arXiv subjects

Claire Loison

Publications and source records attributed to Claire Loison.

8 recordsLinked to original sources

arXiv2025

Interfacial Thermal Conductance Between a Polyethylene Glycol Polymer Chain and Water: A Molecular Dynamics Study

Understanding interfacial heat transfer between polymers and water is crucial for the design of biomaterials, drug delivery platforms, and nanofluidic systems. In this study, we employed all atom molecular dynamics (MD) simulations to quantify the interfacial thermal conductance between a polyethylene glycol (PEG) 36mer chain and explicit water over the temperature range of 280-350 K. To compare the conformational behavior of the PEG chain, we examined its radius of gyration and observed a temperature dependent chain collapse consistent with previous coarse grained models. By employing a transient non equilibrium MD approach, we imposed temperature difference across the interface and analyzed the energy relaxation behavior to compute heat transfer across the polymer water interfaces. Our results demonstrate that both temperature and interfacial interaction strength influence interfacial thermal conductance, with temperature playing the dominant role. Structural factors such as chain conformation and interfacial area were found to mediate the effect of interfacial interaction. Additional analysis of the vibrational density of states (VDOS) and the mean square displacement (MSD) reveal that vibrational coupling has minimal impact on thermal conductance across interfaces, whereas increased water thermal motion enhances energy transfer. These findings highlight the structural and dynamical origins of interfacial thermal conductance and provide atomistic insights into the tuning of interfacial heat transport in molecular systems through temperature and solvent interactions.

cond-mat.soft↗

arXiv2025

The role of confined water in the emergence of electrostatic strong coupling as revealed by nanoseparated charged lipid layers

This study investigates the interplay between Strong Coupling (SC) attraction and hydration repulsion in nanoconfined water between like-charged phospholipid layers. It challenges the assumption that SC attraction requires multivalent counterions by showing that hydration water can enhance electrostatic interactions. We combine reflectivities with numerical simulations to analyze supported phospholipid layers under different relative humidity and surface charge densities. X-ray fluorescence demonstrates that we can control the valence of the associated counterions. Experimental measurement of the water thickness, combined with precise determination of charged surface positions by numerical simulations, enable us to compare our experiments with a theoretical model. It shows that charge-screening by hydration water induces SC attraction, even at moderate surface charge densities with monovalent counterions. Furthermore, hydration repulsion is stronger for DPPS compared to DPPC. These findings offer insights into the forces that control interactions between phospholipid layers and have important implications for biological and colloidal systems.

cond-mat.soft↗

arXiv2025

Dielectric permittivity of water confined in stacks of charged lipid layers: extracting profiles from molecular dynamics simulations using a modified Poisson-Boltzmann equation

Most organic and inorganic surfaces (e.g., glass, nucleic acids or lipid membranes) become charged in aqueous solutions. The resulting ionic distribution induces effective interactions between the charged surfaces. Stacks of like-charged lipid bilayers immersed in multivalent ion solutions exhibit strong coupling (SC) effects, where ion correlations cause counter-intuitive membrane attraction. A similar attraction observed with monovalent ions is explained by SC theory through reduced dielectric permittivity under confinement. To explore this phenomenon, we propose a modified Poisson-Boltzmann (mPB) model with spatially varying dielectric permittivity and explicit Born solvation energy for ions. We use the model to investigate the dielectric permittivity profile of confined water in molecular dynamics simulations of charged lipid layers stacks at varying hydration levels, and compare the results with alternative computational methods. The model captures a sharp decrease in permittivity upon dehydration, converging to a plateau value that we attribute to lipid headgroups. The generic nature of the mPB framework allows application to other systems, such as other biological interfaces or solid walls, provided ions follow Boltzmann statistics. Finally, the increase of the area per lipid in our tension-free simulations of the fluid membranes hints that the permittivity decrease upon dehydration is concomitant with an intermembrane attraction.

cond-mat.soft↗

arXiv2020

Rheology of sliding leaflets in coarse-grained DSPC lipid bilayers

Amphiphilic lipid bilayers modify the friction properties of the surfaces on top of which they are deposited. In particular, the measured sliding friction coefficient is significantly reduced compared with the native surface. We investigate in this work the friction properties of a numerical coarse-grained model of DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine) lipid bilayer subject to longitudinal shear. The interleaflet friction coefficient is obtained from out-of-equilibrium pulling and relaxation simulations. In particular, we gain access to the transient viscoelastic response of a sheared bilayer. The bilayer mechanical response is found to depend significantly on the membrane physical state, with evidence in favor of a linear response regime in the fluid but not in the gel region.

physics.bio-ph↗

arXiv2005

Fluctuations and Defects in Lamellar Stacks of Amphiphilic Bilayers

We review recent molecular dynamics simulations of thermally activated undulations and defects in the lamellar $L_α$ phase of a binary amphiphile-solvent mixture, using an idealized molecular coarse-grained model: Solvent particles are represented by beads, and amphiphiles by bead-and-spring tetramers. We find that our results are in excellent agreement with the predictions of simple mesoscopic theories: An effective interface model for the undulations, and a line tension model for the (pore) defects. We calculate the binding rigidity and the compressibility modulus of the lamellar stack as well as the line tension of the pore rim. Finally, we discuss implications for polymer-membrane systems.

cond-mat.soft↗

arXiv2005

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.

cond-mat.soft↗

arXiv2004

Amphiphiles at Interfaces: Simulation of Structure and Phase Behavior

Computer simulations of coarse-grained molecular models for amphiphilic systems can provide insight into the the structure of amphiphiles at interfaces. They can help to identify the factors that determine the phase behavior, and they can bridge between atomic descriptions and phenomenological field theories. Here we focus on model systems for amphiphilic membranes. After a brief general introduction, we present selected simulation results on monolayers, bilayers, and bilayer stacks. First, we discuss internal phase transitions in membranes and show that idealized models reproduce the generic phase behavior. Then we consider membrane fluctuations and membrane defects. The simulation data is compared with mesoscopic theories, and effective phenomenological parameters can be extracted.

cond-mat.soft↗

arXiv2004

Thermal Fluctuations in a Lamellar Phase of a Binary Amphiphile-Solvent Mixture: A Molecular Dynamics Study

We investigate thermal fluctuations in a smectic A phase of an amphiphile-solvent mixture with molecular dynamics simulations. We use an idealized model system, where solvent particles are represented by simple beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction of 20 % and sufficiently low temperature, the amphiphiles self-assemble into a highly oriented lamellar phase. Our study aims at comparing the structure of this phase with the predictions of the elastic theory of thermally fluctuating fluid membrane stacks [Lei et al., J. Phys. II 5, 1155 (1995)]. We suggest a method which permits to calculate the bending rigidity and compressibility modulus of the lamellar stack from the simulation data. The simulation results are in reasonable agreement with the theory.

cond-mat.soft↗