Biomedical subjects

Alex Travesset

Publications and source records attributed to Alex Travesset.

5 recordsLinked to original sources

PubMed2006

Atomistic simulations of langmuir monolayer collapse.

Monolayers at the vapor/water interface collapse by exploring the third dimension at sufficient lateral compression, either by forming three-dimensional structures or by solubilization into the aqueous solution. In this paper, we provide an atomistic description of collapse from molecular dynamics (MD) simulations. More specifically, we investigate monolayers of arachidic acids spread on pure water and in an aqueous solution with Ca2+ ions in the subphase. In both cases, it is found that the collapsed systems generally lead to the formation of multilayer structures, which in the system with Ca2+ ions, proceeds by an intermediate regime where the monolayer exhibits significant roughness (of the order of 4 A). If no roughness is present, the system forms collapsed structures into the aqueous solution. The computational cost of atomic MD limits our simulations to relatively small system sizes, fast compression rates, and temporal scales on the order of a nanosecond. We discuss the issues caused by these limitations and present a detailed discussion of how the collapse regime proceeds at long time scales. We conclude with a summary of the implications of our results for further theoretical and experimental studies.

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PubMed2006

How accurate is Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

Surface sensitive synchrotron X-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion (Cs+) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H3O+ to the phosphate group, leading to proton-transfer back to the phosphate group and to a reduced surface charge. The increase of Cs+ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating possible deviations from Poisson-Boltzmann theory.

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PubMed2005

Monovalent counterion distributions at highly charged water interfaces: proton-transfer and Poisson-Boltzmann theory.

Surface sensitive synchrotron-x-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion distributions (Cs+) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H3O+ at the interface leads to proton transfer back to the phosphate group by a high contact potential, whereas high salt concentrations lower the contact potential resulting in proton release and increased surface charge density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.

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PubMed2004

Curvature-induced defect unbinding in toroidal geometries.

Toroidal templates such as vesicles with hexatic bond orientational order are discussed. The total energy including disclination charges is explicitly computed for hexatic order embedded in a toroidal geometry. Related results apply for tilt or nematic order on the torus in the one Frank constant approximation. Although there is no topological necessity for defects in the ground state, we find that excess disclination defects are nevertheless energetically favored for fat torii or moderate vesicle sizes. Some experimental consequences are discussed.

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PubMed2002

Formation of vortex loops (strings) in continuous phase transitions.

The formation of vortex loops (global cosmic strings) in an O(2) linear sigma model in three spatial dimensions is analyzed numerically. For over-damped Langevin dynamics we find that defect production is suppressed by an interaction between correlated domains that reduces the effective spatial variation of the phase of the order field. The degree of suppression is sensitive to the quench rate. A detailed description of the numerical methods used to analyze the model is also reported.

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