Microphase membranes model

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

Figure 3. Two models describing the microphases of swollen Nation membranes. Top Gierke s [48] suggestion of aqueous inverse spherical micelles connected by water-filled cylindrical channels. Bottom Yeager and Steck s [49] three-region model of a water/ionomer mixture without regular structure. Regions A, B and C are the hydrophobic polymer, the solvent bridges and the hydrophilic regions, respectively.

These five sets of observations, plus knowledge of the phenomenon of microphase separation in block copolymers leads to a model of reverse osmosis or ion exchange membranes in which the hydrophobic portions of the polymer chains have come together to form one more or less continuous microphase, while the hydrophilic portions of the polymer chains (ionic groups, -OH groups, -NH2 or > NH groups) have "dissolved" in a small amount of water to form another more or less continuous microphase when the meni>rane is swollen in water. The hydrophilic groups, in most cases, probably form clusters but not continuous microphases in the dried membranes. 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

In vesicular solution at pH = 7.1 two emission bands are present in the fluorescence spectra undissociated form and naphtholate anion. At the conditions used in our experiments almost all ArOH molecules are localized in the lipid membrane of vesicles. Described earlier microphase model of microheterogeneous systems can also be applied to vesicles. In such case the term "lipid membrane" means not only lipid bilayer itself but also head group region and adjacent water layers. 

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