The Application of Irreversible Thermodynamics to Transport Processes in Cation-Exchange Membranes

Cameron, Ronald G (1976) The Application of Irreversible Thermodynamics to Transport Processes in Cation-Exchange Membranes. PhD thesis, University of Glasgow.

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A study of the transport properties of normal and expanded forms of a cation exchange membrane in 0.05m calcium chloride has been carried out. Isotopic diffusion coefficients and electrical, salt diffusion and osmotic properties have been determined experimentally and the results have been used to provide an Irreversible Thermodynamic analysis of membrane phase interactions. Such an analysis is extremely rigorous and the conditions governing its validity are stated and strictly observed. In this study the properties of the ion-exchanger are defined by six phenomenological transport coefficients. Under isothermal, isobaric conditions (which were maintained for the major part of this work) only five independent equations may be formulated and so certain assumptions regarding the magnitude of one minor coefficient are necessary. In the system described the salt uptake is small and the salt diffusional flow is predicted accurately from a knowledge of electrical conductivity, transport and transference numbers. Four sets of Onsager frictional coefficients are calculated, each using a different limiting assumption. All four assumptions have as their basis the low co-ion content of the exchanger and three of them are seen to be in good agreement for the major parameters. However, the fourth assumption, that co-ion isotope-isotope interactions are negligible, provides a set of frictional coefficients which correlates poorly with the others and so only three sets of Onsager mobility coefficients are considered. A salt model calculation is presented which is based on the aqueous chloride solution of the membrane counterion. The model successfully predicts both the individual transport coefficients of the irreversible thermodynamic analysis and the measured transport / transport parameters for the sodium forms of this membrane. It becomes obvious however, that there are certain significant differences between the calcium form of the membrane and the corresponding salt model (aqueous calcium chloride). Since the Salt Model Calculation for the sodium form proved successful it would appear that the differences are probably due to ion-association between the calcium and matrix-fixed charge which would lower conductivity, reduce counterion-water friction and increase interaction between counterion and matrix-fixed charge. These conclusions are strengthened by a comparison of coefficients for the sodium and calcium forms when valency differences have been accounted for. Conductivities of mixed ionic forms (calcium and sodium) were obtained and compared with values which were predicted by assuming that the reduction of conductivity (below the ideal mixture relationship) is due to calcium-sodium interionic coupling. The theoretical model was that used successfully by Miller to predict the properties of ternary electrolyte solutions. The predictive value of the Nernst-Planck Theory is investigated for the system described and the anomalies between calculated and observed values are rationalised by making a detailed comparison with the Irreversible Thermodynamic approach.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Physical chemistry, Thermodynamics
Date of Award: 1976
Depositing User: Enlighten Team
Unique ID: glathesis:1976-78717
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:59
Last Modified: 30 Jan 2020 14:59

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