Kinetics and Thermodynamics of Silk Dyeing with Turmeric Extract

Abstract

A kinetics and thermodynamics study of turmeric extract dyeing on Bombyx mori silk fabric was conducted with 1 g/L initial dye concentration at three temperatures (70 °C, 85 °C, and 100 °C). The fabric was pre-mordanted with FeSO₄ and dyed with the turmeric extract at pH 7 and a material liquor ratio (MLR) of 1:20. Pseudo first-order and pseudo second-order kinetics were tested. The adsorption kinetics of turmeric extract dyeing on silk fit the pseudo second-order kinetic model. The activation energy was 94.9 kJ/mol. The enthalpy and entropy of activation were 91.93 kJ/mol and –14.47 J/mol·K, respectively.

Introduction

Silk is a well-known fiber for use in clothing and apparel. It is a soft, delicate, textured, and very smooth fiber originating from silkworm cocoons as a continuous filament of protein-aceous polymer resembling synthetic fibers. Among many varieties, the Mulberry silk (Bombyx mori) is the of the highest quality, and therefore, highly valued.

Efforts have been made to improve the physical properties of silk,^1–7 including dyeability and colorfastness with different classes of dyes (e.g., natural, reactive, acid, basic, and metal complex dyes).^8–12

Earlier studies of turmeric extract dyeing of silk were successfully performed at dyeing temperatures between 60 °C to 100 °C using different mordants (e.g., FeSO₄, alum, tartaric acid, and CuSO₄).^13,14 The color strength increased with the depth of shade, with the best results at pH 7. ¹⁴ The washfastness and crockfastness of turmeric dyed silk were observed to be nearly similar to that of cotton dyed with the same dye, in most cases between the ratings of 2–3 to 3–4. ¹⁴

Kinetic models are very important for a dye house manager or dyeing specialist as it shows how various experimental conditions affects the speed of chemical reaction and thus helps to select preferable conditions for dyeing. But in fact, research on silk dyeing adsorption kinetics was performed for very few dyes up to now.

The adsorption kinetics and thermodynamics of lac dyeing was reported by several researchers.^15–17 Furthermore, equilibrium and kinetic modeling of indigo carmine, sodium copper chlorophyllin dye, and acid dye on silk were also investigated.^18–20 However, the dyeing kinetics and thermodynamics of silk dyed with turmeric from the Curcuma longa root, the most popular natural dye for textiles, has not yet been studied. ²¹

Turmeric is rich in curcuminoids and belongs to the diaroylmethane group named diferuloylemethane. ²² This natural dye is environmentally friendly and has antibacterial properties.^23–28 The active coloring component in turmeric rhizome is curcumin (C.I. number 75300, C.I. Natural Yellow 3). ²⁹ It has a molecular weight of 368.38 g/mol and molecular formula of C₂₁H₂₀O₆ . ³⁰ The chemical structure of curcumin is shown in Fig. 1. ²²

Fig. 1

Chemical structure of curcumin (keto form).

This research focuses on regression analysis for two different kinetic models of dyeing woven silk fabric with turmeric extract. An investigation of the thermodynamic aspects was also made, adopting kinetic rate constants from the best ft kinetic model.

Experimental

Material

In this work, degummed and bleached 100% plain woven silk fabric was supplied by Rajshahi Silk Industries. The fabric specifications are listed in Table I.

Table I

Specification of Silk Woven Fabric

Parameter	Value
Structure	Plain weave
Warp count	160 Ne
Weft count	100 Ne
Ends per inch	118
Picks per inch	95
GSM (g/m2)	60
Silk type	Bombyx mori

Mordanting

Before dyeing, the fabric was mordanted with 0.5 g/L FeSO4 at 70 °C at a 1:20 material liquor ratio (MLR) for 10 min.

Extraction of Turmeric Dye

Turmeric powder was supplied by Square Food and Beverage Ltd. The dye was obtained by extracting 1 g/L powder with deionized water at 95 °C and pH 7 for 90 min. The solution had a yellowish color.

Dyeing Process

Dyeing of silk fabric by turmeric extract was done in a Mathis Labomat laboratory dyeing machine equipped with a programmed temperature controlling system using infrared (IR) heating and a combined air-water cooling unit. Tree different temperatures (70 °C, 85 °C, and 100 °C) were used for dyeing at pH 7 with a 1:20 MLR in a 500 mL dyebath. The dyeing process was continued for 100 min with 1.0 mL of dye solution removed every 2 min for the first 20 min, every 5 min from 20 to 40 min, and every 20 min from 40 to 100 minutes, for the spectroscopic absorbance measurements. After dyeing, a hot wash was carried out at 80 °C in water and then a cold wash was done at room temperature (28 °C). Each fabric was soaked in water at a 1:50 MLR, stirred gently for 5 min, and then taken out for squeezing. Finally, the samples were dried in a dryer for 10 min at 60 °C.

Kinetics Experiments

A UV-Visible spectrophotometer (UV 1800, Shimadzu Corp.) was used for absorbance measurements with quartz cuvette cells of 1-cm path length. Dye concentrations were determined at time zero and at subsequent times from the absorbance values at λ_maxof 419 nm. The concentration of dye in liquor was calculated by the Beer-Lambert equation (Eq. 1).

$$A=\epsilon lc$$

(Eq. 1)

A is absorbance, c is concentration of dye solution (mg/L), l is path length of light (cm), and ε is dye extinction coefficient (L/mg·cm). The dye extinction coefficient for turmeric yellow (0.01031 L/mg·cm) was obtained by calculating the slope of a calibration curve (c versus A) where concentration values were known.

The amount of dye (mg/g) absorbed on silk at any time q_t was calculated from the mass-balance relationship formula in Eq. 2.

$$q_t=(C_0-C_t)\frac{V}{W}$$

(Eq. 2)

C₀ is the initial dye concentration (mg/L), C_t is the dye concentration after time t (mg/L), V is the volume of solution (mL), and W is the weight of the silk fabric (g).

Examination of Fiber Cross Sections

To avoid false equilibrium and confirm that the dyes fully penetrated the inside of the fiber upon reaching equilibrium, cross sections of dyed silk fabrics were observed under a digital microscope (Micros MCX100LED). Only the samples at equilibrium were considered and compared with an undyed silk sample. The images are shown in Fig. 2.

Fig. 2

Cross-sectional view of (a) greige silk, (b) silk dyed at 70 °C for 40 min, (c) silk dyed at 85 °C for 20 min, and (d) silk dyed at 100 °C for 10 min.

Results and Discussion

Temperature Effect on Turmeric Extract Adsorption

The initial dye adsorption rate (h_i) of turmeric on silk before equilibrium was greater at higher temperatures and the time required for reaching equilibrium was less (40 min at 70 °C, 20 min at 85 °C, and 10 min at 100 °C). However, a greater amount of dye was absorbed on silk at a lower temperature (70 °C) after it had reached equilibrium, suggesting that the process is exothermic. The dye adsorption (mg/g of silk) against time is shown in Figs. 3 and 4. Fig. 3 shows the results in the first 20 min, where it was clearly observed that at higher temperatures, dye absorption occurred at a greater speed, whereas at lower temperatures, the speed was comparably less. When considering the whole contact time, a greater amount of dye was absorbed during the lower temperature dyeing.

Fig. 3

The effect of contact time and temperature of turmeric dyeing on silk (0–20 min) at an initial dye concentration of 1 g/L, MLR 1:20, and pH 7.0.

Fig. 4

The effect of contact time and temperature of turmeric dyeing on silk (0–100 min) at an initial dye concentration of 1 g/L, MLR 1:20, and pH 7.0.

Kinetics of Adsorption

Pseudo first-order and second-order kinetic models were applied to the experimental data to represent the adsorption kinetics of turmeric yellow dye on silk. The linear form of the pseudo first-order equation (Lagergren equation) is given in Eq. 3.

$$\ln \left(q_e-q_t\right)=\ln q_e-k_{1}t$$

(Eq. 3)

k₁ is the rate constant of pseudo first-order adsorption (s⁻¹), and q_e and q_t are the amounts of dye adsorbed per gram of silk (mg/g) at equilibrium and at a specific time t. The first-order Lagergren equation generally does not ft for the whole range of contact time and is only applicable for the initial stage of dyeing. ³¹ If the ln(q_e- q_t) versus t plot is linear, it indicates the applicability of the kinetic model to ft the experimental data. k₁ and q_e were derived from the slope and intercept of the plot.

The pseudo second-order kinetic model based on the adsorption equilibrium can be expressed in linear form in Eqs. 4 and 5.^31,32

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$

(Eq. 4)

$$h_i=k_2{q}_e^2$$

(Eq. 5)

k₂ (g/mg·min) is the rate constant for pseudo second-order adsorption and h_i is the initial dye adsorption rate (mg/ g·min). ³³ If the plot of (t/q_t) versus t shows a linear relationship, pseudo second-order kinetics should be applicable. The slope and intercept of (t/q_t) versus t were considered to compute the pseudo second-order rate constant k₂ and q_e. It is probable that the behavior over the whole range of adsorption complies with the chemisorption mechanism being the rate-controlling phase. ³³

Kinetic data, obtained from turmeric yellow dye adsorption in the current study, was analyzed using the pseudo first-order kinetic model (Eq. 3) as well as the pseudo second-order kinetic model (Eq. 4) and is shown in Figs. 5 and 6. The results are depicted in Table II. The correlation coefficients (R ² ) obtained from the pseudo second-order kinetic model were very close to 1 (greater than 0.99) and greater than that of the first-order kinetic model. This is an indication that the adsorption of turmeric dye on silk is unlikely to be a first-order reaction. The calculated q_e values were also close to the experimental q_e value. Therefore, the experimental data of turmeric yellow dyeing on silk fabric ft the pseudo second-order kinetics model.

Fig. 5

Plot of the pseudo first-order equation at various temperatures for the adsorption of turmeric yellow on silk.

Fig. 6

Plot of the pseudo second-order equation at various temperatures for the adsorption of turmeric yellow on silk.

Table II

Comparison of Pseudo First- and Second-Order Adsorption Rate Constant of Turmeric Dyeing on Silk ^a

Temp. (°C)	qe (Experimental) (mg/g)	Pseudo First- Order Model		Pseudo Second-Order Model
		k1 (min–1)	R 2	k2 (g/mg·min)	qe (Calculated)(mg/g)	hi (mg/g·min)	R 2
70	12.32	0.077	0.969	0.0122	13.33	2.16	0.995
85	11.26	0.164	0.952	0.0544	11.63	7.35	0.999
100	8.36	0.306	0.978	0.1762	8.70	13.33	0.999

a

initial dye concentration 1 g/L, MLR 1:20, and pH 7.0

Activation Parameters

From the rate constant of pseudo second-order kinetics k ₂ (Table II), the activation energy (E_a) for the adsorption of turmeric yellow dye on silk was determined using the Arrhenius equation (Eq. 6). ³⁴

$$\ln k=\ln A-\frac{E_a}{RT}$$

(Eq. 6)

A, E_a, T, and R refer to the Arrhenius factor (temperature independent), the Arrhenius activation energy (kJ/mol), the absolute temperature (K), and the gas constant (8.314 J/ mol·K), respectively. The Arrhenius plot of lnk against 1/T for the adsorption of turmeric dye on silk is shown in Fig. 7 and the value of the activation energy derived from the slope of the plot is given in Table III.

Fig. 7

Arrhenius plot for the adsorption of turmeric yellow on silk.

Table III

Activation Parameters for the Adsorption of Turmeric Yellow on Silk ^a

Temp. (°C)	k2(g/mg·min)	Ea (kJ/mol)	R 2	Δ H° (kJ/mol)	ΔS° (J/mol·K)	ΔG° (kJ/mol)	R 2
70	0.0122	94.90	0.998	91.93	–14.47	96.89	0.997
85	0.0544	—	—	—	—	97.11	—
100	0.1762	—	—	—	—	97.33	—

a

initial dye concentration 1 g/L, MLR 1:20, and pH 7.0

The enthalpy (ΔH°) and entropy (ΔS°) of activation were derived using Eyring equation (Eq. 7).

$$\ln \left(\frac{k}{T}\right)=\ln \left(\frac{K_b}{h}\right)+\frac{\Delta S^{\circ }}{R}-\frac{\Delta H^{\circ }}{RT}$$

(Eq. 7)

K_b and h are the Boltzman and Planck constants respectively. The ΔH° and ΔS° of dyeing were derived from the slope and intercept of the graph ln(k₂/T) versus 1/T (Fig. 8).

Fig. 8

Eyring plot for the adsorption of turmeric yellow on silk.

The Gibbs energy of activation (ΔG°) was calculated from Eq. 8.

$$\Delta G^{\circ }=\Delta H^{\circ }-T\Delta S^{\circ }$$

(Eq. 8)

The calculated values are listed in Table III. The ΔS° value was negative, which suggests an interaction between turmeric dye and silk fabric.

Conclusions

The kinetics of turmeric dyeing on silk fabric ft well with the pseudo second-order kinetic model in this study. Initial dye adsorption rates (h_i) were greater at higher temperatures before equilibrium was reached, which suggests that the process was kinetically controlled. The process was exothermic and the activation energy for the adsorption process on silk fabric was found to be 94.90 kJ/mol.