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Jul 19, 2016 - (6-14) One specific salt of interest is sodium benzoate, a bacteriostat and preservative often used in acidic foods such as salad dress...
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Density and Viscosity Study of Interactions of Some Amino Acids in Aqueous Solutions of Sodium Benzoate Abdolhossein Haghani,† Hossein Iloukhani,† and Markus M. Hoffmann*,‡ †

Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamadan 65178-38695, Iran The College at Brockport, State University of New York, Brockport, New York 14420, United States

‡

S Supporting Information *

ABSTRACT: The density (ρ) and viscosity (η) of three amino acids, glycine, L-alanine, and L-valine, have been determined as a function of amino acid concentration in aqueous solutions of 0.1011, 0.3088, and 0.5245 mol·kg−1 sodium benzoate at temperatures 303.15, 308.15, and 313.15 K. Apparent molar volumes (Vϕ), limiting values of apparent molar volumes (V0ϕ), and transfer volumes (ΔtV0ϕ) have been calculated from the density data. The viscosity data could be fit to the Jones−Dole equation, and B coefficients and variation of B with temperature dB/dT were obtained at different concentrations and temperatures. Free energies of activation of the solvent (Δμ10≠) and solute (Δμ20≠) were also calculated by application of the Eyring transition-state theory. The obtained thermophysical data have been interpreted in terms of the structure of the amino acids and their interactions with the sodium benzoate solution.

1. INTRODUCTION The thermodynamic properties of proteins in aqueous solutions play an important role in the stability, dynamics, structural characteristics, and functional activity of these biomolecules.1−4 The study of proteins is difficult because they are large complex molecules. Therefore, amino acids, which are the fundamental substances for proteins and peptides, have been quite useful as models for understanding the thermodynamic behavior and the state of solvation of peptides and proteins in solution.3,4 A small change in the solution composition may result in a significant change in interactions such as hydrogen bonding with water or ionic interactions with dissolved salts. The addition of some substances such as salts may also change the conformational stability and folding behavior of proteins.5 Consequently, many studies on the thermodynamic properties and transport behavior of amino acids in aqueous electrolyte solutions and drug solutions have been reported.6−14 One specific salt of interest is sodium benzoate, a bacteriostat and preservative often used in acidic foods such as salad dressings (vinegar), carbonated drinks (carbonic acid), jams and fruit juices (citric acid), pickles (vinegar), and condiments. It is also used as a preservative in medicines and cosmetics.15,16 However, negative side effects such as allergic responses from excessive use of sodium benzoate have been cited,17 and there is active research going on to understand the human immune response to sodium benzoate and other food preservatives.18 Therefore, there is a need to understand the effects of sodium benzoate on biochemical solutions, and our main purpose of this work is to study the interactional behavior of amino acids with aqueous © XXXX American Chemical Society

sodium benzoate solutions. Although some studies on the thermodynamic properties of aqueous sodium benzoate solution have been reported,19−23 we are not aware of any study investigating volumetric and viscosity properties of amino acids in aqueous sodium benzoate solutions, even though composition and temperature dependent thermophysical properties can lead to helpful insights of the present interactions.24,25 Here, the density and viscosity of glycine, Lalanine, and L-valine in aqueous solutions at fixed sodium benzoate concentrations of 0.1011, 0.3088, and 0.5245 mol· kg−1 at 303.15, 308.15, and 313.15 K were measured. (These sodium benzoate molalities correspond to 0.1000, 0.3000, and 0.5000 molar solutions at 298.15 K.) The temperature conditions were chosen to include a range of about 5 K above and below the body temperature. From the density data, apparent molar volumes (Vϕ,2), limiting apparent molar volumes (V0ϕ), and transfer volume at infinite dilution (ΔtV0ϕ) of amino acids in aqueous solutions of sodium benzoate were computed. The viscosity data were used to calculate B coefficients and free energy of activation per mole of the solvent and solute. The results are interpreted in terms of interactions between amino acids and aqueous sodium benzoate solutions. Received: December 3, 2015 Accepted: July 8, 2016

A

DOI: 10.1021/acs.jced.5b01031 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Specification of Chemicals

All solutes were obtained from Sigma, United States, and used as received; i.e., initial and final purity are the same. bDensities of solids were as provided by vendor. Density of pure water at 298.15 K and 0.1013 MPa from ref 26. a

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The details of the chemicals used in the present work are given in Table 1. The amino acids were stored under vacuum in desiccators over silica gel as a drying agent for a minimum of 48 h before use. Each mixture was prepared by weighing and was immediately used after it was mixed by shaking and brief exposure to an ultrasonic bath. All of the weight measurements were performed on a Mettler electronic digital balance (model AB 204-N) with an uncertainty of 1 × 10−5 g. The standard uncertainties in the reported amino acid and sodium benzoate molalities are estimated to be 2 × 10−4 mol·kg−1. 2.2. Density and Viscosity Measurements. The densities of pure compounds and mixtures were measured by means of an oscillating U-tube density meter with automatic viscosity correction (model DMA 4500, Anton Parr, Austria). The standard uncertainty of the density measurements is estimated to be 1.5 × 10−4 g·cm−3. All measurements were made at a constant temperature. The temperature in the cell was regulated to ±0.01 K by a solid-state thermostat (Peltier) and two integrated Pt 100 platinum thermometers. The apparatus was calibrated once a day with dry air and doubly distilled, freshly degassed water. Viscosities were measured with an Ubbelohde viscometer of 0.6 mm capillary size which was fixed in a water bath, and the temperature was controlled by a thermostat with a precision of 0.01 K. The equation for viscosity according to Poiseuille’s law is ⎛ k⎞ η = ρυ = ρ⎜ct − ⎟ ⎝ t⎠

obtained by measurements of doubly distilled water at 298.15 and 308.15 K. The obtained parameters are k = 1.53447 ± 0.00020 m2·s−2 and c = 0.00534 ± 0.00020 m2. From error propagation, the standard uncertainty of the viscosity measurements is estimated to be 0.04 mPa·s.

3. RESULTS 3.1. Volumetric Properties. The measured densities of the aqueous solutions of glycine, L-alanine, and L-valine with and without varying amounts of sodium benzoate are reported in Table 2. The entries for zero mol·kg−1 of amino acid in absence of sodium benzoate are measurements of pure water, which agree with accepted density data for pure water (0.99565, 0.99403, and 0.99222 g·cm−3 at 303.15, 308.15, and 313.15 K, respectively).26 Measured densities of aqueous solutions of amino acid without sodium benzoate have been reported in the literature before2,4,5,27,28 and are compared with the corresponding densities from this study in Figure 1. Error bars in Figure 1 are omitted as these would be smaller than the size of the symbols. As can be seen in Figure 1, densities from this study are in agreement with the reported literature values,2,4,5,27,28 and deviations are very small (about less than 0.1%) except for the density data reported by Banipal et al. that deviate by as much as about 1%.28 The apparent molar volume of the solutes (V2,ϕ) were computed from experimental density data using eq 2 V2, ϕ =

1000(ρ − ρ0 ) M − ρ mρρ0

(2) −1

where m is the molality of the solution (mol·kg ), M is the molar mass of the amino acid, and ρ0 and ρ are the densities of the solvent (aqueous solution of sodium benzoate) and solution containing amino acid, respectively. The numerical values for apparent molar volumes and their standard uncertainties obtained by means of error propagation are listed in Table S1. All calculated values of V2,ϕ for amino acids in

(1)

where c and k are the viscometer constants and t, η, ρ, and υ are the efflux time, dynamic viscosity, density, and kinematic viscosity, respectively. The efflux time measurements were made with a digital chronometer (Kenko KK-5898) with a standard uncertainty of 0.01 s. The c and k parameters were B

DOI: 10.1021/acs.jced.5b01031 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 2. Experimental Density of Amino Acids in Aqueous Solutions with Varying Amounts of Present Sodium Benzoate at Ambient Pressurea glycine −1

L-alanine

m (mol·kg )

303.15K

308.15K

313.15K

0 0.1001 0.1498 0.1998 0.2497 0.2999 0.3502 0.4005 0.4500 0.4993

0.99564 0.99879 1.00034 1.00188 1.00342 1.00495 1.00644 1.00794 1.00944 1.01087

0.99402 0.99712 0.99865 1.00016 1.00168 1.00319 1.00466 1.00615 1.00762 1.00905

0.99216 0.99514 0.99661 0.99806 0.99950 1.00094 1.00234 1.00374 1.00512 1.00646

0 0.1002 0.1500 0.1997 0.2498 0.3001 0.3494 0.3994 0.4490 0.4976

1.00130 1.00444 1.00598 1.00750 1.00902 1.01053 1.01199 1.01347 1.01492 1.01632

0.99962 1.00275 1.00428 1.00579 1.00730 1.00879 1.01025 1.01170 1.01314 1.01453

0.99751 1.00076 1.00233 1.00387 1.00537 1.00687 1.00829 1.00971 1.01111 1.01243

0 0.1000 0.1502 0.1998 0.2503 0.3003 0.3489 0.3998 0.4495 0.4992

1.01249 1.01561 1.01714 1.01864 1.02015 1.02162 1.02303 1.02450 1.02591 1.02730

1.01060 1.01375 1.01529 1.01679 1.01830 1.01977 1.02118 1.02264 1.02404 1.02542

1.00853 1.01171 1.01326 1.01477 1.01627 1.01773 1.01913 1.02056 1.02193 1.02327

0 0.1003 0.1502 0.1997 0.2500 0.3003 0.3496 0.3996 0.4496 0.4987

1.02351 1.02664 1.02816 1.02965 1.03114 1.03261 1.03403 1.03545 1.03685 1.03820

1.02149 1.02464 1.02616 1.02765 1.02913 1.03059 1.03199 1.03339 1.03476 1.03609

1.01994 1.02318 1.02475 1.02628 1.02781 1.02931 1.03076 1.03221 1.03364 1.03501

−1

m (mol·kg )

303.15K

L-valine

308.15K

313.15K

−1

m (mol·kg )

303.15K

308.15K

313.15K

0 0.1004 0.1489 0.1996 0.2497 0.3001 0.3488 0.3995 0.4501 0.4989

0.99563 0.99827 0.99957 1.00084 1.00210 1.00333 1.00455 1.00574 1.00693 1.00809

0.99402 0.99663 0.99790 0.99916 1.00041 1.00163 1.00284 1.00402 1.00520 1.00634

0.99216 0.99492 0.99625 0.99755 0.99883 1.00005 1.00127 1.00246 1.00361 1.00475

0 0.1000 0.1497 0.1997 0.2499 0.2994 0.3496 0.3989 0.4496 0.4990

1.00130 1.00390 1.00517 1.00643 1.00767 1.00889 1.01010 1.01128 1.01247 1.01362

0.99934 1.00211 1.00344 1.00474 1.00599 1.00721 1.00841 1.00956 1.01071 1.01180

0.99741 1.00024 1.00158 1.00287 1.00411 1.00531 1.00646 1.00759 1.00867 1.00969

0 0.1000 0.1498 0.1995 0.2500 0.3001 0.3494 0.3998 0.4497 0.4994

1.01246 1.01508 1.01633 1.01754 1.01873 1.01988 1.02098 1.02205 1.02311 1.02412

1.01054 1.01325 1.01454 1.01578 1.01699 1.01816 1.01928 1.02038 1.02143 1.02242

1.00827 1.01118 1.01256 1.01388 1.01517 1.01641 1.01759 1.01872 1.01982 1.02089

0 0.1001 0.1497 0.1999 0.2497 0.3001 0.3500 0.3999 0.4494 0.4995

1.02359 1.02609 1.02729 1.02847 1.02962 1.03075 1.03185 1.03292 1.03396 1.03497

1.02161 1.02417 1.02538 1.02656 1.02770 1.02881 1.02987 1.03091 1.03191 1.03288

1.01994 1.02271 1.02401 1.02527 1.02648 1.02764 1.02876 1.02985 1.03087 1.03185

0 0.99563 0.99401 0.99216 0.1001 0.99846 0.99683 0.99500 0.1501 0.99985 0.99821 0.99639 0.1998 1.00123 0.99958 0.99776 0.2502 1.00260 1.00094 0.99911 0.3000 1.00396 1.00229 1.00045 0.3501 1.00532 1.00364 1.00177 0.3986 1.00662 1.00493 1.00307 0.4493 1.00797 1.00627 1.00436 0.4994 1.00928 1.00756 1.00562 0.1011 mol·kg−1 aqueous sodium benzoate 0 1.00131 0.99949 0.99751 0.1002 1.00414 1.00238 1.00044 0.1499 1.00552 1.00378 1.00185 0.1998 1.00689 1.00516 1.00324 0.2497 1.00825 1.00652 1.00460 0.2997 1.00959 1.00787 1.00594 0.3501 1.01093 1.00919 1.00726 0.3985 1.01220 1.01045 1.00852 0.4493 1.01351 1.01175 1.00979 0.4987 1.01478 1.01299 1.01099 0.3088 mol·kg−1 aqueous sodium benzoate 0 1.01249 1.01059 1.00816 0.1002 1.01530 1.01344 1.01123 0.1498 1.01666 1.01481 1.01268 0.1992 1.01800 1.01615 1.01408 0.2500 1.01935 1.01751 1.01546 0.2998 1.02066 1.01881 1.01677 0.3497 1.02195 1.02009 1.01804 0.3992 1.02322 1.02134 1.01927 0.4494 1.02448 1.02258 1.02049 0.4991 1.02572 1.02379 1.02163 0.5245 mol·kg−1 aqueous sodium benzoate 0 1.02361 1.02152 1.01994 0.1001 1.02635 1.02434 1.02289 0.1500 1.02768 1.02570 1.02429 0.1995 1.02898 1.02701 1.02563 0.2496 1.03028 1.02832 1.02695 0.3000 1.03156 1.02960 1.02823 0.3495 1.03280 1.03084 1.02945 0.3998 1.03404 1.03206 1.03065 0.4500 1.03526 1.03326 1.03182 0.4986 1.03643 1.03439 1.03292

Experimental density: ρ, g·cm−3. Ambient pressure: p = 0.0815 ± 0.0005 MPa. All density measurements have an estimated standard uncertainty of 1.5 × 10−4 g·cm−3. An additional decimal place for the densities is shown to allow for calculation of thermophysical parameters derived from densities with less rounding error. Standard uncertainties for temperature and molalities, m, are 0.01 K and 2 × 10−4 mol·kg−1, respectively. a

dependence of V2,ϕ on the amino acid concentration is also steeper when sodium benzoate is present. The observed linear dependence of the apparent molar volumes with respect to amino acid solution molality is well understood in terms of the Masson equation29

different molal concentrations of sodium benzoate are positive and are in the sequence L-valine > L-alanine > glycine. The values of V2,ϕ versus molal concentration of amino acids m are plotted in Figure 2 to facilitate the inspection of the effects of different molalities of aqueous sodium benzoate solutions and changes in temperature. As can be seen from the graphs in Figure 2, V2,ϕ generally increases linearly with m. At 303.15 K, the V2,ϕ values are affected very little by the concentration of sodium benzoate, while at 313.15 K, the added sodium benzoate causes a significant reduction of the V2,ϕ values over the studied range of amino acid concentrations. The

V2, ϕ = V ϕ0 + S Vm

(3)

V0ϕ

where is the limiting value of apparent molal volumes and SV is the experimental slope obtained from linear fitting of V2,ϕ as a function of molality, as shown in Figure 2. It follows from eq 3 that the limiting value of the apparent molar volumes is C

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sodium benzoate, it is consistent to observe in Table S2 that V0ϕ, representing the y-intercept of the Masson equation, generally decreases as the temperature and sodium benzoate concentration increase except, again, for the case of L-valine in 0.5245 mol·kg−1 of sodium benzoate. The observation that V0ϕ generally decreases with both the concentration of sodium benzoate as well as temperature is opposite to what was observed for the same amino acid solutions with added sodium fluoride.35 The difference in V0ϕ in solutions with and without added sodium benzoate is referred to as the transfer volumes (ΔtV0ϕ), as shown by eq 4. Δt V ϕ0 = V ϕ0 (inaqueous sodium benzoate) − V ϕ0 (in water) (4)

The ΔtV ϕ values are included in Table S2 and are predominantly negative for all amino acids at different concentrations of aqueous sodium benzoate solutions. 3.2. Viscosity and Derived Properties. The viscosity data for glycine, L-alanine, and L-valine with sodium benzoate as a function of amino acid concentration and temperature are reported in Table 3. Generally, the viscosities follow expected trends as they increase with increasing amino acid concentration as well as increasing sodium benzoate concentration and decrease with increasing temperature. Jones and Dole introduced in 1929 an empirical expression for the relative fluidity for the concentration dependence of the viscosity of electrolyte solutions36 that, over time, was recast in terms of relative viscosity, as shown in eq 5.37 0

ηr =

η = 1 + AC1/2 + BC η0

(5)

In eq 5, ηr, η, and η0 are the relative viscosity, the viscosity of the solution, and the viscosity of the solvent, respectively, and A and B are coefficients that depend on solute, solvent, and temperature. Even though amino acids are not electrolytes but rather neutral molecules, the Jones−Dole equation has nevertheless also been applied to these nonelectrolyte solutions.1,4−6 Indeed, when the density data in Table 2 are used to convert molality into molarity and the viscosity data fit to eq 5 rearranged to

Figure 1. Comparison of aqueous solution densities at 308.15 K for (a) glycine (○, this work; ▽, ref 2; +, ref 5; ×, ref 27; △, ref 28); (b) L-alanine (○, this work; ▽, ref 2; +, ref 28; ×, ref 53); and L-valine (○, this work; ▽, ref 2; +, ref 5; ×, ref 28).

equal to the infinite dilution apparent molar volume. The SV and V0ϕ values and their standard uncertainties are reported in Table S2 and include V0ϕ values for the aqueous amino acid solutions without added sodium benzoate, which are compared in Table S3 with other prior studies at a temperature of 308.15 K, for which many values are reported. The V0ϕ values reported in this study are in agreement with the literature values in Table S3 within about 2% and mostly within 1%.2,5,11,30−34 It is interesting to observe from Figure 2 and Table S2 that SV generally increases as temperature and the sodium benzoate concentration increase. The only significant exception is observed for L-valine, where the values for SV with 0.5245 mol·kg−1 of sodium benzoate present are smaller than with 0.3088 mol·kg−1 sodium benzoate present, possibly indicating that SV undergoes a maximum with sodium benzoate concentration, at least for the lower temperatures where this trend is more pronounced. The limiting values of the apparent molar volumes (at infinite dilution, V0ϕ) in Table S2 are all positive values in the order of L-valine > L-alanine > glycine. Because SV, representing the slope of the Masson equation, is observed to generally increase with temperature and added

ηr − 1 C

=A+B C

(6)

graphs, as shown in Figure S1, are obtained that fit very well with this linear relation in C1/2. The coefficients A and B are summarized in Table 4 along with some fit statistics. A perusal of Table 4 shows that the magnitudes of the A coefficient values are generally small. The B coefficients in Table 4 follow in magnitude the trend glycine < L-alanine < L-valine but do not show any distinct dependence on the sodium benzoate concentration. The same order of B coefficients of the three amino acids has also been observed for aqueous solutions with other solutes.4,5,38,6,8,35,39 Furthermore, although the studied temperature range of 10 K is rather limited and prohibits too strong of statements about the temperature dependences of the B coefficients, these generally increase with increasing temperature for L-valine and L-alanine (Table 4). Finally, the combined results for the apparent molar volumes and B coefficients allow the evaluation of thermodynamic activation parameters from an extension of the Eyring D

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Figure 2. Apparent molar volumes (V2,ϕ) as a function of amino acid concentration in aqueous solutions of fixed sodium benzoate (●, 0.0 mol·kg−1; □, 0.1011 mol·kg−1; Δ, 0.3088 mol·kg−1; ◊, 0.5245 mol·kg−1). (a) Glycine at T = 303.15 K. (b) Glycine at T = 313.15 K. (c) L-Alanine at T = 303.15 K. (d) L-Alanine at T = 313.15 K. (e) L-Valine at T = 303.15 K. (f) Valine at T = 313.15 K. The different effect of fixed sodium benzoate concentrations on the slopes of the graphs at 303.15 and 313.15 K is evident for all three amino acid solutions.

⎛ η V̅ 0 ⎞ 1 Δμ10 ≠ = RT ln⎜⎜ 0 ⎟⎟ hN ⎝ ⎠

transition-state theory.40 In this theory, the B coefficient is related to the activation free energy by eq 7: B=

(V1̅ 0

V2̅ 0)

− 1000

⎛ V̅ 0 ⎞ Δμ 0 ≠ − Δμ 0 ≠ 1 +⎜ 1 ⎟ 2 RT ⎝ 1000 ⎠

(8)

and the free energy of activation per mole of solute Δμ20≠ can be obtained by rearrangement of eq 7 as follows:

(7)

where V̅ 01 (∑xiMi/ρ0) is the mean volume of the solvent (sodium benzoate solutions) while V̅ 02 (V0ϕ) is the partial molar volume of the solute (amino acid only) at infinite dilution. The xi and Mi values define the mole fraction and molar weight of water and sodium benzoate, respectively, and ρ0 is the density of the solvent. Δμ10≠ and Δμ20≠ are the free energy of activation for the viscous flow per mole of the solvent and the free energy of activation per mole of solute, respectively. The factor 1000 in the denominator of eq 7 accounts for entering the partial molar volumes in units of 10−6 m3·mol−1 to obtain B in units of 10−3 m3·mol−1.37 The free energy of activation per mole of solvent Δμ10≠ is given by

Δμ20 ≠ = Δμ10 ≠ +

RT [1000B − (V1̅ 0 − V2̅ 0)] V1̅ 0

(9)

Table S4 presents the obtained thermodynamic activation parameters and their standard uncertainties obtained by means of error propagation. It is evident from Table S4 that the Δμ10≠ and Δμ20≠ values are all positive, and the Δμ20≠ values are consistently greater than Δμ10≠ values. This has been observed in other studies of amino acids with an added component.41 A perusal of Table S4 also indicates that the Δμ20≠ values for all amino acids in different concentrations of sodium benzoate solutions are in the sequence L-valine > L-alanine > glycine, which is in the same order as that of the B coefficient. This is not surprising, as the Δμ20≠ values are directly obtained from E

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Table 3. Experimental Viscosity of Amino Acids in Aqueous Solutions with Varying Amounts of Present Sodium Benzoate at Ambient Pressurea glycine −1

m (mol·kg )

303.15K

L-alanine

308.15K

313.15K

0 0.1002 0.1500 0.1997 0.2498 0.3001 0.3494 0.3994 0.4490 0.4976

0.845 0.859 0.866 0.872 0.879 0.886 0.893 0.899 0.906 0.913

0.758 0.776 0.783 0.790 0.796 0.803 0.809 0.816 0.823 0.829

0.691 0.710 0.716 0.723 0.729 0.736 0.742 0.749 0.755 0.762

0 0.1000 0.1502 0.1998 0.2503 0.3003 0.3489 0.3998 0.4495 0.4992

0.931 0.940 0.947 0.953 0.960 0.967 0.974 0.981 0.988 0.995

0.837 0.845 0.852 0.859 0.865 0.872 0.879 0.886 0.892 0.899

0.753 0.767 0.773 0.780 0.787 0.793 0.800 0.807 0.813 0.820

0 0.1003 0.1502 0.1997 0.2500 0.3003 0.3496 0.3996 0.4496 0.4987

1.019 1.033 1.040 1.047 1.054 1.061 1.068 1.075 1.082 1.089

0.912 0.926 0.933 0.940 0.947 0.954 0.961 0.968 0.974 0.981

0.817 0.831 0.838 0.845 0.851 0.858 0.865 0.872 0.879 0.885

−1

m (mol·kg )

303.15K

L-valine

308.15K

0.1011 mol·kg−1 aqueous 0 0.840 0.1002 0.859 0.1499 0.871 0.1998 0.883 0.2497 0.895 0.2997 0.907 0.3501 0.919 0.3985 0.931 0.4493 0.943 0.4987 0.955 0.3088 mol·kg−1 aqueous 0 0.920 0.1002 0.945 0.1498 0.957 0.1992 0.969 0.2500 0.982 0.2998 0.994 0.3497 1.006 0.3992 1.018 0.4494 1.031 0.4991 1.043 0.5245 mol·kg−1 aqueous 0 1.024 0.1001 1.049 0.1500 1.062 0.1995 1.074 0.2496 1.086 0.3000 1.099 0.3495 1.111 0.3998 1.124 0.4500 1.136 0.4986 1.149

313.15K

sodium benzoate 0.758 0.686 0.776 0.709 0.788 0.721 0.800 0.733 0.812 0.745 0.824 0.757 0.836 0.769 0.848 0.781 0.860 0.793 0.872 0.805 sodium benzoate 0.821 0.748 0.845 0.772 0.857 0.784 0.869 0.796 0.881 0.808 0.893 0.820 0.905 0.832 0.917 0.844 0.930 0.856 0.942 0.868 sodium benzoate 0.918 0.806 0.937 0.836 0.949 0.848 0.961 0.860 0.974 0.873 0.986 0.885 0.998 0.897 1.011 0.909 1.023 0.921 1.035 0.933

−1

m (mol·kg )

303.15K

308.15K

313.15K

0 0.1000 0.1497 0.1997 0.2499 0.2994 0.3496 0.3989 0.4496 0.4990

0.840 0.875 0.892 0.910 0.927 0.944 0.962 0.979 0.997 1.014

0.757 0.787 0.804 0.821 0.839 0.856 0.873 0.891 0.908 0.925

0.686 0.715 0.732 0.749 0.766 0.784 0.801 0.818 0.835 0.853

0 0.1000 0.1498 0.1995 0.2500 0.3001 0.3494 0.3998 0.4497 0.4994

0.926 0.961 0.979 0.996 1.014 1.032 1.049 1.067 1.084 1.102

0.837 0.872 0.890 0.907 0.925 0.942 0.960 0.977 0.995 1.012

0.748 0.783 0.800 0.818 0.835 0.853 0.870 0.888 0.905 0.923

0 0.1001 0.1497 0.1999 0.2497 0.3001 0.3500 0.3999 0.4494 0.4995

1.013 1.049 1.072 1.096 1.119 1.142 1.166 1.189 1.212 1.236

0.912 0.953 0.976 1.000 1.023 1.046 1.069 1.093 1.116 1.139

0.812 0.853 0.876 0.899 0.922 0.945 0.969 0.992 1.015 1.038

a Experimental viscosity: η, mPa·s. Ambient pressure: p = 0.0815 ± 0.0005 MPa. All viscosity measurements have an estimated standard uncertainty of 0.04 mPa·s. Three decimal places for the viscosities are shown to allow for calculation of thermophysical parameters derived from viscosities with less rounding error. Standard uncertainties for temperature and molalities, m, are 0.01 K and 2 × 10−4 mol·kg−1, respectively.

the B coefficient values.41 Furthermore, the magnitude of Δμ20≠ generally increases as temperature increases, except perhaps for glycine, where the temperature dependence is rather small.

the dissolved benzoate acts as a weak base with a pKb value of 9.846,47 and increases the solution pH. Using a pKb value of 9.8, we obtain calculated pH values in the range of 5.2−6.2, 5.4− 8.9, and 5.9−8.6 for sodium benzoate molarities of 0.1, 0.3, and 0.5, respectively. These pH values are near the isoelectric point that is about 6.0 for the three amino acids in this study.48 While acid−base equilibria are certainly temperature dependent, the temperatures studied here are only slightly higher than 298.15 K, which should not significantly alter the solution speciation. Thus, for the investigated concentrations in this study, the amino acids are predominantly present in their neutral zwitterionic form. With low concentrations of amino acid and sodium benzoate, the pH is slightly below the isoelectric point, while the reverse is the case at high amino acid and sodium benzoate concentrations. As is well-known, liquid water is highly structured due to hydrogen bonding interactions.49 When a solute is placed in water, the water structure could either be broken (structure breaker) or enhanced (structure maker). If neither is the case and the water structure remains unaffected, then volumes of solvent and solute would be additive for obtaining the resulting

4. DISCUSSION To set a framework for discussing the obtained density and viscosity results, it is helpful to point out that aqueous solutions of amino acids and sodium benzoate are in chemical acid−base equilibria. The pKa values for the carboxylic acid and amine functional groups are both very similar for all three studied amino acids, about 2.3 and 9.6 at ambient conditions, respectively.42−44 It is also known that the amino acids in their pure solid form are present as zwitterions, where the proton from the carboxylic acid group is attached to the amine group.45 When dissolving the amino acid in water, the acidity of the protonated amine group is slightly stronger than the basicity of the carboxylate, and the solution pH becomes slightly acidic. Using the corresponding pKa and pKb values of 9.6 and 11.7, we obtain pH values between 5 and 5.5, respectively, for the amino acid concentrations studied here. When sodium benzoate is added to the amino acid solutions, F

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Table 4. Values of A and B Parameters for Amino Acids in Aqueous Solutions of Sodium Benzoate at Ambient Pressurea m (mol·kg−1)

T (K)

0.1011

303.15 308.15 313.15 303.15 308.15 313.15 303.15 308.15 313.15

0.3088

0.5245

0.1011

0.3088

0.5245

0.1011

0.3088

0.5245

303.15 308.15 313.15 303.15 308.15 313.15 303.15 308.15 313.15 303.15 308.15 313.15 303.15 308.15 313.15 303.15 308.15 313.15

A (10−3/2 m3/2·mol−1/2) glycine −0.002 ± 0.028 ± 0.031 ± −0.027 ± −0.030 ± −0.002 ± −0.002 ± −0.002 ± −0.002 ± L-alanine −0.034 ± −0.037 ± −0.007 ± −0.005 ± −0.005 ± −0.005 ± −0.004 ± −0.030 ± 0.024 ± L-valine −0.012 ± −0.043 ± −0.047 ± −0.009 ± −0.010 ± −0.011 ± −0.059 ± −0.039 ± −0.043 ±

being the least polar of the amino acids due to the isopropyl side chain and glycine being the most polar amino acid. For further comparison, the apparent molar volume of sodium benzoate in aqueous solutions at similar concentrations of about 8.5 mL19,21 shows that it uses even less volume (∼10% less) than its neat volume of 9.61 mL. One factor in how far a solute disturbs the solvent structure is certainly its size. This can be seen in this study, insofar that V2,ϕ and V0ϕ follow the order of size of the amino acid, as was also noted by others.4−6,10,39 Treating the sodium benzoate solutions as the “solvent” allows comparison of the amino acids with respect to their intermolecular interactions with the “solvent”. Interactions of the solute with the solvent are dominating compared to interactions between solutes. This can be seen from the very small A coefficient values of the Jones− Dole equation, which suggest weak ion−ion interactions between the amino acids41 as well as the values for SV that are much smaller than the values for V0ϕ.5 However, some solute−solute interactions appear to be present because the SV values in Table S2 are all greater than zero.5,31,32 Addition of sodium benzoate appears to increase solute−solute interactions because the SV values for all three aqueous amino acid solutions are higher when sodium benzoate is present than when it is absent, although further increases in sodium benzoate appear to reverse this effect, especially at higher temperatures (Table S2). A similar behavior for the additive concentration dependence of SV values has also been observed for volumetric studies on amino acids with citrate.5,11 It is interesting that the B coefficients in Table 4 are largest for L-valine. L-Valine should have weaker attractive interactions with water compared to the other two amino acids based on its less polar structure and the apparent molar volume findings. Evidently, the fact that it is largest in size of the three amino acids outweighs the weaker interactions with water. Overall, all three amino acids appear to be structure breakers. First, the Δμ10≠ and Δμ20≠ values in Table S4 are all positive, and the Δμ20≠ values are consistently greater than Δμ10≠ values, which has been observed in other studies of amino acids with an added component41 and suggests that interactions between solute (amino acids) and solvent (aqueous sodium benzoate solutions) are stronger than the solvent−solvent interactions. The increased Δμ20≠ values in the order glycine < L-alanine < Lvaline imply that more energy is required to reach the transition state as the size of the amino acid increases.51 Second, the dependence of the B coefficient with temperature (dB/dT) at fixed concentrations has been classified into two categories. A positive sign of dB/dT indicates that the solutes break the solvent structure, while a negative sign is evidence for building up solvent structure in solution.52,4 The observed positive sign for at least L-valine and L-alanine indicates that these are structure breakers. The Δμ20≠ values, which are directly related to the B coefficient (eq 8), also generally increase with increasing temperature for these two amino acid solutions. Third, the transfer volumes are generally negative, which is indicative of structure breaking.31 The effect of sodium benzoate on the interactions of the amino acid with the solvent is not clear from observation of the B coefficients in Table 4. A consistent trend with increasing sodium benzoate concentration is only observable for the Lalanine solutions, where the B coefficients slightly decrease with increasing sodium benzoate concentration, indicating a weakening of the L-alanine interactions with the solvent upon addition of sodium benzoate. A weakening of solute−solvent

B (10−3 m3·mol−1)

0.001 0.002 0.002 0.001 0.002 0.001 0.001 0.001 0.001

0.167 0.152 0.163 0.177 0.194 0.181 0.141 0.154 0.170

± ± ± ± ± ± ± ± ±

0.001 0.004 0.004 0.002 0.002 0.001 0.001 0.001 0.001

0.001 0.001 0.005 0.001 0.001 0.004 0.001 0.001 0.003

0.332 0.365 0.367 0.278 0.309 0.336 0.250 0.302 0.283

± ± ± ± ± ± ± ± ±

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.004

0.002 0.001 0.001 0.002 0.002 0.002 0.001 0.001 0.001

0.449 0.526 0.578 0.405 0.446 0.499 0.534 0.565 0.632

± ± ± ± ± ± ± ± ±

0.003 0.002 0.002 0.003 0.003 0.002 0.002 0.002 0.002

a Ambient pressure: p = 0.0815 ± 0.0005 MPa. Standard uncertainties for temperature and aqueous sodium benzoate molality, m, are 0.01 K and 2 × 10−4 mol·kg−1, respectively.

solution volume. In this regard, the apparent molar volume represents the effective volume the solute contributes to the resulting solution volume upon addition. Because of intermolecular interactions between the solute and solvent, the volumes of neat solvent and neat solute are usually not simply additive, thus the term “apparent” molar volume. In the case of very strong solute−solvent interactions such as for some strong aqueous electrolyte solutions, negative apparent molar volumes are possible, which means the final solution is less in volume than that of the corresponding neat solvent.50 It is therefore interesting to compare the apparent molar volumes listed in Tables S1 with the volume of the neat added amino acid using the molar masses and densities of the neat solid amino acids included in Table 1. For example, 0.1 mol of solid amino acid corresponds to 4.66, 6.27, and 8.88 mL for glycine, L-alanine, and valine, respectively. When these values are compared with the entries for the 0.1 mol·kg−1 aqueous amino acid solution in Table S1 (i.e., 4.35, 6.08, and 9.09 mL), the resulting solution volume of adding 0.1 mol of L-valine is larger than the coadded volumes of the neat water and amino acid. For glycine and L-alanine, the resulting solution volume is less than the corresponding coadded volumes of neat water and amino acid, with the largest relative change (∼7% less than the neat volume) for glycine. This trend is consistent with L-valine G

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interactions upon addition of sodium benzoate is also indicated by the general decrease of V0ϕ with addition of sodium benzoate (Table S2). In this regard, the interactions of sodium benzoate are best inspected at infinite dilution conditions of the amino acid, where amino acid−amino acid interactions are negligible and amino acid−solvent interactions (i.e., with aqueous sodium benzoate) are dominant. This information is therefore accessible from the transfer volumes.39,53 The interactions between amino acids and sodium benzoate include contributions from (i) ion−ion interaction between Na+ and COO− in amino acids, (ii) io−ion interactions between COO− and NH3+ in amino acids, (iii) hydrophobic−hydrophobic interactions between the cyclic ring of sodium benzoate and the nonpolar group of amino acids, and (iv) the ion−nonpolar group of amino acids. Co-sphere modeling54 predicts that (i) and (ii) interactions lead to positive ΔtV0ϕ, and (iii) and (iv) interactions make negative contributions to the transfer volume. The negative ΔtV0ϕ values obtained in this study suggest that the (iii) and (iv) interactions are predominant interactions in the studied system in contrast to, for example, sodium fluoride, where (i) and (ii) appear to be dominant.35

AUTHOR INFORMATION

Corresponding Author

*E-mail: mhoff[email protected], Tel.: 585-395-5598, Fax: 585-395-5805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Bu-Ali Sina University authorities and the Physical Chemistry department for providing the necessary facilities to carry out the research.

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REFERENCES

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5. CONCLUSIONS New data for the density and viscosity of glycine, L-alanine, and L-valine in aqueous solutions of sodium benzoate (0.1011, 0.3088, and 0.5245 mol·kg−1) at different temperatures (T = 303.15, 308.15, and 313.15 K) are reported. From these data, a number of thermophysical properties have been calculated: the apparent molar volumes (V2,ϕ), the limiting value of apparent molar volumes (V0ϕ), transfer volumes (ΔtV0ϕ), B coefficients, and the free energy of activation of the solvent (Δμ10≠) and solute (Δμ20≠). These quantities were then compared and discussed in terms of the size and interactions of the amino acids with the aqueous sodium benzoate solution. Overall, Lvaline and L-alanine are water structure breakers, while for glycine, a clear determination cannot be made from the data. Despite the fact that the amino acids are predominantly present as zwitterions, the strongest effects on the solute−solvent interactions of the amino acids are from the increasing size of the amino acids and the associated larger nonpolar structural proportion. The presence of sodium benzoate was found to have very little or no impact on the B coefficients, while volumetric properties were notably impacted at higher temperatures. The experimental slopes (SV) for V2,ϕ (m) generally increased, and the corresponding V 0ϕ values decreased as the sodium benzoate concentration increased. The notable exception to these general trends was observed for 0 L-valine, where SV and V ϕ appear to undergo maxima and minima, respectively, an observation that might be of interest to further investigate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b01031. Tabulated values of several properties calculated from the reported densities and viscosities and a typical graph of the rearranged Jones−Dole equation (PDF) H

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