Membrane Surface Charge Property

In 2011, composite PA NF membranes were used in a study of polyelectrolyte adsorption using LbL manner (Malaisamy et al. 2011). The researchers employed measurements to assess the zeta potentials of the polyelectrolyte-modified and 

FIGURE 4.13 Membrane surface morphologies obtained using AFM after mcxlified by (a) 5 bilayers, (b) 10 bilayers, (c) 15 bUayers, and (d) 20 bilayers. (Reprinted with permission from Li, X. etal., Chem. Mater., 20,3876-3883,2008a. Copyright 2008 American Chemical Society.) 

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Zhang, Q. et al.. An improved Parks equation for prediction of surface charge properties of composite ceramic membranes, J. Membr. Sci.. 318, 100, 2008. 

C. Bellona, J. Drewes, The role of membrane surface charge and solute physicochemical properties in the rejection of organic acids by NF membranes. Journal of Membrane Science 2005, 249, 227-234. 

As both membrane surface charge and pharmaceutical properties including hydrophobicity (log D) and charge vary with pH, the pharmaceuticals rejection can be pH dependent. To examine the effect of pH on the pharmaceuticals rejection, the experiments were conducted with feed solutions at different pH levels (3, 6 and 8). TFC-1 and CTA-HW membranes with different materials were used. For both membranes, feed water pH had no effect on the permeate water flux (data not shown). In Figure 14.7a, complete or almost complete rejection of all four pharmaceuticals by TFC-1 membrane was observed over the entire pH range tested. The pH effect on the pharmaceuticals rejection was not noticeable. This indicates that TFC-1 membrane performance is stable over pH 3-8 and that the size exclusion mechanism may dominate over than pH-dependent mechanisms (charge repulsion and adsorption). 

Plasma membrane vesicles are isolated according to their surface charge properties. After phase partition of a homogeneous solution of polyethylene-glycol (5.5%, w/w) and dextran (5.5%, w/w), the plasma membrane has more affinity for the PEG-enriched upper phase whereas other membranes are accumulated at the interface and in the dextran-enriched lower phase. 

Modification of the membranes affects the properties. Cross-linking improves mechanical properties and chemical resistivity. Fixed-charge membranes are formed by incorporating polyelectrolytes into polymer solution and cross-linking after the membrane is precipitated (6), or by substituting ionic species onto the polymer chain (eg, sulfonation). Polymer grafting alters surface properties (7). Enzymes are added to react with permeable species (8—11) and reduce fouling (12,13). 

The properties of membranes commonly studied by fluorescence techniques include motional, structural, and organizational aspects. Motional aspects include the rate of motion of fatty acyl chains, the head-group region of the phospholipids, and other lipid components and membrane proteins. The structural aspects of membranes would cover the orientational aspects of the lipid components. Organizational aspects include the distribution of lipids both laterally, in the plane of the membrane (e.g., phase separations), and across the membrane bilayer (phospholipid asymmetry) and distances from the surface or depth in the bilayer. Finally, there are properties of membranes pertaining to the surface such as the surface charge and dielectric properties. Fluorescence techniques have been widely used in the studies of membranes mainly since the time scale of the fluorescence lifetime coincides with the time scale of interest for lipid motion and since there are a wide number of fluorescence probes available which can be used to yield very specific information on membrane properties. 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Early investigations of peptides in membrane model systems included studies of mel-letin 124,125 220 221 spectra and polarization properties. This water-soluble peptide is found to be structureless in solution at neutral pH but was sensitive to environmental change. The undecapeptide hormone, substance P, a member of the tackykinin family, was also found by Choo et a].1222 to be unstructured in solution at physiological pH and to aggregate at high pH or on interaction with charged lipids. These data were used as counter-evidence to a hypothesis that the membrane surface structured the peptide to facilitate interaction with the receptor. 

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