Layer Analysis of the Structure of Water Confined in Vycor Glass

A Molecular Dynamics simulation of the microscopic structure of water confined in a silica pore is presented. A single cavity in the silica glass has been modeled as to reproduce the main features of the pores of real Vycor glass. A layer analysis of the site-site radial distribution functions evidence the presence in the pore of two subsets of water molecules with different microscopic structure. Molecules which reside in the inner layer, close to the center of the pore, have the same structure as bulk water but at a temperature of 30 K higher. On the contrary the structure of the water molecules in the outer layer, close to the substrate, is strongly influenced by the water-substrate hydrophilic interaction and sensible distortions of the H-bond network and of the orientational correlations between neighboring molecules show up. Lowering the hydration has little effect on the structure of water in the outer layer. The consequences on experimental determinations of the structural properties of water in confinement are discussed.Comment: 6 pages, 8 figures included in the text, one figure added, changes in the tex

Influence of the Environment Fluctuations on Incoherent Neutron Scattering Functions

In extending the conventional dynamic models, we consider a simple model to account for the environment fluctuations of particle atoms in a protein system and derive the elastic incoherent structure factor (EISF) and the incoherent scattering correlation function C(Q,t) for both the jump dynamics between sites with fluctuating site interspacing and for the diffusion inside a fluctuating sphere. We find that the EISF of the system (or the normalized elastic intensity) is equal to that in the absence of fluctuations averaged over the distribution of site interspacing or sphere radius a. The scattering correlation function is $C(Q,t)=\sum_{n} \psi(t)$, where the average is taken over the Q-dependent effective distribution of relaxation rates \lambda_n(a) and \psi(t) is the correlation function of the length a. When \psi(t)=1, the relaxation of C(Q,t) is exponential for the jump dynamics between sites (since \lambda_n(a) is independent of a) while it is nonexponential for diffusion inside a sphere.Comment: 7 pages, 7 eps figure

Sostenibilita' ambientale di tecnologie anti-fouling: studio dell'ecotossicita' di polimeri fouling-release

Con il termine biofouling si intende il fenomeno di accumulo e deposito di organismi viventi, animali e vegetali, unicellulari o pluricellulari su superfici naturali o artificiali immerse; tale aspetto costituisce un problema che deve essere contrastato e controllato quando si presenta l’esigenza di avere superfici efficienti dal punto di vista idrodinamico (es. carene di imbarcazioni, tubazioni). Il biofouling può essere schematicamente rappresentato come una successione ecologica in cui il microfouling (o biofilm), costituito da batteri, alghe unicellulari e cianobatteri, si instaura sulle superfici preparando le stesse all’attacco del macrofouling, costituito dall’insediamento di organismi marini di maggiori dimensioni sia di origine vegetale (macroalghe) che animale (serpulidi, cirripedi, bivalvi, spugne ed altro). Per contrastare l’attacco di organismi con spiccate capacità adesive possono essere usati prodotti vernicianti antivegetativi (antifouling) che contengono al loro interno molecole con azione biocida che vengono rilasciate con tempi e a concentrazioni differenti a seconda delle matrici in cui sono incorporate. Alcune sostanze ad azione biocida ed elevata efficacia impiegate nel corso degli anni hanno mostrato livelli di tossicità elevata nei vari comparti dell’ambiente marino (sedimenti, colonna d’acqua, organismi). Un esempio è dato dai composti organostannici (es. TBT), il cui uso come antivegetativi è stato vietato a seguito delle indicazioni dell’IMO e della convenzione internazionale (AFS) adottate il 5 ottobre 2002 dagli Stati membri dell’Unione Europea. Recentemente anche le vernici a base di composti rameici, ad oggi le più utilizzate, sono state vietate in Svezia. Una alternativa alle vernici contenenti biocidi potrebbe essere l’impiego di polimeri ad azione fouling-release, la cui azione non impedisce la formazione di biofouling ma ne facilita il distacco, a causa delle deboli interazioni che si creano tra la matrice e le strutture di adesione degli organismi. Lo scopo di questa tesi di laurea è quello di valutare l’ecotossicità di polimeri fouling-release per mezzo di saggi biologici su organismi modello quali batteri bioluminescenti (Vibrio fischeri), alghe unicellulari (Dunaliella tertiolecta) e crostacei (Artemia salina). I polimeri oggetto dello studio, sintetizzati dal gruppo del Prof. Giancarlo Galli (UNIPI), sono basati su una matrice di polidimetilsilossano (PDMS) a cui sono legati i differenti copolimeri con diverse caratteristiche di idrofilicità. I polimeri, depositati su vetro, sono stati lisciviati per 15 giorni in acqua di mare naturale seguendo il protocollo descritto da Karlsson & Eklund (2004). Sui lisciviati sono state condotte le prove di tossicità

Spectral Signatures of the Diffusional Anomaly in Water

Analysis of power spectrum profiles for various tagged particle quantities in bulk SPC/E water is used to demonstrate that variations in mobility associated with the diffusional anomaly are mirrored in the exponent of the \onebyf\ region. Monitoring of \onebyf behaviour is shown to be a simple and direct method for linking phenomena on three distinctive length and time scales: the local molecular environment, hydrogen bond network reorganisations and the diffusivity. The results indicate that experimental studies of supercooled water to probe the density dependence of $1/f^\alpha$ spectral features, or equivalent stretched exponential behaviour in time-correlation functions, will be of interest.Comment: 5 Pages, 4 Figure

Crystal-like high frequency phonons in the amorphous phases of solid water

The high frequency dynamics of low- (LDA) and high-density amorphous-ice (HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering (IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of these excitations is in sharp contrast with the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to sound-like excitations which is assimilated to low local disorder.Comment: 4 pages, 3 figure

Liquid-Liquid Phase Transition for an Attractive Isotropic Potential with Wide Repulsive Range

Recent experimental and theoretical results have shown the existence of a liquid-liquid phase transition in isotropic systems, such as biological solutions and colloids, whose interaction can be represented via an effective potential with a repulsive soft-core and an attractive part. We investigate how the phase diagram of a schematic general isotropic system, interacting via a soft-core squared attractive potential, changes by varying the parameters of the potential. It has been shown that this potential has a phase diagram with a liquid-liquid phase transition in addition to the standard gas-liquid phase transition and that, for a short-range soft-core, the phase diagram resulting from molecular dynamics simulations can be interpreted through a modified van der Waals equation. Here we consider the case of soft-core ranges comparable with or larger than the hard-core diameter. Because an analysis using molecular dynamics simulations of such systems or potentials is too time-demanding, we adopt an integral equation approach in the hypernetted-chain approximation. Thus we can estimate how the temperature and density of both critical points depend on the potential's parameters for large soft-core ranges. The present results confirm and extend our previous analysis, showing that this potential has two fluid-fluid critical points that are well separated in temperature and in density only if there is a balance between the attractive and repulsive part of the potential. We find that for large soft-core ranges our results satisfy a simple relation between the potential's parameters

Liquid Polymorphism and Double Criticality in a Lattice Gas Model

We analyze the possible phase diagrams of a simple model for an associating liquid proposed previously. Our two-dimensional lattice model combines oreintati onal ice-like interactions and \"{}Van der Waals\"{} interactions which may be repulsive, and in this case represent a penalty for distortion of hydrogen bonds in the presence of extra molecules. These interactions can be interpreted in terms of two competing distances, but not necessarily soft-core. We present mean -field calculations and an exhaustive simulation study for different parameters which represent relative strength of the bonding interaction to the energy penalty for its distortion. As this ratio decreases, a smooth disappearance of the doubl e criticality occurs. Possible connections to liquid-liquid transitions of molecul ar liquids are suggested