18 research outputs found
Thermodynamics of water confined within hydrophobic Metal-Organic Frameworks and the dynamics of intrusion/extrusion process
No full textQuesta tesi di dottorato è dedicata all'indagine delle interazioni tra acqua e materiali microporosi idrofobici attraverso simulazioni di Dinamica Molecolare (MD). Lo studio delle proprietà dell'acqua confinata in cavità microscopiche e della dinamica dell'intrusione del liquido a scala microscopica è guidato dalla promessa di molte applicazioni tecnologiche interessanti, tra cui: una nuova tecnologia per lo sviluppo di dispositivi per l'accumulo di energia ecologici sotto forma di batterie meccaniche, nonché sistemi di dissipazione dell'energia e, in particolare, ammortizzatori per il mercato automobilistico, e molte altre. Le simulazioni di Dinamica Molecolare ci hanno permesso non solo di indagare il comportamento dell'acqua sotto condizioni di confinamento elevato, ma anche la dinamica nel processo di intrusione/estrusione sotto diverse condizioni termodinamiche. Inoltre, abbiamo fatto luce sul ruolo della morfologia/topologia complessa dei solidi microporosi considerati, che nel presente studio sono Metal-Organic Frameworks idrofobici. Anticipando alcuni risultati, attraverso simulazioni atomistiche eseguite sul MOF Cu2(tebpz) abbiamo mostrato una riduzione senza precedenti della temperatura critica dell'acqua confinata di 200-250 K rispetto al suo valore bulk, confermata anche da analisi sperimentali. Inoltre, esperimenti in-silico e di porosimetria liquida hanno rivelato una fenomenologia in contrasto con le leggi fisiche consolidate, ovvero la legge di Young-Laplace. Una fenomenologia analoga è ststat osservata anche con ZIF-8. Le simulazioni confermano che il quadro teorico sviluppato per Cu2(tebpz) spiega anche i comportamenti di ZIF-8, suggerendo una possibile generalità di questi fenomeni fisici. I calcoli di energia libera tramite Dinamica Molecolare, eseguiti con tecniche avanzate, ci hanno permesso di spiegare come l'idrofobicità di materiali microporosi complessi, come ZIF-8, dipenda non solo dalla sua composizione chimica, ma anche dalla sua rugosità/texture superficiale, fornendo un suggerimento su come regolarne l'idrofobicità. Infine, le simulazioni di Dinamica Molecolare, supportate anche da risultati sperimentali, hanno rivelato la dipendenza della temperatura del calore associato al processo di intrusione dell'acqua e l'influenza che la dimensione dei cristalliti può avere su di essa.This PhD thesis is devoted to the investigation of interactions between water and hydrophobic microporous materials through Molecular Dynamics (MD) simulations. The study of the properties of water confined in microscopic cavities and the dynamics of the liquid intrusion at a microscopic scale is driven by the promise of many interesting technological applications, including: a novel technology for developing eco-friendly
energy storage devices in the form of mechanical batteries, as well as energy dissipation systems and, in particular, shock absorbers for the automotive market, and many more. Molecular Dynamics simulations allowed us not only to investigate the behavior of water under high confinement conditions but also the dynamics in the intrusion/extrusion process under different thermodynamic conditions. In addition, we shed light on the role of the complex morphology/topology of the microporous solids considered, which in the present study are hydrophobic Metal-Organic Frameworks. Anticipating some results, through atomistic simulations performed on the Cu2(tebpz) MOF we showed an unprecedented critical temperature reduction of confined water of 200-250 K with respect to its bulk value, which is further confirmed also by experimental analyses. In addition, in-silico and liquid porosimetry experiments revealed a phenomenology at odds with consolidated physical laws, namely the Young-Laplace law, and intuition. Remarkably, an analogous phenomenology is observed also with ZIF-8. Simulations confirm that the theoretical framework developed for Cu2(tebpz) explains ZIF-8 behaviors as well, suggesting a possible generality of these physical phenomena. Free-energy Molecular Dynamics calculations, performed by advanced MD techniques, allowed us to explain how the hydrophobicity of complex microporous material, like ZIF-8, is driven not only by its chemical composition but also by its surface roughness/texture, providing a hint to tune their hydrophobicity. Finally, Molecular Dynamics simulations, also supported by experimental results, revealed the temperature dependence of the heat associated with the water intrusion
process and the influence that crystallite size can have on it
Photoinduced Proton-Coupled Electron Transfer in Supramolecular SnIV Di(l -tyrosinato) Porphyrin Conjugates
No full textProton-coupled electron transfer (PCET) plays a key role in many biological processes, and a thorough comprehension of its subtle mechanistic complexity requires the synthesis and characterization of suitable artificial systems capable of mimicking this fundamental, elementary step. Herein, we report on a detailed photophysical investigation of conjugate 1, based on a tin(IV) tetraphenylporphyrin (SnTPP) chromophore bound to two l-tyrosinato amino acids, in CH2Cl2 in combination with organic bases of different strength and the preparation of a novel conjugate 3, based on a tin(IV) octaethylporphyrin (SnOEP) in place of the tetraphenyl analogue, and its photophysical characterization in CH2Cl2 in the presence of pyrrolidine. In the case of compound 1 with all bases examined, quenching of both the singlet and triplet excited states is observed and attributed to the occurrence of concerted proton-electron transfer (CPET). Rates and quenching yields decrease with the strength of the base used, consistent with the decrease of the driving force for the CPET process. Conjugate 3 with pyrrolidine is quenched only at the triplet level by CPET, albeit with slower rates than its parent compound 1, ascribable to the smaller driving force as a result of SnOEP being more difficult to reduce than SnTPP. For both systems, the quenching mechanism is confirmed by suitable blank experiments, specific kinetic treatments, and the observation of kinetic isotope effects (KIEs). Differently from what has been previously proposed, a detailed reinvestigation of the triplet quenching of 1 with pyrrolidine shows that no long-lived radical pair state is formed, as diradical recombination is always faster than formation. This is true for both 1 and 3 and for all bases examined. The kinetics of the CPET pathways can be well described according to Marcus theory and point toward the involvement of substantial reorganization energy as typically observed for PCET processes of concerted nature
Hydrophobicity of molecular-scale textured surfaces: The case of zeolitic imidazolate frameworks, an atomistic perspective
No full textHydrophobicity has proven fundamental in an inexhaustible amount of everyday applications. Material hydrophobicity is determined by chemical composition and geometrical characteristics of its macroscopic surface. Surface roughness or texturing enhances intrinsic hydrophilic or hydrophobic characteristics of a material. Here we consider crystalline surfaces presenting molecular-scale texturing typical of crystalline porous materials, e.g., metal-organic frameworks. In particular, we investigate one such material with remarkable hydrophobic qualities, ZIF-8. We show that ZIF-8 hydrophobicity is driven not only by its chemical composition but also its sub-nanoscale surface corrugations, a physical enhancement rare amongst hydrophobes. Studying ZIF-8’s hydrophobic properties is challenging as experimentally it is difficult to distinguish between the materials’ and the macroscopic corrugations’ contributions to the hydrophobicity. The computational contact angle determination is also difficult as the standard “geometric” technique of liquid nanodroplet deposition is prone to many artifacts. Here, we characterise ZIF-8 hydrophobicity via: (i) the “geometric” approach and (ii) the “energetic” method, utilising the Young-Dupré formula and computationally determining the liquid-solid adhesion energy. Both approaches reveal nanoscale Wenzel-like bathing of the corrugated surface. Moreover, we illustrate the importance of surface linker termination in ZIF-8 hydrophobicity, which reduces when varied from sp3 N to sp2 N termination. We also consider halogenated analogues of the methyl-imidazole linker, which promote the transition from nanoWenzel-like to nanoCassie-Baxter-like states, further enhancing surface hydrophobicity. Present results reveal the complex interface physics and chemistry between water and complex porous, molecular crystalline surfaces, providing a hint to tune their hydrophobicity
Phase change material-sand mixtures for distributed latent heat thermal energy storage: interaction and performance analysis
No full textIn this study two phase change materials (PCMs) mixed with sand were evaluated for distributed latent heat thermal energy storage (LHTES) coupled with a novel Flat-Panel ground heat exchanger (GHE) for shallow geothermal applications. N-Octadecane and a commercial paraffin-based PCM were mixed (30% v/v) separately with sand, which is commonly used as backfilling material for GHE. Both two mixtures underwent 16 thermal cycles and specimen’s temperatures and their variation over time were analyzed to evaluate phase change stability and supercooling. Grain size laser diffraction and pore analysis were performed together with optical microscopy, environmental scanning electron microscopy coupled with X-Ray spectrometry (ESEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis to evaluate PCMs-sand dynamic interaction over time and temperature. Results shown that sand addition halves n-Octadecane phase change time, although leading to a limited supercooling equal to 1 °C. Sand addition to commercial PCM leaded to a similar increasing in heat transfer, however in absence of supercooling phenomena. These performances were constant through 16 thermal cycles. Therefore, PCMs mixing in sand as mixture for GHEs backfilling material can be considered a strategy to enhance thermal storage of backfilling material, by increasing the underground thermal energy storage and then the exploitation carried out by shallow geothermal applications
Rivestimento a base di cutina per la protezione dalla corrosione di acciai e leghe di alluminio
No full textLe vernici ed i rivestimenti sono ampiamente utilizzati in molte applicazioni civili e industriali per proteggere le strutture metalliche da fenomeni di degrado come i processi corrosivi.
Negli ultimi decenni, il sistema produttivo di pitture e vernici ha indirizzato i propri sforzi verso lo sviluppo di tecnologie e prodotti più ecologici, in accordo con un approccio più sostenibile. L'Unione Europea produce ogni anno più di 700 milioni di tonnellate di rifiuti agricoli che, potenzialmente, possono essere utilizzati come materia prime in altri processi produttivi come lo sviluppo di sistemi di protezione ecocompatibili. La possibilità di produrre rivestimenti protettivi a partire da scarto e/o sottoprodotti di origine agricola sta destando l’interesse di settori industriali che vanno dall'industria degli imballaggi a quella automobilistica.
In questa ricerca, un rivestimento derivato dalla cutina estratta dalle bucce di pomodoro è stato applicato a freddo, senza pretrattamento, sia sulla lega AA6063 che sull’acciaio ST 1405 laminato a freddo, ed inseguito trattato a 200°C per 10 minuti, ottenendo una grammatura di 6,5 g/ m2.
Il comportamento a corrosione dei campioni rivestiti è stato studiato mediante misure di spettroscopia di impedenza elettrochimica (EIS) durante 28 giorni di immersione in soluzioni di NaCl 0.1M, NaCl 0.6M e pioggia acida artificiale a pH = 4.5, al termine dei quali sono state registrate le curve di polarizzazione. Sono state inoltre eseguite prove di esposizione alla nebbia di pioggia acida della durata di 4 settimane su campioni rivestiti in presenza di una incisione ad X.
I campioni rivestiti, prima e dopo le prove di corrosione, sono state caratterizzati mediante spettroscopia infrarossa a trasformata di Fourier (FTIR) e microscopia elettronica a scansione (SEM) accoppiata con una microsonda a dispersione di energia (EDX), sia sulla superficie che sulla sezione trasversale.
I risultati hanno evidenziato bassa velocità di corrosione dei substrati ed un marcato effetto barriera da parte della bio-lacca a base di cutina, suggerendo che il rivestimento può fornire un’ottima protezione dalla corrosione per l'intero periodo di prova, in tutti gli ambienti testati
Phase Change Material Evolution in Thermal Energy Storage Systems for the Building Sector, with a Focus on Ground-Coupled Heat Pumps
No full textThe building sector is responsible for a third of the global energy consumption and a
quarter of greenhouse gas emissions. Phase change materials (PCMs) have shown high potential
for latent thermal energy storage (LTES) through their integration in building materials, with the
aim of enhancing the efficient use of energy. Although research on PCMs began decades ago,
this technology is still far from being widespread. This work analyses the main contributions to
the employment of PCMs in the building sector, to better understand the motivations behind the
restricted employment of PCM-based LTES technologies. The main research and review studies are
critically discussed, focusing on: strategies used to regulate indoor thermal conditions, the variation
of mechanical properties in PCMs-based mortars and cements, and applications with ground-coupled
heat pumps. The employment of materials obtained from wastes and natural sources was also
taken in account as a possible key to developing composite materials with good performance and
sustainability at the same time. As a result, the integration of PCMs in LTES is still in its early stages,
but reveals high potential for employment in the building sector, thanks to the continuous design
improvement and optimization driven by high-performance materials and a new way of coupling
with tailored envelopes
Experimental Investigation of Steel Bar Corrosion in Recycled Plastic Aggregate Concrete Exposed to Calcium Chloride Cycles
No full textRecycling plastics waste into concrete represents one of the possible approaches for its valorization, offering both economic and environmental benefits. Although numerous studies have explored the mechanical properties of concrete with plastics waste, its durability performance remains largely unexplored. In this context, this study aims to assess the electrochemical behavior of rebars embedded in reinforced concrete modified by partially replacing natural aggregates with recycled plastics, comparing their behavior to that of conventional concrete. The corrosion of reinforcing steel bars was evaluated by wet and dry cycles (w/d) in calcium chloride solutions, monitoring corrosion potential and potentiostatic polarization resistance, and recording electrochemical impedance spectroscopy (EIS) and polarization curves. In addition, the chloride diffusion tendency and the mechanical performances were assessed in unreinforced samples. The findings indicate that in environments with lower chloride concentrations, concrete with plastic granules provides good protection against rebar corrosion. Although the mechanical results of the studied mixes confirmed that incorporating plastic granules as aggregates in the concrete matrix causes a reduction in compressive strength, as known in the literature, the modified concrete also exhibits improved post-cracking behavior, resulting in enhanced ductility and fracture toughness
Cutin-based coatings for corrosion protection of Al alloys
No full textPaint and coatings are widely used to protect the metallic structures from corrosion in many civil and industrial applications. In recent decades, the paint industries have directed their efforts in the search for eco-friendly and more sustainable technologies and products, according to the principles of the circular economy. Since the European Union produces annually more than 700 million tons of agricultural wastes that, potentially, can be used as resources in other production processes, the development of eco-friendly protective systems starting from agricultural wastes and by-products has attracted the interest of many industrial sectors, ranging from packaging to automotive industries.
Similarly, nontoxic eco-friendly painting systems are needed in the Cultural Heritage field which must strictly comply with the requirements of effectiveness, conservation of a natural surface appearance of the protected artwork and reapplicability.
Recently, the aeronautical heritage of the Second World War (WWII), consisting of aircraft or wrecks recovered from sea, lakes or ground, has been officially considered a Cultural Heritage asset, by virtue of its historical and emotional value, but very few studies are still carried out for the development of suitable treatments for a long-term conservation.
In the present paper a room temperature cured cutin-based coating derived from tomato peels was developed and applied on a AA2017 T4 alloy, with corrosion behavior similar to that of alloys used for WWII aircraft. Its protective capability was compared, at similar grammage (6.5g/m2), with that offered by a bio-based coating, also derived from cutin extracted from tomato peels, but formulated for industrial purposes (curing at 200°C for 10 minutes) and applied on AA6063 alloy.
The corrosion resistance of the coated samples was studied by electrochemical techniques (polarization curves and electrochemical impedance spectroscopy) during 15 days of exposure to an artificial acidic rain at pH =4.5 and 30°C. Moreover, acid rain spray tests at 30 °C were carried out for 4 weeks on cross scratched coupons.
The surface morphology and the coating nature were studied before and after the corrosion tests, by Fourier-transform infrared spectroscopy (FTIR) and by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis (EDX), performed on both surface and cross sections.
The results highlighted very low corrosion rates of the substrates and very high barrier effects for both types of cutin-based bio-lacquers, suggesting that they can provide long term resistance to the tested aggressive environment.
This research was carried out within the European Project JPICH - Conservation, Protection and Use - “PROCRAFT, PROtection and Conservation of Heritage AirCRAFT”
Water intrusion in hydrophobic MOFs with complex topology: A glimpse of the intrusion mechanism of Cu2(tebpz)
No full textDespite water intrusion in microporous materials being extensively investigated, obtaining a detailed overview of the intrusion mechanism in materials with more complex morphology, topology, and physical-chemical characteristics, such as metal-organic frameworks (MOFs), is far from trivial. In this work, we present a qualitative study on the mechanism of water intrusion in a crystallite of hydrophobic Cu2(tebpz) (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) MOF. This MOF is characterized by a complex morphology; it consists of primary (main channels) and secondary (lateral apertures) porosities. This is similar to some zeolites, such as the so-called ITT-type zeolite framework, but it presents the additional characteristics of high flexibility of the material and non-uniform hydrophobicity. Interestingly, in Cu2(tebpz), water intrusion occurs first for some of the channels lying tangent to the surface of the MOF's crystallite. This is due to hydrogen bonding bridging with bulk water across the (thin) lateral apertures of these channels. In macroscopic terms, this can be understood as a local reduction of hydrophobicity favoring intrusion. Temperature and pressure influence the average number of hydrogen bonds and the number of intruded water molecules, explaining the effect of these thermodynamic parameters on the intrusion/extrusion characteristics of this porous material. Molecular dynamics simulations allowed us to glimpse liquid intrusion in this complex hydrophobic material, highlighting how the classical models valid for mesoporous systems, namely, Young-Laplace's law, are not quite appropriate to describe intrusion in such materials
