Chapter 4 Part-B: Density and Vapor Pressure Osmometry Studies of Aqueous Solutions of N-Butyl-Pyridinium Bromide at K

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1 Chapter 4 Part-B: Density and Vapor Pressure Osmometry Studies of Aqueous Solutions of N-Butyl-Pyridinium Bromide at K 87

2 4 (B).1.Introduction and Literature survey In the first half of the past century several phenomenon were discovered which are associated with the passage of electric current through a salt solution. The most important discoveries in the field were made by Michael Faraday [227]. He made a thorough study of electrolysis and classifieds substances into electrolytes whose solutions conduct electricity and non-electrolytes whose solution does not conduct electric current. Aqueous solutions of electrolytes have a number of properties that distinguish them from solutions of non-electrolytes. They have high osmotic pressure, higher boiling points and lower freezing points, they conduct electric current, etc. these specific properties can be explained only if we suppose that the molecules of electrolyte fully or partly separate into their constituent parts, ions. As reacting particles, ions take part in dissociation, solvation, ionic sublimation, in electrochemical, oxidation-reduction processes, etc. In other words, along with atoms, molecules and radicals, ions are fundamental structural units of substances. In our everyday life we deal with solutions whose concentrations vary within a very wide range, from very dilute to saturated and supersaturated solutions. Electrolytes solutions are characterized by various properties depending on the concentration, significant qualitative changes are often observed when passing from one region concentration to another. Dilute solutions having an infinitely small concentration of the solute acquire the properties of an ideal solution. Here the dissociation degree is unity and we deal with solutions in which ions play the role of the particles. In addition to ions, solutions of high and medium concentrations can also form molecules, associates, ion pairs, etc., which give new properties to the solution. The situation is even more complicate with unstable supersaturated solutions. The main difficulty in describing the properties of electrolyte solutions is 87

3 that it is impossible to establish the boundaries of concentration and the ranges in which the changes in properties obeys one law. As a rule, the properties of dilute solutions are divided into two groups. One comprises the properties which, for a given solvent, are independent of the nature of the solute. These are the saturated vapour pressure of the solvent over the solution, elevation of the boiling point and lowering of the freezing point of the solution (compared with the solvent), the osmotic pressure, and others. The laws that govern their changes, associated with the variation of the solute concentration, have become the foundation of the physical theory of solution. The classical trio of colligative properties, of which boiling-point elevation and freezing-point depression are the first two members, is completed by the phenomenon of osmotic pressure. In the course of investigating the properties of aqueous solutions of electrolyte in this laboratory, it has become increasingly apparent that reasonably precise thermodynamic data on these solutions are of fundamental importance. At 0 0 C, the determination of the freezing-point lowering is a convenient and accurate method. Unfortunately, many compounds of interest both from a theoretical and from a practical standpoint are quite insoluble in water at 0 0 C. Furthermore, the evidence for the rate of change of activity with temperature is conflicting, making uncertain the extrapolation of properties measured at 0 0 C, to room and higher temperatures [228]. On the other hand the range and scope of applications of vapour pressure osmometry over a large number of aqueous and non-aqueous solutions of electrolytes including salts of drug molecules and non-electrolytes molecules including enzymes and proteins have been investigated and documented by researchers in the field of physical chemistry chemical physics. 88

4 The thermoelectric differential vapour pressure method was first described by Hill, but it was not widely used until the development of convenient means of measuring small temperature differences became available. Brady, Huff, and McBain used thermistors in an apparatus of this type and then proceeded to determine solution properties of surface active solution. Several subsequent workers have built similar devices using thermistors, but the major application has been in the determination of molecular weight Since the observed temperature change in any instrument based on this principle is determined by several mass transfer processes, the significance of the results is uncertain until experimental evidence is provided to show that a definite correlation exists between the observations and thermodynamic solution properties [229]. Although others have made use of the thermoelectric differential vapor pressure method to study solution properties, Prof. K. J. Patil and his colleagues [ ] have extended this to a broader class of electrolytes and have now shown that it is a valid and useful technique for applications to most aqueous and non-aqueous solutions. To date, a number of researchers have studied the thermo-physical properties of aqueous solutions of ILs systems [ ].We were confronted with a problem of hydrophilicity of [bpy][br]. In literature a research article by Crosthwaiteet al. [186] provides an excellent account of the thermo-physical properties like melting temperatures, freezing temperature, glass transition temperatures, etc., of some pyridinium-based ILs. Very surprisingly [bpy][br] salt shows melting temperature 378K and freezing temperature 315K. Even in the mass spectral analysis of [bpy][br], it is not possible to predict the presence water in the compound because the technique deals with high vacuum to remove solvent molecules, and possibly the water attached to [bpy][br] should be removed in this 89

5 region. Therefore we decided to measure the exact molecular weight by performing osmometry. The results obtained are supplemented by FT-IR, TGA and KF titrimetry. The application of vapor pressure osmometry to the determination of molecular weight of [bpy][br] has been thus investigated. A discussion on the results and the structural formula of [bpy][br] is given in following pages. It was also necessary to measure densities of aqueous solutions to convert concentration scales appropriately i.e. to mole fraction, molarity and g.cm -3 etc. 4(B).2 Chemicals Used In the present work, all the solutions were prepared on a molality basis using doubly-distilled water and converted to molarity scale whenever required with the help of density data. The salt NaCl and sucrose were of AR grade and dried under vacuum at 393 K for 24 h before use. These were required for calibration purposes. Synthesized N-butyl-pyridinium bromide was purified and dried in vaccum oven before use. 4(B).2.1 Density Measurements Densities of all the solutions were determined by a high precision ANTON PAAR digital densitometer (Model: DMA 5000). This method is the most accurate and convenient for the density measurements of liquids. The method involves measurement of natural vibrational frequency of the oscillating tube containing liquid under investigation. The natural vibrational frequency of the tube is related to the density of the liquid inside it by --- (1) Where d is the density of the liquid, A and B are the instrumental constants and τ is the oscillation period of the tube. 90

6 The used digital densitometer with the oscillator excitation, amplitude control, and time lapse measuring and data reception circuitry which was described in detail by Leopold [252]. There is a software installed in the instrument by which with the help of calibrating fluids, the constants A and B are obtained and can be used for measuring the density of unknown liquid or solution at a given temperature. A syringe holder is used to hold the syringe while it is being deployed for filling the sample tube. While loading the vibrating tube with liquid sample, the precaution is taken that the introduction of the liquid must be made slowly enough to enable the sample liquid to properly wet the walls of the sample tube. The meniscus of the liquid while filling the sample tube must be concave and not convex, to avoid the trapping of micro air bubbles so that the period readings obtained are stable. The sample tube is completely filled when the liquid meniscus passes the upper enlarged portion of the sample tube. The opening of the upper part of the sample tube is to be closed off with Teflon stopper and the illumination is to be turned off. Once the illumination of the sample is turned off, the tube starts oscillating and the reading τ (tau) will be displayed on the display unit after the interval of selected time period (k). As a check of authenticity of the density measurements, the measurements for aqueous NaCl and sucrose solutions were carried out and compared with the best literature data [253, 254] reported. The plot of 10 3 x (d d 0 ) against molality for aqueous NaCl and sucrose solutions at K are shown in figure 4(B).2.1 and 4(B) (B).2.2 Accuracy Accuracy of the instrument is depends on the temperature control since the A and B constants are temperature dependent. The accuracy of density measurements in the present work was estimated to be of the order of ± kg.m

7 Table 4(B).1: Density data of aqueous NaCl and Sucrose solutions at K NaCl Molality 1000*Δ ρ (m) (gm/cm3) Sucrose Molality 1000*Δ ρ (m) (gm/cm3)

8 10 3 (d-do) / g cm This Work Literature m / mol kg -1 Figure 4(B).2.1: Comparison of density data of aqueous NaCl solutions with literature at K 10 3 (d-do) / g cm This Work Literature m / mol kg -1 Figure 4(B).2.2: Comparison of density data of aqueous Sucrose solutions with literature at K 93

9 4(B).3 Osmotic Vapor Pressure Measurements Vapor pressure is not measured directly due to difficulties in sensitivity, but is measured indirectly by using thermistors to measure voltage changes caused by changes in temperature. In the present work we have used the KNAUER K-7000 Vapor Pressure Osmometer (K-7000) and the complete experimental set-up is shown in figure 4(B).3.1. In this model, two thermistors are placed in measuring chamber with their glass enclosed sensitive bead elements pointed up. The thermistors are covered with pieces of fine platinum screen to ensure a constant volume of the drop of the analyte, which is present on the bead for each measurement. The chamber contains the reservoir of solvent and wicks (the wick provides a large surface area for the solvent and its transfer to the atmosphere inside the chamber) to provide a saturated solvent atmosphere around the thermistors. If pure solvent on one thermistor is replaced by a solution, condensation of solvent into solution from the saturated solvent atmosphere will proceed. Solvent condensation releases heat so the thermistor will be warmed. Condensation will continue until the thermistor temperature raises enough to bring the solvent vapor pressure of solution up to that of pure solvent at the surrounding chamber temperature. 4(B).3.1 Operational Procedures The first step in preparing for a Vapor Pressure Osmometry run is to clean the chamber assembly. The chamber was removed from the oven, revealing the baffles, wicks and thermistors. The platinum gauze covering of the thermistors were carefully removed and placed in a solvent, from which the new samples will be prepared, which causes the wetting of the thermistor probes with the solvent. The uncovered thermistors were fully rinsed with the solvent and new wicks placed in the assembly. The chamber was reassembled with the gauze in place and 20 ml of solvent poured 94

10 into centre cylinder over the thermistors. The injection syringe were then removed, cleaned with solvent, loaded with appropriate solutions or solvent and returned to the apparatus. The temperature control was set to the appropriate number (the cell at 25 0 C and head at 27 0 C) and the instrument was allowed to reach the operating temperature. This was usually started before the cleaning and allowed to warm and equilibrate for at least one hour. 4(B).3.2 Calibration of Vapor Pressure Osmometer Using the material of known molecular weight, one can do the calibration of the vapor pressure osmometer. Requirements for standards are vapor pressure no more than 0.1 % of the solvent, high purity, complete solubility. Sucrose, mannitol and NaCl are excellent standards for aqueous solutions while benzil, sucroseoctaacetate are good for organic solvents. In the present work, we have used aqueous NaCl and sucrose solutions of known osmolality for the calibration and hence determined the instrumental constant, K, with the help of which the osmolality of aqueous solutions of various samples were determined and further used for estimation of practical osmotic coefficient values. The operating temperature for these measurements was ±0.001 K. The calibration constant K is represented by the slope of the regression curve (measurement value as a function of osmolality of aqueous NaCl solutions) passing through origin and is represented by the equation. Kcalib = Measurement value/known Osmolality The osmolality of the sample solution has been calculated with Osmolality = Measurement Value/ K calib. At the time of each measurement, at least 5-6 readings were taken for the fixed time settings and the averaged value was used for further processing. Since KNAUER K vapor pressure osmometer is a commercial instrument and it works above the 95

11 ambient temperature, one cannot do the measurements at K or below this. Thus it was necessary to have a proper cooling assembly so that the surrounding of the instrument is kept colder than the working temperature. Fortunately we had air conditioned lab of which temperature was maintained at 17 0 C., so that measurements could be made at 25 0 C ( K). The osmotic coefficient values (ϕ i ) for different solutions were mole using the equations: Where i is the total number of species ( = 2 for electrolyte) and m i is the molality of the salt. The accuracy in osmotic coefficient (ϕ) measurements was found to be better than ± 1 x Comparison of the osmotic coefficient NaCl and sucrose in water obtained in this work with the literature data [255] is shown in figure 4(B).3.2, which shows the authenticity of the osmotic vapor pressure measurements done in this work. Table 4(B).2: VPO data of aqueous solutions of NaCl and Sucrose at K NaCl Sucrose m 1/2 /mol.kg -1 Osmotic Coefficient (ϕ) m 1/2 /mol.kg -1 Osmotic Coefficient (ϕ)

12 Figure 4(B).3.1: Knauer K-7000 vapor pressure osmometer (K-7000 vpo) and elements and design of measuring cell 97

13 Osmotic Coefficient Osmotic Coefficient Literature Experimental m/mol kg m 1/2 Figure 4(B).3.2: Comparison of osmotic coefficient of aqueous solutions of Sucrose and NaCl at K 98

14 4(B).4.Results and Discussion Molecular Weight determination of [Bpy][Br] by Vapour Pressure Osmometry. We have used aqueous NaCl solutions of known osmolality for the calibration and hence determined the instrumental constant (K calib ).The K calib is represented by the slope of the regression curve (measurement value as a function of osmolality of aqueous NaCl solutions) passing through origin and is represented by the equation: K calib = Measurement value/known Osmolality (1) The osmolality of the sample solution can be calculated with the following equation: Osmolality = Measurement Value/K calib (2) The osmotic pressure (π) is calculated by following equation: π(atm) = Osmolality (mosmol kg 1 ) (L atm K 1 mol 1 ) (K) (3) The concentration c in (g cm 3 ) is calculated with the help of density and weight fraction data. The values of parameter (π/crt) are estimated with help of following equation: π/crt (mol g 1 ) = π (atm)/[c (g cm 3 ) (cm 3 atm K 1 mol 1 ) (K)] In Figure 4(B).5, parameter π/crt is plotted as a function of concentration (cin g cm 3 ) for the studied compound. The intercept of the said plot yielded the value of reciprocal of molecular weight while the slope value gave the measure of osmotic second virial coefficient. The intercept of the plot reveals that the molecular weight of studied compound is g mol -1. The theoretical molecular weight of the compound is g mol -1. Therefore, we concluded that the salt contains four water molecules as water of hydration. This conclusion is also supported by our KF titrimetry and TGA analysis [256].In FT-IR analysis of [bpy][br] (Figure 4(B).7) a broad band found in the spectrum indicates the presence of water, as well KF titrimetry (0.3%) and TGA profile (30%) (Please refer fig. chapter 3.1) also reveal 99

15 water and in conformity with osmotic pressure measurements. These results indicate that the salt is a hydrate containing 4 water molecules, which cannot be easily removed simply by heating. It is the main reason for the difference in melting point and freezing point of this compound. Probably, the IL is in a gel like state between freezing point and melting point region, having a glass transition temperature in between. The calculated molecular weight was used to correct the molalities and collected in Table 4(B).3. The density (d) data as a function of concentration of ionic salt molecule in aqueous solutions at K are reported in Table 4(B).3. The apparent molar volume ( V ) as a function of molality of the drug molecules were calculated by using the following equation: 1000( d0 d) M 2 V (4) mdd d 0 Where m is molality of ionic salt molecules in aqueous solution (mol kg -1 ), d and do are the densities of solution and solvent respectively in kg m -3 and M 2 is the molar mass of the solute (kg mol -1 ). The V data can also be expressed as [257] 0 Where V 0 2 A c B c (5) V V V 1 V is apparent molar volume of the salt at infinite dilution, A V is Debye- Hückel limiting law coefficient (1.868 for 1:1 electrolyte solutions at K), B V is deviation parameter and c is the concentration of the salt on molarity scale. The variation of ( V 1.868c 1/2 ) as a function of concentration of ionic salt(c/mol dm -3 ) in aqueous solutions at K are shown in Figure 4(B). 6. When ( V 1.868c 1/2 ) 100

16 0 values extrapolated to infinite dilution, yield limiting apparent molar volumes ( V ) of ionic salt. Table 4(B).3: Molality (m), Density (d), Apparent Molar Volume, for Aqueous Solutions of [Bpy][Br] at K / m d 10 3 / / mm 3 mol * * Extrapolated value at infinitely dilute solution of [Bpy][Br] at K 101

17 π/crt c / gm cc -1 Figure 4(B).3.3: The plot of parameter ( /crt) against concentration (gm cc -1 ) of 170 [Bpy][Br] in aqueous [Bpy][Br] solutions at K 10 3 ( V c 1/2 ) / mm 3 mol c / mol dm -3 Figure 4(B).3.4: Variation of ( V c 1/2 ) as a function of concentration (c/mol dm -3 ) of [Bpy][Br] in aqueous solutions at K. 102

18 %T CH N Br solid Figure 4(B).5: FT-IR spectrum of n-butyl-pyridinium bromide /cm 103

Soluble: A solute that dissolves in a specific solvent. Insoluble: A solute that will not dissolve in a specific solvent. "Like Dissolves Like"

Soluble: A solute that dissolves in a specific solvent. Insoluble: A solute that will not dissolve in a specific solvent. Like Dissolves Like Solutions Homogeneous Mixtures Solutions: Mixtures that contain two or more substances called the solute and the solvent where the solute dissolves in the solvent so the solute and solvent are not distinguishable