Article pubs.acs.org/jced
L‑Proline
Based Aqueous Biphasic System: Design and Application To Isolate the Alkaline Earths
Arabinda Chakraborty and Kamalika Sen* Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700 009, India ABSTRACT: An aqueous biphasic system (ABS) was obtained after mixing an amino acid (L-proline) solution (5 mol·kg−1) with a 0.90 g·g−1 solution of nonionic surfactant (Triton X-100). The phase diagram of the system was constructed at 296 K. The system was found to work in a more efficient and economic way when the surfactant is enriched with a cosolute, isoamyl alcohol in a ratio of 2:1. The addition of isoamyl alcohol results in a decrease in density of the surfactant phase, much below the density of a dilute solution of proline (0.1 mol·kg−1). This results in very low consumption of the amino acid for the design of the ABS. The tolerance of the ABS toward changes in pH and concentrations have been found to improve drastically in the presence of the cosolute. When sodium alginate is used as a complexing agent, the developed system shows a pH-dependent separation for the alkaline earth metal ions Mg2+, Ca2+, Sr2+, and Ba2+ from one another. Detection of these metal ions after their separations has been achieved with eriochrome black T (EBT; for Mg2+ at pH 9.5) and arsenazo III (for Ca2+, Sr2+, and Ba2+ at pH 7.2) using absorption spectrometry. As different pH values are suitable for the complexation and extraction of different alkaline earths, it is possible to have a sequential separation of the individual elements from a mixture as and when required.
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INTRODUCTION Aqueous biphasic systems (ABSs) are formed when two mutually incompatible polymers, polymer/salt or salt/salt, are dissolved in water above a certain critical concentration. These systems comprise water mass fractions of (80 to 90) % in equilibrium phases, each phase containing mainly one of the components and a small amount of the other. ABSs have been widely used in the separation, concentration, and fractionation of biological solutes and particles such as cells and proteins.1,2 ABSs have also been studied for applications including the selective distribution and separation of metal ions,3 small organic molecules,4,5 nano- and microsolid particulates6,7 and as green reaction media.8−10 Several ABSs have been designed which are mostly based on polymer/polymer, polymer/salt, ionic liquid/salt, and surfactant/salt systems.11,12 An ionic liquid/amino acid system was also designed; however, none of its applications were proposed.13 The mutual coexistence curve or the phase diagram of aqueous biphasic systems are of immense assistance to define the potential working range of the ABS. The phase diagram is a unique feature of a particular system under particular conditions of pH, temperature, and component concentration. Information that can be obtained from such diagrams includes the concentration of phase-forming components, the subsequent concentration of phase components in the top and bottom phases, and the ratio of phase volumes. The diagram bears a binodal curve, which divides a region of compositions that will form two immiscible aqueous phases (the area above the curve) from those that will form one phase (at and below the curve). Coordinates for all possible systems lie on a tie-line which connects two points on the binodal. Moving along the © 2014 American Chemical Society
tie-line coordinates denote systems with differing total compositions and volume ratios, but with the same final concentration of phase components in the top and bottom phases.1 Three methods for the preparation of a phase diagram are possible, turbidometric, cloud point, and node determination. Turbidometric titration is a relatively quick and commonly used method for determination of the binodal. The significance of the tie line length, TLL, lies in the fact that, as tie-line length increases, the top phase becomes more concentrated in the upper part and the bottom phase becomes more concentrated in the lower part of the ABS. As tie-lines decrease in length, they ultimately approach a critical point on the binodal where the TLL = 0. At this point the composition and volume of the two phases theoretically become equal. At trace concentrations, alkaline earth metal ions play crucial roles in deciding chemical and biological processes. Magnesium and calcium are ubiquitous and essential to all known living organisms. Magnesium or calcium ion pumps take part in cellular processes.14 Magnesium functions as the active center in certain enzymes, and calcium salts take a structural role, most notably in bones. Strontium plays an important role in marine aquatic life, especially hard corals, which use strontium to build their exoskeletons. Sr and Ba find uses in medicine, for example “barium meals” in radiographic imaging. In addition, there are reports on the catalytic activity and selectivity of several alkaline-earth metal compounds in the trans-esterification of Received: December 4, 2013 Accepted: March 10, 2014 Published: March 24, 2014 1288
dx.doi.org/10.1021/je401052n | J. Chem. Eng. Data 2014, 59, 1288−1294
Journal of Chemical & Engineering Data
Article
Table 1. List of IUPAC Names of the Compounds with Their Respective Purity in Mass Fraction (g·g−1) IUPAC name
purity (mass fraction, g·g−1)
(2S)-pyrrolidine-2-carboxylic acid 2-[4-(2,4,4-trimethylpentan-2-yl)phenoxy]ethanol 6-(2-carboxy-4,5-dihydroxy-6-methoxyoxan-3-yl)oxy-4,5-dihydroxy-3-methoxyoxane-2-carboxylic acid (3Z,6Z)-3,6-bis[(2-arsonophenyl)hydrazinylidene]-4,5-dioxonaphthalene-2,7-disulfonic acid sodium (4Z)-4-[(1-hydroxynaphthalen-2-yl)hydrazinylidene]-7-nitro-3-oxonaphthalene-1-sulfonate magnesium sulfate heptahydrate calcium diacetate strontium dichloride hexahydrate barium(II) dichloride
0.990 0.999 0.980 0.980 0.980 0.990 0.990 0.980 0.990
compound L-proline
Triton X-100 sodium alginate arsenazo III eriochrome black T MgSO4·7H2O Ca(OAc)2 SrCl2·6H2O BaCl2
maintain the temperature at 296 ± 0.5 K. A Mettler Toledo digital balance correct up to fourth decimal place was used for measuring the weights. Construction of Phase Diagram of the Biphase and Effect of Cosolute. The phase diagram of such a system was constructed by turbidometric titration method.19−22 For construction of phase diagram of the present ABS, the initial weight of the blank centrifuge tube was recorded. Experimental sets were prepared in centrifuge tubes with different weight fractions of L-proline solution (5 mol·kg−1) and Triton-X-100 (0.90 g·g−1), keeping the final weight constant (3 g), and shaken for 5 min in BOD shaker at 296 K. The solutions were centrifuged for 5 min at 4500 rpm at the same temperature. Two distinct phases were observed, and the weight of the tube was taken. Water was added dropwise to the biphasic solution, and the same procedure was repeated until the system turned clear; that is, a single phase was formed. The final weight of the centrifuge tube was noted and amount of water added just prior to one-phase formation was calculated. Now after calculating the system composition of L-proline and Triton, their final composition was measured considering the amount of water added for different sets. Weight percentages of Triton-X-100 and L-proline for variable sets were plotted, and a binodal curve was obtained. By convention, the component predominantly in the bottom phase (L-proline for our system) was plotted in the abscissa, and the component predominantly in the top phase (Triton-X-100 for our system) was plotted as the ordinate. The experimental binodal curve was further fitted by the following equation proposed by Merchuk et al.:23
refined sunflower oil with methanol. This has a profound effect in heterogeneously catalyzed biodiesel production.15 Another important application of alkaline earth metals at nanomolar concentrations is that they act as signal enhancers in photometric and chemiluminescence of gold nanoparticles. This is mainly attributed to specific surface adsorption phenomena of divalent nature of cations.16,17 Separation of these metals from one another at trace scale is therefore important in analytical point of view. The selective distribution of alkaline earth metal ions was observed by Komatsu et al.18 Separation was achieved between carbon tetrachloride and aqueous sodium perchlorate solutions by adding chelate forming ligand (thenoyltrifluoroacetone, TTA) and adduct forming ligand (trioctylphosphine oxide, TOPO) of high concentrations (0.1 mol·kg−1) at 298 K. The separation of alkaline earth metal ions was explained on the basis of solvation of metal ions. All probable pairs of these metal ions were separated using this method. It is evident that the method involved binary separations using hazardous chemicals like volatile organic solvent and sodium perchlorate. The present article describes a much safer method of designing an aqueous biphasic system comprising of the amino acid L-proline together with a nonionic surfactant Triton X-100, the effect of the addition of a cosolute, isoamyl alcohol, on the phase-forming behavior and on the application of the resulting ABS toward mutual separation and isolation of the alkaline earths Mg, Ca, Sr, and Ba from one another at trace concentrations. Be and Ra being rare have not been used for our studies.
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EXPERIMENTAL SECTION
Y = A exp[(BX 0.5) − (CX3)]
Materials. L-Proline (Loba Chemie), Triton-X-100 (TX) (octylphenol polyethoxylene, C34H62O11) (Spectrochem), sodium alginate (SRL), arsenazo (III) (2,2′-(1,8-dihydroxy3,6-disulfonaphthylene-2,7-bisazo) bisbenzenearsonic acid) (Sigma Aldrich), Eriochrome Black T, MgSO4·7H2O, Ca(OAc)2, SrCl2·6H2O, and BaCl2 (Merck). The IUPAC names and purity of all the compounds used are listed in Table 1. Apparatus. The UV visible spectra were obtained using an Agilent 8453 diode array spectrophotometer. Absorbance values are measured up to fifth decimal place. A digital pH/ ion meter Mettler (S 220-K) was used to measure and adjust the pH of different solutions. pH values were precisely measured up to second decimal place. Centrifugation was done using Hermle microprocessor controlled universal refrigerated high speed table top centrifuge (model Z 36 K) operated on 230 V/50 Hz with an adjustable speed range of (200 to 30000) rpm. The density was measured using a Mettler Toledo portable density meter (model 30PX). Density values were measured precisely up to fourth place of decimal. A BOD incubator shaker NOVA model: SHCI 10(D) was used to
(1)
where Y and X are the mass fraction percentages of TX and Lproline, respectively, and A, B, and C are constants obtained by the regression of the experimental binodal data. The tie-lines associated to the phase diagrams were determined by a simple gravimetric method originally proposed by Merchuk et al.23 for a polymer-based ABS and later on applied by Rogers and co-workers to the ionic liquid based ABS. A ternary mixture composed of Triton-X-100 + L-proline + water at the biphasic region was gravimetrically prepared within ± 10−4 g, vigorously agitated, and left to equilibrate for 24 h at (296 ± 0.5) K, aiming at a complete separation of the coexisting phases. Both phases were then carefully separated and individually weighed. Each TL was determined by the lever-arm rule through the relationship between the top phase composition and the overall system composition and for which the following system of four equations (eqs 2 to 5) and four unknown values ([TX]TX, [TX]AA, [AA]TX, and [AA]AA was solved by using MATLAB: 1289
dx.doi.org/10.1021/je401052n | J. Chem. Eng. Data 2014, 59, 1288−1294
Journal of Chemical & Engineering Data
Article
3 [TX]TX = A exp[(B ·[AA]0.5 TX ) − (C · [AA]TX )]
(2)
3 [TX]AA = A exp[(B ·[AA]0.5 AA ) − (C · [AA]AA )]
(3)
⎛ [TX]M ⎞ 1 − α ·[TX]AA [TX]TX = ⎜ ⎟− ⎝ α ⎠ α
(4)
⎛ [AA]M ⎞ 1 − α [AA]TX = ⎜ ·[AA]AA ⎟− ⎝ α ⎠ α
(5)
conditions of this ABS. Sodium alginate, a salt of polysaccharide which naturally occurs in cell walls of brown algae, was found suitable for sequential extraction and separation of these four alkaline earth metals depending on the conditions of pH of the proline phase. The 0.01 g·g−1 sodium alginate solution was treated with 1 mM bivalent Mg, Ca, Sr, and Ba salts at varying pH conditions. A portion of 0.2 mL of the complex was added to the ABS, shaken for 10 min, and centrifuged at 4500 rpm for 5 min. The phases were separated out, adjusted to suitable pH ,and analyzed for the metal ions. The metal ions were detected using the metallochromic dyes, arsenazo III (for Ca, Sr, and Ba at pH 7.2), and EBT (for Mg at pH 9.5) spectrophotometrically after suitable calibrations both in proline and in TX−IAA media. The percentage extractions were calculated using the equation
where subscripts “TX”, “AA”, and “M” designate TX-rich phase, the amino acid-rich phase, and the mixture, respectively, [TX] and [AA] represent respectively the weight fractions of TritonX-100 and L-proline, and α is the ratio between the mass of the top phase and the total mass of the mixture. The system solution results in the composition of TX and L-proline in the top and bottom phases. Each tie-line length (TLL) and slope of the tie-lines (STL) could be obtained using the following equation:24 TLL =
2
%E =
2
[x]bottomphase
·100 (8)
where [x]top phase is the concentration of a particular component in the TX−IAA phase and [x]bottom phase is the concentration of the particular component in the L-proline phase. The extraction profiles were plotted against different pH values of the alginate−metal mixture. Determination of Binding Constants of Metal Complexes. The binding constants of the metal−alginate, metal− arsenazo III, and metal−EBT complexes were obtained from the absorbance values obtained by varying the metal ion concentration in suitable pH media using Benesi−Hildebrand (BH) equations.25
([AA]TX − [AA]AA ) + ([TX]TX − [TX]AA )
(6)
STL = ([TX]TX − [TX]AA )/([AA]AA − [AA]TX )
[x]topphase
(7)
where subscripts “TX” and “AA” symbolize, respectively, the Triton X-100 rich phase and the amino acid-rich phase, and [AA] and [TX] are the weight fraction percentages of L-proline and surfactant. The resulting TLLs and STLs are tabulated in Table 3. However, a practical problem arose at this point. After a certain lower concentration of L-proline (3 mol·kg−1) as well as at drastic conditions of pH (>8 and
