Phases inversion

Phase inversion of emulsions can be one of two types Transitional inversion induced by changing factors that affect the HLB of the system, e.g. temperature and/or electrolyte concentration, and catastrophic inversion, which is induced by increasing the volume fraction of the disperse phase [43]. 

K also decreases sharply at the inversion point since the continuous phase is now 

Earlier theories of phase inversion were based on packing parameters -inversion occurs when exceeds the maximum packing ( 0.64 for random packing and 0.74 for hexagonal packing of monodisperse spheres for polydisperse 

At (f), ri suddenly decreases as the inverted W/O emulsion has a much lower volume fraction, k also decreases sharply at the inversion point as the continuous phase is now oil. 

Tadros, T.F. and Vincent, B. (1983) in Encyclopedia of Emulsion Technology (ed. P. Becher), Marcel Dekker, New York. Tadros, T. (ed.) (2013) Emulsion Formation and Stability, Wiley-VCH Verlag GmbH, Weinheim. 

Deryaguin, B.V. and Landau, L. (1941) Acta Physicochem. USSR, 14, 633. Verwey, E.J.W. and Overbeek, J.T.G. (1948) Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam. 

The phase-inversion process consists of the induction of phase separation in a previously homogeneous jx)lymer solution either by temperature change, by immersing the solution in a nonsolvent bath (wet process) or exposing it to a nonsolvent atmosphere (dry process). 

Only the size of the droplets increases with time. If the demixing path crosses the critical point, going directly into the unstable region (Path B), spinodal decomposition (SD) predominates. 

increasing the viscosity of the casting solution by adding a crosslinking agent 

The growth of a polymer-poor phase by SD or NG is an isotropical process, which takes place as soon as the solvent-nonsolvent contents supply the thermodynamic condition for dembdng. To understand the macrovoid formation, a quite interesting explanation was provided by McKelvey and Koros [28] as de- 

Changing the solvent may act in different ways. Solvents with higher diffusivity across the nucleus walls would be able to leave the nucleus faster, while the nonsolvent is added in, which does not favor macrovoid formation. But even 

The point of phase inversion (where the continuous phase becomes dispersed and the dispersed phase becomes continuous) is generally best detected using a conductivity technique, provided that one phase is a good conductor (e.g., an aqueous solution) and the other is not (most organic liquids). 

The phenomenon of the phase inversion itself has been investigated by Molau (1965), who studied the graft-type polyblending of styrene with 

Of course, the exact point of phase separation depends upon the choice of the polymer pair, the molecular weight control, the temperature, etc. However, for many important systems, phase separation occurs in the region of 5-15% polymerization of polymer II. Phase inversion, brought about by continued mixing, should be made to occur shortly thereafter while the viscosity is still relatively low. A conversion of 30-35 % is usually considered adequate for completion of the phase inversion process. 

The most frequent emulsiflcation using phase inversion is known as the PIT (Phase Inversion Temperature) method [81-83] and occurs through a temperature quench. This method is based on the phase behavior of nonionic surfactants and the correlation existing between the so-called surfactant spontaneous curvature and the type of emulsion obtained. 

The well-known empirical Bancroft s rule [84] states that the phase in which the surfactant is preferentially soluble tends to become the continuous phase. An analogous empirical correlation has been reported by Shinoda and Saito [85]. Eor a nonionic surfactant of the polyethoxylated type [R-(CH2-CH2-0) -0H, where R is an alkyl chain], as temperature increases, the surfactant head group becomes less hydrated and hence the surfactant becomes less soluble in water and more soluble in oil. Its phase diagram evolves as schematically shown in Fig. 1.4. At low 

Based on the pioneering work of Loeb and Sourirajan [4], membranes prepared according to the phase-inversion technique form the most important group of NF/RO-membranes, together with those prepared via interfacial polymerization 

But of prime importance with regard to the final separation process is the nature of the membrane-forming polymer its hydrophihdty, charge density, polymer structure and molecular weight Typical polymers used in this phase-separation process are cellulose esters (most commonly CA), polyamides, poly(amide-hydra-zides), polyimides, (sulfonated) polysulfones, poly(phenylene oxide) and (sulfona-ted) poly(phthalazine ether sulfone ketone). 

In preparing membranes via the phase inversion process for applications in pressure-driven processes, the formation of macrovoids should be avoided completely. These finger-like pores of the type present in the substructure of membranes (b) and (c) of Fig. 3.6-1, severely Hmit the compaction resistance of the membrane. Membranes with a sponge-Hke structure (Fig. 3.6-la) are to be preferred. 

Interfacial polymerization has become a very important and useful technique for the synthesis of thin-film composite RO and NF membranes [5, 13]. Polymerization occurs at the interface between two immiscible solvents that contain the reactants (Fig. 3.6-8). For instance, a UF membrane is immersed in an aqueous diamine solution. The excess of water is removed, and the saturated support is put in contact with an organic phase that contains an acyl chloride. As a consequence, the two monomers react to form a thin layer (1 to 0.1 pm) of PA on top of the U F membrane. 

Polyamides clearly dominate the field of thin-film composites by interfacial polymerization. The composition and morphology of the membranes depend on different parameters, including the concentration of the reactants, their partition coefficients and reactivities, the kinetics and diffusion rates of the reactants, the presence of by-products, competitive side-reactions, cross-linking reactions and postreaction treatment 

It is now well established that the choice of emulsification conditions is an important consideration in determining the ultimate drop size (and hence stability) of an emulsion. Using nonionic surfactants, Shinoda and Saito demonstrated that emulsification at the phase inversion temperature (PIT) followed by cooling led to the formation of stable O/W emulsions of small drop size. Emulsification at temperatures higher than the PIT, initially producing W/O emulsions, resulted in very stable emulsions on subsequent cooling. The inversion process, forming a 

Sagitani has described an alternative method for producing fine 0/W emulsions called surfactant-phase emulsification . Here, the addition of polyols like 1,3-butanediol to the usual emulsion components oil, water and surfactant is necessary. In the method, oil is added dropwise to an isotropic solution of concentrated surfactant in a water/polyol mixture. A clear gel eventually forms which is an oil- 

DENSE CERAMIC OXYGEN-PERMEABLE MEMBRANE REACTORS 

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Theoretical studies of the interaction between an ultrasonic beam and planar defects have been widely carried out and shown that the upper and lower tip diffraction echoes are characterized by phase inversion. In other words, the measurement of 180° phase shift between these two echoes proves the plane nature of the defect that has generated them. 

Note Conversely, it is important to emphasize that a lack of phase inversion between the signals of two superimposed echoes along the depth axis is not necessarily an evidence that the defect is volumetric (diffraction effect on a planar defect could miss if the geometry of the tips are not favorable). 

The estimated VSS and EPD allow for the observation of the tip diffraction effects (phase inversion - Atp = 180° - for the direct and mirror diffraction echoes) for all selected Ascan signals. This proves the plane nature of the OSD and confirm our initial hypothesis. 

The phase-inversion temperature (PIT) is defined as the temperature where, on heating, an oil—water—emulsifier mixture inverts from O/W to a W/O emulsion [23]. The PIT correlates very well with the HLB as illustrated in Fig. XIV-10 [72, 73]. The PIT can thus be used as a guide in emulsifier selection. 

Fig. XIV-10. The correlation between the HLB number and the phase inversion temperature in cyclohexane of nonionic surfactants. (From Ref. 71.)...

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Fig. 27. Scanning electron micrograph (a) and cross-sectional comparison (b) of screen and depth filters both having a nominal particulate cut-off of 0.4 flm. The screen filter (a Nuclepore radiation track membrane) captures particulates at the surface. The phase-inversion ceUulosic membrane traps the...

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

Fig. 27. Phase contrast photomicrographs showing particle formation via phase inversion.

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap Hquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

Most ultrafiltration membranes are porous, asymmetric, polymeric stmctures produced by phase inversion, ie, the gelation or precipitation of a species from a soluble phase (see Membrane technology). 

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

Fig. 12. Permeabilities for a two-phase blend with a phase inversion. Discontinuous phase has aspect ratio of 1.0. See Table 1 for unit conversion.

At low temperature, nonionic surfactants are water-soluble but at high temperatures the surfactant s solubUity in water is extremely smaU. At some intermediate temperature, the hydrophile—Hpophile balance (HLB) temperature (24) or the phase inversion temperature (PIT) (22), a third isotropic Hquid phase (25), appears between the oil and the water (Fig. 11). The emulsification is done at this temperature and the emulsifier is selected in the foUowing manner. Equal amounts of the oil and the aqueous phases with aU the components of the formulation pre-added are mixed with 4% of the emulsifiers to be tested in a series of samples. For the case of an o/w emulsion, the samples are left thermostated at 55°C to separate. The emulsifiers giving separation into three layers are then used for emulsification in order to find which one gives the most stable emulsion. 

If the packing surface is discontinuous in nature, a phase inversion occurs, and gas oubbles through the liquid. The column is not unstable and can be brought back to gas-phase continuous operation by merely reducing the gas rate. Analogously to the flooding condition, the pressure drop rises rapidly as phase inversion occurs. 

Flooding and Loading Since flooding or phase inversion normally represents the maximum capacity condition for a packed column, it is desirable to predict its value for new designs. The first generalized correlation of packed-column flood points was developed by Sherwood, Shipley, and Holloway [Ind. Eng. Chem., 30, 768 (1938)] on the basis of laboratory measurements primarily on the air-water system. 

Chemical Phase Inversion Svmrnetrical phase-inversion membranes (Fig, 22-71) remain the most important commercial MF membranes produced. The process produces tortiioiis-Bow membranes. It involves preparing a concentrated solution of a polvrner in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, iisiiallv w ater, sometimes from the vapor phase. The technique is irnpressivelv v ersatile, capable of producing fairlv uniform membranes wFose pore size rnav be varied within broad limits. 

Thermal Phase Inversion Thermal phase inversion is a technique wFich rnav be used to produce large quantities of MF membrane econornicallv, A solution of polvrner in poor solvent is prepared at an elevated temperature. After being formed into its final shape, a sudden drop in solution temperature causes the polvrner to precipitate, The solvent is then w ashed out. Membranes rnavbe spun at high rates using this technique. 

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

Recently, an in-depth review on molecular imprinted membranes has been published by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be distinguished (i) in-situ polymerization by bulk crosslinking (ii) preparation by dry phase inversion with a casting/solvent evaporation process [45-51] (iii) preparation by wet phase inversion with a casting/immersion precipitation [52-54] and (iv) surface imprinting. 

Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

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