Characterization of electrical charge separation at the interface of two aqueous solutions in the presence of concentration gradients and cation/anion mobility ratio asymmetry

Abstract

Physical consequences of ionic diffusion processes play a major role on the outcome of electrophysiology experiments due to both their contribution to the ionic transmembrane transport and phenomena taking place at the measuring instruments interface. As most of the time heterogenities in biological media with respect to ionic diffusion constants are disregarded, we intended to look upon the general case of ionic diffusion at the interface of two liquids on which gradients of these diffusion constants no longer can be neglected. We developed a theoretical model for the diffusion potential which emerges at an aqueous interface under gradients of concentration and diffusion constants. The experimental validation of our model was achieved through potential difference measurements of the diffusion potential between two solutions containing sodium chloride (NaCl) and glycerine solutions of various concentrations. Within the studied domain of the electrical charge mobility ratio, we noticed that experimental results are in agreement with the theoretically inferred diffusion potential values. This demonstrates that the resulting relationship for the diffusion potential inferred from our model could be applied for other cases, as well. When the ionic solutions contains an indefinite quantity of glycerine or an unknown substance able to modify diffusion constants of sodium and chloride, it was shown that through measurements of the diffusion potential one can infer the unknown concentration of glycerine and the modified ionic mobility ratio. This, in turn, builds up the foundation for a novel yet simple and efficient analitycal sensing device for quantitative determination in the field.

Effect of added monovalent electrolytes on the myelin formation from charged lipids

“…Moreover, the thickness of water layers between the bilayers becomes quite large from the beginning. It implies that this minimum water layer thickness between neighbouring bilayers is at least 2D Na þ (=1.44 nm) according to the steric repulsion (D Na þ defined as the diameter of hydrated Na + is 0.72 nm [30][31][32]). Moreover, Sein and Engberts [33] suggested that besides water molecules, screening ions such as Na + and the corresponding disso- Table 1 The effect of concentrations of added monovalent salts on the myelin formation from charged lipids.…”

“…ciated counterion Cl À would penetrate the membrane from the lamellar roots [2]. The diameters of the water molecule and hydrated Cl À are 0.40 nm and 0.66 nm [30][31][32], respectively. So Na + and the dissociated counterion Cl À and water molecules, possibly occupy volume, schematically shown in Fig.…”

“…Schematic drawing of negative charged lipid bilayers in lamellar structure. (a) Before screening ions penetrate the minimum water layer thickness dw0 ¼ 2D Na þ ¼ 1:44 nm (D Na þ defined as the diameter of hydrated Na + is 0.72 nm [30][31][32]); (b) After screening ions penetrate the minimum water layer thickness dw1 ¼ 3D Na þ ¼ 2:16 nm (the diameter of hydrated Cl À is estimated as 0.66 nm [30][31][32], smaller than that of Na + ). k m represent the growth rate constant.…”