The GGM investigated in this study was isolated from thermomechanical pulp received from Stora Enso (Kvarnsveden, Sweden) and ultrafiltered using 5 kDa membranes. The GGM concentration prior to ultrafiltration was less than 1% w/w. The solid GGM was achieved by precipitation using 95% ethanol (Solveco, Rosersberg, Sweden) in a 2:1 ratio, centrifuged, dried in room temperature, and stored for further usage (Willför et al., 2003). The GGM was modified by hydrophobic side chains, butyl glycidyl ether (BGE; Nypelö et al., 2016) (Figure 1). NaOH pellets (VWR, Sweden) were added to 436 g GGM solution (10% w/w) in mol equivalent to 3:1 NaOH to GGM monomer. BGE, purchased from Sigma-Aldrich (St. Louis, MO, United States), was added dropwise under an inert atmosphere in mol equivalent to 3:1 BGE to GGM monomer, and the solution stood overnight in a 45°C oil bath under magnetic stirring. The solution was then neutralized and dialyzed for approximately 1 week using a Spectra/Por 7 Dialysis membrane with a molecular weight cutoff of 1 kDa (Spectrum Laboratories, Rancho Dominguez, CA, United States). The CNF from softwood via enzymatic pretreatment was received from Stora Enso (Karlstad, Sweden) as a slurry (4.2 wt%). The bleached softwood kraft pulp has a typical glucose content of 85% (Sixta, 2006).

The concentration of the solutions containing GGM was kept constant at 6 wt% and cellulose nanofiber suspensions were added to give 0, 0.2, 0.4, 1, and 2 wt% cellulose nanofiber concentration. The foaming procedure was performed on mixtures containing nanofibers and GGM/modified GGM using 0.5 L siphon (Exxent, Sweden) and 8 g nitrous oxide gas patrons (Exxent, Sweden) in plastic tubes for 30 s. Thereafter, the foams were frozen and freeze-dried (FreeZone^®, Labconco, US) at –40°C for 5 days and stored in a desiccator for further analyses.

The GGM has been characterized by Härdelin et al. (2020). The main carbohydrates in the GGM were galactose, glucose, and mannose, with an average molar ratio of 1:1.4:4.6. The molecular weight (Mw) was 15 kDa. The molar substitution of BGE was reported to be 2.1 (Härdelin et al., 2020). Fourier transform infrared spectroscopy (FTIR) analysis of the GGM grades is presented in Supplementary Figure S1.

Solutions of the modified and unmodified GGMs (1 mg ml⁻¹) were analyzed with dynamic light scattering (DLS) (Beckman Coulter N4 Plus submicron particle size analyser), with 90° scattering angle, 5 min measuring time, and at a temperature of 23°C and reported as number-based distribution.

The surface tension was determined by using an Attension Theta optical tensiometer (KSV instruments, Biolin Scientific, Sweden) and pendant drop shape analysis. The shape was recorded immediately after the maximum drop size was achieved.

Adsorption of the hemicelluloses, both modified and unmodified GGMs, on the CNF were analyzed using a quartz crystal microbalance with dissipation monitoring (QCM-D, Q-Sense AB Gothenburg, Sweden) at a temperature of 22°C. The CNF substrates were prepared following a reported procedure (Palasingh et al., 2021). First SiO₂-coated QCM-D sensors were cleaned by immersion in 10% NaOH solution for 20 s, rinsed with excess of water, and dried with nitrogen gas. The cleaned sensor was then immersed in 1.6 g L⁻¹ of PEI solution to deposit a positively charged anchoring layer. The spin-coating solution was prepared by dispersing CNFs in water (1.7 g L⁻¹) by ultrasound sonication (Sonics VCX-750 Vibra-Cell Ultra sonicator) for 5 min (20% power). The dispersion was then centrifuged for 40 min in 6,000 rpm (Beckman Coulter Optima XL-100K ultracentrifuge). The supernatant was used for spin coating using 3,000 rpm, with an acceleration of 2,100 rpm s⁻¹ for 1 min (SPS Spin 150). The films were then treated in an oven for 10 min at 80°C and stored in a desiccator.

The rheological behavior of unmodified and modified GGM, with or without the nanocellulose, was analyzed using a Discovery Hybrid Rheometer 3 (TA Instruments). The concentrations of unmodified and modified GGM without nanofibers were between 0.2 and 2.0 wt%. The concentration of unmodified and modified GGM was fixed at 6 wt%, when combined with nanofibers, and the concentration of nanocellulose varied between 0 and 2.0 wt%. The measurements were performed at 25°C using the following procedure: first run at 10.0 s⁻¹ for 60 s; thereafter, the shear rate was immediately decreased to 0.03 s⁻¹ and then ramped up to 1,000 s⁻¹ and then decreased from 1,000 to 0.03 s⁻¹.

To assess the foam stability, the foam and the precursor suspension volumes were determined volumetrically as a function over time for 125 min and used for calculating the foam specific volume (V), determined as the foam volume in relation to the initial total mass. Lost volume, V_L, was determined after 15 min as the volume% of the solution that was incorporated in the foam.

N₂O is soluble in hydrophilic and hydrophobic substances (O’Neil et al., 2001; Yokozeki and Shiflett, 2011; Stankov et al., 2021) and hence used for foaming of hydrophobic substances in hydrophilic matrices (e.g., water). Here, we used foaming aided by N₂O following a traditional siphon foaming approach of incorporating liquid N₂O into an aqueous foam precursor in a container (Figures 2A–C). The aqueous foam precursors contained GGM, GGM-BGE alone, or a CNF suspension. The release of the foam precursors containing N₂O (aq) under ambient conditions led to the expansion of N₂O into gas (g) and formation of a foam (Figure 2D).

The hypothesis of this work was that a hydrophobic component is required to facilitate foaming and foam stability. The GGM did not decrease the surface tension of water–air interface. However, the GGM-derivative containing the aliphatic modification (Figure 1), i.e., the GGM-BGE, decreased the surface tension to 30 mN m⁻¹ (Figure 3). These findings comply with the previous work (Nypelö et al., 2016) and indicate that the modified GGM was successfully functionalized with aliphatic side chains, which alters the polarity of the molecule resulting in increased interfacial activity of the molecule in aqueous environment.

Light scattering revealed that the GGM solution (1 mg ml⁻¹) contains structures with a hydrodynamic diameter of 1.3 ± 0.1 nm and the GGM-BGE solution, 87.0 ± 36.6 nm. Wood hemicellulose water solubility depends on their source and the substituting units and degree of substitution, and hence the detectable features in the DLS analysis, are not surprising (Kishani et al., 2018). However, the GGM-BGE clearly features aggregation that we deem to originate from the reduction of its water solubility.

Foam stability of neat GGM and GGM-BGE and both in the presence of CNF was elaborated by analysis of the foam specific volume V and lost foam volume V_L. The specific volume is the ratio of the volume of the foam to the mass of the precursor at time zero and was lower for GGM than for GGM-BGE (2.4 vs. 3.0 ml g⁻¹), indicating a low foaming capacity of GGM compared to that of GGM-BGE (Table 1). We note that although the surface tension analysis (Figure 1) did not indicate the GGM to be interfacially active, foams could also be formed with the GGM, even though with lower V. An amount of 96% of the GGM-BGE foam was lost after 15 min, being on similar level with the value for GGM that was 98%. Hence, both the GGM and GGM-BGE foams were found to be unstable with time.

The specific volume V for the GGM-CNF foams was approximately the same as that of GGM alone (2.1 vs. 2.4 ml g⁻¹), indicating insignificant influence of the CNF to the foaming ability. However, the lost foam volume (Table 1) of GGM foam was 98% while for GGM-CNF it was only 33%, which indicates more stable foam being generated with CNFs. The reference foam prepared using 1 wt% CNF had the lowest specific volume (1.2 ml g⁻¹). Although only 33% of the foam was lost after 15 min, we note that the foam had a gel-like character, being different from the other foams (Supplementary Figure S2). The appearance of the foams after 15 min is presented in Supplementary Figures S2, S3.

Incorporation of CNFs to the GGM-BGE led to the highest foam specific volume (3.1 ml g⁻¹) and the lowest lost foam volume, 28%. It appears that hydrophobic components in the foam formulation led to larger foam volumes. Since N₂O is known to be soluble in hydrophobic matrices (Yokozeki and Shiflett, 2011) and to solubilize hydrophobic components, it may be an underlying factor contributing to the highest foam volume fractions of the foams with hydrophobic components. We speculate that these formulations may be able to incorporate more N₂O (larger Δv in Figure 2C) and hence lead to the highest specific volumes (Table 1).

The foam stability of the wet foams is presented as the foam specific volume (V) at given points of time in comparison to the initial (at 0 min) foam specific volume (V₀) over an observation period of 125 min (Figure 4). The foams with GGM and the GGM-BGE alone collapsed after 15 min, while the pristine CNF foam decreased until reaching a plateau of 45 min after the foaming.

The GGM-stabilized CNF foams, with the lowest CNF concentration of 0.2 wt%, were as unstable as the neat GGM foams (Figures 4A, B). Increase in the CNF concentration to 0.4 and 1 wt% increased the stability, but with 2 wt% CNF increase the foam could not be formed. In the case of the GGM-BGE-CNF foams, the stability increased with increasing CNF concentration. We observe that while the stability of the foams appears equal with 1 wt% CNF concentration for both GGM and GGM-BGE, the increase in the CNF concentration in the foam from 1 to 2 wt% resulted in it not being possible to form a foam when using GGM as the foaming additive, but the foam stability increased for GGM-BGE. The presence of the hydrophobic component seems to be essential for an increase both in foam volume and in the foam stability. The observation during foaming was that the incorporation of CNF into the GGM made the system thicker and hence making foaming in the siphon challenging, which led to larger gas bubbles and, presumably, also to foam instability.

The CNF foam appeared to perform well when the stability over the 125 min period was scrutinized (Figure 4). To further inspect the stability, an investigation of the specific volume V compared to the initial foam mass was performed (Figure 5).This presentation amplifies the stability with respect to the amount of the precursor that was attempted to be foamed, rather than only considering the amount of the formed foam that was retained. In this consideration, the CNF performance decreases, in comparison to when accompanied with the hemicellulose additive, and it becomes evident that GGM-BGE additive is needed to facilitate efficient conversion of the precursor into a foam.

The siphon foaming occurs by depressurizing the precursor–N₂O suspension, abrupt instability, cell nucleation, and growth followed by stabilization of the foamed cellular structure. The latter steps are determined by the rheological characteristics of the solution and hence this characterization was performed. During the bubble growth, a low viscosity of the expanding matter allows elongation of the liquid surrounding the bubble, while, after the formation of foam bubbles, the cellular structure containing polymers leads to greater resistance to the stretching and prevents the collapse of the foamed structure. The viscosity of GGM solutions were higher at all investigated concentrations and exhibited non-Newtonian behavior, while the GGM-BGE solutions were Newtonian and had lower viscosity (Figure 6). This can be one of the reasons for the difficulty to foam the GGM in comparison to GGM-BGE. As expected, inclusion of CNF in solutions increased the viscosity for both the GGM and GGM-BGE solutions (Figure 7).

The wet foams were dried via freezing and lyophilization, which resulted in self-standing foams. The structural investigation of these foams is presented in Supplementary Figures S4–S6. The benefit of incorporation of GGM-BGE, rather than GGM, in the CNF foams was that the foams containing the former were able to withstand the forces (capillary, gravitational, and interfacial) created during the drying process to yield porous dry foams. The foams containing GGM suffered from artifacts caused by drying, which may be the result of the observed lower stability. We deem that the hydrophobicity of GGM was needed for foam formation while the cellulose fibrils were needed for generating a self-standing foam structure.