Biomedical subjects

F Mallamace

Publications and source records attributed to F Mallamace.

At least 19 recordsLinked to original sources

PubMed2006

The fragile-to-strong dynamic crossover transition in confined water: nuclear magnetic resonance results.

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.

Journal Article↗

PubMed2006

Complex viscosity behavior and cluster formation in attractive colloidal systems.

The increase in viscosity that is observed in attractive colloidal systems by varying the temperature or the volume fraction can be related to the formation of structures due to particle aggregation. In particular we have studied the nontrivial dependence of the viscosity from the temperature and the volume fraction in the copolymer-micellar system L64. The comparison of the experimental data with the results of numerical simulations in a simple model for gelation phenomena suggests that this intriguing behavior can be explained in terms of cluster formation and that this picture can be quite generally extended to other attractive colloidal systems.

Journal Article↗

PubMed2002

Observation of a re-entrant kinetic glass transition in a micellar system with temperature-dependent attractive interaction.

We detect in a tri-block co-polymer micellar system an ergodic-to-nonergodic-to-ergodic transition, as a function of temperature, in a range of concentrations, by photon correlation measurements. The shear viscosity is also shown to jump two order of magnitude at these transition temperatures. Surprisingly, the structure factor as measured by small angle neutron scattering shows a marked narrowing at the structural arrest state. Rationalization of these results with the existence of an attractive branch in the phase diagram of an attractive colloid system predicted by mode coupling theory is made.

Journal Article↗

PubMed2001

Separation of scattering and absorption contributions in UV/visible spectra of resonant systems.

Resonance light scattering (RLS) is a phenomenon due to an enhancement of the scattered light in close proximity to an absorption band. The effect is easily detectable in the case of strongly absorbing chromophores, which are able to interact, thus leading to large aggregates (Pasternack, R. F.; Collings, P. J. Science 1995, 269, 935). The measurement of absorption spectra from solutions containing such resonant systems can lead to misleading results. In this paper, a simple method is described to obtain absorption spectra of aggregated species with a fairly good correction of the scattering component. The RLS spectrum, obtained using a common spectrofluorimeter, is correlated to the extinction spectrum of the same sample, allowing for an estimation of the scattering contribution to the total extinction spectrum. The method has been successfully applied both on real samples containing aggregated chromophores, such as porphyrins, chlorophyll a and gold colloids, and by simulating extinction spectra.

Chlorophyll↗

PubMed2000

Kinetic glass transition in a micellar system with short-range attractive interaction

We show that percolation and structural arrest transitions coexist in different regions of the phase diagram of a copolymer-micellar system and relate them to short-range intermicellar attraction. The intermediate scattering function shows a nonergodic transition along a temperature and concentration dependent line. Analyses show a logarithmic time dependence, attributed to a higher-order glass transition singularity predicted by mode-coupling theory, followed by a power law.

Journal Article↗

PubMed1999

Interaction and percolation in the L64 triblock copolymer micellar system.

We present results of analyses of an extensive set of static light scattering (SLS), small angle neutron scattering (SANS), and viscoelastic (frequency dependent complex moduli) measurements of aqueous solutions of a triblock copolymer micellar system. We investigate Pluronic L64 (PEO(13)PPO(30)PEO(13))-water system in a wide range of composition and temperature. We determine phase diagram of the disordered micellar phase, including a cmc-cmt curve, a cloud point curve, the critical concentration, and the critical temperature by means of SLS and SANS. The microstructure and interaction between micelles are determined by analyses of SANS intensities. SANS intensity distributions are well described by combining the cap-and-gown model for the polymer segmental distribution within a micelle and the sticky hard sphere model for the intermicellar structure factor. The existence of percolation loci at well defined poins in the temperature-concentration plane is inferred from an abrupt increase of the stickiness parameter extracted from SANS data and from two order of magnitude jump of the complex moduli at the percolation point. Study of temperature dependence of real (storage) and imaginary (loss) part of the complex modulus at fixed concentration and frequency lends further support to the existence of a percolation line. We observe an increase of some order of magnitude of the real and imaginary part of viscosity at certain temperature and composition, a phenomenon usually ascribed to a gelation process in a polymer solution. The definitive confirmation of the percolation process is obtained by frequency dependent complex viscosity measured in a frequency range 0-160 (rad/sec). From these measurements we clearly observe a well defined frequency scaling behavior of the complex moduli and a loss angle (delta) independent of the frequency. Scaling exponents, determined for frequency-dependent complex moduli satisfy the scaling relations predicted by the scalar elasticity percolation theory.

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PubMed1996