Electro-Osmosis of Water Through a Collodion Membrane

Electro-osmosis of water through a collodion membrane has been studied. The data confirmed the existence of a threshold potential difference in the case of electroosmosis. The data also confirmed that the electro-osmotic velocity is given by an exponential type of relationship. The efficiencies of electro-kinetic energy conversions have been calculated and the results thus obtained have been discussed in the light of Non-Equilibrium Thermodynamics. Introduction In the recent preliminary studies [ 1 ] on electro-osmosis of water through a collodion membrane it was observed that no electro-osmosis takes place unless the applied electrical potential difference exceeds a certain threshold value. Since this observation is quite unusual and has not been reported in literature earlier, the experiments have been repeated for confirmation of the unusual form of the phenomenological relation for the water flux in presence of both, a pressure difference and an electrical potential difference. It has been indicated that the existence of the threshold potential difference may be due to the double layer overlap in the narrow capillaries of the collodion membrane. The data have been further utilized to calculate the efficiencies of electro-kinetic energy conversion for both modes of conversions namely electroosmosis and the streaming potential. The results thus obtained have been discussed in the light of the theory, Non-Equilibrium Thermodynamics delivers for electro-kinetic energy conversion. 1. Experimental Membrane Preparation: The casting solution was prepared by shaking 5.43 gm of pyroxylin (BDH) in a mixture containing 68.48 gm of ether (BDH) and 26.10 gm absolute alcohol. The membrane was prepared by evaporation technique, casting Present address: 1 St. Andrew's College, Goräkhpur, India 2 National Sugar Institute, Kanpur, India J. Non-Equilib. Thermodyn. , Vol. 3, 1978, No. 2 84 R. C. Srivastava, A. K. Jain, K. Upadhyay • r pyroxylin solution on a homogeneous clean glass plate with a spreader knife. The time allowed for evaporation was ~ 30 s. The collodion membrane used in the present studies was 0.0125 mm in thickness and 4.25 cm in area. In order to avoid fluctuations in permeability, the membrane was treated with water at 40 ± 0.1 °C — the temperature of the experiment — for 24 hours, before performing the actual experiments. Water, distilled once over alkaline permangnate in an all glass still was used in the present investigations. Apparatus and Procedure: The electro-osmotic cell used in the present studies and the procedure adopted for the measurement of hydraulic permeability, electro-osmotic velocity, streaming potential and streaming current have already been described in the earlier publications [1—2]. The volume flux of water was measured by noting the rate of advancement of a liquid meniscus in a capillary (diameter 0.213 cm) with a cathetometer reading up to 0.001 cm and a stop watch reading up to 0.1 s. The streaming potential was measured using a vernier potentiometer reading up to 10~ V. In the measurement of the streaming potentials the asymmetry potential of the electrodes was taken into account. Streaming currents were calculated from the streaming potential data and resistance of the system using Ohms Law. The resistance of the system was measured with an electrical conductivity meter (Leitf higkeitsmesser). Electrical potential differences needed for the measurements of electro-osmotic velocity were tapped from an electronically operated and stabilized power supply. All measurements were made at constant temperature by placing the electro-osmotic cell in a thermostat set at 40 ± 1 °C. 2. Results and Discussion The data on the hydraulic permeability, the electro-osmotic velocity, the streaming potential and the streaming current are plotted in Figs, l—3. They confirm the validity of the phenomenological equations J = Ln ÄÑ + Lm (ÄÑ) + L12 [ÄÖ ÄÖ3 ÄÖ-ÄÖ ÄÖ,, A3»a {Iexp(Äö _ Äö* )}] , (1) D 3 and I = L21 ÄÑ + 1,22ÄÖ, (2) reported earlier in our priliminary investigations [1 ]. These equations describe the simultaneous transport of water (J) and electricity (I) respectively in the presence of both, pressure difference ÄÑ and an electrical potential difference ÄÖ. In equations (1) and (2), the quantities Lik are the phenomenological coefficients and the constants ÄÖ3 (3.7 V) and ÄÖ1) (5.2 V) are the threshold potential difference and the extrapolated intercept of the straight line part of the 1 In Figs. 1,3,5,6 the pressure is given as Äñ = pH2o g Ä h with pH2o =: 0,99987 g/cm, g = 9,81 m/s and Ah given in cm. J. Non-Equilib. Thermodyn. , Vol. 3, 1978, No. 2 Electro-osmosis of water 85