The Impact of Hydrogen Bond Basicity of Ionic Liquids on Cotton Cellulose Dissolution: Experimental and Simulation Study

Abstract

This study explores the influence of anion hydrogen-bond basicity, quantified by the Kamlet–Taft β parameter, on cellulose dissolution in imidazolium-based ionic liquids (ILs). A series of ILs sharing the common cation 1-benzyl-3-methylimidazolium were synthesized with varying anions, including chloride, acetate, formate, methoxyacetate, and methylphosphonate. The hydrogen-bond accepting ability (β) of each IL was experimentally determined and correlated with cellulose dissolution performance. Dissolution capability was evaluated by solubilizing 5 wt% cotton cellulose at 90 °C and monitoring under polarized light microscopy. Among the studied systems, 1-benzyl-3-methylimidazolium acetate (β = 1.01) demonstrated the highest dissolution efficiency, highlighting the critical role of strong hydrogen-bond basicity in disrupting the cellulose hydrogen-bonding network. To support the experimental observations, COSMO-RS simulations were conducted to probe the molecular-level interactions between anions and cellulose. Parameters such as anion size, theoretical density, viscosity, and surface charge density distribution were analyzed to elucidate their contributions to dissolution behavior. The regenerated cellulose was further characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy.

1. Introduction

Over the past two decades, ionic liquids (ILs) have attracted significant attention as highly effective solvents for dissolving cellulose. Structurally, ILs are salts made up of organic cations and either inorganic or organic anions. Their physicochemical properties, such as non-flammability, negligible vapor pressure, high chemical and thermal stability, and recyclability, make them especially attractive for biomass processing. Most ILs exhibit melting points lower than 100 °C and are thus categorized as room-temperature ionic liquids [1,2]. Owing to these unique features, ILs have been widely used in cellulose technology, including dissolution, homogeneous functionalization, regeneration, and biomass fractionation [3,4,5,6,7,8,9,10,11,12]. Despite extensive studies, the dissolution mechanism of cellulose in ILs remains a subject of debate. Numerous investigations have explored the roles of individual ionic components in this process [13,14,15,16,17,18,19,20,21,22,23,24,25,26]. The prevailing consensus is that IL anions penetrate the crystalline cellulose matrix and disrupt the extensive hydrogen-bonding network through competitive hydrogen bonding [16,18,27]. In this interaction, the anion generally functions as a hydrogen bond acceptor, whereas the cation serves as a hydrogen bond donor, with the exact bonding dynamics determined by the structural characteristics of the individual IL components [18,27].

Many studies have shown that the physicochemical properties of ILs play a crucial role in cellulose dissolution. Key parameters influencing this process include Kamlet–Taft parameters, viscosity, conductivity, density, and melting point [27,28,29,30,31,32,33]. The Kamlet–Taft parameters characterize solvents by three descriptors: hydrogen-bond donor ability (α), hydrogen-bond acceptor ability (β), and dipolarity/polarizability (π*) [34,35,36]. Among these, the β parameter, which reflects the hydrogen-bond basicity of the anion, has been strongly correlated with cellulose solubility. The hydrogen-bond accepting ability of the anion plays a key role in interacting with the hydroxyl groups of cellulose, breaking the inter- and intramolecular hydrogen bonds [16,28]. Experimental evidence supports the importance of hydrogen-bond basicity in cellulose dissolution [28]. For instance, replacing the hydrogen atom in the acetate anion (CH₃COO⁻) of 1-butyl-3-methylimidazolium acetate ([C₄C₁im][CH₃COO]) with substituents -OH, -NH₂, C₆H₅, and -SH led to a reduction in cellulose solubility [37]. Similarly, ILs containing strongly basic anions, such as carboxylates and phosphates, display superior solvation ability, whereas ILs with weakly basic, non-coordinating anions ([BF₄]⁻, [PF₆]⁻, [N(CN)₂]⁻, [MeSO₄]⁻, [HSO₄]⁻, and [NTf₂]⁻) are poor cellulose solvents [38,39,40,41].

Ionic liquids with different anions exhibit varying hydrogen-bond basicity, which significantly influences their ability to dissolve cellulose. For example, carboxylate- and phosphate-based anions display strong hydrogen-bond basicity and are considered highly effective solvents for cellulose [29,42]. Fukaya et al. reported that 1-allyl-3-methylimidazolium formate could dissolve greater amounts of cellulose at lower temperatures than 1-allyl-3-methylimidazolium chloride due to the formate anion’s hydrogen-bond basicity being 1.2 times higher than that of chloride [42]. Similarly, the effect of the β parameter on cellulose dissolution was investigated for 1-ethyl-3-methylimidazolium ([C₂C₁im]) salts containing methanesulfonate ([MeSO₃]), methylsulfate ([MeOSO₃]), and dimethylphosphate ([(MeO)₂PO₂]) anions [29]. The results showed that [C₂C₁im][(MeO)₂PO₂] (β = 1.00), with the highest hydrogen-bond acceptor capacity, dissolved cellulose more efficiently than [C₂C₁im][MeSO₃] (β = 0.70) and [C₂C₁im][MeOSO₃] (β = 0.61), which possess lower basicity. Furthermore, Xu et al. demonstrated a linear correlation between the β parameter of ILs and the solubility of microcrystalline cellulose (MCC) at 70 °C [37]. Despite these findings, relatively few systematic studies have directly investigated the influence of hydrogen-bond basicity on cellulose dissolution. Therefore, further research is required to elucidate the precise relationship between the β parameter of ILs and their cellulose dissolution capacity. COSMO-RS (Conductor-like Screening Model for Real Solvents) has been widely adopted to gain molecular-level understanding and to predict the performance of ILs in dissolving cellulose and related polysaccharides. COSMO-RS combines quantum-chemical surface charge density calculations with statistical thermodynamics to estimate key thermodynamic properties such as logarithmic activity coefficients (ln γ) and excess enthalpies (Hᵉ), which correlate with solvent affinity and solute solubility. Such descriptors enable systematic screening of thousands of IL candidates without exhaustive experimental testing. For example, Kahlen et al. used COSMO-RS to compute relative solubilities for over 2000 ionic liquid combinations and found good qualitative agreement with experimental cellulose solubility data, identifying that anion identity is the dominant factor controlling IL solvent power. Further computational studies have refined this approach by evaluating ln γ and Hᵉ for various ILs and showing that ln γ at infinite dilution in particular serves as a robust predictor for cellulose dissolution capability [43,44,45]. Beyond simple prediction, COSMO-RS has been successfully applied to guide practical fiber processing strategies. In the field of cellulose fiber functionalization, COSMO-RS screening has identified ILs such as 1-ethyl-3-methylimidazolium diethylphosphate ([Emim]DEP) with strong dissolution ability suitable for preparing antibacterial cellulose fibers, supporting both the incorporation of bioactive agents and fiber regeneration processes [46,47,48]. Similarly, COSMO-RS predictions have been correlated with nanocellulose production performance, where predicted solvent affinity descriptors help explain observed trends in nanocellulose yield and morphology when using IL-assisted dissolution routes [43,49,50]. Mechanistically, COSMO-RS analyses reinforce that hydrogen-bonding interactions between IL anions and cellulose hydroxyl groups are critical to dissolution, a conclusion consistent across studies of cellulose, hemicellulose, and other lignocellulosic fractions [43,49,51]. Integrating COSMO-RS with experimental validation enhances understanding of how structural features of both ionic liquids (e.g., anion basicity, cation structure) and solutes influence the thermodynamics of dissolution. This enables rational design of ILs optimized for the efficient and sustainable processing of cellulose fibers and related biomaterials [45,48,50].

To address this, a series of ionic liquids (ILs) based on the 1-benzyl-3-methylimidazolium ([BnzC₁im]⁺) cation was synthesized with structurally diverse anions, including chloride, acetate, formate, methoxyacetate, and phosphonate see Figure S4. Their hydrogen-bond basicity (β) was experimentally determined and identified as a key parameter governing cellulose dissolution. The ability of these ILs to dissolve 5 wt% cotton cellulose at 90 °C was evaluated. To complement the experimental findings, COSMO-RS simulations were employed to elucidate the molecular origins of anion–cellulose interactions. Analysis of anion size, theoretical density, viscosity, and surface charge density distributions provides mechanistic insight into how anion structure governs disruption of the cellulose hydrogen-bonding network. Together, these results establish clear structure–property relationships and offer guiding principles for the rational design of next-generation cellulose-dissolving ionic liquids. To the best of our knowledge, this study is the first to systematically investigate the influence of hydrogen-bond basicity in aromatic-functionalized methylimidazolium ionic liquids (ILs) on cellulose solubility through both experimental and theoretical approaches. The findings offer valuable insights for the rational design of ILs with tailored properties to achieve efficient cellulose dissolution.

2. Materials and Methods

2.1. Ionic Liquids

1-Methylimidazole (Sigma, St. Louis, MO, USA, 98.9%), 1-Benzylimidazole (Sigma, 99%), 1-benzyl bromide (Sigma, St. Louis, MO, USA, 98%), 1-Benzyl chloride (Sigma, St. Louis, MO, USA, 98%), Amberlite IRA-400 chloride resin (Sigma, St. Louis, MO, USA,), acetic acid (Sigma, St. Louis, MO, USA, 99%), formic acid (Sigma, St. Louis, MO, USA, 95%), and methoxyacetic acid (Sigma, St. Louis, MO, USA, 98%) were used without further purification. The solvents THF and diethyl ether were distilled over Na/benzophenone. A solution of NaOH (1 M) was prepared by using 40 g of NaOH (Fisher, Pittsburgh, PA, USA). The gravitational columns were packed by using Celite (Fisher, Pittsburgh, PA, USA), sand ( Acros, Pittsburgh, PA, USA), aluminium oxide (60 A, activated, basic, 50–200 micron, Acros, Pittsburgh, PA, USA), and glass wool. Acetic acid, formic acid, and methoxyacetic acid were used for neutralization. After isolation, all ILs were dried under vacuum at 50 °C for 4 days. After drying the ILs, water content was measured (less than 1000 ppm) by using Karl Fisher titration (Karl Fisher METTLER TOLEDO C20 coulometric KF titrator, METTLER TOLEDO, Greifensee, Switzerland). After performing the AgNO₃ test, the absence of halide impurities was confirmed. Other impurities were undetectable by NMR with a JEOL 400 MHz spectrometer (JEOL Ltd., Tokyo, Japan). Ionic liquid purity was evaluated using a combination of analytical techniques. Structural integrity and organic purity were confirmed by ¹H NMR and ¹³C NMR spectroscopy. Water content was determined by Karl Fischer titration. Residual halide impurities were analyzed using the AgNO₃ test, while trace organic impurities were assessed by HPLC [52], ¹H, and ¹³C NMR, spectra obtained from a JEOL ECS 400 MHz spectrometer (JEOL Ltd., Tokyo, Japan). For ¹H NMR spectra, tetramethylsilane (TMS, d = 0.00) or the residual protic solvent peak (for (CD₃)₂CO, d = 2.05, DMSO-d₆, d = 2.50) and for ¹³C NMR spectra, tetramethylsilane (TMS, d = 0.00) or the residual solvent peak (for (CD₃)₂CO, d = 29.84, DMSO-d₆, d = 39.50) served as shift references. Coupling constants (J) are reported in hertz (Hz).

2.1.1. Synthesis of Ionic Liquids

1-Benzyl-3-Methylimidazolium Chloride

A 250 mL two-neck Schlenk flask equipped with a magnetic stir bar was charged under nitrogen with 50 mL of acetonitrile, followed by the addition of 1-methylimidazole (11.50 mL, 144 mmol) and benzyl chloride (99%) (17.50 mL, 114 mmol) via syringes. The reaction mixture was stirred at 70 °C for two days. After the reaction went to completion, the crude product was isolated in a good yield of 91%. After purification, the pure compound was left under vacuum and solidified over time. The compound was characterized by ¹H NMR and ¹³C NMR.

¹H NMR (400 MHz; (DMSO-d6); TMS) 3.78 (s, 3H, N-CH₃), 4.47 (s, 2H, N-CH₂-), 7.35–7.45 (m, 5H), 7.70–7.80 (d, 2H, J = 12.0 Hz, NCHCHN), 9.55 (s, 1H, NCHN) ppm. ¹³C NMR (DMSO-d6, 100 MHz), 35.8 (N-CH₃), 51.8 (N-CH₂), 121.9 (NCHCHN), 123.5 (NCHCHN), 128, 129.2, 134.6 (aromatic), 137.80 (NCHN), ppm. Melting point 58 °C.

1-Benzyl-3-Methylimidazolium Phosphonate

A 250 mL two-neck Schlenk flask equipped with a magnetic stir bar was charged under nitrogen with 50 mL of THF, followed by the addition of 1-benzylimidazole (10 mL, 85 mmol) and dimethylphosphite (99%) (9.0 mL, 80 mmol) via syringes. The reaction mixture was stirred at 90 °C for two days. After the reaction reached completion, the solvent was removed and dried under vacuum for a prolonged period. The isolated oil was characterized by ¹H and ¹³C NMR.

¹H NMR (400 MHz; (CD₃)₂CO; TMS) 1.95 (-OCH₃) 3.26 (d, J = 21 Hz 3H, PO-CH₃), 3.90 (s, 3H, J = 7.0 Hz, N-CH₃), 5.10 (t, 2H, J = 7.0 Hz, N-CH₂), 6.09 (d, H, J = 550.0 Hz, P-CH₃) 7.22–7.41 (m, 5H), 7.45 (d, 2H, J = 12.0 Hz, NCHCHN), 10.50 (s, 1H, NCHN) ppm. ¹³C NMR, (CD₃)₂CO, 100 MHz): 36.3 (N-CH₃), 50.6.8 (N-CH₂), 50.0 (POCH₃, 121.6 (NCHCHN), 123.5 (NCHCHN), 138.8 (NCHN) ppm. ³¹P NMR, (CD₃)₂CO, 162 MHz) 3.9 ppm. FTIR (cm⁻¹): 1190 (P=O), 1050 (P–O⁻).

1-Benzyl-3-Methylimidazolium Bromide

A 250 mL two-neck Schlenk flask equipped with a magnetic stir bar was charged under nitrogen with 50 mL of acetonitrile, followed by the addition of 1-methylimidazole (6.30 mL, 79.01 mmol) and benzyl bromide (99%) (9.97 mL, 82.96 mmol) via 10 mL syringes. The reaction mixture was stirred at room temperature overnight. After the reaction reached completion (progress was monitored by ¹H NMR), the reaction flask was removed from the condenser, and the crude product was washed with 60 mL of hexane three times. The isolated crude product was dried under vacuum and dissolved in 60 mL of CH₂Cl₂. Approximately 2 to 3 g of activated carbon was added to this solution, which was then stirred overnight. The black slurry was removed from the stir plate and then filtered through a gravitational column that was packed with glass wool, 10 cm of aluminum oxide (activated, basic, 50–200 micron), and 5 cm of Celite 545. The excess solvent was removed using a rotary evaporator.(Buchi R100, New Castle, DE 19720, USA) The sample was dried under vacuum, and the compound was isolated, giving a yield of 91%. The pure compound, left under vacuum, solidified over time. ¹H NMR and ¹³C NMR were used to characterize the compound.

Note that bromide ILs were converted to hydroxide by passing through the anion exchange column to generate hydroxides. Then, neutralization reactions were performed with acetic acid, formic acid, and methoxy-acetic acid to yield 1-benzyl-3-methylimidazolium acetate, 1-benzyl-3-methylimidazolium formate, and 1-benzyl-3-methylimidazolium methoxyacetate, respectively. The synthesized ILs were isolated and dried under vacuum for a prolonged period and characterized by ¹H and ¹³C NMR.

1-Benzyl-3-Methylimidazolium Acetate

¹H NMR (400 MHz; (DMSO-d₆); TMS) 1.54 (s, 3H, CH₃C=O), 3.86 (s, 3H, N-CH₃), 4.57 (s, 2H, N-CH₂-), 7.40–7.43 (m, 5H), 7.72–7.78 (d, 2H, J = 12.0 Hz, NCHCHN), 9.67 (s, 1H, NCHN) ppm. ¹³C NMR (DMSO-d₆, 100 MHz): 26.0 (CH₃C=O), 36.2 (N-CH₃), 52.2 (N-CH₂), 122.7 (NCHCHN), 124.0 (NCHCHN), 129, 129.4, 135.1 (aromatic), 138.10 (NCHN), 173.2 (C=O) ppm. FTIR (cm⁻¹): 1560 (COO⁻),

1-Benzyl-3-Methylimidazolium Formate

¹H NMR (400 MHz; (DMSO-d₆); TMS) 1.90 (s, H, HC=O), 4.02 (s, 3H, N-CH₃), 4.93 (s, 2H, N-CH₂-), 7.20–7.38 (m, 5H), 8.5–9.16 (d, 2H, J = 12.0 Hz, NCHCHN), 9.20 (s, 1H, NCHN) ppm. ¹³C NMR (DMSO-d₆, 100 MHz): 36.5 (N-CH₃), 53.4 (N-CH₂), 121.2 (NCHCHN), 122.8 (NCHCHN), 129.1, 129.5, 133.2 (aromatic), 140.2 (NCHN), 169.4 (C=O) ppm. IR FTIR (cm⁻¹): 1610 (COO⁻)

1-Benzyl-3-Methylimidazolium Methoxyacetate

¹H NMR (400 MHz; (DMSO-d₆); TMS) 3.17 (s, 3H, CH₃C=O), 3.67 (s, 3H, N-CH₃), 3.87 (s, 2H, N-CH₂-), 5.32 (s, 2H, O-CH₂-CO), 7.20–7.24 (m, 5H), 8.10–8.20 (d, 2H, J = 12.0 Hz, NCHCHN), 9.8 (s, 1H, NCHN) ppm. ¹³C NMR (DMSO-d₆, 100 MHz): 36.2 (N-CH₃), 52.2 (N-CH₂), 58.3 (O-CH₃), 72.1 (O-CH₂C=O), 122.7 (NCHCHN), 138.0 (NCHCHN), 128.7, 129.0, 134.1 (aromatic), 138.30 (NCHN), 175.6 (C=O) ppm. FTIR (cm⁻¹): 1620 (C=O), 1170 (C–O–C)

2.1.2. Measurement of Beta (β) Parameter of Molecular Solvents

The β parameter quantifies the hydrogen-bond acceptability of a solvent and is measured via the solvatochromic comparison method, based on shifts in the absorption bands of two solvatochromic dyes. In this study, the dye pair employed was 4-nitroaniline (99%, Sigma-Aldrich, St. Louis, MO, USA,) and N, N-diethyl-4-nitroaniline (97%, Oakwood Chemical, Estill, SC, USA), used as received. A 1.0 × 10⁻³ M solution of each dye was first prepared in methanol. To prepare a dye/IL or dye/solvent solution, a measured volume of the dye stock solution was transferred to a clean vial, followed by 1 mL of the IL or solvent. For the dye/IL/solvent solution, equimolar amounts of IL or solvent were added to the dye solution. All sample preparations were performed inside an argon-filled glovebox to prevent moisture absorption. Methanol was then carefully evaporated under vacuum at 40 °C for a duration of 8 h. The dye concentration was adjusted to maintain an absorbance below 0.40 to prevent aggregation. Samples were loaded in a quartz cell of 1 mm path length, and absorption spectra were recorded using a Shimadzu UV-1800 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The β values were calculated from the wavelengths of maximum absorption (λmax) using Equations (1)–(3), where NA corresponds to 4-nitroaniline and DENA to N, N-diethyl-4-nitroaniline. The maximum absorption for N, N-diethyl-4-nitroaniline ranged from 402–412 nm, whereas 4-nitroaniline exhibited a maximum peak between 380 and 402 nm.

$$\mathit{\nu }\left(\mathbf{N}\mathbf{A}\right)={\displaystyle \frac{\mathbf{1}}{{\mathit{\lambda }}_{\mathbf{m}\mathbf{a}\mathbf{x}}\left(\mathbf{N}\mathbf{A}\right)\mathbf{\times }{\mathbf{10}}^{-\mathbf{4}}}}$$

(1)

$$\mathit{\nu }\left(\mathbf{D}\mathbf{E}\mathbf{N}\mathbf{A}\right)={\displaystyle \frac{\mathbf{1}}{{\mathit{\lambda }}_{\mathbf{m}\mathbf{a}\mathbf{x}}\left(\mathbf{D}\mathbf{E}\mathbf{N}\mathbf{A}\right)\times {\mathbf{10}}^{-\mathbf{4}}}}$$

(2)

$$\mathit{\beta }={\displaystyle \frac{\mathbf{1.035} \mathit{\nu }\left(\mathbf{D}\mathbf{E}\mathbf{N}\mathbf{A}\right)\mathbf{+}\mathbf{2.64}-\mathit{\nu }\left(\mathbf{N}\mathbf{A}\right)}{\mathbf{2.80}}}$$

(3)

2.2. Cotton Cellulose

2.2.1. Dissolution of Cotton Cellulose in ILs

The cellulose used in this study was carded cotton fibers, received from the Fiber and Biopolymer Research Institute at Texas Tech University. The fibers were initially scoured, bleached, and air-dried. Subsequently, the cotton fibers were ground with a Wiley mill (Thomas Scientific, Swedesboro, NJ, USA) to pass through a 20-mesh screen and then oven-dried overnight at 105 °C prior to the dissolution experiments. Dissolution of cotton cellulose in the five ILs was performed following a recently established protocol [26].

2.2.2. Polarized Light Microscopy (PLM)

The dissolution state of cotton fibers in each ionic liquid was examined using a Nikon (ECLIPSE LV100POL) polarizing light microscope equipped with a 530 nm Lambda plate and two polarizing filters (polarizer and analyzer) (Nikon Instruments Inc., Tokyo, Japan). Representative samples from each solution were collected at 4 time intervals: prior to microwave heating, immediately after microwave heating, after 4 h, and 24 h at 90 °C. The solutions were thoroughly stirred before the samples were collected. Each sample was placed on a clean, dry glass slide and covered with a cover slip. Images of the representative samples were collected at room temperature using10× objective lens.

2.2.3. Regeneration of Cellulose from the Solutions

Each cellulose solution was cast onto a glass plate and gently pressed with the second glass plate to achieve uniform spreading of the mixture. The glass plates containing cellulose/IL mixtures were then immersed in deionized water to regenerate cellulose. Deionized water was replaced every 2 to 3 h over a period of 5 days to thoroughly remove the ILs from the regenerated cellulose. The regenerated cellulose materials were dried using a Labconco FreeZone freeze dryer (4.5 Liter Cascade Benchtop Freeze Dry System) (Kansas City, MO, USA) at a temperature of 102 °C and a pressure of 0.05 mbar for 3 days.

2.2.4. Material Characterization

Scanning Electron Microscopy

Regenerated cellulose samples were mounted on carbon discs without any prior coating. The surface morphology of freeze-dried, regenerated cellulose was analyzed by using a TM-1000 tabletop scanning electron microscope (SEM) (Hitachi, Tokyo, Japan) operated at an accelerating voltage of 15 kV.

Fourier Transform Infrared Spectroscopy

The FTIR spectra of freeze-dried, regenerated cellulose samples and hot-dried ground cotton fibers were recorded using a Spectrum 400 FTIR spectrometer (Perkin-Elmer, Shelton, CT, USA) equipped with a universal attenuated total reflectance (UATR) accessory and a ZnSe-diamond crystal. Prior to analysis, freeze-dried, regenerated cellulose samples and hot-dried cotton fibers were conditioned in the laboratory at 21 ± 1 °C and 65 ± 2% relative humidity for 2 days. A background scan of the clean Zn-diamond crystal was performed before the sample’s measurements. Each sample was placed directly on the clean Zn-diamond crystal without further preparation. The pressure arm of the instrument was used to apply a constant pressure to each sample, and the pressure was monitored by Perkin-Elmer software spectrum 10. The FTIR spectra for all samples were collected in the mid-IR range (650 to 4000 cm⁻¹) at a spectral resolution of 4 cm⁻¹ with 32 co-added scans. The baseline correction and normalization of each spectrum were performed by the Perkin-Elmer software.

2.3. Statistical Analysis

One-way analysis of variance (ANOVA) was conducted for regenerated cellulose obtained from each IL using STATISTICA (Version 13.3; 2017; TIBCO software, Palo Alto, CA, USA). Data for ANOVA was collected from three replicates of each regenerated cellulose sample.

2.4. COSMO-RS Analysis

All quantum chemical calculations were performed using the TURBOMOLE package within the COSMO framework (Karlsruhe, Germany). The molecular geometries of the ionic species were fully optimized in the gas phase using density functional theory at the BP86 level with the def-TZVP basis set. Subsequently, COSMO calculations were carried out using a conductor-like screening model to generate the σ-surfaces required for COSMO-RS analysis.

3. Results

3.1. Polarized Light Microscopy

In this study, cotton-derived cellulose, which exhibits a higher degree of polymerization (DP) than microcrystalline cellulose (MCC), was selected as the cellulose source. Previous research on cellulose dissolution has primarily focused on MCC [29,37,53]. However, cellulose with an elevated DP presents greater challenges for solubilization in conventional solvents, such as N-methylmorpholine N-oxide (NMMO), with dissolution occurring more readily at elevated temperatures [27]. For the present experiments, 5 wt% cotton cellulose was added to vials containing ionic liquids (ILs). The mixtures were initially subjected to heat in a domestic microwave oven and subsequently transferred to a laboratory oven, where they were maintained at 90 °C for 24 h. Initial microwave heating is an important step, as shown by numerous studies where the heating efficiency of ILs was higher under microwaving [38,54]. The progress of the dissolution of cotton cellulose in ILs was monitored periodically using a polarized light microscope (PLM), and the PLM images are depicted in Figure 1.

Figure 1. The PLM images of cotton cellulose in ILs at different time intervals of dissolution.

The analysis emphasizes birefringence, swelling behavior, and morphological changes in the fibers as key indicators for assessing the dissolution capability of the ionic liquids. These parameters offer direct visual evidence of the extent to which the ionic liquids interact with and progressively disrupt the cellulose structure over time. As illustrated in Figure 1, the investigated ionic liquids (ILs) demonstrated differing abilities to dissolve cotton cellulose. Prior to microwave pretreatment, numerous intact fibers were observed in all cotton/IL mixtures under crossed polarized light, reflecting the highly crystalline nature of native cellulose. The observed birefringence contrast arises from the interaction between plane-polarized light and the anisotropic structure of the cotton fibers, which causes the incident light to split into two orthogonally polarized components [55]. Cotton cellulose dissolution was markedly enhanced in [BnzC1im][CH₃COO] under microwave heating, with partial dissolution observed after 4 h and complete dissolution achieved within 24 h. In contrast, [BnzC1im][HCOO], [BnzC1im][Cl], and [BnzC1im][MeOCH₂COO] exhibited only limited solubility after 4 h at 90 °C, while no measurable changes were detected in [BnzC1im][(OMe)(H)(PO₂)] under identical conditions. Polarized light microscopy revealed undispersed fiber aggregates in several systems, indicating incomplete dissolution. Under crossed polarized light, native cellulose fibers displayed strong birefringence, whereas dissolution resulted in loss of both birefringence and fibrillar structure. In [BnzC1im][(OMe)(H)(PO₂)], fibers retained their elongated morphology and birefringence after 24 h at 90 °C, with no evidence of swelling or fragmentation, indicating limited solvent penetration and preservation of the native crystalline structure. Swelling was more pronounced in [Bnzim][Cl] than in [BnzC1im][HCOO] and [BnzC1im][MeOCH₂COO] after 24 h; however, complete dissolution was not achieved in these systems, suggesting the persistence of crystalline domains. In contrast, no detectable swelling was observed in [BnzC1im][(OMe)(H)(PO₂)] over the same period, consistent with its poor solvating ability toward cellulose. Regenerated cellulose obtained after dissolution was subsequently characterized by scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy to evaluate morphological and structural changes.

3.2. Scanning Electron Microscopy

The surface morphologies of the regenerated cellulose materials were examined using scanning electron microscopy (SEM), and representative micrographs are shown in Figure 2. The surface structures varied significantly depending on the ionic liquids (ILs) used for dissolution. Cellulose regenerated from [BnzC1im][CH₃COO] (Figure 2D) exhibited a uniform, nonporous surface with no visible undissolved fibers, indicating complete dissolution of cotton in the acetate-based IL, consistent with polarized light microscopy (PLM) observations. In contrast, cellulose regenerated from [BnzC1im][Cl] and [BnzC1im][HCO₂] (Figure 2A,B) displayed rough, aggregated textures with fibers fused into a homogeneous macrostructure. Meanwhile, samples regenerated from [BnzC1im][(OMe)(H)(PO₂)] and [BnzC1im][MeOCH₂COO] were predominantly composed of undissolved cotton fibers, suggesting that these ILs are comparatively poor solvents for cellulose.

Figure 2. Scanning electron micrographs of cellulose regenerated from [BnzC₁im][Cl] (A), [BnzC₁im][HCO₂] (B), [BnzC₁im][(OMe)(H)(PO₂)] (C), [BnzC₁im][CH₃COO] (D), and [BnzC₁im][MeOCH₂COO] (E) in 500×.

3.3. FTIR Characterization

FTIR spectroscopy analyses were performed to investigate changes in the functional groups of native cotton cellulose following dissolution in various ILs and subsequent regeneration in deionized water. FTIR spectra of freeze-dried regenerated cellulose and hot-dried ground cotton fibers are illustrated in Figure 3. FTIR spectra of cellulose dissolved and regenerated from all ILs closely resemble that of native cotton cellulose, indicating that no chemical reactions occurred during dissolution and regeneration processes. However, there are a few changes in the FTIR spectra that were observed for cellulose regenerated, particularly from [BnzC₁im][CH₃COO] and [BnzC₁im][Cl], compared to both hot-dried ground cotton fiber and cellulose regenerated from other ILs. The broad absorption band between 3700 and 3000 cm⁻¹ in native cellulose corresponds to OH-stretching vibrations involved in hydrogen bonding [56]. For regenerated cellulose from [BnzC₁im][CH₃COO] and [BnzC₁im][Cl] (Figure 3a,e), this region appeared broader compared to that of native cellulose. Moreover, two main vibrations, at 3330 cm⁻¹ (intra-molecular hydrogen bonds) and 3289 cm⁻¹ (inter-molecular hydrogen bonds), in the FTIR cellulose regenerated from [BnzC₁im][CH₃COO] shifted to higher wavenumbers and appeared at 3482 and 3439 cm⁻¹, respectively. These shifts suggest disruption of the native cellulose hydrogen-bonding network during dissolution and regeneration. The vibration at 1428 cm⁻¹ in native cellulose, attributed to CH₂ scissoring, which is indicative of crystalline regions, was reduced and shifted to 1422 cm⁻¹ in cellulose regenerated from [BnzC₁im][CH₃COO], reflecting the disruption of intra-molecular hydrogen bonds at the C6 position [27,56]. The CH₂ rocking vibration at 1315 cm⁻¹ and the C-O stretching vibration at 900 cm⁻¹, associated with amorphous regions [57,58], also exhibited increased intensity and slight redshifts in cellulose regenerated from [BnzC₁im][CH₃COO] and [BnzC₁im][Cl], indicating a more amorphous structure relative to native cellulose. The FTIR analysis indicates a complete dissolution of cotton cellulose in [BnzC₁im][CH₃COO] and a transformation from native cellulose I_β crystalline structure to cellulose II during the dissolution and regeneration. The changes observed in the vibrations of cellulose regenerated from [BnzC₁im][Cl] could be due to the partial dissolution of cotton cellulose in [BnzC₁im][Cl], resulting in a mixture of cellulose I_β and II crystalline structure [59]. In contrast, cellulose regenerated from less effective ILs; [BnzC₁im][HCO₂], [BnzC₁im][(OMe)(H)(PO₂)], and [BnzC₁im][(OMe)(CH₃COO)] showed minimal spectral changes, indicating preservation of the native cellulose I_β structure. Minor additional peaks in the spectra of cellulose regenerated from [BnzC₁im][HCO₂] and [BnzC₁im][(OMe)(CH₃COO)] likely originated from residual ILs that were not completely removed during regeneration.

Figure 3. FTIR spectra of regenerated cellulose from (a) [BnzC₁im][Cl], (b) [BnzC₁im][HCO₂], (c) [BnzC₁im][(OMe)(H)(PO₂)], (d) [BnzC₁im][CH₃COO], and (e) [BnzC₁im][MeOCH₂COO].

The integrated intensity of the 896 cm⁻¹ band in regenerated cellulose showed a highly significant dependence on the ionic liquid (IL) anion (F = 808.66, p < 0.000001; Table 1). Native cellulose and [BnzC1im][(OMe)(H)(PO₂)] exhibited the lowest and statistically similar values (1.06 and 1.07, group a), consistent with limited structural disruption and high crystallinity. A progressive increase in intensity was observed for [Bnzim][MeOCH₂COO] (1.83, b) and [Bnzim][HCOO] (2.60, c), indicating increasing amorphization. The highest values were recorded for [Bnzim][Cl] (3.47) and [Bnzim][CH₃COO] (4.40), both in group d, reflecting the most effective cellulose dissolution. These trends agree with PLM and SEM results and confirm that the 896 cm⁻¹ band intensity is a reliable indicator of IL dissolution efficiency.

Table 1. Variance analysis: effect of different ILs in cellulose dissolution on the integrated intensity of the peak 896 cm⁻¹ (I₈₉₆).

Parameter	df	F Value	Probability
Intercept	1	15,522.10	0.000001
Treatment	5	808.66	0.000001
Error	12	-	-
Sample		Peak Intensity (896 cm−1)
Native cotton		1.06 a
[BnzC1im][(OMe)(H)(PO2)]		1.07 a
[Bnzim][MeOCH2COO]		1.83 b
[Bnzim][HCOO]		2.60 c
[Bnzim][Cl]		3.47 d
[Bnzim][CH3COO]		4.40 d

3.4. Kamlet–Taft Beta Parameter

Table 2 summarizes the experimentally determined Kamlet–Taft hydrogen-bond basicity (β) parameters of the investigated ionic liquids, which span a range from 0.72 to 1.05. A clear dependence on anion structure is observed, with [BnzC₁im][MeOCH₂COO] exhibiting the highest β value (1.05 ± 0.01), followed by acetate (1.01 ± 0.02), phosphonate (0.98 ± 0.01), and formate (0.95 ± 0.01), while chloride shows the lowest basicity (0.72 ± 0.02). This trend is consistent with the increasing electron-donating ability and charge delocalization of the anions, which enhances their hydrogen-bond accepting strength. Such variations in β are known to strongly influence the ability of ionic liquids to disrupt the intermolecular hydrogen-bonding network in cellulose, thereby governing dissolution efficiency, in agreement with previous reports in the literature [60]. The Kamlet–Taft β parameters of the five ILs range from 0.72 to 1.05. Among them, [BnzC1im][OMeCH₃COO] exhibits the highest hydrogen-bond basicity, while [BnzC1im][Cl] shows the lowest.

Table 2. Experimentally determined Kamlet–Taft beta values of ionic liquids.

IL	β
[BnzC1im][Cl]	0.72 ± 0.02
[BnzC1im][HCOO]	0.95 ± 0.01
[BnzC1im][(OMe)(H)(PO2)]	0.98 ± 0.01
[BnzC1im][CH3COO]	1.01 ± 0.02
[BnzC1im][MeOCH2COO]	1.05 ± 0.01

4. COSMO-RS-Derived Molecular Descriptors and Physicochemical Properties of Ionic Liquids and Electrostatic Potential (Surface Charge Density) Maps

The electrostatic surface charge density maps of the investigated ionic liquids (ILs) provide valuable insight into their ability as hydrogen bond acceptors. As shown in Figure 4, regions of high negative charge (red) correspond to electron-rich sites capable of acting as hydrogen bond acceptors, which play a dominant role in disrupting the extensive hydrogen-bonding network within cellulose. Among the studied systems, the acetate-based IL ([BnzC₁im][CH₃COO]) exhibits a highly localized and intense negative charge on the oxygen atoms, indicating strong hydrogen bond basicity and a high propensity to interact with hydroxyl groups in cellulose. On the other hand, chloride is showing relatively low hydrogen bond acceptor ability, which is consistent with the experimentally determined hydrogen bond basicity trend. The electrostatic potential (surface charge density) maps are shown below.

Figure 4. Surface charge density maps of the ionic liquids. (a) Acetate, (b) chloride, (c) formate, (d) methoxy acetate, and (e) phosphonate.

Additionally, the molecular size and physicochemical properties of the investigated ionic liquids were evaluated using the COSMO-RS approach, as shown in Table 3. Key descriptors, including COSMO surface area and COSMO volume, were obtained from quantum chemical calculations and provide a quantitative measure of the effective molecular size of each ionic liquid. These parameters are particularly useful for understanding steric effects and ion–solute interactions. In addition to molecular descriptors, bulk properties such as density and viscosity were included to assess their potential influence on mass transfer and dissolution behavior. The results show a clear increase in both COSMO surface area and volume with increasing anion size, following the trend:

chloride < formate < acetate < methylphosphonate < methoxyacetate

Table 3. Molecular size and physicochemical properties of the investigated ILs.

Ionic Liquid	Anion	Surface Area (A2)	Volume (A3)	Density (kg/L)	Viscosity Pa.S
[BnzC1im][Cl]	Chloride	247.83	260.48	1.146	0.0191
[BnzC1im][HCOO]	Formate	259.26	274.23	1.134	0.142
[BnzC1im][(OMe)(H)(PO2)]	Methyl phosphonate	299.90	318.00	1.152	0.043
[BnzC1im][CH3COO]	Acetate	279.95	295.3	1.107	0.151
[BnzC1im][OMeCH3COO]	Methoxy-acetate	308.98	327.41	1.128	0.091

5. Discussion

Although hydrogen-bond basicity (β) is widely recognized as a key parameter governing the cellulose dissolution capability of ionic liquids (ILs), the present results suggest that β alone does not fully account for dissolution performance. The measured β values of the synthesized ILs followed the order of [BnzC1im][MeOCH₂COO] > [BnzC1im][CH₃COO] > [BnzC1im][(OMe)(H)(PO₂)] > [BnzC1im][HCO₂] > [BnzC1im][Cl], indicating that [BnzC1im][MeOCH₂COO] should possess the strongest hydrogen-bond basicity and, consequently, the highest efficiency for dissolving cotton cellulose. However, polarized light microscopy (PLM) observations over time revealed a contrasting trend. ILs such as [BnzC1im][CH₃COO], [BnzC1im][Cl], and [BnzC1im][HCOO] exhibited superior dissolution capabilities compared to [BnzC1im][MeOCH₂COO]. This discrepancy highlights that cellulose dissolution is a multifaceted process influenced not only by hydrogen-bond basicity but also by additional factors, including anion structure, steric effects, ion–cellulose interactions, and solvent dynamics. In particular, the relatively lower dissolution efficiency of [BnzC1im][MeOCH₂COO], despite its higher β value, may be attributed to steric hindrance and reduced accessibility of the anion to cellulose hydroxyl groups. These findings emphasize the importance of considering a combination of physicochemical properties, rather than relying solely on β, when designing ILs for efficient cellulose dissolution.

Additionally, dissolution capacity was measured quantitatively, and the same trend was observed. A known mass of the cellulose–ionic liquid solution was precipitated in excess deionized water, and the regenerated cellulose was collected by vacuum filtration, washed with water and ethanol, dried under vacuum at 80 °C to a constant weight, and quantified gravimetrically; measurements were performed in triplicate, and the amount of dissolved cellulose was calculated as mean ± standard deviation. Among the investigated ionic liquids, [BnzC1im][CH₃COO] exhibited the most pronounced cellulose dissolution at 90 °C, despite possessing only the second-highest β value. The superior performance of [BnzC1im][CH₃COO] compared to [BnzC1im][MeOCH₂COO] can be attributed to the smaller size and higher accessibility of the acetate anion. This structural advantage facilitates more effective interaction with the hydroxyl groups of cellulose and may enable simultaneous binding at multiple sites along the polymer chain. Such dual interaction mechanisms have been previously reported to enhance the disruption of intermolecular hydrogen bonding networks in cellulose, thereby significantly improving its solubility. Consequently, acetate-based ionic liquids demonstrate exceptional solvating capabilities, reinforcing the critical role of anion size and coordination behavior in cellulose dissolution processes. [8,22,61,62]. Additionally, dissolution kinetics are strongly influenced by the viscosity of the ILs [63]. Our findings are in close agreement with the observations reported by Ohno and co-workers, who showed that a higher hydrogen-bond basicity (β) of ionic liquids (ILs) does not necessarily translate into improved cellulose solubility. In their systematic study, microcrystalline cellulose (MCC) was examined in a series of [C₂C₁im]-based ILs, including [C₂C₁im][(OMe)(H)PO₂] (β = 1.00), [C₂C₁im][(OMe)(Me)PO₂] (β = 1.07), and [C₂C₁im][(OMe)₂PO₂] (β = 1.00). Despite the slightly higher β value of [C₂C₁im][(OMe)(Me)PO₂], MCC exhibited greater solubility in [C₂C₁im][(OMe)(H)PO₂]. This apparent discrepancy highlights that hydrogen-bond basicity alone is not the sole determinant of dissolution performance. Instead, physicochemical properties such as viscosity play a crucial role in governing the dissolution process. The lower viscosity of [C₂C₁im][(OMe)(H)PO₂] likely facilitates improved mass transport and more efficient penetration of the solvent into the cellulose matrix, thereby enhancing disruption of the intermolecular hydrogen-bonding network within MCC. In contrast, the relatively higher viscosity of [C₂C₁im][(OMe)(Me)PO₂] may hinder molecular mobility and slow down the dissolution kinetics, offsetting the potential advantage of its higher basicity [29]. These findings reinforce the notion that cellulose dissolution in ILs is governed by a delicate interplay between thermodynamic factors, such as hydrogen-bonding interactions, and kinetic or transport-related properties, including viscosity and diffusivity. Consequently, the design of effective IL solvents for cellulose processing should consider not only hydrogen-bond basicity but also the overall fluidity and transport characteristics of the system. Consistent with this broader framework, the PLM images presented in Figure 1 show that [BnzC₁im][Cl] exhibits lower dissolution efficiency than [BnzC₁im][CH₃COO] but performs more effectively than [BnzC₁im][HCOO]. These results align with earlier studies by Zavrel et al. and Zhao et al. [39,61]. Zavrel et al. reported that acetate-based imidazolium ILs dissolve cellulose more efficiently than chloride analogues, owing to the higher hydrogen-bond basicity of acetate. In contrast, Zhao et al. found that [C₄C₁im][Cl] outperforms [C₄C₁im][HCOO] at 110 °C, attributing this behavior to the smaller size of the chloride anion relative to formate [62]. Previous studies have further established that chloride is more effective than bromide or iodide for cellulose dissolution due to its smaller thermochemical radius and stronger coordinating ability [38,64]. Taken together, these results suggest that, despite possessing the lowest hydrogen-bond basicity among the ILs investigated, [BnzC₁im][Cl] remains an effective solvent for cellulose. While other anions exhibit higher basicity, their larger size and associated steric hindrance can limit their ability to penetrate the dense, crystalline structure of cellulose. In contrast, the small, highly polar chloride anion can more readily access the intermolecular spaces between cellulose chains and effectively disrupt both inter- and intramolecular hydrogen bonds. Moreover, chloride ions tend to form comparatively weaker ion pairs with the cation, increasing the availability of free anions to interact with cellulose hydroxyl groups. In addition to viscosity and anion characteristics, IL density has also been identified as an important parameter influencing dissolution efficiency. ILs with lower densities generally require higher hydrogen-bond basicity to achieve effective cellulose dissolution, whereas ILs with moderate β values can still perform well if their density is sufficiently high [30]. In this context, 1-benzyl-3-methylimidazolium chloride emerges as one of the most effective ILs studied, as it combines relatively high density with low viscosity, as predicted by COSMO-RS. Its high density reflects strong intermolecular interactions and favorable solvation capacity, while its low viscosity promotes efficient mass transport. This balance between interaction strength and fluidity ultimately enhances its overall dissolution performance relative to the other ILs examined.

The IL [BnzC₁im][HCOO] exhibited greater cellulose dissolution capacity compared to [BnzC₁im][MeOCH₂COO] and [BnzC₁im][(OMe)(H)(PO)₂], despite the latter possessing higher hydrogen-bond basicity. This enhanced performance can be attributed to the smaller size of the formate anion (HCOO⁻), which enables more effective penetration into the compact cellulose structure and facilitates disruption of its hydrogen-bonding network. These observations further confirm that although hydrogen-bond basicity is an important factor influencing cellulose dissolution, it cannot be considered the sole determinant of dissolution efficiency. Instead, the process is governed by a complex interplay of multiple physicochemical properties, including ion size, polarity, viscosity, density, and the nature of intermolecular interactions within the system. Together, these factors influence solvent–solute interactions, mass transport, and the overall thermodynamic and kinetic aspects of dissolution. Consequently, the rational design of ionic liquids (ILs) for cellulose processing should not rely on hydrogen-bond basicity alone as a predictive parameter. Rather, a comprehensive, systems-level approach is required—one that considers the combined effects of molecular structure and macroscopic properties. Such a holistic strategy will enable the development of more efficient ILs with optimized performance, ultimately advancing sustainable biomass processing and the design of next-generation cellulose-based materials.

6. Conclusions

In summary, this study demonstrates that hydrogen-bond basicity alone is insufficient to fully describe or predict cellulose dissolution in 1-benzyl-3-methylimidazolium-based ionic liquids. While acetate-based systems, with near-optimal hydrogen-bond basicity, exhibit superior performance, the notable effectiveness of chloride-based ILs highlights the critical role of complementary physicochemical properties. In particular, a smaller anion size, reduced steric hindrance, and greater availability of free ions significantly enhance the ability of the solvent to penetrate and disrupt the hydrogen-bonding network of cellulose. Moreover, macroscopic properties such as viscosity and density play a decisive role in governing dissolution kinetics by influencing mass transport and solvent accessibility. ILs that combine favorable interaction strength with high fluidity are better suited to facilitate efficient cellulose dissolution. These findings emphasize that the dissolution process is governed by a complex interplay between thermodynamic interactions and transport-related factors, rather than a single dominant parameter. Therefore, the rational design of ionic liquids for cellulose processing requires an integrated, systems-level approach that simultaneously considers hydrogen-bonding capability, ion size, ion-pairing behavior, steric effects, and transport properties. By adopting this comprehensive framework, it becomes possible to more effectively tailor IL structures for enhanced performance, thereby advancing the development of efficient, sustainable solvents for biomass processing and next-generation cellulose-based materials.