Ultrathin membrane

The seminal discovery that transformed membrane separation fi-om a laboratory to an industrial process was the development in the 1960s of the Loeb-Sourirajan process to make defect-free ultrathin cellulose acetate membranes [1]. Loeb and Sourirajan were trying to use membranes to desalt water by reverse osmosis (RO). The concept of using a membrane permeable to water and impermeable to salt to remove salt from water had been known for a long time, but the fluxes of aU the membranes then available were far too low for a practical process. The Loeb-Sourirajan breakthrough was the development of an anisotropic membrane. The membrane consisted of a thin, dense polymer skin 0.2-0.5 pm thick sup- 

The second advance that made industrial membrane-separation processes possible was the development of methods to incorporate large membrane areas into economical membrane packets or modules. Even with the best anisotropic membranes, most industrial processes require several hundred, sometimes several thousand, square meters of membrane to perform the separation. The three most important configurations, shown schematically in Fig. 7.1, are hollow-fme-fiber, capillary-fiber and spiral-wound modules. Tubular and plate-and-frame 

Module type Manufacturing cost (US /m ) Area of standard module (m ) Characteristics 

Hollow-fine- fiber 2-10 100-300 Low cost per of membrane but modules easily fouled. Only suitable for dean fluids. 

Caplllary-fiber 5-50 50-100 Limited to low-pressure applications 200 psi good fouling resistance, can be backflushed. Important in ultraflltration (UP) and microfiltration (MF) applications. 

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Pigment particles to be studied by electron microscopy are sometimes incorporated into ultrathin membranes. At a first glance, a quantitative image analysis of such ultrathin layers seems convenient, since it appears to reflect the distribu-... 

W.R. Browall, Method for Sealing Breaches in Multi-layer Ultrathin Membrane Composites, US Patent 3,980,456 (September, 1976). 

An alternative ED method in 96 well plate format has been reported by Kariv et al. (2000). The authors used a disposable equilibrium dialyser with a 10 KDa ultrathin membrane, co-developed with Amika Corp. (Columbia MD USA). The binding of three well-studied drugs, propranolol, paroxetine and losartan with low, medium, high binding properties, respectively were tested to validate the method. The data of free fraction correlated with the published values determined by conventional ED. 

PW. Gibson, HE. Schreuder-Gibson, C. Pentheny. 1998. Electrospinning technology direct application of tailorable ultrathin membranes. J. of Coated Eabrics,2%.. 63. 

Seufert, M., Fakirov, C., and Wegner G. Ultrathin Membranes of Moleculary Reinforced Liquids on Porous Substrates. Adv. Mater. 7, 52 (1995). 

The thickness of the membranes could be controlled in a range of 20 to 500 nm. Optical interference colors of the ultrathin membrane served as a guide for estimating thickness actual measurements were made by interferometric or gravimetric methods. Ultrathin films could be cast on a water surface from many other commercial polymers for preparation of composite membranes. However, only those formed from cellulosic or related polysaccharide esters exhibited adequate flux for use in reverse osmosis. 

Miniaturization and mass production of biosensors could increase their availability and decrease their unit cost. Technologies such as microlithography, ultrathin membranes, and molecular self-assembly have the potential to facilitate the development and diversification a wide variety of biosensors. Miniature biosensors could be incorporated into food packages to monitor temperature stress, microbial contamination, or remaining shelf life, and to provide a visual indicator to consumers of product state at the time of purchase (3,51,52). 

Tubular Ultrathin Membranes for Water Desalination" in Reverse Osmosis Membrane Research, H.K. Lonsdale and H.E. Podall, Ed., Plenum Press, New York, 1972, p. 419. 

Vesicles have molecular masses in the order of 10 -10 daltons. Van der Waals attraction between such gigantic solutes is very large. Nevertheless, vesicles do not precipitate but stay in solution, because the ultrathin membranes undulate. 

Crystallization of ultrathin membrane structures in nature is also hindered by the presence of molecules with different chain lengths within these molecular assemblies. In the case of gluconamides, pseudo-racemates with alkyl chains of different lengths (e.g., Cj2 and Cg), first separate to produce left- and right-handed helices, which only later pair and rearrange to form racemic platelets within about 30 minutes (Fig. 4.5.10). 

Self-assembling membrane (SAM) is a new type of organic ultrathin membrane emerging in recent years. It consists of specific organic molecules synthesized by... 

Homok V, Erdohelyi A, Dekany I. Preparation of ultrathin membranes by layer-by-layer (LBL) deposition of oppositely charged inorganic colloids. Colloid Polym Sci 2006 284(6) 611-619. 

W.R. Browali, Method for sealing breaches in multilayer ultrathin membrane composites, US Patent 3,980,456, 1976. 

The developtnem of castirtg techniqttes fiir ultrathin membranes comprised of ISO A silicone robber-polycarbonate copolymers pemritted formation of pinhole-fiee selective menrbranes with thicknesses of aroimd 1(XX) A 1 laminating rtruHipie layms. Stt membranes ofiered flux increases of as much as 50-to 100-fold compared to their 1 rtril alicone robber counterparts. The membranes were supported typically in a plate-and-frame fashion on a porous substrate, so that accorarmodation of a large menibrane area in a small module was not feasible. 

Fig. 2 An ultrathin membrane (e.g. BLM) separating two aqueous solutions. Biface is defined as two interconnecting interfaces where material and energy transport are possible. f = A F/tm, P = electrical potential, Arf = potential difference across the membrane = Em, m = lipid bilayer thickness (estimate varies from 5 to 5 nm) (see Fig. 1 and text for details).

Fig. 1 Porous silicon membrane examples (a) ultrathin membrane (Fang et al. 2010), (b) membrane in microfluidic channel (Tjerkstra et al. 2000), (c) suspended mieromaehined membrane (Lammel and Renaud 2000), (d) membrane on...

Seufert M, Fakirov C and Wegner G (1995) Ultrathin membranes of molecularly reinforced liquids on porous substrates, Adv Mater 7 52-55. 

Figure 2. Hypothetical ultrathin membrane constructed from stacked p-sheets of a periodic polypeptide. Large and small circles represent steric requirements of amino acid side chains. Variation in side chain size creates local vacancies - or pores - in the structure functional groups X decorate the membrane surface and control wettability.

Recognizing that the key to prepare a reverse osmosis membrane of high flux is its asymmetric structure, a thin selective layer deposited on top of a porous sublayer, an ultrathin top layer was made by spreading a dilute polymer solution on water. The resultant ultrathin membrane had thicknesses ranging from 140 to 500 nm [28]. 

The ultrathin membranes prepared by the above method arc transferred to the top surface of a porous substrate membrane before being mounted in the permeation cell. Normally, the air-dried side of the membrane is brought into contact with the feed solution. When the other side (the side that faced water during solvent evaporation) is brought into contact with the feed solution, there is no significant change in the reverse osmosis performance of the membrane. Therefore, the ultrathin membrane has no asymmetric structure as far as membrane permeation is concerned. 

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