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SN Applied Sciences

, 1:671 | Cite as

Volumetric and compressibility studies and phase equilibria of aqueous biphasic systems of alcohols using phase diagram

  • Vidhya Jadhav
  • Rajendra Kumbhar
  • Bhaskar Tamhankar
  • Sandeep Shinde
  • Sanjay Kolekar
  • Sandip SabaleEmail author
Research Article
  • 81 Downloads
Part of the following topical collections:
  1. 1. Chemistry (general)

Abstract

The solubility data of Na2S2O3·5H2O in various concentrations of water and ethanol/1-propanol/2-propanol/2-methyl-2-propanol have been obtained to plot triangular phase diagrams and to determine the liquid–liquid–solid equilibria at 298.15 K. The density and sound velocity measurements were carried out for binary systems (water–salt, water–ethanol/1-propanol/2-propanol/2-methyl-2-propanol) and ternary systems (water–ethanol/1-propanol/2-propanol/2-methyl-2-propanol-Na2S2O3) to investigate volumetric and compressibility properties. The obtained data was used to determine the acoustic parameters including adiabatic compressibility (βs), excess molar volume (VE), intermolecular free length (Lf), specific acoustic impedance (Z), relative association (RA), solvation number (Sn) of water–alcohols, water–salt and water–alcohol–salt at 298.15 ± 0.05 K to evaluate equilibrium effects. Based on which, the salting-out of studied alcohols and phase forming ability of Na2S2O3 through its bond making and breaking properties, intermolecular interactions were reported. The density, sound velocity and adiabatic compressibility of ternary system suggest phase separation phenomenon is bond breaking and bond making process with hydrogen bonding and hydrophobic interaction. The phase formation ability of systems composed of different alcohols is in order of 2-methyl-2-propanol > 1-propanol > 2-propanol > ethanol.

Keywords

Phase diagram Acoustic properties Salting out effect Hydrophobic interactions Ion–ion interactions 

1 Introduction

Phase diagram represents a phase boundary, the conditions when two phases may coexist in equilibrium and the preferred physical state of a matter at different temperature and pressure for binary or ternary mixture. This experimental data is important for studying the various aqueous two-phase systems (ATPs), from which the optimization of the operating condition for superior results in separation and solvent extraction can be possible. In addition, it has various applications in the field of biology, medicine, research and industry [1, 2, 3]. Furthermore, the ATPs diagrams offer (salt-aqueous alcohol system) most valuable information about the structural changes in solvent, salting-in/salting-out effect, to understand the hydration and hydrophobic interaction between ions-solvent systems and nature of hydrogen bonding [4, 5]. The challenge to separate the alcohol is due to its many desirable characteristics in fuel additive (gasoline), replacing ethers like methyl tertiary butyl ether (MTBE), beverages, solvent, plasticizer, precursor to detergent. So it needs to distinct two layers as well as recycling the alcohols. The phase separation is depends upon the dielectric constant, ion–ion, ion–dipole interaction, hydrogen bonding and solubility etc.

In our previous study liquid–liquid–solid equilibria in the ternary system Na2S2O3 + ethanol + water and Na2S2O3 + t-butanol + water at ambient pressure and at room temperature (303 ± 2 K) [6, 7] have been studied in detail. Now, our aim is to study the molecular interactions such as solute–solute, solute–solvent, solvent–solvent, which are responsible for phase separation. For these phase separation, the interactions of salt with solvent, the behavior of ions produced from salt plays a crucial role. Generally the ions are categorized into two types such as “kosmotropic” or “chaotropic” based on ‘water structure maker’ or ‘water structure breaker’ giving salting out and salting-in effect respectively [8, 9]. Up to now, various researchers were carrying out the study of ATPS by using a phase diagram in which they used some models and equations to correlate the binodal and tie line in liquid–liquid equilibria (LLE) [10, 11, 12, 13, 14, 15, 16]. In recent years, several researchers focused on the measurement and investigation of thermodynamics of ATPSs composed of an aqueous solution of a short chain aliphatic alcohol in the presence of an electrolyte [10, 11, 17, 18, 19, 20, 21]. However, the detailed study of molecular interactions responsible for phase separation is not reported in detail. Therefore, our aim is to do the study of liquid–liquid and liquid–solid equilibria by using molecular interactions.

Through this paper, we attempt to present, the liquid–liquid–solid phase equilibria of ternary system (water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3) at 298.15 ± 0.05 K. In addition, this paper presents a systematic study of density, ultrasonic velocity and some thermo-acoustic parameters of binary systems such as, water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol, water + Na2S2O3, and ternary systems like water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3. The thermo-acoustic parameters like adiabatic compressibility (βs), apparent molar volume (фV), intermolecular free length (Lf), specific acoustic impedance (Z), relative association (RA), solvation number (Sn) and excess molar volume (VE) estimated from density and speed of sound data. The obtained data further used to explain the molecular interactions such as solute–solute, solute–solvent, solvent–solvent. Moreover, the purpose of this study is to develop the thermo-acoustic model of ATPs system as well as to understand the mechanism of salting out and predict the effect of electrolyte on interactions in a water–alcohol mixture.

2 Materials and method

AR grade ethanol (Changshu Hongsheng Fine Chemical; 99.9%), 1-propanol (Molychem; 99.5%), 2-propanol (Molychem; 99.5%), 2-methyl-2-propanol (Molychem; 99.5%), Sodium thiosulphate (Na2S2O3·5H2O, Lobachemie; 99.0%), were used without further purification. Different percent mass fraction compositions of solutions were prepared by using double distilled water. The weight of solutions required for the preparation of compositions taken on Shimadzu balance (AUW220D) with a precision of 0.0001 g.

3 Experimental procedure

The experimental apparatus employed in this work is double-wall glass vessel with air tide lid. The phase separation data of the phase diagram were obtained using water + various aliphatic alcohols (ethanol/1-propanol/2-propanol/2-methyl-2-propanol) mixture (initially 50:50 wt%) to which successive addition of solid Na2S2O3·5H2O (wt%) in a small increment were made with constant stirring. The point of phase separation (salting-out) was determined by the appearance of turbidity in the solution. The solution was allowed to stand for 1 h to confirm the phase separation and further addition of Na2S2O3·5H2O was continued until solid ceases to dissolve (saturated point of salt in aqueous layer). In a similar way, water + alcohol mixtures of different compositions were used (Table 1) and phase separation data (liquid–liquid equilibrium) and solid separation data (liquid–solid equilibrium) were obtained by adding Na2S2O3·5H2O in these mixtures. The concentrations of the constituent in terms of mass fraction × 100 were plotted on a triangular graph (known as a triangular phase diagram) to determine the mentioned concentrations (Table 1). All concentrations in terms of % mass fraction of the hydrated salt solutions were converted to that of anhydrous salt solutions.
Table 1

Liquid–liquid and liquid–solid equilibria data in terms of mass of water (w1), ethanol/n-propanol/2-propanol/2-methyl-2-propanol (w2) and Na2S2O3 salt (w3)

Liquid–liquid equilibria

Solid–liquid equilibria

100w1

100w2

100w3

100w1

100w2

100w3

Ethanol + Na 2 S 2 O 3  + water

00.00

100

NP*

0.009

99.97

0.02

10.04

89.95

NP*

10.06

89.92

0.03

20.10

79.89

NP*

20.12

79.83

0.05

30.03

69.96

NP*

30.05

69.74

0.20

40.02

58.03

01.95

39.52

50.21

10.27

49.19

47.05

03.76

45.28

32.81

21.90

56.61

34.46

08.92

49.88

2303

27.09

61.14

22.39

16.46

52.94

15.00

32.06

62.89

12.55

24.55

56.02

09.31

34.67

89.78

10.22

NP*

57.03

03.96

39.00

100

00.00

NP*

59.37

00.00

40.63

1-Propanol + Na 2 S 2 O 3  + water

00.00

100

NP*

0.004

99.99

0.008

10.06

89.94

NP*

10.07

89.92

0.014

20.04

79.62

0.33

22.20

69.03

08.77

30.09

69.29

0.62

31.60

52.41

15.77

39.98

59.09

0.92

38.81

40.31

20.88

49.53

48.41

02.06

44.68

30.67

24.65

58.44

37.52

04.03

49.07

21.64

29.29

67.02

27.43

05.55

53.02

14.93

32.05

74.34

17.49

08.17

55.79

08.97

35.24

75.06

07.27

17.66

59.26

04.31

36.43

100

00.00

NP*

59.38

00.00

40.63

2-Propanol + Na 2 S 2 O 3  + water

00.00

100

NP*

0.004

99.99

0.008

10.05

89.95

NP*

10.05

89.93

0.013

20.15

79.54

0.30

21.06

72.86

05.63

30.00

69.49

0.51

31.27

55.45

13.27

39.99

59.11

0.81

38.72

38.96

22.31

49.57

48.25

02.18

44.32

29.17

26.50

58.07

36.71

05.22

49.12

21.64

29.23

65.03

25.65

09.32

52.28

14.27

33.43

69.75

15.50

14.75

55.02

08.68

36.29

70.44

06.36

23.19

56.27

03.72

40.00

100

00.00

NP*

59.37

00.00

40.63

2-Methyl-2-propanol + Na 2 S 2 O 3  + water

00.00

100

NP*

00.00

99.99

0.006

10.06

89.94

NP*

11.18

99.98

0.012

20.13

79.68

0.18

22.38

68.59

09.03

30.18

69.41

0.41

31.37

55.88

12.74

39.93

58.98

01.10

38.94

42.94

18.12

49.62

48.60

01.77

44.53

30.03

25.43

59.01

38.34

02.64

49.09

21.61

29.29

67.50

27.94

04.55

52.56

14.56

32.88

74.71

17.69

07.59

55.87

09.02

35.11

72.05

06.79

21.15

58.17

04.16

37.66

100

00.00

NP*

59.37

00.00

40.63

NP*, no phase separation

The densities of binary systems such as, water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol, water + Na2S2O3·5H2O and for ternary system such, water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3·5H2O were determined using an Anton Paar DMA-4100 M vibrating tube densitometer with an accuracy of 0.1 kg m−3. Calibration was performed periodically under atmospheric pressure, in accordance with the specification using dry air and deionized water. The densities and sound velocities values of pure water, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanolwere found to be in good agreement 298.15 ± 0.05 K with literature data [22, 23, 24, 25, 26, 27] which are given in Table S1 and Fig.S1 [28]. The sound velocity measurements were carried out at 298.15 ± 0.05 K for all mixtures using an ultrasonic interferometer (Mittal Enterprises, New Delhi, India, F-05) operating at 2 MHz frequency. The uncertainty in measured values of sound velocity is found to be ± 0.5 m/s. The density and speed sound velocity measurements for binary systems such as, water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol and water + Na2S2O3·5H2O as well as for ternary system of water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3·5H2O were carried out at 298.15 K.

4 Results and discussion

In the present work, liquid–liquid and liquid–solid equilibria of water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3 systems were carried out in order to achieve a further understanding about the salting out effect. The results obtained for the different alcohols + water were discussed in different subsections such as, construction of phase diagram and liquid–liquid equilibria properties, volumetric and acoustic properties of binary system, volumetric, acoustic, conductometric properties of ternary system etc.

4.1 Construction of phase diagram and liquid–liquid equilibria properties

The triangular phase diagram of water + ethanol + Na2S2O3 (Fig. 1), water + 1-propanol + Na2S2O3 (Fig. 2), water + 2-propanol + Na2S2O3 (Fig. 3) and water + 2-methyl-2-propanol + Na2S2O3 (Fig. 4) have been constructed from solubility data of Na2S2O3 in different compositions of water–alcohol mixtures. All the data has been plotted in terms of percentage mass fraction of concentrations of each components [water (w1), alcohol (w2) and Na2S2O3 salts (w3)] (Table 1). To understand the region of the phase diagram, we used terms like LA, LO and S as aqueous layer, organic layer and solid Na2S2O3 salt respectively.
Fig. 1

Phase diagram of ternary system of water + ethanol + Na2S2O3:(open circle) liquid–liquid phase separation; (filled circle) solid–liquid phase separation at 298.15 ± 0.5 K

Fig. 2

Phase diagram of ternary system of water + 1-propanol + Na2S2O3: (open triangle) liquid–liquid phase separation; (filled triangle) solid–liquid phase separation at 298.15 ± 0.5 K

Fig. 3

Phase diagram of ternary system of water + 2-propanol + Na2S2O3: (open square) liquid–liquid phase separation; (filled square) solid–liquid phase separation at 298.15 ± 0.5 K

Fig. 4

Phase diagram of ternary system of water + 2-methyl-2-propanol + Na2S2O3: (open inverted triangle) liquid–liquid phase separation; (filled inverted triangle) solid–liquid phase separation at 298.15 ± 0.5 K

The various areas in all the four phase diagrams can be described as, the area ‘AabcdA’: a point in this region represents homogeneous mixtures (L) of all three components such as, water, ethanol/1-propanol/2-propanol/2-methyl-2-propanol and Na2S2O3, where point ‘a’ represent the solubility or saturation point of salt in water. The area ‘BabB’: a point in this region (LA + S) represents Na2S2O3 salt in equilibrium with the water rich saturated solution. The area ‘BdCB’: this region (LO + S) represents Na2S2O3 salt in equilibrium with alcohol rich saturated solution. The area ‘dcbd’ (LA + LO) represents the alcohol is separated continuously from the aqueous layer by adding salt (two conjugated solutions of salt in water–alcohol mixture), the area ‘BbdB’ (LA + LO + S): a point in this region represents the saturation of Na2S2O3 salt in aqueous layer.

The curve ‘bcd’ represents the liquid–liquid phase separation curve, which was observed by formation of turbid solution. The solid separation points (line denoted as a solid symbol, i.e. ‘abd’) was determined from the point where the solubility of Na2S2O3 ceases, at this point solid Na2S2O3, liquid LA and LO are coexisting in equilibrium. Further addition of salt could not generate any change in the composition of these liquid layers hence in this region ‘BbdB’, the system is isothermally invariant (LA + LO + S). The triangular phase diagram shows that the point ‘a’ represents the saturation solubility of Na2S2O3 in water, where the composition of Na2S2O3 and water 59.37% and 40.63% respectively. The ‘d’ point is the composition of Na2S2O3 at which salting out starts, these points are different for different alcohols, the values obtained from the phase diagrams are given in Table 2. The ‘d’ point of ethanol is 58.03% alcohol composition with 1.95% Na2S2O3 (Fig. 1). Whereas, ‘d’ point of 1-propanol, 2-propanol and 2-methyl-2-propanol are found to be, ~ 79.61% alcohol composition with ~ 0.27% Na2S2O3 (Figs. 2, 3 4), due to hydrogen bonding interaction and the ion–dipole interactions with alcohols. While ~ 10.22% of ethanol composition was not separated due to strong interaction with water and water has enough space to make bond with salt without salting out the ethanol than the rest of alcohols. From Figs. 1, 3 and 4 it is observed that, the determined phase separation abilities of investigated alcohols from the extent of biphasic areas (dcbd) of water + ethanol + Na2S2O3, water + 1-propanol + Na2S2O3, water + 2-propanol + Na2S2O3 and water + 2-methyl-2-propanol + Na2S2O3 systems were in the order of 2-methyl-2-propanol > 1-propanol > 2-propanol > ethanol. This observed sequence follow the order, which is reported in literature [29, 30]. The effect of the addition of salt in a solution of water–alcohol is very complex, primarily because a large number of different types of interactions come into play between the water–alcohol, water–ion, and water–alcohol–ion. These interactions were explained by using acoustic properties to resolve this mechanism [31, 32, 33, 34].
Table 2

The values of density (ρ/kg m−3), ultrasonic velocity (u/m s−1), adiabatic compressibility (βs/m2 N−1), excess molar volume (VE/m3 mol−1), intermolecular free length (Lf/m), specific acoustic impedance (Z/kg m2 s−1), relative association (RA/mol dm3) of water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol binary mixture at 298.15 ± 0.05 K

x 2

ρ (kg m−3)

u (m s−1)

10−10 βs (m2 N−1)

10−4 VE (m3 mol−1)

10−3 Lf (m)

106 Z (kg m2 s−1)

RA (mol dm3)

Water + ethanol

1.0000

785.2 ± 0.080

1143.44 ± 0.142

9.5628 ± 2.915

0.0000

2.9030

0.91

0.341

0.7777

814.8 ± 0.086

1225.84 ± 0.134

8.3022 ± 2.339

− 7.1397

2.7049

0.99

0.336

0.6095

840.0 ± 0.083

1282.3 ± 0.129

7.4712 ± 2.029

− 9.7922

2.5659

1.06

0.334

0.4763

864.0 ± 0.081

1348.42 ± 0.121

6.3655 ± 1.624

− 1.0647

2.3685

1.17

0.321

0.3695

887.4 ± 0.079

1406.34 ± 0.116

5.6977 ± 1.396

− 1.0685

2.2408

1.25

0.316

0.2809

910.0 ± 0.077

1477.56 ± 0.110

5.0334 ± 1.177

− 1.0026

2.1061

1.34

0.309

0.2066

931.8 ± 0.075

1544.74 ± 0.105

4.4974 ± 1.008

− 8.9284

1.9908

1.44

0.302

0.1535

950.7 ± 0.073

1601.36 ± 0.101

4.1018 ± 0.888

− 7.0409

1.9013

1.52

0.297

0.0893

966.5 ± 0.072

1613.9 ± 0.101

3.9723 ± 0.853

− 4.5876

1.8710

1.56

0.300

0.0420

980.2 ± 0.071

1570.34 ± 0.103

4.1371 ± 0.909

− 1.9143

1.9094

1.54

0.312

0.0000

997.0 ± 0.071

1497.4 ± 0.109

4.4792 ± 1.026

0.0000

1.9841

1.54

0.333

Water + 1-propanol

1.0000

799.8 ± 0.083

1207.48 ± 0.104

8.5234 ± 1.928

0.0000

2.7422

0.97

0.331

0.7258

822.8 ± 0.081

1261.86 ± 0.100

7.6327 ± 1.644

− 4.7662

2.5935

1.04

0.327

0.5458

844.2 ± 0.079

1298.32 ± 0.097

7.0273 ± 1.472

− 6.7257

2.4885

1.10

0.326

0.4107

864.6 ± 0.077

1334.8 ± 0.094

6.4916 ± 1.323

− 6.8365

2.3918

1.15

0.324

0.3095

885.2 ± 0.075

1372.56 ± 0.092

5.9964 ± 1.189

− 6.6974

2.2988

1.21

0.323

0.2306

905.9 ± 0.073

1413.48 ± 0.089

5.5251 ± 1.065

− 6.3228

2.2066

1.28

0.321

0.1665

926.5 ± 0.072

1463.2 ± 0.086

5.0414 ± 0.940

− 5.6643

2.1078

1.36

0.37

0.1138

947.6 ± 0.070

1517.3 ± 0.083

4.5839 ± 0.826

− 4.9972

2.0099

1.44

0.312

0.0696

966.8 ± 0.069

1577.62 ± 0.080

4.1558 ± 0.722

− 3.7870

1.9137

1.53

0.307

0.0322

982.4 ± 0.068

1572.44 ± 0.080

4.1168 ± 0.716

− 1.9034

1.9047

1.54

0.313

0.0000

997.0 ± 0.071

1496.6 ± 0.109

4.4791 ± 0.811

0.0000

1.9840

1.50

0.333

Water + 2-propanol

1.0000

781.2 ± 0.093

1140.06 ± 0.144

9.4209 ± 2.973

0.0000

2.8913

0.92

0.325

0.7284

807.5 ± 0.090

1203.42 ± 0.135

8.5511 ± 2.378

− 4.1750

2.7451

0.97

0.330

0.5448

832.1 ± 0.088

1255.04 ± 0.130

7.6297 ± 2.063

− 7.3463

2.5930

1.04

0.334

0.4108

855.8 ± 0.085

1301.62 ± 0.121

6.8970 ± 1.652

− 8.5776

2.4654

1.11

0.337

0.3099

880.1 ± 0.083

1362.62 ± 0.116

6.1195 ± 1.418

− 9.1367

2.3223

1.20

0.343

0.2305

902.9 ± 0.081

1423.16 ± 0.111

5.4683 ± 1.195

− 8.6107

2.1952

1.28

0.349

0.1664

926.3 ± 0.079

1493.40 ± 0.106

4.8406 ± 1.022

− 7.9804

2.0654

1.38

0.357

0.1138

948.0 ± 0.077

1562.20 ± 0.102

4.3223 ± 0.898

− 6.7276

1.9517

1.48

0.365

0.0699

966.7 ± 0.075

1619.42 ± 0.101

3.9445 ± 0.859

− 4.8227

1.8644

1.57

0.371

0.0325

980.9 ± 0.074

1425.22 ± 0.104

5.0189 ± 0.914

− 2.1132

2.1031

1.40

0.322

0.0000

997.0 ± 0.071

1496.62 ± 0.109

4.4779 ± 1.032

0.0000

1.9841

1.49

0.333

Water + 2-methyl-2-propanol

1.0000

781.5 ± 0.094

1125.61 ± 0.145

10.0075 ± 3.076

0.0000

2.9697

0.89

0.348

0.6812

804.5 ± 0.091

1179.34 ± 0.138

8.9370 ± 2.603

− 2.2303

2.8064

0.95

0.341

0.4929

827.9 ± 0.089

1226.3 ± 0.133

8.0321 ± 2.252

− 5.7125

2.6605

1.02

0.338

0.3907

852.5 ± 0.086

1280.5 ± 0.127

7.1539 ± 1.923

− 7.6233

2.5109

1.09

0.333

0.2668

875.7 ± 0.084

1326.26 ± 0.123

6.4921 ± 1.687

− 7.9133

2.3919

1.16

0.330

0.1954

899.4 ± 0.082

1382.02 ± 0.118

5.8212 ± 1.453

− 7.8219

2.2650

1.24

0.320

0.1394

921.4 ± 0.080

1438.62 ± 0.113

5.2439 ± 1.260

− 6.8197

2.1497

1.33

0.320

0.0942

945.3 ± 0.078

1514.6 ± 0.108

4.6114 ± 1.055

− 6.2012

2.0159

1.43

0.312

0.0577

966.9 ± 0.076

1594.2 ± 0.102

4.0694 ± 0.888

− 4.8709

1.8937

1.54

0.303

0.0261

982.1 ± 0.075

1588.72 ± 0.103

4.0341 ± 0.881

− 2.2991

1.8855

1.56

0.309

0.0000

997.0 ± 0.071

1496.44 ± 0.109

4.4790 ± 1.030

0.0000

1.9842

1.49

0.333

x2—mole fraction of alcohols respectively

Standard uncertainties u are u(ρ) = 0.1 kg m−3 with u(T) = 0.05 K, u(u) = 0.5 m s−1 with u(T) = 0.05 K

4.2 Volumetric and acoustic properties of binary systems (water + alcohol and water + salt)

The density and speed of sound data of water–alcohol mixture at different mole fractions and for water + Na2S2O3 mixture at concentrations from 0.01 to 3 M solutions were determined and given in Tables 2 and 3 respectively. From these experimental data, various acoustical parameters like, adiabatic compressibility (\(\beta s\)), inter molecular free length (Lf), relative association (RA), specific acoustical impedance (Z), Solvation number (Sn) and excess molar volume (VE) were determined using the following equations,
$$\beta s = \left( {\rho u^{2} } \right)^{ - 1}$$
(1)
$$L_{f} = K\sqrt {\beta_{S} }$$
(2)
$$R_{A} = \left( {\rho / \rho_{0} } \right)\left( {u_{0} /u} \right)^{{\frac{1}{3}}}$$
(3)
$$Z = u \times \rho$$
(4)
$$S_{n} = \left( {{\raise0.7ex\hbox{${n_{1} }$} \!\mathord{\left/ {\vphantom {{n_{1} } {n_{2} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${n_{2} }$}}} \right)\left( {\left( {1 - \beta_{S} } \right)/\beta_{S}^{0} } \right)$$
(5)
$$V^{E} = \frac{{\left( {M_{1} X_{1} + M_{2} X_{2} } \right)}}{{\rho_{1,2} }} - \frac{{M_{1} X_{1} }}{{\rho_{1} }} - \frac{{M_{2} X_{2} }}{{\rho_{2} }}$$
(6)
where ρ, ρ0 and u, u0 are the densities and ultrasonic velocities of solutions and solvents respectively. M is the molecular weight of the solute, \(\beta_{S}^{0}\) and \(\beta_{S}\) is the adiabatic compressibility of solvent and solution respectively, K the Jacobson constant, n1 and n2 are the number of moles of the solvent and solute respectively. \(V^{E}\) is excess molar volume of liquid mixture x1 and x2 are mole fraction, M1 and M2 molecular weight of liquid mixtures.
Table 3

The values of density (ρ/kg m−3), ultrasonic velocity (u/m s−1), adiabatic compressibility (βs/m2 N−1),apparent molar volume (фV/m3 mol−1), excess molar volume (VE/m3 mol−1), intermolecular free length (Lf/m), specific acoustic impedance (Z/kg m2 s−1), relative association (RA/mol dm3),solvation number (Sn) of water + Na2S2O3 with different molar concentration at 298.15 ± 0.05 K

M

x 1

x 3

ρ (kg m−3)

u (m s−1)

10−10 βs (m2 N−1)

фV (m3 mol−1)

10−4 VE (m3 mol−1)

10−3 Lf (m)

106 Z (kg m2 s−1)

RA (mol dm3)

S n

0.0092

0.9998

0.0002

998.7 ± 0.1008

1501.08 ± 0.1009

4.4438 ± 1.002

0.1581

− 0.0031

1.9789

1.05

0.3345

31.8014

0.0459

0.9992

0.0008

1003.6 ± 0.1003

1505.00 ± 0.1007

4.3991 ± 0.989

0.1574

− 0.0891

1.9689

1.51

0.3370

18.4903

0.0917

0.9984

0.0016

1010.1 ± 0.0996

1512.54 ± 0.1001

4.3274 ± 0.968

0.1564

− 0.2019

1.9528

1.53

0.3409

19.0225

0.3669

0.9936

0.0064

1047.1 ± 0.0961

1557.96 ± 0.0972

3.9346 ± 0.853

0.1509

− 0.8169

1.8621

1.63

0.3640

18.3832

0.7338

0.9874

0.0126

1094.4 ± 0.0920

1615.92 ± 0.0937

3.4993 ± 0.731

0.1444

− 1.5418

1.7561

1.77

0.3946

17.0470

0.9173

0.9747

0.0253

1117.4 ± 0.0901

1646.78 ± 0.0920

3.3001 ± 0.676

0.1414

− 1.8699

1.7053

1.84

0.4105

02.0105

1.6680

0.9549

0.0451

1178.3 ± 0.0854

1812.88 ± 0.0836

2.5823 ± 0.485

0.1341

− 2.6764

1.5085

2.14

0.4766

08.9282

2.9364

0.9232

0.0768

1285.8 ± 0.0783

1904.78 ± 0.0795

2.1436 ± 0.380

0.1229

− 3.9067

1.3744

2.45

0.5464

06.2502

M is the molarities of salt, x1 and x3—mole fraction of water and Na2S2O3 respectively

Standard uncertainties u are u(ρ) = 0.1 kg m−3 with u(T) = 0.05 K, u(u) = 0.5 m s−1 with u(T) = 0.05 K

From the result, it is observed that the density (ρ) and sound velocity (u) of water + alcohol systems gradually increase with increase in mole fraction of water (Fig. S2-S3), whereas for water–Na2S2O3 (Table 3) data increases with increase in concentration of Na2S2O3 (Figs. S5, S6). This may be attributed to the aggregation of alcohol in water which results in the decrease in volume and rise in mass. The apparent molar volumes of aqueous solutions of sodium thiosulphate were estimated by using Eq. (1) and the data obtained was given in and Table 3.
$$\varphi_{V} = \frac{{M_{2} }}{\rho } + \left( {\frac{{\left( {\rho_{0} - \rho } \right)}}{{m\rho \rho_{0} }}} \right)$$
(7)

In Eq. (1), M2 represents the molar mass of the salt in kg mol−1 and m is the molality in mol kg−1 whereas \(\rho\) and \(\rho_{0}\) represents the densities in kg m−3, for salt in aqueous solution and water respectively.

The apparent molar volume (\(\varphi_{V}\)) data for the water + salt, decreases with increase in concentration is generally attributed to electrostriction of ion (Table 3).

The speed of sound data of alcohol and sodium thiosulphate in aqueous solutions were used to determine the adiabatic compressibility (βs) by using Eq. (1) and the data obtained was given in Tables 2 and 3 respectively.

The decrease in adiabatic compressibility (βs) of aqueous solutions of alcohols and salt decreases with increase in concentration, which indicates that, the water–alcohol and water–salt molecules undergo structural reorganization and they are being compressed (Figs. S4–S7). The values of ρ, u and βs along with corresponding uncertainties are given in Tables 2 and 3. The water molecules have strong hydrogen bonded structure with numerous cavities, whereas, the alcohol or salt molecules make a hydrogen bond and occupy the interstitial space of water. Further, they aggregate and contribute to the increase in volume, while the βs values decreases with the increase in concentration of salt due to association of ions results in compact packing. The value of excess molar volume (VE) becomes negative with concentration for water + alcohols mixture and water + Na2S2O3 solutions are shown in Figs. 5 and S8. The values of VE are negative for the entire composition in both cases, which indicates that, a more efficient packing or attractive interaction occurred in the water–alcohol and water–Na2S2O3 solutions. The nature of graph of water + alcohol system is U-shaped, which ascribed to the equilibria of State effect and steric factor arising from the change of orientation of water molecules with change in its mole fraction [35]. In water + ethanol system shows maximum negative VE value suggested strong molecular interaction between them compare to the rest of alcohols. The decrease in intermolecular free length (Lf) values in both the cases is observed and it is proportional to the adiabatic compressibility, which shows strong ion–solvent and solute–solvent interactions. The Lf values evident that the degree of H-bonding of salt–water is stronger than water–alcohol. Specific acoustic impedance (Z) is a quantitative property, which is depending on the molecular packing of the system. The increase in the Z value suggests the presence of a strong interaction through hydrogen bonding in both systems [36]. Relative association (RA) depends either on, solvation of ions or breaking of solvent molecules associated with electrolyte. It is observed that the RA values of water–alcohol and water–salt increases (Tables 3, 4) due to the strong association between molecules. Solvation number (Sn) determined by using passynski method [31], the obtained values explain the interaction of solvent with solute molecules. This process involves reorganizing solvent and solute into hydration sphere. The maximum Sn value observed at low concentration of salt due to electrostriction of ion while as molarity increases Sn value dropped due to association of ion results in ion–ion interactions. The calculated acoustic values of water–salt system are higher than water–alcohol system as the hydration sphere of ion is stronger than that of alcohol.
Fig. 5

Comparison of excess molar volume (VE) for the alcohol + water system at 298.15 ± 0.05 K: (open circle) ethanol; (open triangle) 1-propanol; (open square) 2-propanol; (open inverted triangle) 2-methyl-2-propanol

Table 4

The values of density (ρ/kg m−3), ultrasonic velocity (u/m s−1), adiabatic compressibility (βs/m2 N−1), apparent molar volume (фV/m3 mol−1), excess molar volume (VE/m3 mol−1), intermolecular free length (Lf/m), specific acoustic impedance (Z/kg m2 s−1), relative association (RA/mol dm3), solvation number (Sn) of ternary water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3 system at 298.15 ± 0.05 K

(x1 + x2)

x 3

ρ (kg m−3)

u (m s−1)

10−10 βs (m2 N−1)

фV (m3 mol−1)

10−4 VE (m3 mol−1)

10−3 Lf (m)

106 Z (kg m2 s−1)

RA (mol dm3)

S n

Ethanol + water + Na2S2O3

1.0000

0.0000

910.1

1497.56

5.0329 ± 0.101

0.0000

0.0000

2.1060

1.3447

0.3333

0.0000

0.9981

0.0019

915.8

1492.24

4.9036 ± 0.096

− 0.6394

− 2.8685

2.0788

1.3666

0.3321

13.2526

0.9961

0.0039

917.4

1484.46

4.9465 ± 0.099

− 0.4328

− 2.5486

2.0879

1.3618

0.3344

4.3696

0.9940

0.0060

920.9

1482.04

4.9438 ± 0.099

− 0.4116

− 3.5728

2.0873

1.3648

0.3363

2.9166

0.9922

0.0078

923.7

1476.5

4.9659 ± 0.099

− 0.3650

− 4.3508

2.0919

1.3638

0.3386

1.6985

0.9905

0.0095

929.5

1472.96

4.9587 ± 0.099

− 0.3718

− 7.4219

2.0904

1.3691

0.3415

1.5412

0.9887

0.0113

934.4

1469.96

4.9528 ± 0.099

− 0.3978

− 9.6693

2.0892

1.3735

0.3440

1.3872

0.9870

0.0130

936.5

1464.5

4.9786 ± 0.099

− 0.3649

− 9.9391

2.0946

1.3715

0.3461

0.8159

0.9851

0.0149

940.2

1460.04

4.9894 ± 0.097

− 0.3600

− 0.1129

2.0969

1.3727

0.3485

0.5725

0.9833

0.0167

942.8

1453.78

5.0186 ± 0.100

− 0.3423

− 0.1181

2.1030

1.3706

0.3510

0.1672

0.9816

0.0184

940.7

1465.58

4.9491 ± 0.986

− 0.2655

− 8.9928

2.0884

1.3787

0.3474

0.8874

1-Propanol + water + Na2S2O3

1.0000

0.0000

905.9

1413.48

5.5251 ± 0.033

0.0000

− 0.6667

2.2066

1.2805

0.3333

0.0000

0.9991

0.0009

906.5

1411.18

5.5394 ± 0.033

− 0.0080

− 0.5596

2.2095

1.2792

0.3341

− 2.7428

0.9979

0.0021

908.8

1410.22

5.5329 ± 0.033

− 0.2026

− 1.9160

2.2082

1.2816

0.3352

− 0.6754

0.9970

0.0030

909.3

1409.42

5.5362 ± 0.033

− 0.1927

− 1.7785

2.2088

1.2816

0.3355

− 0.6771

0.9969

0.0031

910.2

1408.44

5.5384 ± 0.033

− 0.1838

− 2.5174

2.2093

1.2820

0.3361

− 0.7716

0.9950

0.0050

911.5

1407.72

5.5361 ± 0.033

− 0.1705

− 2.3878

2.2088

1.2831

0.3368

− 0.3976

0.9943

0.0057

913.5

1406.06

5.5371 ± 0.033

− 0.1678

− 3.8131

2.2090

1.2844

0.3379

− 0.3817

0.9931

0.0069

914.3

1405.66

5.5354 ± 0.033

− 0.1561

− 3.7006

2.2087

1.2852

0.3383

− 0.2695

0.9922

0.0078

915.4

1404.58

5.5372 ± 0.033

− 0.1558

− 4.0766

2.2090

1.2858

0.3390

− 0.2800

0.9909

0.0091

917.4

1404.08

5.5291 ± 0.033

− 0.1675

− 5.0574

2.2074

1.2881

0.3398

− 0.0798

0.9897

0.0103

916.2

1407.46

5.5098 ± 0.033

− 0.0965

− 3.0726

2.2036

1.2895

0.3386

0.2662

2-Propanol + water + Na2S2O3

1.0000

0.0000

903.1

1427.52

5.4337 ± 0.055

0.0000

0.0000

2.1883

1.2892

3.3333

0.0000

0.9989

0.0011

905.3

1424.8

5.4412 ± 0.055

− 0.0746

− 0.8294

2.1898

1.2899

3.3478

− 1.2413

0.9978

0.0022

907.6

1423.42

5.4380 ± 0.055

− 0.1706

− 2.1192

2.1891

1.2919

3.3596

− 0.3484

0.9966

0.0034

908.4

1422.98

5.4365 ± 0.055

− 0.1639

− 1.9891

2.1888

1.2926

3.3636

− 0.1542

0.9956

0.0044

910.7

1422.56

5.4260 ± 0.055

− 0.1591

− 3.3178

2.1867

1.2955

3.3731

0.3188

0.9945

0.0055

911.3

1422.3

5.4244 ± 0.055

− 0.1418

− 3.0187

2.1864

1.2961

3.3759

0.3082

0.9934

0.0066

914.1

1419.98

5.4255 ± 0.055

− 0.1366

− 4.7916

2.1866

1.2980

3.3918

0.2273

0.9924

0.0076

915.4

1416.9

5.4414 ± 0.055

− 0.1250

− 5.2001

2.1898

1.2970

3.4041

− 0.1827

0.9912

0.0088

916.6

1413.96

5.4569 ± 0.055

− 0.1124

− 5.4106

2.1929

1.2960

3.4156

− 0.4795

0.9902

0.0098

918.5

1408.58

5.4872 ± 0.055

− 0.1087

− 6.4019

2.1990

1.2938

3.4358

− 0.9949

0.9891

0.0109

915.4

1411.3

5.4846 ± 0.055

− 0.0761

− 2.6150

2.1985

1.2919

3.4176

− 0.8488

2-Methyl-2propanol + water + Na2S2O3

1.0000

0.0000

899.4

1382.02

5.8213 ± 0.055

0.0000

0.0000

2.2650

1.2430

0.3333

0.0000

0.9989

0.0011

900.6

1381.62

5.8169 ± 0.055

− 0.1249

− 0.5090

2.2641

1.2443

0.3339

0.7117

0.9982

0.0018

901.4

1379.70

5.8279 ± 0.055

− 0.1172

− 0.8273

2.2663

1.2437

0.3346

− 0.6359

0.9973

0.0027

901.9

1376.42

5.8525 ± 0.055

− 0.1099

− 0.6779

2.2710

1.2414

0.3356

− 1.9969

0.9967

0.0033

902.8

1375.18

5.8572 ± 0.055

− 0.1055

− 1.1685

2.2720

1.2415

0.3363

− 1.8446

0.9953

0.0047

904

1373.70

5.8620 ± 0.056

− 0.1003

− 1.4396

2.2729

1.2418

0.3371

− 1.4881

0.9948

0.0052

904.8

1371.62

5.8746 ± 0.056

− 0.0993

− 1.8875

2.2753

1.2410

0.3379

− 1.7368

0.9938

0.0062

905.4

1368.90

5.8941 ± 0.056

− 0.0976

− 1.8279

2.2791

1.2394

0.3388

− 2.0164

0.9933

0.0067

906.1

1366.56

5.9097 ± 0.056

− 0.0952

− 2.1872

2.2821

1.2382

0.3396

− 2.2526

0.9923

0.0077

906.9

1364.88

5.9190 ± 0.056

− 0.0909

− 2.2604

2.2839

1.2378

0.3403

− 2.1560

0.9915

0.0085

905.7

1366.20

5.9154 ± 0.056

− 0.0274

− 0.2611

2.2832

1.2374

0.3396

− 1.8795

(x1 + x2) and x3—mole fraction of solvent (50% water + 50% alcohol) and Na2S2O3 respectively

Bold values indicate the point of phase separation

Standard uncertainties u are u(ρ) = 0.1 kg m−3 with u(T) = 0.05 K, u(u) = 0.5 m s−1 with u(T) = 0.05 K

4.3 Volumetric, acoustic and conductometric properties of ternary system (water + alcohol + Na2S2O3·5H2O)

It is difficult to understand and discuss the interactions (bond breaking and bond making processes) in ternary water + alcohols + salt system. The volumetric and acoustic properties such as, ρ, u, βs, \(\varphi_{V}\), VE, Lf, Z, RA and Sn were estimated from density and sound velocity data. For this study, the mixture of 50% water + 50% alcohols composition (which shows maximum separation) has been selected as solvent and Na2S2O3 as an electrolyte which acts as solute. The points which are selected to determine the density and sound velocity are before phase separation and last point is the phase separation point of water + alcohol + Na2S2O3 systems and the data obtained are given in Table 4. The density (ρ) increases with addition of small amount of sodium thiosulphate salt in solution up to phase separation point whereas; it suddenly dropped at phase separation point as shown in Fig. S9. Similarly, the speed of sound data (u) decreases and βs values increases with concentration of salt as shown in Figs. 6 and 7. The obtained density data was further used to estimate the apparent molar volume of ternary systems and from this data it is observed that, the ion–solvent interactions are dominant over ion–ion (solute–solute) interactions at lower concentration while at higher concentration ion–ion interactions are dominant over ion–solvent interactions which results in ion association or ion-pair formation (Fig. S10). From the obtained data it is also observed that, the easy separation of alcohols containing more hydrophobic groups from solution due to increase in ion-pair formation. The negative VE values of Na2S2O3 in ternary system are also correlated to the association of ions in water–alcohol mixture. In addition, Lf values and RA values are slowly increasing which sense a breaking of water–alcohol bond and formation of water–electrolyte sheath with increases Na2S2O3. Sn describes bond formation, hydrogen bonding, and Van der Waals forces between solvent and solute. The Sn value of ethanol is positive while 1-propanol, 2-propanol and 2-methyl-2-propanol values are negative. Sn value increases with increase Na2S2O3 and at phase separation point it is higher (Table 4). Sn characterizes the solute–solvent interactions are maximum in water + ethanol + salt than rest studied alcohols due to hydrogen bonding ability of ethanol towards water and therefore this behavior shows that water–salt interactions increases and water–alcohol interaction decreases slowly with salt concentration.
Fig. 6

Effect of Na2S2O3 on velocity (u) of water + alcohol system at 298.15 ± 0.05 K: (open circle) ethanol; (open triangle) 1-propanol; (open square) 2-propanol; (open inverted triangle) 2-methyl-2-propanol

Fig. 7

Effect of Na2S2O3 on adiabatic compressibility (βs) of water + alcohol + system at 298.15 ± 0.05 K: (open circle) ethanol; (open triangle) 1-propanol; (open square) 2-propanol; (open inverted triangle) 2-methyl-2-propanol

The density, speed of sound and isentropic compressibility of ternary systems after phase separation (upper layer and lower layer) including saturation point of studied alcohols with Na2S2O3 were given in Table S2. From the obtained result, it is observed that, the density and sound velocity of upper layer i.e. alcohol rich layer decreases and compressibility increases while density and sound velocity of lower layer i.e. water rich layer increases and compressibility decreases. This can be illustrated that the salt slowly breaks the hydrogen bonding between alcohol and water due to electrostatic interactions of ions produced from the sodium thiosulphate with water. The increase in hydrophobic moieties on the alcohols as we goes from ethanol to tertiary butanol results in ion-pair formation ability due to reducing hydrogen bonding capacity of water towards alcohol. The obtained results are further supported with the degree of dissociation (α) values determined for ternary system containing different (water–alcohol) solvents from conductivity data given in Table 5. From the conductivity data, it is observed that, with addition of salt in solvent, initially conductance increases with increase in concentration of salt due to the dissociation of Na+ and S2O3 ions in solvent. Whereas after phase separation conductance dropped due to ion-pair formation as well as decreasing hydrogen bonding ability of water. The degree of dissociation of salt in each solvent system can be considered as efficient criteria to represent the dissociation of salt [37]. The degree of dissociation of Na2S2O3 is in solvent, the order of water + 2-methyl-2-propanol (0.7766) > water + 1-propanol (0.66710) > water + 2-propanol (0.5993) > water + ethanol (0.5579). The dissociation of salt in each system is different due to different strength of hydrogen bonding between water–alcohol and the decrease in degree of dissociation with increase electron donating group due to association of alcohol and ion-pair formation in aqueous solutions.
Table 5

The value of concentration (N), Observed conductance (Ω−1), Specific conductance (k Ω−1 cm−1), Equivalence conductance (λv Ω−1 cm−1 eq−1), Equivalence conductance at infinity (λ Ω−1 cm−1 eq−1), Degree of dissociation (α), of Na2S2O3 in studied systems

system

N

Ω−1

k (Ω−1 cm−1)

λv−1 cm−1 eq−1)

λ−1 cm−1 eq−1)

α

Water +ethanol

0.3777

0.01124

0.01335

30.0530

53.8643

0.5579

Water + 1-propanol

0.1024

0.00357

0.00361

35.1955

52.7593

0.66710

Water + 2-propanol

0.1522

0.00348

0.00351

23.0987

38.5371

0.5993

Water + 2-methyl-2-propanol

0.0515

0.001268

0.00128

24.85276

32.0001

0.7766

The before phase separation of water–alcohol–salt into water–alcohol, and water–salt phases, various interactions are present. These interactions are due to H-bonding, hydrophobic interactions, electrostatic interactions etc. The addition of salt in 50% water–50% alcohol mixture, results in ion–water interactions dominating over water–alcohol interactions which weaken the H-bonding between water and alcohol. This is confirmed from the acoustic data and increase in conductance of solution before phase separation. From comparative study of different alcohol it is observed that, the alcohol carrying more hydrophobic moieties are separated easily due to dominant hydrophobic interactions over water–alcohol interactions. The order of separation is found to be in correlation with the hydrophobicity of alcohol. It has been shown that the two-phase formation ability of systems composed of different alcohols is in order of 2-methyl-2-propanol > 1-propanol > 2-propanol > ethanol. The density, sound velocity and adiabatic compressibility results suggest that, phase separation phenomenon is bond breaking and bond making process as well as hydrophobic interaction and hydrophobic hydration.

One way to clarify, the mechanism of salting-out or salting-in effect in aqueous alcohol solutions with the addition of salt is studies based on liquid–liquid–solid equilibria, volumetric, acoustic and conductometric properties of water–alcohol, water–salt and water–alcohol–salt systems. This provides useful information of the interactions that exist in these systems and is of great importance to obtain information about the solvation and association behavior of ions in water–alcohol systems.

5 Conclusion

The phase diagrams for the water + ethanol/1-propanol/2-propanol/2-methyl-2-propanol + Na2S2O3·5H2O systems were constructed from the solubility data. The salting-out effect can be ascribed to ion–ion, ion–dipole, dipole–dipole interactions. Acoustic properties of water–alcohol, water–salt and water–alcohol–salt system provides useful information on the interactions that exist in these systems and is of great importance to obtain information about the solvation and association behavior of ion in these systems. Rise in the density and sound velocity results in a decrease in adiabatic compressibility, which signifies greater structural interaction of water with alcohol and salt in system. The negative value of VE, decrease in Lf, Z, and RA suggested a strong interaction of water with salt and alcohol individually through H-bonding and denser packing in binary system. However, the relation is quite changed with the ternary system composed of water + alcohol + salt. Where, water–alcohol hydration sphere has been broken due to kosmotropic salt by forming salt–water hydration sphere. In addition, phase forming ability of alcohol can be explained by the dissociation constant of sodium thiosulphate. It has been shown that the two-phase formation ability of systems composed of different alcohols is in order of 2-methyl-2-propanol > 1-propanol > 2-propanol > ethanol.

Notes

Acknowledgements

Authors are thankful to University Grants Commission, New Delhi, India, for grants under Major Research Project [File No: 43-219/2014 (SR) Dt. 18.08.2015] and Department of Science and Technology, New Delhi, India, for sanctioning grant under DST-FIST Program [No/SR/FST/College-151/2013(C)] to Jaysingpur College, Jaysingpur.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

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Supplementary material 1 (DOCX 1010 kb)

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of ChemistryJaysingpur College (Shivaji University, Kolhapur)JaysingpurIndia
  2. 2.Department of ChemistryWillingdon College (Shivaji University, Kolhapur)SangliIndia
  3. 3.Department of ChemistryShivaji UniversityKolhapurIndia

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