arXiv subjects

Doru Constantin

Publications and source records attributed to Doru Constantin.

35 records · Page 2Linked to original sources

arXiv2015

Structure and interaction potentials in solid-supported lipid membranes studied by X-ray reflectivity at varied osmotic pressure

Highly oriented solid-supported lipid membranes in stacks of controlled number $N \simeq 16$ (oligo-membranes) have been prepared by spin-coating using the uncharged lipid model system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The samples have been immersed in aqueous polymer solutions for control of osmotic pressure and have been studied by X-ray reflectivity. The bilayer structure and fluctuations have been determined by modelling the data over the full q-range. Thermal fluctuations are described using the continuous smectic Hamiltonian with the appropriate boundary conditions at the substrate and at the free surface of the stack. The resulting fluctuation amplitudes and the pressure-distance relation are discussed in view of the inter-bilayer potential.

cond-mat.soft↗

arXiv2015

Structural Transition in the Isotropic Phase of the C$_{12}$EO$_6$/H$_2$O Lyotropic Mixture: A Rheological Investigation

We study the structural changes occurring in the isotropic phase of the C$_{12}$=EO$_6$/H$_2$O lyotropic mixture (up to 35% surfactant weight concentration) upon increasing the concentration and temperature, from small individual micelles to an entangled network which subsequently becomes connected. High-frequency (up to $ω= 6 \times 10^4$ rad/s) rheological measurements give us access to the viscoelastic relaxation spectrum, which can be well described by the sum of two Maxwell models with very different temperature behaviors: the slower one ($τ_1 \simeq 10^{-4}$ s) is probably due to reptation, and its associated viscosity first increases with temperature (micellar growth) and then decreases after reaching a maximum (appearance of connections). The fast mechanism ($τ_2 \simeq 10^{-6}$ s) remains practically unchanged in temperature and can be related to the relaxation of local micellar order, as observed at higher concentration in a previous investigation. This interpretation is confirmed by additional measurements in aqueous mixtures of the related surfactant C$_{12}$EO$_8$ (which forms smaller micelles), where only the fast mechanism -related to local order- is detected.

cond-mat.soft↗

arXiv2015

Electric field unbinding of solid-supported lipid multilayers

We studied by X-ray reflectivity the behaviour of fully hydrated solid-supported lipid multilayers under the influence of a transverse electric field, under conditions routinely used in the electroformation process. The kinetics of sample loss (unbinding) was measured as a function of the amplitude and frequency of the applied field by monitoring the integrated intensity of the Bragg peaks. We also performed a time-resolved analysis of the intensity of the first Bragg peak and characterized the final state of the sample.

cond-mat.soft↗

arXiv2015

High-frequency rheological behaviour of a multiconnected lyotropic phase

High-frequency (up to $ω = 6 \, 10^4$ rad/s) rheological measurements combined with light-scattering investigations show that an isotropic and multiconnected phase of surfactant micelles exhibits a terminal relaxation time of a few μs, much smaller than in solutions of entangled wormlike micelles. This result is explained in terms of the local hexagonal order of the microscopic structure and we discuss its relevance for the understanding of dynamic behaviour in related systems, such as wormlike micelles and sponge phases.

cond-mat.soft↗

arXiv2015

Membrane-mediated repulsion between gramicidin pores

We investigated the X-ray scattering signal of highly aligned multilayers of the zwitterionic lipid 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine containing pores formed by the antimicrobial peptide gramicidin as a function of the peptide/lipid ratio. We are able to obtain information on the structure factor of the pore fluid, which then yields the interaction potential between pores in the plane of the bilayers. Aside from a hard core with a radius close to the geometric radius of the pore, we find a repulsive exponential lipid-mediated interaction with a decay length of 2.5 Å and an amplitude that decreases with the pore concentration, in agreement with the hydrophobic matching hypothesis. In dilute systems, the contact value of this interaction is about 30 k$_B$T. Similar results are obtained for gramicidin pores inserted within bilayers formed by the nonionic surfactant pentaethylene glycol monododecyl ether.

cond-mat.soft↗

arXiv2015

Dynamics of bulk fluctuations in a lamellar phase studied by coherent x-ray scattering

Using x-ray photon correlation spectroscopy, we studied the layer fluctuations in the lamellar phase of an ionic lyotropic system. We measured the relaxation rate of in-plane (undulation) fluctuations as a function of the wave vector. Static and dynamic results obtained during the same experiment were combined to yield the values of both elastic constants of the lamellar phase (compression and bending moduli) as well as that of the sliding viscosity. The results are in very good agreement with dynamic light-scattering data, validating the use of the technique in ordered phases.

cond-mat.soft↗

arXiv2015

Lyotropic Lamellar Phase Doped with a Nematic Phase of Magnetic Nanorods

We report the elaboration of a hybrid mesophase combining the lamellar order of a lyotropic system of nonionic surfactant and the nematic order of a concentrated solution of inorganic nanorods confined between the surfactant layers. Highly aligned samples of this mesophase can be obtained by thermal annealing, and the orientation of the nanorods is readily controlled with a magnetic field. High-resolution synchrotron X-ray scattering and polarized optical microscopy show that, compared to their isolated counterparts, both the nematic and lamellar orders are altered, demonstrating their interplay.

cond-mat.soft↗

arXiv2015

Short-range dynamics of a nematic liquid-crystalline phase

Using x-ray photon correlation spectroscopy, we studied the dynamics in the nematic phase of a nanorodsuspension. The collective diffusion coefficient in the plane perpendicular to the director varies sharply with the wave vector. Combining the structure factor and the diffusion coefficient, we find that the hydrodynamic function of the phase decreases by more than a factor of 10 when going from length scales comparable to the interparticle distance toward larger values. Thus, the collective dynamics of the nematic phase experiences strong and scale-dependent slowing down, in contrast with isotropic suspensions of slender rods or of spherical particles.

cond-mat.soft↗

arXiv2015

The interaction of hybrid nanoparticles inserted within surfactant bilayers

We determine by small-angle x-ray scattering the structure factor of hydrophobic particles inserted within lamellar surfactant phases for various particle concentrations. The data are then analyzed by numerically solving the Ornstein-Zernicke equation, taking into account both the intra- and interlayer interactions. We find that particles within the same layer repel each other and that the interaction potential (taken as independent of the concentration) has a contact value of 2.2 k$_{\text{B}}$T and a range of about 10 Å. If the amplitude is allowed to decrease with increasing concentration, the contact value in the dilute limit is about 5 k$_{\text{B}}$T for a similar range.

cond-mat.soft↗

arXiv2015

Slow dynamics of a colloidal lamellar phase

We used x-ray photon correlation spectroscopy to study the dynamics in the lamellar phase of a platelet suspension as a function of the particle concentration. We measured the collective diffusion coefficient along the director of the phase, over length scales down to the interparticle distance, and quantified the hydrodynamic interaction between the particles. This interaction sets in with increasing concentration and can be described qualitatively by a simplified model. No change in the microscopic structure or dynamics is observed at the transition between the fluid and the gel-like lamellar phases.

cond-mat.soft↗

arXiv2015

Repulsion Between Inorganic Particles Inserted Within Surfactant Bilayers

We study by synchrotron small-angle x-ray scattering highly aligned lamellar phases of a zwitterionic surfactant, doped with monodisperse and spherical hydrophobic inorganic particles as a function of particle concentration. Analysis of the structure factor of the two-dimensional fluid formed by the particles in the plane of the bilayer gives access to their membrane-mediated interaction, which is repulsive, with a contact value of about 4k$_{\text{B}}$T and a range of 14 Å. Systematic application of this technique should lead to a better understanding of the interaction between membrane inclusions.

cond-mat.soft↗

arXiv2015

Morphology of gold nanoparticles determined by full-curve fitting of the light absorption spectrum. Comparison with X-ray scattering and electron microscopy data

UV-Vis absorption spectroscopy is frequently used to characterize the size and shape of gold nanoparticles. We present a full-spectrum model that yields reliable results for the commonly encountered case of mixtures of spheres and rods in varying proportions. We determine the volume fractions of the two populations, the aspect ratio distribution of the nanorods (average value and variance) and the interface damping parameter. We validate the model by checking the fit results against small-angle X-ray scattering and transmission electron microscopy data and show that correctly accounting for the polydispersity in aspect ratio is essential for a quantitative description of the longitudinal plasmon peak.

cond-mat.soft↗

arXiv2015

A two-dimensional nematic phase of magnetic nanorods

We report a hybrid mesophase consisting of magnetic nanorods confined between the non-ionic surfactant bilayers of a lamellar phase. The magnetic field-induced ordering of the nanorods was measured experimentally and modeled by a two-dimensional Onsager theory including the third virial coefficient. The nanorods are strongly confined in layers, with no orientational coupling from one layer to the next. At high volume concentration they exhibit spontaneous in-plane orientational ordering and form a stack of independent two-dimensional nematic systems. This isotropic-nematic transition is first-order.

cond-mat.soft↗

arXiv2015

The interaction of charged nanoparticles at interfaces

We study charged nanoparticles adsorbed onto surfactant bilayers using small-angle scattering of synchrotron radiation. The in-plane interaction of the particles is well described by a DLVO component (measured independently in solution) and a repulsive dipolar interaction due to the presence of the interface, with an amplitude close to the theoretical prediction. We prove that charged nanoparticles at soft interfaces are well described by the classical model of Poisson-Boltzmann and van der Waals terms; as a corollary, they do not experience the like-charge attraction reported in the literature for some systems of micron-sized spheres at interfaces.

cond-mat.soft↗

arXiv2014

Infrared dichroism of gold nanorods controlled using a magnetically addressable mesophase

Gold nanorods have unique optical properties, which make them promising candidates for building nano-structured materials using a "bottom-up" strategy. We formulate stable bulk materials with anisotropic optical properties by inserting gold and iron oxide nanorods within a lamellar mesophase. Quantitative measurements of the order parameter by modelling the absorbance spectra show that the medium is macroscopically aligned in a direction defined by an external magnetic field. Under field, the system exhibits significant absorption dichroism in the infrared range, at the position of the longitudinal plasmon peak of the gold nanorods (about 1200 nm), indicating strong confinement of these particles within the water layers of the lamellar phase. This approach can yield soft and addressable optical elements.

cond-mat.soft↗

arXiv2014

Lamellar Lα Mesophases Doped with Inorganic Nanoparticles

The development of nanostructured hybrid systems is a flourishing area of research, which brings together chemistry, physics and materials science. These systems are composed of nanoparticles with interesting properties (e.g. optical, magnetic, catalytic) dispersed within an organic matrix. Control of both the position and orientation of the particles in a precise and reproducible way is an important goal. Towards this goal, the use of lyotropic liquid crystals as host phases is a promising strategy that has prompted sustained experimental work over the last decade. Here we briefly review this field, with an emphasis on the structure and the physical characterization of these novel materials.

cond-mat.soft↗

arXiv2012

Bilayer elasticity at the nanoscale: the need for new terms

Continuum elastic models that account for membrane thickness variations are especially useful in the description of nanoscale deformations due to the presence of membrane proteins with hydrophobic mismatch. We show that terms involving the gradient and the Laplacian of the area per lipid are significant and must be retained in the effective Hamiltonian of the membrane. We reanalyze recent numerical data, as well as experimental data on gramicidin channels, in light of our model. This analysis yields consistent results for the term stemming from the gradient of the area per molecule. The order of magnitude we find for the associated amplitude, namely 13-60 mN/m, is in good agreement with the 25 mN/m contribution of the interfacial tension between water and the hydrophobic part of the membrane. The presence of this term explains a systematic variation in previously published numerical data.

cond-mat.soft↗