A detailed study of the single-particle dynamics of liquid water in normal and supercooled regime has been carried out by comparing molecular dynamics (MD) simulation results with now available high resolution quasielastic neutron scattering (QENS) data. Simulation runs have been performed at 264, 280, 292, and 305 K, using the extended simple point charge model, well suited for reproducing single-particle properties of H2O. The microscopic dynamics has been probed over a wide range of times and distances. The MD results indicate that a substantial coupling between translational and rotational dynamics exists already at about 1 ps. The decay of the translational dynamic correlations has been phenomenologically analyzed in terms of three exponential components, and the agreement between the parameters thus obtained from experimental and simulation derived datasets is quite satisfactory. Both QENS and MD data can not be described with sufficient accuracy by simple diffusion models over the entire range of examined wave vectors.
Sustained drug delivery requires the use of multifunctional devices with enhanced properties. These properties include responsiveness to external stimuli (such as temperature, pH, ionic strength), ability to deliver suitably designed ligands to specific receptors, enhanced bioadhesion to cells, and cytocompatibility. Microgels represent one of such multifunctional drug delivery devices. Recently, we described the fabrication of a stable colloidal aqueous suspension of cytocompatible microgel spheres based on a poly(vinyl alcohol)/poly(methacrylate-co-N-isopropylacrylamide) network ( Ghugare, S. Mozetic, P. Paradossi, G. Biomacromolecules 2009 , 10 , 1589 ). These microgel spheres undergo an entropy-driven volume phase transition around the physiological temperature, this phase transition being driven by the incorporation of NiPAAm residues in the network. In that study, the microgel was loaded with the anticancer drug doxorubicin. As the microgel shrank, a marked increase in the amount of doxorubicin released was noted. Indeed, dynamic light scattering measurements showed the diameter reduction to be about 50%. In the present paper, we focus on some fundamental issues regarding modifications of the hydrogel architecture at a nanoscopic level as well as of the diffusive behavior of water associated with the polymer network around the volume phase transition temperature (VPTT). Sieving and size exclusion effects were studied by laser scanning confocal microscopy with the microgel exposed to fluorescent probes with different molecular weights. Confocal microscopy observations at room temperature and at 40 degrees C (i.e., below and above the VPTT) provided an evaluation of the variation of the average pore size (from 5 nm to less than 3 nm). Using quasielastic neutron scattering (QENS) with the IRIS spectrometer at ISIS, UK, the diffusive behavior of water molecules closely associated to the polymer network around the VPTT was investigated. A clear change in the values of diffusion coefficient of bound water was observed at the transition temperature. In addition, the local dynamics of the polymer itself was probed using the QENS spectrometer SPHERES at FRM II, Germany. For this study, the microgel was swollen in D(2)O. An average characteristic distance of about 5 A for the localized chain motions was evaluated from the elastic incoherent structure factor (EISF) and from the Q-dependence of the Lorentzian width.
We have performed a detailed analysis of the conformational and segmental dynamics in lipid-based vesicles using quasielastic neutron scattering. Our data evidence the presence of dynamical heterogeneities: the hydrogens in the headgroups and in the initial part of the hydrophobic chains perform slow diffusive motions in a confined volume; those belonging to the end part of the chains show a faster confined diffusivity together with torsional isomerization transitions. A model with three hydrogen populations has been proposed to describe the main features of this complex dynamics. The addition of a charged polysaccharide component to the vesicles mostly slows the dynamics of the headgroups and of the upper part of the acyl chains without significantly affecting the isomerization dynamics.
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