Heterogeneous membranes

The glass membrane of the electrodes discussed above may be replaced by other materials such as a single crystal or a disc pressed from finely divided crystalline material it may be advantageous to incorporate the crystalline material into an inert carrier such as a suitable polymer thus producing a heterogeneous-membrane electrode. 

Ion-selective electrodes are membrane systems used as potentiometric sensors for various ions. In contrast to ion-exchanger membranes, they contain a compact (homogeneous or heterogeneous) membrane with either fixed (solid or glassy) or mobile (liquid) ion-exchanger sites. 

Y. N. Kalia, F. Pirot, and R. H. Guy. Homogeneous transport in a heterogeneous membrane Water diffusion across human stratum comeum in vivo. Biophys. J. 

Figure 2.2 Schematic diagram of the physical model for passive transport of solutes across the intestinal membrane. The bulk aqueous solution with an aqueous boundary layer (ABL) on the mucosal side is followed by a heterogeneous membrane consisting of lipoidal and aqueous channel pathways and thereafter by a sink on the serosal side. (Adapted from Ho et al. [5]).

Schematic depiction of the structural evolution of polymer electrolyte membranes. The primary chemical structure of the Nafion-type ionomer on the left with hydrophobic backbone, side chains, and acid head groups evolves into polymeric aggregates with complex interfacial structure (middle). Randomly interconnected phases of these aggregates and water-filled voids between them form the heterogeneous membrane morphology at the macroscopic scale (right).

W. R. Cattral, Heterogeneous-membrane, carbon-supported and coated-wire... 

A. K. Covington, Heterogeneous membrane electrodes. Chapter 3 of Ion-Selective... 

The thiocyanate ion-selective electrode [21,247] This has a heterogeneous membrane containing AgCNS. It is used for the determination of the activity of SCN in suspensions of submitochondrial particles and chromatophores from Rhodospirillum rubrum in order to measure the membrane potential of these particles [200], as well as for other applications. 

The Ag2 S ISEs used in practice are of three kinds with a heterogeneous membrane containing Ag2 S precipitate (preferably precipitated from excess sulphide or by the action of H2 S [228]) in a matrix of silicon rubber [346] or a thermoplastic material [248], with a pressed pellet as a membrane [235,... 

Problems 2 and 3 are of direct relevance for an adequate understanding of concentration polarization at, respectively, composite heterogeneous and homogeneous permselective membranes. The main difference between these formulations is that in Problem 2, relevant for a composite heterogeneous membrane, the motion in a pore of the support is induced by the electro-osmotic slip due to the interaction of the applied electric field with the space charge of the electric double layer which is present already at equilibrium. 

Heterogeneous membrane Similar to solid-state type but active material dispersed in inert matrix. Cf, Br, I, S2, Ag+... 

Heterogeneous membranes of the ion-exchange resin plus binder... 

Heterogeneous Membranes of the Ion-Exchange Resin Plus Binder Type... 

Measurements at heterogeneous membranes, the potentials measured being compared with the Nernst potentials, have been carried out by A. G. Winger, G. W. Bodamer and R. Kunin (181) and by M. R. J. Wyllie and H. W. Patnode (184). 

Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

Ion-exchange membranes can also be divided, according to their structure and preparation procedure, into homogeneous and heterogeneous membranes [4]. 

Despite the fact that the skin is a heterogeneous membrane, Fick s laws of diffusion have been successfully used to analyze skin permeation data. Solutions to the second law have been used in mechanistic interpretations (see later) and in considering concentration profiles within the skin. Fick s first law has been used to analyze steady-state diffusion rates and in the development of predictive models for skin permeability. 

The bond graph method of network thermodynamics is widely used in studying homogeneous and heterogeneous membrane transport. Electroosmosis and volume changes within the compartments are the critical properties in the mechanism of cell membrane transport, and these properties can be predicted by the bond graph method of network thermodynamics. In another study, a network thermodynamics model was developed to describe the role of epithelial ion transport. The model has four membranes with series and parallel pathways and three transported ions, and simulates the system at both steady-state and transient transepithelial electrical measurements. 

Network thermodynamics has also been applied to nonstationary diffusion through heterogeneous membranes concentration profiles in the composite membrane and change of the osmotic pressure have been calculated with the modified boundary and experimental conditions. 

A calcium-sensitive heterogeneous membrane electrode has been made by evaporating an alcohol-ether solution of collodion and calcium dioctylphosphate. ... 

---