Transport Studies with Aqueous Solutions of Urea and Thiourea Across an Ionexchange Membrane

Abstract

Experimental results for the measurement of hydrodynamic permeability, electro-osmotic velocity, electro-osmostic pressure and streaming potential for different aqueous solutions of urea and thiourea across the cation-exchange membrane (Indion 236, Sd fine chemicals, India) at 30 °C and voltages up to 25 V are reported. The results have been analysed in the light of thermodynamics of irreversible processes. The cation-exchange membrane has been characterised in terms of effective cross-sectional area, equivalent pore radius, and frictional coefficient. The permeability coefficient L P , has been found to be independent of the hydraulic pressure but is a characteristic property of the membrane and depends upon the concentration of the solute. The reflection coefficient σ, has also been evaluated. Onsager's reciprocity relation has not been found to hold well and this may be attributed to the higher polarization in case of electro-osmosis and also to the highly charged character of the membrane. The efficiencies of maximum electro-kinetic energy conversion took place at half the value of the electroosmotic pressure in each case but the value of maximum energy conversion has been found to be different in both cases thereby also indicating the non-validity of Onsager's reciprocity relation.

Electrokinetic Energy Conversion in Heterogeneous Ion-Exchange Plugs

“…s and equation (4) general it can be seen that the maximum efficiency is extremely low because the cross-phenomenological coefficient L 21 , which represents the electricalmechanical coupling, is also very small (-ºÏ" 11 m 3 /Vs), [7]. Similar results have been found with both weakly charged and ion-exchange membranes [1][2][3][4][5][6]. This implies that a large part of the input energy is dissipated in the electrokinetic processes.…”

“…Thus, for plugs with different structural characteristics (5) may be written as: (6) where D is a constant which includes the parameters characterizing the liquid permeant: (7) …”

“…In this case, the η ηιΛΧ variations are a consequence of the changes in the solid-liquid interface and the plug structure, besides the variation of the characteristics of the liquid phase. Therefore, equation (5) cannot be reduced for the last results (as can (6) or (8)) because all the parameters appearing in that equation vary with X a in a complex way.…”

“…Nevertheless, it is worthwile characterizing electrokinetic processes from an energy conversion point of view in order to find out which parameters influence *7max-Several papers have been published recently [1][2][3][4][5][6], analyzing the variation of ^m ax in non-equilibrium thermodynamics terms. These papers deal meanly with the effect of the permeant composition on f/ max when both weakly charged [1][2][3][4] and ion-exchange membranes [5,6] are used as porous media. But */ max depends as much on the structure of the membrane as on the permeant composition and few papers have been published dealing with the effects of these structural factors of The aim of this work, therefore, is to understand the variations in f/ max under all experimental conditions, by discovering the effect of the structural characteristics (composition, compactness and D.V.B.…”

Activation Energy Parameters for Hydrodynamic Permeability of Binary Aqueous Solutions of Divalent Transition Metal Sulphates and Magnesium Sulphate Through an Inorganic Membrane of Aluminium Oxide

“…occurring across the ionic membranes which have been described by Spiegler [20]. Blokhra et al [21][22][23][24][25][26] have also reported the transport phenomenon of various electrolytes and non-electrolytes across different types of diaphragm using the thermodynamics of irreversible processes. It is expected that the electrical double layer in highly charged membranes would be more sensitive to the change in characteristics depending on the nature of the permeants which would naturally be reflected in transport properties, For better understanding and prediction of a membrane's permeation characteristics, an insight into the structural details, e.g.…”