Pakistan Research Repository

INVESTIGATION OF ELECTROLYTE SOLUTIONS IN STRONGLY INTERACTING LIQUIDS

REHMAN, HAFIZ-UR- (2008) INVESTIGATION OF ELECTROLYTE SOLUTIONS IN STRONGLY INTERACTING LIQUIDS. PhD thesis, Quaid-i-Azam University, Islamabad.

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Abstract

From some previous NMR studies water-ethanol-LiCI ternary system (the system) is known for its constituent species exhibiting strong mutual interactions which also lead to modification of the internal dynamics of ethanol molecules. Features of the system are miscibility of the solvent components over the entire composition range, considerable high solubility of LiCI and sustainability over fairly wide temperature range; all of these features make the system important for chemistry, industry, chemical engineering and technology etc. With the above considerations, the present work is devoted to systematic study of some thermodynamic and transport properties of the system over its entire mixture composition, LiCI concentration upto almost maximum solubility and within a range well above and below the ambient temperature (from -5 °c to +50 °C). The properties determined include density, excess molar volume (of the solvent mixtures), viscosity and electrical conductivity. To allow rational comparisons of various behaviours between solutions having different solvent compositions, temperature independent aquamolality (m, moles of solute per 55.5 moles of solvent) scale has been mostly employed but molarity has been also determined for incorporation wherever required. The measured values showed good agreement with those reported in literature. The investigation has been supported by the proton NMR spectra, and visible region spectra of some selected solutions after adding trace amount of CoC12. Density of the system showed quite normal behaviour with LiCI concentration and temperature; density could be very well fitted to second order polynomials in m. Density index, g(x1) has been introduced in the present work as an important solution parameter; for a solution prepared in the solvent mixture having water mole fraction x1, g(X1) can reproduce density of the system at any given m and temperature employing the solvent density, Po(X1, T). Excess molar volumes of the solvent mixtures determined at all the temperatures show minima for X1 = 0.6 which indicate that temperature variation within the investigated range causes no substantial change in the overall solvent structure. Behaviour of density with temperature for the entire system has been explained through the temperature average co-efficient of thermal expansion,β. A cubic equation in m was required to fit the viscosity data of the system over the temperature range of investigation. Since most of the viscosity models are restricted to one component solvents, ( quite) dilute solutions and single temperatures, an attempt was made to fit the data to a more versatile Lencka's model (1998); this model explains the viscosity change (compared to the solvent mixture) as sum of contributions arising from various kinds of interactions operating within the solution. Experimental results of the solvent mixtures and comparatively dilute solutions (m ≤2) could be reproduced by the model quite well at all the temperatures but strong deviations were observed at higher LiCl concentrations. However, by incorporating a temperature dependent dynamic term (∆ηD) fairly good agreement between the experimental and calculated data has been achieved even for the concentrated solutions. Temperature dependence of viscosity could be explained by the Arrhenius equation involving single flow activation energy for each system. For a given solvent (mixture) the activation energy always increased with LiCI concentration however, the increase was more prominent for the ethanol-rich mixtures. Activation energy-solvent composition plot passes through a maximum at water mole fraction X1 = 0.8. Plots of electrical conductivity against m for all the cases (for various mixtures and at different temperatures) could be fitted to third order polynomials but the coefficient of the cubic term was always negligible; these plots also passed through maxima which shifted from 4.5 m to 6 m for water rich mixtures. For the sake of brevity this behaviour has been explained by a simple model in which the conductivity at any point is considered to be comprising of two sets oppositely acting terms, namely Kup and Kdown. Whereas the first term (Kup) is directly related to the number density of the ionic species and should increase with m, Kdown has been considered to include contributions arising due to ion-ion interactions, sharing of solvation spheres by the same solvent molecules, electrostriction etc. Subsequently, these two terms have been compared with the coefficients of second order polynomial in m. Casteel & Amis equation for KlKmax as a function of m could be employed over the entire composition, temperature, concentration ranges. Temperature dependence of the electrical conductivity has been correlated with the thermal co-efficient of conductivity, Ү which increased with the salt concentration and passed through a maximum at X1= 0.8. The Walden product (∆oxηo) also passed through maximum at X1≈ 0.9 indicating that the solvent viscosity determined the conductivity when the solvates were mutually interacting. However, the Fuoss-Hsia equation has been used for the higher LiCI concentrations to incorporate the interactions between solvates as they tend to approach each other. Also from the molar conductivity data at different temperatures, so-called conductivity activation energies have been determined by employing the Arrhenius relationship; the activation energy showed a steady increase with water mole fraction for relatively dilute solutions and passed through a maximum at X1 ≈0.8. Visible region spectra measured after adding trace amount of CoC12 indirectly demonstrated selective solvation of Li+ by the water molecules. The complexes of C02+ in pure water and ethanol exhibited the absorbance-λmax at the known positions i.e. 520 and 670 nm, respectively. However, when added to 6 m or higher LiCI solutions in water (having no ethanol), a prominent broad peak around 670 nm was also observed indicating that C1- too started taking part in the cation solvation at higher electrolyte concentrations. This ion-pair formation is also responsible for the decrease of electrical conductivity beyond the maxima. The 1H-NMR spectra have demonstrated increase of the intermolecular proton exchange rate and decrease of the proton residence time on the ethanol molecule with addition of water. Collapse of the OH-triplet and coalescence of the OH-water peaks in the solvent mixtures were observed at X1≈0.3 (דres-value of 80 ms) and X1≈ 0.7 (Kexch= 107 s-1) respectively; the later also happened to be the composition at which minimum of the excess molar volume occurred. Concentration of LiCI did affect the spectrum; in this regard one significant finding of the present study is that water and LiCI work in opposite directions. As Li+ -ions are preferentially solvated by water molecules hence the phenomenon should make water molecules less available for exchange with alcohol molecules.

Item Type:Thesis (PhD)
Uncontrolled Keywords:electrolyte solutions, strongly interacting liquids, water-ethanol-lici ternary system, intermolecular interactions, ion solvation
Subjects:Physical Sciences (f) > Chemistry(f2)
ID Code:2653
Deposited By:Mr. Javed Memon
Deposited On:18 Jul 2009 10:44
Last Modified:10 Oct 2009 16:16

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