Ultrafiltration membranes characterization methods

Sinee this technique is based on the Kelvin equation, it is applicable only to mesopores, limited by the gas and working pressure. However, these limits can be considered as ranging from 2 to 50 mn. The method has been used and eompared wifli other ultrafiltration membrane characterization methods, with very good results [54,117—119]. 

Information about the porous support layer rather than the skin layer. The techniques used by these authors, as well as those reviewed by Pusch and Welch (21), provide valuable Insight Into the mechanism of membrane formation and thus may assist membrane scientists In developing better membranes. However, many of these techniques do not characterize the membrane under the conditions of application for example, the ultrafiltration membranes (23,24) are dried prior to gas sorption studies and microscopy. Therefore, caution must be exercised In Interpreting the results of these characterization methods and relating them to membrane performance and transport mechanisms. 

The method of thermoporometry, developed by Brun, Lallemand, Quinson and Eyraud( ), represents another method applicable, at least in principle, to characterization of pore volume in ultrafiltration membranes (, ). However, for asymmetric membranes, pore volumes explored by thermoporometry may not be the volumes associated with membrane skins and "functional" pores. 

Smolders CA, Vugteveen E (1985) New characterization methods for asymmetric ultrafiltration membranes. In Lloyd DR (ed) Materials science of synthetic membranes. ACS Symposium Series 269. American Chemical Society, Washington, DC, p 327... 

The pore size distribution can also be estimated by using two immiscible liquids, one of them soaking the membrane structure and the other a permeating liquid, which expels the wetting one [89, 100]. This technique allows determination of pore sizes from 5 to 140 nm by using immiscible liquids with appropriately low surface tensions. This method permits characterization of ultrafiltration membranes. The main advantage of this technique is the absence of high applied pressures (always below 10 bar). Examples of typically used pairs of liquids are distilled water-water saturated isobutanol (y = 1.7 mN/m), distilled water-mixture of isobutanol, methanol, and water (5 1 4 v/v) (y = 0.8 mN/m), or distilled water-mixture of isobutanol, methanol, and water (15 7 25 v/v) (y = 0.35 mN/m) [89]. 

This method has been used to characterize porous materials and specifically ultrafiltration membranes, giving good results for pore size distributions in the range 2—30 nm [118, 123—125], In Fig. 17 the pore size distribution of a y-alumina plug calculated from results obtained by Brun et al. [126], by using benzene thermoporom-etry, is shown. By N2 desorption a quite similar distribution was obtained with an average pore size of 7.2 nm. 

Because NMR spectroscopy is a nuclei-specific technique and has the ability to distinguish between similar compounds, it is an excellent method for identifying similar species in complex matrices. Thus, 31P FT-NMR spectroscopy is ideal for the identification and characterization of the hydrosphere DOP. Even so, NMR spectroscopy is fairly insensitive and requires high sample concentrations. Low DOP concentrations are increased to 31P FT-NMR detection limits by using ultrafiltration and reverse osmosis membranes. Not only is the DOP concentrated, but it is fractionated according to its molecular size. Compared to other concentration and molecular size fractionation techniques for DOP, ultrafiltration and reverse osmosis are relatively rapid and easy. 

The most important property characterizing a microporous membrane is the pore diameter (d). Some of the methods of measuring pore diameters are described in Chapter 7. Although microporous membranes are usually characterized by a single pore diameter value, most membranes actually contain a range of pore sizes. In ultrafiltration, the pore diameter quoted is usually an average value, but to confuse the issue, the pore diameter in microfiltration is usually defined in terms of the largest particle able to penetrate the membrane. This nominal pore diameter can be 5 to 10 times smaller than the apparent pore diameter based on direct microscopic examination of the membrane. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

The formation of ceramic membranes for microfiltration, ultrafiltration or nanofiltration by association of various granular layers is now a common procedure [10]. Each layer is characterized by its thickness, h, its porosity, 8, and its mean pore diameter, dp. These parameters are controlled by the particle size, d, and the synthesis method. Each layer induces a resistance which may be predicted through the classical Carman-Kozeny model ... 

Micro filtration (MF) and ultrafiltration (MF) are typical low-driven pressure membrane processes widely applied in various chemical and biochemical processes thanks to their advantages over traditional filtration methods. They are generally a thermal and simple in concept and operation and do not involve phase changes or chemical additives. Additionally, they are modular, easy to scale-up and characterized by low energy consumptions (Mulder, 1998). 

A general problem of polymer analysis is the distribution of molecular mass which should be as uniform as possible. A certain limitation is given by the purification methods Ultrafiltration is carried out with membranes which are characterized by a certain molecular mass exclusion. That means no significant change of distribution characteristics of the support during synthesis can be assumed although this has not been confirmed by published studies yet. 

Zhang PY, Xu ZL, Yang H, Wei YM, Wu WZ, Chen DG (2013) Preparation and characterization of PVDF-P(PEGMA-r-MMA) ultrafiltration blend membranes via simplified blend method. Desalination 319 47-59... 

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