Droplets esterification reactions

The success of this esterification can be attributed to the fact that the surfactant-type catalysts and organic substrates (carboxylic adds and alcohols) in water form droplets with a hydrophobic interior. Catalytic species, such as a proton, concentrate at the polar surface of the droplets, where reaction takes place. 

The ultrasonic treatment of PET/LCP blends leads to improvements in the mechanical properties of the homopolymers and some blends. The increase in the viscosity and mechanical properties of neat PET and LCP, as well as some of their blends at certain amplitudes, points toward homo- and copolymerization. The increase in viscosity of PET after ultrasonic treatment is attributed to possible esterification reaction. The presence of hairy structures on the surface of LCP droplets in the core region of 90/10 PET/LCP blends after ultrasonic treatment at an amplitude of 7.5 otm illustrates the improved adhesion between the PET and LCP phases, and possible copolymer formation under the action of ultrasound. Low residence times in the ultrasonic zone and energy saving due to reduced pressure render ultrasonic extrusion a viable method for the compatibilization of thermoplastic polymer/thermotropic LCP blends. Further refinement of processing conditions could allow one to achieve greater enhancements in the performance of LCP, and its blends with PET. 

The amounts of DBSA used were also found to affect the equilibrium position (Table 13.6). Each equilibrium position was confirmed by conducting both esterification of the carboxylic acid with the alcohol and hydrolysis of the ester. Table 13.6 clearly shows that increase of the amount of DBSA resulted in decrease of the yield of the ester at the equilibrium position. This result may be attributable to the size difference of the emulsion droplets that were formed by the hydrophobic substrates and the surfactant in water. As the amount of the surfactant-type catalyst increases, the size of each droplet may decrease, because the emulsion system may become a microemulsion system where the substrates are solubilized in water by a large amount of the surfactant. In fact, while 10 mol% DBSA gave the white turbid mixture, the reaction mixture was almost clear in the presence of 200 mol% DBSA, indicating that the size of the droplets became smaller. The smaller the droplets, the larger the sum of surface area of the droplets. As a result. 

Fig. 8. A schematic diagram showing cellular processes known to require SCP2. The reactions in cholesterol biosynthesis and esterification have been shown for liver. The reactions involving cholesterol transport from cytoplasmic lipid inclusion droplets to mitochondria have been demonstrated in endocrine tissues. Choi and C. cholesterol ACAT, acyl-CoA cholesterol acyl transferase C.E., cholesterol ester SEH, sterol ester hydrolase (hormone-dependent) P-450s,, cytochrome P-450 cholesterol side-chain cleavage enzyme PREG, pregnenolone.

Fig. 1. The cycle of CE from plasma through a fibroblast-like cdl. LDL containing CE bind to apo B/E receptors on the cell surface, whereupon the lipoprotein-receptor complexes are internalized by endocyto-sis. The lipoprotein CE is then hydrolyzed within lysosomes by an add CEH. The UC and fatty acids (FA) released diffuse through the lysosomal membrane to the cytoplasm, where they may be re-esterified by the ACAT reaction on the endoplasmic reticulum. The CE accumulates in fat droplets by an unknown mechanism or it can be hydrolyzed to UC and FA by neutral CEH in a futile cycle involving successive re-esterification (ACAT) and hydrolysis (neutral CEH) reactions. Ultimately, UC may be utilized for membrane formation and/or excreted from the cell. To be excreted, an appropriate extracellular acceptor - here shown as HDL - may bind to receptors on the cell surface, which specifically recognize apo A, and thereafter adsorb UC from the cell surface. HDL UC may then be esterified by LCAT to form CE which may be transferred by CETP to VLDL as the HDL returns to the circulation. VLDL thus acquire CE and eventually become converted to LDL, completing the cycle.

The droplets can be regarded as individually acting nanoreactors, suitable for a wide variety of different reactions. It has been shown that organic reactions like esterification and saponification [1,2], crystallization processes [3-6] and sol-gel reactions [7] can efficiently be performed in miniemulsions. 

Even in the presence of large amounts of water, the miniemulsion process permits the synthesis of hydrophobic polyesters in a very simple manner and at very low temperatures in order to obtain stable polyester dispersions. The influence of several parameters on the esterification yield has been studied. On the one hand, any modification of the dispersed phase such as the hydrophobicity of the components, viscosity, and the reactant nature results in different yields. With increasing hydrophobicity of the monomers or decreasing viscosity, the yield increases. On the other hand, any modification of the surrounding environment of the droplets such as the interface nature, the ionic strength and the interface area, has no influence on the equilibrium. From a thermodynamic point of view, this polymerization presents the characteristics of a bulk or solution polymerization. Independently of the dispersion state in the range studied, the equilibrium is the same as in bulk or in solution polymerization with an organic phase saturated with water. It is however very unlikely that the reactions occur exclusively in the core of the particle, but in order to provide an answer to this question it would be necessary to conduct a kinetic study related to the interface area. 

The cholesterol esters present in the intestinal lumen during digestion are rapidly hydrolyzed to the free alcohol and fatty acids[5]. Vahouny et al. showed that a pancreatic enzyme - cholesterol esterase or cholesterol esters hydrolase - is responsible for both the hydrolysis of cholesterol esters in the intestinal lumen and the esterification of the absorbed cholesterol in the intestinal mucosa[6,26] Because of the pH, the nature of the substrate and other undefined factors the reaction is directed towards hydrolysis in the lumen and towards esterification in the mucosa[5]. Although the ideal substrate for hydrolysis seems to be micellar cholesterol [27], an action of the enzyme at the oil-water interface of the lipid droplets cannot be excluded. The pancreatic origin of the rat cholesterol esterase has been repeatedly suggested, since in absence of pancreatic juice both hydrolysis of cholesterol esters and cholesterol absorption are significantly lower[28]. 

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