Long-Range Interaction between Heterogeneously Charged Membranes

Despite their neutrality, surfaces or membranes with equal amounts of positive and negative charge can exhibit long-range electrostatic interactions if the surface charge is heterogeneous; this can happen when the surface charges form finite-size domain structures. These domains can be formed in lipid membranes where the balance of the different ranges of strong but short-ranged hydrophobic interactions and longer-ranged electrostatic repulsion result in a finite, stable domain size. If the domain size is large enough, oppositely charged domains in two opposing surfaces or membranes can be strongly correlated by the elecrostatic interactions; these correlations give rise to an attractive interaction of the two membranes or surfaces over separations on the order of the domain size. We use numerical simulations to demonstrate the existence of strong attractions at separations of tens of nanometers. Large line tensions result in larger domains but also increase the charge density within the domain. This promotes correlations and, as a result, increases the intermembrane attraction. On the other hand, increasing the salt concentration increases both the domain size and degree of domain anticorrelation, but the interactions are ultimately reduced due to increased screening. The result is a decrease in the net attraction as salt concentration is increased

Electrostatic Interactions of Asymmetrically Charged Membranes

We predict the nature (attractive or repulsive) and range (exponentially screened or long-range power law) of the electrostatic interactions of oppositely charged and planar plates as a function of the salt concentration and surface charge densities (whose absolute magnitudes are not necessarily equal). An analytical expression for the crossover between attractive and repulsive pressure is obtained as a function of the salt concentration. This condition reduces to the high-salt limit of Parsegian and Gingell where the interaction is exponentially screened and to the zero salt limit of Lau and Pincus in which the important length scales are the inter-plate separation and the Gouy-Chapman length. In the regime of low salt and high surface charges we predict - for any ratio of the charges on the surfaces - that the attractive pressure is long-ranged as a function of the spacing. The attractive pressure is related to the decrease in counter-ion concentration as the inter-plate distance is decreased. Our theory predicts several scaling regimes with different scaling expressions for the pressure as function of salinity and surface charge densities. The pressure predictions can be related to surface force experiments of oppositely charged surfaces that are prepared by coating one of the mica surfaces with an oppositely charged polyelectrolyte

The interaction between colloids in polar mixtures above Tc

We calculate the interaction potential between two colloids immersed in an aqueous mixture containing salt near or above the critical temperature. We find an attractive interaction far from the coexistence curve due to the combination of preferential solvent adsorption at the colloids' surface and preferential ion solvation. We show that the ion-specific interaction strongly depends on the amount of salt added as well as on the mixture composition. Our results are in accord with recent experiments. For a highly antagonistic salt of hydrophilic anions and hydrophobic cations, a repulsive interaction at an intermediate inter-colloid distance is predicted even though both the electrostatic and adsorption forces alone are attractive.Comment: 9 pages, 6 figure

Viscoelasticity of two-layer-vesicles in solution

The dynamic shape relaxation of the two-layer-vesicle is calculated. In additional to the undulation relaxation where the two bilayers move in the same direction, the squeezing mode appears when the gap between the two bilayers is small. At large gap, the inner vesicle relaxes much faster, whereas the slow mode is mainly due to the outer layer relaxation. We have calculated the viscoelasticity of the dilute two-layer-vesicle suspension. It is found that for small gap, the applied shear drives the undulation mode strongly while the slow squeezing mode is not much excited. In this limit the complex viscosity is dominated by the fast mode contribution. On the other hand, the slow mode is strongly driven by shear for larger gap. We have determined the crossover gap which depends on the interaction between the two bilayers. For a series of samples where the gap is changed systematically, it is possible to observe the two amplitude switchings

Attractive instability of oppositely charged membranes induced by charge density fluctuations

We predict the conditions under which two oppositely charged membranes show a dynamic, attractive instability. Two layers with unequal charges of opposite sign can repel or be stable when in close proximity. However, dynamic charge density fluctuations can induce an attractive instability and thus facilitate fusion. We predict the dominant instability modes and timescales and show how these are controlled by the relative charge and membrane viscosities. These dynamic instabilities may be the precursors of membrane fusion in systems where artificial vesicles are engulfed by biological cells of opposite charge

Alignment of Rods and Partition of Integers

We study dynamical ordering of rods. In this process, rod alignment via pairwise interactions competes with diffusive wiggling. Under strong diffusion, the system is disordered, but at weak diffusion, the system is ordered. We present an exact steady-state solution for the nonlinear and nonlocal kinetic theory of this process. We find the Fourier transform as a function of the order parameter, and show that Fourier modes decay exponentially with the wave number. We also obtain the order parameter in terms of the diffusion constant. This solution is obtained using iterated partitions of the integer numbers.Comment: 6 pages, 4 figure

Zero-range processes with saturated condensation: the steady state and dynamics

We study a class of zero-range processes in which the real-space condensation phenomenon does not occur and is replaced by a saturated condensation: that is, an extensive number of finite-size "condensates" in the steady state. We determine the conditions under which this occurs, and investigate the dynamics of relaxation to the steady state. We identify two stages: a rapid initial growth of condensates followed by a slow process of activated evaporation and condensation. We analyze these nonequilibrium dynamics with a combination of meanfield approximations, first-passage time calculations and a fluctuation-dissipation type approach.Comment: 21 pages, 12 figure

Nonionic and ionic surfactants at an interface

A Ginzburg-Landau theory is presented on surfactants in polar binary mixtures, which aggregate at an interface due to the amphiphilic interaction. They can be ionic surfactants coexisting with counterions. Including the solvation and image interactions and accounting for a finite volume fraction of the surfactant, we obtain their distributions and the electric potential around an interface in equilibrium. The surface tension is also calculated. The distribution of the adsorbed ionic surfactant is narrower than that of the counterions. The adsorption is marked for hydrophilic and hydrophobic pairs of ionic surfactant and counterions.Comment: 6 figure

Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models

We investigate pores in fluid membranes by molecular dynamics simulations of an amphiphile-solvent mixture, using a molecular coarse-grained model. The amphiphilic membranes self-assemble into a lamellar stack of amphiphilic bilayers separated by solvent layers. We focus on the particular case of tension less membranes, in which pores spontaneously appear because of thermal fluctuations. Their spatial distribution is similar to that of a random set of repulsive hard discs. The size and shape distribution of individual pores can be described satisfactorily by a simple mesoscopic model, which accounts only for a pore independent core energy and a line tension penalty at the pore edges. In particular, the pores are not circular: their shapes are fractal and have the same characteristics as those of two dimensional ring polymers. Finally, we study the size-fluctuation dynamics of the pores, and compare the time evolution of their contour length to a random walk in a linear potential

Hypertensive patients' perceptions of their physicians' knowledge about them: a cross-sectional study in Japan

<p>Abstract</p> <p>Background</p> <p>In order to evaluate the difference in quality of primary care provided by physicians between the types of medical institutions in Japan, we examined whether the physicians' comprehensive knowledge of their patients is perceived differently by the patients seen at clinics and hospitals.</p> <p>Methods</p> <p>Patients with prescriptions for hypertensive drugs were approached sequentially at 13 pharmacies, and were administered a questionnaire on their perception of their physician's knowledge about them. Data were obtained for 687 patients (362 from clinics and 325 from hospitals). A physician's knowledge of his or her patients was assessed according to six aspects: their medical history, their current medications, history of allergy, what worries patients most about their health, patients' values and beliefs on their health, and patients' roles and responsibilities at work, home, or school. Responses were scored from 1 through 6 (1: knows very well; 6: doesn't know at all).</p> <p>Results</p> <p>Patients treated in clinics were seen more frequently, for a longer period, and had fewer complications than the patients who were treated in hospitals. Among the six aspects of physicians' knowledge assessed, 79.3% of the patients reported that their physicians knew their complete list of medications "very well or well," while 28.3% reported the same about their roles and responsibilities at work, home, or school. Physicians in clinics were considered to know their patients' worries about their health (p = 0.004) and the roles and responsibilities of the patients at work, home, or school (p = 0.028) well. Multiple regression analysis showed that the type of medical institutions remained as a significant variable only for the aspect of patients' worries about their health. The factor that consistently affected the patients' perception of physicians' knowledge about them was the patients' age.</p> <p>Conclusions</p> <p>Hypertensive patients' perceptions of their physicians' knowledge about them did not differ significantly between clinics and hospitals in Japan for most of the aspects. In order to differentiate the roles of physicians in hospitals and clinics better and ensure the quality of primary care, the establishment of a standardized educational system to train primary care physicians better is recommended.</p