International Journal of Chemical and Pharmaceutical Review and Research

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1 ISSN No: Int. J. Chem. Pharm. Rev. Res. Vol (1), Issue (2), 2015, Page Research Article International Journal of Chemical and Pharmaceutical Review and Research Tetrabutylammonium bromide (Bu4NBr), Sodium tetraphenylborate(nabph4) and Sodium bromide (NaBr) in Acetonitrile(1) + Water(2) mixtures at (288.15, , , , , and )K A Conductance Study Ngasepam Bhogenjit Singh and Gopal Chandra Bag* Department of Chemistry, Imphal College, Imphal , Manipur, India A R T I C L E I N F O Article history: Received 6 May 2015 Accepted 4 June 2015 Available online 14 June 2015 Keywords: Electrolytic conductance, Shedlovsky equation, ion-association, acetonitrile- water, solvation. A B S T R A C T Molar conductances of Bu4NBr, NaBPh4 and NaBr have been measured in acetonitrile(1) + water(2) mixtures containing 0, 20, 40, 60 and 80 mass% acetonitrile at temperatures ranging from K to K at an interval of 5K. The conductance data have been analyzed by Shedlovsky method to obtain Ʌ and KA values of the electrolytes. The ionic contributions to the limiting molar conductance have been estimated using tetrabutyl ammonium tetraphenyl borate (Bu4NBPh4) as the reference electrolyte. Thermodynamic quantities for the ion-association process have been derived along with Walden product. All these electrolytes are highly associated in the acetonitrile rich region of the solvent system within the studied temperature ranges. It has been found that solvated radii (ri) of Bu4N+ and Ph4B-decreases with increasing acetonitrile percentage while that of Na+ and Br- show an increase with acetonitrile concentration. Specific interactions of the ions with the solvent molecules have been found to exhibit a profound influence on their mobilities besides the effect of viscosity and permittivity of the media. IJCPRR All rights reserved. 1. Introduction Frequently, investigations were made for the binary mixture of water and some organic solvents in varying compositions.binary mixture of acetonitrile with water is also one of the commonly studied mixtures which is non-ideal and constitutes three different regions3-5. It may affect significantly the ion-pair formation and solvation of ions. Studies on the transport properties of electrolytes in different solvent media are of great importance in obtaining information as to the solvation and association behavior of ions in solutions1. The conductometric method is well suited to investigate the ion-ion and ion-solvent interactions in electrolyte solutions. From the concentration dependence of the electrolyte conductivity, different quantities strongly influenced by solvent properties can be derived. Their changes with mixed solvent composition may thus reflect the change in solvent structure and ion-solvent interactions2. Conductometric studies on a number of electrolytes have been carried out in acetonitrile(1) + water(2) mixture. There are only a few studies on Bu4NBr, NaBPh4 and NaBr in a very limited range of temperature and composition in this solvent system1. The present paper reports the molar conductivities of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate(nabph4) and sodium bromide (NaBr) in acetonitrile(1) + water(2) * Corresponding author. E mail address: gopalbag53@gmail.com Present address: Assoc. Prof., Department of Chemistry, Imphal College, Imphal , Manipur, India. 1

2 mixtures with an acetonitrile mass percentage of 0, 20, 40, 60 and 80 at seven temperatures from K to K with an interval of 5K. In our reports, we included an extended work in acetonitrile rich region and broader range of temperatures so that a clear trend can be observed. The observed data were treated in order to obtain Ʌ (limiting molar conductivity) and (ion association constant) which further enables to calculate the Walden product (Ʌ η )and other thermodynamic properties (ΔG, ΔH and ΔS ) for the process of ion association. The precise temperature-dependent single-ion conductivities were also obtained because such data are practically very less in mixed-solvent media. 2. Experimental HPLC grade acetonitrile (SRL) was used without further purification. Double distilled water of specific conductance of ~ Scm -1 at K was used throughout the experiments. Conductance measurements were carried out on Systronics-306 Conductivity Bridge using a dip-type cell of cell constant 1.0cm -1 having an accuracy of ±0.1%. Solvent mixtures were prepared by mass percent and that of electrolyte solutions for the conductance runs were prepared in molarities using a balance (Mettler Toledo, Model No. ME204) of g accuracy. All the gathered permittivity, viscosity and density values were taken from the literature 6-15 and by graphic interpolation of the reported values. They are enlisted in Table 1. All the salts were SRL AR grade. Tetrabutylammonium bromide(bu 4NBr) and sodium tetraphenylborate (NaBPh 4) were recrystallized from acetone and then dried under vacuum. Sodium bromide (NaBr) was recrystallized from water and dried in vacuum for 72 hours immediately prior to use. Measurements were made in a refrigerated water bath maintained within ±0.01K of the desired temperature. The details of the experimental procedure have been described elsewhere 16. Due correction was made for the specific conductance of the solvent at all temperatures., where D is the permittivity of the medium; η the viscosity co-efficient of the medium. The degree of dissociation (τ) is related to S(Z) by the equation, f ± is the mean activity coefficient of the free ions and was calculated using eqn. (2) where and (2) R is the maximum centre to centre distance between the ions in the ion-pair. There exists at present no precise method 17 for determining the value of R. In order to treat the data in the present system, the R value is assumed to be R =a + d, where a is the sum of crystallographic radii of the ions and d is the average distance corresponding to the side of a cell occupied by a solvent molecule. The distance d is given by 18 d = (M/ρ) 1/3 Å where M is the molecular mass of the solvent and ρ is the density of the solution. For mixed solvents M is replaced by mole fraction average molecular weight. 3. Results and Discussion 3.1 Evaluation of Λ 0, and Λ 0η o The measured conductance data of all the electrolytes have been analyzed by using Shedlovsky equation: (1) Where Ʌ is the molar conductance at a concentration C mol.dm -3, Ʌ o, the limiting molar conductance and, the observed association constant. The other symbols are given by x 1 is the mole fraction of acetonitrile of molecular weight M 1 and x 2 that of water of molecular weight M 2.The values of association constant () and the limiting molar conductance (Ʌ o) have been calculated using eqn. 1 by an iterative procedure 19. All the calculations have been carried out on IBM PC. The results of Ʌ 0 and for NaBr, NaBPh 4 and Bu 4NBr at different acetonitrile-water compositions and temperatures are collected in Tables

3 Table 1: Relative permittivity, density and viscosity of water-acetonitrile mixture at different acetonitrile mass percent and temperatures. %ACN : K c c c c c f e e e e g b b b b K c c c c c f a a g b b b b K c c c c c f d d d d g d d d d K c c c c c f a a g b b b b K c c c c c f h e e e g b b b b K c c c c c f j j i i g b b b b K c c c c c f h e e e g b b b b a - calculated values, b reference 1, c reference 2, d reference 3, e reference 6, f reference 14, g reference 15, h reference 16, i reference 4, j reference 7 3

4 Table 2: Λ o Scm 2 mol -1 and dm 3 mol -1 values of NaBr at different acetonitrile- water compositions and temperatures. %ACN T/K Table 3: Λ Scm 2 mol -1 and KA dm 3 mol -1 values of Ph4BNa at different acetonitrile-water compositions and temperatures. T/ K %ACN

5 Table 4: Λ Scm 2 mol -1 and KA dm 3 mol -1 values of Bu4NBr at different acetonitrile-water compositions and temperatures. %ACN T/K The variation of Ʌ 0 with temperature (Fig. 1) and solvent composition is similar for Ph 4BNa and Bu 4NBr. Throughout the composition ranges, trends in Ʌ 0 values show a minimum in the 20%acetonitrile region. However for NaBr, the decrease is up to 40% in ( )K temperature range and above K, it decreases further to 60% and then increases. Figure 1: Plot of Ʌ vs T for NaBr, Ph4BNa and Bu4NBr at 40% acetonitrile-water mixture Ph4BNa NaBr and Bu4NBr. These trends clearly depict the dependence of Ʌ o of electrolytes to the viscosity of the solvents. Viscosities of acetonitrile(1) + water(2) show a maxima in 20% acetonitrile (Table 1) region, the effect of which is manifested in the molar conductance values of all the electrolytes where a drop in the Ʌ o values is observed in this particular region. Ʌ o values of all the electrolytes increase with increasing temperature which is attributed to the decrease in viscosities of the solvent mixture. A comparison of Ʌ o values of all the electrolytes show that Ʌo of NaBr>Bu 4NBr>NaBPh 4. This trend can be explained based on the differences in the ionic mobilities of the ions contributing to the total conductivity of the respective electrolytes. In most of the composition and temperature range, the association constant () values follow the order: NaBr>NaBPh 4 Bu 4NBr. Minimum association occurs in aqueous medium for all the electrolytes and increases with increasing acetonitrile percentage and temperature. The association constant for Bu 4NBr in pure water is practically negligible (i.e. < 10) at all the temperatures and hence the trends in their values is not important 20. In water, it almost exists as free ions 5

6 in the studied range of temperatures. Ghosh and Das 1 also reported a very low value of of all the electrolytes in two compositions (viz & 0.40 volume fractions of acetonitrile in water) at (308.15, & )K. Table 5: Calculated λ Scm 2 mol -1 values of,, and ions at different acetonitrile-water compositions and temperatures. %ACN : K K K K K K K

7 Table 6: Calculated ri Å values of,, and ions at different acetonitrile-water composition and temperatures. %ACN : K K K K K K K 7

8 Table 7: Thermodynamic parameters for the association of Bu4NBr in different acetonitrile-water composition and at different temperatures. %ACN T/K Comparable large sizes and similar charge distribution of Bu 4N + and Ph 4B - lead to comparable extent of association, which are lower than NaBr in the corresponding temperature and composition of the solvent. It may be attributed to the lower charge densities of Bu 4N + and Ph 4B - owing to their large sizes as compared to the ions of NaBr. The increasing association with increasing temperature and acetonitrile percentage in the medium is well explained by the decrease in the dielectric constant of the medium. For NaBPh 4 and Bu 4NBr,the decreasing compatibility of solvent molecules (acetonitrile) to interact with the large Bu 4N + and Ph 4B - ions with increasing acetonitrile percentage, leading to strengthening of short-range attraction between the counterions and thereby contributing to the increase in the association constant is worth mentioning here 22. 8

9 3.2 Limiting ionic molar conductance (λ i) and effective ionic radius (r i) Limiting ionic molar conductivities of all the ions were determined by resolving the experimentally determined Ʌ values for corresponding electrolytes on the basis of Bu 4NBPh 4 assumption 1 using the following relationships Ʌ (Bu 4NBPh 4) = λ (Bu 4N + )+ λ (Ph 4B - ) (3) (4) using the salt tetrabutylammonium tetraphenylborate (Bu 4NBPh 4) as the reference electrolyte and 5.35Å, 5.00Å are the crystallographic radii of Ph 4B - and Bu 4N + ion, respectively. The Ʌ values of Bu 4NBPh 4 have been obtained by combining those of NaBr, NaBPh 4 and Bu 4NBr using Kohlrausch s additive rule: Ʌ (Bu4NBPh4)= Ʌ (Bu4NBr) +Ʌ (NaBPh4) Ʌ (NaBr)--(5) The calculated λ values for Bu 4N +, Ph 4B -, and at various acetonitrile-water compositions and temperatures are reported in Table5. In mixed solvent media, the limiting ionic equivalent conductances decrease in the order: λ Br- >λ Na+>λ Ph4B- λ Bu4N+ at each temperature indicating that sizes of ions as they exist in solutions follow the order < < Ph 4B - Bu 4N +. It is observed that the limiting molar ionic conductances of and decrease from 0% (pure water) to 40%acetonitrile up to K while it decreases up to 60% acetonitrile at higher temperatures. Whereas, for Ph 4B - and Bu 4N + ions, a decrease in individual ion conductance occurs up to 20%acetonitrile and then increases with increasing acetonitrile percentage in all the solvent compositions and at all the experimental temperatures. A similar trend had been reported earlier 1. The trend indicates the influence of specific interaction of the ions with the solvent media on their mobilities. The corresponding effective ionic radii (r) have been calculated using Stokes 23 law radius.the trends show that Bu 4N +, Ph 4B - and ions are solvated both in aqueous medium and in solvent mixture throughout the whole composition range, though the extent of solvation and its variation with percentage of acetonitrile are different for different ions. For Bu 4N + and Ph 4B -, the extent of solvation decreases with increasing acetonitrile percentage which reflects the weak interaction of these ions with acetonitrile which was mentioned earlier. is more solvated in aqueous medium than in 20% acetonitrile-water mixture; the extent of solvation further increases with increase in acetonitrile percentage. Water being a stronger solvator of than acetonitrile 24 the decreasing concentration of water in 20% acetonitrile decreases the solvation of the ion leading to a decrease in the effective ionic radii. Protophobic nature, capability to behave as a ligand by acetonitrile 25 altogether with comparatively small size of with respect to Bu 4N + results in increasing interaction between and acetonitrile molecules. The lower r i values of than the corresponding crystallographic radii may be explained by the effect of polarization of the ion by which it loses its spherical shape, the extent of which decreases with increasing acetonitrile content 22,25. The ion-solvent interactions can be studied by analyzing the variation of Walden product (Ʌ η ) with solvent composition which indicates the change in total solvation 26. The variation of the Walden product (Ʌ η ) with acetonitrile percentage at K for all the electrolytes is shown in Fig. 2. As the fraction of acetonitrile increases, smaller ions are solvated stronger but the differences become smaller. (6) where r is expressed in Å, η o in poise and λ o in S cm 2 equiv -1. The Stokes radii of,bu 4N +, Ph 4B - and at different temperatures and compositions are listed in Table6.The sizes of the ions are greater than their corresponding crystallographic radii except for the bromide ion where it is less than its crystallographic Figure 2: Plot of λ η vs acetonitrile mass percent for NaBr, Ph4BNa and Bu4NBr at 30 o C NaBr, Bu4NBr and Ph4BNa. 9

10 Table 8: Thermodynamic parameters for the association of Ph4BNa in different acetonitrile-water composition and at different temperatures. %ACN T/K Table 9: Thermodynamic parameters for the association of NaBr in different acetonitrile-water composition and at different temperatures. %ACN T/K

11 Evaluation of thermodynamic parameters The free energy change (ΔG o ) for the association process is calculated from the relation ΔG o = -RTln (7) The heat of association (ΔH o ) is obtained from the slope of the plot of log vs 1/T(Fig. 3) and the entropy change ΔS o is then calculated from the Gibbs-Helmholtz equation, ΔG o =ΔH o -TΔS o (8) Figure 3: Plot of logkavs 1/T for NaBr, Ph4BNa and Bu4NBr at 60% acetonitrile-water mixture NaBr, Bu4NBr and Ph4BNa. The values of these thermodynamic functions for the association of Bu 4NBr, Ph 4BNa and NaBr are given in Tables 7, 8 and 9, respectively. It is expected that more strongly solvated ions will be less inclined to association. This is confirmed by the order of the ΔG o values: Bu 4NBr <NaBPh 4< NaBr. All the values are uniformly descending with increasing acetonitrile content, indicating a greater degree of association at lower relative permittivity. An observation of ΔH o values for all the salts show that only a significant entropy gain can make an endothermic reaction spontaneous. In this case, all the possible structuremaking processes accompanying the ionic association are more than compensated for by weakening of the solvation shells. The positive ΔS values of all the salts make the (-TΔS o ) term greater than the ΔH o term in eqn (8), making the process of ion-association favourable. 4. Conclusion In mixed solvent media, the limiting ionic equivalent conductances decreases in the order: λ Br- >λ Na+>λ Bu4N+ λ Ph4B- at each temperature indicating that sizes of ions as they exist in solutions follow the order: < <Bu 4N + Ph 4B -. The values of the Stokes radii also follow the same order. Another interesting observation is that the limiting ionic equivalent conductances of Bu 4N + and Ph 4B - ions first decrease up to 20% ACN and then increase with increasing % of ACN. The λ values for and ions decrease from 20% to 40% acetonitrile mixture and then increase at all temperatures. This indicates that besides the relative permittivity and viscosity of the media the specific interaction of the ions with the solvent media has a profound influence on the mobilities of the ions. 11

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