Electric bilayers

In the field of electrochemistry, several materials are known to induce a stable electric bilayer when attached to another material, without any power supply, so we looked for suitable materials to induce electric potential on the skin surface. Here I will describe our recent work on ionic polymers and barium sulfate as examples, because they are used as ingredients for cosmetic products. 

FIGURE 15.2 Schematic illustration of electric bilayers induced by ionic polymers on the surface of the skin. When the counter ion of the anionic polymer is sodium, the skin surface is negatively charged because of the diffusion of sodium ions. Calcium and magnesium ions do not diffuse as easily as sodium ions. Thus, when the counter ion is calcium or magnesium, an electric bilayer does not form. When the counter ion of a cationic polymer is chloride, an oppositely charged electric bilayer is induced, and the skin surface is positively charged. 

Procarione W L and Kauffman J W 1974 The electrical properties of phospholipid bilayer Langmuir films Chem. Phys. Lipids 12 251-60... 

Simple considerations show that the membrane potential cannot be treated with computer simulations, and continuum electrostatic methods may constimte the only practical approach to address such questions. The capacitance of a typical lipid membrane is on the order of 1 j.F/cm-, which corresponds to a thickness of approximately 25 A and a dielectric constant of 2 for the hydrophobic core of a bilayer. In the presence of a membrane potential the bulk solution remains electrically neutral and a small charge imbalance is distributed in the neighborhood of the interfaces. The membrane potential arises from... 

Figure 7 The electric potential relative to the hydrocarbon ( dipole potential) as a function of distance from the center of a fully hydrated DPPC bilayer.

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

On the experimental front, Burrows and Forrest 155] have measured the electric field and thickness dependence of the current and radiance from bilayer devices with various HTLs and Alqs as the ETL. The data were analyzed in temis of trap-limited transport in the Alq t layer, with the assumption that the voltage drop across the HTL is negligible. However, this assumption was challenged by Vestweber and Riess [ I56 and Giebcler et al. 1157], who demonstrated that HTL plays an important role in determining the efficiency of bilayer OLEDs. 

The axonal membrane is a lipid bilayer in the nerve fibre. Ionic channels and other proteins are located in the membrane to achieve electrical activity. Action potentials are generated and conducted along the membrane. 

Figure 25. Movement rate of bilayer devices (along an angle of 90°) with different dimensions (different polypyrrole weights) versus applied electrical current per mass unit (mA mg ). (Reprinted fromT. F. Otero and J. M. Sansinana, Bilayerdimensions and movement of artificial muscles. Bioelectrochem. Bioener-genetics 47, 117, 1997, Fig. 4. Copyright 1997. Reprinted with permission from Elsevier Science.)...

Thus the formation of tilted analogues of the smectic A phases, i.e. monolayer Cl and bilayer C2, is possible for mesogens with relatively large electric quadrupoles. In the case of strongly sterically asymmetric molecules (e.g., zigzag shaped or dumbell shaped molecules, Fig. 3b) these quadrupolar interactions may be steric in origin. From this point of view observation of molecular tilt in the molecular dynamics simulations for a one-layer film of DOBAMBC in the absence of electrostatic interactions is not so surprising [106]. 

FIG. 11 Schematic illustration of the electric potential profiles inside and outside a nanopore with lipid bilayer membranes separating the internal and external electrolyte solutions. The dotted line is a junction potential representation where the internal potential is shifted. 

Groves, J. T, Boxer, S. G. and McCormeh, H. M. (1997) Electric field-induced reorganization of two-component supported bilayer membranes. Proc. Natl. Acad. Sci. USA, 94, 13390-13395. 

Groves, J. T., Wiilfing, C. and Boxer, S. G. (1996) Electrical manipulation of glycan-phosphatidyl inositol-tethered proteins in planar supported bilayers. Biophys.J., 71, 2716-2723. 

GMO bilayers Polar head/acyl core Electrical time constant 30-37 451... 

Cevc, G. Marsh, D. Properties of the electrical double layer near the interface between a charged bilayer membrane and electrolyte solution Experiment vs. theory, J. Phys. Chem. 87, 376-379 (1983). 

Purely electrochromic dyes. Transient membrane potentials have typical amplitudes on the order of 100 mV across a typical 4 nm thick bilayer. This means an average electric field within the membrane of 2.5 x 107 Vm 1 or 2.5 x 105 V cm While this may seem like a very large field, in fact it produces a small wavelength shift of only about 0.4 nm (20 cm-1) in A,max for typical dyes absorbing... 

Fig. 6 The electrical potential, ij/, profile across a lipid bilayer. The transmembrane potential, Aij/, is due to the difference in anion and cation concentrations between the two bulk aqueous phases. The surface potential, ij/s, arises from charged residues at the membrane-solution interface. The dipole potential, J/d, results from the alignment of dipolar residues of the lipids and associated water molecules within the membrane...

A variety of methods have been developed to study exocytosis. Neurotransmitter and hormone release can be measured by the electrical effects of released neurotransmitter or hormone on postsynaptic membrane receptors, such as the neuromuscular junction (NMJ see below), and directly by biochemical assay. Another direct measure of exocytosis is the increase in membrane area due to the incorporation of the secretory granule or vesicle membrane into the plasma membrane. This can be measured by increases in membrane capacitance (Cm). Cm is directly proportional to membrane area and is defined as Cm = QAJV, where Cm is the membrane capacitance in farads (F), Q is the charge across the membrane in coulombs (C), V is voltage (V) and Am is the area of the plasma membrane (cm2). The specific capacitance, Q/V, is the amount of charge that must be deposited across 1 cm2 of membrane to change the potential by IV. The specific capacitance, mainly determined by the thickness and dielectric constant of the phospholipid bilayer membrane, is approximately 1 pF/cm2 for intracellular organelles and the plasma membrane. Therefore, the increase in plasma membrane area due to exocytosis is proportional to the increase in Cm. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

Figure 8.16 Illustration of symmetry of Soto Bustamante-Blinov achiral antiferroelectric smectic LC with finite number of layers. Such systems can be studied using DRLM technique with thin freely suspended smectic films, (a) With even number of bilayers, film has local C2 symmetry, and therefore no net electric polarization, (b) With odd number of bilayers, film has local Cnv symmetry and is therefore polar, with net spontaneous electric polarization in plane of layers.

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