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Ind. Eng. Chem. Res. 1997, 36, 2474-2482
Partition Coefficients and Solubilities of Glycine in the Ternary Solvent System 1-Butanol + Ethanol + Water Mos van Berlo, Michael T. Gude,† Luuk A. M. van der Wielen,* and Karel Ch. A. M. Luyben Department of Biochemical Engineering, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands
For solvent extraction of biomolecules, usually optimized blends of one or more solvents and/or other compounds are required. Often the amount of experimental data and relevant thermodynamic models is very limited. In this report, we give partition coefficients and solubilities of glycine in the solvent system 1-butanol + ethanol + water at a temperature of 25 °C. Additionally, at the same temperature, liquid-liquid equilibria are given for mixtures of 1-butanol + ethanol + water + glycine as well as densities of the coexisting phases. The experimental results have been correlated to a one-parameter excess Gibbs model, using both the phase behavior of the glycine-free solvent system and the solubilities of glycine in the single solvents. Considering the simplicity of the model, its predictions were found to be in good agreement with the experimental results. 1. Introduction Solvent extraction is a popular method for the recovery of biomolecules because it concentrates the desired species as well as purifies it from salts, sugars, and other contaminants. However, the common occurrence of polar as well as apolar moieties on biomolecules usually demands optimized mixtures rather than singlecomponent solvents. An example is the use of a multicomponent extraction system for the production of carboxylic acids, such as lactic acid and citric acid. Conventional extractants like water-immiscible alcohols, ketones, or ethers are relatively inefficient for carboxylic acid recovery from the dilute aqueous acid solutions obtained in most fermentation streams. More suitable are blends of one or more solvents with either organophosphorus or alkylamine compounds. These blends are used in almost all composition ratios (Wardell and King, 1978; Kertes and King, 1986). Similar extraction systems can efficiently purify the antibiotic penicillin G (Likidis and Schu¨gerl, 1987), while amino acids can be purified using a mixture of an organic solvent and a quaternary ammonium salt (Hano et al., 1991). Liquid-liquid chromatography is another application of multicomponent solvent systems. For instance, in centrifugal partition chromatography (CPC) multicomponent solvent systems are used either in isocratic or gradient mode in order to obtain an optimal separation. Some characteristic examples of solvent systems which are used in CPC are ethyl acetate + hexane + methanol + water and diethyl ether + butanol + water (Martin, 1995; van Buel et al., 1996), which allow control over the solvation power of both phases. The bioproduct which was used in this work, the R-amino acid glycine, is a typical biomolecule which shares many similarities in its chemical and phase behavior with more complex molecules such as small peptides and amino antibiotics (Hou and Poole, 1969; McMeekin et al., 1935) despite its relative chemical * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Current address: Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. S0888-5885(96)00762-2 CCC: $14.00
simplicity. In fact, amino acids, peptides, and proteins contain a structural resemblance that sets them apart from other organic molecules. In unbuffered aqueous solutions, at their isoelectric point, they are dominantly present as zwitterions, overall neutral, but locally charged molecules which carry a dipole (Bjerrum, 1923; Cohn and Edsall, 1943; Greenstein and Winnitz, 1961). Among all amino acids glycine has the simplest structure, having just a hydrogen atom as a specific group. Early studies into the solubilities of R-amino acids in pure water and in several pure organic solvents were compiled by Cohn and Edsall (1943). Cohn et al. (1934), McMeekin et al. (1935), Dunn and Ross (1938), and Nozaki and Tanford (1971) measured the solubilities of a number of amino acids in ethanol-water mixtures at 25 °C. Gude et al. (1996b) determined the solubilities of seven R-amino acids in aqueous 1-butanol solutions. Studies on the partitioning behavior of amino acids in partially miscible solvent systems are very scarce. An early study (England and Cohn, 1935) presents the partition coefficients of amino acids in 1-butanol + water but does not include changes in phase compositions caused by the addition of an amino acid to the solvent systems nor densities of the coexisting phases. Recently, additional experimental data on the partitioning of R-amino acids in aqueous 1-butanol solutions have been published (Gude et al. 1996b). Gude and co-workers (1996a,b) developed an excess Gibbs energy model, combining a combinatorial term based on Flory-Huggins theory and a Margules residual expression to describe both solubility and partitioning of amino acids in aqueous alkanol solutions. Unlike the UNIQUAC and NRTL models, this model requires only one unique parameter for various ternary systems consisting of an alkanol, water, and an amino acid. It was speculated that even for a quaternary system consisting of 1-butanol, ethanol, water, and an amino acid only the same single parameter is required (Gude et al., 1996a). Experimental data on partitioning of several R-amino acids in aqueous 1-butanol solutions have already been correlated with the model (Gude et al., 1996b), which gave similar fits of the data as the © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2475
UNIQUAC and NRTL models. The unique parameter increases regularly with polarity in the homologous series of amino acids. In the biotechnological and pharmaceutical industry, aqueous alkanol systems are important for processes such as extraction and precipitation (Glatz, 1990; Schu¨gerl, 1993; Rothstein, 1994). The phase compositions of solute-free ternary solvent systems of 1-butanol + ethanol + water had been studied at various temperatures by Drouillon (1925). More recently, Solomko et al. (1962) and Ruiz et al. (1984) provided liquid-liquid equilibrium data at 25 °C. To extend the limited experimental data base available for process design and thermodynamic modeling, in this work partitioning coefficients and solubilities of glycine in aqueous alkanol solvent systems of 1-butanol + ethanol + water were measured at 25 °C. At the same temperature, the phase compositions and densities of the liquid phases of these mixtures were determined as well. An important aspect of this work was to test the accuracy of the model when fitted to these experimental data. 2. Materials and Methods 2.1. Chemicals. Glycine (>99% purity) was obtained from Sigma. 1-Butanol and ethanol (stated purities >99.0% and >99.8%) were purchased from Baker and Merck. Water was distilled and deionized with a Milli-Q Water System (Millipore). 2.2. Procedure. Partitioning Experiments. The partitioning experiments were performed in 80 mL glass flasks with a screw top and two side ports with septums allowing the direct sampling of both liquid phases. Each of the nine sets of experiments had a different solvent composition prepared by weighing in a large volume of the solvents which was then equilibrated. From this mixture equal volumes of both phases were added to the 80 mL flasks. Various predetermined amounts of glycine were weighed into these bottles. For each set of experiments the same amounts were used: 0, 0.6, 1.2, and 2.4 g and an amount sufficient to saturate the system with glycine (solid phase present). The bottles were then immersed in a thermostated bath and magnetically stirred for more than 24 h to ensure complete equilibration. The thermostated bath was controlled by a Julabo U3 Thermostate (Julabo Labortechnik) with a temperature stability better than (0.02 °C, which was set at 25 °C for all experiments. The bath temperature was measured with a Unisystem U1410 PT100 temperature meter (Unisystem AB) with a stated accuracy of (0.015 °C. After equilibration of the systems in the bottles, samples were taken with syringes through 0.2 µm filters (Gelman Sciences) to avoid entrainment of solids. Three sets of experiments were performed in a slightly different way because the prepared stock solutions were fully miscible. The above mentioned amounts of glycine were weighed into four bottles containing 80 mL of the stock solution. The concentrations of glycine in the aqueous phases were determined gravimetrically. Approximately 20 mL of the aqueous phase was weighed and dried in an oven, and the remaining solids were weighed again. The oven was set at a temperature of 110 °C so that no color change indicating degradation of the glycine occurred. To ensure complete removal of the solvents, the solid was redissolved in water and dried again until aconstant weight was obtained. The
weights were determined on a Mettler Toledo balance, Model AB204 (Mettler) with a resolution of 0.0001 g and repeatability found to be better than 0.0003 g. The measured amounts of glycine dissolved in the aqueous phase were reproducible within 2%. The concentration of glycine in the organic phases could not be measured gravimetrically due to the small amounts in this phase and were therefore determined by HPLC analysis. A Waters System with a Waters Nova-Pak 4 µm C18 column and a Waters 470 scanning fluorescence detector was used and controlled by a Millennium 2010 Chromatography Manager (all Waters). For detection in HPLC, glycine was derivatized by a chemical reaction with an aqueous reactant solution. Before derivatizing, a sufficient amount of water (50-250 mL) was added to the samples (∼3 mL) to form a single aqueous phase because otherwise adding the aqueous reactant solution to a sample would have caused a phase split. The analysis protocol is essentially equivalent to that presented by Cohen and Michaud (1993), and R-aminobutyric acid served as the internal standard. The concentrations of glycine in the organic phases determined with this method were reproducible within 5%. The amounts of 1-butanol and ethanol in the samples were determined by gas chromatography using a Chrompack CP 9001 (Chrompack) gas chromatograph with a HPInnowax (Hewlett Packard) column and a flame ionization detector. The internal standard applied was 3-methyl-1-butanol (isopentanol). The amounts of water were calculated by a mass balance. The accuracy of the composition measurements by gas chromatography was estimated to be 0.3%. The densities of both phases were measured using a Paar DMA 48 density meter (Anton Paar) with a resolution of 1 × 10-5 g/mL and a stated accuracy of (1 × 10-4 g/mL. Each time before doing measurements the density meter was calibrated against the known densities of air and water at 25 °C. No calculation was made with respect to a mass balance on glycine or any other component, because the phase volumes after glycine addition were unknown. Moreover, all phase compositions were determined experimentally with good accuracy. Glycine-Free Experiments. Experiments were performed in order to determine the phase behavior of the ternary solvent system 1-butanol + ethanol + water in the absence of glycine. The relevant methods and equipment used were the same as those described above, including the GC method to measure the phase compositions. Solubility Experiments. The solubility experiments of glycine in single phase solvent mixtures were performed in a similar way and with the same equipment as the experiments described above; however, the solubility experiments were performed with glass flasks without any side ports. All solutions were saturated with glycine, and the solubilities were measured using the same HPLC method as that described above. 3. Modeling In unbuffered solutions of glycine (at the isoelectric point), over 99.9% of the dissolved glycine is present as zwitterions. These overall neutral species carry a strong dipole. Consequently, the model used in this work only considers isoelectric solutions of glycine and neglects the amount of overall charged species (j N
Ajixjxi ∑ j)1
(1)
(2)
(xj′ ln xi,jsat.) ∑ j)1
Cjli
o
and xi,jsat. are the solubilities of glycine in the individual solvents. γi* denote activity coefficients of the solute defined in the unsymmetric convention (Prausnitz et al., 1986) approaching unity in the limit of infinite dilution. xiE,a/o are excess solubilities of glycine in phase a or o defined as (Orella and Kirwan, 1991):
ln xiE ≡ ln xisat. -
ri 2.671
sion containing a single adjustable parameter for each ternary system:
RT
where xj′ are the solute-free mole fractions of the solvents in the two phases:
xj′ )
glycine
)
( ) ( )| ( )| xiE,o
solute (i)
gE
N
a
Table 1. Parameters Used in Equation 4
(4)
To calculate the excess solubilities of glycine in both phases, the following excess Gibbs energy expression was used, consisting of a combinatorial term based on Flory-Huggins theory and a Margules residual expres-
(5)
where Ajl and Aji are solvent-solvent and solventsolute parameters, respectively. The phase behavior of butanol-ethanol systems is close to ideal (Gmehling and Onken, 1977), so Ajl involving 1-butanol and ethanol is assumed to be zero. Here, φk ) xkrk/r, with r ) ∑kxkrk, is the so called segment fraction which is a volume fraction variable of component k based on normalized van der Waals volumes rk, that are tabeled as UNIQUAC volume parameters (Prausnitz et al., 1986). Cjli is a ternary interaction parameter specific for an amino acid in a binary solvent. Gude et al. (1996a) found that values for Cjli involving water and an alkanol appear to be approximately independent of the alkanol involved and are specific for an amino acid. Hence, both relevant ternary interaction parameters (Cbut.,wat,i and Ceth,wat,i) are assumed to be equal. It should be noted that this approximation has not been extensively tested. Table 1 gives the volume and interaction parameters used in the present study (Gude et al., 1996a). On the basis of eq 5, an expression for the excess solubilities for glycine in both phases can be derived: N
ln xiE ) ln r′ -
∑ j)1
xj′ ln rj + ri
(
N
1
r′
N N
-
)
xj′
∑ j)1 r
j
+
∑ ∑[Ajlxj′xl′(1 + Cjli)] j)1 l>j
(6)
where r′ is the glycine-free value of r. The first three terms on the right side of eq 6 form the combinatorial part and the last term containing Ajl forms the residual part of this expression for the excess solubility. Values for Ajl are estimated from literature vapor-liquid equilibrium (VLE) data on the solute-free solvent system (Gmehling and Onken, 1977; Sørensen and Arlt, 1980) as indicated by Gude et al. (1996a). Equation 6 can be substituted into eq 4 (ln xiE,o and ln xiE,a) to give the thermodynamic partition coefficient of glycine. 4. Experimental Results 4.1. Liquid-Liquid Equilibria of 1-Butanol + Ethanol + Water. Table 2 gives the experimental results for phase compositions of equilibrated organic and aqueous phases of the ternary system 1-butanol + ethanol + water. In this table, x1, x2, and x3 are mole fractions of 1-butanol, ethanol, and water, respectively. Figure 1 shows a ternary phase diagram for the data given in Table 2 along with literature data for the same system. The data from the present study match well with data from literature (Solomko et al., 1962; Ruiz et al., 1984); moreover, it is more extensive, especially close to the plait point.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2477 Table 2. Phase Compositions of Coexisting Phases of Mixtures of 1-Butanol (1) + Ethanol (2) + Water (3) at 25 °Ca organic phase
aqueous phase
x1
x2
x3
x1
x2
x3
0.485 26 0.465 04 0.452 71 0.429 14 0.393 58 0.392 34 0.362 47 0.338 50 0.337 42 0.275 51 0.264 44 0.238 55 0.231 47 0.190 40 0.155 04 0.141 99 0.127 53 0.120 17
0 0.010 21 0.018 11 0.029 82 0.045 81 0.045 73 0.058 14 0.067 55 0.067 44 0.083 98 0.085 70 0.090 11 0.089 05 0.090 88 0.087 83 0.083 85 0.082 10 0.078 87
0.514 74 0.524 75 0.529 18 0.541 04 0.560 61 0.561 93 0.579 39 0.593 95 0.595 14 0.640 51 0.649 86 0.671 34 0.679 48 0.718 72 0.757 13 0.774 16 0.790 37 0.800 96
0.018 96 0.022 07 0.019 50 0.019 34 0.020 37 0.020 09 0.020 81 0.021 82 0.021 51 0.024 78 0.025 75 0.027 72 0.029 30 0.036 74 0.048 59 0.040 16 0.059 13 0.058 88
0 0.003 16 0.005 86 0.010 10 0.016 17 0.016 16 0.021 47 0.026 00 0.025 53 0.035 88 0.037 85 0.042 30 0.042 47 0.050 12 0.057 28 0.055 08 0.061 55 0.060 87
0.981 04 0.977 61 0.974 64 0.970 56 0.963 46 0.963 75 0.957 72 0.952 18 0.952 96 0.939 34 0.936 40 0.929 98 0.928 23 0.913 14 0.894 13 0.904 76 0.879 32 0.880 25
a The number of decimal places does not reflect the accuracy of the data; this also applies to Tables 3-5.
Figure 3. Phase diagram of the system 1-butanol + ethanol + water in the presence of various amounts of glycine. Solid tie lines connect coexisting phases of solute-free solvent systems; dotted tie lines connect coexisting phases of mixtures containing glycine.
the mole fraction of ethanol in the aqueous phase of the stock solution xeth,0a, with the subscript 0 denoting the absence of glycine. The last three datasets are indexed by xbut.,0 and xeth,0, which are the mole fractions of 1-butanol and ethanol in the (single phase) stock solutions. Table 3 shows that the solvent mixture with xbut.,0 ) 0.1059, xeth,0 ) 0.0847 already separated into two phases at the lowest glycine addition (0.6 g), while the solvent mixture with xbut.,0 ) 0.0591, xeth,0 ) 0.0790 only demixed at the highest glycine addition (saturated). The last stock solution did not demix at all, even when saturated with glycine. The molar partition coefficients Kic (molarity basis) and thermodynamic partition coefficients Ki (mole fraction basis) are given in Table 4, calculated from the experimental data with
Kic ≡ cio/cia Figure 1. Phase diagram of the system 1-butanol + ethanol + water with data from this work and from literature (sources of data: this work; Solomko et al., 1962; Ruiz et al., 1984).
Figure 2. Thermodynamic partition coefficient of glycine versus the mole fraction of glycine in the aqueous phase.
4.2. Partitioning Experiments. Table 3 gives experimentally determined compositions and densities (F) of the coexisting aqueous and organic phases. x1′, x2′, and x3′ are solute-free mole fractions of 1-butanol, ethanol, and water, defined by eq 2. The data for the experiments without ethanol originate from Gude et al. (1996b). All five compositions of a data set were prepared with the same stock solution of solvents and with an increasing amount of glycine. The stock solution of each successive dataset has an increased ethanol concentration, and every data set is characterized by
and
Ki ≡ xio/xia
(7)
where cio and cia are molarities of glycine in the two phases. The values of the partition coefficients at infinite solute dilution, also given in Table 4, are extrapolated values from the experimental data. Values for the partition coefficients obtained with the model are also included and will be discussed below (see “Correlation”). Since glycine is very hydrophilic, its concentration in the aqueous phase is much larger than that in the organic phase. The partition coefficient of glycine decreases as a function of the amount of glycine added, which is shown in Figure 2. This figure shows the thermodynamic partition coefficient as a function of the mole fraction of glycine in the aqueous phase which is almost proportional to the amount of glycine added. The decrease of the partition coefficient is related to the influence of glycine on the composition of the two phases, as will be shown here. Figures 3 and 4 display the experimental partitioning data in terms of solvent compositions as ternary phase diagrams and make it clear that glycine, despite its relatively low solubility in both phases, has a large influence on the solvent compositions. Glycine is very hydrophilic and consequently causes the removal of 1-butanol and ethanol from the aqueous phase. At the same time the organic phase is enriched in both 1-butanol and ethanol. The markers to the far left (organic phase) and far right (aqueous phase) denote solutions saturated with glycine. The solid curved lines in Figures 3 and 4 are polynomial fitted lines representing the binodal of the solute-free ternary solvent system. The solid straight lines are tie lines between coexisting
2478 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 3. Partitioning Data of Glycine (i) in 1-Butanol (1) + Ethanol (2) + Water (3) at 25 °C (Data Set with x2,0a ) 0 from Gude et al., 1996b)a organic phase x1′
x2′
xi
aqueous phase
F (kg‚m-3)
ci (mol‚L-1)
x1′
x2′
xi
F (kg‚m-3)
ci (mol‚L-1)
0 0 0 0 0 0 0 0
0 3.054 [-3] 9.203 [-3] 1.900 [-2] 1.906 [-2] 3.215 [-2] 3.649 [-2] 5.074 [-2]
986.0 991.6 1002.3 1018.8 1018.9 1039.3 1046.4 1067.5
0 0.1577 0.4741 0.9742 0.9775 1.6299 1.8427 2.5327
0.4853 0.4966 0.5114 0.5359 0.5360 0.5603 0.5664 0.5909
0 0 0 0 0 0 0 0
0 1.52 [-4] 3.62 [-4] 5.82 [-4] 5.87 [-4] 6.48 [-4] 6.69 [-4] 7.79 [-4]
844.1 843.1 841.6 839.1 839.1 836.6 836.0 833.7
x2,0a ) 0 0 0.0190 2.79 [-3] 0.0185 6.53 [-3] 0.0168 1.017 [-2] 0.0143 1.025 [-2] 0.0143 1.097 [-2] 0.0124 1.124 [-2] 0.0119 1.271 [-2] 0.0093
0.4527 0.4640 0.4778 0.4868 0.5355
0.0181 0.0194 0.0197 0.0210 0.0244
0 2.45 [-4] 4.32 [-4] 6.63 [-4] 1.041 [-3]
846.6 845.2 843.9 841.9 836.3
x2,0a ) 0.0059 0 0.0195 4.64 [-3] 0.0184 8.04 [-3] 0.0170 1.216 [-2] 0.0155 1.786 [-2] 0.0099
0.0059 0.0059 0.0056 0.0057 0.0048
0 3.855 [-3] 7.709 [-3] 1.513 [-2] 4.488 [-2]
983.6 990.6 997.5 1010.2 1056.9
0 0.1966 0.3933 0.7684 2.2343
0.3923 0.3893 0.4150 0.4357 0.4801
0.0457 0.0456 0.0491 0.0525 0.0609
0 3.81 [-4] 6.14 [-4] 8.82 [-4] 1.397 [-3]
851.5 849.6 848.1 845.6 839.8
x2,0a ) 0.0162 0 0.0201 7.86 [-3] 0.0187 1.219 [-2] 0.0176 1.697 [-2] 0.0154 2.513 [-2] 0.0105
0.0162 0.0159 0.0156 0.0146 0.0130
0 3.915 [-3] 7.781 [-3] 1.521 [-2] 4.092 [-2]
979.5 986.6 993.5 1006.3 1046.7
0 0.1958 0.3891 0.7601 2.0148
0.3374 0.3436 0.3671 0.3869 0.4104
0.0674 0.0681 0.0734 0.0775 0.0861
0 5.12 [-4] 8.86 [-4] 1.228 [-3] 1.690 [-3]
857.8 854.9 853.0 849.7 843.6
x2,0a ) 0.0255 0 0.0215 1.117 [-2] 0.0198 1.856 [-2] 0.0184 2.489 [-2] 0.0159 3.277 [-2] 0.0114
0.0255 0.0245 0.0247 0.0234 0.0207
0 3.897 [-3] 7.762 [-3] 1.528 [-2] 3.755 [-2]
975.4 982.7 989.6 1002.8 1038.4
0 0.1911 0.3810 0.7507 1.8286
0.2385 0.2687 0.2828 0.3091 0.3582
0.0901 0.0984 0.1025 0.1108 0.1308
0 6.37 [-4] 1.081 [-3] 1.562 [-3] 1.769 [-3]
871.7 867.6 864.5 859.7 851.2
x2,0a ) 0.0423 0 0.0277 1.542 [-2] 0.0245 2.541 [-2] 0.0221 3.483 [-2] 0.0186 3.600 [-2] 0.0127
0.0423 0.0412 0.0401 0.0385 0.0341
0 3.374 [-3] 6.987 [-3] 1.331 [-2] 3.295 [-2]
965.8 973.1 980.4 992.4 1025.1
0 0.1584 0.3300 0.6332 1.5710
0.1420 0.1981 0.2269 0.2558 0.3099
0.0839 0.1012 0.1110 0.1211 0.1473
0 1.121 [-3] 1.846 [-3] 2.340 [-3] 2.776 [-3]
890.0 879.9 874.1 866.8 856.5
x2,0a ) 0.0551 0 0.0402 3.081 [-2] 0.0351 4.749 [-2] 0.0258 5.652 [-2] 0.0204 6.000 [-2] 0.0134
0.0551 0.0529 0.0495 0.0469 0.0419
0 3.836 [-3] 7.381 [-3] 1.420 [-2] 3.055 [-2]
952.9 966.2 975.2 989.2 1021.1
0 0.1711 0.3388 0.6611 1.4429
0.1548 0.1797 0.2279 0.2783
0.1000 0.1102 0.1295 0.1544
1.679 [-3] 2.315 [-3] 2.816 [-3] 3.183 [-3]
908.5 887.0 875.2 861.2
x1,0 ) 0.1059; x2,0 ) 0.0847 5.152 [-2] 0.0428 0.0620 6.564 [-2] 0.0317 0.0579 7.134 [-2] 0.0223 0.0531 7.200 [-2] 0.0146 0.0472
3.646 [-3] 7.536 [-3] 1.483 [-2] 3.015 [-2]
946.2 966.1 985.9 1015.6
0.1544 0.3337 0.6783 1.4043
0.2183
0.1586
4.915 [-3]
873.0
x1,0 ) 0.0591; x2,0 ) 0.0790 1.229 [-1] 0.0187 0.0588
2.697 [-2]
1003.4
1.2205
x1,0 ) 0.0395; x2,0 ) 0.0848 fully miscible, even when saturated with glycine a Each last row of a set of data represents values for the solvent system saturated with glycine. y [-z] denotes y × 10-z; this also applies to Tables 4 and 5.
phases in the solute-free stock solutions for each set of data; the dotted lines are tie lines between coexisting phases of mixtures containing glycine which originate from single phase stock solutions. Because of the relatively low glycine solubility in both phases, the dotted tie lines nearly go through the plus signs denoting the corresponding single-phase stock solutions. The plait points for both the solute-free solvent system and the mixture saturated with glycine were estimated graphically by extrapolating the lines connecting the middle of the tie lines to the probable location of the binodals. The mole fractions of 1-butanol, ethanol, and water for the plait point of the solute-free system were estimated to be xbut. ) 0.08, xeth ) 0.07, and xwat ) 0.85, and for the system saturated with glycine the plait point corresponds approximately with xbut.′) 0.07, xeth′ ) 0.82, and xwat′ ) 0.05. It is difficult to generate data closer to the plait point than the data shown in Figures 1 and 3 because of the small density differences between the two phases.
Small fluctuations in temperature cause large changes to the phase compositions and can even cause remixing of the two-phase system. Figure 5 displays the relative changes of the mole fractions of a solvent component in both phases as a function of the relative mole fractions of glycine in the aqueous phase for one stock solution. These figures use the values from the data set characterized by xeth,0a ) 0.0551. It is obvious that the addition of glycine enlarges the two-phase area of the ternary phase diagram and consequently enriches both phases in their dominant component. This salting-out phenomenon can change solvent mole fractions as much as 70% and hence cause a decrease of the partition coefficient. Figures 6 and 7 show the densities of the organic and the aqueous phases. As a consequence of the glycine effect on the solvent compositions of both phases, the densities of the organic phases decrease due to the addition of glycine, while the densities of the aqueous phases increase.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2479 Table 4. Experimental and Model Values for Partition Coefficients of Glycine (i) in 1-Butanol (1) + Ethanol (2) + Water (3) at 25 °C (Data Set with x2,0a ) 0 from Gude et al., 1996b) x1′
organic phase xi
x1′
aqueous phase xi
expt Kic
Ki
model Ki
relative model error
0.4853 0.4966 0.5114 0.5359 0.5360 0.5603 0.5664 0.5909
0 1.52 [-4] 3.62 [-4] 5.82 [-4] 5.87 [-4] 6.48 [-4] 6.69 [-4] 7.79 [-4]
0.0190 0.0185 0.0168 0.0143 0.0143 0.0124 0.0119 0.0093
0 3.054 [-3] 9.203 [-3] 1.900 [-2] 1.906 [-2] 3.215 [-2] 3.649 [-2] 5.074 [-2]
x2,0a ) 0 2.030 [-2] 1.770 [-2] 1.376 [-2] 1.044 [-2] 1.048 [-2] 6.732 [-3] 6.102 [-3] 5.017 [-3]
5.646 [-2] 4.966 [-2] 3.930 [-2] 3.063 [-2] 3.077 [-2] 2.015 [-2] 1.833 [-2] 1.535 [-2]
3.815 [-2] 3.464 [-2] 3.026 [-2] 2.417 [-2] 2.414 [-2] 1.928 [-2] 1.821 [-2] 1.435 [-2]
0.32 0.30 0.23 0.21 0.22 0.04 0.01 0.07
0.4527 0.4640 0.4778 0.4868 0.5355
0 2.45 [-4] 4.32 [-4] 6.63 [-4] 1.041 [-3]
0.0195 0.0184 0.0170 0.0155 0.0099
0 3.855 [-3] 7.709 [-3] 1.513 [-2] 4.488 [-2]
x2,0a ) 0.0059 2.966 [-2] 2.360 [-2] 2.044 [-2] 1.583 [-2] 7.994 [-3]
7.445 [-2] 6.350 [-2] 5.609 [-2] 4.385 [-2] 2.320 [-2]
4.459 [-2] 4.001 [-2] 3.518 [-2] 3.203 [-2] 1.959 [-2]
0.40 0.37 0.33 0.27 0.16
0.3923 0.3893 0.4150 0.4357 0.4801
0 3.81 [-4] 6.14 [-4] 8.82 [-4] 1.397 [-3]
0.0201 0.0187 0.0176 0.0154 0.0105
0 3.915 [-3] 7.781 [-3] 1.521 [-2] 4.092 [-2]
x2,0a ) 0.0162 5.063 [-2] 4.014 [-2] 3.133 [-2] 2.233 [-2] 1.247 [-2]
1.179 [-1] 9.723 [-2] 7.887 [-2] 5.801 [-2] 3.415 [-2]
6.167 [-2] 6.246 [-2] 4.920 [-2] 3.966 [-2] 2.437 [-2]
0.48 0.36 0.38 0.32 0.29
0.3374 0.3436 0.3671 0.3869 0.4104
0 5.12 [-4] 8.86 [-4] 1.228 [-3] 1.690 [-3]
0.0215 0.0198 0.0184 0.0159 0.0114
0 3.897 [-3] 7.762 [-3] 1.528 [-2] 3.755 [-2]
x2,0a ) 0.0255 7.250 [-2] 5.845 [-2] 4.872 [-2] 3.315 [-2] 1.792 [-2]
1.561 [-1] 1.315 [-1] 1.141 [-1] 8.039 [-2] 4.500 [-2]
8.492 [-2] 7.930 [-2] 6.292 [-2] 5.086 [-2] 3.727 [-2]
0.46 0.40 0.45 0.37 0.17
0.2385 0.2687 0.2828 0.3091 0.3582
0 6.37 [-4] 1.081 [-3] 1.562 [-3] 1.769 [-3]
0.0277 0.0245 0.0221 0.0186 0.0127
0 3.374 [-3] 6.987 [-3] 1.331 [-2] 3.295 [-2]
x2,0a ) 0.0423 1.244 [-1] 9.735 [-2] 7.700 [-2] 5.500 [-2] 2.292 [-2]
2.304 [-1] 1.889 [-1] 1.547 [-1] 1.173 [-1] 5.370 [-2]
1.714 [-1] 1.262 [-1] 1.078 [-1] 8.029 [-2] 4.353 [-2]
0.26 0.33 0.30 0.32 0.19
0.1420 0.1981 0.2269 0.2558 0.3099
0 1.121 [-3] 1.846 [-3] 2.340 [-3] 2.776 [-3]
0.0402 0.0351 0.0258 0.0204 0.0134
0 3.836 [-3] 7.381 [-3] 1.420 [-2] 3.055 [-2]
x2,0a ) 0.0551 2.444 [-1] 1.754 [-1] 9.593 [-2] 6.807 [-2] 4.158 [-2]
3.613 [-1] 2.922 [-1] 2.501 [-1] 1.648 [-1] 9.087 [-2]
4.121 [-1] 2.373 [-1] 1.654 [-1] 1.180 [-1] 5.911 [-2]
0.14 0.19 0.34 0.28 0.35
0.1548 0.1797 0.2279 0.2783
1.679 [-3] 2.315 [-3] 2.816 [-3] 3.183 [-3]
0.0428 0.0317 0.0223 0.0146
x1.0 ) 0.1059; x2,0 ) 0.0847 3.646 [-3] 3.337 [-1] 7.536 [-3] 1.967 [-1] 1.483 [-2] 1.052 [-1] 3.015 [-2] 5.127 [-2]
4.604 [-1] 3.072 [-1] 1.898 [-1] 1.056 [-1]
3.611 [-1] 2.552 [-1] 1.434 [-1] 7.450 [-2]
0.22 0.17 0.24 0.29
0.2183
4.915 [-3]
0.0187
x1.0 ) 0.0591; x2,0 ) 0.0790 2.697 [-2] 1.007 [-1]
1.822 [-1]
1.255 [-1]
0.31
x1.0 ) 0.0395; x2,0 ) 0.0848 fully miscible, even when saturated with glycine
solubility of glycine in these mixtures and the density of the saturated solutions. The solubilities of glycine in the pure solvents and in the binary solvent mixtures were taken from literature. Calculated solubilities obtained with the model are included as well and will be discussed below (see “Correlation”). As expected, the solubility decreases with increasing mole fractions of ethanol and 1-butanol, the latter component causing the largest decrease. 5. Correlation
Figure 4. Phase behavior of the aqueous phase of the system 1-butanol + ethanol + water in the presence of various amounts of glycine.
4.3. Solubility Experiments. The solvent compositions of all single-phase mixtures of 1-butanol, ethanol, and/or water, both from previous literature and from this work, in which the solubility of glycine has been determined are given in Table 5. This table gives the
Table 5 gives solubility data for glycine in a number of fully miscible solvent mixtures of 1-butanol, ethanol, and/or water. Both experimentally determined data as well as solubility values calculated with eqs 3 and 6 are included. The relative error that is given in Table 5 is defined as the absolute difference between measured and experimental solubilities divided by the measured solubility. The difference between experimental and model solubilities is illustrated in Figure 8. This figure also includes solubility data obtained from the parti-
2480 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 5. Experimental and Model Values for Solubilities of Glycine in Fully Miscible Mixtures of 1-Butanol (1) + Ethanol (2) + Water (3) at 25 °C xisat x1′
x2′
expt
model
F (kg‚m-3)
cisat. (mol‚L-1)
sourcea
relative model error
1 1 0.9212 0.8204 0.7654 0.6886 0.6144 0.5909 0.0093 0 0 0 0 0 0 0 0 0 0 0 0.0400 0.1027 0.1033 0.1016 0.1017 0.3176 0.3199 0.3064 0.5214 0.5096 0.7100
0 0 0 0 0 0 0 0 0 0 0 1 0.7350 0.5521 0.3161 0.1704 0.0715 0.0516 0.0331 0.0160 0.0869 0.2048 0.4032 0.6067 0.7928 0.2101 0.3737 0.6019 0.2152 0.4061 0.2011
8.80 [-6] 9.51 [-6] 2.09 [-5] 6.67 [-5] 1.23 [-4] 2.73 [-4] 6.14 [-4] 7.90 [-4] 5.074 [-2] 5.667 [-2] 5.660 [-2] 2.29 [-5] 2.60 [-4] 1.09 [-3] 4.68 [-3] 1.237 [-2] 2.860 [-2] 3.464 [-2] 4.149 [-2] 4.897 [-2] 2.043 [-2] 4.96 [-3] 1.18 [-3] 2.19 [-4] 4.71 [-5] 1.39 [-3] 3.12 [-4] 3.99 [-5] 1.63 [-4] 3.15 [-5] 4.96 [-5]
8.80 [-6] 8.80 [-6] 2.35 [-5] 7.54 [-5] 1.37 [-4] 3.00 [-4] 6.09 [-4] 7.54 [-4] 5.252 [-2] 5.660 [-2] 5.660 [-2] 2.29 [-5] 2.53 [-4] 1.11 [-3] 6.42 [-3] 1.761 [-2] 3.457 [-2] 3.962 [-2] 4.499 [-2] 5.064 [-2] 2.323 [-2] 6.58 [-3] 1.53 [-3] 2.99 [-4] 5.64 [-5] 1.28 [-3] 3.12 [-4] 4.18 [-5] 1.97 [-4] 3.33 [-5] 3.03 [-5]
806.7 805.9 810.0 815.6 819.5 824.9 831.5 833.7 1067.5 1082.2 1083.1 785.1 825.4 859.8 910.7 961.1 1014.0 1030.7 1046.4 1064.6 971.6 895.2 854.4 824.1 799.8 846.3 826.2 803.3 823.6 806.2 810.4
9.59 [-5] 1.036 [-4] 2.43 [-4] 8.51 [-4] 1.66 [-3] 3.98 [-3] 9.73 [-3] 1.289 [-2] 2.533 2.887 2.886 3.9 [-4] 5.56 [-3] 2.787 [-2] 1.573 [-1] 5.077 [-1] 1.344 1.671 2.043 2.457 8.352 [-1] 1.498 [-1] 2.864 [-2] 4.42 [-3] 8.19 [-4] 2.811 [-2] 5.55 [-3] 6.15 [-4] 2.52 [-3] 4.38 [-4] 6.33 [-4]
1b 2 2 2 2 2 2 2 2 2 1b 3b 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4
0.00 0.07 0.12 0.13 0.11 0.10 0.01 0.05 0.04 0.00 0.00 0.00 0.03 0.02 0.37 0.42 0.21 0.14 0.08 0.03 0.14 0.33 0.30 0.37 0.20 0.08 0.00 0.05 0.21 0.06 0.39
a Sources: 1, McMeekin et al. (1936); 2, Gude et al. (1996b); 3, Cohn et al. (1934); 4, this work. b Pure component solubility data from these rows have been used in the correlations.
Figure 5. Influence of glycine on the mole fraction of 1-butanol, ethanol, and water in both phases (x2,0a ) 0.0551).
Figure 6. Density of the organic phase in partition measurements.
tioning experiments. Figure 8 shows that the model predicts very well the solubility of glycine in solvent systems containing relatively little water; however, Tables 4 and 5 show that it generally overestimates the solubility of glycine in solvent mixtures containing mainly water. This deviation can be primarily subscribed to the disruption of the specific water-alkanol interactions due to the addition of solute components to water-rich aqueous alkanol solutions. This unusual solubility behavior leading to a repulsion of the solute has earlier been observed and measured for various solutes including amino acids (Nozaki and Tanford, 1971; Orella and Kirwan, 1991; Gude et al., 1996b). Using eq 7, thermodynamic partition coefficients can be calculated from the experimental data presented in
Table 3. Table 4 gives these experimentally determined partition coefficients, together with thermodynamic partition coefficients calculated with the correlation based on eq 4. For these model calculations, solvent mole fractions from Table 3 were used. The relative is defined in a manner similar to that for Table 5. Figure 9 shows the comparison between the experimentally measured thermodynamic partition coefficients and the calculated values. The model seems to predict the partition coefficients of glycine in the solvent systems 1-butanol + ethanol + water quite well, although it generally underestimates the partition coefficients. This appears to be a consequence of the overestimation of the glycine concentration in the aqueous phase which has already been discussed above. Regarding the data
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2481
Figure 7. Density of the aqueous phase in partition measurements.
densities of the phases. Additionally, compositions of the solute-free solvent system were determined. The model used to correlate the experimental data, regarding the partition coefficient and solubility, was a one-parameter excess Gibbs model, primarily developed for systems of amino acids in water-alkanol systems. The model only needs solvent compositions and solubilities of glycine in the pure solvents. The partition coefficients and solubilities were predicted quite well with this model, especially considering its simplicity, also with respect to the parameter(s) needed. The universality of the model is slightly limited by a small underestimation of the partition coefficients, which is due to an overestimation of the solubility of glycine in the aqueous phase. Apparently, small amounts of alkanols change the solvent properties of a mainly aqueous solution more than calculated with this model. Acknowledgment The authors would like to thank Corrie Erkelens and Max Zomerdijk for performing the analyses. This work was financially supported by the Ministry of Economic Affairs, the Ministry of Education, Culture and Science, and the Ministry of Agriculture, Nature Management and Fishery in the framework of an industrial relevant research program of the Netherlands Association of Biotechnology Centers in the Netherlands. M.T.G. is grateful to the European Union for providing a Human Capital and Mobility postdoctoral fellowship at the TU Delft.
Figure 8. Solubilities of glycine calculated with eqs 2 and 5 versus experimental values (parity plot).
Nomenclature A ) binary interaction parameter C ) ternary interaction parameter Ci ) concentration solute (glycine, mol‚L-1) g ) molar Gibbs energy (J‚mol-1) Ki ) thermodynamic partition coefficient Kci ) molar partition coefficient N ) number of components R ) gas constant (J‚mol-1‚K-1) r ) UNIQUAC constant T ) temperature (K) x ) mole fraction Greek Symbols
Figure 9. Thermodynamic partition coefficients of glycine calculated with eq 3 versus experimental values (parity plot).
in Table 4, it is obvious that especially at low concentrations of glycine the partition coefficients are underestimated. This deviation is probably due to the fact that the model does not contain a correction for solutions of finite solute concentration other than saturated solutions. The model only contains solubilities of glycine in all pure solvents. The partition coefficient of glycine apparently depends on the glycine concentration, even if the solute-free phase compositions would remain constant when changing the glycine concentration. 6. Conclusions Partition coefficients and solubilities of glycine in the solvent system 1-butanol + ethanol + water at a temperature of 25 °C were measured. Additionally, liquid-liquid equilibria were given for mixtures of 1-butanol + ethanol + water + glycine, as well as
γ* ) activity coefficient of solute φ ) UNIQUAC constant Subscripts 0 ) in solution without solute a ) aqueous phase but. ) 1-butanol E ) excess eth ) ethanol o ) organic phase sat. ) saturated wat ) water ′ ) solute-free Superscripts i ) solute (glycine) j, l ) solvent component k ) component tot. ) total
2482 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
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Received for review December 2, 1996 Revised manuscript received February 27, 1997 Accepted March 11, 1997X IE960762W X Abstract published in Advance ACS Abstracts, May 1, 1997.
