87

Chapter 4 Part-B: Density and Vapor Pressure

Osmometry Studies of Aqueous Solutions of N-Butyl-Pyridinium Bromide at

298.15 K

87

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

88

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 00C, 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 00C.

Furthermore, the evidence for the rate of change of activity with temperature is

conflicting, making uncertain the extrapolation of properties measured at 00C, 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.

89

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 [230-

235] 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 [236-251].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

90

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.

91

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 103 x (d – d0) against molality for

aqueous NaCl and sucrose solutions at 298.15 K are shown in figure 4(B).2.1 and

4(B).2.2.

4(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 ±5 × 10-3

kg.m-3

.

92

Table 4(B).1: Density data of aqueous NaCl and Sucrose solutions at 298.15K

NaCl Sucrose

Molality

(m)

1000*Δ ρ

(gm/cm3)

Molality

(m)

1000*Δ ρ

(gm/cm3)

0.00000 0.000 0.00000 0.000

0.00515 0.201 0.01173 1.518

0.00700 0.259 0.01856 2.409

0.01006 0.370 0.02367 3.076

0.01960 0.724 0.02954 3.826

0.02630 1.000 0.03573 4.643

0.03979 1.588 0.04134 5.361

0.05139 2.066 0.04663 6.027

0.06219 2.509 0.05024 6.478

0.08090 3.342 0.10058 12.907

0.10366 4.231 0.20227 25.395

0.11951 4.889 0.30320 37.644

0.15118 6.177 0.40137 48.260

0.20123 8.201 0.50130 59.028

0.25065 10.200 0.80320 89.780

0.30245 12.255 1.00007 106.499

0.35221 14.232 1.25043 127.502

0.40100 16.092

0.44960 18.091

0.50685 20.311

0.55237 22.137

0.59971 24.053

0.65466 26.151

0.70523 28.147

0.71871 28.912

93

Figure 4(B).2.1: Comparison of density data of aqueous NaCl solutions with

literature at 298.15 K

Figure 4(B).2.2: Comparison of density data of aqueous Sucrose solutions

with literature at 298.15 K

0

5

10

15

20

25

30

35

40

45

0 0.2 0.4 0.6 0.8 1

10

3 ∙

(d-d

o)

/ g

∙cm

-3

m / mol∙kg-1

This Work

Literature

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1 1.2 1.4

10

3 ∙ (

d-d

o)

/ g

∙cm

-3

m / mol∙kg-1

This Work

Literature

94

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

95

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

250C and head at 27

0C) 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, sucrose-

octaacetate 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 298.15±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/ Kcalib.

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- 7000 vapor pressure osmometer is a commercial instrument and it works above the

96

ambient temperature, one cannot do the measurements at 298.15 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 170C., so that measurements

could be made at 250C (298.15 K). The osmotic coefficient values (ϕi) for different

solutions were mole using the equations:

∑

Where i is the total number of species (= 2 for electrolyte) and mi is the molality of

the salt.

The accuracy in osmotic coefficient (ϕ) measurements was found to be better

than ± 1 x 10-3

. 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 298.15 K

NaCl Sucrose

m1/2

/mol.kg-1

Osmotic Coefficient

(ϕ)

m1/2

/mol.kg-1

Osmotic Coefficient

(ϕ)

0.00000 1.00000 0.00000 1.00000

0.16217 0.95704 0.05024 1.00199

0.19947 0.94886 0.10058 1.00776

0.22669 0.93986 0.20227 1.01568

0.28443 0.93758 0.30320 1.02759

0.32196 0.93188 0.40137 1.03048

0.38882 0.92695 0.50130 1.03812

0.44859 0.92290

0.50065 0.92141

0.54995 0.91992

0.59325 0.91908

0.63325 0.91862

97

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

98

Figure 4(B).3.2: Comparison of osmotic coefficient of aqueous solutions of Sucrose and NaCl at 298.15 K

1

1.01

1.02

1.03

1.04

1.05

1.06

0 0.2 0.4 0.6 0.8

Osm

oti

c C

oef

fici

ent

m/mol kg-1

Literature

Experimental

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

0 0.2 0.4 0.6 0.8 1

Osm

oti

c C

oef

fici

ent

m1/2

99

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 (Kcalib).The Kcalib 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 (1)

The osmolality of the sample solution can be calculated with the following equation:

Osmolality = Measurement Value/Kcalib (2)

The osmotic pressure (π) is calculated by following equation:

π(atm) = Osmolality (mOsmol∙kg‐1)∙0.082056 (L∙atm∙K‐1∙mol‐1)∙298.15(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)∙82.056 (cm3∙atm∙K‐1∙mol‐

1)∙298.15 (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 283.53 g∙mol-1

. The theoretical molecular weight of the

compound is 216.12 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

100

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 298.15 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:

d

M

mdd

ddV

2

0

0 )(1000

(4)

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 M2 is the molar mass

of the solute (kg∙mol-1

).

TheV data can also be expressed as [257]

cBcA VVVV 21

0 (5)

Where 0

V is apparent molar volume of the salt at infinite dilution, AV is Debye-

Hückel limiting law coefficient (1.868 for 1:1 electrolyte solutions at 298.15 K), BV 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 298.15 K are shown in Figure 4(B). 6. When (V – 1.868c

1/2)

101

values extrapolated to infinite dilution, yield limiting apparent molar volumes ( 0

V ) of

ionic salt.

Table 4(B).3: Molality (m), Density (d), Apparent Molar Volume , for Aqueous

Solutions of [Bpy][Br] at 298.15 K

m

/

d

/

103 /

mm3 mol

-1

0.00000 0997.043 162.03*

0.01984 0998.117 162.14

0.02735 0998.523 162.07

0.04333 0999.388 161.93

0.05223 0999.870 161.85

0.06294 1000.450 161.76

0.07738 1001.231 161.63

0.09862 1002.381 161.45

0.12224 1003.659 161.24

0.16454 1005.949 160.88

0.23438 1009.729 160.27

0.31217 1013.394 159.61

0.39501 1018.424 158.90

*Extrapolated value at infinitely dilute solution of [Bpy][Br] at 298.15 K

102

Figure 4(B).3.3: The plot of parameter ( /cRT) against concentration (gm∙cc-1

) of

[Bpy][Br] in aqueous [Bpy][Br] solutions at 298.15 K

Figure 4(B).3.4: Variation of (V - 1.868c1/2

) as a function of concentration

(c/mol∙dm-3

) of [Bpy][Br] in aqueous solutions at 298.15 K.

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0 0.02 0.04 0.06 0.08 0.1 0.12

π/c

RT

c / gm∙cc-1

150

155

160

165

170

0 0.1 0.2 0.3 0.4

10

3 ∙ (

V -

1.8

68

c1/2

) /

mm

3m

ol-1

c / moldm-3

103

Figure 4(B).5: FT-IR spectrum of n-butyl-pyridinium bromide

5007501000125015001750200025003000350040001/cm

0

7.5

15

22.5

30

37.5

45

52.5

%T

3454

.62

3215

.44

2935

.76

2870

.17

2717

.79

2416

.89

2052

.33

1882

.59

1631

.83

1579

.75

1485

.24

1381

.08

1321

.28

1215

.19

1172

.76

1066

.67

952.

87

887.

28

771.

55 734.

90

684.

75

solid

N+

CH3 Br-