Overcoming timescale and finite-size limitations to compute nucleation rates from small scale Well Tempered Metadynamics simulations

Condensation of a liquid droplet from a supersaturated vapour phase is initiated by a prototypical nucleation event. As such it is challenging to compute its rate from atomistic molecular dynamics simulations. In fact at realistic supersaturation conditions condensation occurs on time scales that far exceed what can be reached with conventional molecular dynamics methods. Another known problem in this context is the distortion of the free energy profile associated to nucleation due to the small, finite size of typical simulation boxes. In this work the problem of time scale is addressed with a recently developed enhanced sampling method while contextually correcting for finite size effects. We demonstrate our approach by studying the condensation of argon, and showing that characteristic nucleation times of the order of magnitude of hours can be reliably calculated, approaching realistic supersaturation conditions, thus bridging the gap between what standard molecular dynamics simulations can do and real physical systems.Comment: 9 pages, 7 figures, additional figures and data provided as supplementary information. Submitted to the Journal of Chemical Physisc

Atom Transfer Radical Polymerization of Polar Monomers and Synthesis of Block Copolymers for Industrial and Biomedical Applications

The aim of this thesis is to push forward the synthesis of well-defined materials containing polar monomers. The ATRP of polar monomers was investigated with the aim to obtain living and well-defined materials. Block copolymers with pre-determinable composition and unimodal distribution of molecular weight were synthesized. Furthermore, the Atom Transfer Radical Co-Polymerization of NVCL and NVP with non-polar monomers was investigated with the aim to obtain amphiphilic material with tunable polarity. The ATRP of vinyl acetate (VAc), which was poorly optimized, was studied trying to obtain poly(VAc) with low polydispersity (<1.25), pre-determinable molecular weight and living character. The optimization of the ATRP of VAc and the synthesis of several block copolymers, synthesized in presence of different experimental conditions, can significantly expand the field of materials and applications of poly(VAc) and poly(vinyl alcohol)-based products. Moreover, the synthesis of pH and temperature polymers was investigated with the aim to obtain products suitable for the development of drug-delivery systems which can be applied in anti-cancer applications. For this purpose Pluronic F127, which is thermosensitive, and poly(ethylene glycol)s were modified with pH poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA), poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA) and poly[2-(N,N-diisopropylamino)ethyl methacrylate] (PDIAEMA). The methacrylic moieties have different pKa, and they give to the synthesized materials the desired pH responsiveness. The gelation behavior of the obtained products was investigated by rheological measurements; the dimension of the polymeric aggregates in water solutions at different pH was studied by DLS and the drug-incorporation as a function of pH was determined in systems with stable pH and in systems in which the pH was decreased progressively. All the cited investigation allowed to well-characterized the behavior and the structure of polymeric aggregates in water solution and they also allowed to determine their pH and temperature responsiveness

Crystal nucleation from solution: design and modelling of detection time experiments

Crystal nucleation is the process responsible for the appearance of a thermodynamically stable phase from a metastable parent solution. Given its activated nature, nucleation is affected by stochasticity which, despite originating at the molecular level, affects heavily also the macroscopic behaviour of the system. Being far too small to be observed directly, nuclei are detected by indirect methods, which correlate the formation of the new phase with a measurable change in a property of the system, hence a model linking nuclei formation and crystals detection is always needed. We have previously presented a model describing nucleation in macroscopic systems as a stochastic Poisson process. The model, despite its general character, can describe industrially relevant processes, e.g. batch cooling at different operating conditions. The different scales influenced by the stochastic nature of nucleation demand appropriate theoretical and experimental investigations, particularly for applying the model to industrial scale-up, optimisation, and control. Using statistical tools, we have looked into the issue of estimating stochastic processes by collecting a representative, but limited number of data, produced from a homogeneous set. Moreover, using our model, we analysed the sensitivity of crystallising systems on initial and boundary conditions, with particular emphasis on the effect of supersaturation, temperature and detection conditions. Finally, in light of the stochastic nature of nucleation, we also applied statistical meta-analysis to assess the agreement between the fitting and its parameters and experiments, to gain further insight into the quality of the model. Experimentally, we have first investigated the conditions to perform homogeneous and reproducible measurements, necessary to understand the fundamental physical features and ultimately to estimate reliable kinetic parameters. A second aspect we have explored concerned the size of the crystallising systems. Since in macroscopic reactors various phenomena occur simultaneously (nucleation, growth, breakage, agglomeration) we chose to work with two main system sizes, 1-3 mL reactors (mesoscale) and 1-60 nL reactors (microscale, i.e. microscopic droplets), where at least some of such phenomena could be decoupled. In the mesoscale crystallisers, one can perform experiments where temperature and transmissivity could be measured online, hence monitoring the appearance and disappearance of crystals. Additionally, the influence of fluid-dynamics, typically turbulent in these reactors, was investigated. In the microfluidic chips, on the other hand, a very high through-put (thousands of replicas of the same reactor) can be potentially achieved and, thanks to their very small size, high supersaturations, outside of usual experimental reach, could be explored. Additionally, within the microscopic droplets the fluid motion is generally diffusive or laminar convective, hence hindering breakage and agglomeration. One could thus observe systems where nucleation and growth of single crystals (or of few crystals) occur unperturbed. Nevertheless, some main challenges, which we have been addressing, must be tackled before performing reliable crystallisation experiments: the characterisation and the reproducibility of shape and size of the droplets and their stability (i.e. the loss of mass due to evaporation and perspiration through the chip). In conclusion, we demonstrate that, even if the data are reproducible and reliable, robust probability estimations can be obtained only with a sufficiently large number of experiments, which require careful design to avoid sensitivity regions and data processing to reject the non-homogeneous data. The different sizes investigated have permitted to gain a better insight into the fundamental phenomena occurring in a crystallising system between the first formation of nuclei until crystal detection, which is of utmost importance for understanding the design of the experiments at an industrially relevant scale. Moreover, appropriate mathematical tools allowed to assess the reliability of the fitting obtained from independent measurements of the same system at different conditions

Optimization of low-carbon multi-energy systems with seasonal geothermal energy storage: The Anergy Grid of ETH Zurich

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Net-zero emissions chemical industry in a world of limited resources

The chemical industry is responsible for about 5% of global CO2 emissions and is key to achieving net-zero targets. Decarbonizing this industry, nevertheless, faces particular challenges given the widespread use of carbon-rich raw materials, the need for high-temperature heat, and the complex global value chains. Multiple technology routes are now available for producing chemicals with net-zero CO2 emissions based on biomass, recycling, and carbon capture, utilization, and storage. However, the extent to which these routes are viable with respect to local availability of energy and natural resources remains unclear. In this review, we compare net-zero routes by quantifying their energy, land, and water requirements and the corresponding induced resource scarcity at the country level and further discuss the technical and environmental viability of a net-zero chemical industry. We find that a net-zero chemical industry will require location-specific integrated solutions that combine net-zero routes with circular approaches and demand-side measures and might result in a reshaping of the global chemicals trade.</p

Net-zero emissions chemical industry in a world of limited resources

The chemical industry is responsible for about 5% of global CO2 emissions and is key to achieving net-zero targets. Decarbonizing this industry, nevertheless, faces particular challenges given the widespread use of carbon-rich raw materials, the need for high-temperature heat, and the complex global value chains. Multiple technology routes are now available for producing chemicals with net-zero CO2 emissions based on biomass, recycling, and carbon capture, utilization, and storage. However, the extent to which these routes are viable with respect to local availability of energy and natural resources remains unclear. In this review, we compare net-zero routes by quantifying their energy, land, and water requirements and the corresponding induced resource scarcity at the country level and further discuss the technical and environmental viability of a net-zero chemical industry. We find that a net-zero chemical industry will require location-specific integrated solutions that combine net-zero routes with circular approaches and demand-side measures and might result in a reshaping of the global chemicals trade