Comparative Investigation of NaOH and KOH Treatments for Cellulose Purity and Polymorph Control in Pineapple Leaf Fibers: Crystallinity Changes and Electrochemical Impedance Signatures

Abstract

A large percentage of pineapple leaves is now being disposed of as agricultural waste, but this biomass holds a lot of conversion potential to value-added products. This study systematically compares NaOH and KOH treatments for the selective extraction of cellulose microcrystals from pineapple leaves. It investigates how the concentration of alkali varies and affects the cellulose content, crystallinity, and extraction yield. Alkali treatment can be used to remove the noncellulosic constituents efficiently and thus improve cellulose purity. As a result, it enhances the crystallinity of the extracted cellulose by effectively dissolving the amorphous regions. The results from Fourier transform infrared (FTIR) and X-ray diffraction (XRD) confirm structural changes in the cellulose matrix due to alkaline treatment, with XRD analysis indicating the onset of the Cellulose I-to-Cellulose II transformation between 2 and 3 M NaOH and between 3 and 4 M KOH under the investigated treatment conditions. Electrochemical impedance spectroscopy (EIS) revealed that the structural transitions of cellulose significantly modulate the electrochemical properties. Resistance and relaxation behaviors are determined by crystallinity and polymorphic transitions between Cellulose I, amorphous phases, and Cellulose II, with Cellulose I having a greater resistive response, amorphous domains having better conductivity and capacitance, and Cellulose II having dense resistive behavior when dry and capacitive capacities after rehydration. Overall, this study demonstrates that agricultural waste can be converted into highly pure cellulose with tunable crystallinity and electrochemical characteristics, providing mechanistic insights that may inform the future design of cellulose-based electroactive materials. These findings suggest potential relevance to areas such as energy storage, biosensing, and environmentally friendly electronic materials, which warrants further investigation.

Introduction

The optimization of alkali pretreatment for cellulose extraction is a critical process in biomass valorization, especially for the abundant and sustainable agricultural residue like pineapple leaves. Despite its comparatively low lignin and hemicellulose fraction, pineapple leaf fibers (PALF) generate large quantities of fibers after fruit harvest and are usually discarded as waste because it contains a high cellulose content (69.5–82%). − This composition renders PALF a potentially valuable, low-cost feedstock for cellulose-based biocomposites, biofuels, and nanomaterials, which in turn facilitates waste valorization and the wider transition toward sustainable development of biobased materials. ,

Efficient cellulose extraction requires selective removal of noncellulosic components, primarily hemicellulose and lignin, while preserving the cellulose framework. Alkali treatment is one of the most widely used pretreatment methods as it disrupts hydrogen bonding between the components of the lignocellulosic matrix, dissolves the amorphous noncellulosic constituents, and thereby increases the purity and crystallinity of cellulose. − However, there needs to be careful control of the treatment severity, as an excessive alkali concentration can lead to degradation of the cellulose substrate itself, which in turn will reduce yield and affect structural integrity. ,

The two most commonly used alkalis for this purpose are sodium hydroxide (NaOH) and potassium hydroxide (KOH), but they cannot be substituted in chemical reactions with respect to cellulose. NaOH is particularly effective at disrupting interchain hydrogen bonding, which facilitates efficient removal of hemicellulose and lignin and yields a highly crystalline cellulose I product under moderate treatment conditions. , KOH is a relatively strong cation source with lower hydration energy, which leads to different swelling behavior in the fiber matrix and has been reported to induce a polymorphic transformation of cellulose from the native cellulose I to the more stable cellulose II form. , This polymorphic distinction is fundamental in terms of chain packing (parallel vs antiparallel), hydrogen-bond density, and, as has been recently demonstrated, in electrical response (dielectric constant and bulk resistance), which is typically higher for cellulose II than for cellulose I due to more segmental mobility in the cellulose II structure. , Moisture further modulates this behavior by acting as a plasticizer that enhances ionic conduction. The scientific basis for the direct comparison of NaOH and KOH is the mechanistic differences between the two, which are related to the ionic size, hydration, and swelling behavior.

However, the majority of studies on pineapple leaf fiber cellulose extraction focused on the use of NaOH or KOH in isolation, in different types of biomasses, concentrations, and process conditions, which is not appropriate for making a direct comparison among different studies. − Thus, there is no systematic, side-by-side description of the differential influences of NaOH and KOH (with the same source of fibers and under the same extraction conditions) on cellulose yield, purity, degree of crystallinity, polymorph ratio, and the polymorph transition of the cellulose and, critically, the resulting electrochemical behavior of the extracted cellulose.

This study aims to fill this gap by systematically investigating the NaOH and KOH treatments over a broad range of concentrations (0–5 M) on pineapple leaf fibers under similar processing conditions. The resulting cellulose was characterized through yield and chemical composition analysis, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) for purity, delignification, and polymorphic transformation, and, as a key novel contribution of this work, through electrochemical impedance spectroscopy (EIS) to establish, for the first time in this system, the relationship between the changes in purity and the changes in the resistive and capacitive behavior of the cellulose. This study directly ties the chemistry of the alkali, degree of concentration, crystallinity, and polymorph identity of cellulose to its electrical response, giving mechanistic insight beyond traditional structural characterization and guiding tailoring of pineapple leaf-derived cellulose for novel applications, including biosensing, energy storage, and environmentally friendly electronic materials.

Materials and Methods

Materials

Pineapple leaves (PLs) were collected from local sources. The reagents used for the chemical treatment of pineapple leaves were NaOH (Analytical grade, Research-Lab Fine Chem Industries, India), KOH (Analytical grade, Daejung Chemicals & Metals Co., Ltd, Korea), and H₂SO₄ (Analytical grade, Research-Lab Fine Chem Industries, Mumbai 400 002 (India)).

Methods

Pretreatment of Raw Materials

The PL was washed with distilled water and oven-dried at 60 °C for 12 h. They were then ground into a fine powder. The pineapple leaf fiber was then sieved using a sieve set for a uniform size of about 0.063 mm.

Alkali Treatment

PL fibers were treated with varying NaOH and KOH concentrations (0, 0.1, 0.5, 1, 2, 3, 4, and 5 M) at 90 °C for 2 h, maintaining a 1:20 g/mL fiber-to-solution ratio. Subsequently, Alkali-Treated Fibers (ATF) were thoroughly washed with distilled water to neutralize pH and eliminate excess alkali deposits. After washing, the fibers were dried in a freeze-dryer for 24 h to remove residual moisture.

Electrochemical Impedance Spectroscopy

Cellulose samples (∼0.05 g) were compressed into pellets with a diameter of approximately 7.0–7.2 mm and a thickness of 1.0–1.2 mm. EIS measurements were performed on cellulose pellets under two sample conditions. As-prepared samples were analyzed without additional drying, then the same samples were dried at 100 °C for 1 h and cooled in a desiccator before analysis. The pellets were sandwiched between two stainless-steel electrodes, and EIS measurements were carried out using a Gamry Reference 3000S instrument.

Characterization

Yield Composition Analysis

The extraction yield percentage of ATF was calculated using eq , considering the mass of raw PL powder and the mass of ATF.

$$\mathrm{y}\mathrm{ield}(\%)=\frac{M_1-M_2}{M_3}\times 100$$ 1

M ₁ = Mass of treated ATF within a weighing container

M ₂ = Mass of weighing container

M ₃ = Mass of raw PL

All experiments were performed in triplicate (n = 3), and results are reported as mean ± standard deviation (SD).

Chemical Composition Analysis

The process’s primary steps involve the removal of extractives, strong acid hydrolysis of hemicellulose under nonheat settings, and high-temperature dilute acid hydrolysis. First, 120 mL of distilled water and 1 g of dried sample (A) were heated in a 1 h water bath at 100 °C. Then, the solution was filtered and the residue was washed with hot water. The dried residue was put in an oven until a constant weight was achieved to yield the residue (B). To the residue (C), 150 mL of 1 N H₂SO₄ was added and heated in a water bath at 100 °C for 1 h. After that, the solution was filtered and washed with distilled water; the dried residue was kept until it reached a constant weight (C). The residue was later soaked with 10 mL of 72% H₂SO₄ at room temperature in the water bath for 4 h. After that, 150 mL of 1 M H₂SO₄ was added to the solution and further heated at 100 °C in the water bath for 2 h. The obtained residue was filtered and washed with distilled water until it reached a neutral pH. The obtained residue was then kept in the oven at 105 °C until a constant weight was obtained (D). The dried residue was then kept in the furnace at 600 °C until it turned to ash; this was weighed (E). The percentage of the chemical composition of ATF was calculated using eqs –.

$$\mathrm{h}\mathrm{ot\; water}‐\mathrm{soluble}(\%)=\frac{A-B}{A}\times 100$$ 2

$$\mathrm{h}\mathrm{emicellulose}(\%)=\frac{B-C}{A}\times 100$$ 3

$$\mathrm{c}\mathrm{ellulose}(\%)=\frac{C-D}{A}\times 100$$ 4

$$\mathrm{l}\mathrm{ignin}(\%)=\frac{D-E}{A}\times 100$$ 5

$$\mathrm{a}\mathrm{sh}(\%)=\frac{E}{A}\times 100$$ 6

Fourier Transform Infrared (FTIR) Analysis

The ATF samples were analyzed for their functional groups using Fourier transform infrared spectroscopy (Bruker α II FTIR Spectrometer) with wavenumbers from 4000 to 400 cm^–1.

X-ray Diffraction (XRD) Analysis

The crystallinity of the ATF samples was examined using X-ray diffraction with Cu Kα radiation (λ = 1.5406 Å) generated by the Rigaku Ultima IV X-ray Diffractometer. The samples were scanned at an operating voltage of 40 kV and 30 mA in the 2θ range of 10°–80°.

The crystallinity index (CI) of cellulose samples was calculated from X-ray diffraction (XRD) data using the Segal method, as given in eq

$$\mathrm{C}\mathrm{I}(\%)=\frac{I_{002}-I_{\mathrm{a}\mathrm{m}}}{I_{002}}\times 100$$ 7

where I ₀₀₂ is the maximum intensity of the (002) crystalline plane and I _am is the intensity of the amorphous background at 2θ ≈ 18°. For Cellulose I-dominant samples, the maximum intensity of the principal crystalline reflection at approximately 2θ = 22–23° and the amorphous intensity at approximately 2θ = 18° were used. For Cellulose II-dominant samples, the maximum intensity of the principal crystalline reflection at approximately 2θ = 20–22° and the corresponding amorphous intensity at approximately 2θ = 16° were used. Samples within the polymorphic transition region were interpreted with caution because of possible overlap between Cellulose I and Cellulose II reflections.

Results and Discussion

Yield Composition Analysis

Table shows the fiber yield percentages obtained using NaOH and KOH at different alkali concentrations. According to the results, yield decreased progressively with increasing alkali concentration for both treatments. The untreated sample was taken as 100% yield, while 0 M treatment (distilled water) reduced it slightly to 88.13 ± 0.25%, reflecting the removal of water-soluble impurities. Yield dropped more sharply with increasing NaOH and KOH concentration, from 74.55 ± 0.35% and 78.13 ± 0.22% at 0.1 M to 22.84 ± 0.15% and 28.64 ± 0.28% at 5 M, respectively. KOH consistently maintained higher fiber yield than NaOH across all concentrations. This behavior can be attributed to differences in the ionic size and hydration characteristics of K⁺ and Na⁺, which influence their accessibility to the cellulose structure and the extent of fiber swelling. The smaller Na⁺ ion can more readily access the cellulose microstructure and promote swelling and disruption of intermolecular hydrogen bonding, leading to greater structural disruption and fiber mass loss. In comparison, the larger K⁺ ion shows more limited accessibility within the cellulose structure, resulting in less extensive structural disruption and consequently higher fiber mass retention under the investigated conditions. This progressive yield loss reflects the increasing removal of noncellulosic components at higher alkali concentrations, the compositional details of which are discussed in relation to Table .

1. Comparison of the Yield Percentage of Untreated and ATF Samples by NaOH and KOH.

	Yield Percentage (%) ± SD
Sample Concentration (mol/L)	NaOH	KOH
Raw	100.00 ± 0.00	100.00 ± 0.00
0 M (Water)	88.13 ± 0.25	88.13 ± 0.25
0.1 M	74.55 ± 0.35	78.13 ± 0.22
0.5 M	71.04 ± 0.02	75.60 ± 0.37
1 M	32.14 ± 0.85	35.45 ± 0.33
2 M	28.71 ± 0.20	30.05 ± 0.85
3 M	25.21 ± 0.10	29.37 ± 0.56
4 M	25.01 ± 0.60	29.19 ± 0.08
5 M	22.84 ± 0.15	28.64 ± 0.28

2. Comparison of the Chemical Composition of Untreated and ATF Samples by NaOH and KOH.

	Lignocellulosic Composition (%)
	Hot Water-Soluble		Hemicellulose		Cellulose		Lignin		Ash
Sample Concentration (mol/L)	NaOH	KOH	NaOH	KOH	NaOH	KOH	NaOH	KOH	NaOH	KOH
Plant Powder	14.26	14.26	19.95	19.95	51.02	51.02	13.99	13.99	0.78	0.78
0 M	14.86	14.86	18.99	18.99	51.68	51.68	13.75	13.75	0.72	0.72
0.1 M	11.95	13.32	17.15	15.26	58.59	59.31	11.75	11.55	0.56	0.56
0.5 M	10.37	11.83	15.61	14.12	62.88	63.26	10.59	10.25	0.55	0.54
1 M	1.19	1.23	7.71	5.07	80.51	83.27	10.09	9.92	0.50	0.52
2 M	1.39	2.74	9.40	5.63	79.01	81.48	9.74	9.66	0.46	0.48
3 M	1.71	3.11	10.67	6.54	77.77	80.39	9.63	9.52	0.22	0.44
4 M	2.23	4.84	11.16	6.47	76.87	79.05	9.55	9.23	0.19	0.41
5 M	2.82	6.24	13.96	8.67	73.85	75.65	9.28	9.08	0.09	0.36

Chemical Composition Analysis

Table discusses the lignocellulosic composition analysis in pineapple leaf powder that was independently treated with varying concentrations of NaOH and KOH. Both alkalis showed a consistent trend up to 1 M: hot water-soluble, hemicellulose, lignin, and ash decreased, while cellulose increased, peaking at 80.51% (NaOH) and 83.27% (KOH). This indicates effective removal of noncellulosic components, concentrating cellulose in the fiber. Beyond 1 M, the cellulose content gradually decreased (to 73.85% and 75.65% at 5 M). This suggests that excessive alkali concentration causes partial cellulose degradation via alkaline peeling reactions, in which cellulose chain ends are progressively hydrolyzed, releasing shorter carbohydrate fragments detected as hemicellulose or hot-water-soluble contents.

Lignin decreased steadily across all concentrations (from 13.99% to ∼9.1–9.3% at 5 M), indicating continuous delignification, while ash content also declined progressively. Comparing the alkalis, KOH consistently retained higher cellulose and lower hemicellulose content than NaOH at every concentration, indicating slightly more selective hemicellulose removal. This aligns with the higher fiber yields observed for KOH in Table . Overall, 1 M represents the optimal alkali concentration for maximizing cellulose purity, with higher concentrations promoting cellulose degradation despite continued lignin removal.

Besides, for the cases of NaOH and KOH at 1 M, a higher degree of delignification is observed, with the lignin content decreasing to 10.09% and 9.92%, respectively, which is highly significant for enhancing cellulose purity. Table demonstrates that optimizing alkali concentrations throughout the cellulose extraction is crucial to obtaining a maximum yield and purity. This will allow the production of biopolymers more efficiently and enhance the variety of potential uses for extracted cellulose.

These results highlight an inherent trade-off between the cellulose yield and cellulose purity. While increasing alkali concentration progressively improves cellulose purity by removing hemicellulose, lignin, and other noncellulosic components (Table ), this comes at the cost of decreasing overall fiber yield (Table ), as more aggressive alkaline conditions also promote partial cellulose degradation and dissolution. For instance, at 1 M concentration, both NaOH and KOH achieve high cellulose purity (80.51% and 83.27%, respectively) but at a substantially reduced yield compared with lower concentrations. This trade-off underscores the importance of selecting an alkali concentration that balances acceptable purity with practical yield retention depending on the intended application.

FTIR Analysis

Figures and indicate a comparative analysis of FTIR spectra of NaOH and KOH alkali treatment at varying concentrations used in cellulose extraction, thus showing both similarities and significant differences in their respective performances. Both FTIR spectra demonstrate effective delignification and removal of hemicellulose, evident by the decrease in peak intensities around 1727 cm^–1 and 1617 cm^–1, which correspond to the carbonyl (CO) groups in hemicellulose and lignin, as the alkali concentration increases. Further evidence supporting the cellulose I to cellulose II transition is observed in the O–H stretching region (3300–3400 cm^–1). In untreated and lightly treated samples, the O–H stretching band appears centered around 3340–3350 cm^–1, reflecting the intramolecular hydrogen-bonding pattern characteristic of native cellulose I. With increasing alkali concentration, this band broadens and shifts toward slightly lower wavenumbers (∼3300–3320 cm^–1), indicating reorganization of the intra- and intermolecular hydrogen-bonding network associated with the structural transformation toward Cellulose II. In addition, the distinctive peak at 1246 cm^–1, indicative of C–O stretching in lignin, illustrates that higher alkali concentrations correspond to decreased lignin content. Fourier transform infrared (FTIR) spectra demonstrate the successful elimination of noncellulosic elements following the optimization of alkali concentrations. KOH specifically can be considered more efficient in removing hemicellulose and lignin compared to other types, which makes it more applicable in those applications that require a high level of cellulose purity. Therefore, the FTIR spectra qualitatively support the removal of lignin and hemicellulose by both alkali treatments, while the quantitative compositional data presented in Table indicate slightly greater removal of noncellulosic components with KOH under the investigated conditions. These observations can be used to emphasize the importance of careful fine-tuning of the alkali concentration to achieve an optimal cellulose extraction.

1.

FTIR spectra of PL fibers and ATF treated with NaOH solutions at various concentrations.

2.

FTIR spectra of PL fibers and ATF treated with KOH solutions at various concentrations.

XRD Analysis

The XRD results of alkali treatments induced distinct modifications in the crystalline structure of cellulose fibers at different alkali concentrations of NaOH and KOH, as shown in Figures and , respectively. Fibers treated with NaOH at 0, 0.1, 0.5, 1, and 2 M concentrations had three characteristic peaks of Cellulose I crystal structure at 2θ around 16.5°, 22.5°, and 34.5°, corresponding to the crystallographic planes of (110), (200), and (004), respectively. The XRD patterns indicated the onset of the Cellulose I to Cellulose II transformation between 2 and 3 M NaOH. KOH-treated samples exhibited a similar overall trend; however, the onset of this transformation occurred at a higher concentration range, between 3 and 4 M KOH (Figures and ).

3.

XRD spectra of PL fibers and ATF treated with NaOH.

4.

XRD spectra of PL fibers and ATF treated with KOH.

This concentration-dependent difference, along with the correspondingly lower fiber yield obtained with NaOH relative to KOH at equivalent concentrations (Table ), reflects the distinct ion-exchange and lattice-penetration behavior of Na⁺ and K⁺, rather than differences in hydroxide basicity, which is essentially equivalent for both alkalis. Na⁺ has a smaller ionic radius but a more strongly bound, larger hydrated shell than K⁺, allowing it to intercalate efficiently between adjacent cellulose chains and form a soda-cellulose (alkali-cellulose I) complex. This intercalation swells the crystal lattice and disrupts the native hydrogen-bonding network at comparatively low NaOH concentrations, driving both the earlier onset of cellulose I→II reorganization and, before that reorganization stabilizes, greater dissolution of shorter cellulose chains into the amorphous phase and into solution (consistent with alkaline peeling of reducing chain-end groups), explaining the lower yield under NaOH. K⁺, being larger and weakly hydrated with lower charge density, penetrates the tightly packed cellulose I lattice less readily at the same molar concentration; consequently, a higher KOH concentration is needed to achieve equivalent lattice disruption, resulting in a delayed I→II transition, better-preserved crystalline continuity, and higher retained yield even at elevated concentrations. Overall, NaOH offers faster, more cost-effective structural transformation at lower concentrations but causes greater fiber loss, while KOH requires higher concentrations (increasing reagent use) but better preserves fiber yield and structural integrity. The choice between the two should therefore depend on whether the application prioritizes processing efficiency (favoring NaOH) or fiber yield retention (favoring KOH).

In both KOH and NaOH treatments, the crystallinity index increases with the concentration because all amorphous crystals are dissolved in the highly concentrated alkali medium. However, a slight reduction in the crystallinity index with the normal pattern was observed before the threshold concentration of phase transitions, because at the transition condition, the amorphous stage is engaged before the phase transition; as a result, the crystallinity index gets reduced, and this variation can be observed at 2 M concentration in NaOH treatment and 3 M concentration in KOH treatment in Table .

3. Crystallinity Index of Samples Treated with Different Concentrations of NaOH and KOH.

	Crystallinity Index (%)
Sample	NaOH	KOH
Raw	29.77	29.77
0 M	30.94	30.94
0.1 M	41.44	38.12
0.5 M	47.53	51.88
1 M	51.00	51.61
2 M	50.09	46.64
3 M	63.73	42.86
4 M	70.22	45.92
5 M	70.72	63.74

The crystallinity index (CI) of the ATF is shown in Table . This study examines the effect of NaOH and KOH concentrations on the CI in alkali treatment. The raw sample exhibited a CI of 29.77%, slightly increasing to 0 M (30.94%) for both alkalis. A significant improvement in CI was noted at a 5 M NaOH concentration (70.72%) and 5 M KOH concentration (63.74%) with an increased alkali concentration. After the treatment was carried out with the use of KOH, the crystallinity index (CI) exhibited a significant reduction, with the lowest value of 42.86 at a concentration level of 3 M, followed by an increase to 63.74 at a level of 5 M. In comparison, treatment with NaOH showed a continuous progressive rise in CI, especially at high levels. It is relevant to note that CI indeed dropped at the 2 and 3 M concentrations, which is explained by the change of cellulose I to cellulose II. This is because hydroxide ions disrupt the hydrogen-bonding network in the cellulose I lattice, causing swelling and the temporary formation of a less-ordered intermediate structure before reorganization into cellulose II. Since this reorganization is not instantaneous, the intermediate stage shows reduced order, producing the observed dip in the CI. The deeper dip with KOH at 3 M may reflect differences in the ionic radius between K⁺ and Na⁺, affecting how the alkali penetrates the fiber. This structural transition temporarily decreases crystallinity as the cellulose framework undergoes rearrangement before stabilizing at higher alkali concentrations. Overall, the results indicate that NaOH is more effective in increasing crystallinity beyond a 3 M concentration, whereas KOH exhibits a more variable trend with intermediate decreases in CI. The results suggest that both the alkali type and concentration significantly influence cellulose structure, which is important for optimizing pretreatment conditions in biomaterial applications.

EIS Analysis

EIS was performed on the NaOH-treated samples to study how changes in the cellulose crystallinity and polymorphic structure affect its impedance behavior. The impedance responses for cellulose samples treated at varying concentrations of NaOH were analyzed using the Nyquist plots shown in Figure . The equivalent circuit model fixed with Nyquist plots consists of charge-transfer resistance (R _ct), relaxation resistance (R _re), a constant phase element (CPE), and the double-layer capacitance (C _dl). R _ct reflects interfacial ion and electron transfer at the cellulose and electrolyte boundary, which is lowered by swelling, ionic residues, and amorphous regions that enhance accessibility due to its high wettability. , R _re describes the dielectric relaxation of the bound water and dipolar groups within the matrix. The CPE describes the heterogeneous capacitive storage. The magnitude (Q) value of the CPE tends to increase with moisture uptake and polymer swelling due to enhanced ionic mobility and heterogeneous charge distribution, but it may decrease upon drying or when crystallinity is high because the system becomes more rigid and homogeneous. The n value of the CPE, which reflects the deviation from ideal capacitive behavior, may increase with crystallinity because crystalline domains provide more uniform dielectric environments. C _dl corresponds to the true electrostatic charge storage at the interface.

5.

Nyquist plots of cellulose samples treated with different concentrations of NaOH, showing (a) nondried samples at room conditions, (b) dried samples, and (c) the equivalent circuit model applied for impedance analysis. Symbols represent the actual data, and the line represents the fitted curve.

Table shows the values of components of the equivalent circuit of all moisturized and dried samples. According to Table , distinct differences were observed between Cellulose I, amorphized states, and Cellulose II.

4. Values of Components of the Equivalent Circuit of All Moisturized and Dried Samples.

			CPE		C dl
Sample	R ct	R re (MΩ)	Q × 10–12/S·s n	n (−)	(pF)
RC- 1M	222.1 kΩ	37.24	904.0	0.514	14.36
RC-2M	471.0 kΩ	11.30	580.0	0.578	14.96
RC-3M	652.9 kΩ	46.64	278.0	0.593	14.81
RC-4M	23.38 kΩ	340.2	226.9	0.553	13.58
RC-5M	130.1 Ω	99.91	691.9	0.502	14.28
DC-1M	4.033 MΩ	353.9	48.70	0.720	13.30
DC-2M	1.56 MΩ	339.0	264.2	0.535	13.90
DC-3M	1.752 MΩ	148.7	300.0	0.577	33.12
DC-4M	2.729 MΩ	278.6	29.73	0.730	15.00
DC-5M	54.51 MΩ	341.5	23.00	0.856	12.90

R _ct represents interfacial ion/electron transfer. Moisturized Cellulose I samples (1 M–2 M) displayed high R _ct values of 222.1 kΩ and 471.0 kΩ, respectively, and they decreased significantly when converted to Cellulose II (23.38 kΩ at 4 M). The lower R _ct in swollen Cellulose II is attributed to improved electrolyte access and restructured fibril interfaces. After drying, however, R _ct increased again due to the removal of swelling water and compaction of the crystalline network, reducing effective ion transport channels. This observation indicates a direct dependence of R _ct on moisture accessibility and polymorphic form, where swelling water promotes charge transfer and drying hinders it.

R _re explains the dielectric relaxation within the bulk sample. Moisturized samples showed broad variation (11.3 to 340.2 MΩ), with a general increase for higher NaOH concentrations, reflecting the emergence of more ordered Cellulose II crystallites. When the sample is dried, R _re values increased (148.7 to 353.9 MΩ), suggesting that restricted segmental mobility in crystalline domains controls the dielectric response. The data confirm that R _re is strongly influenced by crystallinity, with Cellulose I, allowing greater relaxation, while Cellulose II exhibits more rigid dielectric behavior.

In a CPE, the magnitude is expressed by the Q value, while the heterogeneity index (n) reflects the degree of deviation from ideal capacitive behavior. Moisturized samples showed relatively high Q values (904.0 × 10^–12 S·s ⁿ at MOIS 1 M), indicating strong hydration and heterogeneous ion storage with enhanced capacitive response. When the sample is dried, Q decreased sharply (23.0 × 10^–12/S·s ⁿ at UM 5 M), consistent with the removal of porosity and reduced water-mediated charge storage. Interestingly, higher crystallinity also led to a reduction in Q, as the tightly packed structure limited the ability of swelling water molecules to participate in charge storage. The n values for moisturized samples ranged from 0.502 to 0.593, while dried samples displayed a broader distribution (0.535–0.856). Higher n values in dried, crystalline samples suggest a more homogeneous dielectric response with less dispersion of relaxation times, whereas lower n values in swollen Cellulose I reflect greater structural heterogeneity introduced by hydration and amorphous domains. When comparing UM 1 M (Cellulose I) with UM 4 M (Cellulose II), it becomes evident that the higher crystallinity of 4 M results in an n value closer to that of low-crystallinity Cellulose II. This indicates that the highly ordered assembly of Cellulose I crystals has a stronger tendency toward higher n values, moving closer to ideal capacitive behavior than Cellulose I.

C _dl remained relatively stable in both conditions (13–33 pF) because it corresponds to the charge storage at the interface. However, it showed slight enhancement in the values based on the CI% due to the contribution of swelling water molecules as the wettability increased, respectively, in Cellulose I, II, and amorphous stages, suggesting increased interfacial surface area due to partial disruption of the fibrillar matrix during swelling. After drying, C _dl generally decreased or stabilized (12.9–15 pF), consistent with the collapse of pores and reduced accessibility of ionic sites. This indicates that double-layer capacitance in cellulose systems is controlled primarily by accessible surface area, which is moisture-dependent but less sensitive to polymorphic transitions than other parameters.

These results clearly demonstrate that the electrochemical behavior of cellulose is not governed by a single factor but by the intricate interplay between crystallinity, polymorphic stage, hydration level, and the presence of ionic residues. Moisturized Cellulose II, for example, exhibited markedly lower R _ct and an enhanced capacitive response, highlighting how swelling water molecules open up ion transport pathways and facilitate charge storage. These trends reveal that crystallinity stiffens the dielectric environment and limits charge accommodation, while hydration and amorphous regions soften the structure and promote ion conduction. Importantly, the comparative analysis between Cellulose I and II shows that although Cellulose II becomes dense and resistive when dried, it can recover strong capacitive properties under hydrated conditions, while Cellulose I maintains a more resistive but less polarizable character. Such mechanistic insights align with previous studies − on the coupling of hydration, crystallinity, and ion transport in biopolymeric materials, and emphasize how controlled structural transitions can be leveraged to tune cellulose into a versatile electroactive platform.

Conclusion

This paper analyzes the impact of alkali treatments on the crystallinity, yield, and purity of cellulose extracted from pineapple leaves. The findings suggest that both sodium hydroxide (NaOH) and potassium hydroxide (KOH) are effective in eliminating noncellulosic components (removing lignin and hemicellulose), with optimal cellulose purity and crystallinity at a concentration of 1 M. This concentration provided the highest cellulose content with the least amount of residual hemicellulose and lignin, as confirmed by the chemical composition analysis. Yield analysis indicates that the yield of extracted cellulose gradually decreased with an increase in the alkali concentration. There is a significant difference in the yield retention of treatments with KOH compared to that with NaOH, with the former showing slightly better retention, which may be due to changes in reactivity. However, an alkali amount of 1 M ensures the purity of cellulose, but a higher concentration also promotes depolymerization of fibers and thus reduces the yields. The FTIR and XRD results indicated that alkali treatments enhanced the crystallinity and induced structural changes of cellulose materials at elevated concentrations. The implications of these findings are the need to standardize the alkali concentration to obtain the maximum recovery of cellulose, its purity, and its crystallinity. As a renewable and cost-effective agricultural waste stream, pineapple leaves represent a promising source of cellulose that could potentially be valorized for the development of environmentally friendly materials. Pineapple leaf-derived alkali-treated cellulose microcrystals have potential applications such as biocomposites, biofuels, and nanomaterials. Electrochemical studies on cellulose show that its electrochemical behavior is affected by several factors, including crystallinity, polymorphic structure, water absorption, and the presence of ionic residues. Higher moisture content and a greater proportion of amorphous regions tend to decrease the resistance parameters (R _ct and R _re) while increasing the capacitance values (Q and C _dl). In contrast, drying and increased crystallinity led to higher resistance and lower capacitive behavior in the cellulose structure. Considering the crystallinity, cellulose II becomes denser and more resistive when dried but regains strong capacitive behavior upon hydration. In contrast, cellulose I remains relatively electrically resistant and exhibits lower polarizability under similar conditions.

Further studies that include alternative treatments, the use of alkalis with various chemicals, and the environmental aspects of different waste streams would add more viability and sustainability to the proposed methodology. These findings may contribute to future efforts toward valorizing agricultural waste in support of sustainability and circular-economy goals.

Glossary

Abbreviations

FTIR

Fourier Transform Infrared Spectroscopy

XRD

X-ray Diffraction

PL

Pineapple Leaves

ATF

Alkali-Treated Fibers

THERE

Crystallinity Index

ICE

Electrochemical impedance spectroscopy

CPE

constant phase element

G.M.M.DissanayakaDepartment of Biosystems Technology, Faculty of Technological Studies, Uva Wellassa University of Sri Lanka, Badulla, Sri Lanka. (First Author: Materials development, methodology, investigation, data curation, formal analysis, writing original draft) K.G.C.Senarathna–Professor, Department of Biosystems Technology, Faculty of Technological Studies, Uva Wellassa University of Sri Lanka, Badulla, Sri Lanka. (Second Author: Conceptualization, review and editing, project administration) K.M.K.S.JayasinghaDepartment of Biosystems Technology, Faculty of Technological Studies, Uva Wellassa University of Sri Lanka, Badulla, Sri Lanka. (Third Author: EIS data)

The authors declare no competing financial interest.