Effects of CuCl₂·6H₂O and ZnCl₂·6H₂O on the viscosity of aqueous ethanol mixtures

Abstract

The effects of CuCl₂ and ZnCl₂ on the viscosity in aqueous ethanol mixtures (10%–50% v/v) were studied in the concentration range 1.0×10^-2–8.0×10^-2 mol·dm^-3 at different temperatures. It was found that the viscosities increased with an increase in the concentration of the salts and percent composition of ethanol content, whereas it decreased with an increase in temperature. Ion-ion and ion-solvent interactions are determined with the help of BoldItalic- and BoldItalic-coefficients of Jones-Dole equation. The values of BoldItalic- and BoldItalic-coefficients are irregular and increase with a rise in temperature and also with an increase in ethanol contents for both salts. Negative values of BoldItalic-coefficients show that ion solvent interactions is comparatively small and suggest that CuCl₂ and ZnCl₂ behave as structure breakers in aqueous ethanol mixtures. Thermodynamic parameters like the energy of activation (BoldItalic_η) and change in entropy of activation (ΔBoldItalic*) were also evaluated which confirm the structure breaker behavior of salts in aqueous ethanol mixtures.

Keywords

viscosity / Jones-Dole equation / ionic interactions / transition metal chlorides / thermodynamic parameters

Introduction

In our earlier publication [1] viscosity of transition metal chlorides (cobalt chloride and nickel chloride) in aqueous ethanol mixtures at different temperatures has been reported. Ion-solvent and ion-ion interactions were studied by analyzing the viscosity data with the help of Jones-Dole Eq. (2).

(1){\eta }_{\mathrm{s}\mathrm{p}}/\sqrt{C}=A+B\sqrt{C}

where η_sp is the specific viscosity, \sqrt{C} is the square root of the concentration of salt and A and B are the coefficient viscosity which are respectively obtained from the intercept and slope of linear plots of {\eta }_{\mathrm{s}\mathrm{p}}\sqrt{C} vs. \sqrt{C}. The values of A-coefficient get increased with the increase of salt, percent composition of the ethanol mixtures and temperature. Negative values of B-coefficient reveal that ion-solvent interaction decreases and suggest that these metal salts behave as a structure breaker in aqueous ethanol mixtures. Furthermore, the energy of activation was also determined, which decreases with an increase in the concentration of salt and increases with a rise in percent composition of ethanol mixture. The present paper deals with the effect copper chloride (CuCl₂) and zinc chloride (ZnCl₂) on the viscosity of aqueous ethanol.

Experimental

Analar grade copper chloride (mol.wt 134.5 g/mol) and zinc chloride (mol.wt 136 g/mol), 99.9% pure ethanol of BDH are freshly prepared in double distilled water (conductivity 0.06 μS·cm^-1) were used.

Viscosity measurements were accompanied according to the procedure given elsewhere [1]. Aqueous ethanol mixtures of different composition (v/v) were prepared in double distilled water. Solutions of metal chloride were prepared in different percent composition of aqueous ethanol .Viscosities of these solutions were measured with the help of Ostwald Viscometer (type Techniconominal constant 0.1 (Cs/s) capillary ASTMD 445) by taking time of flow of solvents and solutions at different temperatures, 30°C–50°C with the interval of 5°C. The temperature of the solutions were kept constant throughout the experimental work with the help of thermostatic water bath (VWP Scientific model 1120 SER9143791). Reproducibility of the observations was made sure.

Results and discussion

Table 1 includes the viscosities and densities of aqueous ethanol (10%, 20%, 30%, 40% and 50%) in the absence of salt. The observations are collected at 30°C. The viscosities of aqueous ethanol solvents increased with the increase in percentage concentration of aqueous ethanol, whereas the density gets decreased with the decrease in percentage concentration of aqueous ethanol. The time of flow for CuCl₂ and ZnCl₂ was measured in 10%, 20%, 30%, 40% and 50% aqueous ethanol at various temperatures (30°C–50°C)±0.1°C with a difference of 5°C. The results are summarized in Tables 2 and 3 which indicate that the viscosity values increased with the increase in concentration of salts whereas it decreased with the rise in temperature. Similar behavior was also noted in the case of CoCl₂ and NiCl₂ [1] and electrolyte of group I of the periodic table in aqueous ethanol system [3] and strong electrolyte like HCl, NaCl and NaOH on the viscosity values of edible oils, e.g sunflower, maize, canola and soybean in 1,4 dioxane [4–7].

The viscosities of metal chloride in aqueous ethanol are also interpreted in terms of ion-ion and ion solvent interactions using Jones-Dole Eq. (1) to characterize the behavior of ectrolytes. The Jones- Dole equation contains two coefficients A and B which are obtained graphically ({\eta }_{\mathrm{s}\mathrm{p}}\sqrt{C} vs. \sqrt{C}). Here η_sp defines specific viscosity of the solutions and \sqrt{C} is the square root of the concentration of metal chloride in aqueous ethanol. The slope and intercept of the plot gives the value of both B- and A-coefficient respectively. The representative plots for CuCl₂ and ZnCl₂ are shown in Figs. 1 and 2 and the results are summarized in Tables 4 and 5. The A-coefficient of the equation indicates the ion-ion interaction. The values get increased with the rise in temperature and also with an increase in ethanol content for both salts. This shows that ion-ion interactions increase with the temperature and percent composition of ethanol with some variations. As the concentration of the organic solvent is increased the dielectric constant of the medium decreases, and the electrostatic ion-ion interaction (A-coefficient) shows an increase with some expectations. A solute alters the viscosity of the solvent as the ions and water molecules exert a viscous drag on the rest of the solvent and the change in viscosity occurs in mixed solvent system. In ethanol the oxygen of alcohol has a tendency of holding water molecules, i.e, in liquid alcohol an oxygen atom carries one proton and two lone pairs of electron. This may lead to intermolecular hydrogen bonded polymer instead of formation of cluster. With the increase in percent composition of aqueous ethanol content, the viscosities of salt vary because of the formation of the complex. Variation in A-values is attributed to the size of the ion which differs in their degree of hydration.

In mixed solvent systems such as aqueous ethanol mixtures, H-bonding to O-H continuously changes the structure of water because of both their cluster forming and cluster distributing character [8]. Water forms strong coordinating bonds with the transition metal ring and also form strong hydrogen bond with negative ions. When the solute is dissolved into the solvent two types of interactions ion-ion and ion-solvent occur. This may be due to the inter penetration effect of cation-cation and cation-anion which brings ions together and is responsible for the increase of ion-ion interactions [9–15].

Similarly the values of B-coefficient of Jones Dole equation were obtained from the slope of the plot {\eta }_{\mathrm{s}\mathrm{p}}\sqrt{C} vs. \sqrt{C}. The observations are found to be effected upon both the temperature and composition of aqueous ethanol mixtures. This B-coefficient describes the ion-solvent interactions. In both CuCl₂ and ZnCl₂ systems, the values are negative and the variation is irregular. The negative values of B-coefficient in aqueous ethanol reveal the structure breaking effect [11,12]. The variation of B values with a change in percent composition of the solvent represents the electrostatic ion solvent interaction in an aqueous ethanol mixture. The smaller the ion is, the stronger the electrostatic interaction, and hence the greater is the size of the solvent ion. On the other hand, the solute with the positive values of B-coefficient in a given solvent is expected to have the structure making effect. Furthermore, a solute with less positive or negative values of B-coefficient in a given solvent is considered to be a structure breaker [11]. A large solute shows a large obstructive effect meaning a bending of the stream lines around a large solute particle. In such a solvent B-coefficient may be always negative irrespective of how it behaves with the solvent as in the present case ethanol (C₂H₅OH) is not very bulky. Besides this the above mentioned irregularities in the values of B-coefficient is due to the degree of hydrolysis in different percent compositions of aqueous ethanol.

The viscosity of a liquid generally decreases with the rise in temperature. This has been explained in terms of “the hole theory of liquids” [15–20]. Intermolecular distances are relatively bigger in liquid than in solid. A liquid molecule therefore needs some activation energy to move in a hole. As the activation energy increases with the rise in temperature, a liquid can flow more easily at high temperature. The coefficient of viscosity thus falls appreciably with the rise of temperature. Arrhenius [13] pointed out the effect of temperature on viscosity by means of an exponential equation measuring the activation energy,

(2)\log \eta =\log A+E_{\eta }/\mathrm{2.303}RT

where η is the coefficient of viscosity; A is called the pre-exponential factor constant for a given liquid; E_η is the energy of activation; R is the gas constant and T is the absolute temperature. The values of energy of activation were obtained from the slopes of linear plots of log η vs. 1/T for the aqueous ethanol for CuCl₂ and ZnCl₂. The values of activation energy (E_η) as a function of salt concentration and solvent composition are tabulated in Tables 6 and 7. The representative plot of energy of activation is shown in Fig. 3. Here both the concentrations of salt and percent composition of solvent influence the value of energy of activation. There is an irregular variation in activation energy observed with the increment of the concentration of salt. At a CuCl₂ concentration of 1×10^-2 mol·dm^-3, the activation energy is determined as 11.647 kJ·mol^-1 in 10% aqueous ethanol mixture, whereas in the same composition for an increased concentration of CuCl₂as 2 × 10^-2 mol·dm^-3, the value of activation energy increases to 12.384 kJ·mol^-1. Furthermore, the increase in concentration of salt, until up to a concentration of 6×10^-2 mol·dm^-3, decreases regularly the value of activation energy. Ultimately at concentrations of 7×10^-2, 8×10^-2 and 9×10^-2 mol·dm^-3 the values of activation energy are obtained respectively as 12.167, 12.140, 11.965 kJ·mol^-1. So the irregular behavior of concentration of salt and composition of solvent is also observed for ZnCl₂ as shown in Table 7. This behavior may also be seen in another composition of solvent. The increase in activation energy may be due to the fact that at higher solvent composition there is a decrease in mobility of ions, which will make it difficult to produce vacant sites in the solvent matrix resulting in the high energy of activation. As a principle the activation energy which are slightly higher than the threshold activation energy at higher temperature provides an additional activation energy which helps the free movement of the liquid and suppresses the friction of liquid in the capillary, thereby the coefficient of viscosity falls appreciable with the rise of temperature.

Entropy is an important thermodynamic parameter which can be calculated from the following expression:

(3)\mathrm{\Delta }S*={\displaystyle \frac{E_{\eta }-\mathrm{\Delta }G}{T}}

Here ΔS* is defined as the change in entropy, E_η represents the activation energy, T is the absolute temperature and ΔG is the change in free energy. The values of free energy of different solutions control the rate of flow in fluid process. This flow process is governed by the ability of a molecule to move into the prepared hole and the readiness with which the holes are prepared in the liquid. The changes in entropy values (ΔS*) at different concentrations of salts, percentages composition of aqueous ethanol mixtures and different temperatures from 30°C to 50°C are summarized in Tables 8 and 9 for CuCl₂ and ZnCl₂ respectively. The results indicate that the values of change in entropy of activation are negative. There is an irregular increase or decrease with the increase of concentration of salts and with percentage composition of the aqueous ethanol mixtures from 10%–50%.

Conclusion

Ion-solvent and ion-ion interactions in the presence of transition metal chloride were evaluated in terms of Jones-Dole coefficient A and B respectively. The results revealed that CuCl₂ and ZnCl₂ behave as structure breakers in aqueous ethanol solvents. The irregular increase or decrease of the values of activation energy and change in entropy also support that these metal chlorides act as structure breakers.

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