Browsing by Author "Podgornik, R."

Now showing 1 - 4 of 4

Open Access
Contribution of dipolar bridging to phospholipid membrane interactions: A mean-field analysis
(A I P Publishing LLC, 2021-06-11) Büyükdağlı, Şahin; Podgornik, R.
We develop a model of interacting zwitterionic membranes with rotating surface dipoles immersed in a monovalent salt and implement it in a field theoretic formalism. In the mean-field regime of monovalent salt, the electrostatic forces between the membranes are characterized by a non-uniform trend: at large membrane separations, the interfacial dipoles on the opposing sides behave as like-charge cations and give rise to repulsive membrane interactions; at short membrane separations, the anionic field induced by the dipolar phosphate groups sets the behavior in the intermembrane region. The attraction of the cationic nitrogens in the dipolar lipid headgroups leads to the adhesion of the membrane surfaces via dipolar bridging. The underlying competition between the opposing field components of the individual dipolar charges leads to the non-uniform salt ion affinity of the zwitterionic membrane with respect to the separation distance; large inter-membrane separations imply anionic excess, while small nanometer-sized separations favor cationic excess. This complex ionic selectivity of zwitterionic membranes may have relevant repercussions on nanofiltration and nanofluidic transport techniques
Open Access
Interactions between zwitterionic membranes in complex electrolytes
(American Physical Society, 2020) Büyükdağlı, Şahin; Podgornik, R.
We investigate the electrostatic interactions of zwitterionic membranes immersed in mixed electrolytes composed of mono- and multivalent ions. We show that the presence of monovalent salt is a necessary condition for the existence of a finite electrostatic force on the membrane. As a result, the mean-field membrane pressure originating from the surface dipoles exhibits a nonuniform salt dependence, characterized by an enhancement for dilute salt conditions and a decrease at intermediate salt concentrations. On addition of multivalent cations to the submolar salt solution, the separate interactions of these cations with the opposite charges of the surface dipoles makes the intermembrane pressure more repulsive at low membrane separation distances and strongly attractive at intermediate distances, resulting in a discontinuous like-charge binding transition followed by the membrane binding transition. By extending our formalism to account for correlation corrections associated with large salt concentrations, we show that membranes of high surface dipole density immersed in molar salt solutions may undergo a membrane binding transition even without the multivalent cations. Hence, the tuning of the surface polarization forces by membrane engineering can be an efficient way to adjust the equilibrium configuration of dipolar membranes in concentrated salt solutions.
Open Access
Like-charge polymer-membrane complexation mediated by multivalent cations: one-loop-dressed strong coupling theory
(American Institute of Physics, 2019) Büyükdağlı, Şahin; Podgornik, R.
We probe the electrostatic mechanism driving adsorption of polyelectrolytes onto like-charged membranes upon the addition of tri- and tetravalent counterions to a bathing monovalent salt solution. We develop a one-loop-dressed strong coupling theory that treats the monovalent salt at the electrostatic one-loop level and the multivalent counterions within a strong-coupling approach. It is shown that the adhesive force of the multivalent counterions mediating the like-charge adsorption arises from their strong condensation at the charged membrane. The resulting interfacial counterion excess locally maximizes the screening ability of the electrolyte and minimizes the electrostatic polymer grand potential. This translates into an attractive force that pulls the polymer to the similarly charged membrane. We show that the high counterion valency enables this adsorption transition even at weakly charged membranes. Additionally, strongly charged membranes give rise to monovalent counterion-induced correlations and intensify the interfacial multivalent counterion condensation, strengthening the complexation of the polymer with the like-charged membrane, as well as triggering the orientational transition of the molecule prior to its adsorption. Finally, our theory provides two additional key features as evidenced by previous adsorption experiments: first, the critical counterion concentration for polymer adsorption decreases with the rise of the counterion valency and, second, the addition of monovalent salt enhances the screening of the membrane charges and suppresses monovalent counterion correlations close to the surface. This weakens the interfacial multivalent counterion condensation and results in the desorption of the polymer from the substrate.
Open Access
Orientational transition and complexation of DNA with anionic membranes: weak and intermediate electrostatic coupling
(American Physical Society, 2019) Büyükdağlı, Şahin; Podgornik, R.
We characterize the role of charge correlations in the adsorption of a short, rodlike anionic polyelectrolyte onto a similarly charged membrane. Our theory reveals two different mechanisms driving the like-charge polyelectrolyte-membrane complexation: In weakly charged membranes, repulsive polyelectrolyte-membrane interactions lead to the interfacial depletion and a parallel orientation of the polyelectrolyte with respect to the membrane; while in the intermediate membrane charge regime, the interfacial counterion excess gives rise to an attractive “salt-induced” image force. This furthermore results in an orientational transition from a parallel to a perpendicular configuration and a subsequent short-ranged like-charge adsorption of the polyelectrolyte to the substrate. A further increase of the membrane charge engenders a charge inversion, originating from surface-induced ionic correlations, that act as a separate mechanism capable of triggering the like-charge polyelectrolyte-membrane complexation over an extended distance interval from the membrane surface. The emerging picture of this complexation phenomenon identifies the interfacial “salt-induced” image forces as a powerful control mechanism in polyelectrolyte-membrane complexation.