Biomedical subjects

Rudi Podgornik

Publications and source records attributed to Rudi Podgornik.

6 recordsLinked to original sources

PubMed2006

Ion induced lamellar-lamellar phase transition in charged surfactant systems.

We propose a model for the liquid-liquid (L(alpha)-->L(alpha(') )) phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the L(alpha)-->L(alpha(') ) phase transition of didodecyldimethylammonium bromide (DDABr), but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters.

Adsorption↗

PubMed2005

Long-range many-body polyelectrolyte bridging interactions.

We investigate polyelectrolyte bridging interactions mediated by charged, flexible, polyelectrolyte chains between fixed cylindrical macroions of opposite charge in a two-dimensional hexagonal crystalline array. We show that in the asymptotic regime of small macroion density, the polyelectrolyte-mediated attraction is long range, falling off approximately linearly with the macroion array density. We investigate the polyelectrolyte free energy as a function of the macroion density and derive several analytic limiting laws valid in different regimes of the parameter space.

Journal Article↗

PubMed2004

Van der Waals interactions in a dielectric with continuously varying dielectric function.

We formulate and evaluate the van der Waals part of the free energy due to a dielectric profile that varies continuously throughout the space between two interacting bodies. Not considering the work needed to create the inhomogeneous dielectric profile, focusing only on that part of the free energy affected by the inhomogeneity, we compare the ensuing interaction free energy with that of the original Lifshitz formulation with its step function changes at material boundaries and uniform dielectric medium. Rather than the monotonically varying attraction between like bodies given by the original formulation, the inhomogeneous continuous dielectric function leads to attractions as well as repulsions. The Lifshitz result emerges naturally in the limit of separations much larger than the thickness of the interfaces.

Chemical Phenomena↗

PubMed2004

Electrostatic contribution to the persistence length of a semiflexible dipolar chain.

We investigate the electrostatic contribution to the persistence length of a semiflexible polymer chain whose segments interact via a screened Debye-Hückel dipolar interaction potential. We derive the expressions for the renormalized persistence length on the level of a 1/D-expansion method already successfully used in other contexts of polyelectrolye physics. We investigate different limiting forms of the renormalized persistence length of the dipolar chain and show that, in, general, it depends less strongly on the screening length than in the context of a monopolar chain. We show that for a dipolar chain the electrostatic persistence length in the same regime of the parameter phase space as the original Odijk-Skolnick-Fixman (OSF) form for a monopolar chain depends logarithmically on the screening length rather than quadratically. This can be understood solely on the basis of a swifter decay of the dipolar interactions with separation compared to the monopolar electrostatic interactions. We comment also on the general contribution of higher multipoles to the electrostatic renormalization of the bending rigidity.

Journal Article↗

PubMed2003

Universal thermal radiation drag on neutral objects.

We compute the force on a small neutral polarizable object moving at velocity v--> relative to a photon gas equilibrated at a temperature T. We find a drag force linear in v-->. Its physical basis is related to that in recent formulations of the dissipative component of the Casimir force, i.e., the change in photon momentum in emission and absorption between the moving body and the stationary thermal bath. We estimate the strength of this universal drag force for different dielectric response functions and comment on its relevance in various contexts, especially to radiation-matter coupling in the cosmos.

Journal Article↗

PubMed2003

Osmotic properties of poly(ethylene glycols): quantitative features of brush and bulk scaling laws.

From glycosylated cell surfaces to sterically stabilized liposomes, polymers attached to membranes attract biological and therapeutic interest. Can the scaling laws of polymer "brushes" describe the physical properties of these coats? We delineate conditions where the Alexander-de Gennes theory of polymer brushes successfully fits the intermembrane distance versus applied osmotic stress data of Kenworthy et al. for poly(ethylene glycol)-grafted multilamellar liposomes. We establish that the polymer density and size in the brush must be high enough that, in a bulk solution of equivalent monomer density, the polymer osmotic pressure is independent of polymer molecular weight (the des Cloizeaux semidilute regime of bulk polymer solutions). The condition that attached polymers behave as semidilute bulk solutions offers a rigorous criterion for brush scaling-law behavior. There is a deep connection between the behaviors of semidilute polymer solutions in bulk and polymers grafted to a surface at a density such that neighbors pack to form a uniform brush. In this regime, two-parameter unconstrained fits of the Alexander-de Gennes brush scaling laws to the Kenworthy et al. data yield effective monomer lengths of 3.3-3.6 A, which agree with structural predictions. The fitted distances between grafting sites are larger than expected from the nominal mole fraction of poly(ethylene glycol)-lipids; the chains apparently saturate the surface. Osmotic stress measurements can be used to estimate the actual densities of membrane-grafted polymers.

Computer Simulation↗