Studies on Molecular Interactions of Some Electrolytes in Water by Volumetric and Viscometric Measurements at T = (303.15 to 323.15 K)

The viscosities and densities of potassium chloride, potassium nitrate, magnesium chloride, and magnesium nitrate have been measured at 303.15, 308.15, 313.15, 318.15 and 323.15 K in aqueous solution. The viscosity data were analyzed by using Jones–Dole equation. The values of apparent molar volume, limiting apparent molar volume have been evaluated from the density data. The results were interpreted in the light of ion–ion and ion–solvent interactions and of structural effects of the solutes in solution.


Introduction
The volumetric behavior of solutes has been proven to be very useful in elucidating the various interactions occurring in aqueous solutions [1].Studies on the apparent and partial molar volumes of electrolytes and the dependence of viscosity on concentration of solutes and temperature of solutions have been employed as a function of studying ion-ion and ion-solvent interactions [2].It has been found by a number of workers [3][4][5] that the addition of electrolyte could either break or make the structure of a liquid.Because a liquid's viscosity depends on the intermolecular forces, the structural aspects of the liquid can be inferred from the viscosity of solutions at different concentrations and temperatures.
interactions between studied electrolytes and protein denaturing agents such as sodium dodecyl sulfate, dodecyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide, etc.The addition of electrolytes to the surfactant solution could effectively reduce the critical micelle concentration of surfactants and also increase the detergency.Considering this, the present investigation have been undertaken to provide better understanding of the nature of these electrolytes at lower concentration (∼0.003 to ∼ 0.01 molkg -1 ) in aqueous medium and to through light on ion-solvent interactions.

Density measurements
The reagents were always placed in the desiccators over P 2 O 5 to keep them in dry atmosphere.Freshly distilled water with specific conductance of ~10 −6 Ω −1 .cm−1 was used for preparing aqueous solution.All the aqueous solutions of electrolytes were made by weight and molalities (m) were converted into molarities (C) using the standard expression [6].It is to be mentioned, water of hydration was taken into consideration for calculating the molalities of MgCl 2 and Mg(NO 3 ) 2 : where ρ and M 2 are the solution density and molecular weight of an electrolyte.A Mettler PM-200 electronic balance with an accuracy of ±0.0001 g was used for the mass determination.It is apparent that densities changed only in the fourth decimal.This is attributed to the very low concentration of the electrolytes.
The densities (ρ) were measured with an Ostwald-sprengel type pycnometer having bulb volume of 10 × 10 -6 m 3 .The pycnometer was calibrated at 303.15 to 323.15 K with doubly distilled water.Averages of triplicate measurements were taken into account.

Viscosity measurements
The viscosities were measured at the stuided temperature using an Ostwald`s suspended level type viscometer which has a flow time of about 214s for water at 303.15 K.The viscometer was calibrated with water.The density and viscosity measurements were carried out in the same water bath (±0.01K).

Apparent molar volume
The measured densities of electrolytes in water are listed in Table 1 as functions of concentration and temperature.The apparent molal volumes of electrolytes in aqueous systems at 303.15 K to 323.15 K are shown in Table 1.The apparent molar volume of a solute in solution, generally denoted by ϕ v was calculated by the following equation: The apparent molar volume at infinite dilution, (ϕ 0 v ) was calculated using least square fit to the linear plots of experimental values of ϕ v versus square root of molal concentration ( m ) using the following Masson equation [7]: where S v is the experimental slope, which is sometimes considered to be the volumetric pairwise interaction coefficient [8][9].The limiting apparent molal volume (ϕ 0 v ) and S v values along with standard error are given in Table 3.It is evident from the table that the values of S v are positive for electrolytes in water at different temperatures.Since S v is a measure of ion-ion interactions, the result indicates the presence of very strong ion-ion interactions.The limiting apparent molal volume (ϕ 0 v ) which is taken to be the partial molal volume at infinite dilution of electrolytes in aqueous solutions reflects the true volume of the solute and the volume change arising from solute-solvent interactions.
Among the electrolytes studied (KCl, KNO 3 , MgCl 2 and Mg(NO 3 ) 2 ), the ϕ 0 v value obtained for Mg 2+ Cl -/NO 3 -is observed to be abnormally higher than those of the other electrolytes (shown in Table 3).This abnormality may be accounted for by the fact that Mg 2+ can form an octahedral complex with water.The variation of ϕ 0 v with the molality of electrolytes can be rationalized in terms of cosphere overlap model [10].According to the model, the overlap of the cospheres of two ions or polar groups or an ion with that of hydrophilic groups always produces a positive volume change.On the other hand, the overlap of the cospheres of an ion with that of hydrophobic groups results in a negative volume change.The temperature dependence of limiting apparent molal volume, ϕ 0 v for electrolyte plus water solutions can be expressed by the following expression: where, T is temperature in Kelvin.The limiting apparent molar expansibilities (ϕ 0 E ) can be obtained by differentiating Eq. ( 3) with respect to temperature.
The limiting apparent molar expansibilities (ϕ 0 E ) for electrolytes plus water obtained using Eq. ( 4) at different temperatures are given in Table 4.It is found that ϕ 0 E values decrease with rising temperature for KCl and KNO 3 .This may be considered as an indication of the fact that the structure of the solvent is weakened by the elevation of temperature, that is, some solvent molecules may be released from the loose solvation layers of the solute.The effect is that the removal of solvent molecules favors solutesolute interactions causing less electrostriction around electrolytes.On the other hand, opposite behavior has been observed for MgCl 2 and Mg(NO 3 ) 2 .(kgm -3 ) Hepler [11] developed a technique of examining the sign of [ ∂ φ 0 E /∂T] P for various solutes in terms of the long range structure making or breaking ability of solutes in aqueous solution using the general thermodynamic expression: The sign of i.e. second derivative of limiting apparent molal volume of solution with respect to temperature at constant pressure which corresponds to structure making or breaking properties of solution was determined.The value of has been found to be negative for KNO 3 and KCl suggesting structure breaking property and the positive value for MgCl 2 and Mg(NO 3 ) 2 suggesting structure making property.Roy et al.

Viscosity B-coefficient
The viscosities of KCl, KNO where η r is the relative viscosity and C is the molar concentration, A and B are the constants characteristic of ion-ion and ion-solvent interactions respectively.The value of η increases with the increase in molarity.As a result, the dynamic equilibrium between monomer water molecule and aggregated water molecule [nH 2 O = (H 2 O) n ] shifts towards right.For The evaluation of the coefficients A and B from the Jones-Dole Eq. ( 6), plots of (η r -1) versus C were constructed and were found to be linear within experimental uncertainty over the whole concentration range of electrolyte studied.The values of A and B-coefficients for the electrolytes are estimated by computerized least-square method and presented in the Table 3, along with uncertainty.Table 3 shows that the values of A coefficients are either positive or negative and very small for all the four electrolytes indicating the presence of weak ion-ion interactions, which of course further improve with the rise in temperatures.It is also evident from Table 3 that the values of B-coefficient, for MgCl 2 and Mg(NO 3 ) 2 aqueous solutions are positive and fairly large thereby suggesting the strong ion-solvent interactions.On the other hand, negative values of B-coefficient have been found for KCl and KNO 3 which reflects the structure rupturing property.The negative Bcoefficient value was reported by A. Hammadi et al. [14] in aqueous system for KCl.Further the values of B-coefficient in aqueous KCl and KNO 3 solutions decrease with the increase in temperature, thereby suggesting that the ion-solvent interactions are weakened with the increase of studied temperatures.On the other hand, the values of the Bcoefficient for MgCl 2 and Mg(NO 3 ) 2 in aqueous solution increase with the rise in temperature, thereby showing the ion-solvent interactions further improve with the increase in temperature.Recently it has been emphasized by many workers that dB/dT is a better criterion [15] for determining the structure making/breaking nature of any electrolyte rather than simply the B-coefficient.The values of dB/dT were calculated from the slope of the curve obtained by plotting the B-coefficient value against temperatures and these values are given in Table 4.It is evident from Table 4 that the positive values of dB/dT for KCl and KNO 3 showing their structure breaking nature.A perusal Table 3 shows that the B-coefficient values of MgCl 2 and Mg(NO 3 ) 2 decreases with rising temperature.So derivative of temperature (dB/dT) is negative.We can classify MgCl 2 and Mg(NO 3 ) 2 acts as structure maker in aqueous solution.Haque et al. [15] noticed that dB/dT value is negative for MgCl 2 in methanol.These are in excellent agreement with the conclusions drawn from [∂ϕ 0 v 2 /∂T 2 ] p as discussed earlier.

Table 1 .
Densities and apparent molar volume of studied electrolytes in water at different temperatures.

Table 2 .
Viscosity of studied electrolytes in aqueous solutions as a function of concentration at various temperatures.

Table 3 .
Standard partial molar volume, experimental slope, values of the parameter A and B of the Jones-Dole equation for studied mineral electrolytes in water at various temperatures.

Table 2 .
[13]viscosity data were analyzed in terms of the semiempirical Jones-Dole equation[13], after arranging it into a straight line form given below;

Table 4 .
Values of various co-efficient of Eq. 3, Hepler constant and dB/dT for studied electrolytes.