Head-group zones

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

Surfactants are amphophilic molecules, which consist of a hydrophobic carbohydrate part and a hydrophilic head group. In capillary zone electrophoresis (CZE), different types, i.e., anionic, cationic, but also neutral, tensides are employed. The ability of such molecules to interact with ionic and nonionic species has been used in ion chromatography and, in particular, in SDS-poly-(acrylamide) gel electrophoresis (PAGE) (15). 

The reversal of the direction of the electro-osmotic flow by the adsorption onto the capillary wall of alky-lammonium surfactants and polymeric ion-pair agents incorporated into the electrolyte solution is widely employed in capillary zone electrophoresis (CZE) of organic acids, amino acids, and metal ions. The dependence of the electro-osmotic mobility on the concentration of these additives has been interpreted on the basis of the model proposed by Fuerstenau [6] to explain the adsorption of alkylammonium salts on quartz. According to this model, the adsorption in the Stern layer as individual ions of surfactant molecules in dilute solution results from the electrostatic attraction between the head groups of the surfactant and the ionized silanol groups at the surface of the capillary wall. As the concentration of the surfactant in the solution is increased, the concentration of the adsorbed alkylammonium ions increases too and reaches a critical concentration at which the van der Waals attraction forces between the hydrocarbon chains of adsorbed and free-surfactant molecules in solution cause their association into hemimicelles (i.e., pairs of surfactant molecules with one cationic group directed toward the capillary wall and the other directed out into the solution). 

The formation of the spherocylindrical shapes is not well understood. They are observed not only on lipid vesicles but also on polymersomes (vesicles made of diblock copolymers [123, 124]) [107]. Therefore, lipid-specific effects, for example partial head group charge and membrane thickness, as a possible cause for the observed cylindrical deformations are to be excluded. One possible explanation could be that ions flatten the equatorial zone of the deformed vesicle. During the pulse there is an inhomogeneity in the membrane tension due to the fact that the... 

The model system has also showed much lesser hydration of phosphate groups, a feature probably contributing to easier penetration by water, since the water molecules do not have to remain bound within the hydration shells. The atom charge distribution across the membrane showed that phospholipid head-groups provide an electrostatic environment conductive to penetration of the headgroup zone by water molecules and sodium ions. This conclusion was also borne out by the fact that water diffused faster in the interface region of the DTPS membrane than in the DPPC membrane. 

Fig. 5. a Bilayer organisation of the stratum corneum lipids. In the corneocyte, a protein envelope has replaced the lipid cell membrane of the cell of the viable epidermis and some lipids are covalently bound to this envelope (not represented here). Between the hydrophilic head groups of the stacked bilayers resides a water tablet, within which diffusion transport may take place parallel to the skin surface, b A water molecule passing through the skin barrier is visualised to take a conspicuously meandering way through the liquid-crystalline zones of the... 

Interestingly, in addition to spherical reverse micelles, even very long, threadlike micelles with a water-rich core have been documented [63], indicating that a comparatively rigid zone of tightly packed hydrocarbon chains is present next to the polar head groups, granting mechanical stability as discussed previously in Section II.D. 

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