Microporous ultrafilter

Suppose now that is sufficiently small compared to i.e. the membrane allows a substantial amount of species 2 to pass through. Therefore, unless rj and V2 are far apart, the integral in the numerator on the right-hand side of (6.3,147e) is small compared to the denominator. The selectivity of a "diffuse cut off microporous ultrafilter is not very high unless the macrosolutes differ considerably in the values of their solvated radii. Traditionally, therefore, fractionation of macrosolutes like proteins by ultrafiltration is limited to systems where the two macrosolutes differ in size by about seven to ten times (Cherkasov and Polotsky, 1996). 

Microfilters differ from ultrafilters primarily in the pore size range and construction of the membranes used to achieve a separation. The polymeric, flat sheet types are usually of a symmetric construction and exhibit either microporous or track-etched forms to facilitate either depth or surface filtration. These membranes are manufactured with pore ratings of 0.02-10 pm. 

Improved membranes have been the key to recent advances in ultrafiltration. The finest niter papers have pore diameters of as small as 1000 nm (1 micron) whereas ultrafilter membranes can be made with pore diameters from 1000 nm to as small as 2-3 nm. For many years cellophane or freshly formed films from collodion (nitrocellulose) were used, but now a number of manufacturers supply strong, flexible, and durable membranes of remarkably uniform pore size yet with high porosity, permitting rapid flow of water. Porous glass membranes have also been developed as well as porous carbon. Po rous ceramic with a microporous coating provides an ultrafilter highly resistant to high temperature and chemical attack. 

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