Temperature and composition dependences of volumetric properties of (water + 1,2-propanediol) binary system

Journal of Molecular Liquids 222 (2016) 656–662

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Temperature and composition dependences of volumetric properties of (water + 1,2-propanediol) binary system Dmitriy M. Makarov ⁎, Gennadiy I. Egorov, Arkadiy M. Kolker G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Ivanovo, Russia

a r t i c l e

i n f o

Article history: Received 15 April 2016 Received in revised form 13 July 2016 Accepted 21 July 2016 Available online 25 July 2016 Keywords: 1,2-Propanediol Aqueous mixture Density Partial molar properties Temperature of maximum density

a b s t r a c t The experimental data on the density of aqueous solutions of 1,2-propanediol over the whole concentration range are performed at different temperatures (278.15, 288.15, 298.15, 308.15, 323.15 and 333.15) K and atmospheric pressure. The density was measured also for the diluted solutions at temperatures from 274.15 to 283.15 K with 1 K step for examination of 1,2-propanediol influence on the temperature of maximal density (TMD) of water. The apparent molar volume of the mixture components, Vφ ,i, the partial molar volumes, Vi, and the partial molar isobaric expansions, Ep,i, were calculated depending on concentration and temperature. The following volumetric parameters were determined for infinite diluted solutions: the partial molar volumes ∞ of the mixture components, V∞ i , and the partial molar isobaric expansions, EP , i. The results obtained were discussed in terms of solute-solvent and solute-solute interactions. The literature data on the above volume properties of (water + 1,2-propanediol) mixture were reviewed. The character of disturbing influence of 1,2propanediol on the structure of water was established using different approaches to analysis of the volumetric properties. It was shown that the criteria of hydrophobicity examined resulted in contradictory conclusions. Therefore those criteria are to be considered simultaneously for drawing reasonable conclusions. © 2016 Elsevier B.V. All rights reserved.

1. Introduction 1,2-propanediol is of interest from both theoretical and applied points of view. It is one of the simple model structural units of polyols, on the one hand, and is an important object for different technological processes, on the other hand. For example, the propanediol-based systems are widely used as antifreezes, food and fuel additives, as co-solvents of parenteral drugs, etc. The aqueous solutions of diols are of great interest due to their anomalous behavior especially at low concentrations of non-aqueous component. 1,2-Propanediol molecules vary the structure of water significantly. From one side the presence of two hydroxyl groups in 1,2propanediol molecule allows to form the hydrogen bond network with water. From the other side the molecules of 1,2-propanediol due to CH3-group presence are able to order the water structure and to form clathrate-like cage. Such phenomenon was established on the base of NMR spectroscopy data [1] and ab initio calculations [2]. Therefore the interest to the mixture of water – 1,2-propanediol is quite reasonable. The goal of such studies is to distinguish the contributions of hydrophilic and hydrophobic fragments of 1,2-propanediol molecules into the processes of hydration and their mutual influence on water structure. ⁎ Corresponding author. E-mail address: [email protected] (D.M. Makarov).

http://dx.doi.org/10.1016/j.molliq.2016.07.095 0167-7322/© 2016 Elsevier B.V. All rights reserved.

The volumetric properties give the valuable information on the system structure packing and on the processes of solvation in solutions. These properties are sensitive to the whole spectrum of solute-solvent interactions. In order to understand in details the nature of the processes occurring in solutions, the studies of their volumetric properties within the wide temperature range are necessary. The volumetric properties of aqueous solutions of 1,2-propanediol have been studied carefully. There are numerous experimental data on the density and other volumetric properties of the system at different temperatures [3–19]. However in many cases the density values were obtained either at three temperatures at most [3–6,14–17] that makes difficult the calculation of their temperature derivatives, or for diluted solutions only [8,9,12,13]. The temperature region around the temperature of maximal density of water is still unexplored. Moreover the literature data on the system volumetric properties are sometimes inconsistent. For example in some works [6,11,14] the positive slope of the derivative of 1,2-propanediol partial molar volume with respect to its concentration, (∂ V2/∂ x)x → 0 (x - 1,2-propanediol molar fraction), was observed within the water-enriched composition region, but the negative slope was observed in works [4,9,13]. It is one more reason for the system to be investigated more thoroughly. In continuation of our investigations of aqueous mixtures of monoatomic alcohols and polyols within the wide range of state parameters [20–24] we present the volumetric properties of (water + 1,2propanediol) mixture depending on the composition over the wide

D.M. Makarov et al. / Journal of Molecular Liquids 222 (2016) 656–662

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temperature range T = (278.15 to 333.15) K. The above temperature interval covers also the temperature range around the temperature of maximal density of water. 2. Experimental 1,2-Propanediol (CAS No. 57-55-6, Aldrich, mass fraction purity 0.995) was used as purchased without any additional purification. The water content was determined by Karl Fisher titration and did not exceed 5 · 10−5 M fraction. Bidistilled and degasified water with conductivity b 2 · 10−6 S·cm−1 was used for the solutions preparation. The mixtures were prepared by gravimetric method using an analytical balance LLB200 model, Russia (resolution of 10− 5 g, uncertainty of 5 · 10−5 g). Mass of each prepared solution was about 25 g. The uncertainty of molar fraction values was estimated to be within ±5 · 10−5. The density was measured with an Anton Paar DMA-4500 densimeter. The densimeter was calibrated with dry air and bidistilled water before every measurement. All measurements were conducted under atmospheric pressure, p = (101.3 ± 0.5) kPa, within the temperature interval from 274.15 to 333.15 K. The density of diluted aqueous solutions of 1,2-propanediol was measured at temperatures from 274.15 to 283.15 K (with 1 K increment). The density was measured over the whole composition range at the following temperatures: 278.15, 288.15, 298.15, 308.15, 323.15, and 333.15 K. The temperature was kept with accuracy of 0.02 K. The density reproducibility was equal to 1 · 10−5 g·cm−3. The total uncertainty of the density measurement did not exceed 5 · 10−5 g·cm−3.

Fig. 1. Relative deviations of density experimental values obtained in this work from different literature data: (□) Geyer et al. [7]; (○) George and Sastry [10]; (Δ) Jiménez and Martínez [11]; (▼) Romero et al. [13]; (◄) Maximino [15]; (►) Zemánková et al. [18]; (◊) Khattab et al. [19]; (♦) Zorębski et al. [25]; (■) Li et al. [26]; (●) Atilhan and Aparicio [27]; (▲) Geyer et al. [28].

The apparent molar volumes of water, Vφ , 1, and 1,2-propanediol, Vφ , 2, at different temperatures were calculated by the following equations:

V φ;1 ¼

M2 xðρ2 −ρÞ M 1 þ ð1−xÞρ2 ρ ρ

ð1Þ

V φ;2 ¼

M1 ð1−xÞðρ1 −ρÞ M 2 þ xρ1 ρ ρ

ð2Þ

3. Results and discussion 3.1. Densities and apparent molar volumes of the mixture The experimental data on the density of 1,2-propanediol are listed in Table 1 at every temperature studied along with the literature data available and in Fig. 1 the relative deviations of 1,2-propanediol density obtained in this work from literature data are plotted. It should be emphasized that the densities at Т = (274.15, 275.15, 276.15, 277.15, 279.15, 280.15, 281.15, 282.15, and 283.15) K are performed for the first time. As one can see our data are in a good agreement with the data by Zorębski et al. [25] over the temperature range from 288.15 to 333.15 K, as well as with the results by Atilhan et al. [27] at temperatures from 278.15 to 308.15 K. The largest deviation (0.66%) is observed from the data by Khattab et al. [19] at 323.15 K. The other literature data satisfactorily coincide with our results and their relative deviations are within −0.06% to +0.04%. The experimental densities of (water + 1,2-propanediol) mixture at all temperatures studied and at atmospheric pressure are performed in Table 2.

where x was the molar fraction of 1,2-propanediol, ρ was the mixture density, M1, ρ1 and M2, ρ2 were the molar weights and densities of water and 1,2-propanediol, respectively. The apparent molar volumes obtained are shown in Table S1 and S2 (Appendix A. Supplementary data). The concentration behavior of 1,2-propanediol in its diluted solutions at atmospheric pressure and at 298.15 K is shown in Fig. 2 along with the results obtained from the literature data on the density of the solutions [4,10,13,17–19]. Our results agree well with Vφ , 2 values obtained on the basis of the literature data [4,18]. But some disagreement is observed between the experimental Vφ,2 values and the values calculated from the densities performed in works [10,13,17,19]. The difference is the most pronounced at low concentrations of the solute. In the work by George et al. [10] there is no minimum at all within the region of low 1,2-proapnediol concentrations.

Table 1 Experimental and literature values of 1,2-propanediol density, ρ, at various temperatures and atmospheric pressure.a Literature source

274.15

275.15

276.15

277.15

278.15

279.15

280.15

281.15

282.15

283.15

K

K

K

K

K

K

K

K

K

K

This work Reference [7] Reference [10] Reference [11] Reference [13] Reference [15] Reference [18] Reference [19] Reference [25] Reference [26] Reference [27] Reference [28]

1.04877 1.04856 1.04814 1.04759 1.04697 1.04629 1.04560 1.04489 1.04416 1.04346 1.03977 1.0499 1.0397

a

1.0471

Standard uncertainties, u, of measured values: u(T) = 0.02 K, u(ρ) = 5 · 10−5 g·cm−3.

288.15 K

298.15 K

1.03254 1.0322 1.03277 1.0328 1.04045 1.03271 1.03275 1.03286 1.0323 1.039844 1.032544 1.03286 1.0399 1.0325 1.0404 1.0330

308.15 K

323.15 K

333.15 K

1.02510 1.0252 1.02540 1.0251 1.02513

1.01359

1.00576

1.0231 1.0069 1.025088 1.013610 1.005758 1.02557 1.01374 1.0253 1.0255

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Table 2 Densities, ρ, of (water + 1,2-propanediol) mixtures at different temperatures.a ρ/g·cm−3

x

0.00000 0.00455 0.00926 0.01403 0.02661 0.03433 0.04240 0.05602 0.06992 0.07412 0.09101 0.11588 0.14965 0.20108 0.25462 0.29799 0.35131 0.40326 0.45876 0.50807 0.55392 0.60694 0.65363 0.68183 0.79304 0.85410 0.93149 0.95389 0.97780 0.98355 a

274.15 K

275.15 K

276.15 K

277.15 K

278.15 K

279.15 K

280.15 K

281.15 K

282.15 K

283.15 K

288.15 K

298.15 K

308.15 K

323.15 K

333.15 K

0.99990 1.00142 1.00300 1.00462 1.00904 1.01188 1.01494 1.01949

0.99994 1.00145 1.00302 1.00462 1.00905 1.01180 1.01487 1.01939

0.99996 1.00146 1.00302 1.00461 1.00899 1.01171 1.01473 1.01920

0.99997 1.00146 1.00300 1.00458 1.00893 1.01161 1.01459 1.01900

0.99996 1.00144 1.00297 1.00454 1.00884 1.01150 1.01444 1.01880 1.02320 1.02446 1.02936 1.03537 1.04174 1.04784 1.05197 1.05349 1.05461 1.05482 1.05457 1.05427 1.05390 1.05320 1.05235 1.05184 1.05003 1.04905 1.04790 1.04758 1.04726 1.04719

0.99994 1.00140 1.00293 1.00448 1.00874 1.01136 1.01427 1.01858

0.99990 1.00135 1.00287 1.00441 1.00864 1.01122 1.01410 1.01835

0.99985 1.00129 1.00280 1.00433 1.00851 1.01107 1.01391 1.01813

0.99978 1.00122 1.00271 1.00422 1.00840 1.01091 1.01373 1.01787

0.99970 1.00113 1.00260 1.00411 1.00825 1.01073 1.01351 1.01762

0.99909 1.00047 1.00192 1.00337 1.00730 1.00969 1.01232 1.01620 1.02009 1.02120 1.02551 1.03072 1.03640 1.04166 1.04530 1.04661 1.04758 1.04776 1.04745 1.04706 1.04648 1.04579 1.04510 1.04460 1.04279 1.04185 1.04070 1.04040 1.04007 1.03999

0.99704 0.99834 0.99970 1.00107 1.00470 1.00689 1.00928 1.01276 1.01623 1.01721 1.02102 1.02566 1.03049 1.03556 1.03848 1.03957 1.04036 1.04035 1.04008 1.03970 1.03912 1.03846 1.03772 1.03722 1.03546 1.03454 1.03345 1.03314 1.03282 1.03275

0.99403 0.99526 0.99654 0.99784 1.00120 1.00324 1.00540 1.00858 1.01169 1.01257 1.01588 1.02008 1.02458 1.02880 1.03140 1.03230 1.03296 1.03290 1.03260 1.03217 1.03158 1.03087 1.03020 1.02968 1.02794 1.02703 1.02597 1.02568 1.02538 1.02531

0.98803 0.98917 0.99034 0.99152 0.99459 0.99636 0.99827 1.00109 1.00371 1.00448 1.00722 1.01082 1.01420 1.01809 1.02020 1.02099 1.02145 1.02129 1.02096 1.02058 1.01995 1.01925 1.01858 1.01809 1.01648 1.01558 1.01450 1.01420 1.01388 1.01380

0.98319 0.98428 0.98537 0.98644 0.98916 0.99075 0.99235 0.99495 0.99739 0.99798 1.00044 1.00364 1.00704 1.01059 1.01251 1.01323 1.01352 1.01343 1.01301 1.01253 1.01198 1.01126 1.01063 1.01017 1.00857 1.00771 1.00663 1.00634 1.00603 1.00596

Standard uncertainties, u, of the values measured: u(T) = 0.02 K, u(x) = 0.00005, u(ρ) = 5 · 10−5 g·cm−3.

3.2. Partial molar properties of the mixture The partial molar volumes of water, V1, and 1,2-propanediol, V2, were calculated by the following equations:  V 1 ¼ M1  V 2 ¼ M2

 1 ∂ð1=ρÞ −w ρ ∂w

ð3Þ

 1 ∂ð1=ρÞ −ð1−wÞ ρ ∂ð1−wÞ

ð4Þ

where w was the mass fraction of 1,2-propanediol in the mixture.

The concentration dependences of the partial molar volumes V1 and V2 are shown in Fig. 3 at all temperatures studied and at atmospheric pressure. As one can see the calculated values of 1,2-propanediol partial molar volumes, V2, decrease over the diluted concentration range with the diol concentration rising and reach the minimum at about 0.1 M fraction. The minimum position shifts to the region of more diluted solutions with temperature rising. Then V2 values increase monotonically. The negative slope of the concentration dependences, (∂V2/∂x)x → 0, at all temperatures studied signifies, according to the conclusions by Franks [29], the hydrophobic properties of the solute molecules. However in some works [6,11,14] the concentration dependences of the partial molar volumes of 1,2-propanediol have no minima and their slopes are positive. This fact either makes those experimental results questionable or some calculation errors could be committed there during the experimental dependences differentiation. In accordance with the Gibbs-Duhem relation the partial molar volumes of water, V1, pass through the maxima at all temperatures studied within the same concentration range where the minima on the concentration dependences of V2 are observed (Fig. 3a). Further the dependences of the excess partial molar volumes of water, VE1, calculated by Eq. (5), will be considered. V E1 ¼ V 1 −V 01

Fig. 2. Concentration dependences of the apparent molar volume of 1,2-propanediol, Vφ,2, within the water-enriched composition region depending on the diol molar fraction at 298,15 K: (□) data by Nakanishi et al. [4]; (○) data by George et al. [10]; (Δ) data by Romero et al. [13]; (▼) data by Rodrigues et al. [17];(◄) data by Zemánková et al. [18]; (◊) data by Khattab et al. [19], (■) this work.

ð5Þ

where V01 is the molar volume of water. V1 values at high water concentrations give the important information on the water structure influenced by different solute molecules [30,31]. According to Marcus [31] VE1 is the quantitative measure of changes of the specific weight of different volume regions, provided that the water structure is considered to consist of two different states. So those states are characterized either by low density (bulky state) or by higher density (dense state) of water. If VE1 values are positive within the region of high water content than the extent of the bulky domains increase and the water becomes more structured. Thus the solute exhibits its hydrophobic properties.

D.M. Makarov et al. / Journal of Molecular Liquids 222 (2016) 656–662

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Fig. 3. Concentration dependences of the partial molar volumes of water, V1 (a) and 1,2-propanediol, V2 (b) in (water + 1,2-propanediol) mixture at various temperatures: (■ −278.15 K, ● −288.15 K, ▲ −298.15 K, ▼ −308.15 K, ◄ −323.15 K, and ► −333.15 K).

When VE1 b 0 the solute behaves like hydrophilic substance and does not make the water more structured. The maxima regions on V1 =f(x) functions (Fig. 3a) are composed by positive VE1 values (the data on V1Emax at xmax are listed in Supplementary data, Table S3). V1Emax values decrease with temperature growth, but the extreme position shifts to the region of more diluted solutions. Thus 1,2-propanediol reveals the hydrophobic properties. However in the work by Marcus [31] this diol at 298.15 K is referred to strongly hydrophilic compounds. Such conclusion was wrong because faulty data on the density [10] were taken for processing. The inaccuracy of the data [10] was demonstrated in Fig. 2. The partial molar isobaric expansions of the mixture components were calculated by the following equation:

Ep;i

  ∂V i ¼ ∂T

conclusions by Tan with co-workers [33] the anomalous concentration behavior of amphiphiles is determined by the following structural transformations. At x b 0.1 (region I), where the partial molar volume and partial expansion of water decrease, the hydration shells around 1,2-propanediol grow up and the fraction of bulky water decreases. Within the region II (0.1 b x b 0.3), where the extreme is observed and the partial properties of the mixture change significantly, the dehydration starts, i.e. the number of water molecules around hydrophobic CH3-group decreases, the hydration shells come in contact and overlap, but the bonds of water with OH-groups remain. At x N 0.3 (region III) the hydration of OH-groups of 1,2-propanediol starts to decrease. 3.3. Partial molar properties at infinite dilution

ð6Þ

The concentration dependences of the partial molar isobaric expansions of water, Ep,1, and 1,2-propanediol, Ep,2 (Fig. 4) have extremes at low concentrations of the solute. The phenomenon of “negative expansion” of water, which is observed at x about 0.1 M fraction at 278.15 K (Fig. 4a), rates this diol among the strongly hydrophobic substance, like tert-butanol [32], dissolved in water. Such hydrophobic substances demonstrate similar dependences within the diluted concentration range and cause the water compression under temperature increasing up to 288.15 K. Such phenomenon was not observed for aqueous solutions of polyatomic alcohols studied by us earlier (ethylene glycol [20] and glycerol [21]), which molecules had no unbound hydrophobic CH3-groups. Though the volumetric parameters obtained do not permit to draw definite conclusions on the solution structure, but the similar behavior of the partial molar properties of 1,2-propanediol and monoatomic alcohols assumes the same structural effects within the diluted concentration ranges over the limited temperature interval. According to

The limiting partial molar volume (or the standard molar volume), Vi∞, coincides with the apparent molar volume at infinite dilution, Vφ∞,i. Both these values were determined by linear extrapolation of the system apparent molar volumes dependences vs. 1,2-propanediol molal concentration to its zero concentration at all temperatures studied: V φ;i ¼V ∞i þbV m

ð7Þ

where bv was the limiting slope (adjustable parameter). The obtained values of the partial molar volumes of water, V∞ 1 , and 1,2-propanediol, V∞ 2 , at infinite dilution are listed in Table 3. As one can see in Table 3 the obtained values of V∞ 2 are in a good agreement with the data [8] over the wide concentration range and with the data [12,34] at 298.15 K. Unfortunately there is lack of the data on V∞ 1 values in literature sources at any temperature studied. The limiting excess partial molar volumes, VE,∞ i , calculated by Eq. (8), are presented in Fig. 5: ¼ V ∞i −V 0i V E;∞ i

ð8Þ

Fig. 4. Concentration dependences of the partial molar isobaric expansions of water, Ep,1 (a) and 1,2-propanediol, Ep,2 (b) in (water + 1,2-propanediol) mixture at various temperatures: (■ −278.15 K, ● −288.15 K, ▲ −298.15 K, ▼ −308.15 K, ◄ −323.15 K, and ► −333.15 K).

660

D.M. Makarov et al. / Journal of Molecular Liquids 222 (2016) 656–662

Table 3 a ∞ The partial molar volumes of water, V1∞, 1,2-propanediol, V2∞, and the partial molar isobaric expansions of water, E∞ p,1, and 1,2-propanediol, Ep,2, at infinite dilution. 3 −1 V∞ 1 /cm ·mol

T/K 274.15 275.15 276.15 277.15 278.15 279.15 280.15 281.15 282.15 283.15 288.15 298.15 308.15 323.15 333.15 a b

2 3 −1 E∞ ·K−1 p,1·10 /cm ·mol

16.29

1.14

16.38 16.53 16.66 16.81 16.98

1.18 1.22 1.26 1.31 1.35

3 −1 V∞ 2 /cm ·mol

2 3 −1 E∞ ·K−1 p,2·10 /cm ·mol

70.16 70.20 70.24 70.28 70.32, 70.39 [8]b 70.36, 70.43 [8] 70.40, 70.47 [8] 70.43, 70.50 [8] 70.47, 70.54 [8] 70.51, 70.58 [8] 70.70, 70.78 [8], 71.22 [13] 71.14, 71.21 [8], 71.79 [13], 71.23 [12], 71.7 [4], 71.22 [34] 71.59, 71.69 [8], 72.51 [13], 71.68 [34] 72.36, 72.48 [8] 72.90, 73.07 [8]

3.69 3.72 3.75 3.78 3.81 3.83 3.86 3.89 3.92 3.95 4.11 4.43 4.78 5.34 5.74

3 −1 Standard uncertainty u is u(T) = 0.02 K and the combined expanded uncertainties Uc are Uc(V∞ and Uc(E∞ i ) = 0.04 cm ∙mol p,i) = 0.04 (0.95 level of confidence). In ref. [8] the limiting partial molar volumes were determined at p = 0.5 MPa.

where V0i is the molar volume of component i. ViE,∞ value characterizes the difference of the volumes of component i in its pure state and in infinite dilute solution in a certain solvent. The limiting partial molar volumes of water, VE1 , ∞ and 1,2propanediol, VE2 , ∞, are negative at all temperatures studied. It means that the solute (diol or water) occupies less volume as compared with its pure liquid state. Such volume decrease implies that the solute- solvent interactions are stronger in infinite diluted solutions than in pure components [35]. With temperature growth VE,∞ values increase, whereas VE,∞ = f(T) 1 2 function is more complicated and has the maximum close to the temperature of maximal density of water. The limiting isobaric molar expansion, E∞ p,i, is of independent significance as it does not include temperature-independent contributions, namely the own volume of the molecules dissolved. E∞ p,i values were calculated by the following equation: E∞p;i

¼


∂V ∞i =∂T p

ð9Þ

E∞ p,i values at different temperatures are listed in Table 3 and shown in Fig. 6. ∞ E∞ p , 1 and Ep , 2 values increase with temperature rising. The sign of ∞ ∂ Ep , 2/∂T derivative, according to the approach by Hepler [36], can be considered as a criterion how the solute molecule influences the 2 water structure. So the positive magnitudes of (∂2V∞ 2 /∂T )p imply the water structure hardening (structure making effect), and the derivative negative values are connected with the water structure breaking by the solute molecules (structure breaking effect). For the system under investigation the positive value of the derivative was obtained at

Fig. 5. Temperature dependences of the limiting excess partial molar volumes of water, VE,∞ (■), and 1,2-propanediol, VE,∞ 1 2 (●).

−3 298.15 K ((∂E∞ cm3 ∙mol−1 ∙K−2) that indiP,2/∂T)p = (0.5 ± 0.3) ∙ 10 cated the water structure hardening due to the hydrophobic properties of 1,2-propanediol.

3.4. The temperature of maximum density of the aqueous solution The influence of solutes nature on the temperature of water maximal density (TMD) is a subject of many investigations [37,38] and is expected to reveal the effect of different solutes on the water structure. In accordance with those results the variation of water TMD, Δθ, under solute influence can be expressed as following:   ∂V E =∂T xV 02 α 02 x Δθ ¼ ðT min −277:13Þ ¼ − − ð1−xÞV 1 2α 01 ð1−xÞV 1 2α 01

ð10Þ

where α01 and α02 are the coefficients of volume thermal expensibility of water and 1,2-propanediol, respectively; V1⁎ is the molar volume of water at 277.13 K, V02 is the molar volume of 1,2-propanediol. The first term in the right side of Eq. (10) can be referred to “ideal” displacement of TMD, Δθid, and the second term is regarded as “structural” shift of TMD, Δθstr. The ideal component, Δθid, is negative as ∂V20/∂T value is positive at all temperatures for almost all substances. All effects of solute-solvent interactions enter into the structural term, which sign is applied as a criterion of solutes classification. Wada and Umeda [37] suggested the equation for Δθstr description depending on a solute concentration: Δθstr ¼ ax þ bx2

ð11Þ

∞ Fig. 6. Limiting isobaric molar expansions of water, E∞ p,1 (■) and 1,2-propanediol, Ep,2 (●) depending on temperature.

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where a and b are adjustable coefficients. The coefficient a is a characteristics of disturbing influence of the solute on water; its positive values are typical for hydrophobic substances and the negative coefficients correspond to hydrophilic ones. The following values of displacement of water TMD, Δθ, caused by different amounts of 1,2-propanediol were obtained: Δθ = − 0.63 K at x = 0.0046, Δθ = − 1.33 K at x = 0.0093, and Δθ = − 2.93 K at x = 0.0140. As one can see 1,2-propanediol results in the negative displacement of water TMD with respect to pure water and Δθ absolute values increase with 1,2-propanediol concentration rising. The coefficients in Eq. (11) are following: a = −22 K and b = −5280 K. As coefficient a is negative than, according to classification by Wada et al. [37], 1,2-propanediol is referred to hydrophilic substances. For other polyatomic alcohols studied earlier this coefficient was also negative and equal to −89 K for ethylene glycol and −85 K for glycerol [37], whereas for monoatomic alcohols the coefficient was positive and equal to 173 K (methanol), 273 K (ethanol), 263 K (propanol), and 605 K (tert-butanol). Thus coefficient a for 1,2-propanol have values which correspond to the intermediate position between strong hydrophobic and strong hydrophilic substances. 4. Conclusions The volume properties of (water + 1,2-propanediol) mixture and the limiting properties of its pure components in the appropriate solutions have been investigated over the wide temperature range at atmospheric pressure. In the work presented the effect of 1,2-propanediol on water structure was estimated on the basis of several thermodynamic criteria of hydrophobicity. Hepler's criterion reflects the hydrophobic “structuremaking” properties of 1,2-propanediol. The values of the derivative of 1,2-propanediol partial molar volume on the mixture composition within the region of low concentrations of the alcohol (the limiting slope coefficient) are negative that also is the evidence of 1,2propanediol hydrophobic nature. Whereas the displacement of the temperature of maximal density of water is negative that is typical for hydrophilic substances. Summing these criteria it can be concluded that 1,2-propanediol displays neither strong hydrophobic nor strong hydrophilic character and can be referred to amphiphilic molecules. The same was resumed by Koga [39] according to his own thermodynamic procedure of classification of solutes behavior in aqueous solutions. Acknowledgments This work was partially financially supported by the Russian Foundation for Basic Research (projects 15-43-03092-r_centre_а and 1543-03093-r_centre_а). The density measurements were carried out with equipment of Inter-laboratory center of scientific equipment “The upper Volga region center for physical-chemical researches”. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2016.07.095. References [1] R.A. Klein, V. Pacheco, Binary diol-water systems studied by 17O nuclear magnetic resonance spectroscopy. Interpretation of the effect of diol structure on 17O-water chemical shift. Formation of networks of water molecules stabilized by weak CH···O interactions, J. Phys. Chem. A 105 (2001) 9298–9304. [2] M.M. Deshmukh, N.V. Sastry, S.R. Gadre, Molecular interpretation of water structuring and destructuring effects: hydration of alkanediols, J. Chem. Phys 121 (2004) 12402–12410. [3] G. MacBeth, A.R. Thompson, Densities and refractive indexes for propylene glycolwater solutions, Anal. Chem 23 (1951) 618–619. [4] K. Nakanishi, N. Kato, M. Maruyama, Excess and partial volumes of some alcoholwater and glycol-water solutions, J. Phys. Chem 71 (1967) 814–818.

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