Article pubs.acs.org/jced
Activity of Water and Osmotic Coefficients for Two- and Three-Basic Amino Acid Ternary Solutions Elena N. Tsurko,*,† Roland Neueder,‡ and Werner Kunz‡ †
Research Institute of Chemistry, Karazin National University, Svoboda Sq. 4, 61022 Kharkiv, Ukraine Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany
‡
ABSTRACT: Osmotic coefficients of ternary systems amino acid−electrolyte−water; aminoethanoic acid (glycine) + NaCl + H2O; aminoethanoic acid + KCl + H2O; aminoethanoic acid + NaNO3 + H2O; aminoethanoic acid + NaSCN + H2O; aminoethanoic acid + NaCOOCH3 + H2O; (S)-2aminopentanedioic acid sodium salt (sodium L-glutamate) + NaCl + H2O; (S)-2-aminobutanedioic acid sodium salt (sodium L-aspartate) + NaCl + H2O; (S)-2-aminopentanedioic acid sodium salt + KCl + H2O; (S)-2-aminobutanedioic acid sodium salt + KCl + H2O, are inferred from vapor pressure osmometry measurements performed at T = 310.15 K.
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INTRODUCTION In recent years, several thermodynamic properties of ternary amino acid−salt−water systems have been published.1−4 In contrast to two-basic amino acids, three-basic amino acids were less investigated. Apparent molar volumes of (S)-2-aminobutanedioic acid (L-aspartic acid), (S)-2-aminopentanedioic acid (L-glutamic acid), L-lysine monohydrate, (S)-2-amino-5guanidinopentanoic acid (L-arginine) in water and aqueous solutions of sodium acetate, sodium propionate, and sodium butyrate ((0.1 to 0.5) mol/kg) at (288.15 to 318.15) K from density measurements have been evaluated by Banipal et al.5 Hydration numbers and partial molar expansibilities of amino acids in water and in the presence of electrolyte were calculated, and the interactions between amino acids and organic salts were discussed. In our previous work,6−8 binary solutions of three basic amino acids, namely, (S)-2-aminopentanedioic acid, (S)-2aminobutanedioic acid, (S)-2-amino-2(4)imidazolpropionic acid (histidine), and their salts have been investigated. In the present work, the study is extended to ternary systems containing aminoethanoic acid (Gly), (S)-2-aminobutanedioic acid anion (Asp−), and (S)-2-aminopentanedioic acid anion (Glu−) as amino acids, Na+ or K+ as cations, and acetate Ac−, Cl−, NO3−, or SCN− as anions. Osmotic coefficients are inferred from vapor pressure osmometry measurements that were performed at T = 310.15 K for the ternary systems amino acid−electrolyte−water; aminoethanoic acid (0.25 mol/kg) + NaCl ((0.006 to 1.3) mol/kg) + H2O; aminoethanoic acid (0.25 mol/kg) + KCl ((0.008 to 1.6) mol/kg) + H2O; aminoethanoic acid (0.25 mol/kg) + NaNO3 ((0.01 to 1.3) mol/kg) + H2O; aminoethanoic acid (0.25 mol/kg) + NaSCN ((0.01 to 1.3) mol/kg) + H2O; aminoethanoic acid (0.25 mol/kg) + NaCOOCH3 ((0.01 to 0.9) mol/kg) + H2O; (S)-2-aminopentanedioic acid sodium salt (0.25 mol/kg) + NaCl ((0.005 to 1.3) mol/kg) © 2012 American Chemical Society
+H2O, (S)-2-aminobutanedioic acid sodium salt (0.25 mol/kg) + NaCl ((0.005 to 1.3) mol/kg) + H2O; (S)-2-aminopentanedioic acid sodium salt (0.25 mol/kg) + KCl ((0.005 to 1.3) mol/kg) + H2O; (S)-2-aminobutanedioic acid sodium salt (0.25 mol/kg) + KCl ((0.005 to 1.3) mol/kg) + H2O. The concentration dependence of osmotic coefficients of binary solutions of aqueous (S)-2-aminopentanedioic acid sodium salt (mi = (0.005 to 0.45) mol/kg) and aqueous (S)2-aminobutanedioic acid sodium salt (mi = (0.005 to 0.4) mol/ kg) at T = 310.15 K have been determined in our previous work.6
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EXPERIMENTAL SECTION Materials. Water from the Millipore purification system with a specific conductivity of 8 × 10−8 S·cm−1 (298.15 K) was used for the preparation of solutions. The chemicals used were stored under dry nitrogen atmosphere prior to use. Their suppliers and purities are given in Table 1. Solutions were prepared by weight with the use of a Mettler AT 201 balance (max 200 g) with an uncertainty of ± 0.001 g, a Mettler AE 240 balance (max 240, max 41 g) with an uncertainty of ± 0.0001 g, or a Mettler P 1210 balance (max 1200 g) with an uncertainty of ± 0.010 g depending of the amount of solution. All solutions are prepared under pure dry nitrogen atmosphere to avoid contact with atmosphere and CO2. Special precautions are exercised to keep the composition of hydrates. To avoid the contact with moisture in the case of hygroscopic salts like NaSCN and with CO2 in the case of NaGlu and NaAc, the work was done in a glovebox in the dry nitrogen atmosphere. The measured NaGlu solutions were transparent. Received: June 26, 2012 Accepted: October 16, 2012 Published: October 26, 2012 3123
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Table 1. Chemicals Used in this Work chemical name aminoethanoic acid (glycine)
source
sodium chloride
SigmaAldrich Merck
potassium chloride
Merck
sodium nitrate
Merck
sodium thiocyanate
Fluka
sodium acetate
Merck
(S)-2-aminopentanedioic acid sodium salt (L-glutamic acid monosodium salt hydrate, sodium L-glutamate) (S)-2-aminobutanedioic acid sodium salt (L-aspartic acid sodium salt monohydrate, sodium L-aspartate)
Sigma Fluka
ϕ(m2) = 1 + preparation method dried at 80 °C dried at 120 °C dried at 120 °C dried at 80 °C dried at 120 °C dried at 80 °C culture tested
purity mass fraction
∫0
m2
md ln γ±
(3)
and
0.98
ln γ± = (ϕ − 1) −
0.995
∫0
m
(1 − ϕ) dm m
(4)
If the solvent is the only volatile component, the liquid−gas equilibrium is given by
0.995 0.995
μ10,g = μ10,L + RT ln a1
0.98
(5)
μ0,g 1
where is the chemical potential of the solvent in the gas phase, and μ0,L 1 is the chemical potential of the solvent in the liquid phase. The apparatus was calibrated using aqueous chloride solutions, yielding a function that correlates with panel readings to the corresponding concentrations of the sodium chloride solutions. Then, the measurements for different amino acid solutions were carried out. From indirect vapor pressure osmometry, the values of the osmotic coefficient ϕ of amino acid (AA) and salt (MA) solution have been obtained with the use of calibration data on NaCl solutions in a given concentration range according to
0.99 0.99 0.99
Vapor Pressure Measurements. The vapor pressure data were obtained from indirect vapor pressure osmometry with the help of a Knauer Osmomat K-7000 that permits precise data recording down to solute concentrations as low as m = 0.005 mol/kg. The vapor pressure was measured by using two thermistors to obtain voltage changes caused by a change in temperature. A comparison between results obtained with this equipment and those from direct vapor pressure lowering on the classical precise equipment9 has been done elsewhere10 and showed good agreement of both techniques. Special care was taken to keep the drop size and shape equal on both thermistors. For each solution, 9 determinations (3 determinations, zero point adjustment, and repeating) were performed, and the mean measured value was calculated. The measured values MW (the panel reading of the apparatus) are obtained from 3 × 3 measurements. MW values were from 1 to 100 units for binary solutions, from 30 to 400 for ternary solutions. Standard deviation σ total 0.1 to 1 units; relative error δ = (0.05 to 0.5) % usually, only in some cases till 1 %. The calibration of the apparatus was done with the help of NaCl solutions at 310.15 K in the NaCl concentration range of (0.009 to 2) mol/ kg.
ϕ=
ν(NaCl)m(NaCl)ϕ(NaCl) ν2m(MA) + ν3m(AA)
(6)
where m(NaCl) is the molality of a sodium chloride solution showing the same instrument reading (MW, “measured values” (English) = “Messwert” (German)) as the amino acid and salt solution; that means that the vapor pressure and therefore the solvent activity is equal in both solutions. m(MA) and m(AA) are the molalities of the salt and amino acid or amino acid salt with their stoichiometric coefficients ν2 and ν3, respectively. ϕ(NaCl) is the respective osmotic coefficient calculated with the help of the following equation set developed by F. Gibbard and G.Scatchard:11 ϕ=1−
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SZ + a
S = 1.17284 −
RESULTS Activity of Water and Osmotic Coefficient. The chemical potential of the solvent in an electrolyte solution is connected to the solvent activity via μ1(p , T ) = μ10 (p , T ) + RT ln a1
1 m2
4
∑ Djm j (7)
j=1
6202.357τ
(
TS2 1 +
τ TS
)
⎛ τ⎞ + 54.4251 ln⎜1 + ⎟ TS ⎠ ⎝
− 0.161993τ + 8.59609· 10−5(2TSτ + τ 2)
(8)
where Z = (1 + X − (1/(1 + X) − 2 ln(1 + X))/X2; τ = T − TS; X = a(m)1/2; TS = 298.15 K; a = 1.5. The coefficients Dj of the power series in m of eq 7 are given by
(1)
where μ1(p,T) is the chemical potential of the solvent in solution, μ10(p,T) is the chemical potential of the pure solvent, R is the universal gas constant, T is temperature, and a1 is the activity of the solvent in solution. The osmotic coefficient ϕ can be defined in molalilty scale as 1000 ϕ=− ln a1 νmM1 (2)
3
Dj = Dj(s) − 0.2516103 ∑ k=0
Dj(s)
Dj(k) k!
∫0
τ
tk dt (t + TS)
(9)
Dj(k)
with coefficients and based on precise measurements and given in the paper of F.Gibbard and G.Scatchard.11 The activity of water, a1, can be calculated according to
where m is the solute molality, ν the stoichiometric coefficient of the solute, and M1 the molar mass of the solvent. In a binary solution, the osmotic coefficient can be related to the mean activity coefficient γ± of the salt through
ln a1 = −
ϕmM1ν 1000
(10)
According to the law of error distribution 3124
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Sϕ =
Article
2 ⎛ ∂f ⎞2 2 ⎛ ∂f ⎞ ⎟⎟ Sϕref 2 ⎜ ⎟ Sm + ⎜⎜ ⎝ ∂m ⎠ ⎝ ∂ϕref ⎠
Table 4. Experimental Values of Osmotic Coefficients ϕ at Molalities m1 for the Systems of Sodium Chloride (1) and Potassium Chloride (1) in Solutions of (S)-2Aminobutanedioic Acid Sodium Salt (2) + Water (3) at Molality m2 = 0.2545 mol/kg and at Temperature T = 310.15 Ka
(11)
where Sm is a standard deviation of solution molality, Sϕref is a standard deviation of osmotic coefficient of NaCl solutions used as external standard. For low concentrations of m about 0.25 mol/kg; Sm about 0.001; ϕ about 1; and Sϕref about 0.001, i.e., Sϕ reaches 0.004; for high concentrations of m about 2 mol/kg; Sm about 0.001; ϕ about 0.5; Sϕref about 0.001, i.e., Sϕ reaches 0.002. That is, the discrepancy in ϕ is stipulated by the error in molality at low concentrations and by the error in standard osmotic coefficient at high concentrations. Osmotic coefficients of aqueous solutions of NaNO3, NaCH3COO, NaSCN with a background concentration of aminoethanoic acid (m = 0.2420 mol/kg), aqueous solutions of NaGlu + KCl, NaGlu + NaCl, and aqueous solutions of NaAsp + KCl and NaAsp + NaCl at T = 310.15 K are presented in Tables 2 to 4.
sodium chloride (1)+ (S)-2aminobutanedioic acid sodium salt (2)
a b
sodium acetate (1) + aminoethanoic acid (2)
(m1/m°)b
ϕ
(m1/m°)b
ϕ
0.595 0.889 1.173 1.547
0.949 0.945 0.942 0.934
0.645 1.010 1.267 1.606
0.936 0.912 0.913 0.903
a Standard uncertainties u are u(m) = 0.001 mol/kg and u(ϕ) = 0.0015. bm° = 1 mol/kg.
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DISCUSSION To illustrate the precision of the reading of the measured values (MW), Figure 1 shows the experimental results of the
Table 2. Experimental Values of Osmotic Coefficients ϕ at Molalities m1 for the Systems of Sodium Nitrate (1), Sodium Acetate (1), and Sodium Thiocyanate (1) in Solutions of Aminoethanoic Acid (2) + Water (3) at Molality m2 = 0.2420 mol/kg and at Temperature T = 310.15 Ka sodium nitrate (1) + aminoethanoic acid (2)
potassium chloride (1)+ (S)-2aminobutanedioic acid sodium salt (2)
sodium thiocyanate (1) + aminoethanoic acid (2)
(m1/m°)b
ϕ
(m1/m°)b
ϕ
(m1/m°)b
ϕ
0.108 0.369 0.527 0.960 0.989 1.743
0.946 0.882 0.858 0.846 0.844 0.820
0.106 0.264 0.286 0.375 0.569 0.786
0.981 0.968 0.974 0.974 0.982 0.987
0.116 0.381 0.448 0.716 0.972 1.023 1.718
0.964 0.913 0.913 0.916 0.921 0.935 0.950
Standard uncertainties u are u(m) = 0.001 mol/kg and u(ϕ) = 0.003. m° = 1 mol/kg.
Table 3. Experimental Values of Osmotic Coefficients ϕ at Molalities m1 for the Systems of Sodium Chloride (1) and Potassium Chloride (1) in Solutions of (S)-2Aminopentanedioic Acid Sodium Salt (2) + Water (3) at Molality m2 = 0.2499 mol/kg and at Temperature T = 310.15 Ka sodium chloride (1) + (S)-2aminopentanedioic acid sodium salt (2)
a b
potassium chloride (1) + (S)-2aminopentanedioic acid sodium salt (2)
(m1/m°)b
ϕ
(m1/m°)b
ϕ
0.011 0.112 0.197 0.391 0.695 0.893 1.358 1.656
0.950 0.955 0.941 0.927 0.922 0.925 0.924 0.931
0.010 0.113 0.486 1.152 1.414 1.720
0.970 0.941 0.893 0.889 0.893 0.893
Figure 1. (top) Osmometer MW of aqueous NaCl (▶), KCl (●), and of aqueous mixtures NaCl + aminoethanoic acid (□), KCl + aminoethanoic acid (○), T = 310.15 K. (bottom) Osmometer MW of aqueous sodium salt mixtures with aminoethanoic acid (NaCl (▶), NaNO3 + Gly (★), NaAc + Gly (▲), NaSCN + Gly (△), NaCl + Gly (□)), T = 310.15 K.
Standard uncertainties u are u(m) = 0.001 mol/kg and u(ϕ) = 0.004. m° =1 mol/kg.
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Figure 4. Osmotic coefficients of aqueous salt mixtures with background of (S)-2-aminobutanedioic acid sodium salt (m = 0.2545 mol/kg) from salt molality (NaCl, data from ref 12 (solid line); KCl, data from ref 12 (dashed line); NaCl + NaAsp (□); KCl + NaAsp (○)), T = 310.15 K.
Figure 2. Osmotic coefficients of aqueous sodium salt mixtures with background of aminoethanoic acid (m = 0.2420 mol/kg) from salt molality (NaNO3 + Gly, our data, (★); NaNO3, data from ref 12 (dashed line); NaAc + Gly, our data (▲); NaAc, data from ref 12 (solid line); NaSCN + Gly, our data (△); NaSCN, data from ref 12 (dotted line)), T = 310.15 K.
acid and NaNO3. By contrast, the osmotic coefficients of sodium acetate show slightly higher values in the presence of aminoethanoic acid compared to the pure salt solution. This is an indication of additional repulsive interactions between acetate and aminoethanoic acid. NaSCN in aminoethanoic acid solutions show the opposite trend. Obviously, SCN− ions have a tendency to slightly associate with aminoethanoic acid molecules. In Figures 3 and 4, osmotic coefficients of sodium and potassium chloride in solutions of (S)-2-aminopentanedioic acid sodium salt and (S)-2-aminobutanedioic acid sodium salt are drawn, respectively. Note that, here, the osmotic coefficients of the binary background amino acid solutions are 0.939 (0.250 mol/kg NaGlu in water) and 0.938 (0.255 mol/kg NaAsp in water).
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Figure 3. Osmotic coefficients of aqueous salt mixtures with background of (S)-2-aminopentanedioic acid sodium salt (m = 0.2499 mol/kg) from salt molality (NaCl, data from ref 12 (solid line); KCl, data from ref 12 (dashed line); NaCl + NaGlu (□); KCl + NaGlu (○)), T = 310.15 K.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful to Dr. Rainer Müller for helpful discussions. J.T. is very grateful to the German Academic Exchange Service (DAAD) for sponsoring of the Osmometer.
aminoethanoic acid containing salt solutions and the reference sodium chloride solution as a function of salt molality. With the help of the reference values, the readings can be transformed into osmotic coefficients, as explained above. Figure 2 contains the results for the measured aminoethanoic acid systems. It should be noted that the value of a pure 0.242 mol/kg aminoethanoic acid solution (the background concentration in the present ternary systems) without added salt is 0.970. As can be seen, the MW of NaCl−Gly mixtures are slightly higher than the data of pure NaCl solutions but are almost parallel. Concerning NaNO3 and the corresponding aminoethanoic acid containing solutions, it seems that the ternary system osmotic coefficients are very close to the values of the osmotic coefficients of pure NaNO3 aqueous solutions, a fact that hints at very small interactions between aminoethanoic
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REFERENCES
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