The role of mechanical pressure difference in the generation of membrane voltage under conditions of concentration polarization.

Sławomir Grzegorczyn,Andrzej Ślęzak

doi:10.1007/s10867-016-9413-8

Abstract

The mechanical pressure difference across the bacterial cellulose membrane located in a horizontal plane causes asymmetry of voltage measured between electrodes immersed in KCl solutions symmetrically on both sides of the membrane. For all measurements, KCl solution with lower concentration was above the membrane. In configuration of the analyzed membrane system, the concentration boundary layers (CBLs) are created only by molecular diffusion. The voltages measured in the membrane system in concentration polarization conditions were compared with suitable voltages obtained from the model of diffusion through CBLs and ion transport through the membrane. An increase of difference of mechanical pressure across the membrane directed as a difference of osmotic pressure always causes a decrease of voltage between the electrodes in the membrane system. In turn, for mechanical pressure difference across the membrane directed in an opposite direction to the difference of osmotic pressure, a peak in the voltage as a function of mechanical pressure difference is observed. An increase of osmotic pressure difference across the membrane at the initial moment causes an increase of the maximal value of the observed peak and a shift of this peak position in the direction of higher values of the mechanical pressure differences across the membrane.

Highlights

The bacterial cellulose membranes, similar to other polymer membranes, undergo relatively strong concentration polarization [1, 2] and they are convenient materials for study of diffusive and convective processes near membrane areas [3,4,5,6]
The measurements were carried out in the membrane system presented in Fig. 1a with bacterial cellulose membrane (Biofill), oriented in horizontal plane, between two chambers with the volume of 1.75 × 10−4 m3 each, filled with aqueous solutions of KCl (Cl < Ch)
For positive pressure difference, time characteristics of K+ ion activities are shifted to higher values and for negative pressure difference to smaller values in comparison to characteristics obtained without pressure difference on the membrane

Summary

Introduction

The bacterial cellulose membranes, similar to other polymer membranes, undergo relatively strong concentration polarization [1, 2] and they are convenient materials for study of diffusive and convective processes near membrane areas [3,4,5,6]. The mechanical pressure difference (ΔP) across the membrane and volume flux (Jv) through the membrane cause changes in thickness of the CBLs [31] and distribution of solute near the membrane [32]. It can be expected that fixing ΔP across the membrane different from zero will cause a significant change in volume and solutes fluxes through the membrane, influencing concentrations in CBLs near the membrane. Changes of CBLs caused by pressure difference across the membrane were visualized by interferometric methods [33]. The influence of pressure difference across the membrane on the voltage characteristics measured in the membrane system are shown in conditions of concentration polarization of membrane. This model was verified on the basis of experimental data

Theory

Experiment

Numerical calculations

Results and discussion

Conclusions