Membrane Surface Hydrophilicity

FIGURE 4.9 Membrane surface hydrophilicity study based on the number of polyelectrolyte bilayers. (Reprinted with permission from Malaisamy, R. et al., Sep. Purif. Technol.. 77, 367-374, 2011.) 

FIGURE 4.10 Variation in the contact angle with the deposition of PDADMAC/PSS on a PSF substrate prepared by different methods. (Reprinted with permission from Baowei, S. U. et al., J. Memb. Sci., 2012.) 

Dynamic deposition method is also energy and time consuming in comparison to other two methods. At the cnrrent stage, dynamic deposition method using polyelectrolytes is still not snitable to be regarded as the best technique to improve the membrane surface hydrophilicity. In brief, it can be postulated that the membrane final contact angle values are strongly dependent on the type of polyelectrolyte deposited. 

---

For hydrophobic membranes treating aqueous feeds, consider pretreating the membrane to make the membrane surfaces hydrophilic. 

By considering forthcoming new environmental regulations, nanofiltration and ultrafiltration using hydrophilic membranes have become even more important for applications in water treatment such as oil-water emulsions. Conventional membranes are effective in the removal of oily microemulsions from water, but they often suffer from low flux due to limited permeability and surface fouling [66, 67]. Membrane surface hydrophilicity is widely accepted as a dominant factor that... 

Develop membranes with lower surface roughness to reduce the accumulation of foulants onto the membrane surface Hydrophilic NF membranes can be developed through grafting process using hydrophilic materials Biofilm formation or biofouling can be avoided through membrane modification using nanoparticles... 

Su et al. [120] used both static and dynamic adsorption for LbL deposition of PDADMAC and PSS on the surface of UF PS membranes. It was shown that the method used to deposit the polyelectrolyte layers did not significantly alter the membrane surface hydrophilicity, and it was found that the membrane hydrophilicity was only greatly improved when more than five bilayers were adsorbed onto the membrane surface. 

The most likely way for pardaxin molecules to insert across the membrane in an antiparallel manner is for them to form antiparallel aggregates on the membrane surface that then insert across the membrane. We developed a "raft"model (data not shown) that is similar to the channel model except that adjacent dimers are related to each other by a linear translation instead of a 60 rotation about a channel axis. All of the large hydrophobic side chains of the C-helices are on one side of the "raft" and all hydrophilic side chains are on the other side. We postulate that these "rafts" displace the lipid molecules on one side of the bilayer. When two or more "rafts" meet they can insert across the membrane to form a channel in a way that never exposes the hydrophilic side chains to the lipid alkyl chains. The conformational change from the "raft" to the channel structure primarily involves a pivoting motion about the "ridge" of side chains formed by Thr-17, Ala-21, Ala-25, and Ser-29. These small side chains present few steric barriers for the postulated conformational change. 

Nissen, J., Gritsch, S., Wiegand, G. and Radler, J. O. (1999) Wetting of phospholipid membranes on hydrophilic surfaces — Concepts towards self-healing membranes. Eur. Phys.J. B, 10, 335—344. 

The formulator must be aware of the potential for binding when filtering protein solutions. Because of the cost of most protein materials, a membrane should be used that minimizes protein adsorption to the membrane surface. Typical filter media that minimize this binding include hydrophilic polyvinylidene difluoride and hydroxyl-modified hydrophilic polyamide membranes [17a]. Filter suppliers will evaluate the compatibility of the drug product with their membrane media and also validate bacterial retention of the selected membrane. 

An alumina matrix may be prepared with high pore density (more than 60 %) and pore diameters ranging from 5 to 250 nm. Ruiz-Hitzky et al. [214] immobilized GOD in nanoporous alumina membranes with regular hexagonal arrays of highly ordered cylindrical pores aligned perpendicularly to the membrane surface. GOD was anchored in the membrane by the highly hydrophilic chitosan biopolymer. Full activity was maintained for at least 50 hours. 

Figure 22.3 The basic construction of phosphodiglyceride molecules within lipid bilayers. The fatty acid chains are embedded in the hydrophobic inner region of the membrane, oriented at an angle to the plane of the membrane surface. The hydrophilic head group, including the phosphate portion, points out toward the hydrophilic aqueous environment.

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

As a rather strongly hydrophilic anion, nitrate requires an ISE membrane containing a strongly hydrophobic cation, as described on p. 169. This function was fulfilled in the first nitrate electrode from Orion Research by cation V [180] in nitro-p-cymene 5. The electrode can be used in the pH range 4-7. In other commercial electrodes, the ion-exchanger ion is a tetra-alkylammonium salt, for example in the electrode from Coming Co., substance XIII in solvent 6 [27]. An ISE with a renewable membrane surface was found to be very useful (see section 4.1 and fig. 4.4), in which the ion-exchanger solution contains the nitrate of crystal violet VII dissolved in nitrobenzene [191]. The NOj ISE also responds to nitrites that can be removed by addition of aminosulphonic acid. 

Amido alcohol functional groups increase the surface hydrophilicity and thereby reduce interactions with biological membranes or proteins. The amino alcohols chosen (HNR1R2, where Rj and R2 each have at least one hydroxyl group) must provide the best compromise between hydrophilic coverage and viscosity while retaining high solubility. 

---