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Surface Tension of Glycine, Alanine, Aminobutyric Acid, Norvaline, and Norleucine in Water and in Aqueous Solutions of Strong Electrolytes at Temperatures from (293.15 to 313.15) K Diana M. Rodríguez and Carmen M. Romero* Departamento de Química, Universidad Nacional de Colombia, Bogotá 111311, Colombia ABSTRACT: The surface tension of glycine, alanine, 2-aminobutyric acid (AABA), norvaline, and norleucine in water and in aqueous solutions of five strong electrolytes (LiCl, NaCl, KCl, (NH4)2SO4, and Na2SO4) was determined in the temperature range between 293.15 and 313.15 K with a LAUDA TVT2 tensiometer using the drop volume method. As the temperature rises, the surface tension decreases as the cohesion between molecules becomes weaker. The limiting slopes of the surface tension as a function of mole fraction of the amino acids in water show that glycine and alanine are hydrophilic solutes whereas α-aminobutyric acid, norvaline, and norleucine behave as hydrophobic solutes, and this behavior is not affected by the addition of the salts. The presence of electrolytes increases the surface tension of the α-amino acid solutions; however, it is not possible to classify the effect of the anions and cations on this property because the surfaces of the mixed solvents are different and a comparison between them is not pertinent.

1. INTRODUCTION Amino acids are organic mixed solutes that play an important role in several biological and industrial processes. For this reason and because they can be used as model systems to understand the behavior of proteins and their stability in water, the study of the thermodynamic properties of these solutes is very interesting and relevant.1−4 A literature survey shows that several studies have been performed on the behavior of aqueous solutions of amino acids in the presence of salts. However, information about the surface tension of amino acids in water and in electrolyte aqueous solutions is limited, especially at temperatures different from 298.15 K. Several authors have considered that the limiting experimental slopes of surface tension with respect to concentration are related to the hydrophobic or hydrophilic character of the solute because the limiting slope of the curve of surface tension depends on the type of interaction predominant on the surface.5−7 Bull and Breese, for example, suggested in their work that surface tension can be used to build a hydrophobicity scale.7 As a continuation of earlier works on the thermodynamic properties of amino acids in aqueous solution,8−15 in this article we present a study of the behavior of the surface tension (σ) of DL-2-aminoetanoic acid (glycine), DL-2-aminopropanoic acid (alanine), DL-2-aminobutanoic acid (AABA), DL-2-aminopentanoic acid (norvaline), and DL-2-aminohexanoic acid (norleucine) in water and in aqueous solutions of strong electrolytes (LiCl, NaCl, KCl, (NH4)2SO4, and Na2SO4) in the temperature range of (293.15 to 313.15) K at 75 kPa. The α-amino acids were chosen because they have a lineal hydrocarbon chain that increases its length by one methylene group © XXXX American Chemical Society

per amino acid, which enables it to associate the change in surface tension with this systematic modification in the molecular structure. The salts used are found in living organisms, so the mixed solvents could simulate the natural environment in which α-amino acids and other solutes with biological interest are present. The influence of the temperature on the behavior of the surface tension is discussed along with the effect of the salt on solute hydration.

2. MATERIALS AND METHODS 2.1. Materials. The characteristics of the reagents used are presented in Table 1. The mole fraction purity is reported Table 1. Sample Descriptions chemical name glycine DL-alanine DL-2-aminobutanoic

CAS no.

mole fraction purity

source

purification method

56-40-6 302-72-7 2835-81-6

Aldrich Sigma Aldrich

>0.98 >0.99 >0.99

none none none

760-78-1 616-06-8 7447-41-8 7647-14-5 7447-40-7 7783-20-2 7757-82-6

Sigma Sigma Sigma Merck Merck Carlo Erba Merck

>0.99 >0.99 >0.99 = 0.995 = 0.995 = 0.99 = 0.99

none none none none none none none

acid (AABA) DL-norvaline DL-norleucine

lithium chloride sodium chloride potassium chloride ammonium sulfate sodium sulfate

Received: May 14, 2017 Accepted: September 25, 2017

A

DOI: 10.1021/acs.jced.7b00433 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 2. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine in Water (1) as a Function of Mole Fraction x2 from (293.15 to 313.15) K at 75 kPaa glycine x2 × 10

alanine

σ/mN·m¯

x2 × 10

0 1.75 3.49 6.98 10.43 13.94 17.40 20.87 26.41

72.45 72.47 72.49 72.52 72.57 72.62 72.68 72.73 72.81

0 1.75 3.49 6.98 10.43 13.94 17.40 20.87 26.41

AABA ¯1

σ/mN·m

x2 × 10

0 1.76 3.53 7.05 10.55 14.06 17.52 21.05 26.42

72.50 72.52 72.53 72.55 72.58 72.59 72.62 72.64 72.68

0 1.77 3.51 7.02 10.53 14.03 17.54 21.03 26.41

71.73 71.75 71.79 71.86 71.92 71.99 72.05 72.11 72.20

0 1.76 3.53 7.05 10.55 14.06 17.52 21.05 26.42

71.73 71.83 71.86 71.90 71.95 72.00 72.07 72.10 72.18

0 1.77 3.51 7.02 10.53 14.03 17.54 21.03 26.41

0 1.75 3.49 6.98 10.43 13.94 17.40 20.87 26.41

71.00 71.04 71.12 71.23 71.33 71.43 71.54 71.64 71.76

0 1.76 3.53 7.05 10.55 14.06 17.52 21.05 26.42

71.04 71.11 71.15 71.25 71.35 71.44 71.50 71.61 71.72

0 1.77 3.51 7.02 10.53 14.03 17.54 21.03 26.41

0 1.75 3.49 6.98 10.43 13.94 17.40 20.87 26.41

70.22 70.26 70.38 70.52 70.66 70.81 70.95 71.09 71.25

0 1.76 3.53 7.05 10.55 14.06 17.52 21.05 26.42

70.30 70.40 70.42 70.57 70.70 70.83 70.94 71.09 71.28

0 1.77 3.51 7.02 10.53 14.03 17.54 21.03 26.41

0 1.75 3.49 6.98 10.43 13.94 17.40 20.87 26.41

69.45 69.49 69.63 69.79 69.95 70.11 70.27 70.43 70.61

0 1.76 3.53 7.05 10.55 14.06 17.52 21.05 26.42

69.42 69.48 69.59 69.73 69.87 70.02 70.20 70.36 70.57

0 1.77 3.51 7.02 10.53 14.03 17.54 21.03 26.41

4

1

4

norvaline ¯1

σ/mN·m

4

x2 × 10

4

norleucine ¯1

σ mN·m

x2 × 10

4

σ mN·m¯1

293.15 K 72.50 72.49 72.49 72.48 72.47 72.46 72.45 72.44 72.43

0 1.73 3.47 5.61 10.40 13.86 17.33 20.79 26.41

72.58 72.55 72.51 72.43 72.37 72.31 72.25 72.16 72.06

0 1.56 3.11 5.24 6.99 8.73 10.48 12.22 14.11

72.50 72.40 72.21 72.02 71.85 71.71 71.48 71.28 71.09

71.73 71.80 71.79 71.77 71.75 71.74 71.72 71.71 71.69

0 1.73 3.47 5.61 10.40 13.86 17.33 20.79 26.41

71.73 71.84 71.80 71.71 71.63 71.53 71.47 71.41 71.25

0 1.56 3.11 5.24 6.99 8.73 10.48 12.22 14.11

71.73 71.55 71.28 71.14 71.00 70.82 70.59 70.42 70.18

71.10 71.08 71.07 71.05 71.02 71.00 70.98 70.96 70.93

0 1.73 3.47 5.61 10.40 13.86 17.33 20.79 26.41

71.21 71.16 71.13 71.01 70.91 70.79 70.71 70.65 70.50

0 1.56 3.11 5.24 6.99 8.73 10.48 12.22 14.11

71.00 70.74 70.54 70.43 70.22 70.06 69.79 69.65 69.44

70.27 70.25 70.24 70.21 70.19 70.15 70.13 70.09 70.07

0 1.73 3.47 5.61 10.40 13.86 17.33 20.79 26.41

70.40 70.35 70.31 70.20 70.10 70.02 69.85 69.78 69.63

0 1.56 3.11 5.24 6.99 8.73 10.48 12.22 14.11

70.23 69.90 69.80 69.62 69.44 69.23 69.00 68.80 68.59

69.50 69.47 69.45 69.42 69.39 69.35 69.31 69.28 69.22

0 1.73 3.47 5.61 10.40 13.86 17.33 20.79 26.41

69.46 69.39 69.34 69.22 69.07 69.00 68.86 68.75 68.58

0 1.56 3.11 5.24 6.99 8.73 10.48 12.22 14.11

69.42 69.20 69.02 68.91 68.68 68.47 68.18 68.02 67.87

298.15 K

303.15 K

308.15 K

313.15 K

Standard uncertainties are u(P) = 1 kPa, u(T) = 0.01 K, and u(x2 × 104) = 0.02. The expanded uncertainty for σ is max U(σ) = 0.12 mN·m−1 (0.95 level of confidence). a

according to the certificates of analysis. Amino acids and salts were used without further purification. They were dried under vacuum at room temperature and kept in a desiccator for at least 48 h prior to use. Water was purified using a Barnstead Easy-Rodi DI 3321 system and degassed before use; the resulting water had a conductivity of less than 1.5 μS·m−1.16,17

All solutions were prepared by weight using a dual-range Mettler AT-261 balance with a readability of 1 × 10−5 g and a reproducibility of better than 1 × 10−4 g in the lower range. The concentration of the electrolyte aqueous solutions used as mixed solvents was the same; the selected mole fraction for the salts was 0.0027, equivalent to 0.1500 mol·kg−1, which is high enough to B



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that the hydrophilic nature of the solutes changes with concentration. Because there is no evidence of such change, we suggest that there is a problem with the measurement method used by the author. Surface tension is very sensitive to the influence of temperature. As the temperature increases, the surface tension of the solutes decreases for all five amino acids. This result was expected because a rise in temperature leads to an increase in kinetic energy, lowing the intermolecular cohesion and therefore the surface tension of the solution.26 The sign and magnitude of the limiting slopes (∂σ/∂x2lim) of surface tension with respect to concentration are related to the hydrophobic or hydrophilic character of the solute as it reflects the type of interaction predominant on the surface.5,6 The limiting slopes were determined using the data corresponding to the very dilute region and are presented in Table 3.

cause a change in the amino acid−water interaction without masking it, as found in previous work for α-amino acids in aqueous NaCl.18 To compare the effect of each salt on the surface tension, it was necessary to have a common concentration, so we decided to use x3 = 0.0027 in all systems; some preliminary tests showed that this was appropriate for the aims of this study. 2.2. Methods. The surface tension of the solutions was measured at intervals of 5 K at temperatures of between (293.15 and 313.15) K using a LAUDA TVT2 tensiometer with temperature control better than ±0.01 K. The tensiometer, which uses the drop volume method, was checked with pure water at 297.15 K as recommended by the manufacturer19 and at the five working temperatures by comparison with the literature.16,20 To calculate the surface tension, the LAUDA TVT2 tensiometer requires the density of the solutions. This property is reported in our previous work,21 and it was determined by means of an Anton Paar DSA 5000M vibrating U-tube densimeter with a working frequency of 3 MHz and temperature control of better than ±0.001 K. The densimeter was calibrated with dry air and purified water at 293.15 K. Each reported density data point is an average of three independent measurements that were reproducible to within 5 × 10−3 kg·m−3 with an uncertainty of 0.150 kg·m−3. The volume of the syringe used for the measurements was 1.00 cm3, and the inner radius of the capillary was 1.380 mm. Each reported values is the average of at least nine measurements with a reproducibility better than 0.05 mN m−1, and the standard uncertainty of the surface tension u(σ) is the standard deviation of the measurements.22

Table 3. Limiting Slopes (∂σ/∂x2lim) of the Surface Tension of the Aqueous Solutions of α-Amino Acids as a Function of Temperature from T = (293.15 to 313.15) K at 75 kPaa ∂σ/∂x2lim/mN m−1 T/K

glycine

alanine

AABA

norvaline

norleucine

293.15 298.15 303.15 308.15 313.15

140.26 182.41 292.49 401.85 452.97

64.22 142.60 250.40 365.08 441.01

−26.09 −44.72 −61.84 −77.26 −100.79

−192.79 −230.81 −267.90 −294.64 −326.35

−1031.99 −1040.38 −1034.37 −1075.71 −1105.58

a

Standard uncertainties are u(P) = 1 kPa, u(T) = 0.01 K, and u(∂σ/∂x2lim) = 0.70 mN m−1 (max).

3. RESULTS AND DISCUSSION The experimental data for the surface tension of the aqueous solutions of all five α-amino acids at T = (293.15 to 313.15) K is presented in Table 2. Figure 1 shows the behavior of the surface

The positive values of ∂σ/∂x2lim for glycine and alanine are typical for electrolytes and very polar hydrophilic compounds,27 and they can be explained as a result of the favorable interaction between their zwitterionic group and water. On the other hand, AABA, norvaline, and norleucine have longer hydrocarbon chains; as a result of their hydrophobic nature they go easily to the liquid−air interface, where their adsorption is favorable; and as their concentration increases, the surface tension of the solution decreases. According to this fact, it is possible to use the slope of the surface tension as a function of mole fraction to classify glycine and alanine as hydrophilic solutes and AABA, norvaline, and norleucine as hydrophobic solutes. Values of ∂σ/∂x2 can be used to calculate the surface excess Γ12. However, activity coefficients have not been reported for amino acids in dilute aqueous solutions, so it is not possible to determine the surface activity based on the Gibbs adsorption theory.28 The surface tension of the solutions of α-amino acids in the mixed solvents is presented in Tables 4−8. Figure 2 shows that in the presence of the salts the behavior of the amino acids in aqueous solution is similar to that observed in water; the surface tension of the solutions of glycine and alanine increases as their mole fraction increases, whereas for AABA, norvaline, and norleucine, as the amino acid concentration becomes larger, the surface tension decreases. This implies that the hydrophilic or hydrophobic nature of the solutes is not changed by the presence of the cosolutes. The behavior of the surface tension of the solutions is the same at all considered temperatures. Previous works1,21,29−33 have suggested that the cations and anions of salts interact directly with the zwitterionic group of

Figure 1. Surface tension σ of ◆, glycine; □, alanine; ▲, AABA; ■, norvaline; and ●, norleucine in water as a function of the amino acid mole fraction x2 at 298.15 K and 75 kPa.

tension of the amino acids in aqueous solution as a function of mole fraction at 298.15 K. As can be seen, the surface tension of the aqueous solutions of glycine and alanine increases as the concentration of the amino acids increases, and the surface tension of the solutions of AABA, norvaline, and norleucine decreases as their concentration increases. The same behavior is observed at the other temperatures and is in good agreement with the trend reported by Matubayasi23 and Pappenheimer.24 However, these results contradict some of the data of Chandra25 for glycine and alanine, according to which, after a certain concentration, the surface tension of aqueous solutions of these amino acids decreases as the concentration increases, suggesting C



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Table 4. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine as a Function of Mole Fraction x2 in an Aqueous Solution of LiCl (x3 = 0.0027 ± 0.0003) from (293.15 to 313.15) K at 75 kPaa glycine

alanine

AABA

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

x2 × 104

0 1.79 3.58 7.17 10.77 14.39 18.00 21.63 27.65

72.82 72.84 72.85 72.89 72.92 72.96 72.99 73.03 73.08

0 1.79 3.58 7.01 10.76 14.36 17.98 21.61 27.37

72.82 72.82 72.85 72.86 72.89 72.91 72.93 72.95 72.98

0 1.78 3.56 7.14 10.48 14.33 17.95 21.54 27.38

0 1.79 3.58 7.17 10.77 14.39 18.00 21.63 27.65

72.11 72.12 72.13 72.17 72.20 72.24 72.27 72.30 72.36

0 1.79 3.58 7.01 10.76 14.36 17.98 21.61 27.37

72.10 72.10 72.11 72.12 72.13 72.15 72.17 72.21 72.24

0 1.78 3.56 7.14 10.48 14.33 17.95 21.54 27.38

0 1.79 3.58 7.17 10.77 14.39 18.00 21.63 27.65

71.35 71.36 71.37 71.41 71.44 71.47 71.50 71.54 71.59

0 1.79 3.58 7.01 10.76 14.36 17.98 21.61 27.37

71.37 71.37 71.37 71.39 71.41 71.43 71.44 71.46 71.50

0 1.78 3.56 7.14 10.48 14.33 17.95 21.54 27.38

0 1.79 3.58 7.17 10.77 14.39 18.00 21.63 27.65

70.56 70.58 70.59 70.62 70.65 70.68 70.72 70.75 70.80

0 1.79 3.58 7.01 10.76 14.36 17.98 21.61 27.37

70.60 70.60 70.60 70.62 70.64 70.66 70.68 70.70 70.73

0 1.78 3.56 7.14 10.48 14.33 17.95 21.54 27.38

0 1.79 3.58 7.17 10.77 14.39 18.00 21.63 27.65

69.73 69.75 69.76 69.79 69.82 69.85 69.88 69.91 69.96

0 1.79 3.58 7.01 10.76 14.36 17.98 21.61 27.37

69.77 69.77 69.77 69.79 69.81 69.83 69.84 69.86 69.90

0 1.78 3.56 7.14 10.48 14.33 17.95 21.54 27.38

norvaline

σ/mN·m¯1

norleucine

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

72.80 72.79 72.78 72.77 72.76 72.74 72.73 72.72 72.70

0 1.76 3.52 7.06 10.61 14.17 17.74 21.34 27.37

72.79 72.82 72.80 72.78 72.75 72.70 72.62 72.54 72.38

0 0.89 1.77 3.55 5.34 7.12 8.91 10.70 12.79

72.80 72.84 72.75 72.51 72.32 72.18 71.93 71.63 71.24

72.11 72.10 72.09 72.08 72.07 72.06 72.04 72.02 71.99

0 1.76 3.52 7.06 10.61 14.17 17.74 21.34 27.37

72.11 72.10 72.09 72.06 72.00 71.95 71.88 71.81 71.64

0 0.89 1.77 3.55 5.34 7.12 8.91 10.70 12.79

72.08 72.12 71.98 71.78 71.61 71.40 71.13 70.84 70.64

71.39 71.38 71.37 71.35 71.34 71.33 71.31 71.28 71.25

0 1.76 3.52 7.06 10.61 14.17 17.74 21.34 27.37

71.35 71.36 71.35 71.32 71.21 71.11 71.02 70.85 70.61

0 0.89 1.77 3.55 5.34 7.12 8.91 10.70 12.79

71.42 71.38 71.25 71.04 70.86 70.63 70.33 70.06 69.75

70.56 70.54 70.53 70.51 70.49 70.47 70.44 70.40 70.36

0 1.76 3.52 7.06 10.61 14.17 17.74 21.34 27.37

70.58 70.60 70.58 70.55 70.52 70.46 70.37 70.23 70.05

0 0.89 1.77 3.55 5.34 7.12 8.91 10.70 12.79

70.56 70.57 70.42 70.19 70.00 69.79 69.51 69.33 68.99

69.77 69.75 69.75 69.70 69.68 69.66 69.61 69.55 69.50

0 1.76 3.52 7.06 10.61 14.17 17.74 21.34 27.37

69.75 69.77 69.74 69.71 69.69 69.64 69.57 69.45 69.26

0 0.89 1.77 3.55 5.34 7.12 8.91 10.70 12.79

69.75 69.80 69.66 69.42 69.28 69.02 68.77 68.47 68.23

293.15 K

298.15 K

303.15 K

308.15 K

313.15 K


α-amino acids, removing water molecules from the hydration sphere of those solutes; this dehydration ability of the ions is in good agreement with the Hofmeister series,34 which is an

empirical organization of the effect of anions and cations on the stability of proteins. Even though it is clear that in the presence of salts the surface tension of the solutions is higher D



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Table 5. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine as a Function of Mole Fraction x2 in an Aqueous Solution of NaCl (x3 = 0.0027 ± 0.0001) from (293.15 to 313.15) K at 75 kPaa glycine

alanine

AABA

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

x2 × 104

0 1.80 3.59 7.20 10.81 14.41 18.05 21.68 27.72

72.81 72.82 72.84 72.87 72.91 72.94 72.98 73.01 73.06

0 1.79 3.58 7.18 10.79 14.42 17.43 21.61 27.44

72.81 72.81 72.82 72.84 72.86 72.88 72.90 72.91 72.94

0 1.79 3.58 7.17 10.77 14.39 17.62 21.65 27.44

0 1.80 3.59 7.20 10.81 14.41 18.05 21.68 27.72

72.07 72.08 72.10 72.13 72.16 72.20 72.23 72.26 72.31

0 1.79 3.58 7.18 10.79 14.42 17.43 21.61 27.44

72.07 72.07 72.08 72.10 72.11 72.13 72.15 72.17 72.20

0 1.79 3.58 7.17 10.77 14.39 17.62 21.65 27.44

0 1.80 3.59 7.20 10.81 14.41 18.05 21.68 27.72

71.59 71.60 71.61 71.64 71.68 71.71 71.74 71.77 71.82

0 1.79 3.58 7.18 10.79 14.42 17.43 21.61 27.44

71.59 71.59 71.60 71.60 71.62 71.64 71.66 71.68 71.72

0 1.79 3.58 7.17 10.77 14.39 17.62 21.65 27.44

0 1.80 3.59 7.20 10.81 14.41 18.05 21.68 27.72

70.79 70.80 70.81 70.84 70.87 70.91 70.94 70.97 71.01

0 1.79 3.58 7.18 10.79 14.42 17.43 21.61 27.44

70.79 70.79 70.80 70.80 70.82 70.84 70.85 70.88 70.91

0 1.79 3.58 7.17 10.77 14.39 17.62 21.65 27.44

0 1.80 3.59 7.20 10.81 14.41 18.05 21.68 27.72

69.86 69.87 69.92 69.97 70.02 70.07 70.12 70.13 70.20

0 1.79 3.58 7.18 10.79 14.42 17.43 21.61 27.44

69.86 69.86 69.86 69.87 69.89 69.91 69.92 69.94 69.97

0 1.79 3.58 7.17 10.77 14.39 17.62 21.65 27.44

norvaline

σ/mN·m¯1

norleucine

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

72.81 72.80 72.79 72.77 72.75 72.73 72.70 72.69 72.65

0 1.75 3.50 7.02 10.55 14.09 17.57 21.20 27.30

72.81 72.80 72.78 72.74 72.69 72.62 72.54 72.41 72.25

0 0.89 1.78 3.56 5.34 7.13 8.92 11.12 12.82

72.81 72.68 72.58 72.38 72.22 71.87 71.63 71.24 70.96

72.07 72.06 72.05 72.03 72.00 71.98 71.96 71.93 71.88

0 1.75 3.50 7.02 10.55 14.09 17.57 21.20 27.30

72.07 72.04 72.01 71.99 71.90 71.84 71.75 71.68 71.52

0 0.89 1.78 3.56 5.34 7.13 8.92 11.12 12.82

72.07 71.93 71.80 71.60 71.37 71.07 70.83 70.54 70.17

71.59 71.57 71.56 71.54 71.52 71.49 71.47 71.44 71.39

0 1.75 3.50 7.02 10.55 14.09 17.57 21.20 27.30

71.59 71.54 71.48 71.39 71.33 71.21 71.09 70.93 70.77

0 0.89 1.78 3.56 5.34 7.13 8.92 11.12 12.82

71.59 71.33 71.19 70.86 70.57 70.33 69.99 69.61 69.31

70.79 70.78 70.75 70.72 70.70 70.67 70.65 70.59 70.54

0 1.75 3.50 7.02 10.55 14.09 17.57 21.20 27.30

70.79 70.72 70.70 70.61 70.51 70.40 70.28 70.11 69.91

0 0.89 1.78 3.56 5.34 7.13 8.92 11.12 12.82

70.79 70.56 70.42 70.05 69.82 69.55 69.30 68.91 68.63

69.86 69.86 69.85 69.82 69.80 69.76 69.73 69.69 69.62

0 1.75 3.50 7.02 10.55 14.09 17.57 21.20 27.30

69.86 69.81 69.79 69.74 69.66 69.59 69.49 69.33 69.14

0 0.89 1.78 3.56 5.34 7.13 8.92 11.12 12.82

69.86 69.64 69.55 69.27 69.02 68.74 68.48 68.09 67.82

293.15 K

298.15 K

303.15 K

308.15 K

313.15 K

Standard uncertainties are u(P) = 1 kPa, u(T) = 0.01 K, and u(x2 × 104) = 0.03. Expanded uncertainty for σ is max U(σ) = 0.08 mN·m−1 (0.95 level of confidence). a

that exerts the larger increase in the surface tension of glycine solutions is (NH4)2SO4; for alanine, AABA, and norvaline it is LiCl, and for norleucine it is Na2SO4. The difference in the

than in pure water (which was expected because electrolytes increase the surface tension of water27,35), this increase changes depending on the amino acid: for example, the salt E



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Table 6. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine as a Function of Mole Fraction x2 in an Aqueous Solution of KCl (x3 = 0.0027 ± 0.0003) from (293.15 to 313.15) K at 75 kPaa glycine x2 × 10

alanine ¯1

AABA ¯1

σ/mN·m

x2 × 10

σ/mN·m

0 1.80 3.61 7.22 10.84 14.47 18.11 21.73 27.78

72.63 72.64 72.66 72.69 72.73 72.76 72.80 72.83 72.89

0 1.79 3.59 7.19 10.75 14.42 18.05 21.62 27.49

72.68 72.68 72.68 72.70 72.72 72.74 72.76 72.77 72.80

0 1.80 3.60 7.21 10.83 14.45 18.10 21.71 26.94

0 1.80 3.61 7.22 10.84 14.47 18.11 21.73 27.78

71.96 71.97 71.99 72.02 72.06 72.09 72.12 72.16 72.21

0 1.79 3.59 7.19 10.75 14.42 18.05 21.62 27.49

71.87 71.87 71.88 71.89 71.91 71.93 71.95 71.97 72.01

0 1.80 3.60 7.21 10.83 14.45 18.10 21.71 26.94

0 1.80 3.61 7.22 10.84 14.47 18.11 21.73 27.78

71.23 71.24 71.25 71.28 71.32 71.35 71.38 71.41 71.47

0 1.79 3.59 7.19 10.75 14.42 18.05 21.62 27.49

71.15 71.15 71.16 71.16 71.18 71.20 71.22 71.24 71.27

0 1.80 3.60 7.21 10.83 14.45 18.10 21.71 26.94

0 1.80 3.61 7.22 10.84 14.47 18.11 21.73 27.78

70.47 70.48 70.49 70.52 70.56 70.59 70.62 70.65 70.70

0 1.79 3.59 7.19 10.75 14.42 18.05 21.62 27.49

70.45 70.45 70.46 70.47 70.48 70.50 70.52 70.54 70.57

0 1.80 3.60 7.21 10.83 14.45 18.10 21.71 26.94

0 1.80 3.61 7.22 10.84 14.47 18.11 21.73 27.78

69.70 69.71 69.72 69.75 69.78 69.81 69.84 69.87 69.93

0 1.79 3.59 7.19 10.75 14.42 18.05 21.62 27.49

69.66 69.66 69.67 69.67 69.69 69.71 69.73 69.74 69.78

0 1.80 3.60 7.21 10.83 14.45 18.10 21.71 26.94

4

4

x2 × 10

norvaline

σ/mN·m

4

¯1

x2 × 10

4

norleucine

σ/mN·m

¯1

x2 × 10

σ/mN·m¯1

4

293.15 K 72.69 72.68 72.66 72.64 72.61 72.61 72.56 72.55 72.53

0 1.77 3.55 7.11 10.69 14.28 17.87 21.50 27.49

72.65 72.67 72.67 72.63 72.59 72.53 72.46 72.36 72.19

0 0.90 1.78 3.57 5.36 7.15 8.94 10.76 12.85

72.71 72.71 72.58 72.40 72.14 71.97 71.68 71.54 71.24

71.95 71.93 71.92 71.89 71.86 71.84 71.81 71.79 71.74

0 1.77 3.55 7.11 10.69 14.28 17.87 21.50 27.49

71.87 71.88 71.88 71.84 71.79 71.74 71.68 71.59 71.44

0 0.90 1.78 3.57 5.36 7.15 8.94 10.76 12.85

72.00 72.00 71.86 71.64 71.45 71.27 71.02 70.80 70.48

71.24 71.22 71.21 71.18 71.16 71.13 71.11 71.08 71.03

0 1.77 3.55 7.11 10.69 14.28 17.87 21.50 27.49

71.17 71.18 71.18 71.16 71.11 71.05 70.95 70.85 70.68

0 0.90 1.78 3.57 5.36 7.15 8.94 10.76 12.85

71.22 71.23 71.11 70.88 70.65 70.47 70.21 69.99 69.67

70.49 70.47 70.47 70.45 70.42 70.40 70.38 70.34 70.29

0 1.77 3.55 7.11 10.69 14.28 17.87 21.50 27.49

70.50 70.52 70.51 70.47 70.40 70.31 70.20 70.11 69.89

0 0.90 1.78 3.57 5.36 7.15 8.94 10.76 12.85

70.45 70.46 70.38 70.11 69.92 69.72 69.44 69.31 68.94

69.76 69.74 69.73 69.70 69.67 69.64 69.61 69.57 69.49

0 1.77 3.55 7.11 10.69 14.28 17.87 21.50 27.49

69.78 69.79 69.78 69.72 69.63 69.56 69.45 69.33 69.14

0 0.90 1.78 3.57 5.36 7.15 8.94 10.76 12.85

69.80 69.67 69.63 69.32 69.15 68.91 68.61 68.44 68.10

298.15 K

303.15 K

308.15 K

313.15 K


results can be explained by the fact that the surfaces are different because even though the concentration of the electrolytes is the same (x3= 0.0027) the number of molecules of each salt in the surface is completely different as it depends on the

shape, the size and the interaction of each cation and anion with water and with the amino acids.36,37 Therefore, it is not possible to compare the surface behavior of the amino acids among solvents because there are different numbers of ions F



Article

Table 7. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine as a Function of Mole Fraction x2 in an Aqueous solution of (NH4)2SO4 (x3 = 0.0027 ± 0.0002) from (293.15 to 313.15) K at 75 kPaa glycine

alanine

AABA

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

x2 × 104

0 1.82 3.63 7.28 10.93 14.58 18.26 21.93 28.02

72.95 72.96 72.97 73.01 73.04 73.07 73.11 73.14 73.19

0 1.81 3.63 7.12 10.72 14.57 18.23 21.91 27.74

72.94 72.94 72.95 72.96 72.98 73.00 73.02 73.03 73.05

0 1.81 3.62 7.25 10.90 14.55 18.22 21.90 27.74

0 1.82 3.63 7.28 10.93 14.58 18.26 21.93 28.02

71.98 71.99 72.00 72.04 72.07 72.10 72.14 72.17 72.21

0 1.81 3.63 7.12 10.72 14.57 18.23 21.91 27.74

72.00 72.00 72.01 72.02 72.03 72.05 72.07 72.09 72.11

0 1.81 3.62 7.25 10.90 14.55 18.22 21.90 27.74

0 1.82 3.63 7.28 10.93 14.58 18.26 21.93 28.02

71.53 71.54 71.55 71.59 71.62 71.65 71.68 71.71 71.76

0 1.81 3.63 7.12 10.72 14.57 18.23 21.91 27.74

71.51 71.51 71.52 71.53 71.54 71.56 71.58 71.60 71.62

0 1.81 3.62 7.25 10.90 14.55 18.22 21.90 27.74

0 1.82 3.63 7.28 10.93 14.58 18.26 21.93 28.02

70.67 70.68 70.69 70.72 70.75 70.78 70.82 70.85 70.89

0 1.81 3.63 7.12 10.72 14.57 18.23 21.91 27.74

70.66 70.66 70.67 70.67 70.69 70.71 70.72 70.74 70.78

0 1.81 3.62 7.25 10.90 14.55 18.22 21.90 27.74

0 1.82 3.63 7.28 10.93 14.58 18.26 21.93 28.02

69.90 69.91 69.92 69.95 69.98 70.01 70.04 70.07 70.12

0 1.81 3.63 7.12 10.72 14.57 18.23 21.91 27.74

69.91 69.90 69.91 69.92 69.93 69.95 69.97 69.98 70.02

0 1.81 3.62 7.25 10.90 14.55 18.22 21.90 27.74

norvaline

σ/mN·m¯1

norleucine

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

72.92 72.90 72.89 72.86 72.84 72.82 72.79 72.77 72.75

0 1.80 3.61 7.23 10.87 14.53 18.19 21.88 28.02

72.92 72.95 72.90 72.82 72.73 72.63 72.55 72.45 72.30

0 0.91 1.82 3.65 5.48 7.31 9.14 10.99 13.10

72.92 72.86 72.75 72.61 72.38 72.13 71.89 71.60 71.21

71.97 71.95 71.93 71.91 71.88 71.86 71.84 71.81 71.76

0 1.80 3.61 7.23 10.87 14.53 18.19 21.88 28.02

72.13 72.13 72.12 72.10 72.03 71.95 71.87 71.80 71.63

0 0.91 1.82 3.65 5.48 7.31 9.14 10.99 13.10

71.97 71.93 71.92 71.81 71.59 71.40 71.10 70.82 70.59

71.53 71.52 71.51 71.50 71.47 71.46 71.44 71.41 71.36

0 1.80 3.61 7.23 10.87 14.53 18.19 21.88 28.02

71.46 71.47 71.44 71.44 71.37 71.30 71.22 71.12 70.96

0 0.91 1.82 3.65 5.48 7.31 9.14 10.99 13.10

71.50 71.39 71.29 71.03 70.83 70.62 70.38 69.99 69.71

70.68 70.66 70.65 70.62 70.60 70.57 70.55 70.51 70.45

0 1.80 3.61 7.23 10.87 14.53 18.19 21.88 28.02

70.69 70.72 70.70 70.67 70.62 70.57 70.49 70.38 70.20

0 0.91 1.82 3.65 5.48 7.31 9.14 10.99 13.10

70.66 70.61 70.49 70.25 70.05 69.82 69.56 69.31 69.04

69.92 69.91 69.91 69.90 69.88 69.85 69.83 69.78 69.72

0 1.80 3.61 7.23 10.87 14.53 18.19 21.88 28.02

70.07 70.09 70.04 69.97 69.90 69.84 69.74 69.64 69.45

0 0.91 1.82 3.65 5.48 7.31 9.14 10.99 13.10

69.91 69.84 69.76 69.53 69.32 69.08 68.81 68.56 68.18

293.15 K

298.15 K

303.15 K

308.15 K

313.15 K


4. CONCLUSIONS The surface tension of glycine, alanine, AABA, norvaline, and norleucine in water and in aqueous solutions of five strong

on each surface, which completely changes the interactions of the solutes, meaning that there is no benchmark between solvents. G



Article

Table 8. Surface Tension σ of Glycine, Alanine, AABA, Norvaline, and Norleucine as a Function of Mole Fraction x2 in an aqueous solution of Na2SO4 (x3 = 0.0027 ± 0.0001) from (293.15 to 313.15) K at 75 kPaa glycine

alanine

AABA

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

x2 × 104

0 1.82 3.64 7.29 10.86 14.62 18.29 21.98 28.06

72.92 72.94 72.95 72.98 73.01 73.05 73.08 73.11 73.16

0 1.82 3.63 7.28 10.93 14.60 18.27 21.95 27.77

72.90 72.91 72.92 72.94 72.96 72.98 73.00 73.01 73.03

0 1.81 3.62 7.26 10.91 14.56 18.22 21.91 27.78

0 1.82 3.64 7.29 10.86 14.62 18.29 21.98 28.06

72.05 72.06 72.07 72.10 72.13 72.17 72.20 72.23 72.27

0 1.82 3.63 7.28 10.93 14.60 18.27 21.95 27.77

72.04 72.04 72.05 72.06 72.08 72.10 72.11 72.13 72.15

0 1.81 3.62 7.26 10.91 14.56 18.22 21.91 27.78

0 1.82 3.64 7.29 10.86 14.62 18.29 21.98 28.06

71.50 71.51 71.52 71.55 71.58 71.61 71.65 71.68 71.72

0 1.82 3.63 7.28 10.93 14.60 18.27 21.95 27.77

71.47 71.47 71.48 71.48 71.50 71.52 71.53 71.55 71.58

0 1.81 3.62 7.26 10.91 14.56 18.22 21.91 27.78

0 1.82 3.64 7.29 10.86 14.62 18.29 21.98 28.06

70.69 70.70 70.71 70.74 70.77 70.80 70.83 70.86 70.90

0 1.82 3.63 7.28 10.93 14.60 18.27 21.95 27.77

70.65 70.65 70.65 70.66 70.68 70.69 70.71 70.72 70.75

0 1.81 3.62 7.26 10.91 14.56 18.22 21.91 27.78

0 1.82 3.64 7.29 10.86 14.62 18.29 21.98 28.06

69.85 69.86 69.87 69.90 69.93 69.96 69.99 70.02 70.06

0 1.82 3.63 7.28 10.93 14.60 18.27 21.95 27.77

69.88 69.87 69.88 69.89 69.90 69.92 69.93 69.95 69.98

0 1.81 3.62 7.26 10.91 14.56 18.22 21.91 27.78

norvaline

σ/mN·m¯1

norleucine

x2 × 104

σ/mN·m¯1

x2 × 104

σ/mN·m¯1

73.03 73.00 72.99 72.96 72.92 72.90 72.87 72.84 72.78

0 1.81 3.62 7.26 10.90 14.56 18.24 21.93 28.06

72.99 72.92 72.89 72.81 72.74 72.66 72.59 72.43 72.22

0 0.91 1.82 3.64 5.47 7.30 9.14 10.95 13.11

73.00 72.90 72.84 72.67 72.53 72.31 72.04 71.61 71.32

71.99 71.97 71.96 71.94 71.91 71.88 71.86 71.82 71.77

0 1.81 3.62 7.26 10.90 14.56 18.24 21.93 28.06

72.07 72.05 72.04 72.00 71.93 71.85 71.79 71.68 71.52

0 0.91 1.82 3.64 5.47 7.30 9.14 10.95 13.11

72.01 72.11 72.05 71.98 71.74 71.46 71.21 70.89 70.49

71.53 71.51 71.48 71.44 71.40 71.37 71.34 71.30 71.24

0 1.81 3.62 7.26 10.90 14.56 18.24 21.93 28.06

71.59 71.54 71.47 71.36 71.26 71.16 71.06 70.89 70.69

0 0.91 1.82 3.64 5.47 7.30 9.14 10.95 13.11

71.56 71.58 71.38 71.22 70.90 70.69 70.42 70.15 69.76

70.76 70.73 70.70 70.67 70.62 70.59 70.56 70.51 70.43

0 1.81 3.62 7.26 10.90 14.56 18.24 21.93 28.06

70.77 70.74 70.68 70.57 70.51 70.43 70.28 70.11 69.90

0 0.91 1.82 3.64 5.47 7.30 9.14 10.95 13.11

70.72 70.79 70.67 70.45 70.23 69.99 69.64 69.34 69.06

69.94 69.91 69.88 69.84 69.80 69.77 69.73 69.66 69.59

0 1.81 3.62 7.26 10.90 14.56 18.24 21.93 28.06

69.97 69.95 69.90 69.80 69.72 69.61 69.52 69.32 69.13

0 0.91 1.82 3.64 5.47 7.30 9.14 10.95 13.11

69.94 69.98 69.89 69.66 69.40 69.16 68.82 68.55 68.24

293.15 K

298.15 K

303.15 K

308.15 K

313.15 K


interaction with water; in this case, glycine and alanine exhibit positive slopes corresponding to hydrophilic solutes whereas AABA, norvaline, and norleucine have negative slopes characteristic

electrolytes was determined in the range of (293.15 to 313.15) K. The sign of the limiting slopes of surface tension as a function of concentration can be used to classify solutes by their predominant H



Article

Figure 2. Surface tension σ of solutions of ◆, glycine; □, alanine; ▲, AABA; ■, norvaline; and ●, norleucine as a function of the amino acid mole fraction x2 in the mixed solvents at 298.15 K and 75 kPa.

■

of hydrophobic solutes. The addition of salts leads to an increase in the surface tension of the solutions, and it does not affect the hydrophilic or hydrophobic character of the solutes. It is not possible to produce a scale of the effect of each ion as a result of the differences in the surfaces of each mixed solvent.

■

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carmen M. Romero: 0000-0003-0356-3872 Notes

The authors declare no competing financial interest. Funding

This work was supported by the Universidad Nacional de Colombia and the Instituto Colombiano para el Desarrollo de la Ciencia y la Tecnologia,́ Francisco José de Caldas, COLCIENCIAS, through the Young Researchers National Program and the Universidad Nacional de Colombia (grants DIB: 23605 and 746-2015 COLCIENCIAS). I



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