Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 455–460

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Aprotic solvents effect on the UV–visible absorption spectra of bixin Winda Rahmalia a,b, Jean-François Fabre a, Thamrin Usman b, Zéphirin Mouloungui a,c,⇑ a

Université de Toulouse, INP-ENSIACET, LCA (Laboratoire de Chimie Agro-industrielle), 4 Allée Emile Monso, CS 44362, 31030 Toulouse Cedex 4, France Departement of Chemistry, Mathematic and Natural Science, Tanjungpura University, Jl. Ahmad Yani, Pontianak 78124, West Kalimantan, Indonesia c INRA, UMR 1010 CAI, F-31030 Toulouse, France b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 We analyze bixin UV–visible changes

spectrum in various solvents.  We use Onsager cavity model and

Hansen theory to explain bixin– solvent interactions.  The S0 ? S2 transition energies of bixin depend on the solvent refractive index and dispersion forces.  We report bixin have higher absorption coefficient in aprotic polar solvents.

a r t i c l e

i n f o

Article history: Received 9 December 2013 Received in revised form 9 March 2014 Accepted 29 March 2014 Available online 13 April 2014 Keywords: Absorption coefficient Bixin Dispersion Refractive index

a b s t r a c t We describe here the effects of aprotic solvents on the spectroscopic characteristics of bixin. Bixin was dissolved in dimethyl sulfoxide, acetone, dichloromethane, ethyl acetate, chloroform, dimethyl carbonate, cyclohexane and hexane, separately, and its spectra in the resulting solutions were determined by UV–visible spectrophotometry at normal pressure and room temperature. We analyzed the effect of aprotic solvents on kmax according to Onsager cavity model and Hansen theory, and determined the approximate absorption coefficient with the Beer–Lambert law. We found that the UV–visible absorption spectra of bixin were found to be solvent dependent. The S0 ? S2 transition energy of bixin in solution was dependent principally on the refractive index of the solvents and the bixin–solvent dispersion interaction. There was a small influence of the solvents dielectric constant, permanent dipole interaction and hydrogen bonding occurred between bixin and solvents. The absorbance of bixin in various solvents, with the exception of hexane, increased linearly with concentration. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Bixin (methyl hydrogen 90 -cis-6,60 -diapocarotene-6,60 -dioate, C25H30O4; Fig. 1) is a carotenoid-based dye extracted from the seeds of the tropical shrub annatto (Bixa orellana L.) [1]. It is highly ⇑ Corresponding author at: Université de Toulouse, INP-ENSIACET, LCA (Laboratoire de Chimie Agro-industrielle), 4 Allée Emile Monso, CS 44362, 31030 Toulouse Cedex 4, France. Tel.: +33 534323520; fax: +33 534323598. E-mail address: [email protected] (Z. Mouloungui). http://dx.doi.org/10.1016/j.saa.2014.03.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

stable in the dark, in the presence or absence of oxygen, but is degraded when illuminated directly [2,3]. Bixin is soluble in most polar organic solvents, to which it imparts a yellow to red color. It is, therefore, widely used in industry for cosmetic, medicinal purposes, and as food coloring and textile dye [4–8]. Given its photoactive properties, bixin has recently been explored as a sensitizing dye in solar cells [9,10] and in photodynamic therapy [11]. Bixin is less expensive than other natural colors, and is therefore of considerable commercial relevance [12].

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W. Rahmalia et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 455–460

Fig. 1. Structure of cis-bixin.

Bixin is obtained from annatto seeds by solvent extraction. This process can be facilitated by use of a soxhlet extractor, microwaves, ultrasounds, and accelerated solvent extraction instruments. Bixin is hydrophobic with limited solubility in water. Efforts have, therefore recently been made to identify the most appropriate solvents for this molecule, to increase extraction efficiency [13–16]. Furthermore, the analytical models for the determination of bixin yield most frequently used are UV–visible spectroscopy and high performance liquid chromatography (HPLC). Various solvents have also been used as the mobile phases in HPLC analysis for the quantification of bixin. This quantification is typically achieved by measuring the absorbance of stock solutions, and using the values obtained to calculate the concentration of bixin. The values obtained with this approach are dependent on the absorption coefficient of bixin in the solvents used [17–19]. Various solvents have been used for bixin extraction and quantification, but no systematic studies of bixin–solvent interactions and their impact on extraction and quantification has been reported. The solute–solvent interactions occurring in solvation shell of the solutes have physical and chemical effects on many processes which can be determined by investigating the interactions of the solvent with solute, through observation of changes in the electronic absorption spectra obtained [20–25]. For this reason we decided to investigate the effects of different solvents on the UV–visible absorption spectrum of bixin. We previously reported that a mixture of acetone and cyclohexane (2:3 v/v) was an appropriate solvent for optimizing bixin extraction yield by the accelerated solvent extraction method. Purification by flash chromatography with gradients of acetone and hexane yielded 99.86% of bixin, consisting of 88.11% of cis-bixin and 11.75% di-cis-bixin [16]. In this study, we investigated the behavior of bixin in different solvents, by considering kmax shifts and the absorption coefficient in particular solvents. We selected two families of aprotic solvents for this study: aprotic polar solvents, such as dimethyl sulfoxide, acetone, dichloromethane, ethyl acetate, chloroform, and dimethyl carbonate, and secondly aprotic non-polar solvents, such as cyclohexane and hexane. We used an experimental and modeling approach based on the Onsager cavity model and Hansen theory. The information about kmax shifts is particularly important if we are to understand the modification of the S0 ? S2 transition energies of bixin different solvents, whereas the absorption coefficient can be used as a diagnostic tool, particularly for estimations of total bixin concentration in a mixture or an extract. These changes provide important information about the various physical and organic reaction of bixin, which have become important in various fields of pure and applied chemistry, such as solvent extraction and the purification of bixin, the design and synthesis of new sensitizers based on bixin for medicinal purposes (e.g. photodynamic therapy) and sensitizing dyes for solar cells. The sensitizing dyes which have the higher absorption coefficient and lower S0 ? S2 transition

energy will give higher photoelectric conversion efficiencies of dyes sensitized solar cells [9,26]. Materials and methods Materials Bixin crystals containing 88.11% cis-bixin and 11.75% di-cisbixin and 0.14% unknown compound were obtained by the extraction and purification processes explained elsewhere by Rahmalia et al. [16]. All the solvents used were HPLC grade and were supplied by Sigma–Aldrich, Germany. Methods A double-beam Shimadzu UV-1800 UV–visible spectrophotometer was used to record the absorption spectra over a wavelengths 200–600 nm, at normal pressure and room temperature. Quartz cuvettes with a 1 cm path length were used for measurements in solution. The estimated experimental error was 1 nm on the band maximum. A 5 mg/l stock solution of bixin was prepared by dissolving a few crystals in solvent. Four dilutions were then prepared, to give concentrations of 4, 3, 2, and 1 mg/l, respectively. All the experiments were performed in a room with the lights turned off and the blinds lowered. The solvent effect on the S0 ? S2 transition energy of bixin was analyzed with the Onsager cavity model and Hansen parameters, and an approximate absorption coefficient was obtained with the Beer–Lambert law. Theory and calculation Solvent effects on the S0 ? S2 transition energy The strong visible absorption band of carotenoids is known to be associated with the one photon allowing the S0(11Ag) ? S2(11Bu+) transition, which can be determined from the maximum wavelength of the absorption spectrum [27]. By using the cavity model and reaction field theory, Onsager [28] found that the band shift of absorption spectra in solutions was related to the refractive index function R(n2), the dielectric constant function R(e) of solvents and the solvent polarity function R(e):

RðeÞ ¼ ðe  1Þ=ðe þ 2Þ

ð1Þ

Rðn2 Þ ¼ ðn2  1Þ=ðn2 þ 2Þ

ð2Þ

RðeÞ  Rðn2 Þ ¼ ðe  1Þ=ðe þ 2Þ  ðn2  1Þ=ðn2 þ 2Þ

ð3Þ

e: dielectric constant, n: refractive index.

W. Rahmalia et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 455–460

This transition is shifted to lower energies when gaseous molecules are solvated, and it should display a linear dependence on the refractive index function of the solvents [29]:

tðsolventÞ ¼ tðgasÞ  kðRðn2 ÞÞ

ð4Þ

t: S0 ? S2 transition energy (cm1), k: constant for a given polyene transition (cm1). The use of Hansen solubility parameters (HSP) provide an explanation for the variable solubility observed in this work. These parameters reveal that the total energy of vaporization of a liquid consists of several individual parts. Based on this theory, there are three major types of interactions in common organic materials [30]: (1) the dispersion interactions/dD, (2) the permanent dipole–permanent dipole interactions/dP, and (3) hydrogen bonding interactions/dH. Solvent effects on the absorption coefficient of bixin In solutions of low concentration, bixin dissolves almost completely into monomers. Hence, the absorption spectra are determined by the contribution of the absorption of bixin molecules and the bixin–solvent interaction. Bixin–bixin (solute–solute) interactions are negligible because of the large mean distance between bixin molecules. The absorption spectra obey the Beer– Lambert law, i.e. there is a linear relationship between the absorbance and concentration of an absorbing species [19]:

A¼kcl

ð5Þ 1

A: absorbance, k: absorption coefficient (l g length (cm), c: concentration (g l1).

1

cm

), l: optical path

Results and discussion Solvent effect on the S0 ? S2 transition energy The conjugated double bond system constitutes the lightabsorbing chromophore of bixin, resulting in the strong absorption band above 400 nm (Fig. 2). This transition is mediated by the

8

absorbance

7

6 5

4 3 2

250

350

450

1 550

wavelength (nm) Fig. 2. UV–visible spectra of bixin in dimethyl sulfoxide (1), acetone (2), dichloromethane (3), ethyl acetate (4), chloroform (5), dimethyl carbonate (6), cyclohexane (7) and hexane (8).

457

S0(11Ag) ? S2(11Bu+) electronic-dipole allowed, and is oriented along the long axis of the molecule. There are three typical peaks in the absorption band of bixin in aprotic solvents, arising from the level of vibration (v0, v1, v2, v3, etc.) associated with the initial and final electronic states [31]. In aprotic polar solvents, the absorption spectra of bixin have their maximum intensity (kmax) in the 0–1 band position. The presence of additional small peaks in aprotic non-polar solvents may be due to the formation of aggregates caused by lack of solubility of bixin in hexane and cyclohexane [32]. When the vibrational structure was not apparent, a similar spacing was assumed for each vibrational progression. The results of deconvolution shown in the supporting information indicate that the kmax of bixin in hexane and cyclohexane could be associated with a v0–v3 band position. The other peak is observed between 250 and 330 nm, which is assigned to the 11Ag ? 21Bu+ transitions. In all-trans-carotenoids, this gives rise to no net dipole moment, and thus no absorption band. However, the presence of one or more cis double bonds may result in a dipole moment for this transition, and the appearance of a so-called cis peak in the spectrum of these cis-carotenoids. The energy and intensity of this peak depend on the position and number of cis bonds [33–35]. In dimethyl sulfoxide, dichloromethane, ethyl acetate, chloroform, dimethyl carbonate, cyclohexane and hexane, the cis peak of bixin appeared at 285 nm, 287 nm, 273 nm, 293 nm, 280 nm, 291 and 293 nm respectively. No cis peak was observed for bixin in acetone, because of the solvent cut off at 330 nm. A comparison of the UV–visible spectra of bixin obtained in these solvents revealed a pronounced dependence on the solvent, a feature typical of the electronic absorption spectra of polyene compounds. The S0 ? S2 transition energy of bixin was lowest in dimethyl sulfoxide and the highest in hexane. Hexane, which has R(n2)  R(e), dP and dH values of 0.0, does not interact with the bixin molecule through dipole–dipole interactions or hydrogen bonding in either the ground or excited state, hence the hypsochromic shift in the spectrum of bixin in this solvent (see Table 1). Fig. 3 shows the different behavior of bixin in two families of solvents. Overall, in aprotic polar solvents, bixin has a much lower S0 ? S2 transition energy than in aprotic non-polar solvents. The structure of bixin, with 25 carbons atoms and acid and methyl ester end-groups, is more polar than that of the carotenoids usually found in foods (chains with 40 carbons atoms), and this molecules has a greater affinity for medium polar solvents [37,38]. Fig. 3a illustrates the strong linear relationship between S0 ? S2 transition energy and solvent refractive index function obtained in aprotic polar solvents. An increase in solvents refractive index value leads to bathochromic shift of the UV–visible absorption peaks. We can predict, from Eq. (4), that the S0 ? S2 transition energy of bixin in the gaseous phase is 24,934 cm1, and that the constant for a given polyene transition of bixin is 13,543 cm1. The use of Hansen theory greatly increased our understanding of the variable solubility of bixin in this work. Increasing of dD values are associated with a bathochromic shift of the UV–visible absorption peaks (Fig. 3c). This suggests that the increasing of dispersion interactions decreases the S0 ? S2 transition energy of the polyene chromophore. These spectral observations highlight the effect of solvent microenvironment on the spectroscopic behavior of the bixin chromophore. However, the S0 ? S2 transition energy of bixin was found to be less dependent on solvent dielectric constant function, permanent dipole–permanent dipole interactions, and hydrogen bonding interactions. The S0 ? S2 transition energies of bixin were found to be located in four different wavelength regions (Fig. 3b, d and e): (1) dimethyl sulfoxide, dichloromethane, and chloroform; (2) acetone, ethyl acetate, and dimethyl carbonate; (3) cyclohexane;

W. Rahmalia et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 455–460

a

25

b

8

8 7

24

(1000/cm)

25

23

4

22

y = -13.543x+24.934 r = 0.9651

6

2

7

24

(1000/cm)

458

3

5

23

6

22

4 5

1

0.25

0.2

0.3

1

0.6

d

25

25

8

8

24

7

(1000/cm)

24

23

6

22

1

R( )

R (n2)

(1000/cm)

3

21

21 0.2

c

2

7

23

4

22

4 3

2

5

1

6

2

3

5

1

21

21 14

15

16

17

18

0

19

3

6

(1000/cm)

e

9

12

15

18

P

D 25

8 24

7

23

2

22

5

4

6

3

1

21 0

3

6

9

12

H Fig. 3. The S0 ? S2 transition energies of bixin against the physical constants of solvents: dimethyl sulfoxide (1), acetone (2), dichloromethane (3), ethyl acetate (4), chloroform (5), dimethyl carbonate (6), cyclohexane (7), hexane (8).

(4) hexane. Considering the differences of dD value between dimethyl sulfoxide and dimethyl carbonate, and the high contribution of Cl atoms to dispersion forces [39,40] bixin may have lower S0 ? S2 transition energies in sulfur or chlorine based solvents than in equivalent carbon based ones. The long conjugated chain of bixin may be responsible of the preponderance of dispersion forces interactions. Provided hydrogen bonding and permanent dipole–permanent dipole interactions can exist between the solvent and the bixin ester and acid functions, the S0 ? S2 transition energy of bixin was found to be more independent on these parameters. The solvent refractive index and dielectric constant are often predicted from idealized theories, as a quantitative measurement of solvent polarity. However, this approach is often inadequate, because these theories regard the solvents as a non-structured isotropic continuum, rather than as being composed of individual solvent molecules, each involved in solvent–solvent interactions. These theories therefore do not take into account specific solute– solvent interactions, such as dispersion, permanent dipole, and hydrogen-bonding interactions, which often play a dominant role in solute–solvent interactions [30,41].

Solvent effect on absorption coefficients of bixin The absorbance at kmax of bixin increased linearly with increasing concentration in dimethyl sulfoxide, acetone, dichloromethane, ethyl acetate, chloroform, dimethyl carbonate and cyclohexane, but not in hexane, in which it increased non-linearly (Fig. 4). This may be due to the poor solubility of bixin in hexane, as observed experimentally. Early studies by Zang et al. [42] showed the poor solubility of zeaxanthin in hexane has caused the increase nonlinearly of absorbance with concentration. This is in a good agreement with the results of Silva et al. [37] and Chiste et al. [38] who reported that bixin shows more affinity for medium polar solvents. Table 2 shows the parameters for the evaluation of validity for the standard. This finding is consistent with those of Khachik et al. [43] who stated that the correlation coefficient should be greater than 0.99, the intercept should be very close to zero, and the relative standard deviation of the regression (standard error of the estimate divided by mean concentration of standards multiplied by 100) should be less than 5%. Based on Eq. (5), the slope given in Table 2, can be taken as a reasonable estimate of the value of the absorption coefficient of bixin. These values are slightly lower

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W. Rahmalia et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 455–460 Table 1 Physical constants of the solvents and parameters of the absorption spectra. Onsager cavity modela

Solvent

2

Dimethyl sulfoxide Acetone Dichloromethane Ethyl acetate Chloroform Dimethyl carbonate Cyclohexane Hexane a b

Hansen parametersb 2

S0 ? S2 transition energy

R(n )

R(e)

R(e)  R(n )

dD

dP

dH

kmax (nm)

t (103 cm1)

0.284 0.220 0.255 0.227 0.267 0.226 0.256 0.229

0.938 0.872 0.728 0.626 0.565 0.412 0.254 0.229

0.654 0.652 0.473 0.399 0.298 0.186 0.002 0.000

18.4 15.5 17.0 15.8 17.8 15.5 16.8 14.9

16.4 10.4 7.3 5.3 3.1 8.6 0.0 0.0

10.2 7.0 7.1 7.2 5.7 9.7 0.2 0.0

473 457 467 456 470 456 417 408

21.24 21.88 21.41 21.93 21.28 21.93 23.98 24.51

Value obtained from Ref. [36]. Value obtained from the HSPiP application notes program.

1.6 5 2 4

absorbance

1.2

6 3 1

0.8

0.4

solvents than in aprotic non-polar solvents. Bixin showed the poor solubility in aprotic non-polar solvents. According to the Onsager cavity model and Hansen theory, these solvents effects were dependent principally on the refractive index of the solvents and the bixin–solvent dispersion interaction. This information can be used to identify the most appropriate solvents will be used in application methods which have become important in different fields, such as solvent extraction, purification, and quantification of bixin, design and synthesis of new sensitizing dye based on bixin. Since dimethyl carbonate has the value of S0 ? S2 transition energy and absorption coefficient is almost similar with acetone, it would be used in our future work as green solvent with benefit health.

7 8

0 0

1

2

3

4

5

concentration (mg/L) Fig. 4. Relationship between absorbance at kmax and concentration of bixin in dimethyl sulfoxide (1), acetone (2), dichloromethane (3), ethyl acetate (4), chloroform (5), dimethyl carbonate (6), cyclohexane (7) and hexane (8).

Acknowledgments This study was financially supported by Indonesian Directorate General of Higher Education (DIKTI). We also thank Dr. Leenawaty Limantara (Machung University) and Deni Ferdian, M.Sc (University of Indonesia) for helpful discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.03.119.

Table 2 Parameters for evaluating the validity of the standards. Solvent

Slope  103

Intercept

Correlation coefficient

Dimethyl sulfoxide Acetone Dichloromethane Ethyl acetate Chloroform Dimethyl carbonate Cyclohexane

185.40 285.49 226.66 269.26 300.91 241.84 75.59

0.0020 0.0183 0.0109 0.0012 0.0034 0.0743 0.0013

0.9994 0.9992 0.9989 1.0000 1.0000 0.9958 0.9989

than previously published data reporting bixin to have an absorption coefficient of 2826 (282.6 l g1 cm1) at 467 nm in dichloromethane and 3230 (323.0 l g1 cm1) at 470 nm in chloroform [44]. If the possible presence of a small amount of residual extraction solvent may be considered, this difference seems to come from a different sourcing of the dye, modifying isomer composition. These values can be used as a reference for the comparison of absorption coefficient values for bixin in different solvents. Conclusions At similar concentrations, bixin has highly variable optical densities in different solvents. In aprotic polar solvents, bixin has a much lower S0 ? S2 transition energy than in aprotic non-polar solvents. Bixin has a higher absorption coefficient in aprotic polar

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Aprotic solvents effect on the UV-visible absorption spectra of bixin.

We describe here the effects of aprotic solvents on the spectroscopic characteristics of bixin. Bixin was dissolved in dimethyl sulfoxide, acetone, di...
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