Diffusion and hydrodynamic instabilities in membrane systems with water solutions of NaCl and ethanol

by Sławomir Grzegorczyn, Iwona Dylong, Paweł Dolibog, Andrzej Ślęzak

The characteristic manifestations of instability were observed in the form of voltage pulsations measured between electrodes immersed directly in solutions of membrane system chambers, in different configurations of membrane systems. The reason for this type of voltage pulsations is Rayleigh-Benard type instabilities of near-membrane layers caused by density gradients of solutions in these layers. The time of build-up of the concentration boundary layer, after which hydrodynamic instability appears is one of important parameters of these phenomena. The concentration characteristics of these times, measured for one- and two-membrane systems, are nonlinear. With increasing differences in the density of solutions on the membrane at the initial moment, the times of build-up of concentration boundary layers were reduced. In two-membrane systems containing ternary solutions (water, NaCl, ethanol), ethanol was used to control the initial differences in the density of solutions on the membrane. The times of hydrodynamic instabilities in two-membrane system were symmetrical due to the concentration of ethanol, for which the densities of solutions on both sides of the membrane were the same at the initial moment. This dependence is similar for both configurations of the membrane system and is characterized by two nonlinear curves converging to the concentration of ethanol at which, at the initial moment, the densities of the solutions in the chambers of the two-membrane system are the same. In turn, the steady-state voltages of the two-membrane system as a function of the initial concentration of ethanol in the middle chamber with the same initial NaCl concentration in the middle chamber, are a complex function depending on the membrane arrangement. These voltages are characterized by a transition in the ethanol concentration range, for which, at the initial moment, the densities of the solutions in the chambers of the two-membrane system are comparable.