Direct observation through the membrane

Li H, Fane AG, Coster HGL, and Vigneswaran S, An assessment of depolarisation models of crossflow microfiltration by direct observation through the membrane, J. Memb. Sci. 2000 172 135-147. 

Critical Coagulation Concentration Chromatographable Organic Carbon Chloral Hydrate Forming Potential Disinfection By-Product Diethylaminoethyl Diffusion Limited Aggregation Dissolved O anic Carbon Dissolved Organic Matter Direct Observation through the Membrane Technique Diffusive Reflectance Fourier Transform Infrared Spectroscopy Electronic Conductive Carbon Black Electron Dispersive Spectra Ethylene Diamine Tetra Acetic Acid Fulvic Acid... 

DOTM Direct observation through the membrane n Viscosity... 

Figure 2.3 Schematic of a crossflow microfiltration device employing direct observation through the membrane. (From Li et al. [10]).

A significant advance in optical characterization was made with the introduction of an approach now known as direct observation through the membrane (DOTM) [5, 6]. The microscope objective is positioned on the permeate side and focused through the permeate channel and the membrane onto the membrane surface on the feed side. Thus the membranes used must be transparent. Although this is a major limitation, significant findings have been made, as discussed elsewhere. 

We have given perhaps undue attention to the mobile carrier mechanism because at one time it was assumed that the Na and K transport in excitable cell membranes occurred precisely via this mechanism. In 1965, Chandler and Meves undertook an experiment to assess the aforementioned specifics of the high-frequency conductance. A nerve fiber was placed in a solution containing no Na or K ions. This precluded direct current through the membrane. However, if there had been any mobile charged carriers in the membrane, the authors would have detected current on application of a variable field. The authors did not observe a detectable current under these conditions, from which it could be deduced that the transport systems of excitable membrane are structured as ion channels whose conductance is controlled by electric field. 

Figure 18.34. Components of the Proton-Conducting Unit of ATP Synthase. The c subunit consists of two a helices that span the membrane. An aspartic acid residue in the second helix lies on the center of the membrane. The structure of the a subunit has not yet been directly observed, but it appears to include two half-channels that allow protons to enter and pass partway but not completely through the membrane.

As described in the Introduction, the process of diffusion of a solvent through a semipermeable membrane from a less-concentrated solution into a more-concentrated solution is osmosis. This results in the development of a hydrostatic pressure head on the more-concentrated solution side of the membrane. Alternatively, pressure may be applied to the moreconcentrated solution side of the semipermeable membrane to prevent the diffusion of solvent. This applied pressure on the concentrated solution is identical to the hydrostatic pressure head that may develop owing to osmosis. It is known as the osmotic pressure and is directly proportional to the solute concentration in an ideal solution. A semipermeable membrane is one that allows the movement of only solvent molecules, and if the membrane is not semipermeable, osmosis may not be observed because the solute will diffuse quickly through the membrane to equalize the concentration on two sides of the membrane. 

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