Pommella, Angelo (2013) Deformation of surfactant vesicles in shear flow. [Tesi di dottorato]

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Abstract

Vesicle dynamics is a multidisciplinary topic of growing scientific interest spanning from physics to engineering and biology. While most studies have been so far addressed to unilamellar phospholipid vesicles, which can be regarded as a model system of living cells, less attention has been devoted to multilamellar vesicles, which are an intriguing example of soft matter found in surfactant-based everyday life products, such as detergents, foodstuff, and cosmetics. In this work, we present the first quantitative investigation of the flow behaviour of single SMLVs. We found that SMLVs are deformed and oriented by the action of shear flow while keeping constant volume, and exhibit complex dynamic modes (i.e., tumbling, breathing and tank-treading). Moreover, SMLV deformation and orientation scale with radius R in analogy with emulsion droplets and elastic capsules (instead of R3 such as in unilamellar vesicles). However one of the key issues is how the packed, onion-like microstructure of SMLVs is capable of undergoing the large tank-treading deformations which are observed under simple shear. By rheo-optical experiments in a parallel-plate shear device, we show that tank-treading vesicle deformation is associated with a core-shell microstructure, where the former is a pool of an isotropic micellar phase and the latter is made of smectic lamellar layers. The outer shell is rearranged by convection with accumulation at the vesicle ends and thinning in the central part, where formation of typical liquid crystalline defects (focal conical domains) is found. These results are further supported by the scaling analysis of vesicle retraction upon cessation of flow, which can be explained in terms of a dilational energy in the same way as the focal conical domain defects. A possible application of the physical insight provided by this work is in the rationale design of processing methods of surfactant-based systems. These systems can be used in the pharmaceutical applications where different catalytic reactions are currently run in batch and homogenous conditions. In fact there are different studies aiming to run several catalytic reactions in continuous flow reactors and heterogeneous conditions in order to increase the efficiency of the process. One of these reactions is the Buchwald-Hartwig aryl amination that is run in batch and homogenous conditions in the industrial applications. In this work the Buchwald-Hartwig reaction is reported in a home-made continuous plug flow tube microreactor, using a homogeneous well-defined palladium-N-heterocyclic carbene [Pd(NHC)] complex. The microreactor enabled a 100% conversion of the reagents within 30 minutes, even at very low catalyst concentrations. A direct comparison between batch and continuous flow reactions is described and shows that the Buchwald-Hartwig reaction is faster in the microreactor than in the batch case. An investigation of the influence of the operating parameters of the microreactor on the reaction was carried out. Increasing temperature allowed a faster conversion of the reagents; moreover, no effect on microreactor performance was found by changing tube diameter. The dependence of reaction kinetics on reagents and [Pd(NHC)] pre-catalyst concentrations was investigated based on initial reaction rates. The resulting expression for the rate of reaction showed some similarities with the one reported for palladium-phosphine catalyst, but also some important differences.

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