Synthetic membranes, phases

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

Cations are known to be transported through membranes by synthetic macrocyclic polyethers as well as by antibiotics. When the rate-determining step is the ion extraction from the IN aqueous phase to the membrane phase, the transport rate increases with the increasing stability constant. On the other hand, when the rate-determining step is the ion-release from the membrane phase to the OUT aqueous phase, the carrier must reduce the stability constant in order to attain efficient decomplexation. Some polyether antibiotics feature... 

Figure 3.15 Polypropylene structures, (a) Type I open cell structure formed at low cooling rates, (b) Type II fine structure formed at high cooling rates [37]. Reprinted with permission from W.C. Hiatt, G.H. Vitzthum, K.B. Wagener, K. Gerlach and C. Josefiak, Microporous Membranes via Upper Critical Temperature Phase Separation, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, Washington, DC. Copyright 1985, American Chemical Society and American Pharmaceutical Association...

J.G. Wijmans and C.A. Smolders, Preparation of Anisotropic Membranes by the Phase Inversion Process, in Synthetic Membranes Science, Engineering, and Applications, P.M. Bungay, H.K. Lonsdale and M.N. de Pinho (eds), D. Reidel, Dordrecht, pp. 39-56 (1986). 

Strathmann, H., Production of microporous media by phase inversion processes. In Material Science of Synthetic Membranes, Lloyd, D.R., Ed., American Chemical Society, ACS Symposium Series 269, Washington, DC, 1985, p. 165. 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

To achieve high selectivity, a substrate-specific receptor must be present in the membrane phase, in which it can act as a carrier between source and receiving phase. Whereas in biological membranes this task is fulfilled by ionophores such as vahnomycin (1), in artificial membranes we rely on the realm of synthetic macrocyclic receptors developed during the past two decade [83]. 

The stereoselective release behaviors of low-swelling molecularly imprinted polymer bead matrices in pressed-coat tablets were studied using either R- or S-propranolol selective MIPs. The in vitro release profiles of the low-swelling matrices showed a difference in the release of enantiomers, in that the nontemplate isomer was released faster than the template isomer. However, in the last phase of dissolution this difference was reduced and later reversed [64]. Stereoselectivity of release profiles for propranolol enantiomers were identified in MIP synthetic membranes from tablet formulations with significant differences between enantiomers [65]. Release of the enantiomer used as the print was always faster than the... 

Synthetic membrane processes perform versatile functions with the membrane acting as a barrier interface between feed and product. In liquid separations, for example, they are used to separate particles that span four orders of magnitude from dissolved ions to bacteria (Figure 1.1). Virtually all membrane processes are pressure driven, do not involve a phase change, and consume much less energy than alternate separation processes. 

There are four main types of polymeric membranes (a) Loeb—Sourirajan phase separation RO, UF and MF membranes, (b) interfacial composite RO and NF membranes, (c) solution-coated composite GS membranes, and (d) other anisotropic membranes such as plasma polymerisation coated. Several methods of manufacturing synthetic membranes are given in Table 1.5. Each method produces different membrane morphology porosity, pore size distribution, and ultrastructure. Membrane formulation techniques are discussed in detail in several texts [8, 16—18]. 

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