# Tuning morphology and properties of cellulose-based aerogels and xerogels by the introduction of tosyl moieties

Loris Gelas · Martin Gericke · Thomas Heinze ·
Tatiana Budtova

Received: 1 December 2024 / Accepted: 6 May 2025
© The Author(s) 2025

**Abstract** Aerogels are nanostructured materials of high porosity and high specific surface area. While the first cellulose II aerogels were made about 20 years ago, practically nothing is known about the influence of the degree of substitution of cellulose ethers and esters on aerogel properties. Model hydrophobic moieties (here, tosyl) were introduced on the cellulose backbone by homogeneous derivatisation with the goal to investigate the influence of the degree of substitution (DS) on the morphology and properties of aerogels and xerogels. The materials were

produced via the dissolution-coagulation route followed by drying with supercritical (sc) CO₂ (named aerogels) or low-vacuum evaporation (named xerogels). Reference materials were made from microcrystalline cellulose (MCC). The goal was to investigate how the introduction of a bulky tosyl group influences the self-assembly of polymer chains during non-solvent-induced phase separation, and to assess the impact of the DS on aerogel and xerogel properties. The DS of tosyl cellulose was varied from 0.22 to 1.26, and different non-solvents (water, ethanol, acetone) were used to test their influence on the materials' properties. Aerogel densities ranged from 0.04 to 0.28 g/cm³, while xerogel densities were between 0.09 and 1.52 g/cm³. Drying only with sc CO₂ resulted in materials with high specific surface area, up to 360 m²/g. The increase in DS resulted in materials with a coarser morphology and a significant rise in hydrophobicity, as evidenced by a water contact angle reaching 130°.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10570-025-06585-w.

L. Gelas · T. Budtova (✉)
Mines Paris, Center for Materials Forming (CEMEF),
UMR CNRS 7635, CS 10207, PSL University,
06904 Sophia Antipolis, France
e-mail: Tatiana.budtova@minesparis.psl.eu

M. Gericke · T. Heinze
Institute of Organic Chemistry and Macromolecular Chemistry, Centre of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraße 10, 07743 Jena, Germany

M. Gericke
Robert Boyle Institute e. V., Im Steinfelde 10, 07751 Jena,
Germany

T. Heinze
Posanova GmbH, Humboldtstraße 29, 99523 Weimar,
Germany

**Keywords** Aerogels · Tosylation · Density · Surface area · Supercritical drying · Evaporative drying

## Introduction

In the early twenty-first century, a new class of bio-based materials, bio-aerogels, emerged: they possess characteristics of traditional inorganic and synthetic polymer aerogels, such as low density, high specific
surface area and nanostructure (Aegerter et al. 2023), but they are derived from renewable resources, e.g. polysaccharides. This makes bio-aerogels suitable for a diverse range of life-science applications, as drug delivery systems (Zou and Budtova 2023), tissue engineering (Quraishi et al. 2015) and wound healing (Rostamitabar et al. 2022), and also for adsorption and absorption of chemicals or fluids (Fan et al. 2021; Nguyen et al. 2013), in electrochemical devices (Thomas et al. 2021), for catalysis (Maldonado-Hòdar et al. 2000) and thermal insulation (Zou and Budtova 2021).

Opposite to classical inorganic or synthetic polymer aerogels which synthesis is based on sol–gel chemistry with the “building blocks” being colloidal particles or monomers, bio-aerogels are derived from polymers or materials produced by Nature: either via polymer dissolution (Buchtová and Budtova 2016; Druel and Budtova 2023), or from a suspension of nanoparticles (nanocellulose or nanochitin) (Jiang and Hsieh 2017; Korhonen et al. 2011), or by the top-down processing of wood (Garemark et al. 2020; Wang et al. 2023). In the first two cases a network is formed via gelation or non-solvent induced phase separation (coagulation). Drying is usually performed with supercritical (sc) CO₂ as it is the best way to preserve network morphology from pores’ collapse occurring upon evaporative drying. Few examples are known where aerogels, i.e. materials of low density and high surface area, were made using freeze-drying or low-vacuum evaporative drying (Borisova et al. 2015; Druel and Budtova 2023; Wang et al. 2023; Gelas and Budtova 2024; Zou and Budtova 2023). In the following, only cellulose aerogels produced by dissolution-coagulation pathway will be considered being in the scope of this work.

While the first cellulose II aerogels were made about 20 years ago, and they were produced by dissolution-coagulation-supercritical CO₂ drying pathway (Cai et al. 2008; Gavillon and Budtova 2008), surprisingly very few studies have reported on aerogels based on cellulose ethers or esters. Methyl cellulose aerogels (DS and molecular weight not reported, density 0.18 g/cm³ and surface area 270 m²/g) were suggested for bio-medical applications (Iglesias-Mejuto et al. 2024). Carboxymethyl cellulose (DS 0.7 and 0.9, molecular weight (1.0–2.8)×10⁵ g/mol) aerogels of density 0.06–0.25 g/cm³ and surface area 30–150 m²/g were explored as drug delivery devices (Yu and

Budtova 2024). Cellulose acetate aerogels (DS 2.4, molecular weight 3×10⁴ and 5×10⁴ g/mol) were obtained by dissolution in acetone and crosslinking with polymethylene polyphenylpolyisocyanate; the density was 0.25–0.75 g/cm³ and surface area around 140–250 m²/g (Fischer et al. 2006). Cellulose laurates were synthesised to make aerogels via tert-butanol sublimation or sc drying, specific surface area was around 100–150 m²/g and materials were used for the absorption of pyridine and chlorobenzene from wastewater (Wang et al. 2017). Zhang et al. utilized cellulose glycerol ether to prepare aerogels with densities below 0.1 g/cm³ and surface area of 180 m²/g; the morphology was tuned by solvent mixture composition (Zhang et al. 2015). Hydrophobic trityl cellulose aerogels were of density around 0.08 g/cm³ and surface area ranging from 250 to 350 m²/g (Demile-camps et al. 2016); the same formulations were also dried by ambient pressure evaporative drying and resulted in materials of low density (0.1–0.2 g/cm³), high water contact angle of 140° but low specific surface area (Pour et al. 2015). Practically nothing is known on the influence of the degree of substitution (DS) on the morphology and properties of cellulose-based aerogels made by dissolution-coagulation-sc CO₂ drying route. The use of cellulose derivatives as a starting polymer may significantly alter the arrangement of the polymer chains during the exposure to non-solvents, which in turn may influence aerogel final morphology and properties.

The objective of this work was to introduce bulky hydrophobic tosyl moieties onto cellulose backbone in order to investigate the influence of the DS on the morphology and properties of aerogels produced by the dissolution-coagulation route. Tosylation was chosen as under homogeneous conditions this reaction enables direct and precise control over the DS, in particular, at low DS values (≤1.5). The latter were targeted to study how and if a steady increase in tosyl groups on cellulose chain influences the self-assembly of the polymer during non-solvent-induced phase separation, which was the first step in aerogels’ preparation. Tosylation reaction is regioselective and is known to occur on C6 position. Another question was if tosyl groups may prevent pores’ collapse during evaporative drying, as in the case of ambient pressure dried silylated silica aerogels (Schwertfeger et al. 1998). In terms of future follow-up studies, the tosylated aerogels can be further functionalized by
selective methods to tune their properties as the tosyl moiety is reactive towards nucleophilic compounds such as azides and amines. Also, the tosyl moieties are stable under moderate acidic and basic aqueous conditions, which may be important for potential application of the aerogel materials.

Tosyl cellulose (TOSC) with different DS values was synthesised and used to produce cellulose-based aerogels via drying with sc CO₂, and so-called xerogels via low-vacuum evaporative drying. Microcrys-talline cellulose was used as reference. The influence of the DS on the materials’ density, specific surface area, morphology and water contact angle were inves-tigated and discussed.

## Materials

Microcrystalline cellulose (MCC, particle average size 50 µm, degree of polymerization 280 (Gelas and Budtova 2024)), N,N-dimethylacetamide (DMA, 99.8%), dimethyl sulfoxide (DMSO, 99+%), lithium chloride, triethylamine, ethanol (99+%) and acetone (99%) were purchased from ThermoFisher Scien-tific. 4-Toluenesulfonyl chloride was obtained from Sigma-Aldrich. 1-Ethyl-3-methylimidazolium acetate (EMIMAc) was from Iolitec, and deuterated dime-thyl sulfoxide (DMSO-d₆) was from Deutero GmbH. Water was distilled using Direct-Q® 3UV. Lithium chloride was dried at 150 °C under vacuum for one day prior use.

## Methods

### Synthesis of tosyl cellulose (TOSC)

Cellulose tosylation was chosen because the reac-tion is well established and regioselective (McCor-mick et al. 1990; Rahn et al. 1996; Klemm et al. 1998; Koschella et al. 2011; Gericke et al. 2012). The homogeneous synthesis of cellulose p-toluene-sulfonic acid esters (tosyl cellulose or TOSC in the following) was performed following the procedure described previously (Gericke et al. 2012), see Fig-ure S1 in the Supporting Information. Briefly, MCC was dispersed in anhydrous DMA at 130 °C for 2 h. The mixture was then cooled to 100 °C, and 7 molar equivalents of LiCl per anhydroglucose unit

were added. The mixture was stirred overnight and allowed to cool to 25° C; cellulose was completely dissolved. The tosylation reaction was carried out at 25 °C by the dropwise addition of triethylamine, fol-lowed by the addition of different amounts of p-tolu-esulfonyl chloride (see Table S1 in the Supporting Information). The solution was allowed to react for 3.5 h at 25 °C. The reaction products were obtained by precipitation in ethanol and filtration. They were washed three times with ethanol and water each and dried under vacuum for 48 h at 60 °C. The resulting white powders were characterized by NMR spectros-copy and elemental analysis (see “Methods” section). It should be noted that tosylation in DMA/LiCl was chosen because it enables an efficient control over the DS values, which was the primary focus of this work. In the context of application of the aerogels, in particular, in biomedical areas, synthesis of TOSC in more environmentally friendly systems such as aqueous NaOH or ionic liquids might be more suited (Schmidt et al. 2014; Gericke et al. 2012).

### Preparation of xerogels and aerogels

TOSC was dissolved in DMSO. To make refer-ence samples, MCC was dissolved in a mixture of EMIMAc:DMSO=1:1 v/v. Polymer concentration in all solutions was adjusted in such a way that the amount of polysaccharide repeating units was iden-tical (0.204 mol, Table 1). Two grams of each solu-tion were poured into cylindrical molds (diameter: 20 mm) and 10 mL of non-solvent were added on the solution surface. Several cellulose non-solvents, all miscible with DMSO and EMIMAc, were selected: ethanol, water and acetone. Non-solvent and solvent were allowed to diffuse in and out of the samples, respectively, triggering the coagulation of cellulose or TOSC. After 24 h storage at room temperature, the liquid above cellulose solution was removed and replaced with 10 mL of a fresh non-solvent. This pro-cess was repeated four times. Solvent exchange was performed every 24 h for 5 days. Samples coagulated in water were split into two groups, with one being further exchanged with ethanol following the same procedure. The preparation pathways are summarized in Fig. 1.

Two different methods were applied to dry the coagulated cellulose and TOSC: with sc CO₂ (“aerogels”) and under low vacuum by evaporation
<table><caption>**Table 1** Overview of the formulations prepared from MCC and TOSC of different DS</caption><thead><tr><th>Starting cellulose matter</th><th>Molar mass<sub>repeating unit</sub> (g/mol)</th><th>Concentration (mol<sub>repeating unit</sub>/L)</th><th>Mass concentration (wt%)</th><th>Solvent</th></tr></thead><tbody><tr><td>MCC</td><td>162</td><td>0.204</td><td>3.00</td><td>EMIMAc/<br>DMSO<br>(1:1)</td></tr><tr><td>TOSC 0.22</td><td>196</td><td>0.204</td><td>3.63</td><td>DMSO</td></tr><tr><td>TOSC 0.56</td><td>250</td><td>0.204</td><td>4.63</td><td>DMSO</td></tr><tr><td>TOSC 1.26</td><td>358</td><td>0.204</td><td>6.63</td><td>DMSO</td></tr></tbody></table>

<figure><img src="image_1.png" /><figcaption>**Fig. 1** Scheme of the preparation of aerogels and xerogels from microcrystalline cellulose (MCC) or tosyl cellulose (TOSC), coagulated in ethanol, acetone or water, and dried either by evaporation ("xerogels") or with sc CO₂ ("aerogels")</figcaption></figure>

("xerogels"), see Fig. 1. To use drying with sc CO₂, alcogels and organogels were placed in tea bags, put into a 2 L autoclave (center PERSEE, Mines Paris), and covered with the same non-solvent to prevent drying during system pressurization. The autoclave was then filled with CO₂, pressurized to 5 MPa at 37 °C, and the excess non-solvent was drained. Subsequently, the autoclave was pressurized to 8 MPa to reach the supercritical condition of carbon dioxide, and the samples were dynamically washed with sc CO₂ for 4 h with an output of 5 kg CO₂ per hour. Depressurization was carried out at a rate of 0.5 MPa per hour.

Hydrogels, alcogels and organogels were also dried by evaporation of the non-solvent at 50 °C under low vacuum (5 kPa) using Heraeus oven until constant mass, resulting in "xerogels" (Fig. 1). All dry

samples, aerogels and xerogels, were stored in a desiccator containing silica gel to prevent water uptake due to ambient humidity.

## Characterization techniques

NMR spectra of TOSC samples were recorded at 25 °C using a Bruker Avance 250 MHz spectrometer (Billerica, MA, USA) at TOSC concentration of 50 mg/mL, with deuterated DMSO-d₆ as the solvent (Figures S2, S3). Elemental analysis of TOSC was performed using a VARIO EL III CHNS analyzer (Elementaranalysensysteme GmbH, Langenselbold, Germany), and the data were employed to determine the DS with tosyl groups and deoxy-chloro groups according to literature (see Formulas S1 and S2 in the Supporting Information).
Volume shrinkage of samples during each processing step, i.e. during coagulation, during second solvent exchange (if any) and during drying was determined as follows:

$$ \text{Shrinkage} = \frac{V_1 - V_2}{V_1} \qquad (1) $$

where $V_1$ corresponds to the volume of solution or before drying (after solvent exchange) and $V_2$ to the volume after solvent exchange or drying, respectively. The volume after solvent exchange (hydrogels, alcohols and organogels, see Fig. 1) was calculated from dimensions of the samples in the shape of disks using calliper (±10 µm). The volume of dry materials was measured using Geopyc 1360 Envelope Density Analyzer (Micromeritics) with DryFlo powder, with chamber of 19.1 mm diameter and applied force of 19 N.

The density of dry materials was determined as a ratio of sample mass, $m$, measured with high precision balance (±0.01 mg), and sample volume, using Geopyc 1360 Envelope Density Analyzer as described above.

The specific surface area was determined by nitrogen adsorption using an ASAP2020 (Micromeritics), applying the Brunauer-Emmet-Teller (BET) method. Prior to measurement, samples were degassed under vacuum (1 Pa) at 70 °C for 10 h. For materials with specific surface area above some tens m²/g, around 0.2–0.3 g were used to obtain reliable values. Samples with surface area below 5 m²/g were considered as materials with negligible surface area. It should be noted that it is not possible to apply standard approaches (Barrett, Joyner and Halenda, BJH) using nitrogen adsorption for the determination of pore size distribution in bio-aerogels (Groult and Budtova 2018; Négrier et al. 2023; Robitzer et al. 2011; Rudaz et al. 2014). BJH approach considers pores sizes in the mesopores and small macropores region, which takes in account only around 10–20% of the total pore volume (Groult and Budtova 2018; Négrier et al. 2023; Robitzer et al. 2011; Rudaz et al. 2014).

The morphology of cellulose materials was analyzed using a Scanning Electron Microscope (SEM) MAIA (Tescan), equipped with a Field Emission Gun. The samples were sectioned and mounted on sample holders using carbon suspension. The exposed cross-sections were coated with a 14 nm layer of

platinum using a Q150T Quorum metallizer. The imaging was performed at an acceleration voltage of 3 kV.

Contact angles with water were recorded using a Krüss DropAnalyser DSA 100 goniometer (Kruss) at 20 °C with a 2 µL droplet. The samples were cut to access the inner morphology, and the droplet was deposited on a macroscopically flat surface; contact angles were measured after the drop shape had equilibrated. To minimize the influence of the sample texture, water contact angles were measured with films that were prepared as follows. Solutions of TOSC and of MCC (all at 7 wt%) were prepared in DMSO or EMIMAc/DMSO, respectively, and cast in Petri dish. The solutions were coagulated in ethanol and subjected to solvent exchange until complete removal of the solvent, followed by drying at ambient pressure and temperature under addition of few milliliters of excess ethanol.

## Results and discussion

### Synthesis and characterization of tosyl cellulose

The goal of this work was to study the effect of the increasing degree of substitution (DS) of tosyl cellulose (TOSC) on the properties of aerogels and xerogels. The distribution of the substituting moieties along the polymer chain can have a strong impact on the solubility of the polysaccharide derivatives. TOSC was synthesised under the homogeneous conditions in order to ensure an even distribution of the substituting groups (Gericke et al. 2012). The DS was gradually increased from 0.22 (TOSC 0.22) to 0.56 (TOSC 0.56) and then to 1.26 (TOSC 1.26) by variation of the reaction conditions (see Table S1). All samples were fully characterized by ¹H- (Figure S2) and ¹³C-NMR spectroscopy (Figure S3) as well as by elemental analysis. The latter was used to calculate the DS values (Eq. S1 and S2). All samples featured a small amount of deoxy-chloro moieties that were introduced into the polymer backbone by nucleophilic displacement side reactions (Table S1). These DS<sub>Cl</sub> values were small (0.02–0.07) in comparison to the DS<sub>tosyl</sub> values (0.22–1.26), and, consequently, were not considered in the subsequent discussions. All samples, including TOSC 0.22 (which had a very low DS), were fully soluble in DMSO.
The distribution of hydrophobic tosyl groups within the repeating unit is another relevant factor that has to be considered. It can potentially impact the interactions between polymer chains, which in turn can influence the solubility as well as the supramolecular organization and “packing” of polymer chains during coagulation. As expected, the TOSC showed a regioselective substitution pattern with a predominant functionalization of the primary position C-6. The corresponding ¹³C-NMR spectra (Figure S3) shows that the intensity of the peak associated with an unmodified position C-6 (around 60 ppm) decreased gradually with increasing DS<sub>tosyl</sub>. At the same time, a new peak emerged (around 68 ppm) that was assigned to a C-6 position that was modified with a tosyl moiety (Gericke et al. 2012).

Visual appearance, shrinkage during processing and density of tosyl cellulose aerogels and xerogels

All synthesized TOSC were fully soluble in DMSO at the concentration used (Table 1). To keep the chemical environment as close as possible to TOSC solvent, MCC was dissolved in EMIMAc:DMSO 50:50 mixture; at this composition MCC is fully soluble up to 15 wt% (Le et al. 2014).

TOSC at all DS used in this work and MCC are insoluble in ethanol, water and acetone; the addition of these non-solvents resulted in polymer coagulation.

After solvent exchange, hydrogels (non-solvent: water), alcogels (non-solvent: ethanol), or organogels (non-solvent: acetone) were formed (see “Methods” and Fig. 1). The diffusion of the non-solvent from the top of the solution induced phase separation into polymer-rich and polymer-lean phases. Above the overlap concentration (around 1 wt% for MCC dissolved in EMIMAc (Gericke et al. 2009)), the polymers formed a continuous network with non-solvent in the pores. Coagulated samples were dried either with sc CO₂ (named aerogels) or by evaporation under low vacuum (named xerogels), see details in “Methods” section.

All aerogels prepared by coagulation in ethanol largely preserved a disk-like shape (Fig. 2). Aerogels derived from dissolved MCC coagulated in ethanol exhibited better shape retention compared to TOSC materials. The latter tended to flatten as the DS increased. For aerogels produced by coagulation in water, followed by exchange to ethanol, some fractures were observed (Fig. 2). These fractures were not a result of the drying process but rather due to the adhesion of coagulated TOSC samples to the polypropylene mould, complicating sample handling. Aerogels made by coagulation in acetone displayed well-defined disk shapes at intermediate DS values (0.22 and 0.56). However, for native cellulose and TOSC with a DS of 1.26, sample warping occurred, resulting in irregular shapes. Some yellowish colour

<table><caption>**Fig. 2** Visual appearance of aerogels and xerogels prepared from MCC and tosyl cellulose of different DS and by different coagulation pathways. The scale is the same for all images</caption><thead><tr><th rowspan="2">DS<br>Non-Solvent</th><th colspan="4">Aerogels</th><th colspan="4">Xerogels</th></tr><tr><th>0</th><th>0.22</th><th>0.56</th><th>1.26</th><th>0</th><th>0.22</th><th>0.56</th><th>1.26</th></tr></thead><tbody><tr><th>Ethanol</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1 cm</td></tr><tr><th>Acetone</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>H<sub>2</sub>O</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>H<sub>2</sub>O -<br>Ethanol</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
can be attributed to the formation of chromophores; if being polar, they are not washed out when using acetone as non-solvent compared to ethanol or water (Ostonen et al. 2016; Parviainen et al. 2015; Sun et al. 2009).

Macroscopic observations of samples dried by evaporation revealed significant differences depending on the DS (Fig. 2). MCC- and TOSC 0.22 based samples tended to collapse independent of the non-solvent used for coagulation, producing non-porous, sometimes translucent materials. Xerogels made from TOSC of high DS coagulated in ethanol preserved disk shape, while when coagulated in water or acetone warped or broke. When high-DS TOSC was coagulated in water, water exchanged to ethanol and drying performed from ethanol, xerogels were rather porous but of irregular shape.

Shrinkage occurring during each processing step (solvent exchange, drying) and the total shrinkage (from solution to dry sample) was systematically studied (Fig. 3). As a general trend, shrinkage during solvent exchange increased with increasing DS (Fig. 3A). The only exception was coagulation of MCC-based solution in acetone. Here, the samples were distorted, and shrinkage was more pronounced compared to TOSC 0.22 and similar to TOSC 0.56.

A similar phenomenon was documented for films made from cellulose-EMIMAc solutions coagulated in acetone (Tiihonen et al. 2024). The increasing shrinkage with the increasing DS can be attributed to several factors. It is suggested that coagulation of cellulose and cellulose derivatives is, in part, driven by the re-formation of intra- and intermolecular hydrogen bonds that were disrupted during the dissolution (Lindman et al. 2017; Medronho and Lindman 2015). From this perspective, the conversion of the hydroxy group on C-6 position of the anhydroglucose unit, which can form both intra- and intermolecular hydrogen bonds, is expected to significantly impact the aggregation of TOSC chains upon the addition of a non-solvent. Lower number of hydroxy groups resulted in weaker stabilization of the chains upon the addition of non-solvent: a weak network was formed from collapsed chains leading to stronger shrinkage.

The type of non-solvent influenced shrinkage (Fig. 3). For MCC dissolved in EMIMAc:DMSO the shrinkage during coagulation (Fig. 3A) was the lowest in ethanol (13%) followed by water (23%) and was the highest in acetone (73%). For TOSC shrinkage followed the same order: in water (from 23 to 52%) < in ethanol (from 46 to 75%) < in acetone (from 48 to 87%). This difference was less pronounced

<figure><img src="image_2.png" /><figcaption>**Fig. 3** Shrinkage during solvent exchange (A) and total shrinkage after drying (B), the latter either with supercritical CO₂ (in yellow, aerogels) or by low-vacuum evaporation</figcaption></figure>

(in green, xerogels), as a function of the degree of substitution with tosyl groups as well as for different solvent-exchange routes
for low DS (0.22) and it increased with the increasing tosyl content. The second solvent exchange from water to ethanol did not show any significant impact on sample volume.

Drying also induced some shrinkage (Fig. 3B), even if performed in supercritical conditions: it is within 5–25%, leading, nevertheless, to a good preservation of the network structure. Theoretically, no meniscus and thus no capillary pressure (Eq. 2) develops in the supercritical state:

$$P = \frac{2\gamma}{r_m} = \frac{2\gamma\cos\theta}{r_p} \quad (2)$$

where $\gamma$ is the surface tension between the vapor and the liquid, $r_m$ the radius of the meniscus curvature, $\theta$ the contact angle between the solid and the liquid and $r_p$ the pore radius. Certain shrinkage during drying with sc CO₂ occurs due to a strong difference in the polarity of cellulose-based materials and CO₂.

For samples dried by evaporation in low vacuum, the lower the shrinkage during solvent exchange, the higher the shrinkage during drying. Samples with low initial shrinkage (below 50%, cases of low and zero DS) exhibited extensive shrinkage during evaporative drying (Fig. 3) with total shrinkage exceeding 95%. Conversely, samples with higher shrinkage during coagulation (above 50%, cases of high DS) tend to show reduced shrinkage during evaporative drying. Several factors can account for this phenomenon. Samples that undergo significant initial shrinkage are more likely to have thicker pore walls due to polymer aggregation, resulting in a less refined network with larger pores, and, consequently, lower capillary pressure (Eq. 2) during non-solvent evaporation. This effect is combined with network increased mechanical properties due to the higher density, which also allows the material to better withstand capillary pressure.

The density of all prepared samples is presented in Fig. 4. For aerogels it ranged from 0.04 to 0.28 g/cm³ and for xerogels from 0.09 to 1.52 g/cm³. Density followed the same trend as shrinkage (Fig. 3). Aerogels made from MCC and low-DS TOSC exhibited lower densities compared to xerogels derived from the same precursors, as expected considering capillary pressure (Eq. 2) rising during evaporation. However, the density of aerogels and xerogels based on TOSC of higher DS, 0.56 and 1.26, was practically the same as

<figure><img src="image_3.png" /><figcaption>Fig. 4 Density of aerogels (in yellow) and xerogels (in green) as a function of DS for different non-solvents</figcaption></figure>

was the shrinkage (see the discussion above). Except for coagulation in acetone in the case of MCC, higher DS resulted in higher density of the aerogels. This was expected as higher polymer concentration in solution results in higher aerogel density (Buchtová and Budtova 2016; Budtova 2019). Higher DS also led to higher shrinkage (Fig. 3), presumably because a lower number of hydroxy groups was available to form hydrogen bonds that can stabilize the polymer chains in a network upon coagulation.

Morphology and specific surface area of aerogels and xerogels

The internal morphology of the materials was examined by SEM (Fig. 5). Aerogels’ morphology was strongly dependent on the DS. Aerogels derived from MCC and TOSC 0.22, coagulated in ethanol or in water, possessed a fine network with pore sizes ranging from tens to hundreds of nanometres (Fig. 5A). The morphology of MCC-based aerogels consisted of a network of sub-micron "hairy beads," as reported previously for cellulose aerogels made from cellulose dissolved in ionic liquids (Buchtová and Budtova 2016; Demilecamps et al. 2015; Négrier et al. 2023; Pircher et al. 2016; Sescousse et al. 2011). When coagulated in acetone, MCC-based aerogel
<figure><img src="image_4.png" /><figcaption>**Fig. 5** SEM images of aerogels (A) and xerogels (B) made from MCC and tosyl celluloses of different DS and by different coagulation pathways</figcaption></figure>
morphology was very rough with large channels, resulting in macroscopically distorted shape (Fig. 2). As the DS increased, the size of aggregates and pores expanded across all non-solvents used, ranging from few to hundreds of microns. Overall, an increase in DS led to a coarser morphology with larger aggregates and pores. Coagulation in acetone resulted in heterogeneous morphology and shape of the materials. With the increase of DS, less hydrogen bonds (compared to MCC) were available to form polymer network; consequently, the network was formed of collapsed polymer.

Drying with sc CO₂ preserves the morphology of the “wet” network (before drying) reasonably well due to the absence of the capillary pressure. Thus, we hypothesize that it is possible to predict xerogels’ morphology from that of the aerogel counterpart. For example, the morphology of aerogels derived from DS 0 and 0.22 (coagulation in ethanol) is with fine pore walls and small pores; capillary pressure (Eq. 2) generated during evaporative drying should be high leading to high shrinkage and pores closure. This was indeed the case of xerogels made from MCC and low DS TOSC (Fig. 5B): the coagulated polymer network collapsed during evaporative drying resulting in xerogels with nearly non-porous morphology (Fig. 5B) and high density (Fig. 3). Conversely, aerogels with coarser morphologies, i.e. with thicker pore walls and larger pores (and thus the same of its “wet” counterpart) should experience lower capillary pressures, making the network more resistant to shrinkage during evaporative drying. This was the case for xerogels obtained from high DS TOSC (Fig. 5B): these xerogels exhibited coarse morphology similar to their aerogel counterparts.

The specific surface area was characterized using the BET approach through nitrogen adsorption. The results, shown in Fig. 6, revealed that the specific surface area ranged from “zero” (non-measurable value) to 360 m²/g. Only aerogels possessed specific surface areas above 100 m²/g.

For aerogels based on dissolved MCC, the specific surface area was around 340 m²/g when coagulated in ethanol and was similar, around 330 m²/g, when coagulated in water-exchanged-to ethanol (Fig. 6). Similar values were reported in literature for cellulose aerogels made by dissolution in ionic liquids (Buchtová and Budtova 2016; Négrier et al. 2023). The specific surface area of the MCC-based aerogels

prepared using EMIMAc/DMSO as cellulose solvent did not depend on the type of non-solvent or the non-solvents' sequence. This result is different from aerogels made using aqueous NaOH as cellulose solvent (Druel and Budtova 2023; Gelas and Budtova 2024). In the latter case, coagulation in water yielded nearly twice higher surface area compared to coagulation in ethanol. A strong influence of the type of non-solvent on aerogel morphology was also recorded when acetone was used (Fig. 6). The specific surface area of this aerogel is negligible, as expected from shrinkage and morphology data (Figs. 3, 5). This particular influence of acetone as non-solvent on cellulose dissolved in EMIMAc was interpreted by a formation of an “asymmetric structure” consisting of alternating densely packed and macroporous cellulose (Tiihonen et al. 2024). The combination of cellulose solvent and non-solvent sequence determines the morphology of coagulated cellulose materials, and altering any component can lead to different morphologies, which in turn will influence the properties of the dry materials. The miscibility of solvent and non-solvent is also known to play an important role on the structure of the coagulated network (Guillen et al. 2011): cellulose chains packing in crystalline or amorphous matter depend on the non-solvent (Isobe et al. 2011; Yamane et al. 2006), and a potential formation of

<figure><img src="image_5.png" /><figcaption>**Fig. 6** Specific surface area (SSA) of aerogels (in yellow) and xerogels (in green) as a function of DS for different non-solvents</figcaption></figure>
“spacers” (Druel and Budtova 2023; Gunnarsson et al. 2017) should also be taken into account.

The specific surface area of aerogels derived from TOSC 0.22 was slightly higher than that of aerogels made from MCC, with values of around 360 m²/g when coagulated in ethanol and 270 m²/g when coagulated in a water-exchanged-to-ethanol. When coagulated in acetone, TOSC 0.22 aerogel showed a surprisingly high specific surface area of 270 m²/g. A low number of substituted groups may change the way of chains packing. As the tosyl content increases, coarser network was formed (Fig. 5), leading to a marked decrease in the specific surface area of aerogels, all exhibiting values below 100 m²/g. As mentioned above, tosyl groups may prevent the formation of hydrogen bonds between cellulose chains leading to a weak network made of collapsed polymer chains. We speculate that an optimal value of “enough” hydrogen bonds is needed to stabilise cellulose-based network with few bulky hydrophobic groups acting as “spacers” not allowing cellulose chains to tightly pack and thus increasing specific surface area.

Xerogels' surface areas were all below 25 m²/g. These values can be explained by the large macropores formed upon solvent exchange and/or pore closure upon evaporative drying. Overall, specific surface area correlates well with the morphologies of the aerogels and xerogels.

## Aerogel and xerogel water contact angles

To quantify the impact of tosylation on material hydrophilicity, contact angle with water was evaluated for the aerogels and xerogels. In addition, TOSC films were prepared to evaluate the influence of the surface roughness of the gel samples (see “Meth-ods”). The results are presented in Fig. 7. Some formulations are not shown (MCC-Acetone-Xerogel, MCC-Acetone-Aerogel, TOSC 0.22-Water-Ethanol-Xerogel, TOSC 0.22-Ethanol-Xerogel, TOSC 0.22-Acetone-Xerogel, TOSC 1.26-Acetone-Aerogel and TOSC 1.26-Acetone-Xerogel) as these samples exhibited cracks or warping making contact angle determination impossible.

The water contact angles of the films increased with the DS, ranging from 43° for unmodified cellulose to 114° for TOSC-based films with a DS = 1.26 (Fig. 7, solid line). The impact of tosylation at a low DS 0.22 was minimal, with a contact angle of 48°;

<figure><img src="image_6.png" /><figcaption>**Fig. 7** Contact angle with water measured for the films (black line) and for aerogels and xerogels. Non-solvent and non-solvent sequence (if any) are indicated for each sample</figcaption></figure>

it became significant at a DS = 0.56, reaching 108°. The increase in contact angle did not linearly correlate with the DS value: a sharp transition from hydrophilic to hydrophobic behavior was observed between a DS of 0.22 and 0.56 (Figure S4).

In the case of porous materials, porosity and surface roughness can significantly impact the contact angle. It is generally accepted that roughness decreases low contact angles and increases high ones (Ryan and Poduska 2008). The threshold is typically set at a contact angle of 90° according to Wenzel equation (Ryan and Poduska 2008; Wenzel 1936). For the aerogels derived from MCC and TOSC 0.22 (density below 0.1 g/cm³), no measurements were possible as the water droplet was immediately absorbed by these porous materials. This is consistent with the claim regarding the impact of texture for materials exhibiting contact angle below 90°: at low DS, porosity tends to reduce the contact angle. Low contact angles were recorded for the xerogels obtained from MCC when coagulated in ethanol (Ethanol-Xerogel), and when coagulated in water-exchanged-to ethanol, both dried from ethanol (Water-Ethanol-Xerogel). The values were the same as for films made with the same processing steps, as expected. For TOSC 0.22 based materials, drop deposition onto the aerogels led to instant absorption, and in the case of xerogels the surface
was not suitable for contact angle measurements. At DS = 0.56, the contact angles of the porous materials (aerogels and xerogels, density below 0.2 g/cm³, coagulation in ethanol or in acetone) were 126°–133°, which is higher than those observed for films, 108° (Fig. 7). Here, porosity increased the contact angle. Further increase of the DS to 1.26 led to materials with contact angles between 77° and 92°. This lower contact angle of aerogels and xerogels derived from TOSC 1.26 compared to TOSC 0.56 is due to large pores in the former, ranging from tens to hundreds of microns (Fig. 5). When the pores sizes become comparable to the droplet sizes, fluid penetration into the pore occurs leading to the decrease of contact angles (Choi and Liang 2016).

It is important to note that the type of the first non-solvent used for coagulation also played an important role in the surface properties of the materials. Water droplets were immediately absorbed by TOSC 0.56 aerogels and xerogels when made by coagulation in water; this was likely due to the increase of hydrophilicity of the polymer chain. We assume that the tosyl groups are more likely exposed at cellulose surface when coagulation was performed in ethanol or acetone. On the opposite, hydrophilic hydroxy groups were more likely to appear on the chain surface when coagulation was performed in water. The contact angle of non-porous MCC-based xerogel was around 42° when cellulose was coagulated in ethanol compared to around 0°–10° when cellulose was coagulated in water. A favourable exposure of the cellulose hydrophilic plane when coagulated in water has already been reported for regenerated cellulose (Yamane et al. 2006).

To visualize the impact of cellulose tosylation on materials hydrophobicity, coloured water and oil droplets were deposited on aerogels derived from MCC and TOSC 0.56, both samples were made by coagulation in ethanol (Fig. 8). In the case of MCC-based aerogel, both oil and water were absorbed, similar to previous reports on cellulose aerogels and cryogels (Ciuffarin et al. 2023). Moreover, the water droplet induced a significant deformation of the materials. For aerogels based on TOSC 0.56, the oil drop was absorbed while the water drop remained on the surface showing high hydrophobicity, as expected from the contact angle measurements (Fig. 7).

<figure><img src="image_7.png" /><figcaption>**Fig. 8** Comparison of oil and water absorption by aerogels made from non-modified and tosyl cellulose TOSC 0.56, both coagulated using ethanol and dried using supercritical CO₂. Water was dyed with methylene blue and silicon oil was dyed with oil red</figcaption></figure>

## Conclusions

This work studied how the chemical modification of cellulose with hydrophobic tosyl groups affects the formation and final properties of aerogels and xerogels. For this purpose, cellulose tosylation in homogeneous conditions was performed; the degrees of substitution (DS) were 0.22, 0.56 and 1.26 and microcrystalline cellulose was as a reference.

The influence of the DS on materials’ shrinkage during processing, and on materials’ density, surface area, morphology and water contact angle was studied comprehensively. Shrinkage of samples during each processing step was monitored. Higher DS increased shrinkage during solvent exchange, which was interpreted by lower number of hydroxyl groups available to rebuild hydrogen bonds upon cellulose coagulation and stabilise the network. Although shrinkage during solvent exchange was greater at high DS, this led to reduced shrinkage during evaporative drying, ultimately resulting in lower total shrinkage. Larger pores and thicker pore walls enabled these structures to withstand capillary pressure more effectively. Thus, several high-DS tosyl cellulose aerogels were of low density, around 0.1–0.2 g/cm³.

High specific surface area was obtained only for materials dried with supercritical CO₂ and of low DS. Microcrystalline- and low-DS tosyl cellulose-based aerogels exhibited fine morphology and high surface area (250–350 m²/g); higher DS led to coarser morphology and lower surface area. Increasing DS
resulted in significative increase in materials’ hydrophobicity with water contact angle reaching 130°.

As mentioned in the Introduction, the influence of the DS on the properties of cellulose aerogels made via dissolution-coagulation-sc CO₂ drying route is not studied and understood, and our work is just a first step in this direction. It should also be noted that coagulated tosyl cellulose provide a versatile platform for further functionalization. The tosyl group can be substituted with reactive click-chemistry groups, affinity ligands, bioactive molecules, or catalytic functional groups, expanding aerogel potential applications (e.g., targeted adsorption of drugs or pollutants, as catalyst support, or biomedical scaffolds for cells and tissue growth).

Low-density highly hydrophobic tosyl cellulose aerogels of DS > 0.50 was produced by a simple dissolution-coagulation-evaporative drying route. By carefully optimizing the incorporation of reactive groups, material properties can be tailored to suit specific applications.

**Acknowledgments** This work was supported by Agence National de Recherche funding (project "Bio-gel", ANR-21-CE43-0017-01) and COST Action CA18125 "Advanced Engineering and Research of aeroGels for Environment and Life Sciences" (AERoGELS), funded by the European Commission. Authors are grateful to Dr. Julien Jaxel (PERSEE, Mines Paris) for supercritical drying. The authors thank the Région Sud - Provence-Alpes-Côte d'Azur for its support in the acquisition of the MAIA 3 SEM (Research Call 2019).

**Author contributions** Conceptualization: Loris Gelas, Martin Gericke, Tatiana Budtova; Methodology: Loris Gelas, Martin Gericke, Tatiana Budtova; Formal analysis and investigation: Loris Gelas, Martin Gericke, Tatiana Budtova; Writing—original draft preparation: Loris Gelas; Writing—review and editing: Loris Gelas, Martin Gericke, Thomas Heinze, Tatiana Budtova; Funding acquisition: Tatiana Budtova; Resources: Thomas Heinze, Tatiana Budtova; Supervision: Tatiana Budtova.

**Funding** Open access funding provided by Mines Paris - PSL. Agence National de Recherche funding (project "Bio-gel", ANR-21-CE43-0017-01) and COST Action CA18125 "Advanced Engineering and Research of aeroGels for Environment and Life Sciences" (AERoGELS), funded by the European Commission.

**Data availability** Data are available upon request.

**Declarations**

**Conflict of interest** The authors have no financial or non-financial interests to disclose. The authors have no competing interests to declare that are relevant to the content of this article.

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