The investigation on dark dyeing properties of silk using Dioscorea cirrhosa Lour. tuber extracts with varied molecular weights

Abstract

This work reported the dark dyeing properties of Dioscorea cirrhosa Lour. tuber extracts with different molecular weights on silk. The study on component analysis indicates that the molecular weight of most condensed tannins and polyphenols in D. cirrhosa L. tuber extracts is higher than 1000. The kinetic study demonstrates that the adsorption processes of D. cirrhosa L. tuber extract components with molecular weights higher than 1000, and components with molecular weights lower than 1000 toward silk fabrics are fitted with the pseudo-second order model, and the survey of adsorption isotherms reveals that multiple interactions occurred between dyes and silk, encompassing hydrogen bonding and van der Waals forces. The dyeing research shows that the dyed silk achieves its darkest color when employing D. cirrhosa L. tuber extract components with molecular weights lower than 1000 with a Fe²⁺ mordant concentration of 4 g/L and a pH value of 5. The K/S and L* values of dyed silk are 9.05 and 30.77, respectively. Under this dyeing condition, the complex, Fe element content, and Fe²⁺ ratio on the silk dyed with D. cirrhosa L. tuber extract components with molecular weights lower than 1000 surface are higher than those of D. cirrhosa L. tuber extracts and D. cirrhosa L. tuber extract components with molecular weights higher than 1000. The present study indicates that condensed tannins in D. cirrhosa L. tuber extracts are not the sole active compounds involved in the silk dark dyeing process. Polyphenols with molecular weights below 1000 in D. cirrhosa L. tuber extracts are shown to influence significantly and, in some cases, dominate the silk dark dyeing process.

Keywords

molecular weights dark dyeing property silk fabric

Synthetic dyes are a prevalent choice for textile dyeing processes in the textile industry.¹ However, synthetic dyes derived from petrochemical sources emit numerous harmful chemicals into the environment throughout their production and application time.² Therefore, eco-friendly natural extracts are gaining popularity in textile fields due to their biodegradability and environmental compatibility.³ Nevertheless, natural extracts demonstrate poor fastness properties. Mordants are frequently employed to address this issue.^4,5 With the addition of mordants, the color of natural extracts undergoes a change. Borah et al.⁶ reported green tea extract as a natural dye for staining silk textiles to achieve a wide range of colors by employing various mordants. Zhang and Jia⁷ developed a new bio-colorant using the chestnut leaf as a raw material for tussah silk, and comprehensively characterized the thermodynamic interactions of the chestnut leaf extract with tussah silk employing the Langmuir isotherm model. Numerous reports indicate the successful utilization of various natural extracts in textile dyeing processes, such as grape seed,⁸ persimmon,⁹ Cinnamomum camphora (L.) presl fruit,¹⁰ blackcurrants,¹¹ and so on. These reports detail the techniques employed for extracting dyes from plants, and evaluate the color variations in fabrics dyed under diverse conditions, encompassing pH, mordant treatments, and fiber types.

Dioscorea cirrhosa Lour. tuber (DL) contains numerous polyphenols, with condensed tannin (the structure is shown in Figure 1) being the primary constituent.^12,13 Silk fabrics can be dyed dark colors using Dioscorea cirrhosa Lour. tuber extracts (DLEs) as colorants. The shiny blackness of Gambiered Guangdong silk is dyed with DLE. Researchers widely believe that condensed tannins in DLE play a crucial role in the dyeing process.^14
–16 The polymerization degree of condensed tannins in DLE ranges from 2 to 8, and its molecular weight (Mw) ranges from 500 to 2500.¹⁷ However, condensed tannins with a high degrees of polymerization are challenging to dye textiles, and this is also one of the reasons for the poor dyeing effect of natural dyes.¹⁸ The composition of DLE is intricate and comprises various polyphenols aside from condensed tannins.¹⁹ These polyphenols with low Mw are easier to get inside the textile fibers and dye them. However, the role of these polyphenols in dyeing processing is often ignored.²⁰ There are only a few reports about the impact of different components within DLE on dyeing textiles. To explore this, finding a method to isolate the condensed tannins and polyphenols is desirable. Currently, diverse methods are employed for multicomponent isolation of natural extracts, such as macro porous resin adsorption,²¹ gel chromatography,²² liquid–liquid extraction methods,²³ and so on. These methods have some drawbacks with isolation efficiency,^24,25 and do not apply to the content of this study. Dialysis is a common separation technique used to isolate substances in solution into components of different Mw.²⁶ This method holds significant value in separating components because of the high separation efficiency and low cost.^27,28 Nevertheless, there are few reports about using the dialysis method for isolating and preparing the different Mw of natural extracts.

Figure 1.

Chemical structure of condensed tannin.

DLE is a complex mixture containing predominantly condensed tannins with a Mw exceeding 1000.^10,17,19 Therefore, a dialysis bag with a Mw cut-off of 1000 was employed to separate DLE into constituents with Mw higher than 1000 (H-DLE), and those constituents with Mw lower than 1000 (L-DLE) in this investigation. Subsequently, the chemical compositions of DLE, H-DLE, and L-DLE were determined and analyzed using ordinary methods. The adsorption kinetics of DLE, H-DLE, and L-DLE dyeing silk were investigated, and their respective adsorption isotherms were constructed. The silk fabrics were dyed using these three components, and their effects on the color characteristics of the dyed fabrics concerning mordant types (Fe²⁺, Fe³⁺, Al³⁺, and Cu²⁺), mordant concentrations, and pH values were investigated. In addition, the morphologies of the Fe²⁺ mordanted silk fabrics were investigated. The content and valence states of elements on the surface of the Fe²⁺ mordanted silk fabrics were detected. Although the use of DLE has a long history in the textile industry, to the best of our knowledge, this study represents the first attempt to isolate DLE using the dialysis method into two parts with different Mw, and investigate polyphenols aside from condensed tannins impacts on dyeing silk fabrics. The present research is expected to provide novel thoughts on exploring DLE dyeing silk fabrics.

Experimental materials and instruments

Materials

Dry Dioscorea cirrhosa Lour. tuber (sourced from Guizhou province, commercially available), silk fabric of crepe de Chine (58 g/m², commercially available), ethanol (purity 95%, San-WEI Disinfectant Co., Ltd.), hydrochloric acid (Kermel Co., Ltd.), vanillin (Kermel Co., Ltd.), nitric acid (Kermel Co., Ltd.), CH₃COO₂Na·3H₂O (Kermel Co., Ltd.), CH₃COOH (Kermel Co., Ltd.), Folin-Ciocalteu reagent (Macklin Reagents), FeSO₄ · 7H₂O (Macklin Reagents), FeCl₃ · 6H₂O (Macklin Reagents), KAl(SO₄)₂ · 12H₂O (Macklin Reagents), CuSO₄·5H₂O (Macklin Reagents), dialysis bags (JIELEPU Co., Ltd.), (+)-Catechin (Macklin Reagents) and the above chemicals are analytical reagents, KBr (spectral purity, Sinopharm Reagents), methanol (spectral purity, Kermel Co., Ltd.) flaked soap (commercially available, pH of 2 g/L solution is 12.5), detergent for silk and wool (Shanghai Zhengzhang Laundering and Dyeing Co. Ltd., pH of 2 g/L solution is 6.5).

Instruments

High-speed centrifuge (Xiang Yi Co., Ltd.), DZKW-4 electronic constant temperature water bath (Yong Guang Ming Medical Instruments Co., Ltd.), YB-150 high-speed mill (Su Feng Industry and Trade Co., Ltd.), RE52CS rotary evaporator (Shanghai Yarong Biochemical Instrument Factory), Carry-5000 UV-visible spectrophotometer (Agilent), Axis Supra+ X-ray photoelectron spectrometer (Shimadzu), Phenom pure type desktop scanning electron microscope (Holland Phenom), X-rite color i7 desktop spectrophotometer (X-Rite, Inc.), 1260 high-performance liquid chromatograph (Agilent), Aglient 5110 ICP-AES (Aglient), XWB-DR-2524 dual-temperature oscillating dyeing machine (Jingjiang Xinwang Dyeing Equipment Factory), Y571L dyeing rubbing fastness tester (Laizhou Electronic Instrument Co., Ltd.), LGJ-10 vacuum freeze drier (Beijing Songyuanhuaxing Technology Develop Co., Ltd.) SW-24E soap wash color fastness tester (Darong Textile Instrument Co., Ltd.), Q-SUN Xe-2-H xenon lamp weathering tester (Shanghai Luozhong Technology Development Co., Ltd.), tensor37 infrared spectrometer (Bruker).

Experimental method

Preparation of DLE

A mill first crushed the dry DL (10 g) into powder. After this, the dry DL powder (10 g) was mixed with 60% ethanol (200 mL). Then, the afore-mentioned mixed liquor was heated in the sealed glass container in a water bath at 60°C for 1 h. The extraction process was conducted twice. Subsequently, the extracting solution was allowed to be combined, centrifuged deposit at 12,000 rpm for 5 min, and filtered with a 0.45 µm filter membrane. Finally, the extraction solution was concentrated and freeze dried. The resulting powder was collected and designated as DLE.

Preparation of H-DLE and L-DLE

Take dry DL powder (10 g) and repeat the identical extraction operation described. The extracting solution of dry DLE powder was diluted to a constant volume of 200 mL. Subsequently, the as-prepared extracting solution was transferred into a dialysis bag with a Mw cut-off of 1000 and subjected to dialysis (1:25) for 120 h, with regular replacement of the dialysis solution every 24 h. The dialysate obtained after the dialysis process is named L-DLE, while the components remaining in the bag are denoted as H-DLE. Finally, the extracting solution of H-DLE and L-DLE was concentrated and freeze dried.

Adsorption experiments

Silk fabrics were immersed in dye baths containing DLE, H-DLE, and L-DLE at an o.w.f (on the weight of silk) of 10%, a liquor ratio of 1:50, and maintained at a constant temperature of 60°C for 460 min to investigate the adsorption rate of DLE, H-DLE, and L-DLE on silk, as shown in formula (1). The adsorption kinetics curves of silk dyed on DLE, H-DLE, and L-DLE were fitted to examine the pseudo-first order and pseudo-second order kinetic formulas on silk, as shown in formulas (2) and (3). In addition, silk fabrics were placed in dye baths containing DLE, H-DLE, and L-DLE with o.w.f ranging from 1% to 40% at 60°C for 400 min to examine the adsorption isotherms on silk, as shown in formula (4). Each set of samples was carried out at least three times, and the results were averaged:

Q_{t} = (C_{0} - C_{t}) \frac{V}{m}

(1)

{ln (Q}_{m} - Q_{t})=ln Q_{m} - k_{1} t

(2)

\frac{T}{Q_{t}} = \frac{1}{Q_{m}} + \frac{1}{k_{2} Q_{m}^{2}}

(3)

The Freundlich isotherm model is shown as²⁹

Q_{e} = K_{F} Q_{m}^{n}

(4)

where C₀ represents the initial mass concentration of dye; C_t represents the mass concentration of dye in the residue at time t; V represents the total volume of dye solution; m stands for silk quality; Q_m represents the adsorption saturation values of dye on silk; Q_t represents the dye concentration on the silk at time t, k₁ and k₂ represent the pseudo-first order and pseudo-second order kinetic adsorption rate constant. K_F is the Freundlich constant, Q_e and C_e are the dye concentrations on silk and in solution at adsorption equilibrium, respectively.

Dyeing process

The metachrome process was used in the dyeing experiment. The silk was put into the 60°C dyeing solution. Mordant was added after 30 min and maintained for 30 min. Finally, the silk fabrics were removed, rinsed thoroughly in 60°C distilled water, and dried in the open air. The silk-to-dyeing solution ratio was set at 1:50. The dye-to-silk ratio (o.w.f) was set at 10%. The dyeing solution was preheated to 60°C and the silk was immersed in it. The mordant was added after 30 min and maintained for 30 min.

Four kinds of mordant, including FeSO₄·7H₂O (Fe²⁺), FeCl₃·6H₂O (Fe³⁺), KAl(SO₄)₂·12H₂O (Al³⁺), and CuSO₄·5H₂O (Cu²⁺), were employed to study the effect of different types of mordant, and the other parameters (4 g/L mordant concentration, no addition of pH buffer) were fixed. Five mordant concentrations (0.5, 1, 2, 4, and 6 g/L) were employed to study the effect of different mordant concentrations, and the other parameters (Fe²⁺ and Fe³⁺ mordant, no addition of pH buffer) were fixed. Three pH values (4, 5, and 6) were employed to study the effect of different mordant concentrations, and the other parameters (4 g/L Fe²⁺ mordant concentration) were fixed. In investigating the pH effect, pH was controlled using a buffer solution comprising CH₃COOH and CH₃COONa. For the remaining experiments involving other parameters, no pH regulator was utilized.

Experimental scheme

The DLE was prepared through extraction, while H-DLE and L-DLE were prepared through dialysis (Figure 2). Subsequently, the composition of DLE, H-DLE, and L-DLE was tested and analyzed. The adsorption kinetics mechanism of DLE, H-DLE, and L-DLE on silk was studied, and the dyeing kinetics curve and adsorption isotherm were fitted. This study utilized the metachrome process to explore the DLE, H-DLE, and L-DLE dyed silk properties by changing the mordant types, concentration, and pH values. Ultimately, the dyed silk fabric surface was observed. The subsequent measurement of the Fe element content and valence was conducted to explore the correlation between the dark color of Fe²⁺ mordant silk fabrics.

Figure 2.

Experimental scheme.

Test method

UV-visible spectrum

The ultraviolet-visible (UV-vis) spectrum of DLE, H-DLE, and L-DLE solutions (0.05 and 0.20 g/L) were measured in the 250–760 nm wavelength range.

Fourier transform infrared spectrum

The Fourier transform infrared (FTIR) spectrum of the DLE, H-DLE, and L-DLE was determined using the KBr pellet method. The ratio of samples to the KBr pellet was 1:100.

High-performance liquid chromatography

DLE, H-DLE, and L-DLE compositions were investigated using high-performance liquid chromatography (HPLC). Chromatographic condition A represents water (H₂O) and was set in the stationary phase, and B represents methanol (CH₃OH) and was set in the mobile phase. Gradient elution: 00.00–20.00 min, 15% B; 20.00–35.00 min, 15–40% B; 35.00–35.10 min, 40–55% B; 35.10–45.00 min, 55–85% B; 45.00–50.00 min, 85–95% B; 50.00–60.00 min, 95% B. Detection wavelength was 279 nm, temperature was 25°C, the sample load was 60 µL. This chromatographic condition was used to test DLE, H-DLE, and L-DLE solutions.

Total phenolic content determination

Aqueous DLE, H-DLE, and L-DLE solutions were prepared at a concentration of 0.015 mg/L for subsequent analysis of polyphenol content using the Folin–Ciocalteu colorimetric method.³⁰ Each dye set was carried out at least three times, and the results were averaged.

Condensed tannin content determination

Methanol solutions of DLE, H-DLE, and L-DLE were prepared at a concentration of 0.2 mg/L. The condensed tannin content will be determined using the vanillin-HCl reagent method. Each set of samples was carried out at least three times, and the results were averaged.

Scanning electron microscope observation

The surface morphologies of the dyed silk fabrics were observed by scanning electron microscope (SEM). Before analysis, all the samples were sputter-coated with a gold layer.

Inductively coupled plasma-atomic emission spectroscopy

The dyed silk fabrics used nitric acid to dispose of through microwave digestion. Then, the Fe element content of the dyed silk was analyzed through inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

X-ray photoelectron spectroscopy

The surface of dyed silk was analyzed for the valence states of the Fe element using the X-ray photoelectron spectroscopy (XPS) technique.

Color parameters and color fastness

An X-Rite spectrophotometer was utilized to measure color characteristic values in silk, including apparent K/S, L*, a*, b*, C*, and h° values. Each set of dyed silk was carried out at least four times, and results were averaged. The silk fabric’s rubbing fastness was assessed following the national standard GB/T3920-2008. The silk fabric’s washing fastness was evaluated based on the national standard GB/T3921-2008. The silk fabric’s light fastness was determined following the ISO 105 B05 standard.

Results and discussion

UV-vis spectrum analysis

Figure 3(a) shows that DLE, H-DLE, and L-DLE exhibit one characteristic peak at 279 nm, the distinct absorption peak of polyphenols.^31,32 According to Figure 3(b), the color depth of the aqueous solutions follows the order L-DLE > H-DLE > DLE, all exhibiting a red-brown hue. L-DLE shows a more pronounced absorption trend within the 400–451 nm wavelength range than H-DLE, indicating more yellow light in the L-DLE solution, and relatively less yellow light in the H-DLE solution. This phenomenon suggests that polyphenolic compounds are the primary constituents of DLE, H-DLE, and L-DLE.

Figure 3.

Ultraviolet-visible (UV-vis) solution spectrum of Dioscorea cirrhosa Lour. tuber extract (DLE), DLE with molecular weight (Mw) higher than 1000 (H-DLE), and DLE with Mw lower than 1000 (L-DLE) of 0.05 g/L (a) and 0.2 g/L (b).

FTIR spectrum analysis

Figure 4 illustrates the FTIR spectra of DLE, H-DLE, and L-DLE, showing characteristic absorption bands at 3388.8 and 1361.2 cm⁻¹, attributed to the hydroxyl group of polyphenols (OH stretching and flexural vibration). Notably, the absorption bands at 1109.1 and 1059.7 cm⁻¹ indicate the C-O stretching of the ether bridge of condensed tannin. In Figure 4(b), DLE and L-DLE display a prominent absorption band at 1724.3 cm⁻¹, related to the carbonyl group of pectin (C=O stretching),^19,33 while H-DLE lacks this absorption band. This phenomenon shows that the Mw of pectin from DLE is below 1000. Distinct vibrations associate with the benzene ring skeleton are detected at 1611.2, 1525.0, and 1444.1 cm⁻¹.^34,35 The FTIR spectrum obtained in the study aligns well with the previous report, indicating that the primary constituents of the DLE, H-DLE, and L-DLE are condensed tannin compounds.³³

Figure 4.

Fourier transform infrared (FTIR) spectrum of Dioscorea cirrhosa Lour. tuber extract (DLE), DLE with molecular weight (Mw) higher than 1000 (H-DLE), and DLE with Mw lower than 1000 (L-DLE) (a); local magnification of 1650–1850 cm⁻¹ (b); and 1350–1700 cm⁻¹ (c).

HPLC analysis

Figure 5 shows the HPLC chromatogram of DLE, H-DLE, and L-DLE. Notably, H-DLE and L-DLE display a retention time difference within the 0–33 min range, in which components contained in H-DLE do not appear within this retention time. It could be related to its high Mw The Mw of polyphenols and condensed tannins is lower, and the polarity is higher. The high polar components in the DLE were below 1000 Mw This result is consistent with previous reports.^36,37

Figure 5.

High-performance liquid chromatography (HPLC) chromatogram of Dioscorea cirrhosa Lour. tuber extract (DLE), DLE with molecular weight (Mw) higher than 1000 (H-DLE), and DLE with Mw lower than 1000 (L-DLE).

Analysis of polyphenols and condensed tannins contents

Figure 6(a) shows that the polyphenols content in DLE, H-DLE, and L-DLE is determined to be 87.3%, 96.9%, and 76.1%, respectively. The condensed tannins content in DLE, H-DLE, and L-DLE is 42.3%, 61.9%, and 18.3%, respectively. The content of polyphenolic compounds and condensed tannins is highest in H-DLE, whereas L-DLE exhibits the lowest range. It could be deduced that most condensed tannin and polyphenolic compounds within DLE have a large Mw above 1000. This finding obtained in our work agrees with that in the previous reports.^17,19,33

Figure 6.

Polyphenols and condensed tannins contents of Dioscorea cirrhosa Lour. tuber extract (DLE), DLE with molecular weight (Mw) higher than 1000 (H-DLE), and DLE with Mw lower than 1000 (L-DLE).

Adsorption rates and kinetics of adsorption

Studying DLE, H-DLE, and L-DLE adsorption kinetics is crucial for determining the processing time and efficiency. Figure 7(a) presents the adsorption kinetics of dyes on silk, while Figure 7(b) and (c) demonstrates the utilization of pseudo-first order and pseudo-second order kinetic formulae to model the adsorption rates of DLE, H-DLE, and L-DLE.

Figure 7.

The adsorption kinetics curves of the Dioscorea cirrhosa Lour. tuber extract (DLE), DLE with molecular weight (Mw) higher than 1000 (H-DLE), and DLE with Mw lower than 1000 (L-DLE) dyes on silk; the model for pseudo-first order (a) and pseudo-second order (a) of the DLE, H-DLE, and L-DLE dyes on silk.

Figure 7(a) illustrates a gradual increase in the adsorption of DLE, H-DLE, and L-DLE onto silk over an extended duration, implying a slow adsorption rate. In addition, after 400 min, the adsorption approaches equilibrium.

The coefficient of correlation coefficients (R²) for the pseudo-second order absorbance kinetic (DLE, H-DLE, and L-DLE) exceed 0.99, while Q_m, cal values are closer to the Q_{m, exp} as indicated by Tables 1 and 2. Hence, the pseudo-second order absorbance kinetic offered a more precise depiction of the investigated process. The results demonstrate that chemical adsorption played a dominant role.¹⁰ Furthermore, L-DLE exhibited the highest equilibrium adsorption quantity and adsorption rate constant as presented in Table 1. This phenomenon could be related to the easier diffusion of L-DLE with low Mw into the silk fibers.³⁸

Table 1.

Parameters of pseudo-first order absorbance kinetic of DLE, H-DLE, and L-DLE dyes toward silk

	Q_{m, exp}/(mg·g⁻¹)	Parameters of pseudo-first order absorbance kinetic
	Q_{m, exp}/(mg·g⁻¹)	k₁/min⁻¹	Q_{m, cal}/(mg·g⁻¹)	R ²
DLE	25.96	2.20 × 10⁻²	25.84	0.9691
H-DLE	15.59	1.91 × 10⁻²	15.35	0.9490
L-DLE	34.25	3.31 × 10⁻²	33.14	0.9571

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

Q_{m, exp} represents the experimental result; Q_{m, cal} represents the calculation result.

Table 2.

Parameters of pseudo-second order absorbance kinetic of DLE, H-DLE, and L-DLE toward silk

	Q_{m, exp}/(mg·g⁻¹)	Parameters of pseudo-second order absorbance kinetic
	Q_{m, exp}/(mg·g⁻¹)	k₂/(g·(mg·min)⁻¹)	Q_{m, cal}/(mg·g⁻¹)	R ²
DLE	25.96	2.47 × 10⁻³	25.86	0.9978
H-DLE	15.59	0.85 × 10⁻³	15.56	0.9922
L-DLE	34.25	2.84 × 10⁻³	33.88	0.9989

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

Q_{m, exp} represents the experimental result; Q_{m, cal} represents the calculation result.

Adsorption isotherm

Investigating the adsorption isotherms of DLE, H-DLE, and L-DLE aids in comprehending the interactions between these dyes and silk. Figure 8(a) illustrates the adsorption isotherms of DLE, H-DLE, and L-DLE on silk. The equilibrium adsorption data were modeled using the Freundlich adsorption model.

Figure 8.

Freundlich adsorption model plots (a). Scatchard plots of adsorption isotherm (b).

The coefficient of correlation coefficients (R²) of the Freundlich model (DLE, H-DLE, and L-DLE) exceeded 0.98, indicating adherence to the Freundlich model in the adsorption process of these three dyed toward silk as presented in Table 3. The high fit of the Freundlich model illustrated the van der Waals force between silk and these three.³⁹ The Freundlich exponent n being less than 1 signifies the heterogeneous nature of the silk surface.⁴⁰ The Scatchard curves show two linear plots (Figure 8(b)), suggesting the adsorption of DLE, H-DLE, and L-DLE reveals the presence of two distinct binding categories.⁴¹ In fact, the DLE, H-DLE, and L-DLE have multiple hydrogen groups in their polyphenol structure, and silk fiber bears a large amount of amide, amino, and carboxyl groups.^33,42 Hydrogen bonding can occur between silk fiber and these three. Thus, the hydrogen bonding and van der Waals forces between silk and these three contribute to the Freundlich adsorption.⁴³

Table 3.

Freundlich adsorption parameter of DLE, H-DLE and L-DLE on silk

Freundlich adsorption parameter	DLE	H-DLE	L-DLE
K_F (g/(mg·min))	23.800	12.186	26.076
n	0.4013	0.4706	0.9444
R ²	0.9899	0.9929	0.99811

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

Effects of different types of mordant

Metal mordants are frequently employed in silk dyeing with natural dyes.^7,44 They facilitate the formation of coordination complexes between metal ions, dye molecules, and silk fiber molecules. The involved functional groups include the hydroxyl group of the dye molecule and the carboxyl and amino groups of the silk fiber molecule.⁴⁵ This complexation reaction immobilizes the dye onto the fibers, as depicted in Figure 9. Moreover, the application of mordants alters the dye’s color, and enhances the natural dye’s bonding strength to the silk.^4,5 This process improves color fastness by way of dye–metal ion–fiber ligand bonding.^3,46

Figure 9.

Possible structure of condensed tannin with metal ion on silk. M represents metal ion.

The impact of various mordants (Fe²⁺, Fe³⁺, Al³⁺, and Cu²⁺) on the K/S, L*, a*, b*, C*, and h° values of dyed silk are presented in Table 4. The silk fabrics dyed without mordant result in a lighter brown shade due to lower dye uptake. Conversely, silk fabrics subject to a mordanted process exhibited diverse darker shades.

Table 4.

K/S and color characteristic values of silk dyed with different mordants

	Types of mordant	K/S value	L*	a*	b*	C*	h°
DLE	DM	0.80	71.53	10.34	13.81	17.25	53.17
	Fe²⁺	4.86	38.59	0.99	3.00	3.15	71.77
	Fe³⁺	3.21	52.03	3.83	12.12	12.71	72.46
	Al³⁺	1.10	66.39	8.17	13.96	16.18	59.65
	Cu²⁺	2.67	54.33	6.87	14.51	16.05	64.67
H-DLE	DM	0.74	73.20	11.13	15.88	19.39	54.98
	Fe²⁺	4.38	41.64	1.63	5.01	5.27	71.95
	Fe³⁺	2.67	55.13	4.18	11.85	12.56	70.58
	Al³⁺	0.87	69.32	5.09	11.77	12.82	66.60
	Cu²⁺	2.57	55.55	5.77	14.88	15.96	68.81
L-DLE	DM	1.26	69.45	11.31	17.41	20.76	56.97
	Fe²⁺	7.90	31.54	1.38	2.43	2.79	60.35
	Fe³⁺	5.88	43.08	3.20	11.47	11.91	74.43
	Al³⁺	2.37	60.80	11.89	19.64	22.96	58.81
	Cu²⁺	2.67	48.69	9.08	18.24	20.38	63.52

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

DM represents dyed silk without mordant.

The descending sequence of the K/S values from different mordant dyeing follows as Fe²+ > Fe³+ > Cu²+ > Al³⁺, as presented in Table 4. The use of Al³⁺ as a mordant has minimal impact on the color of the dyed silk based on the discriminative indices K/S and L values. Employing Cu²⁺ as a mordant increases the K/S value and decreases the L* value of the dyed silk, resulting in a deeper shade. However, the ability of Cu²⁺ to dye dark color is not as profound as that achieved with Fe²⁺ and Fe³⁺. This result is highly consistent with past reports.^7,11 The reasons for this phenomenon are that Fe²⁺ and Fe³⁺, acting as transition metal mordants, create numerous complexes with the dye molecules, predominantly octahedral ones with a coordination number 6.⁴⁶ Consequently, specific coordination sites remain vacant on interaction with the silk fiber, allowing functional groups such as amino and carboxylic groups on the silk fiber to occupy these empty sites. Consequently, Fe²⁺ and Fe³⁺ can establish a ternary complex at one spot with the silk and another with the dye. This occurrence leads to the dark color in dyed silk fabrics employing Fe²⁺ and Fe³⁺ as mordants.^2,7

Effects of different mordant (Fe²⁺ and Fe³⁺) concentrations

The impact of different mordant (Fe²⁺ and Fe³⁺) concentrations on dyed silk’s K/S, L*, a*, b*, C*, and h° values are presented in Table 5.

Table 5.

K/S and color characteristic values of silk dyed with different Fe²⁺ mordant concentrations

	Fe²⁺ concentration (g/L)	K/S value	L*	a*	b*	C*	h°
DLE	0.5	2.38	49.13	1.31	3.03	3.3.0	66.72
	1.0	2.83	46.69	1.14	2.98	3.19	69.11
	2.0	4.39	39.89	0.74	2.64	2.75	74.39
	4.0	5.25	38.59	0.99	3.00	3.15	71.77
	6.0	4.86	39.88	0.78	2.63	2.75	73.49
H-DLE	0.5	1.91	53.26	1.67	4.00	4.33	67.38
	1.0	3.40	46.04	2.00	5.95	6.28	71.41
	2.0	4.35	41.66	1.71	5.18	5.45	71.78
	4.0	4.38	41.64	1.63	5.01	5.27	71.95
	6.0	4.18	42.40	1.57	5.02	5.26	72.64
L-DLE	0.5	4.11	40.16	1.33	1.63	2.11	50.77
	1.0	5.86	35.77	1.16	2.45	2.71	64.75
	2.0	6.86	33.40	1.19	2.29	2.58	62.50
	4.0	7.90	31.54	1.38	2.43	2.79	60.35
	6.0	6.23	34.31	1.03	1.78	2.05	59.92

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

The K/S value peaked at a Fe²⁺ concentration of 4.0 g/L, as indicated by Table 5. Moreover, the highest K/S value corresponded to a Fe³⁺ mordant of 6, as given by Table 6. Fe³⁺ exhibits weaker dark dyeing capabilities than Fe²⁺. This phenomenon could be attributed to the high complexation ability of Fe³⁺, leading to the formation of precipitates that inadequately bind to the silk after attaching to the dye molecules.¹¹

Table 6.

K/S and color characteristic values of silk dyed with different Fe³⁺ mordant concentrations

	Fe³⁺ concentration (g/L)	K/S value	L*	a*	b*	C*	h°
DLE	0.5	2.25	51.24	2.83	4.54	5.35	58.00
	1.0	2.36	53.62	3.92	8.86	9.69	66.13
	2.0	2.31	56.24	4.72	11.52	12.45	67.74
	4.0	3.21	52.03	3.83	12.12	12.71	72.46
	6.0	3.98	50.14	3.32	13.30	13.71	76.00
H-DLE	0.5	1.78	55.51	3.20	5.72	6.55	60.76
	1.0	1.72	58.75	4.53	10.07	11.04	65.77
	2.0	1.97	58.02	5.38	11.63	12.81	65.17
	4.0	2.57	55.13	4.18	11.85	12.56	70.58
	6.0	3.86	49.86	3.16	12.40	12.79	75.71
L-DLE	0.5	3.90	41.68	1.67	2.72	3.19	58.34
	1.0	4.50	40.79	1.94	3.91	4.37	63.67
	2.0	5.51	42.48	2.92	9.54	9.98	72.97
	4.0	5.88	43.08	3.20	11.47	11.91	74.43
	6.0	6.32	43.46	3.51	12.90	13.37	74.76

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

Effects of different pH values

The impact of different pH values on the K/S, L*, a*, b*, C*, and h° values of dyed silk under 4 g/L Fe²⁺ mordant concentration is presented in Table 7.

Table 7.

K/S and color characteristic values of silk dyed with different pH values

	pH value	K/S value	L*	a*	b*	C*	h°
DLE	NP	5.25	38.59	0.99	3.00	3.15	71.77
	4.0	5.67	36.40	1.35	3.41	3.67	68.48
	5.0	6.67	34.71	1.20	3.33	3.54	70.19
	6.0	5.89	35.18	1.27	2.24	2.57	60.34
H-DLE	NP	4.38	41.64	1.63	5.01	5.27	71.95
	4.0	5.89	36.86	2.03	4.88	5.28	67.38
	5.0	5.89	36.68	1.69	4.17	4.5	67.97
	6.0	4.55	39.36	1.90	3.31	3.81	60.10
L-DLE	NP	7.90	31.54	1.38	2.43	2.79	60.35
	4.0	6.20	34.75	1.29	2.78	3.07	65.17
	5.0	9.05	30.77	1.27	2.87	3.14	66.18
	6.0	8.76	31.03	1.81	3.50	3.94	62.57

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

NP represents no addition of pH buffer.

The highest K/S and lowest L* values correspond to a pH of 5, as presented in Table 7. Dyed silk obtained its darkest color when employing L-DLE as a colorant in this dyeing condition. The K/S and L* values of dyed silk are 9.05 and 30.77, respectively. The obtained result, indicating a darker color under weak acid conditions, resembles previous reports on dyed silk fabrics with natural extracts.^7,47 This phenomenon may be related to Fe²⁺ exhibiting a reduced propensity for oxidation into Fe³⁺ under weak acidic conditions. In addition, the order from high to low silk surface darkness level from different dyes follows as L-DLE >DLE > H-DLE.

Surface morphology and Fe element content of the dyed silk

Figure 10 shows the surface morphologies and the Fe element content of the silk fabrics dyed with DLE, H-DLE, and L-DLE. The undyed silk displays a clean and smooth surface. The silk fabrics treated without Fe²⁺ mordant show the deposition of some particles on their surface. Employing Fe²⁺ mordant leads to more particle depositions to adhere silk fibers, and these depositions are complexes of condensed tannins and Fe²⁺. Figure 10(c), (e), and (f) suggests that the order from high to low deposit content from different dyes follows as L-DLE > DLE > H-DLE. Figure 10(h) indicates the Fe element content of the dyed silk. The order from high to low deposit content from different dyes follows as L-DLE > DLE > H-DLE. These observations align with the order of dyed silk fabrics’ dark color intensity. From this result, we could infer a specific correlation between these three.

Figure 10.

Scanning electron microscope (SEM) images of silk (a); unmordanted silk fabrics of Dioscorea cirrhosa Lour. tuber extract (DLE) (b); DLE with molecular weight (Mw) higher than 1000 (H-DLE) (d); DLE with Mw lower than 1000 (L-DLE) (f); 4 g/L Fe²⁺ mordanted silk fabrics (pH of dye bath was 5) of DLE (c); H-DLE (e); L-DLE (g); the Fe element content of 4 g/L Fe²⁺ mordanted silk fabrics (pH of dye bath was 5) of DLE, H-DLE, L-DLE (h).

The valence states of the Fe element in silk fabrics surface

The results obtained by XPS are shown in Figure 11. The Fe element of 4 g/L Fe²⁺ mordanted silk surface (pH value of dye bath was 5) exhibits two valence states (2 and 3). Figure 11(d) shows that the different orders of Fe²⁺ proportion, from highest to lowest, were L-DLE, DLE, and H-DLE. This result is consistent with the order of dark color from high to low observed in the dyed silk fabrics under these dyeing conditions. This phenomenon suggests that the dark color of Fe²⁺ mordanted silk might have a certain relation with the valence state of the Fe element on the Fe²⁺ mordanted silk surface. This finding is similar to the conclusion from the previous research conducted by Pan et al.¹⁶

Figure 11.

Peaks for Fe 2p in Dioscorea cirrhosa Lour. tuber extract (DLE) (a); DLE with molecular weight (Mw) higher than 1000 (H-DLE) (b); and DLE with Mw lower than 1000 (L-DLE) (c) on the surface of dyed silk fabrics, Fe²⁺ and Fe³⁺ ratio (d).

Color fastness testing

As presented in Table 8, the mordanted silk fabrics have better color fastness because mordants enhanced the bond between the dye and the silk fabric. Disparities were observed in washing fastness. Dyed silk fabrics show poor washing fastness under alkaline conditions. This phenomenon could be related to the instability of natural extracts under alkaline conditions.⁴⁴ The light fastness of dyed silk fabrics is about 3. The poor light resistance of natural extracts has always been an urgent problem to be solved.⁴⁸ These results show the satisfactory color fastness of mordanted silk.

Table 8.

Color fastness of unmordanted and 4 g/L Fe²⁺ mordanted silk fabrics (pH value of dye bath was 5).

	Fe²⁺ concentration (g/L)	Washing (neutral)		Washing (alkaline)		Rubbing		Light
	Fe²⁺ concentration (g/L)	CC	SC	CC	SC	Dry	Wet	Light
DLE	0	3–4	5	2–3	5	3–4	3–4	2–3
DLE	4	4–5	5	3–4	5	4	4–5	3
H-DLE	0	3–4	5	2–3	4–5	4	3–4	2–3
H-DLE	4	5	5	3–4	5	4–5	4–5	3
L-DLE	0	3	4–5	2–3	4–5	4	4	3
L-DLE	4	4–5	5	3	5	5	4–5	3–4

DLE: Dioscorea cirrhosa Lour. tuber extract; H-DLE: DLE with molecular weight (Mw) higher than 1000; L-DLE: DLE with Mw lower than 1000.

CC represents color change; SC represents staining on cotton.

Conclusions

This study is the first attempt to isolate DLE using the dialysis method into two parts with different Mw. These two components are studied for their composition and their adsorption kinetics as well as dark dyeing properties on silk, comparing the experimental results with those of DLE. The constituent analysis reveals that most condensed tannins and polyphenols are present in DLE above 1000 Mw, exhibiting lower polarities. DLE, H-DLE, and L-DLE adsorption kinetics on silk conform to the pseudo-second order kinetic model, and several interactions occur between these three and silk, including hydrogen bonding and van der Waals forces through the adsorption isotherms study. The dyeing research reveals that silk obtained its darkest color when using L-DLE with a Fe²⁺ mordant concentration of 4 g/L and a pH value of 5. The K/S and L* values of dyed silk are 9.05 and 30.77, respectively. The Fe complex, Fe element content, and Fe²⁺ ratio on the silk dyed with L-DLE surface are higher than those of DLE and H-DLE. The results taken together indicate that the condensed tannins in DLE are not the only active substances in the dyeing silk process; the polyphenols and condensed tannins with Mw above 1000 in DLE, it is challenging to enter the silk fibers’ interior and stain them. Polyphenols with a Mw lower than 1000 would significantly impact and even dominate the silk dark dyeing process. It is worth mentioning that the valence states of the Fe element on the surface of dyed silk fabrics could also be a factor affecting the dark color of DLE dyeing when Fe²⁺ was employed as a mordant.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Qing Wang

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The investigation on dark dyeing properties of silk using Dioscorea cirrhosa Lour. tuber extracts with varied molecular weights

Abstract

Keywords

Experimental materials and instruments

Materials

Instruments

Experimental method

Preparation of DLE

Preparation of H-DLE and L-DLE

Adsorption experiments

Dyeing process

Experimental scheme

Test method

UV-visible spectrum

Fourier transform infrared spectrum

High-performance liquid chromatography

Total phenolic content determination

Condensed tannin content determination

Scanning electron microscope observation

Inductively coupled plasma-atomic emission spectroscopy

X-ray photoelectron spectroscopy

Color parameters and color fastness

Results and discussion

UV-vis spectrum analysis

FTIR spectrum analysis

HPLC analysis

Analysis of polyphenols and condensed tannins contents

Adsorption rates and kinetics of adsorption

Adsorption isotherm

Effects of different types of mordant

Effects of different mordant (Fe2+ and Fe3+) concentrations

Effects of different pH values

Surface morphology and Fe element content of the dyed silk

The valence states of the Fe element in silk fabrics surface

Color fastness testing

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Effects of different mordant (Fe²⁺ and Fe³⁺) concentrations