient method to
tune the surface properties and functionality of CNF-based aerogels. In
this study, two types of wet-stable CNF-based aerogel-like structures
were prepared through a freeze-linking technique: i) hemiacetal cross­
linked aerogel-likes, where dialdehyde-containing CNFs are chemically
crosslinked through hemiacetal bonds formed between the individual
CNFs during freezing, and ii) calcium ion cross-linked aerogel-likes, in
which calcium ions physically crosslink the structure. Both aerogel-likes
(hereafter called aerogel) were then modified through m-LBL deposition
of trimesoyl chloride (TMC) and m-xylylene diamine (MXD), resulting in
an ultrathin polyamide film covering all surfaces throughout the aero­
gels. The chemical structure, atomic composition, thickness and
roughness of the deposited film was characterized and the effect of mLBL modification on morphology, wet and dry mechanical properties,
acid-base resistance, thermal degradation and surface properties of the
aerogels was investigated. Finally, the modified aerogels were examined

for potential use as oil-water separators for water purification.

2. Experimental section
2.1. Materials
Carboxymethylated CNFs with a total charge of 600 ± 50 ueq/g were
produced according to a previously reported method (Wågberg et al.,
2008) and provided by RISE Bioeconomy AB in the form of 20 g/l
aqueous gel. Branched polyethylenimine (PEI) with the molecular
weight of 25,000 Da, poly(allylamine hydrochloride) (PAH) with the
molecular weight of 17,500 Da, molecular sieve beads (pore diameter of
3 Å and 4 Å, 8–12 mesh), trimesoyl chloride, m-xylylene diamine, oil red
O, toluene (ACS reagent, ≥99.5%), calcium chloride, and sodium hy­
droxide were purchased from Sigma Aldrich. Sodium metaperiodate
(99%) was purchased from Acros Organic, Belgium. Acetone was pur­
chased from VWR International (Radnor, PA, USA).
Prior to use, water was carefully removed from acetone and toluene
with the aid of the molecular sieves. All other chemicals were used
without further purification.
2.2. Sample preparation
2.2.1. Synthesis of calcium ion crosslinked aerogels
CNF gels with a concentration of 7.5 g/l were mixed with CaCl2 using
an Ultra Turrax (IKA Werke GmbH & Co. KG, Staufen, Germany) at
12000 rpm for 5 min. Total concentration of CaCl2 in the CNF gel was
13.5 mM to achieve a 3:1 ratio of calcium ions to CNF charge groups.
The gels were then placed into cylindrical polystyrene molds and frozen
in a freezer (− 18 ◦ C) overnight to ensure a complete freezing. The frozen
samples were then thawed and solvent exchanged in acetone followed
by ambient drying. Calcium ion crosslinked aerogels are abbreviated as
Ca-AG in the text.
2.2.2. Synthesis of hemiacetal crosslinked aerogels

Hemiacetal crosslinked aerogels (HA-AG) were prepared according
to an earlier described method (Erlandsson et al., 2016). Briefly, dia­
ldehyde CNFs were prepared by mixing a 7.5 g/l CNF gel with sodium
metaperiodate to a concentration of 60 mM, using an Ultra Turrax
disperser at 12000 rpm for 5 min. The mixture was covered with
aluminum foil to prevent exposure to light. After 1 h of reaction, the
obtained gel was quickly transferred to a mold and frozen (− 18 ◦ C)
overnight, thawed and solvent exchanged to acetone, dried under
ambient conditions and stored for further use. Before use, the aerogels
were thoroughly rinsed with water until the conductivity of the washing
water was below 5 μS/cm.
2.2.3. Preparation of CNF dispersions
CNF dispersions were prepared according to an earlier described
method (Erlandsson et al., 2018). A 20 g/l CNF gel was diluted to 2 g/l
and homogenized with an Ultra Turrax at 12000 rpm for 10 min. The
dispersion was then ultrasonicated at 300 W for 10 min using an ul­
trasonic probe (Sonics VCX 750, Newton, USA) followed by centrifu­
gation at 4500 rpm for 1 h. The stable CNF supernatant was collected
and stored in the fridge for further use. The dry content was obtained
gravimetrically.
2.2.4. Molecular layer-by-layer deposition on cellulose aerogels
CNF-based aerogels were modified by three cycles of m-LBL depo­
sition of TMC and MXD molecules to form a thin polyamide film on the
surface. Each bilayer was deposited using a four-step procedure as
shown schematically in Fig. 1. First, aerogels were soaked in a solution
of 1 wt% TMC in toluene for 2 min to ensure that the reactants have
penetrated the entire aerogel and reacted with the cellulose surface. The
aerogels were then soaked in toluene with being the solvent replaced
every 5 min a total of three times. The aerogels were then dried by
vacuum filtration to remove excess toluene as to not dilute the reactants

in the next step. The dried aerogels were then soaked in a 1 wt% solution
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Fig. 1. a) Molecular layer-by-layer deposition of TMC and MXD on a CNF-based aerogel. b) Schematic illustration of the formation of polyamide film by sequential
reaction of TMC and MXD monomers with the cellulose surfaces during the deposition. The term lamellae in the figure refers to the internal walls of the aerogels that
were formed during the preparation of the aerogels.

of MXD, in toluene, for 2 min. The MXD modified aerogel was then
soaked in acetone with the solvent being replaced every 5 min a total of
three times, followed by vacuum filtration. These steps repeated three
times to achieve three bilayers on the surface of the aerogels.

deposition and scratched and imaged by atomic force microscopy
(AFM). Solutions of 1 wt% TMC and MXD in toluene were prepared. CNF
model surfaces were soaked in TMC solution for 30 s followed by
excessive washing with toluene. The CNF surfaces were then soaked in
MXD solution for 30 s, washed excessively with acetone followed by
drying under a gentle flow of nitrogen gas. These steps were repeated to
achieve the desired number of bilayers. The same procedure was used to
adsorb TMC/MXD bilayers on the free standing CNF films.

2.2.5. Preparation of CNF films
CNF films were prepared by vacuum filtration of 75 ml of 2 g/l CNF
dispersion through a 0.45 μm membrane (Durapore, Merck Millipore).
After filtration, the obtained wet film was dried for 15 min at 93 ◦ C and

95 kPa using the dryer of a Rapid-Kă
othen sheet former (PTI, Austria).

2.3. Analysis

2.2.6. Preparation of CNF model surfaces
The formation of m-LBL layers inside the three-dimensional CNFbased aerogel structures is difficult to study due to the thin nature of the
deposited film (<5 nm) and complex structure of the aerogels. To
circumvent this problem, model CNF surfaces were prepared to inves­
tigate m-LBL film formation. CNF model surfaces were prepared on sil­
icon oxide surfaces with a pre-adsorbed layer of cationic anchoring
polymer. Specifically, silicon oxide surfaces (Addison Engineering, Inc.,
USA) were plasma cleaned for 3 min (Harrick PDC-002, Harrick Scien­
tific Corporation, Ithaca, USA) and activated in 10 wt% NaOH solution.
Silicon oxide surfaces were immersed in 0.1 g/l PAH containing 10 mM
NaCl at pH 7 for 30 min followed by excessive rinsing with MilliQ water.
CNFs were then adsorbed on the surface by dipping the silicon surfaces
in 0.1 g/l solution of CNF at pH 7 for 5 min followed by washing with
MilliQ water. Subsequently, PEI was adsorbed on the surface by soaking
the silicone surfaces in 0.1 g/l aqueous pH 7 solution. This process was
continued until five bilayers of PEI and CNFs were adsorbed onto the
surface, yielding a smooth, tightly packed cellulose surface.

2.3.1. Carbonyl content determination
Aldehyde content within HA-AGs was measured using a titration
method described earlier (Larsson et al., 2008; Lopez Duran et al.,
2018). Specifically, hydroxylamine hydrochloride reacts with all avail­
able aldehyde groups in the aerogel to release a stoichiometric amount
of protons, which are then titrated by hydroxyl ions. HA-AGs were cut
into approximately 80 mg pieces and added to 25 ml solutions con­

taining 10 mM NaCl and pH adjusted to pH 4. An amount of 25 ml of
0.25 M hydroxylamine hydrochloride in 10 mM NaCl was then added to
each aerogel mixture to initiate a reaction between aldehydes and hy­
droxylamine. After 2 h, the mixture was titrated with 0.1 M NaOH back
to pH 4. The aldehyde content of HA-AGs was calculated as the amount
of NaOH consumed per gram of sample. Samples were tested in triplicate
for each type of aerogel.
2.3.2. Aerogels density and porosity measurements
Aerogels were prepared using cylindrical polystyrene molds. The
density of the aerogels was obtained by dividing their weight by their
volume and their porosity was estimated according to:
(
)
ρ*
porosity = 1 −
× 100
(1)

2.2.7. m-LBL deposition on CNF model surfaces and CNF films
To study the layer growth and changes to the surface roughness, CNF
model surfaces were modified with 5, 10, and 15 bilayers of m-LBL

ρs

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Cressington 208 HR sputter coater (Cressington Scientific Intruments,
Watford, UK).

where ρ* is the calculated density of the aerogels and ρs is the pore wall
density (Lopez Duran et al., 2018). For neat cellulose aerogels, ρs was
estimated to be cellulose density (1500 kg⋅m− 3) while for modified
aerogels it was calculated according to the mass of deposited polyamide
layer:

ρs =

mc + mPA
mPA
mc
ρ + ρ
c

2.3.8. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) spectra were recorded using
a Kratos AXIS UltraDLD x-ray photoelectron spectrometer (Kratos
Analytical, Manchester, UK). The aerogel samples were analyzed using a
monochromatic Al x-ray source and the analysis area was approximately
0.7 × 0.3 mm2. Detailed spectra of each individual element was recor­
ded and quantified in order to obtain the relative elemental composition
of the surface expressed in atomic %.

(2)

PA

where mc, mPA, ρc, and ρPA are the initial mass of cellulose aerogel, mass
of deposited polyamide layer, density of cellulose and density of poly­
amide, respectively.

2.3.9. Thermogravimetric analysis
Thermogravimetric analysis (TGA) of the aerogels was performed
using a Mettler Toledo TGA/DSC 1 STARe System at a temperature range
of 30–700 ◦ C and a heating rate of 10 ◦ C/mins under constant nitrogen
flow. Specimens were conditioned at room temperature before mea­
surement and a minimum of 3.5 mg was used for each measurement.

2.3.3. Fourier transform infrared spectrometry
Fourier transform infrared spectrometry (FTIR) was performed
before and after m-LBL modification of the aerogels using a PerkinElmer
Spectrum 100 FTIR equipped with a Golden Gate attenuated total
reflection accessory (Specac Ltd., Kent, England) with a set resolution of
4 cm− 1 in the spectral region of 4000–600 cm− 1. Spectra were baseline
corrected and normalized by the PerkinElmer software at 1030 cm− 1, a
wavenumber corresponding to the C–O stretch band in the cellulose
backbone.

2.3.10. Absorption and reusability study
The oil absorption capacity of the aerogels was measured by
immersing the aerogels in 20 ml of organic solvent for 30 s. The aerogels
were taken out and weighed after the superficial solvent was wiped off
by a filter paper. The absorption capacity was calculated as the mass of
oil absorbed per mass of dry aerogel prior to immersion. Five replicates
were used for each type of aerogel.
To study the reusability of HA-AGs in absorbency applications, the

absorbed solvent was removed from the aerogel, and collected, using
vacuum filtration. The aerogels were then dried followed by a repeated
absorption test. This absorption-removal cycle was repeated five times.
Four replicates were performed.
To test the separation efficiency, multi-functional samples were
assembled such that the center was constituted by m-LBL modified
aerogel and the surrounding area by unmodified CNF-based aerogel, and
vice versa. Samples were prepared by punching the middle of the un­
modified aerogels out and filling the created void with a piece of
modified aerogel. The multi-functional samples were then soaked in a
stirred mixture of toluene (dyed with oil red) and water (dyed with
brilliant blue).

2.3.4. Contact angle analysis
The water contact angle of a 3 μl droplet of ultra-pure water on the
CNF films before and after the deposition of three m-LBL layers was
measured under conditions of 25 ◦ C and 50% relative humidity by a KSV
instrument CAM 200 equipped with a Basler A602f camera. All mea­
surements were performed in triplicate.
2.3.5. Mechanical properties
The mechanical properties of the aerogels were measured using an
Instron 5566 universal testing machine (Norwood, USA) equipped with
a 500 N load cell. The aerogel samples were in cylindrical form with
average diameter of 28 mm and thickness of 8 mm. Dry compressive
tests were performed for the aerogels conditioned at 50% RH and 23 ◦ C
for 48 h, with a strain rate of 10%/min until they reached 90%
compression. To determine the mechanical properties of wet aerogels,
samples were immersed in deionized water for 48 h prior to testing. The
wet aerogels were then compressed to 80% compression, followed by
gradual removal of the force applied by again separating the clamps at a

rate of 10%/min. The obtained mechanical data for each aerogel was
then normalized to its post-conditioned density. Each type of aerogel
was tested in triplicate.

3. Results and discussion
3.1. Principles of aerogel preparation and functionalization
The impact of m-LbL deposition on covalently and physically cross­
linked CNF-based aerogels, prepared by the freeze-linking technique,
was examined. Covalently crosslinked aerogels (HA-AG) consisted of
dialdehyde CNFs crosslinked through hemiacetal linkages formed in the
aerogel lamellae during ice templating. Crosslinking specifically
occurred during the formation and growth of ice crystals where the CNFs
are forced together to form strong interconnected pore walls that can
withstand capillary pressures following solvent exchange and ambient
drying (Erlandsson et al., 2018). The HA-AGs contained 1.2 ± 0.1
mmol⋅g− 1 aldehyde, corresponding to a degree of oxidation of approx­
imately 10% of all glucose units of the cellulose in the fibrils. The density
and porosity of these aerogels was 13.9 kg⋅m− 3 and 99.1%, respectively.
The second group of aerogels (Ca-AG) consisted of CNFs physically
crosslinked by calcium ions using the same freeze linking method. The
Ca-AGs had nearly the same low density and high porosity as the HAAGs (14.3 kg⋅m− 3 and 99.0%).
The prepared aerogels were then modified with three bilayers of
TMC and MXD. After modification, both physically and covalently
crosslinked aerogels yellowed in color (the color was more significant in
HA-AG-3LBL) and became hydrophobic enough to float on water
(Fig. 2a,c). To assess the effect of m-LBL modification on the hydro­
phobicity, water contact angle measurements were performed on free-

2.3.6. Atomic force microscopy
CNF model films on silicon oxide substrates were imaged under

ambient conditions using a MultiMode 8 AFM in ScanAsyst mode
(Bruker, Santa Barbara, CA). Root-mean-squared (rms) roughness of the
surfaces was calculated over eight individual 2 × 2 μm2 areas. The
thickness of the CNF films before and after m-LBL deposition was
determined via scratch test according to a previously established
method (Reid et al., 2016). Briefly, in contact mode (with a deflection
set point of approximately 1 V) an AFM cantilever was scanned over a 1
× 1 μm2 area to detach CNFs from the surface. The applied force was set
to be sufficient to detach the CNFs from the surface without damaging
underneath silica substrate (Reid et al., 2016). The scratched area was
then reimaged in tapping mode to obtain the cross-sectional profile.
Images were analyzed using NanoScope Analysis software.
2.3.7. Scanning electron microscopy
The microstructure of the aerogels was studied using an S-4800 field
emission scanning electron microscope (SEM; Hitachi, Tokyo, Japan).
Aerogels were frozen in liquid nitrogen and cut into thin rectangular
shapes and mounted on the sample holder using carbon tape. To mini­
mize charging, samples were coated with a Pt–Pd alloy for 40 s using a
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Fig. 2. a) Photograph of the CNF-based aerogels before and after three bilayers of m-LBL deposition. b) Water contact angle with CNF films before and after three
bilayers of m-LBL deposition. c) Photograph of Ca-AG (left) and Ca-AG-3LBL (right) in contact with water. d) FTIR spectra of the aerogels before and after three mLBL depositions.

standing CNF films. The water contact angle of the unmodified CNF film
was 55◦ , slightly higher than, but in the same range as several reported

values in literature (Aulin et al., 2009; Kang et al., 2017; Sethi et al.,
2019). Nonetheless, after three m-LBL deposited bilayers, the water
contact angle increased from 55◦ to 77◦ (Fig. 2b). According to earlier
investigations the contact angle of interfacially polymerized PA thin
films from TMC and m-phenylenediamine is 79◦ (Rajaeian et al., 2013).
Hence, a contact angle of 77◦ indicates a good coverage of the surface
with a thin PA film after three m-LBL bilayers. In addition to the
increased contact angle, modified HA-AGs and Ca-AGs showed an in­
crease in mass of 24.1% and 49.3% and a volume reduction of 21.5%
and 24.9%. Thus, the final HA-AG-3LBL and Ca-AG-3LBL had a density
of 21.9 kg⋅m− 3 and 28 kg.m− 3 and porosity of 98.5% and 97.9%,
respectively. Furthermore, no mass loss was observed after soaking the
modified aerogels in toluene for 1 week demonstrating that the depos­
ited layers are stable and don't dissolve or detach in the solvent. FTIR
spectra of the modified aerogels show characteristic amide signals at
–O
1540, 1610 and 1645 cm− 1, assigned to N–H bending, H-bonded C–
– O stretching modes, respectively (Johnson et al., 2012; Reid
and C–
et al., 2019) (Fig. 2d). However, the signal intensity was higher for CaAG-3LBL, in agreement with the larger weight increase of the aerogels.
It is worth mentioning that since the last molecular layer used in this
process was MXD, the surface of the aerogels was covered with a high
concentration of amine groups which can be utilized as a platform for
further chemical reactions or other applications requiring amine func­
tionality such as CO2 capturing (Jiang, Oguzlu, & Jiang, 2021; Zhu et al.,
2020). Moreover, by using TMC as the final layer or by utilizing other
types of monomer, the surface functionality can be tailored. This indeed
makes m-LBL deposition a valuable and efficient method for controlling
surface functionality.

3.2. Characterization of the molecular layers
To better characterize the build-up of the molecular layers on the
CNF-based aerogels, CNF model surfaces were prepared and subjected to
5, 10 and 15 bilayers of m-LBL deposition. AFM images clearly
demonstrate the formation and growth of a smooth and uniform layer on
the CNF surfaces. Fig. 3a,b shows that after deposition of five molecular
bilayers, the CNF surface becomes smoother with the surface roughness
decreasing from 3.2 to 1.8 nm (Table S1). Continuing m-LBL deposition
to 15 bilayers (Fig. 3d), the deposited layer grew thicker making it
difficult to clearly distinguish the supporting CNF surface. The deposited
film remained smooth and uniform with a roughness of 2.1 nm, lower
than the roughness of the initial CNF surface. To measure the thickness
of deposited molecular layers, scratch tests were performed for the CNF
surfaces before and after 5, 10, and 15 cycles of m-LBL deposition.
Fig. 3e,g displays the scratched CNF surfaces after 15 m-LBL cycles and
the average cross sectional analysis of the film. The scratched image
shows that CNFs in the scratched areas are detached from the substrate
and accumulate as ridges around the scratched area. The total thickness
of the layer was measured as the height difference between the scratched
substrate and the unaffected modified CNF surface. The thickness of the
deposited molecular layer was then measured as the thickness difference
of the CNF surface before and after m-LBL deposition.
Fig. 3f presents the thickness of the deposited film per cycle of m-LBL
deposition. Unlike m-LBL modification of nearly atomically flat surfaces
(Chan, Lee, Chung, & Stafford, 2012; Johnson et al., 2012), the thickness
growth of the deposited polyamide film on the CNF surfaces is not linear.
This non-linear growth is most probably due to 3D structure and the
roughness of initial CNF surface. During the m-LBL modification, TMC
and MXD molecules react with the surface and gradually grow, filling
the empty areas between the fibrils and decreasing the surface rough­

ness. With each successive layer the surface area ideally increases and
thus more and more molecules deposited on the surface, in turn creating
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Fig. 3. a-d) AFM images of CNF surfaces with 0, 5, 10, and 15 bilayers of m-LBL deposition. e) AFM image of a scratched CNF surface after 15 m-LBL cycles. f)
Thickness of the deposited layers per cycle of m-LBL deposition. g) Average cross-section-height analysis of the scratched surface. d) Schematic view of the growth of
CNF dimensions by m-LBL deposition leading to the decrease in surface roughness and nonlinear thickness growth.

a larger surface for further modification (Fig. 3 h). Similar, non-linear
growth has been reported for more conventional LBL assembly of
polyelectrolyte films (Haynie et al., 2011). According to Fig. 3f, the
thickness of the deposited polyamide film after three bilayers is
approximately 1 nm and thus the thickness of deposited film on the pore
wall of the HA-AG-3LBL and Ca-AG-3LBL is predicted to be in a similar
range.

No sign of monomer aggregation or inhomogeneity was observed after
m-LBL modification. These results are in line with AFM measurements
which showed that the thickness of the coated PA film after three bi­
layers is approximately 1 nm and thus, no significant difference is
noticeable in the SEM images.
SEM images of Ca-AG before and after three m-LBL modification is
shown in Fig. 5. Comparing the microstructure of the Ca-AG before
(Fig. 5a,b) and after m-LBL deposition (Fig. 5c-f) demonstrates that in
contrast to the HA-AGs, modification of Ca-AGs has resulted in a

somewhat inhomogeneous pattern. In some areas, the deposited film is
very thin and homogeneous (Fig. 5c,d) while in some other parts TMC
and MXD molecules are polymerized to form a thick and inhomogeneous
PA layer on the surface (Fig. 5e,f). This potentially could be ascribed to
the less accessible areas where extra monomers were not effectively
washed away and polymerization has occurred on the surface. This

3.3. Characterization of the aerogels
The microstructure of the aerogels was investigated by SEM imaging.
Fig. 4 shows the microstructure of HA-AGs before and after three bi­
layers of m-LBL deposition. No significant difference is observed in the
microstructure of HA-AGs following three bilayers of m-LBL deposition,
demonstrating that the deposited film is very thin, smooth and uniform.
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Carbohydrate Polymers 282 (2022) 119098

Fig. 4. Micrographs of a,b) HA-AG and c,d) HA-AG-3LBL at two different magnifications.

could also possibly be the reason for the higher weight increase of Ca-AG
after m-LBL deposition.
XPS was performed to measure the surface chemical composition of
the aerogels before and after m-LBL modification. Atomic composition
of oxygen, carbon, and nitrogen indirectly reveal the extent of the re­
action of TMC and MXD molecules with the cellulose surface. Specif­
ically, the atomic ratios can be used to determine linear or branched
growth of the deposited molecular layers (Chan et al., 2013). Scheme S1

shows the two possible extremes of a fully branched and a fully linear mLBL reaction. If all the three acid chloride groups in each TMC molecule
react with an MXD molecule, a fully branched structure with a C/O ratio
of 8.3 and an O/N ratio of 0.8 is obtained. At the other extreme, if each
TMC molecule reacts with only two other MXD molecules a linear
structure with a C/O ratio of 4.3 and an O/N ratio of 2 is obtained,
assuming that the unreacted acid chloride is converted to carboxylic
acid due to the humidity of ambient conditions. Therein, by comparing
the XPS results of the aerogels with the theoretical calculations, the
chemical structure of the deposited molecular layer can be predicted.
From Table 1 it can be seen that the C/O and O/N atomic ratios of the
HA-AG-3LBL aerogel are very similar to the theoretical values for the
fully branched 3-mLBL structure. In comparison, the atomic ratios of the
Ca-AG-3LBL aerogel are closer to the linear 3-mLBL structure. For CaAG-3LBL the O/N ratio of 3.7 is much larger than that of theoretical
values for three linear growth bilayers, suggesting that the supporting
CNF layer is likely contributing to the detected signal. This in turn in­
dicates that the reaction between TMC and MXD is not as complete as for
the HA-AG-3LBL aerogels. The exact reason for this difference is not
known, but the Ca2+ ions may hinder the reaction between the TMC and
MXD, at least for very thin layers. A lower nitrogen content for the CaAG-3LBL may seem contradictory to the FTIR and weight

measurements; however, the minor areas of TMC and MXD polymeri­
zation observed in SEM helps to clarify this contradiction. These areas of
polymerization are expected to be the result of the reaction of unwashed
excess molecules, which lead to a higher weight increase and more
intense amide peaks in FTIR. Since XPS is a surface sensitive technique
with an analysis depth of 5–10 nm (Laine et al., 1994) the results from
XPS will be less affected by, poor washing and larger polymer aggregates
within the bulk of the aerogel. As a result, a lower concentration of N is
predicted to be due to less effective m-LBL reaction on Ca-AG surfaces.
To have a better understanding of the structure of HA-AG-3LBL,

which showed uniform layer deposition, high-resolution XPS C 1S
spectra of the HA-AG, before and after three m-LBL modification, were
collected and deconvoluted into five sub-peaks corresponding to the
chemical shifts in the carbon signals due to the different functional
groups (Fig. S1). The atomic percent of carbon linked to different
functional groups was calculated (Table 2).
Theoretically, deconvolution of C 1s spectra of pure cellulose shows
two peak at 286.7 eV (C2) and 288.3 eV (C3) corresponding to the C–O
at alcohol and ether groups and O—C—O at acetal moieties, respectively
(Khiari et al., 2017). Comparing the deconvolution results of HA-AG
with theoretical values, a decrease in C2 atomic percent and an in­
crease in C3 is observed due to the conversion of some alcohol groups to
aldehydes during the oxidation of CNFs and formation of hemiacetals
linkages during the freezing-induced crosslinking (Guigo et al., 2014).
The presence of C1b peak is attributed to the C–H and C–C groups in
the residual lignin and hemicelluloses associated with the CNFs
(Johansson & Campbell, 2004; Khiari et al., 2017; Yao et al., 2017) and
the C4 peak is attributed to the carboxylic groups of the CNFs, since we
used fibers that were subjected to a carboxymethylation pre-treatment
to facilitate fibrillation to CNFs.
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Carbohydrate Polymers 282 (2022) 119098

Fig. 5. Microstructure of Ca-AG and Ca-AG-3LBL. a,b) microstructure of Ca-AG c,d) Representative microstructure of Ca-AG-3LBL, showing neat features. e,f)
Example of random, non-representative, inhomogeneous microstructure of Ca-AG-3LBL occurring at some positions in the modified aerogel.

According to Fig. S1 and Table 2, after deposition of three m-LBL
bilayers, a significant peak at C1 appeared. This peak, which is equiv­
– C groups present in
alent to 46.7 atomic percent, is attributed to the C–
both TMC and MXD, demonstrating the high efficiency of m-LBL reac­
tion on HA-AG. Moreover, the fact that the atomic amount of C4 carbon,
corresponding to carboxylic groups, is zero shows that all the acid
chloride groups in the TMC molecules have reacted with MXD and a
fully branched structure is obtained (Scheme 1). By comparing the
percentage of C1 and C2 carbon within the HA-AG-3LBL with the
theoretical data for three fully branched m-LBL bilayers, it can be
concluded that the amount of C2‑carbon is higher while the C1 carbon is
lower for HA-AG-3LBL. This can be explained by the signals detected
from the supporting cellulose surface. For example, since there is no
– C group in cellulose, the intensity of C1 peak in the HA-AG-3LBL is
C–
lower than the theoretical value. On the other hand, the intensity of C2

–O
peak is higher than the theoretical value due to the O-C-O and C–
signals coming from the supporting dialdehyde cellulose layer.
3.4. Mechanical properties
The dry and wet mechanical properties of the aerogels before and
after m-LBL deposition was evaluated by uniaxial compression test
(Fig. 6). Similar to the mechanical properties previously reported for the
same type of aerogels (Luo et al., 2020; Wang, Li, et al., 2020), the
compressive-stress–strain curve of all the aerogels exhibited three
deformation regions. Specifically: i) a linear elastic region below 10%
strain due to the elastic bending of lamellae, ii) a relatively flat stress
plateau region (10% < strain < 60%) due to plastic deformation and

failure of the lamellae, and iii) a densification region above 60% strain
where a dramatic increase in the stress was observed due to the overall
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Carbohydrate Polymers 282 (2022) 119098

resulting in a stronger interconnected network structure. Additionally,
water is a well-known plasticizer for cellulose-based materials and thus
coverage of the surface with a more hydrophobic polyamide film re­
duces swelling and improves wet integrity. While m-LBL modified aer­
ogels show good improvement in the wet state, Fig. 6b shows large
hysteresis after m-LBL deposition indicating the mechanical failure of
the aerogels. This is potentially due to the rupture of the thin polyamide
film at high compressive strain, which allows water to enter the un­
derlying CNF network and weaken the pore walls.
Fig. 6c shows a comparison of the dry compressive modulus of HAAG-3LBL and Ca-AG-3LBL as a function of density with relevant CNFbased aerogels reported in the literature. Considering the density of
the aerogel, m-LBL modified aerogels show higher compressive modulus
compared to most chemically modified CNF, microfibrillated cellulose
(MFC) and CNC-based aerogels. Their modulus is also comparable with
several CNF-based composites. The fact that aerogels are weaker in
comparison with 3D printed or the strongest direction in directionally
freeze dried aerogels demonstrates the advantage of 3D printing and
directional freezing in design of light-weight materials. The main
advantage of m-LBL modification on the mechanical properties can be
seen in the wet state. There are only a few studies in the literature clearly
reporting the wet modulus of the light-weight cellulose-based structures
(Fig. 6d). Considering the low density of m-LBL modified aerogels pre­

pared in this work, they have greater wet stiffness compared to CNF,
CNC, and CNF-alginate aerogels. However, 3D printed light-weight
cellulose structures still show superior wet properties.
In addition to mechanical properties, the stability of aerogels under
acidic and basic aqueous solutions (pH 0–13) was tested. All the aerogels
were stable in acidic and mild basic conditions (pH 0–8.5), showing no
visual degradation during 1 week of incubation. However, in basic
conditions of above pH 10, the HA-AG disintegrated, while all the other
aerogels were stable (Fig. S2 a,b). This is due to the fact that hemiacetal
linkages break at high pHs since the equilibrium shifts toward free
aldehyde and alcohol groups (Erlandsson et al., 2018). Additionally, the
stability of aerogels was tested at extremely harsh acidic environment
(37% HCl solution) where the unmodified aerogels disintegrated within
2 min while the m-LBL modified aerogels were visually intact up to 5 h
(Fig. S3). This stability demonstrates that the molecular layers deposited
on the aerogel surfaces have covered and crosslinked the cellulose sur­
faces and prevented the hydrolysis of the CNFs by HCl. The acid/base
resistance of the m-LBL modified aerogels make them promising can­
didates for applications needing long term exposure to acid and alkalis.
The effect of m-LBL modification on thermal stability of the aerogels
was tested by TGA and is shown in Fig. S4 and discussed in detail in
Supplementary Data. Notably, while modified aerogels showed a
decrease in the onset of degradation temperature they maintained
significantly higher residual weight at 700 ◦ C. Strong interactions be­
tween deposited layer and CNF, and the high thermal stability of poly­
amides is potentially reason for the improved thermal stability of
modified aerogels at temperatures >350 ◦ C.

Table 1
Atomic composition analysis of aerogels before and after m-LBL modification by

XPS.
Substrate

C (%)

O (%)

N (%)

C/O

O/N

Cellulosea
3 LBL linear growtha
3 LBL branched
growtha
Ca-AG

54.5
73.9
78.1

45.5
17.4
9.4

0
8.6
12.5

1.20
4.3
8.3

–
2.0
0.8

55.6 ±
0.4
55.6 ±
0.4
72.2 ±
1.0
77.0 ±
0.7

43.5
0.4
43.6
0.3
19.3
1.6
10.6
1.9

±

0

±

–

±

0

±

–

±

5.2 ± 1.5

±

±

10.4 ±
0.4

1.3
0.0
1.3
0.0
3.7
0.4

7.3
1.3

3.7 ±
1.3
1.0 ±
0.2

HA-AG
Ca-AG-3LBL
HA-AG-3LBL

±

a

Theoretical calculation (assuming that unreacted acid chloride is turned to
carboxylic acid due to ambient humidity).

densification of the structure after collapse of the pores, leading to an
increased number of contact points. According to Table 3, the ultimate
strength and the compressive modulus of both aerogels in the dry state
has increased significantly after three cycles of m-LBL deposition.
However, by normalizing the data with density, it was observed that the
specific mechanical properties did not change significantly after modi­
fication. Comparing CNF films and interfacially polymerized polyamide
thin films, the specific strength and modulus of CNF films are signifi­
cantly higher in the dry state (Benítez & Walther, 2017; Roh et al.,
2002). Thus, the thin layer of polyamide on CNF-based aerogels is ex­
pected to either; i) reduce the specific mechanical properties of the

aerogel via the addition of a weaker thin film, or ii) increase the specific
mechanical properties by improving joint strength between the indi­
vidual CNFs as well as the larger lamellae. The slightly increased specific
modulus and strength of HA-AG after three bilayers indicate that
improvement of the joint strength between the structural elements is
greater than the lower intrinsic mechanical properties of the polyamide
film. This is in good agreement with the SEM images, whereby ultrathin
uniform polyamide layers were observed on the HA-AG-3LBL. On the
other hand, the specific strength of Ca-AG reduced slightly after m-LBL
deposition suggesting that improvement of CNF joint strength was less
effective. The SEM images show that in some parts of Ca-AG-3LBL thick
and inhomogeneous polymerized layers were deposited, which reduce
the specific compressive strength of the treated aerogels.
To assess the mechanical properties in the wet state, the aerogels
were soaked in water for 48 h to ensure that both unmodified and
modified aerogels were sufficiently hydrated. According to Table 3, the
wet mechanical properties were improved significantly after m-LBL
modification. Comparing Ca-AG and Ca-AG-3LBL, the specific modulus
increased 2.6 times from 0.32 kPa⋅m3/kg to 0.84 kPa⋅m3/kg. The same
trend was observed for HA-AGs where their specific modulus increased
2.5 times from 0.4 kPa⋅m3/kg to 1.0 kPa⋅m3/kg and specific ultimate
strength increased from 0.5 kPa⋅m3/kg to 1.0 kPa⋅m3/kg. This
improvement can potentially be attributed to the chemical crosslinking
of the CNFs at the surface of lamellae following m-LBL deposition,

Table 2
Atomic percent of carbon in different functional groups calculated from deconvoluted high-resolution C 1 s spectra.
C groups atomic percent and their corresponding binding energy (eV)
Sample

C tot

C1
284.7
–C
C–

C 1b
285.0
C–C, C—H

C2
286.1–286.7
C–O,
C—N

C3
287.9–288.3
–O, O=C—N
O—C—O, C–

C4
289.3
O—C=O

Cellulosea
3 m-LBL, branched growtha
HA-AG
HA-AG-3LBL

54.5
78.1
55.6 ± 0.4
77 ± 0.7

–
56.2
–
46.7 ± 1.1

–
–
2.9 ± 0.2
–

45.5
12.5
38.7 ± 0.2
22.0 ± 0.1

9.0
9.4
12.7 ± 0.1
8.3 ± 0.5

–
–
1.4 ± 0.2
–

a

Theoretical calculation.
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Carbohydrate Polymers 282 (2022) 119098

Fig. 6. Compressive stress–strain curve of HA-AG and Ca-AG before and after three bilayers of m-LBL deposition; tested in a) dry and b) wet state. c) A summary of
dry compressive modulus vs density of the modified aerogels in the present work ( ), and earlier investigations comprising MFC or CNF aerogels/foams ( )
(Buchtov´
a et al., 2019; Dilamian & Noroozi, 2021; Gonz´
alez-Ugarte et al., 2020; Li et al., 2017; Mariano et al., 2021; Verdolotti et al., 2019; Zhang et al., 2019),
directionally freeze-dried pure and hybrid CNF aerogels parallel to the freezing direction ( ) (Chen et al., 2016; Wu et al., 2020; Zhang et al., 2020; Zhang, Wang,
et al., 2020), 3D printed CNF aerogels ( ) (Jiang, Oguzlu, & Jiang, 2021; Li et al., 2018), CNF composite aerogels ( ) (Franỗon et al., 2020; Gonz´
alez-Ugarte et al.,
2020; Khlebnikov et al., 2020; Verdolotti et al., 2019; Wang et al., 2019; Wang et al., 2020; Wang, Li, et al., 2020; Zhang et al., 2016; Zhou et al., 2019), and CNCbased aerogels ( ) (Gong et al., 2019; Li et al., 2019). d) A Summary of the wet compressive modulus vs density of the modified aerogels of this work, MFC or CNF
aerogels/foams ( ) (Mariano et al., 2021; Verdolotti et al., 2019), 3D printed CNF aerogels ( ) (Jiang, Kong, et al., 2021; Li et al., 2018), CNC based aerogels ( ) (Li
et al., 2019), cellulose fiber based aerogels ( ) (Lopez Duran et al., 2018), and CNF-alginate aerogels ( ) (Franỗon et al., 2020).
Table 3
Mechanical properties of aerogels in the dry and the wet state.
Aerogel
type

Dry compressive properties
Ultimate
strength (kPa)

Elastic
modulus (kPa)

Specific
ultimate
strength
(kPa⋅m3/kg)

Specific modulus
(kPa⋅m3/kg)

Wet compressive properties
Ultimate
strength (kPa)

Elastic
modulus (kPa)

Specific ultimate
strength (kPa⋅m3/kg)

Specific Modulus
(kPa⋅m3/kg)

Ca-AG
HA-AG
Ca-AG3LBL
HA-AG3LBL

248 ± 24

268 ± 58
465 ± 23

118 ± 14
131 ± 18
258 ± 58

17.4 ± 0.7
19.3 ± 1.7
16.5 ± 0.9

8.3 ± 0.3
9.6 ± 1.1
9.2 ± 1.9

5.3 ± 0.7
5.0 ± 0.9
17.5 ± 0.6

4.2 ± 0.6
4.2 ± 0.5
21.1 ± 2.9

0.41 ± 0.06
0.47 ± 0.1
0.7 ± 0.03

0.32 ± 0.03
0.40 ± 0.05
0.84 ± 0.12

497 ± 56

229 ± 13

22.5 ± 1.9

10.4 ± 0.3

20.0 ± 1.6

20.0 ± 1.1

1.0 ± 0.1

1.00 ± 0.10

3.5. Oil absorption properties

density, porosity, and surface properties, as well as the characteristics of
the solvent including density, hydrophobicity, molecular dimension,
and surface tension (Li et al., 2014). Fig. 7b demonstrates that the ab­
sorption capacity of m-LBL aerogels is largely dependent on the density
of the solvent with hexane, which has the lowest density, absorbing the
least and xylene and toluene, which have the highest density, absorbing
the most.
HA-AG-3LBL showed higher absorption capacity (25–36 g/g)

To assess the applicability of the aerogels in applications such as
water purification, the absorption capacity of the aerogels for six

different organic solvents (toluene, hexane, hexadecane, octane, xylene,
and decane) was investigated. Fig. 7a shows that the maximum ab­
sorption capacity differs with each organic solvent. Generally, absorp­
tion capacity depends on both the characteristics of the aerogel, such as
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Carbohydrate Polymers 282 (2022) 119098

Fig. 7. a) Absorption capacity of HA-AG-3LBL and Ca-AG-3LBL for six different solvents. b) Gravimetric absorption capacity of HA-AG-3LBL as function of density of
the solvent. c) Reusability study of aerogel. Five cycles of toluene absorption and desorption in a HA-AG-3LBL; the aerogel was emptied by vacuum filtration and
reused. d) Absorption of toluene (dyed with oil red) from water surface by Ca-AG-3LBL e) Partially m-LBL modified aerogel soaked in a mixture of toluene (red) and
water (blue), demonstrating how the modified materials are capable of efficiently separating two immiscible liquids. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)

compared to Ca-AG-3LBL (16–26 g/g) due to their lower density and,
most likely, to a more open porous structure. Nonetheless, both types of
aerogels showed comparable or even higher absorption capacity than
previously reported cellulose-based adsorbents. For example,
epoxidized-soybean-oil-modified cellulose aerogels (Xu et al., 2019),
CNC/poly (vinyl alcohol) aerogels (Gong et al., 2019), styrene-acrylic-

modified cellulose aerogels (Mulyadi et al., 2016), super hydrophobic
cellulose/chitosan composite aerogels (Meng et al., 2017), cellulose
acetate monolith (Zhang, Wang, et al., 2020), and cold plasma treated
cellulose aerogels (Lin et al., 2015). However, the absorption capacity
was lower than that of those aerogels with ultralow density (3–10 mg/
cm3), such as octadecylamine coated cellulose aerogels (Gao et al.,

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Carbohydrate Polymers 282 (2022) 119098

2018), and silanized CNF aerogels (Zhou et al., 2016). While m-LBL
aerogels may not have the highest reported absorption capacity, the fact
that the aerogels in this study were prepared using an energy-efficient
method (no freeze-drying), and have significantly better mechanical
properties than most alternatives make them suitable candidates for
cyclic oil absorption applications as they are able to tolerate the vacuum
pressure needed to quickly remove absorbed liquid. Fig. 7c presents the
cyclic absorption of toluene using HA-AG-3LBL. The aerogel retained
more than 90% of its absorption capacity after five cycles, clearly
showing the reusability of the aerogels. Moreover, the absorbed solvent
can be easily collected for further uses by vacuum filtration. Absorption
of toluene (dyed red) from water surface with Ca-AG-3LBL is shown in
Fig. 7d and a video showing the fast and efficient removal of toluene
from water surface is included in Supplementary Data.
In addition to cyclability, m-LBL modified aerogels specifically
absorb non-polar solvents and can be used to separate oil-water mix­
tures. To demonstrate the separation efficiency, multi-functional sam­
ples were prepared such that the center contained m-LBL modified
aerogel and the surrounding area contained unmodified CNF-based
aerogel, and vice versa. Upon soaking the multi-functional aerogels in
a toluene-water mixture where toluene was dyed red and water dyed
blue, it can be seen that the unmodified aerogel region absorbed water,
turning blue, and the modified region absorbed the oil, turning red

(Fig. 7e). This simple demonstration shows separation effectiveness of
m-LBL modified aerogels and the potential to tune CNF-based aerogel
behavior for specific clean-up applications.

CRediT authorship contribution statement
Zhaleh Atoufi: Methodology, Formal analysis, Investigation,
Conceptualization, Writing – original draft. Michael S. Reid: Method­
ology, Investigation, Writing – review & editing. Per A. Larsson: Su­
pervision, Conceptualization, Writing – review & editing. Lars
Wågberg: Supervision, Conceptualization, Methodology, Writing – re­
view & editing, Project administration, Funding acquisition.
Declaration of competing interest
None.
Acknowledgements
We thank the Swedish Foundation for Strategic Environmental
Research (MistraTerraClean programme, Project No. 2015/31). RISE
Research Institutes of Sweden, department of material and surface
design, is thanked for performing the XPS analyses, Mikael Sundin for
help with the XPS runs and Lic. Marie Ernstsson for interpretation of
data and discussions of results.
Appendix A. Supplementary data
Thickness and roughness of m-LBL deposited layers on CNF surfaces;
Schematic chemical structure of fully branched and fully linear m-LBL
deposited layer; Deconvoluted high-resolution spectra of HA-AG and
HA-AG-3LBL; Stability of aerogels at aqueous acid and base solutions,
and their stability at extremely harsh acidic environment; Thermogra­
vimetric analysis of aerogels before and after m-LBL modification (PDF).
Supplementary data to this article can be found online at doi: https://
doi.org/10.1016/j.carbpol.2022.119098.

4. Conclusion
Surface modification of freeze-linked, CNF-based aerogels was
accomplished via a novel, efficient and precise m-LBL deposition tech­
nique. In this process, physically and chemically crosslinked CNF-based
aerogels were prepared, and modified with three cycles of m-LBL
deposition of TMC and MXD. Characterization of the deposited poly­
amide demonstrated that the layer is uniform and ultrathin, with a
thickness of approximately 1 nm. Moreover, it was observed that the
type of crosslinking within the aerogels affects the efficiency and
structure of the deposited layers, with Ca2+ and hemiacetal crosslinking
yielding linear and branched polyamide layers, respectively. Critically,
an open-porous structure within the aerogel is needed to avoid poly­
merization of unwashed excess monomers and to maintain a homoge­
neous deposition of the molecular layers. Although the deposited
coating was approximately 1 nm thin, the modified aerogels showed an
improvement in specific wet modulus and strength of up to 2.6 times, as
well as a high resistance toward acidic and alkaline conditions.
Furthermore, after modification the water contact angle increased from
55◦ to 77◦ and aerogels changed from being a hydrophilic to hydro­
phobic with high selectivity toward oils and organic solvents. Modified
aerogels showed an absorption capacity of 16–36 g/g depending on the
density of the aerogel and the density of the solvent, as well as good
cyclability, with more than 90% of their absorption capacity retained
after five cycles. Additionally, multi-functional aerogels showed good
selectivity, with modified and unmodified regions absorbing non-polar
and polar solvents, respectively, making them promising candidates
for oil-water separation. Overall, the results demonstrate that m-LBL
assembly is a potential method for depositing functional molecules on
cellulose aerogels in order to tailor the properties in a homogeneous and
in a highly controlled manner.

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Funding sources
Swedish Foundation for Strategic Environmental Research (Mis­
traTerraClean programme, Project No. 2015/31).

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