Partial molar volumes of aluminium chloride, aluminium sulphate and aluminium nitrate in water-rich binary aqueous mixtures of tetrahydrofuran

Introduction

Partial molar volume is one of the important thermodynamic property which has been proved a very useful tool in determining the various types of interactions like ion-ion, ion – solvent and solvent –solvent interactions occurring in aqueous as well as non-aqueous solutions^1-11.These interactions are very helpful in determining the structure and properties of the solutions. On the addition of an organic solvent to water, it brings a change in the solvation of ions. Recently, appreciable work has been done on thermodynamic properties of various electrolytes in different binary aqueous mixtures but studies of aluminium salts in binary aqueous mixtures of tetrahydrofuran are still missing. Hence, the present study seeks an attempt to understand the interactions of aluminium salts in water and THF+ water system.

Table 1: partial Molar Volumes (ΦVo) And Experimental Slopes (Sv*) For Aluminium Sulphate, Aluminium Nitrate And Aluminium Chloride In Different Compositions of Thf+Water At 303.15 K Temperature

Composition of                                                                             THF- Water(wt.% )	ALUMINIUM SULPHATE			ALUMINIUM  NITRATE			ALUMINIUM CHLORIDE
	Φvo (cm3 mol-1)	Sv* (cm3 lt1/2 mol-3/2)	∆V0tr ( cm3 mol-1)	Φvo (cm3 mol-1)	Sv* (cm3 lt1/2 mol-3/2)	∆V0tr (cm3 mol-1)	Φvo (cm3 mol-1)	Sv* (cm3 lt1/2 mol-3/2)	∆V0tr ( cm3 mol-1)
0	599.83	-2.572	–	307.14	-2.668	–	166.08	-1.948	–
5	610.80	-3.637	10.97	316.22	-3.139	09.08	179.10	-2.398	13.02
10	617.92	-3.935	18.09	335.75	-3.562	28.61	184.81	-2.414	18.73
15	633.60	-4.391	33.77	355.80	-3.731	48.66	196.03	-2.664	29.95
20	644.32	-4.885	44.49	366.20	-3.886	59.06	202.09	-2.749	36.01

 

Experimental

The reagents, aluminium chloride, aluminium sulphate and aluminium nitrate were taken of AR grade. All these reagents were used after drying over calcium oxide desiccator to keep them in dry atmosphere. Fresh triple distilled water was used as standard solvent for preparing binary THF + water mixture. The density of dioxane was found to 1.02250 gcm^-3 at 303.15 K, is in good agreement with literature value of THF density value¹² (d = 1.02230 gm^-3).

The binary aqueous mixtures of tetrahydrofuran and six different concentrations of above mentioned aluminium salts were prepared by weight and the conversion of molality (m) into molar concentration(c) was done by using the standard expression:

Where, d is the density of solution and M₂ is the molecular weight of aluminium salts.

The density was measured with the help of an apparatus similar to the one reported by Ward and Millero¹³. The accuracy in the density measurements was 1 x 10^-4 gcm^-3.

The apparent molar volumes (Φ_v ) were calculated from the density data using the following expression¹⁴

where d­₀ is the density of THF +water as solvent and d is the density of solution; c is the molar concentration of aluminium salts and M₂ is the molecular weight of aluminium salts. The density measurements were taken in a well stirred water bath with a temperature control of ± 0.01 K.

Table – 2 Partial Molar Volumes (Φ_V^o) And Experimental Slopes (S_v^*) For Aluminium Sulphate, Aluminium Nitrate, Aluminium Chloride In 5% (W/W) Thf+Water At Different Temperatures

TEMPERATURE (K)	ALUMINIUM SULPHATE		ALUMINIUM  NITRATE		ALUMINIUM   CHLORIDE
	Φv0 (cm3 mol-1)	Sv* (cm3lit1/2mol-3/2)	Φv0 (cm3 mol-1)	Sv* (m3lit1/2mol-3/2)	Φv0 (cm3 mol-1)	Sv* (cm3lit1/2mol-3/2)
298.15	599.94	-3.142	303.46	-2.554	165.03	-1.919
303.15	610.80	-3.637	316.22	-3.139	179.10	-2.398
308.15	617.06	-3.728	329.25	-3.645	184.24	-2.455
313.15	631.49	-4.277	351.34	-3.970	195.54	-2.761
318.15	638.37	-4.489	361.39	-4.205	207.79	-3.276

 

Results and Discussion

The measured values of densities of aqueous solutions of aluminium salts in different compositions of THF + water (5,10,15,20%) at 303.15 K , have been used to calculate the limiting molar volumes Φ_v. the plot of Φ_v verses √c were found to be linear with positive slopes in different compositions of THF + water. A sample plot for aluminium nitrate in different compositions of THF +water at 303.15K is shown in fig.1

The limiting molar volumes Φ_v and experimental slopes (S_v*) were calculated by using Masson equation

Table 1 shows the values of Φ_v and S_v* calculated in different compositions of binary aqueous mixture of THF at 303.15 K. It is clear from table 1, that the values of slopes ( ) are negative in water as well as in THF+ water mixture at 303.15K in all the compositions. The negative slopes values of aluminium salts, indicates the presence of weak ion- ion interactions. Also, from Table 1, it is clear that the magnitude of S_v* values of aluminium salts decreases with increase in the amount of THF in water, which shows that ion-ion interactions are further weakened which and results in the increase in solvation. Also from Table 1, it is clear that the Φ_v^o values are positive and increase in magnitude, in all the compositions of THF + water at 303.15K for aluminium salts taken for study. The Φ_v^o values which is  a measure of ion-solvent interaction increases for each aluminium salts with the increase of THF amount in water, shows that ion- solvent interactions enhanced on the addition of THF in water which indicates that solvent has less affinity for water. On comparing the magnitudes Φ_v^o values with the values of S_v*, Φ_v^o values are more in magnitudes than that of S_v* of the same aluminium salts. This concludes that ion-solvent interactions dominate over the ion-ion interactions in water + THF mixture at 303.15 K.

The volume of transfer (∆v⁰_(tr)) has also been calculated by using the relation:

Here Φ_v^o _(MS) and Φ_v^o _(W) are the partial molar volumes of aluminium salts in mixture of THF + water and water as solvents respectively. The values of volume of transfer of aluminium salts were recorded in table 1.It is clear from table 1 that the ∆v⁰(tr) values continuously increase in magnitude with the increase in the content of THF in water. The increase in Φ_v^{o and ∆v0_(tr)} may be trait to the decrease in electrostriction in the presence of THF. The electrostriction effect, which leads to the contraction in the volume of the solvent, is lowered in the mixed solvents as compared with that in pure water. This electrostriction effect, again confirming the earlier conclusion of lesser affinity of THF for water. ∆v0_(tr) values , for each aluminium salts rise with the increase of THF amount in water ; which may be trait to the decrease in solvent – solvent interactions between THF + water.

 

Fig1: Plot of фv Vs c1/2 of aluminium nitrate in different compositions of  THF + water at 303.15K  Click here to View figure

 

Since the behaviour of each electrolyte was found to be linear and same in different composition of THF + water at 303.15 K, so only one composition system (5% w/w) has been selected for studying the effect of temperature. The experimentally determined values for the different concentrations of aluminium salts at different temperatures (298.15K – 315.15K) have been used to calculate the limiting molar volume Φ_v^o of of the salts. The plot of apparent molar volume Φ_v against √c were found to be linear with the positive slopes in water as well as in aqueous solution of THF as shown in fig. 2 for one of aluminium salts (aluminium nitrate) as sample plot.

It is evident from table 2, that the values of S_v* are negative for aluminium salts in 5% aqueous mixture of THF at all temperatures. The negative values with rise in temperature from 298.15 K to 318.15 K of aluminium salts indicates the presence of weak ion-ion interactions and  attributes the increase in the solvation of aluminium salts with rise in temperature. Also it is clear from table 2 that the values of Φ_v^o increases with increase in temperature for the selected aluminium salts in THF + water (5% w/w) ,thereby showing that ion – solvent interactions are strengthened with increase in temperature. The increase inΦ_v^o values may be due to increase in solvation of aluminium salts with rise in temperature.

 

Fig2: Plot of фvVs c1/2 for aluminium nitrate in 5% binary mixture of THF + water at different temperatures  Click here to View figure

 

It has been found that only S_v* is not only one parameter for determining the structure making or structure breaking nature of any solute. Partial molar volume expansibilities (Φ_E^o) is another parameter, which determines the making or breaking capacity of any solute in any of the solvent developed by Hepler¹⁵.

The temperature dependence of Φ_v^o THF + water for selected aluminium salts can be expressed by the following relations:

Φ_v^o =  – 6395.90 + 17.80 T – 0.026 T2 for aluminium sulphate     (5)

Φ_v^o =  – 2283.58 + 14.64 T – 0.020 T2 for aluminium nitrate        (6)        

Φ_v^o =  – 3177.60 + 19.47 T – 0.027 T2 for aluminium chloride       (7)

Partial molar volume expansibilities Φ_E^o = [aΦ_v^o/aT] p which is temperature dependence function of Φ_v^o is calculated for the above mentioned aluminium salts by using relations 5,6 and 7 are given in table 3. From table 3 Φ_E^o values for aluminium salts at different temperatures are positive but decrease in magnitude with rise in temperature showing that these aluminium salts are not behaving  like symmetrical tetra alkyl ammonium salts 16 and but like common salts17,18 as in the case of common electrolytes the molar volume expansibilities should decrease with the rise in temperature. The decrease in the magnitude of Φ_E^o values indicates the absence of “packing effect”15. The plot of Φ_E^o verses temperature is found to be linear for all aluminium salts  in 5% w/w THF + water mixture as shown in fig.3.

 

Fig3: Plot of фE0 verse temperature for aluminium sulphate, aluminium nitrate, aluminium chloride in 5% THF + water.  Click here to View figure

 

Table – 3 Partial Molar Volume Expansibilities (Φ_E^o) For Aluminium Sulphate, Aluminium Nitrate And Aluminium Chloride In 5% (W/W) Thf + Water  Mixture At Different Temperatures

TEMPERATURE(K)	ALUMINIUM SULPHATE	ALUMINIUM  NITRATE	ALUMINIUM   CHLORIDE
	ΦEo( cm3 mol-1 K-1)	ΦEo( cm3 mol-1 K-1)	ΦEo( cm3 mol-1 K-1)
298.15	2.296	2.714	3.367
303.15	2.036	2.514	3.099
308.15	1.776	2.314	2.829
313.15	1.516	2.114	2.559
318.15	1.256	1.914	2.289

 

In last few years, it has been observed by the number of workers that S_v* is not the only criterion for determining the structure making or breaking nature of any solute. Hepler¹⁴ has given a method of finding the sign of (a²Φ_v^o/aT² )p for different types of solutes in forms of long range structure making or structure breaking capacities of the solutes in aqueous solutions using the general thermodynamic relation:

On the basis of equation 8, It has been found that the positive values indicates the structure making nature of solutes and negative values indicates the structure breaking nature of solutes. In our present study, it is found from equations 5 to 7 (a²Φ_v^o/aT² )p  values are negative for aluminium salts in 5% THF + water suggests that the all the aluminium salts taken in present study act as structure breakers in 5% aqueous solution of THF. This indicates that the addition of aluminium salts break the structure of THF + water.

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