Interfacial membrane formation

In the book, Condensation Polymers By Interfacial and Solution Methods, by P.W. Morgan,67 interfacial polyamide formation is stated to occur in the organic phase, that is, on the organic solvent side of the interface. Several proofs are presented in support of this statement. For instance, monofunctional acyl halides added to the difunctional acyl halide in the organic phase always lowered polymer molecular weights. However, monofunctional amines added to the difunctional amines in the aqueous layer did not always show this effect. In this latter case, partition coefficients became a factor, particularly when relative reactivities of the amines were comparable. Mass transfer rate of diamine across the interface into the organic phase was noted to be the rate-controlling step at all concentrations of diamine. 

In the case of a polymeric amine such as polyethylenimine or polyepiamine, the solubility of the polymer in the organic phase would be very low. Reaction would take place at the interface to form an extremely thin, crosslinked network. This network would block the transport of further polymeric amine from the aqueous phase into the organic phase. Continued buildup of the membrane material on the organic side becomes impossible. Thereafter, the growth in thickness of the barrier layer is controlled by the much slower diffusion of acyl halide or isocyanate into the aqueous phase. As a result, composite membranes made by the NS-100/NS-101 type of approach will naturally tend to have very thin barrier layers-typically 200 to 250 angstroms thick. 

In the case of monomeric amines such as piperazine or 1,3-benzenediamine, transport of the amine across the water-solvent interface takes place readily. Furthermore. the interfacially formed polymer film remains rather porous to salts and small molecules (until dried or heat-cured), so that membrane material continues to form on the organic side. Therefore, barrier layer thicknesses as high as 2500 angstroms are readily produced. 

Monomeric amines have two advantages over polymeric amines in interfacial composite membrane fabrication. First, monomeric amines can be obtained in most cases as pure crystalline compounds, identical in lot after lot. Polymeric amines, on the other hand, will show variations in purity, molecular weight, chain branching and viscosity from lot to lot. This adds an element of variability to the membrane fabrication process. Second, monomeric amines lead to thicker barrier layers, which consequently tend to show better abrasion resistance and greater tolerance to chemical attack. By contrast, a membrane such as PA-300 is normally overcoated with a protective layer of water-soluble polyvinyl alcohol to minimize abrasion and salt rejection losses during spiral element assembly. 

In the patent by Kurihara, Uemura and Okada,38 combinations of a polymeric amine with a monomeric amine were used to produce composite polyamide membranes having high salt rejections. The membranes were described as having a bilayer polyamide barrier film a surface polyamide zone rich in monomeric amine, and a subsurface polyamide zone incorporating both monomeric and polymeric amine. This patent disclosure demonstrated an understanding of the mechanism of interfacial polyamide barrier layer formation. 

---

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. 

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 neutralized amine groups did not react with the acyl chloride reagents in the interfacial reaction step and did not undergo condensation reactions during the heat cure step of membrane formation. 

Zydowicz, N. Chaumont, P. Soto-Portas, M. L. Formation of aqueous core polyamide microcapsules obtained via interfacial polycondensation—Optimization of the membrane formation through pH control. Journal of Membrane Science (2001), 189(1), 41-58. 

Song, Y., Sun, R, Henry, L.L. and Sun, B. 2005. Mechanisms of structure and performance controlled thin film composite membrane formation via interfacial polymerization process. Memh. 251 67-79. 

Many researchers have studied the interfacial science and technology of laminar flow in microfluidics [8]. Interfacial polymerization and the subsequent formation of solid micro structures, such as membranes and fibers in a laminar flow system, are very interesting techniques because the bottom-up method through polymerization is suitable for the formation of miniature structures in a microspace [3]. The development of such microstructure systems plays an important role for the integration of various microfluidic operations and microchemical processing [9]. For instance, membrane formation in a microchannel and further modification has a strong potential for useful functions such as microseparation, microreaction and biochemical analysis [8-10]. Here, we will introduce several reports on polyamide and protein membrane formation through interfadal polycondensation in a microflow. 

Research has shown that the interfacial region, which is a transition phase between the continuous polymer and dispersed sieve phases, is of particular importance in successful mixed-matrix membrane formation.The type of morphology that forms at the interfacial region has a direct impact on a membrane s separation properties, and its abUity to reach the predicted Maxwell model properties. As shown in Figure 30.4, the ideal mixed-matrix membrane will exhibit both an increase in selectivity and permeability as the solid-phase volume fraction is increased, and the Maxwell model ean be used to estimate these separation properties (as discussed earlier). 

Section 33.6 reviewed the work of Kools et al. (1998) who published the only study to date of the use of UTDR to characterize membrane formation processes. This powerful appUcation of UTDR should be used to smdy other membrane formation processes such as wet-casting, thermally induced phase separation, vapor-induced phase separation, and interfacial polymerization. Indeed, UTDR characterization can provide invaluable insight into precisely how membranes acquire their unique permselective properties during the fabrication process. This knowledge in turn will help develop improved and new membranes. 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

The oscillation at a liquid liquid interface or a liquid membrane is the most popular oscillation system. Nakache and Dupeyrat [12 15] found the spontaneous oscillation of the potential difference between an aqueous solution, W, containing cetyltrimethylammo-nium chloride, CTA+CK, and nitrobenzene, NB, containing picric acid, H" Pic . They explained that the oscillation was caused by the difference between the rate of transfer of CTA controlled by the interfacial adsorption and that of Pic controlled by the diffusion, taking into consideration the dissociation of H Pic in NB. Yoshikawa and Matsubara [16] realized sustained oscillation of the potential difference and pH in a system similar to that of Nakache and Dupeyrat. They emphasized the change of the surface potential due to the formation and destruction of the monolayer of CTA" Pic at the interface. It is... 

It follows from the above that the mechanism for electrical potential oscillation across the octanol membrane in the presence of SDS would most likely be as follows dodecyl sulfate ions diffuse into the octanol phase (State I). Ethanol in phase w2 must be available for the transfer energy of DS ions from phase w2 to phase o to decrease and thus, facilitates the transfer of DS ions across this interface. DS ions reach interface o/wl (State II) and are adsorbed on it. When surfactant concentration at the interface reaches a critical value, a surfactant layer is formed at the interface (State III), whereupon, potential at interface o/wl suddenly shifts to more negative values, corresponding to the lower potential of oscillation. With change in interfacial tension of the interface, the transfer and adsorption of surfactant ions is facilitated, with consequent fluctuation in interface o/ wl and convection of phases o and wl (State IV). Surfactant concentration at this interface consequently decreased. Potential at interface o/wl thus takes on more positive values, corresponding to the upper potential of oscillation. Potential oscillation is induced by the repetitive formation and destruction of the DS ion layer adsorbed on interface o/wl (States III and IV). This mechanism should also be applicable to oscillation with CTAB. Potential oscillation across the octanol membrane with CTAB is induced by the repetitive formation and destruction of the cetyltrimethylammonium ion layer adsorbed on interface o/wl. Potential oscillation is induced at interface o/wl and thus drugs were previously added to phase wl so as to cause changes in oscillation mode in the present study. 

---