Interfacial membrane

The immense interfacial area separating dispersed globules from the dispersion phase is of critical Importance in determining their stability. For example, it is estimated that a typical emulsion has approximately 7 X 10 cra interfacial area per liter (3 ). Thus, those factors controlling the properties of the interfacial membrane are extremely Important in determining the stability of the emulsion. 

Figure D3.5.10 Effect of charge of interfacial membrane on location and distribution of metal catalyst.

Interfacial phenomena with special reference to biological systems. Chapter discussions include properties of proteins, polar lipids and polysaccharide at interfaces. A variety of spectrophotometric methods to resolve interfacial membrane structures are described in detail. 

When two or more emulsion droplets come into contact, a thin film of the liquid continuous phase forms between them. Coalescence is the process whereby liquid droplets merge to form a single larger droplet as a result of the rupture of both this liquid film and the interfacial membrane of the droplets. Coalescence moves an emulsion towards a thermodynamically stable state, because it reduces the contact area between the phases (Tadros and Vincent, 1983 Walstra, 1996 McClements, 1999). 

Improved stability of lipophilic bioactives may be obtained by tailoring the interfacial membranes of oil droplets. Rosenberg and Lee (2004) coated a primary whey protein-based oil-in-water emulsion containing paprika oleoresin (as the model core) with calcium alginate to enhance the stability and control the core release. 

Klinkesom, U., Sophanodora, P., Chinaclioti, P., Decker, E.A., and McClements, D.J. (2006). Characterization of spray-dried tuna oil emulsified in two-layered interfacial membranes prepared using electrostatic layer-by-layer deposition. Food Res. Int. 39,449-457. 

A broad-based effort in both the United States and Japan has since taken place to find other polymeric amine reactants that would contribute to better properties. Perhaps the most significant outcome of this effort to date has been the discovery of composite interfacial membranes based on "polyepiamine" by Wrasidlo,31 and their development into the PA-300 and RC-100 commercial forms by Riley and coworkers.32 33... 

OTHER INTERFACIAL MEMBRANES BASED ON POLYMERIC AMINES... 

Yaginuma patented interfacial membranes made by condensation of poly-alicyclic diisocyanates and diacyl halides with polyethylenimine or polyepia-mine.44 This approach was claimed to provide high organic rejections simultaneously with low salt rejections, whereas comparative data for typical aromatic diisocyanates or diacyl halides showed high rejections for both types of solutes. However, only a wastewater product, naphthalenesulfonic acid/formaldehyde condensate, was used in the testing of such membranes. 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

Synthesis of piperazine-terminated oligomers as prepolymers for interfacial membrane formation was also examined.46 48 Excess piperazine was reacted with di- and triacyl chlorides in an inert solvent such as 1,2-dichloroethane. The resulting amine-terminated polyamide oligomers had low solubility in the solvent system and precipitated. This served to limit the degree of polymerization of the oligomer. Even so, a portion of the product was insoluble in water and was filtered out during preparation of the aqueous oligomeric amine solution for the interfacial reaction step. 

Table 5.5 Reverse Osmosis Properties of Interfacial Membranes Formed of Piperazine Oligomers and Isophthaloyl Chloride...

Cadotte discovered that aromatic diamines, interfacially reacted with triacyl halides, gave membranes with dramatically different reverse osmosis performance characteristics than membranes based on aliphatic diamines. 56 Before that time, the area of aromatic amines in interfacial membrane formation had been neglected because of two factors (a) the emphasis on chlorine-resistant compositions, which favored use of secondary aliphatic amines such as piperazine, and (b) poor results that had been observed in early work on interfacial aromatic polyamides. The extensive patent network in aromatic polyamide (aramid) technology may also have been a limiting factor. 

The prepolymer approach which worked well for piperazineamide interfacial membranes was not useful in the case of FT-30 and its analogs. Poor solubility of aromatic amide precursors in aqueous media was the main obstacle. A patent application has appeared on the use of prepolymer formed from 1,3-benzenediamine and trimellitic anhydride acid chloride.66 This prepolymer contains a free carboxylate group and is soluble in water as the sodium salt. A membrane is obtained by reaction of this intermediate with trimesoyl chloride, followed by a curing step at 110° to 130°C. Salt rejections of 98.5 to 99.1% on 2,000 ppm sodium chloride solution at 200 psi were obtained fluxes were 4 to 11 gfd. 

In addition to being characterized by the area per amphiphile, the interfacial membrane is also characterized by its thickness. A, which can also change under deformations of the film. For simplicity, we assume that the equation of state of the flat membrane determines the thickness as a function of the area... 

Klinkesorn U, Sophanodora P, Chinachoti P et al. (2005) Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes. Journal of Agricultural and Food Chemistry 53 8365-8371. 

These molecules are actually supposed to penetrate the interfacial film surrounding the water droplets and hereby to alter the rheological properties of the film material. From the low dosage levels used, i.e., 5-20 ppm, one can conclude that these molecules are extremely efficient as film modifiers. A critical and decisive step for the HMW demulsifiers to perform optimally is the time requirement for the diffusion to the interfacial membrane and for the reorientation movement inside the film until local equilibrium is attained. 

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