Measurement and correlation of solubility of Acid Red 57 in supercritical carbon dioxide by ion-pairing with hexadecyltrimethylammonium bromide

J. of Supercritical Fluids 37 (2006) 23–28

Measurement and correlation of solubility of Acid Red 57 in supercritical carbon dioxide by ion-pairing with hexadecyltrimethylammonium bromide ∗ , Adnan Ozcan ¨ ¨ A. Safa Ozcan Department of Chemistry, Faculty of Science, Anadolu University, 26470 Eski¸sehir, Turkey Received 23 July 2004; received in revised form 1 June 2005; accepted 15 June 2005

Abstract Solubility of C.I. Acid Red 57 (AR57) in supercritical carbon dioxide was measured by ion-pairing with hexadecyltrimethylammonium (HDTMA) bromide. The solubility measurements of AR57 and AR57-HDTMA in supercritical carbon dioxide without/with methanol as a modifier solvent were carried out at the temperature range from 35 to 75 ◦ C and for pressures from 250 to 325 bar. The solubility of AR57 and AR57-HDTMA was examined in terms of pressure and temperature of supercritical carbon dioxide. Even though Acid Red 57 is insoluble both in supercritical carbon dioxide and methanol-modified supercritical carbon dioxide, AR57-HDTMA can easily dissolve in methanol-modified supercritical carbon dioxide. The hydrophobic ion-pairing of HDTMA provides a possibility to dissolve a hydrophilic dye in supercritical carbon dioxide. A semi-empirical equation was used to correlate the obtained experimental solubilities of AR57-HDTMA by means of the density of carbon dioxide in methanol-modified supercritical carbon dioxide. © 2005 Elsevier B.V. All rights reserved. Keywords: Acid dye; Supercritical carbon dioxide; Solubility; Ion-pairing; HDTMA

1. Introduction Conventional dyeing of textile fibers is generally performed in water based media. However, this process has intrinsic environmental problems including water pollution due to the inevitable use of an excess amount of water and the discharge of various chemical additives. Moreover, a subsequent drying process with high energy consumption is necessary [1,2]. Due to this environmental problems and economic consideration, a new attractive dyeing method has been developed in recent years, in which supercritical fluid dyeing replaces a conventional water-based dyeing process [3–7]. Since supercritical fluids have very low surface tension and high diffusion, they can easily enable penetration into the textile fibrous structure. The supercritical fluid dyeing method has some advantages to water-based dyeing, such as ∗ Corresponding author. Tel.: +90 222 3350580/5781; fax: +90 222 3204910. ¨ E-mail address: [email protected] (A.S. Ozcan).

0896-8446/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2005.06.010

solubility of dyes can be controlled by pressure, allowing possible control of the dyeing strength and color, contaminated wastewater streams are not produced, and washing and then drying of the dyed fabric is not necessary, supercritical fluid and the remain in the cell of solid textile dye can be reused after dyeing. So, the dyeing procedure is shorter than that of conventional methods [8,9]. Even though several substances are useful as supercritical fluids, CO2 has been the most widely used because it is inexpensive, essentially nontoxic, nonflammable, recyclable, abundant, chemically inert under most conditions, and has easily accessible critical conditions (31 ◦ C, 73.8 bar) [10,11]. Solubility of dyes requires the optimization and design of the textile dyeing in supercritical CO2 . The non-polar disperse dyes are easily soluble in the likewise non-polar supercritical CO2 , and many studies about the solubilities of disperse dyes in supercritical CO2 have been carried out [2,6,12–23]. On the contrary, water soluble polar dyes such as acid dyes and reactive dyes do not dissolve in nonpolar supercritical CO2 and data available in the literature are still

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few [24–29]. Dyeing of textile fibres in supercritical CO2 , therefore, has been limited to synthetic fibres using disperse dyes but the dyeing of natural fibres in this medium is still under development. In order to dye natural fibres, it is necessary to make such dyes to be soluble in supercritical fluids for supercritical dyeing procedure. For this reason, the polar dyes are reacted with hydrophobic ion-pairing reagent and then they can be solubilized in supercritical CO2 . The aim of this research was to evaluate the measurement and correlation of the solubility of Acid Red 57 (AR57) by ion-pairing with hexadecyltrimethylammonium (HDTMA) bromide in supercritical CO2 .

2. Experimental 2.1. Materials A commercial grade textile dye C.I. Acid Red 57 (AR57) (Nylosan Red EBL) (purity 83.0%) was supplied by ClairentSwitzerland and used without further purification. Hexadecyltrimethylammonium (HDTMA) bromide purchased from Merck was used as ion-pairing reagent and was used without further purification. The chemical structures of substances were given in Fig. 1. Carbon dioxide (purity 99.9%) was obtained from Habas, Turkey. HPLC-grade methanol (Riedel-de Ha¨en) was used as received.

Fig. 1. Chemical structures of AR57 (a); and HDTMA bromide (b).

Table 1 Elemental analysis of AR57 and AR57-HDTMA Dye

C%

H%

O%

N%

S%

AR57 By calculation By analysis

54.48 48.03

4.18 4.63

18.52 29.09

10.65 8.39

12.17 9.86

AR57-HDTMA By calculation By analysis

63.78 59.54

7.79 7.95

11.87 19.35

8.65 6.62

7.91 6.54

2.2. Preparation of ion-pairing 0.01 mol AR57 was mixed with 500 ml water and 10 ml 1 M NaOH to get an AR57 solution. 0.01 mol of HDTMA bromide was dissolved in 500 ml water. The alkaline AR57 solution was dropped into HDTMA bromide solution and then acidified with HC1 solution to form the ion-pair. After filtration, the solid ion-pair precipitate was washed with 30 ml water for three times, dried at 50 ◦ C for 6 h and kept in a dryer with CaCl2 as the drying agent [28]. Elemental analysis of AR57 and AR57-HDTMA is given in Table 1. The quantities of oxygen were obtained by difference in weight. As can be seen from Table 1, the percentage of C was drastically increased from 48.03 to 59.54, but the

percentage of S was decreased from 9.86 to 6.54 by the modification of AR57 with HDTMA bromide. These results are well agreed with FTIR results and indicate the interaction between AR57 and HDTMA bromide. FTIR spectra were carried out (KBr) on a Jasco Model FT/IR-300E Fourier transform infrared spectrophotometer to observe the interaction between HDTMA bromide and AR57. 2.3. Supercritical fluid extraction (SFE) A dynamic analytical apparatus (Fig. 2) was used for the measurement of the solubility in supercritical CO2 . The apparatus was operated using pure CO2 (99.9%) and an Isco

Fig. 2. Schematic diagram of the experimental apparatus. (1) CO2 cylinder; (2) CO2 pump; (3) modifier pump; (4) modifier reservoir; (5) oven; (6) vessel; (7) restrictor; (8) solvent heater; (9) solvent reservoir; (10) solvent pump; (11) back pressure regulator; (12) collection vial.

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Model 260 D syringe pump (266 ml capacity). An amount of 2 g of AR57-HDTMA was mixed with 2 g of prewashed sea sand and placed into a stainless steel cell (10.4 ml total volume) and the rest of the cell was then filled with sand. The extraction cell was placed inside an oven, the temperature of which could be easily controlled. The flow rate of the supercritical fluid through the extraction cell at a rate of 1.0 cm3 min−1 was measured at the CO2 pump and 15 cm long stainless steel restrictor (30 ␮m i.d.) was connected to the oven. The restrictor was heated to prevent it from plugging during the extraction procedure. The cell was purged with a flow of supercritical CO2 . The CO2 pressure was controlled using a back-pressure regulator (Go Inc., USA). After equilibrium at the desired temperature and pressure was reached, extracted samples were collected by placing the outlet of the back-pressure regulator into a vial containing 5 ml of methanol. Dynamic extractions were sequentially performed for 15 min periods. Flow rate of methanol was 0.1 cm3 min−1 . Solubilities were measured dynamically at 35, 55 and 75 ◦ C, and at 250, 275, 300 and 325 bar. The accuracy of the temperature was ±1◦ C, the accuracy of the pressure measurement was ±2% of the full scale of the pump (510 atm of the maximum pressure), and errors in flow rate were ±0.5%. The extracts were analyzed by maximum absorbance at 517.5 nm using a model 2101 Shimadzu UV–vis spectrophotometer. The calibration curve was obtained with the standard solutions (0.4–1.3 × l0−5 mol dm−3 ) of HDTMA-AR57 dissolved in methanol. The solubility of the dye in supercritical CO2 was then calculated by means of mole fraction by using the calculated densities [30] of the supercritical fluid in the pump and the volume of supercritical CO2 used during each collection and displayed at the pump. Measurements were carried out in quadruplicate. 2.4. Test of solubility data The disperse dye, C.I. Disperse Orange 30 (DO30), is examined to test the apparatus and compare the solubility with literature data [6,31]. The comparison of results is given in Table 2. The solubility value of DO30 at 200 bar and 80 ◦ C in the present study is very close to the value that in the literature given by Ozcan et al. [6] and the values at the pressures of 200 and 300 bar and at a temperature of 100 ◦ C are higher than those in the literature [31]. These results show that the

comparison is reasonably well with respective literature data. Furthermore, the solubility of Disperse Orange 30 in supercritical CO2 by Baek et al. [32] was measured by using a closed-loop (batch) solid-fluid equilibrium apparatus at temperatures between 40 and 120 ◦ C and at pressures between 110 and 330 bar. Their solubility result at 90 ◦ C and 290.5 bar was 49.78 × 10−6 . This result is higher than that of our result at 100 ◦ C and 300 bar.

3. Results and discussion 3.1. FTIR analysis Since there is evidence for IR bands from the formation of AR57-HDTMA, FTIR spectra were recorded in the region of 400–4000 cm−1 (Fig. 3). As can be seen from Fig. 3, a pair of strong bands at 2854 and 2923 cm−1 was observed for AR57-HDTMA and can be assigned to the symmetric and asymmetric stretching vibrations of the methylene groups (νCH2 ), and their bending vibrations are between 1362 and 1437 cm−1 , supporting the interaction between HDTMA and AR57. 3.2. Solubility of AR57-HDTMA Although hydrophobic disperse dyes dissolve in supercritical CO2 , water soluble hydrophilic polar acid dyes, here AR57, do not dissolve in supercritical CO2 or methanol-modified supercritical CO2 at the pressure range of 250–325 bar and the temperature range of 35–75 ◦ C. Therefore, AR57 was modified with HDTMA bromide by forming a hydrophobic ion-pair, and the ion-pair formed by replacing the cation in AR57 with HDTMA bromide is insoluble in water. The original structures of the ions to be paired will not change during the ion-pairing process. The AR57-HDTMA ion-pair is likewise not soluble in supercritical CO2 , but it is solubilized in a small amount of methanol-modified supercritical CO2 , because the solvent power and the polarity of supercritical CO2 is then increased. Furthermore, interaction

Table 2 The comparison of solubility data for DO30 in CO2 with published data t (◦ C)

p (bar)

Ozcan et al. [6]

Draper et al. [31]

80

200.0

5.49

5.70

–

90

190.6 290.5

– –

– –

– –

100

200.0 300.0

34.7 37.1

– –

17.5 17.9

106 x This study

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Fig. 3. FTIR spectra of AR57 and AR57-HDTMA.

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Table 3 Solubilities in mole fraction, x, for AR57-HDTMA in 10 vol% methanolmodified supercritical CO2 107 x

p (bar)

250 275 300 325

35 ◦ C

55 ◦ C

75 ◦ C

2.32 7.77 9.39 10.7

0.76 4.28 6.14 7.02

0.20 1.11 2.19 5.02

between AR57-HDTMA and modifier via hydrogen bonding can also play an important role in solubility enhancement [33,34]. Surface tension between the CO2 -phase and AR57HDTMA might also be decreased. In addition, the viscosity of the CO2 -phase may also increase, having a positive effect on the solubility of AR57-HDTMA [35]. The effects of temperature and pressure on the solubility of AR57-HDTMA were determined in supercritical CO2 containing 10 vol% of the modifier methanol. The measurements of solubility of AR57-HDTMA were performed to enable data correlation and are given in Table 3. The accuracy of the solubility measurements for AR57-HDTMA was determined by obtaining the percentage relative standard deviation, these varied from 2.8 to 8.6% in the worst case for AR57-HDTMA. When the pressure is increased at a constant temperature, the solubility of AR57-HDTMA increases due to increasing in the fluid density. On the contrary, as the temperature is increased at a constant pressure, the solubility of AR57HDTMA decreases (Table 3). 3.3. Correlation of the solubility data In the literature, several semi-empirical models were used to describe and correlate the solubility data, and a simple semi-empirical method [36] was used to correlate the experimental data in this study. Although the fit to data is not as good as could be desired, only three parameters for the compound are needed to predict a value at any pressure and temperature in the experimental range. The correlation equation is given in Eq. (1). ln(xp/pref ) = A + c(ρ − ρref )

Table 4 Parameters used in the calculation of solubilities of AR57-HDTMA from Eq. (3) a

b (K−1 )

c (g dm−3 )

Hv (kJ mol−1 )

31.64

−1.54 × 104

4.39 × 10−2

127.87

where T is the absolute temperature. Combining Eqs. (1) and (2), the correlation equation becomes ln(xp/pref ) = a + b/T + c(ρ − ρref )

From the experimental data, each isotherm was fitted using Eq. (1) to obtain values of A and c. The values for c were then averaged for AR57-HDTMA, and the results are given in Table 4. Afterward, the isotherms were fitted to obtain new values of A using the averaged values of c. The new best values of A were then plotted against 1/T, as shown in Fig. 4, and values for a and b were obtained from the intercept and the slope of the straight line of graph, which are also given in Table 4. Finally, predicted solubilities were compared with experimental values in Fig. 5, using Eq. (3). Although the fitting of individual isotherms to Eq. (3) was very good, the agreement between the calculated and experimental values for AR57-HDTMA is less successful, as can be seen. However, solubilities can approximately be calculated at any temperature and pressure within the experimental range using Eq. (3) and the parameters of Table 4. The parameters b is approximately related to the enthalpy of vaporization of the solid state, Hv , and it is given by Hv = −R b

b T

(1)

(2)

(4)

where R is the gas constant. The estimated Hv value [36] is also included in Table 4.

where x is the mole fraction solubility, p is the pressure, A and c are constants, pref is the standard pressure of 1 bar, ρ is the density of the solution, and ρref is a reference density for which a value of 700 g dm−3 was used for calculations. The reason for using ρref is to make the value of A much less sensitive to experimental error in the data and to avoid the large variations caused by extrapolation to zero density. A is given by A=a+

(3)

Fig. 4. The plot of A vs. 1/T according to Eq. (2).

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Fig. 5. Comparison of predicted curves (lines) and experimental solubility (points) in mole fraction for the complex AR57-HTDMA at three temperatures.

4. Conclusions Although disperse dyes are soluble in supercritical CO2 , a polar dye like Acid Red 57 (AR57) is insoluble in supercritical CO2 . The most current method to dissolve polar substances in supercritical CO2 is to make an addition of a small amount of methanol as a modifier to increase the polarity and therefore the solvent power. However, AR57 is still insufficiently soluble both in supercritical and methanolmodified CO2 . Additionally, even the ion-pair form of AR57, AR57-HDTMA is essentially insoluble in supercritical CO2 and requires the addition of 10 vol% methanol to achieve a measurable solubility. The highest solubility was obtained as 1.07 × 10−6 (in mole fraction) at 325 bar and 35 ◦ C by AR57-HDTMA in methanol-modified supercritical CO2 . The correlation of solubility data of AR57-HDTMA in methanol-modified supercritical CO2 was carried out. The solubility of the ion-pairing in this medium is strongly dependent on the operation pressure, temperature and the density of supercritical CO2 . The correlation of the solubility data of AR57-HDTMA is satisfactorily correlated by a semiempirical equation at lower temperatures.

Acknowledgement The authors gratefully acknowledge for the financial support of Turkish State Planning Commission (DPT 2003K 120170).

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