Adsorption Kinetic and Thermodynamic Studies of Silk Dyed with

Jun 6, 2012 - Key Laboratory of Science & Technology of Eco-Textiles, Ministry of ... Recently, there has been revived interest in dyeing textiles wit...
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Adsorption Kinetic and Thermodynamic Studies of Silk Dyed with Sodium Copper Chlorophyllin Xiuliang Hou,†,∥ Ruiling Yang,† Helan Xu,‡ and Yiqi Yang*,†,‡,§ †

Key Laboratory of Science & Technology of Eco-Textiles, Ministry of Education, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu, 214122, China ‡ Department of Textiles, Clothing and Design and §Department of Biological Systems Engineering, 234, HECO Building, University of Nebraska−Lincoln, Lincoln, Nebraska 68583-0802, United States ∥ Jiangsu Sunshine Group, Sunshine Industrial Park, Jiangyin, Jiangsu, 214126, China ABSTRACT: In this study, silk was dyed into a bright green shade with a natural dye derivative, sodium copper chlorophyllin (SCC). Kinetics and thermodynamics of SCC adsorption on silk in dye bath with and without sodium chloride at different temperatures were investigated. The pseudo second-order kinetic model fitted experimental data well. Addition of sodium chloride increased SCC adsorption at equilibrium qe, increased the diffusion coefficient D and decreased the half dyeing time t1/2. The Langmuir isotherm agreed well with a high correlation coefficient (R2 > 0.99) for dyeing temperatures of 70, 80, and 90 °C with SCC concentration varying from 0.1 to 2.9 g/L. The thermodynamic parameters of SCC adsorption with and without sodium chloride showed significant difference. The adsorption affinity (−Δμ°) and enthalpy change (ΔH°) of SCC dye on silk indicated that the adsorption is a spontaneous and exothermic process. SCC could be a good candidate as a natural green dye for protein fibers.

1. INTRODUCTION Recently, there has been revived interest in dyeing textiles with natural dyes considering their biodegradability, renewability, compatibility with the environment, and low allergic reaction.1−5 Natural dyes also exhibit other preferable properties, such as antimicrobial activity and UV protection.1−3 Green remains a rare color in natural dyestuffs due to the lack of plants that can yield stable green colorants. Natural dyes, such as woad and indigo, had been used in combination with yellow dye to obtain dull green shades on textiles through a two-step overdyeing method in ancient times.6 The overdyeing method requires good compatibility of two dyes, otherwise, it would lead to problems in color matching and shade change after laundering. Therefore, obtaining stable green dyes from natural and renewable resources could be of major significance. Chlorophyll is a green pigment that captures the light energy for photosynthesis in most plants. Efforts have been made to utilize chlorophyll as a natural green dye for practical applications. However, the magnesium ions of the chlorophyll molecules are easily lost during extraction and processing. The resulting compound, pheophytin, exhibits a dull yellow or brownish color which is not suitable for coloration.7 A feasible approach to obtain a bright green color with good stability from chlorophyll is to form a complex of pheophytin and copper.7 As a derivative of natural chlorophyll, sodium copper chlorophyllin (SCC) is a water-soluble bright green colorant with good stability.7 SCC has great potential to be used as a green dye for natural fibers with various beneficial properties for textiles. Dyeing properties of SCC on cotton,8 flax,9 silk,10 and wool11 fabrics have been investigated. The SCC dyed fabrics showed good color fastness to washing but poor color fastness to light ranging from grade 2 to 3. However, the SCC dyed cotton © 2012 American Chemical Society

fabric that had been pretreated with chitosan showed light fastness of grade 4 and noticeably good antimicrobial activity.12 SCC has also been extensively used as a common dietary food supplement.7,13 It was reported that SCC has antimutagenic and antioxidative properties.14−16 Hossain et al.17 investigated the fabrication of dye-sensitized solar cells with nanocrystalline TiO2 films and SCC dye. All these features make SCC capable of achieving beneficial functions for textiles. Fundamental studies on the adsorption kinetics and thermodynamics of the dyeing process are important to understanding the dyeing mechanisms and improving the dyeing properties of natural dyes. There have been a few papers18−22 which studied the adsorption kinetics and thermodynamics of natural lac or laccaic acid dye on cotton and silk fibers. The investigations on the adsorption kinetics of SCC dyeing on wool23 and silk24 without sodium chloride showed that the amount of SCC dye adsorbed on silk fabrics at equilibrium was obviously lower than that on wool. In order to improve the adsorption of SCC dye on silk fabric, this paper first optimized the dyeing conditions of SCC dye on silk to obtain a vivid green shade and then investigated the effect of sodium chloride on the adsorption kinetics and thermodynamics of SCC dye on silk.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Commercial grade sodium copper chlorophyll (SCC) powder was purchased from Shandong Guangtongbao Pharmaceuticals Co. Ltd., China Received: Revised: Accepted: Published: 8341

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and was used as is. The main colorants in SCC are disodium copper chlorophyllin (C34H30O5N4CuNa2, MW = 684.16) and trisodium copper chlorophyllin (C34H31O6N4CuNa3, MW = 724.17). Both disodium copper chlorophyllin and trisodium copper chlorophyllin in SCC have planar space structures as shown in Figure 1.13,25 In this study, the two components were

Figure 2. Absorbance spectra of SCC dye liquor with 0.02 g/L SCC at pH 8 after 3 h of treatment at 90 °C.

2.3.1. pH. To determine the effect of pH on the stability of SCC in dye bath, 0.3 g/L SCC dye liquors with different pH values were stored at room temperature for 24 h before measurement. Selection of pH was based on comparison of SCC stability and color characteristics of the dyed silk fabrics. The pH of SCC dye solution was adjusted between 6 and 9 by adding potassium hydrogen phthalate, mixed phosphate, and borax buffer, respectively. 2.3.2. Electrolytes. The effect of electrolytes in dye bath was studied with NaCl concentrations varying from 1.2 to 18.0 g/L in 0.3 g/L SCC dye bath at pH 8 and 80 °C. The color depth (K/S) and percent uptake were major parameters to be investigated. 2.4. Evaluation of Color Characteristics and Dye Uptake. Color Characteristics, L*, a*, b*, c*, and h° and color depth (K/S) of the dyed samples were measured by a ColorEye 7000A Spectrophotometer (Gemini BV, Gretag Macbeth company, USA) using an illuminant D65 and 10° standard observer. The dyed silk fabrics were folded into four layers and three different positions were measured. L*, a*, b*, c*, and h° are lightness, redness−greenness, yellowness− blueness, saturation, and hue, respectively. The hue of 180 represents green in CIE 1976 color spaces.26 The percent uptake of SCC dye on silk fabric was calculated according to eq 1

Figure 1. Chemical structures of (a) disodium copper chlorophyllin and (b) trisodium copper chlorophyllin.

considered to be sorbed onto silk at the same rate. The weight average molecular weight of SCC was calculated to be 689.87, assuming that SCC was constituted of 85% disodium copper chlorophyllin and 15% trisodium copper chlorophyllin.12 The purity of SCC of 52% was calculated based on the total copper content of 4.8% and the dissociative copper content of 0.02%. The woven silk fabric was provided by Jiangsu Fu’an Cocoon & Silk Joint-Stock Co. Ltd. in China. The silk fabric was degummed in 1.0 g/L sodium carbonate solution at 100 °C for 30 min with a bath ratio of 1:50. The degummed silk fabric was washed in 95 °C hot water, 60 °C hot water, and cold water in sequence and then dried at room temperature. Sodium chloride (AR) was produced by Sinopharm Chemical Reagent Co. Ltd. China. Potassium hydrogen phthalate, phosphate buffer, and borax buffer for pH adjustment were purchased from Degussa-AJ Initiators Co, Ltd. Shanghai, China. 2.2. Determination of SCC Concentration in the Dye Liquor. Considering that heat during the dyeing process may affect light absorbance of SCC,13 SCC dye liquors with known concentrations were treated at 90 °C for 3 h before absorbance measurement. The absorbance spectra of the SCC dye liquor was measured with the UV−visible spectrophotometer TU1901 (Purkinje General Instrument Co. Ltd., Beijing, China) using a quartz cell with path length of 1 cm. As shown in Figure 2, SCC has a strong absorbance peak at 405 nm and a weak one at 629 nm. The concentration of SCC in the dye bath was measured at 405 nm based on the Lambert−Beer law. The calibration curve had the relation of y = 67.86x with an R2 value of 0.999. 2.3. Optimization of Dyeing Conditions. The degummed silk fabric samples were dyed in a thermostatted shaker water bath (Rapid Precision Machinery Co., Ltd., Xiamen, China) under different dyeing conditions.

% uptake =

(A 0 − A t ) × 100% A0

(1)

Where A0 and At are the absorbance at λmax 405 nm of the initial dye liquor and the dye liquor at time point t, respectively. 2.5. Calculation of Adsorption Kinetics. The adsorption kinetics of SCC dye on silk with and without NaCl were compared. The adsorption of SCC dye onto silk was performed at pH 8, with a fabric to bath ratio of 1:50, at 70, 80, and 90 °C for 5−180 min. The dye concentration qt (g/kg silk) in the silk fiber at time t was calculated using eq 2. V (2) W Where C0 and Ct are the initial SCC concentration (g/L) in the dye liquor and the dye concentration (g/L) at dyeing time t, respectively, V is the volume of dye solution (L), and W is the weight of silk fabric (kg). 2.6. Calculation of Adsorption Isotherm. The amounts of SCC dye adsorbed by silk fabric were investigated with dye concentrations varying from 0.1 to 2.9 g/L at pH 8 for 3 h with a bath ratio of 1:50. The adsorption isotherms of the SCC qt = (C0 − Ct )

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Figure 3. Effect of dye bath pH on the stability of SCC dye liquor: (original) “fresh” SCC dye liquor; (after 24 h) SCC dye liquor stored at room temperature for 24 h.

The effect of dye bath pH on % uptake of SCC on silk fabric and the colors of the dyed silk fabrics are shown in Table 1.

dyeing process with NaCl (9.0 g/L) and without NaCl at 70, 80, and 90 °C were compared. The amount of SCC dye adsorbed per unit weight of silk fiber at equilibrium qe (g/kg silk) was calculated using eq 3. qe = (C0 − Ce)

V W

Table 1. Effect of Dye Bath pH on Percent Uptake of SCC Dye on Silk and the Colors of the Dyed Silk Fabricsa characteristics of colors

(3) % uptake

L*

6 7 8 9

32.97 34.01 33.90

60.05 59.03 58.65 58.96

6 7 8 9

73.93 74.75 74.73

52.67 50.05 49.07 49.64

pH

Where C0 is the initial dye concentration (g/L), Ce is the equilibrium dye concentration (g/L), V is the volume of dye solution (L), and W is the weight of silk fabric (kg).

3. RESULTS AND DISCUSSION 3.1. Optimal Conditions for SCC Dyeing of Silk. The effects of dyeing parameters, such as pH of dye bath, addition of NaCl, temperature, and dyeing time were investigated to obtain the optimal conditions for adsorption of SCC dyes onto silk fabrics. 3.1.1. Effect of pH on Stability of SCC Liquor and Colors of Dyed Silk. Figure 3 shows the stability of SCC in dye liquor with pH 6, 7, 8, and 9 after being stored at room temperature for 24 h. Comparing to original SCC dye liquor, SCC dye liquor with pH 6 showed the highest decrease of absorbance at both 405 and 629 nm and that with pH 7 showed obvious decrease of the absorbance at 405 nm. The loss of absorbance may be caused by the precipitation of SCC (Figure 3a and b). However, for SCC liquor with pH 8 and 9, the curves before and after storage were overlapped significantly, indicating the retaining concentration of SCC at pH 8 and 9 after 24 h. In solutions with higher pHs, more carboxylic groups of SCC could dissociate into RCOO− anions, resulting in higher solubility of SCC. Dye bath with pH 8 could be selected since SCC has good solubility which facilitates achieving deep color, while silk may not be damaged seriously at this pH.

a*

b*

without NaCl −14.60 18.25 −15.01 17.72 −16.5 15.38 −14.90 16.05 with NaCl (6.0 g/L) −14.64 18.47 −17.53 17.38 −19.38 17.24 −19.06 17.34

c*

h°

23.38 23.22 23.58 21.90

128.66 130.26 137.06 132.87

23.57 24.68 26.18 25.64

128.40 135.24 138.23 137.87

Dyeing temperature 80 °C, initial SCC dye concentration 0.3 g/L, bath ratio 1:50, and dyeing time 60 min. The percent uptake was not calculated due to the precipitation of SCC at pH 6.

a

The dye bath pH has no significant effect on % uptake of SCC dye on silk fabric. The dyeing sites for carboxylic groups of SCC on silk decrease at higher pH conditions, while the solubility of SCC at higher pH increases. As a result, the dye uptake was not influenced by pH significantly. The % uptake of fabric dyed at pH 6 was not measured since SCC precipitated during dyeing process; therefore, eq 1 cannot be used. Color characteristics, L*, a*, b*, c*, and h° varied at different dye bath pHs, inferring that pH had an impact on the colors of dyed fabrics. For all the dyed silk at pH 6−9 with and without NaCl, the a* values were negative and the b* values were positive, which showed that all the dyed silk fabrics were green with 8343

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yellow hue, which is in agreement with the measured h° values varying from 128 to 138. With and without NaCl, the dyed silk fabrics at pH 8 had minimum a*, b*, maximum c*, and highest h°, presenting the greenest shades. The percent uptake of SCC onto silk dyed with NaCl more than doubled that without NaCl. For naturally dyed textiles, there is also a practical concern regarding the problem of color change induced by high temperature and alkaline pH during laundering. However, since silk textiles are either washed in mild conditions with neutral pH and low temperature or dry cleaned, the color of silk dyed at pH 8 should be stable and might not change after cleaning. Considering the comprehensive effects of dye bath pH on the stability of SCC solution, percent uptake of SCC dye on silk fabrics and the color characteristics of the dyed silk fabrics, pH 8 was used in the following dyeing experiments. 3.1.2. Effect of NaCl Amount on SCC Dye Uptake and Color Depth. The effect of the NaCl amount on the uptake of SCC dye on silk and color depth (K/S) of dyed silk fabrics is shown in Figure 4. Uptake of SCC dye and K/S values of the

Therefore, the NaCl concentration for the following adsorption study was fixed at 9.0 g/L. 3.1.3. Effect of Time and Temperature on the Adsorption of SCC onto Silk. Figure 5 shows the adsorption kinetic curves

Figure 5. Adsorption kinetic curves of SCC on silk: pH 8, bath ratio 1:50, initial concentration of dye 0.3 g/L.

of SCC on silk at the dyeing temperatures of 70, 80, and 90 °C. The amount of dye on silk (qt) increased quickly during the initial dyeing process. After 10 min, the adsorption rate of SCC dyeing on silk fiber decreased. After 120 min, the qt curve flattened, indicating that the dyeing process reached equilibrium. Figure 5 also shows the qt at higher dyeing temperature was lower than that at lower temperature after initial dyeing process, the maximum qt was 8.5 g/kg without NaCl and 12.79 g/kg with 9.0 g/L NaCl at dyeing temperature of 70 °C. NaCl can significantly increase the amount of dye absorbed by silk. 3.2. Adsorption Kinetics. In order to investigate the mechanism of adsorption, the pseudo second-order equation was used to test the experimental data of temperature and electrolyte effects. The second-order kinetic model is expressed as eq 4 and integrated as eq 5.18,19

dqt dt

= k(qe − qt )2

t 1 t = + 2 qt qe kqe

(4)

(5)

Where qt (g/kg silk) and qe (g/kg silk) are the amount of SCC dye adsorbed per unit weight of silk fiber at time t and equilibrium, k (kg/g per min) is the rate constant of the second-order adsorption, and t is the dyeing time. Figure 6 shows adsorption kinetic fitted curves of SCC on silk with eq 5. The slopes and intercepts of plots of t/qt versus t are used to calculate the equilibrium adsorption amount qe,cal and the second-order rate constant k. The straight lines in plot t/qt versus t showed good agreement of experimental data with the second-order kinetic model. For all the cases, the correlation coefficients were higher than 0.999, indicating that the adsorption of SCC on silk is a second-order kinetic model. Increasing dyeing temperature decreased the calculated adsorption amount qe,cal and the experimental adsorption amount qe,exp of SCC dye on silk. This was probably due to the negative adsorption enthalpy of SCC dye on silk. Three common dyeing kinetic parameters, dyeing rate constant k, the diffusion coefficient D, and the half dyeing time t1/2 are listed in Table 2. The diffusion coefficient D of the SCC dye to the silk fibers at 70, 80, and 90 °C were calculated

Figure 4. Effect of NaCl amount on (a) the percent uptake of SCC dye on silk and (b) color depth (K/S) of dyed silk fabrics: dyeing temperature 80 °C, pH 8, bath ratio 1:50, concentration of dye 0.3 g/ L, dyeing time 3 h.

dyed silk fabrics increased with increase of NaCl concentration from 1.0 to 9.0 g/L. Since the main components of SCC, disodium copper chlorophyllin has two carboxylic groups in its structure and trisodium copper chlorophyllin has three as shown in Figure 1, the SCC was negatively charged in pH 8 dye bath. When the silk protein with an isoelectric point of pH 427 also carried negative charge in the dye bath of pH 8, there was a repulsive electrostatic force between SCC dye molecules and silk protein molecules. With addition of NaCl, the repulsion can be decreased and adsorption of SCC dye on silk fabrics can be improved. The increasing trends were not obvious with the increase of NaCl concentration from 9.0 to 18.0 g/L. 8344

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adsorption isotherm is fundamental in describing the interactive behavior between solute and adsorbent. The Langmuir isotherm has been successfully applied in many adsorption processes and is expressed as eq 8:19−21,27−29 1 1 1 = + qe Q QbCe (8) Where qe is the amount of adsorption dye per kilogram of silk (g/kg silk) at equilibrium, Q is the maximum amount of the absorption dye per unit weight of silk fiber to form a complete monolayer coverage on the surface bound at equilibrium dye concentration Ce, and b is the Langmuir constant related to the affinity of binding sites. Another isotherm, the Freundlich model, is expressed as eq 9:27,28 1 ln qe = ln Q f + ln Ce (9) n

Figure 6. Fitted curves of SCC on silk with the pseudo second-order model: pH 8, bath ratio 1:50, initial concentration of dye 0.3 g/L.

Table 2. Effect of NaCl on the Adsorption Kinetic Parameters of SCC on Silk at Different Temperaturesa temp (°C)

qe,exp (g/kg)

70 80 90

8.50 6.47 4.91

70 80 90

12.79 11.84 10.75

qe,cal (g/kg)

k (kg/g per min)

without NaCl 8.88 14.04 × 10−3 6.73 20.24 × 10−3 5.06 41.95 × 10−3 with NaCl (9 g/L) 13.18 14.45 × 10−3 12.19 13.70 × 10−3 11.20 14.77 × 10−3

D (10−13 m2/ min)

t1/2 (min)

3.11 3.15 3.34

7.26 5.91 5.21

3.80 4.25 4.37

4.52 3.82 3.82

Where qe and Ce are the same with those in eq 8, Qf, an adsorption constant, is roughly an indicator of the adsorption capacity, and 1/n is that of the adsorption intensity. Langmuir and Freundlich isotherms were used to fit the equilibrium data of SCC dye adsorption on silk, and their correlation coefficients R2 are listed in Table 3. The high Table 3. Correlation Coefficient R2 of Fitting the Adsorption of SCC Dye on Silk with Langmuir and Freundlich Isotherm Models and Langmuir Isotherm Constants at Different Temperaturesa

a

qe,exp and qe,cal are respectively the experimental and calculated amounts of adsorption dye on silk at equilibrium.

Langmuir

using eq 6 and were obtained from the slope of straight lines in plots of qt/qe versus √t.28−30 The half dyeing time t1/2 is the time required to reach 50% of equilibrium sorption and was calculated from the regression of the experiment data using eq 7.31 It can be used to express the dyeing rate of SCC dye on the silk fabric.

temp (°C)

R2

70 80 90

0.9984 0.9983 0.9976

70 80 90

0.9996 0.9994 0.9995

Q (g/kg silk) without NaCl 17.86 14.49 11.11 with NaCl (9 g/L) 49.75 43.67 39.06

Freundlich b (L/g)

R2

3.11 2.96 2.73

0.9497 0.9361 0.9298

5.29 4.49 4.00

0.9230 0.9563 0.9445

⎡ Dt ⎤1/2 = 4⎢ 2 ⎥ ⎣ πr ⎦ qe

(6)

t1/2 = t |(q = q /2)

(7)

Initial SCC dye concentration 0.1−2.9 g/L, pH 8, bath ratio 1:50, dyeing time 3 h.

Where qt (g/kg silk) and qe (g/kg silk) are the amount of SCC dye adsorbed per unit weight of silk fiber at time t and equilibrium time, the same with 2.5 and 2.6, D is the diffusion coefficient, r is the radius of silk fiber (7.73 μm) and t1/2 is the half dyeing time. As seen in Table 2, higher dyeing temperature with and without NaCl led to higher diffusion coefficient D. This was because SCC dye molecules moved faster both in the solution and in the silk fibers at higher temperatures. It also can be seen from Table 2 that at the same dyeing temperature, the experimental and calculated adsorption amount at equilibrium qe,exp and qe,cal, diffusion coefficients D of SCC on silk with NaCl were higher than those without NaCl, while the half dyeing time t1/2 was lower. These differences were due to the stronger interaction between SCC dye molecules and silk protein molecules in the presence of NaCl, which also indicated that NaCl can significantly improve dyeing properties of SCC on silk fabric. 3.3. Adsorption Isotherms and Thermodynamic Parameters. 3.3.1. Adsorption Isotherms. The equilibrium

correlation coefficients (R2 > 0.99) showed that the Langmuir equation agreed well with the equilibrium isotherm in the entire dye concentration ranging from 0.1 to 2.9 g/L for the three dyeing temperatures of 70, 80, and 90 °C. However, the low correlation coefficients (R2 < 0.96) indicated poor agreement of Freundlich isotherm with the experimental data. Figure 7 is the Langmuir adsorption isotherm of SCC dye on silk at different temperature without and with NaCl (9.0 g/L) in dye bath. As seen in Figure 7, lower temperatures led to higher SCC dye adsorption in the range of 70−90 °C. This was due to the exothermic adsorption of SCC dye on silk fibers. Although the higher temperatures may create more space in silk fibers, the space effect for SCC dye was negligible comparing to the effect of adsorption enthalpy. It can also be seen from Figure 7 that the adsorption capacity of SCC dye on silk in the presence of NaCl was much larger than that without NaCl at the same temperature. By transforming eq 8, a linear plot of 1/qe versus 1/Ce is obtained. Q and b were calculated from the slopes and

qt

t

e

a

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Table 4. Effect of NaCl on the Adsorption Thermodynamic Parameters of SCC Dye on Silk at Different Temperaturesa temp (°C)

b (L/g)

70 80 90

3.11 2.96 2.73

70 80 90

5.29 4.49 4.00

−Δμ° (kJ/mol)

ΔH° (kJ/mol)

without NaCl 3.24 3.18 −6.32 3.03 with NaCl (9 g/L) 4.75 4.41 −13.74 4.18

ΔS° (J/ (mol·K))

R2

−9.01

0.9726

−26.28

0.9921

a

Initial SCC dye concentration 0.1−2.9 g/L, pH 8, bath ratio 1:50, dyeing time 3 h.

Figure 7. Langmuir adsorption isotherms of SCC dye on silk at 70, 80, and 90 °C: pH 8, bath ratio 1:50, dyeing time 3 h.

4. CONCLUSIONS In this study, a natural green dyestuff, sodium copper chlorophyllin (SCC), was used to dye silk to a vivid green shade. The effects of pH and NaCl in the dyeing bath were studied. The kinetics and adsorption isotherms for the SCC dyeing on silk were also studied. With and without addition of NaCl, an optimal green shade could be obtained at pH 8. NaCl could significantly increase the uptake of SCC and color depth of silk. The pseudo second-order kinetic model agreed well with the adsorption system of SCC on silk with and without NaCl in the dye bath. Adding NaCl in the dye bath increased the amount of adsorption at equilibrium qe, the diffusion coefficient D, and decreased the half dyeing time t1/2. Raising dyeing temperature decreased the amount of adsorption at equilibrium qe, increased diffusion coefficient D, and decreased the half dyeing time t1/2. The Langmuir equation agreed well with the equilibrium isotherm for the dyeing temperatures of 70, 80, and 90 °C in the entire SCC dye concentration range (0.1−2.9 g/L) with and without NaCl in the dye bath. The adsorption affinity (−Δμ°) and enthalpy change (ΔH°) of SCC dye on silk showed that the adsorption process was an exothermic and spontaneous process. Addition of NaCl increased affinity and enthalpy change, indicating that NaCl could increase interactive attraction between SCC dye and silk and therefore increased the adsorption capacity. SCC could be a potentially viable natural green dye for protein fibers.

intercepts of the above plots at different temperatures. The calculated Q and b are also shown in Table 3. The maximum monolayer adsorption capacity Q of SCC dye on silk in dye bath with NaCl is 39.06−49.75 g/kg, about 2.7−3.5 times of that without NaCl in dye bath at the same dyeing temperature. The presence of NaCl can decrease repulsive electrostatic force between SCC dye molecules and silk protein molecules. Moreover, water molecules are attracted by NaCl; therefore, the number of water molecules that can freely move decreased. Consequently, the effective concentration of SCC dye increased accordingly. Also as seen in Table 3, the maximum monolayer adsorption capacity Q decreased with increasing dyeing temperature. 3.3.2. Thermodynamic Parameters. The adsorption thermodynamic parameters, including the adsorption affinity (−Δμ°), enthalpy change (ΔH°), and entropy change (ΔS°), were calculated. The Langmuir constant b is related to the affinity of binding sites (b ∝ (exp(−H°/RT))). The adsorption affinities were estimated using eq 10.19,20 −Δμ° = RT ln(b)

(10)

Where −Δμ° is the adsorption affinity, R is the ideal gas constant, and T is the temperature. The apparent sorption enthalpy (ΔH°) was calculated according to eq 11 and was obtained by the slope of the linear regression of Δμ0/T vs 1/T.19,20 ΔH °/T = −Δμ° /T + C

(11)

■

Where ΔH° is the apparent sorption enthalpy and C is a constant. The sorption entropy (ΔS°) was calculated using eq 12.19,20 Δμ° = ΔH ° − T ΔS°

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 402-472-5197. Fax: +1 402-472-0640. E-mail address: [email protected].

(12)

The calculated dyeing affinity (−Δμ°), enthalpy change (ΔH°), and entropy change (ΔS°) under the different dyeing conditions are shown in Table 4. All the values of Δμ° and ΔH° are negative under the dyeing conditions used in this study, indicating that the adsorption process of SCC dye on silk fiber is a spontaneous and exothermic process. Increasing dyeing temperature decreased the dyeing affinity. The adsorption affinity of SCC dye on silk fiber with NaCl was about 1.5 times without NaCl. The adsorption enthalpy decreases from −6.32 to −13.74 kJ/mol. This reveals that NaCl in the dye bath can increase interaction between SCC dye and silk fibers.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project was financially supported by Program for Changjiang Scholars and Innovative Research Team in University (No.IRT1135) and the Agriculture Scientific Support Program of Jiangsu Province (No. BE2011404). We also thank the Fundamental Research Funds for the Central Universities (No. JUSRP21102) and for the Returned Overseas Chinese Scholars from State Education Ministry in China. 8346

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dx.doi.org/10.1021/ie300201j | Ind. Eng. Chem. Res. 2012, 51, 8341−8347