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Evaluation of limiting molar conductance, Walden product, association constant and thermodynamic properties of sulfacetamide sodium in water + EtOH mixtures
Journal of Molecular Liquids 160 (2011) 140–143
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Evaluation of limiting molar conductance, Walden product, association constant and thermodynamic properties of sulfacetamide sodium in water + EtOH mixtures J. Ishwara Bhat a,⁎, M. Kishore Shetty b a b
Department of Chemistry, Mangalore University, Mangalagangothri, Mangalore 574199, Karnataka, India Department of Chemistry, Alva's Institute of Engineering and Technology, Mijar, Moodbidri, Karnataka, India
a r t i c l e
i n f o
Article history: Received 12 February 2011 Received in revised form 1 March 2011 Accepted 16 March 2011 Available online 2 April 2011 Keywords: Sulfacetamide sodium Electrical conductance Water–ethanol mixtures Association constant Ion solvation
a b s t r a c t The ion solvation behavior of sulfacetamide sodium in water and various volume fractions of ethanol (EtOH) in water in the range of 283 to 313 K, using electrical conductivity principle have been studied. The conductance data were analyzed according to Kraus–Bray and Shedlovsky models of conductivity. The limiting molar conductance λom, association constant Ka, energy of activation of the rate process (Ea), and related thermodynamic parameters have been determined. Using viscosity of the solvent, Walden product (λomη0) and Stokes molecular radius have been determined. Standard thermodynamic parameters of association (ΔGa, ΔHa,) were calculated and discussed. The limiting molar conductance sharply decreased for the increased amount of ethanol suggesting the increased ion–solvent and solvent–solvent interactions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The electrical conductivity study of electrolytes in mixed solvents is known to be influenced by a number of factors like viscosity, temperature, relative permittivity, hydrogen bonding, ion-solvation and solvent–solvent interactions [1–8]. Literature reveals that nature of the ion–solvent interactions and ion–ion interactions or the behavior of the electrolytes in a solvent of different relative permittivity can be informative depending on the conductance and ionic mobility of these electrolytes in solutions [9–14]. Water+EtOH mixtures at different temperatures exhibit a wide range of relative permittivity (ε), viscosity (η) and a high degree of hydrogen bonding effect. Varied interaction between water and ethanol because of mixing them at different quantities may be studied by the measurement of conductivity over that composition range. Appropriate conductivity measurements provide useful indications of ion–solvent interaction, proton–anion and proton–solvent association, and solvent structure. In continuation with our earlier report [15] on ion association and solvation of sulfonamide drug; sulfathiazole sodium in aqueous and partial aqueous media, we report herein the solvation behavior of another sulfonamide drug sulfacetamide sodium in water + ethanol in view of evaluation of limiting molar conductance, Walden product, association constant and thermodynamic properties. Study of solvation behavior of medicinal compounds under varying biological conditions is an important field of studies which revealed much information to medicinal chemist [16]. The conductivity values may be ⁎ Corresponding author. Tel.: + 91 824 2287262. E-mail address: [email protected] (J.I. Bhat). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.03.005
useful to have an appropriate idea regarding drug diffusion and change in the nature of hydrophobic interactions of drug with the change in free energy involved while moving from water to non-aqueous solvents [17]. The sulfonamides constitute an important class of drugs and have extensively been documented for their wide variety of pharmacological activities such as antimicrobial, insulin releasing antibiotic, diuretic, ant carbonic anhydrase, anti-thyroid, hypoglycemic, anti HIV and antitumor activity. Sulfacetamide sodium is a sulfonamide [18] antibacterial and is applied in infections or injuries of the eyes. The structure of sulfacetamide sodium is given in Fig. 1. We report in this communication the electrical conductivity of sulfacetamide sodium in partial aqueous media by varying the compositions (v/v) of water + ethanol at four different temperatures. 2. Materials Sodium acetyl[(4-aminophenyl)sulfonyl]azanide (Sigma-Aldrich, USA, 99%, b 0.05% H2O) was used as such, without further purification. The purity was checked from the melting point. (Melting point obtained in the lab: (531–533) K; Literature value [18]: 530 K). The conductivity of water used in this study was obtained by deionization and triple distillation with specific conductance in the range of (1 to 3)·10− 7 S·cm− 1. Ethanol conductivity 10− 7 S− 1·cm− 1 was used. 3. Solutions Mixed solvents, ϕ2 = 0.1, ϕ2 = 0.2, ϕ2 = 0.4, and ϕ2 = 0.6 were prepared by mixing known quantities of water and EtOH in volume ratio at laboratory temperature. The calculated values of mass fractions
J.I. Bhat, M.K. Shetty / Journal of Molecular Liquids 160 (2011) 140–143
O -
However, this model does not include any correction for inter-ionic effects or for the activities of the ions. Therefore, Shedlovsky [21] relation was used to evaluate the absolute limiting molar conductance and association constant Ka.
+
S N Na C CH3 O O
H2N
141
Fig. 1. The structural formula of sodium acetyl[(4-aminophenyl)sulfonyl]azanide.
1 1 Cλm SfF2 Ka = o + Sλm λm λom 2
ð3Þ
For measurement of electrical conductivity, the digital conductivity meter (CM-180, Elico India) and immersion type cells (cell constant=0.998 cm− 1) were used. Cell constant was determined to be within 0.01% precision at various times by calibration with aqueous potassium chloride [19] solutions. The solution of known concentration was taken in a double walled vessel and kept in a thermally stabilized water bath with good thermal regulation (±0.1 °C) for approximately 20 to 30 min and its specific conductivity was noted. All conductivity measurements were repeated thrice and it was found to be reproducible to 0.1%.
Required relative permittivity (ε) and viscosity (η) values were obtained from the literature [22–24]. The f± is the mean ionic activity coefficient of the electrolyte. λom and Ka were obtained from the intercept and slopes of the plot of 1/Sλm vs CλmSf2±. The values of limiting molar conductance evaluated using Kraus–Bray and Shedlovsky models at different temperatures having uncertainty of ±0.5% are presented in Table 2. Limiting molar conductance (λom) was found to be higher in water and then decreased sharply on adding ethanol to it, despite the maximum in viscosity near ϕ2 = 0.5 ethanol. This behavior may be due to the formation of hydrogen bond between ethanol and water molecules resulting in the association of ethanol and water molecules, which in turn reduces the solvated ionic mobility and decreases the limiting molar conductance values with higher percentage of ethanol in the solvent mixture. The variation in viscosity (η) is not only the factor controlling the ionic mobility, which is evidenced by the Walden product. The limiting molar conductance of sulfacetamide sodium increased with increase in temperature from 283 K to 313 K for all volume fractions of co-solvent due to the increase in ionic mobility. Increase in thermal energy breaks more number of hydrogen bonding that of water decreasing the solvated ionic size and hence increases the mobility of the species.
5. Results and discussion
5.2. Association constant
5.1. Limiting molar conductance
The calculated values of association constant having uncertainty of ± 2% were presented in Table 3. The values of Ka of the studied system at the same temperature were found to increase as the proportions of solvent ethanol increase with water. The extent of ion association depends on the nature of ion–ion interaction in the solution, relative permittivity of the medium and also intermolecular hydrogen bonding between the solvent molecules. This suggests the fact that the values of Ka increased with the increase of EtOH in water. The association constant for the studied salt was found to decrease with the increase in temperature; increased thermal motion probably breaks more bonds between ions and makes them to move away from each other, where it gets solvated. Hence the mobility if ions should increase. This may account for the decrease in Ka for the increased temperature. To identify the association Fuoss [25] suggested the following equation
of ethanol in water at studied temperatures are given in Table 1. Solutions of sulfacetamide sodium with a concentration range of (0.001 to 0.01) mol·L− 1 were prepared in water or water + EtOH as and when needed. Taking into account the sources of error, we estimate the uncertainty in molar concentration to be within ± 5.0⋅10− 4 mol⋅L− 1. Work could not be carried out beyond ϕ2 = 0.6 and pure ethanol due to solubility problem. 4. Instrument and methods
The specific conductances of solutions of sulfacetamide sodium with a concentration range of (0.001–0.01) mol·L− 1 inϕ2 = 0.0, ϕ2 = 0.1, ϕ2 = 0.2, ϕ2 = 0.4, and ϕ2 = 0.6 EtOH in water at 283, 293, 303 and 313 K were measured. The molar conductance for all the studied systems was calculated using Eq. (1). λ=
κ C
ð1Þ
where C is the molar concentration, κ is the specific conductance of sulfacetamide sodium solution from which the specific conductance of the used solvent was subtracted. The experimental molar conductivities were analyzed as per Kraus–Bray conductivity equation [20]. 1 1 λ C = o + o m2 λm λm λm K c :
ð2Þ
The limiting molar conductances, λom and dissociation constant, Kc were obtained from the intercept and slope of the plot 1/λmvsλmC.
Table 1 Calculated values of mass fractions (W2) of ethanol in water from T = 283 K to 313 K. T/K
T/K = 283 T/K = 293 T/K = 303 T/K = 313
W2 ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
0.0988 0.0987 0.0987 0.0987
0.1963 0.1960 0.1958 0.1956
0.3889 0.3877 0.3866 0.3857
0.5790 0.5773 0.5759 0.5745
2 0 Cλm = KC λm
ð4Þ
According to Fuoss, the slope of the plot log λm vs log C is around −0.5, it is an indication of the presence of ion-pairs in equilibrium with the salt. In the present case, the slope was found to be −0.11. The higher value of the slope hints the absence or minor amount of ionpair formation. Moreover it also supports the fact that higher ion aggregates (ion–ion and triplet ion pair) from 1:1 salts cannot occur in higher permittivity media [26,27] (ε N 10). 5.3. Variation of Walden products with solvent composition If ionic mobility depends on bulk viscosity as per the Stokes' law, the Walden products [28] would appear reciprocal to the ionic radius. The variations in Walden product with solvent composition and
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Table 2 Limiting molar conductance for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K. Property
ϕ2= 0.0 EtOH
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
T = 283 K λοm (Kraus–Bray) λοm (Shedlovsky)
λοm / (S⋅cm2·mol− 1) 52.07 53.47
37.87 39.13
30.16 31.15
19.88 21.27
18.31 20.74
T = 293 K λοm (Kraus–Bray) λοm (Shedlovsky)
64.51 65.78
50.50 52.35
40.65 42.46
25.38 27.00
23.69 26.32
T = 303 K λοm (Kraus–Bray) λοm (Shedlovsky)
78.12 80.00
64.51 66.60
52.35 54.00
37.59 38.90
31.54 34.48
T = 313 K λοm (Kraus–Bray) λοm (Shedlovsky)
92.59 95.20
80.00 81.86
65.79 67.06
48.78 51.94
38.16 43.41
Table 3 Association constant Ka for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K.
Table 4 Energy of activation Ea and change in enthalpy of association ΔHa for sulfacetamide sodium in water + EtOH.
T/K
ϕ2 = 0.0 EtOH
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
ϕ2 = 0.0 EtOH
T/K = 283 T/K = 293 T/K = 303 T/K = 313
8.530 8.525 8.518 8.215
8.809 8.771 8.436 8.005
9.561 9.119 8.921 8.830
12.691 11.174 11.031 9.845
32.41 30.69 29.17 27.78
Ea/(kJ·mol− 1) 13.90 15.69 ΔHa/(kJ·mol− 1) 1.98 3.03
temperature might then be explained through changes in ion solvation alone. o
λm ηο =
Z eo F 6π r
ð5Þ
where eο is the electronic charge, F is Faraday constant; and Z is the charge on the ion and r is the effective Stokes molecular or ionic radius. The most satisfactory interpretation of the Walden products is based on the structure of water–ethanol mixtures and the effect of ions on this structure. The thermodynamic, kinetic, ultrasonic and transport data indicate that addition of simple alcohols to water initially enhances the structure of the solvent water [29]. The structure enhancement (structure former) appears to reach a
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
19.15
20.69
21.59
3.52
5.65
8.96
maximum near ϕ2 = 0.2 to ϕ2 = 0.3 ethanol [29]. At higher volume fractions of ethanol, the water structure progressively gets reduced (structure breaker). The maxima in the Walden products near ϕ2 = 0.3 ethanol may thus be a consequence of the greater disruption of water structure in the vicinity of the ions for solvent of this composition. This viewpoint also predicts that Walden products should decrease with increase in temperature. Both this predictions are in agreement with the Walden products as depicted in Fig. 2. 5.4. Thermodynamic parameters Since, the conductance of an ion depends on its movement; it is quite reasonable to equate the conductance process with rate process [30] and is given by the equation, o
−
λm = Ae
Ea =RT
ð6Þ
λ0η0 /(S·cm2·mol-1 kg·m-1·s-1)
0.75
where Ea is energy of activation of the rate process, which is characteristic of the rate of movement of ions in solution, R is the gas constant, A is the Arrhenius factor and T is the absolute temperature. The value of Ea was calculated from the slope of the plot of log λom vs 1/T. The value of energy of activation increased with increase in composition of ethanol in water as shown in Table 4, suggesting the involvement of high
0.70 0.65 0.60 0.55
Table 5 Computed change in Gibb's energy (ΔGa) for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K.
0.50 0.45
T/K
0.40 0.0
0.2
0.4
0.6
0.8
1.0
φ2 Fig. 2. Plots of Walden product λomηo/(S·cm2 mol− 1 kg·m− 1 s− 1) against volume fraction φ2 for sulfacetamide sodium in water + ethanol from T = 283 K to 313 K.
φ2 = 0.0 EtOH
-ΔGa/(kJ·mol− 1) T/K = 283 2.85 T/K = 293 2.89 T/K = 303 2.89 T/K = 313 2.95
φ2 = 0.10 EtOH
φ2 = 0.20 EtOH
φ2 = 0.40 EtOH
φ2 = 0.60 EtOH
3.38 3.33 3.38 3.40
4.04 3.98 4.02 4.03
5.09 5.07 4.92 5.00
8.33 8.33 8.32 8.26
J.I. Bhat, M.K. Shetty / Journal of Molecular Liquids 160 (2011) 140–143
energy for the movement of ions or the existence of the drifting force effect on the conducting ion. Hence conductance should decrease with the increase in the amount of ethanol in water. This theory holds well with the observed result of λom as shown in Table 2. From the slope of plot log Ka vs 1/T, which was linear, the change in enthalpy of association (Δ Ha) was calculated. The change in free energy (ΔGa) and change in entropy (λomSa) were also calculated at all temperature and compositions and the values of Δ Ga are shown in Table 5. Δ Ha is positive indicating the system to be endothermic in character for all the compositions, which is supported by the negative values of Δ Ga in all the cases proposing the spontaneity of the association process. The entropy of association (Δ Sa) is small in all the studied systems suggesting the specific interaction required for the formation of ion-association of all the thermodynamic functions describing the solvation process and the ion-association occurs without bringing much disorderliness of the system. References [1] [2] [3] [4] [5] [6]
J. Ishwara Bhat, T.N. Sreelatha, Indian J. Chem. 45A (2006) 1165–1169. J. Zielkiewicz, J. Chem. Eng. Data 43 (1998) 650–652. D.S. Gill, J. Chem. Soc. Faraday Trans. 77 (1981) 751–758. D.M. Rao, C. Kalidas, Z. Naturforsh. 45A (1990) 1006–1009. L.P. Safonova, A.M. Kolker, Russ. Chem. Rev. 61 (1992) 959–973. J. Ishwara Bhat, T.P. Mohan, C.B. Susha, Indian J. Chem. 35A (1996) 825–831.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Evaluation of limiting molar conductance, Walden product, association constant and thermodynamic properties of sulfacetamide sodium in water + EtOH mixtures J. Ishwara Bhat a,⁎, M. Kishore Shetty b a b
Department of Chemistry, Mangalore University, Mangalagangothri, Mangalore 574199, Karnataka, India Department of Chemistry, Alva's Institute of Engineering and Technology, Mijar, Moodbidri, Karnataka, India
a r t i c l e
i n f o
Article history: Received 12 February 2011 Received in revised form 1 March 2011 Accepted 16 March 2011 Available online 2 April 2011 Keywords: Sulfacetamide sodium Electrical conductance Water–ethanol mixtures Association constant Ion solvation
a b s t r a c t The ion solvation behavior of sulfacetamide sodium in water and various volume fractions of ethanol (EtOH) in water in the range of 283 to 313 K, using electrical conductivity principle have been studied. The conductance data were analyzed according to Kraus–Bray and Shedlovsky models of conductivity. The limiting molar conductance λom, association constant Ka, energy of activation of the rate process (Ea), and related thermodynamic parameters have been determined. Using viscosity of the solvent, Walden product (λomη0) and Stokes molecular radius have been determined. Standard thermodynamic parameters of association (ΔGa, ΔHa,) were calculated and discussed. The limiting molar conductance sharply decreased for the increased amount of ethanol suggesting the increased ion–solvent and solvent–solvent interactions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The electrical conductivity study of electrolytes in mixed solvents is known to be influenced by a number of factors like viscosity, temperature, relative permittivity, hydrogen bonding, ion-solvation and solvent–solvent interactions [1–8]. Literature reveals that nature of the ion–solvent interactions and ion–ion interactions or the behavior of the electrolytes in a solvent of different relative permittivity can be informative depending on the conductance and ionic mobility of these electrolytes in solutions [9–14]. Water+EtOH mixtures at different temperatures exhibit a wide range of relative permittivity (ε), viscosity (η) and a high degree of hydrogen bonding effect. Varied interaction between water and ethanol because of mixing them at different quantities may be studied by the measurement of conductivity over that composition range. Appropriate conductivity measurements provide useful indications of ion–solvent interaction, proton–anion and proton–solvent association, and solvent structure. In continuation with our earlier report [15] on ion association and solvation of sulfonamide drug; sulfathiazole sodium in aqueous and partial aqueous media, we report herein the solvation behavior of another sulfonamide drug sulfacetamide sodium in water + ethanol in view of evaluation of limiting molar conductance, Walden product, association constant and thermodynamic properties. Study of solvation behavior of medicinal compounds under varying biological conditions is an important field of studies which revealed much information to medicinal chemist [16]. The conductivity values may be ⁎ Corresponding author. Tel.: + 91 824 2287262. E-mail address: [email protected] (J.I. Bhat). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.03.005
useful to have an appropriate idea regarding drug diffusion and change in the nature of hydrophobic interactions of drug with the change in free energy involved while moving from water to non-aqueous solvents [17]. The sulfonamides constitute an important class of drugs and have extensively been documented for their wide variety of pharmacological activities such as antimicrobial, insulin releasing antibiotic, diuretic, ant carbonic anhydrase, anti-thyroid, hypoglycemic, anti HIV and antitumor activity. Sulfacetamide sodium is a sulfonamide [18] antibacterial and is applied in infections or injuries of the eyes. The structure of sulfacetamide sodium is given in Fig. 1. We report in this communication the electrical conductivity of sulfacetamide sodium in partial aqueous media by varying the compositions (v/v) of water + ethanol at four different temperatures. 2. Materials Sodium acetyl[(4-aminophenyl)sulfonyl]azanide (Sigma-Aldrich, USA, 99%, b 0.05% H2O) was used as such, without further purification. The purity was checked from the melting point. (Melting point obtained in the lab: (531–533) K; Literature value [18]: 530 K). The conductivity of water used in this study was obtained by deionization and triple distillation with specific conductance in the range of (1 to 3)·10− 7 S·cm− 1. Ethanol conductivity 10− 7 S− 1·cm− 1 was used. 3. Solutions Mixed solvents, ϕ2 = 0.1, ϕ2 = 0.2, ϕ2 = 0.4, and ϕ2 = 0.6 were prepared by mixing known quantities of water and EtOH in volume ratio at laboratory temperature. The calculated values of mass fractions
J.I. Bhat, M.K. Shetty / Journal of Molecular Liquids 160 (2011) 140–143
O -
However, this model does not include any correction for inter-ionic effects or for the activities of the ions. Therefore, Shedlovsky [21] relation was used to evaluate the absolute limiting molar conductance and association constant Ka.
+
S N Na C CH3 O O
H2N
141
Fig. 1. The structural formula of sodium acetyl[(4-aminophenyl)sulfonyl]azanide.
1 1 Cλm SfF2 Ka = o + Sλm λm λom 2
ð3Þ
For measurement of electrical conductivity, the digital conductivity meter (CM-180, Elico India) and immersion type cells (cell constant=0.998 cm− 1) were used. Cell constant was determined to be within 0.01% precision at various times by calibration with aqueous potassium chloride [19] solutions. The solution of known concentration was taken in a double walled vessel and kept in a thermally stabilized water bath with good thermal regulation (±0.1 °C) for approximately 20 to 30 min and its specific conductivity was noted. All conductivity measurements were repeated thrice and it was found to be reproducible to 0.1%.
Required relative permittivity (ε) and viscosity (η) values were obtained from the literature [22–24]. The f± is the mean ionic activity coefficient of the electrolyte. λom and Ka were obtained from the intercept and slopes of the plot of 1/Sλm vs CλmSf2±. The values of limiting molar conductance evaluated using Kraus–Bray and Shedlovsky models at different temperatures having uncertainty of ±0.5% are presented in Table 2. Limiting molar conductance (λom) was found to be higher in water and then decreased sharply on adding ethanol to it, despite the maximum in viscosity near ϕ2 = 0.5 ethanol. This behavior may be due to the formation of hydrogen bond between ethanol and water molecules resulting in the association of ethanol and water molecules, which in turn reduces the solvated ionic mobility and decreases the limiting molar conductance values with higher percentage of ethanol in the solvent mixture. The variation in viscosity (η) is not only the factor controlling the ionic mobility, which is evidenced by the Walden product. The limiting molar conductance of sulfacetamide sodium increased with increase in temperature from 283 K to 313 K for all volume fractions of co-solvent due to the increase in ionic mobility. Increase in thermal energy breaks more number of hydrogen bonding that of water decreasing the solvated ionic size and hence increases the mobility of the species.
5. Results and discussion
5.2. Association constant
5.1. Limiting molar conductance
The calculated values of association constant having uncertainty of ± 2% were presented in Table 3. The values of Ka of the studied system at the same temperature were found to increase as the proportions of solvent ethanol increase with water. The extent of ion association depends on the nature of ion–ion interaction in the solution, relative permittivity of the medium and also intermolecular hydrogen bonding between the solvent molecules. This suggests the fact that the values of Ka increased with the increase of EtOH in water. The association constant for the studied salt was found to decrease with the increase in temperature; increased thermal motion probably breaks more bonds between ions and makes them to move away from each other, where it gets solvated. Hence the mobility if ions should increase. This may account for the decrease in Ka for the increased temperature. To identify the association Fuoss [25] suggested the following equation
of ethanol in water at studied temperatures are given in Table 1. Solutions of sulfacetamide sodium with a concentration range of (0.001 to 0.01) mol·L− 1 were prepared in water or water + EtOH as and when needed. Taking into account the sources of error, we estimate the uncertainty in molar concentration to be within ± 5.0⋅10− 4 mol⋅L− 1. Work could not be carried out beyond ϕ2 = 0.6 and pure ethanol due to solubility problem. 4. Instrument and methods
The specific conductances of solutions of sulfacetamide sodium with a concentration range of (0.001–0.01) mol·L− 1 inϕ2 = 0.0, ϕ2 = 0.1, ϕ2 = 0.2, ϕ2 = 0.4, and ϕ2 = 0.6 EtOH in water at 283, 293, 303 and 313 K were measured. The molar conductance for all the studied systems was calculated using Eq. (1). λ=
κ C
ð1Þ
where C is the molar concentration, κ is the specific conductance of sulfacetamide sodium solution from which the specific conductance of the used solvent was subtracted. The experimental molar conductivities were analyzed as per Kraus–Bray conductivity equation [20]. 1 1 λ C = o + o m2 λm λm λm K c :
ð2Þ
The limiting molar conductances, λom and dissociation constant, Kc were obtained from the intercept and slope of the plot 1/λmvsλmC.
Table 1 Calculated values of mass fractions (W2) of ethanol in water from T = 283 K to 313 K. T/K
T/K = 283 T/K = 293 T/K = 303 T/K = 313
W2 ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
0.0988 0.0987 0.0987 0.0987
0.1963 0.1960 0.1958 0.1956
0.3889 0.3877 0.3866 0.3857
0.5790 0.5773 0.5759 0.5745
2 0 Cλm = KC λm
ð4Þ
According to Fuoss, the slope of the plot log λm vs log C is around −0.5, it is an indication of the presence of ion-pairs in equilibrium with the salt. In the present case, the slope was found to be −0.11. The higher value of the slope hints the absence or minor amount of ionpair formation. Moreover it also supports the fact that higher ion aggregates (ion–ion and triplet ion pair) from 1:1 salts cannot occur in higher permittivity media [26,27] (ε N 10). 5.3. Variation of Walden products with solvent composition If ionic mobility depends on bulk viscosity as per the Stokes' law, the Walden products [28] would appear reciprocal to the ionic radius. The variations in Walden product with solvent composition and
142
J.I. Bhat, M.K. Shetty / Journal of Molecular Liquids 160 (2011) 140–143
Table 2 Limiting molar conductance for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K. Property
ϕ2= 0.0 EtOH
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
T = 283 K λοm (Kraus–Bray) λοm (Shedlovsky)
λοm / (S⋅cm2·mol− 1) 52.07 53.47
37.87 39.13
30.16 31.15
19.88 21.27
18.31 20.74
T = 293 K λοm (Kraus–Bray) λοm (Shedlovsky)
64.51 65.78
50.50 52.35
40.65 42.46
25.38 27.00
23.69 26.32
T = 303 K λοm (Kraus–Bray) λοm (Shedlovsky)
78.12 80.00
64.51 66.60
52.35 54.00
37.59 38.90
31.54 34.48
T = 313 K λοm (Kraus–Bray) λοm (Shedlovsky)
92.59 95.20
80.00 81.86
65.79 67.06
48.78 51.94
38.16 43.41
Table 3 Association constant Ka for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K.
Table 4 Energy of activation Ea and change in enthalpy of association ΔHa for sulfacetamide sodium in water + EtOH.
T/K
ϕ2 = 0.0 EtOH
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
ϕ2 = 0.0 EtOH
T/K = 283 T/K = 293 T/K = 303 T/K = 313
8.530 8.525 8.518 8.215
8.809 8.771 8.436 8.005
9.561 9.119 8.921 8.830
12.691 11.174 11.031 9.845
32.41 30.69 29.17 27.78
Ea/(kJ·mol− 1) 13.90 15.69 ΔHa/(kJ·mol− 1) 1.98 3.03
temperature might then be explained through changes in ion solvation alone. o
λm ηο =
Z eo F 6π r
ð5Þ
where eο is the electronic charge, F is Faraday constant; and Z is the charge on the ion and r is the effective Stokes molecular or ionic radius. The most satisfactory interpretation of the Walden products is based on the structure of water–ethanol mixtures and the effect of ions on this structure. The thermodynamic, kinetic, ultrasonic and transport data indicate that addition of simple alcohols to water initially enhances the structure of the solvent water [29]. The structure enhancement (structure former) appears to reach a
ϕ2 = 0.10 EtOH
ϕ2 = 0.20 EtOH
ϕ2 = 0.40 EtOH
ϕ2 = 0.60 EtOH
19.15
20.69
21.59
3.52
5.65
8.96
maximum near ϕ2 = 0.2 to ϕ2 = 0.3 ethanol [29]. At higher volume fractions of ethanol, the water structure progressively gets reduced (structure breaker). The maxima in the Walden products near ϕ2 = 0.3 ethanol may thus be a consequence of the greater disruption of water structure in the vicinity of the ions for solvent of this composition. This viewpoint also predicts that Walden products should decrease with increase in temperature. Both this predictions are in agreement with the Walden products as depicted in Fig. 2. 5.4. Thermodynamic parameters Since, the conductance of an ion depends on its movement; it is quite reasonable to equate the conductance process with rate process [30] and is given by the equation, o
−
λm = Ae
Ea =RT
ð6Þ
λ0η0 /(S·cm2·mol-1 kg·m-1·s-1)
0.75
where Ea is energy of activation of the rate process, which is characteristic of the rate of movement of ions in solution, R is the gas constant, A is the Arrhenius factor and T is the absolute temperature. The value of Ea was calculated from the slope of the plot of log λom vs 1/T. The value of energy of activation increased with increase in composition of ethanol in water as shown in Table 4, suggesting the involvement of high
0.70 0.65 0.60 0.55
Table 5 Computed change in Gibb's energy (ΔGa) for sulfacetamide sodium in water + EtOH from T = 283 K to 313 K.
0.50 0.45
T/K
0.40 0.0
0.2
0.4
0.6
0.8
1.0
φ2 Fig. 2. Plots of Walden product λomηo/(S·cm2 mol− 1 kg·m− 1 s− 1) against volume fraction φ2 for sulfacetamide sodium in water + ethanol from T = 283 K to 313 K.
φ2 = 0.0 EtOH
-ΔGa/(kJ·mol− 1) T/K = 283 2.85 T/K = 293 2.89 T/K = 303 2.89 T/K = 313 2.95
φ2 = 0.10 EtOH
φ2 = 0.20 EtOH
φ2 = 0.40 EtOH
φ2 = 0.60 EtOH
3.38 3.33 3.38 3.40
4.04 3.98 4.02 4.03
5.09 5.07 4.92 5.00
8.33 8.33 8.32 8.26
J.I. Bhat, M.K. Shetty / Journal of Molecular Liquids 160 (2011) 140–143
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