Bilayer surface tensions

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

Bilayers have received even more attention. In the early studies, water has been replaced by a continuous medium as in the monolayer simulations [64-67]. Today s bilayers are usually fully hydrated , i.e., water is included exphcitly. Simulations have been done at constant volume [68-73] and at constant pressure or fixed surface tension [74-79]. In the latter case, the size of the simulation box automatically adjusts itself so as to optimize the area per molecule of the amphiphiles in the bilayer [33]. If the pressure tensor is chosen isotropic, bilayers with zero surface tension are obtained. Constant... 

It follows from Eqs. (73) and (74) that the only stabilizing force for a-modes at long X is the membrane tension, and critical voltage vanishes as cr 0. In experiments with black lipid membranes the surface tension a arises from the contact of the bilayer with the bulk phase contained in the surrounding rim and is typically < 0.002 N/m. Then choosing... 

For lipid bilayers, equation (4) can be simplified. Above we have seen that the flat unsupported bilayer is without tension, i.e. y(0, 0) = 0, and therefore the first two terms must cancel y0 = — kcj. As argued above, JQ = 0, and thus also the third term drops out. The remaining two terms are proportional to the curvature to the power two. For a cylindrical geometry only, the term proportional to J2 is present. For spherical vesicles, the two combine into one ( kc + k)J2. The curvature energy of a homogeneously curved bilayer is found by integrating the surface tension over the available area ... 

Figure 20. (a) The (dimensionless) lateral compressibility (dilatational modulus, elastic area expansion modulus) (left ordinate) and the dimensionless area per molecule (right ordinate) as a function of the tail length (t) of the PC lipids in equilibrium bilayer membranes. The conversion to real compressibilities and areas per molecule is discussed in the text, (b) The (dimensionless) surface tension and the (dimensionless) lateral compressibility as a function of the relative expansion for the C PC lipid... 

In Figure 20 the surface tension of the bilayer is given as a function of the relative expansion of the bilayer. Of course, when the surface area is increased, the surface tension invariably goes up. The slope of this curve decreases slightly with increasing relative expansion. From this, it is seen that the membrane compressibility increases when the membrane is stretched. 

Lindahl, E. and Edholm, O. (2000). Spatial and energetic-entropic decomposition of surface tension in lipid bilayers from molecular dynamics simulations, J. Chem. Phys., 113, 3882-3893. 

Feller, S. E. and Pastor, R. W. (1999). Constant surface tension simulations of lipid bilayers the sensitivity of surface areas and compressibilities, J. Chem. Phys., Ill, 1281-1287. 

Chiu, S. W., Clark, M., Balaji, V., Subramaniam, S., Scott, H. L. and Jakobsson, E. (1995). Incorporation of surface tension into molecular dynamics simulation of an interface a fluid phase lipid bilayer membrane, Biophys. J., 69,1230-1245. 

One characteristic property of surfactants is that they spontaneously aggregate in water and form well-defined structures such as spherical micelles, cylinders, bilayers, etc. (review Ref. [524]). These structures are sometimes called association colloids. The simplest and best understood of these is the micelle. To illustrate this we take one example, sodium dode-cylsulfate (SDS), and see what happens when more and more SDS is added to water. At low concentration the anionic dodecylsulfate molecules are dissolved as individual ions. Due to their hydrocarbon chains they tend to adsorb at the air-water interface, with their hydrocarbon chains oriented towards the vapor phase. The surface tension decreases strongly with increasing concentration (Fig. 3.7). At a certain concentration, the critical micelle concentration or... 

Solubility 168 Surface Tension 169 LaPlace s Law 171 Surfactants 175 Monolayers 176 Bilayers 176 Micelles 177 Viscosity 178 Vapor Pressure 178 Boiling Point 182 Melting Point 184... 

The influence of a cut-off relative to the full treatment of electrostatic interactions by Ewald summation on various water parameters has been investigated by Feller et al. [33], These authors performed simulations of pure water and water-DPPC bilayers and also compared the effect of different truncation methods. In the simulations with Ewald summation, the water polarization profiles were in excellent agreement with experimental values from determinations of the hydration force, while they were significantly higher when a cut-off was employed. In addition, the calculated electrostatic potential profile across the bilayer was in much better agreement with experimental values in case of infinite cut-off. However, the values of surface tension and diffusion coefficient of pure water deviated from experiment in the simulations with Ewald summation, pointing out the necessity to reparameterize the water model for use with Ewald summation. 

The surfactant properties of polymeric silicone surfactants are markedly different from those of hydrocarbon polymeric surfactants such as the ethylene oxide/propylene oxide (EO/PO) block copolymers. Comparable silicone surfactants often give lower surface tension and silicone surfactants often self-assemble in aqueous solution to form bilayer phases and vesicles rather than micelles and gel phases. The skin feel and lubricity properties of silicone surfactants do not appear to have any parallel amongst hydrocarbon polymeric surfactants. 

Necessary Conditions for Stability. In a system with a fixed number of layers, such as the phospholipid bilayers, the equilibrium position (corresponding to the minimum of the free energy, F, of the whole system) is obtained when the free energy per unit area for the pair water/bilayer, f, is a minimum. This is no longer true when the number of pairs of layers is variable. In this case, at thermodynamic equilibrium one should use eq 3 c. From this equation, if the interactions between lamellae are known, one can calculate the surface tension y as a function... 

The results on formation and stability of black foam films, on the first place those on bilayer foam films (NBF) (see Sections 3.4.1.2 and 3.4.4) have promoted the development of methods which enable lung maturity evaluation. The research on stability of amphiphile bilayers and probability for their observation in the grey foam films laid the grounds of the method for assessment of foetal lung maturity created by Exerowa et al. [20,24]. Cordova et al. [25] named it Exerowa Black Film Method. It involves formation of films from amniotic fluid to which 47% ethanol and 7-10 2 mol dm 3 NaCl are added [20,24]. In the presence of alcohol the surface tension of the solution is 29 mN m 1 and the adsorption of proteins from the amniotic fluid at the solution/air interface is suppressed, while that of phospholipids predominates. On introducing alcohol, the CMC increases [26], so that the phospholipids are present also as monomers in the solution. The electrolyte reduces the electrostatic disjoining pressure thus providing formation of black foam lipid films (see Sections 3.4.1.2 and 3.4.4). 

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