Solid compressive pressure membrane

The solid material is placed into a small vial that is introduced into a thermostated high-pressure chamber (volume 79 ml). The vial is attached to a suspension device that is magnetically coupled to the balance during measurement. The device is lowered to a neutral position when no recording of a mass point takes place, thus allowing for the balance to be tared before each measurement. This is of particular importance in slow processes in which thermodynamic equilibrium is attained only in the course of several hours or even days. The chamber may be evacuated with a vacuum pump at the beginning and at the end of each experiment. The dense gas is taken from a cylinder and, if necessary, compressed to system pressure with the help of a membrane piston pump. 

Dead-ended filtration of fermentation broth is complicated by the problem of low porosity and compressibility of the accumulated solids, which results in gradually decreasing permeability during the filtration cycle. With compressible solids, an increase in the differential pressure across the membrane can actually lead to a reduced permeation rate. This problem can be reduced by the use of fitter aids added to the broth and onto the filter paper as a precoat. The two most widely used filter aids are the diatomaceous earths and the perlites. 

A number of methods have recently been applied to determining the properties of polymer membranes. These include the spreading of mono-layers at an air-water interface and also the production of vesicles or liposomes. The former method allows for the compression of the two-dimensional film between movable barriers. At the same time a film balance registers the change in surface pressure as a function of molecular area, thereby producing a pressure-area isotherm. Various states of molecular orientation can be determined from the shapes of the plot, typified by the diagram in Fig. 5.20. Such states are analogous to those in three-dimensional systems, i.e. solid, liquid, gas. 

Due to its large compressibility, silver sulfide serves, for many manufacturers, as a basis material for halide as well as metal electrodes. In this case only about 30% of the corresponding silver halide is used relative to silver sulfide, and the mixture is compressed at higher pressures and temperatures. It is, strictly speaking, no longer a homogeneous solid-state membrane. 

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