Salt (chemistry)

Summary

In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions. The component ions in a salt compound can be either inorganic, such as chloride (Cl−), or organic, such as acetate (CH3COO−). Each ion can be either monatomic, such as fluoride (F−), or polyatomic, such as sulfate (SO42−). Types of salt Salts can be classified in a variety of ways. Salts that produce hydroxide ions when dissolved in water are called alkali salts and salts that produce hydrogen ions when dissolved in water are called acid salts. Neutral salts are those salts that are neither acidic nor alkaline. Zwitterions contain an anionic and a cationic centre in the same molecule, but are not considered salts. Examples of zwitterions are amino acids, many metabolites, peptid

Official source

https://en.wikipedia.org/wiki/Salt_(chemistry)

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Field-based Strategies for Solid-Liquid Separation

Thierry Chappuis

The ever increasing demand for large quantities of bio-engineered pharmaceuticals and the tendency for doses to become larger stimulate the development of more productive and reliable mammalian cell culture technologies. In this context, perfusion bioreactors appear an efficient means to achieve high volumetric productivity and high product concentration. Since large-scale implementation of perfusion is limited by the reliability and scale-up potential of the device used to achieve cell retention, the present work proposes to tackle the problem bottom-up from the fundamentals of physics in order to develop, characterize and model novel particle retention strategies able to overcome the limitations of state-of-the-art systems. The first part of the present thesis focuses on the identification of physical force fields chosen for their ability to drive particles or cells into highly localized regions of a suspension. The ability of ultrasonic, dielectrophoretic and magnetophoretic interactions to make cells levitate over a surface was identified as an innovative way of implementing a Boycott effect (similar to that occurring in inclined sedimentation channels) while maintaining the flow channel in a vertical position. Therefore, inclined settlers were chosen as a reference system and the Ponder-Nakamura-Kuroda theory describing their performance was extended to take into account ultrasonic, dielectrophoretic and magnetic effects. The second part of the work provides deep modeling insight into the mechanisms underlying mammalian cell retention in these new filter implementations. Potentials and limitations are discussed in details. In particular, coupling dielectrophoresis with the Boycott effect proved to be successful in separating polystyrene particles from their moderately electrically conductive suspending medium. However, in highly conductive media, this process was shown to be very sensitive to order-disruptive electrokinetic effects such as ac electroosmosis and electrothermal fluid flow. Heat dissipation is however not specific to dielectrophoresis. This phenomenon is also a known limitation of the ultrasonic or magnetophoretic implementations, in certain conditions. The last part focused its work on the development of magnetophoretically-enhanced sedimentation, which was proposed as a means to partly overcome the heat dissipation problem observed with dielectrophoresis in highly conductive culture media. For such a magnetic approach to work correctly with intrinsically nonmagnetic materials, a paramagnetic suspending medium has to be used. The current implementation proposes to prepare media with fair quantities of gadolinium(III) or dysprosium(III) salts dissolved in them. Using such a paramagnetic buffer, the feasibility of magnetically retaining polystyrene particles could be verified in a vertical separation channel, hence providing first demonstration of the principle. The use of paramagnetic substances in mammalian cell culture media is naturally a matter of concern. The present work addresses this question as well and highlights the feasibility of growing Chinese Hamster Ovary cells in Gd(III)- or Dy(III)-rich environments. In contrast, attempts to develop low electrically conductive culture media failed and replacement of the NaCl content by D-mannitol or glycerol demonstrated severe growth inhibition effects.

EPFL2010

The increase of bacterial resistance against various disinfection processes is a worrying phenomenon. As a consequence, the search for alternative techniques for fighting micro-organisms has become one of the major issues of these last few years. In this domain, water treatment is an important preoccupation for many states, either to guarantee the quality of drinking water supply or to detoxify industrial effluents, charged with chemically or biologically pollutants. The financing of this thesis project was provided by the European project AQUACAT. This project aims at evaluating an emerging technology, the titanium dioxide (TiO2) photocatalysis, as an alternative to the traditional water treatments. This technique is based on the illumination of the catalyst, which thus generates reactive oxygen species (ROS), particularly hydroxyls radicals. TiO2 photocatalysis can be used as a water treatment by detoxifying organic compounds and inactivate micro-organisms. Moreover, its bactericidal activity have a promising biomedical future, by killing cancerous cells for example. Although the bactericidal action of the TiO2 photocatalysis is well documented, its mode of action on bacteria was not yet well defined. The objective of this thesis is to contribute to a better comprehension of the mode of action of the TiO2 photocatalysis on the bacterium Escherichia coli. Few microbiologists ever worked on this subject. A review of published articles on the mode of action of TiO2 photocatalysis showed some experimental failures. Moreover, dogmas related to TiO2 photocatalysis showed contradictions, which will be discussed in this thesis. This thesis is divided in four chapters, including two major sections: i) interaction between TiO2 particles, bacteria, biomolecules and salts and ii) the defence mechanisms of the bacterial cells against TiO2 photocatalysis. The first chapter shows that bacterial cells are adsorbed onto TiO2 particles. So as to demonstrate this feature, bacterial cells and TiO2 particles were mix in the presence of two salts affecting at different degrees the effectiveness of TiO2 photocatalysis rate. In the presence of NaCl-KCl, photocatalysis was very effective and the cultivability of the cells decreased at the beginning of the illumination phase. On the contrary, in a phosphate solution, a latency phase of 20 minutes was observed. Using flow cytometry and microscopic observations, we observed very distinctly that the bactericidal effect begun when the cells started to be adsorbed on TiO2 particles. Bacterial adsorption on TiO2 particles could be correlated to the loss of cell cultivability and also to the loss of membrane integrity, measured by flow cytometry. We then studied the interaction between biomolecules (proteins and DNA), bacterial cells and TiO2 particles. Biomolecules strongly adsorbed onto TiO2 particles in a NaCl-KCl solution (up to 60 µg protein per ml of catalyst). A desorption buffer composed of phosphate (50mM, pH 7.0) and SDS (0.1%) is able to desorb 12% only of the proteins. When bacterial cells were in presence of a cell crude extract corresponding to the quantity of proteins belonging to their population (equal to 12µg protein/ml for 2×107 bacteria /ml), it was observed that the effectiveness of the treatment was decreased, suggesting that cell crude extract protected bacterial cells from the photocatalytical treatment. So as to improve our understanding of the mode of action of the TiO2 on the bacterial cells, we compared the survival of E. coli mutants with the wild type one. Knowing that TiO2 generated ROS, we assayed mutant affected in their defence mechanisms against these molecules. Thus, the sensitivity of mutants defected in the induction of DNA repair and of DNA protection systems suggested the induction of cytoplasmic damage by the photocatalytical process. We could also highlight the importance of the Fenton reaction and the formation of hydroxyls radicals in this inactivation process. The hypothesis is that these hydroxyl radicals were also produced in the cells, following an increase in reactive iron and hydrogen peroxide. Interestingly, the loss of E. coli cultivability after TiO2 illumination appeared not only during the illumination, but also when cells were incubate on Petri plates, whereas TiO2 was not illuminated. Thus bacteria underwent a secondary stress, leading also to their loss of cultivability. Finally, we identified genes expressed in the response to the treatment (primary stress) and after a short incubation in a rich culture medium to 37 °C (secondary stress). Thus after an illumination time (100% of bacteria were cultivable) and an other one which show first signs of letality (80% of the cultivable bacteria), no difference in expression of the mRNA could be observed with the transcriptome analysis (DNA chips) of the E. coli genome. On the other hand, as soon as the cell were resuspended in a rich medium after the treatment during only 5 minutes, we could observe differences, by a factor higher than two, in the induction of certain genes. The totality of genes induced during the TiO2 photocatalysis were repressed by Fur and were implied in the iron transport.

EPFL2006

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

In UV-curable printing technology, an ink or a varnish is exposed to the UV-light source within a fraction of a second. Subsequent polymerization therefore proceeds in the dark, referred to as dark curing. Because dark curing can be critical and contributes to a large part of the overall conversion and to the final properties of the printed ink, the aim of this work was to get a better understanding of the dark curing process. Because an ink is a complex mixture of components, simpler systems representative of free radical and cationic chemistry were studied. Special attention was given to free radical polymerization with a detailed kinetic modeling. For cationic systems, the study was more application oriented. Attenuated-Total-Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy was found to be the method of choice to follow in real-time the progress of the reaction under UV irradiation and in the dark. This technique allowed the study of curing conditions close to those encountered in industry. Dark curing of free radical was examined in systems consisting of a monomer (multifunctional acrylate) and a photoinitiator. The reactivities of different monomers were investigated and it was found that the chemistry of the spacer between the acrylate functional groups had an influence on the extent of dark curing. For example, TPGDA (tripropylene glycol diacrylate) led to higher overall conversion than HDDA (hexanediol diacrylate) although both are diacrylate monomers. Several explanations were put forward, such as the effect of dissolved oxygen, primary cyclization and initiation efficiency. The efficiency of photoinitiators (PIs) was examined and different behaviours were highlighted. For example, it was observed that DMPA (dimethoxyphenyl acetophenone) has a better quantum yield for α-cleavage compared to HCPK (1-hydroxycyclohexylphenyl ketone), whereas HCPK is more efficient in initiating the reaction, resulting in a better overall conversion in the dark. It is suggested that the two primary radicals resulting from the cleavage of the PI behave differently for DMPA and HCPK. One radical effectively initiates the reaction and the second is mainly involved in the termination process. These two different behaviours were observed under specific experimental conditions: (i) by covering the sample with a quartz plate and taking into account dissolved molecular oxygen and (ii) by curing under inert conditions with nitrogen. Photo-crosslinking polymerization reactions are strongly influenced by diffusion effects due to the formation of an infinite network. Therefore, it was proposed that a redistribution of reactivity within the system occurs, such as the diffusion of unreacted photoinitiators towards concentrations of unreacted monomers. This diffusion process has two positive effects: (i) for the same UV dose, but different intensities, a higher extent of conversion was reached with longer irradiation times under the condition investigated; (ii) multiple exposures led to a higher overall conversion than for a single exposure, for a fixed overall UV dose. Other parameters were explored, such as temperature and diffusing oxygen from air. With temperature, different trends were observed depending on the photoinitiator and the start of dark curing, either starting before the maximum rate of polymerization (Rp,max), or after Rp,max. The development of a kinetic model to describe dark curing of free radical polymerization was proposed. Existing models based on physical parameters, such as those incorporating diffusion-controlled phenomena and free-volume are relatively complex and require extensive information on the system studied. Therefore, a semi empirical approach was chosen in order to study the variation of fitting parameters when changing curing parameters, such as the photoinitiator concentration and type, the irradiation time, the UV light intensity and the temperature. The model was derived from kinetic data and it was assumed that rate coefficients are time-dependent and that termination, which was considered to be a first-order process, is propagation dependent. The kinetic equation has the form of a stretched exponential function and incorporates four fitting parameters. The model was evaluated for a large number of experimental data and very good fits were obtained with a random distribution of residuals. The exponent β was found to be sensitive to the termination process and reflected different reactivities in the dark as in the case of DMPA and HCPK. Dark curing of cationic systems continues until complete monomer conversion but at a slower rate. Thus systems with high reaction rates, both during UV irradiation and during the first seconds of the dark period, are of most importance for UV-curable printing technology. The most widely used diepoxide monomer in cationic ink formulation is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (DECC). In order to improve its reactivity during the first seconds of dark curing, the influence of several parameters was examined, such as the addition of an oxetane co-monomer (TMPO), the influence of relative humidity, UV dose, light intensity, temperature, and film thickness. The enhanced reactivity of DECC with TMPO was confirmed. During the cationic curing process, two reaction mechanisms compete; an activated chain end (ACE) mechanism and an activated monomer (AM) mechanism. It was observed that these two processes depend on the presence of hydroxyl containing compounds such as TMPO monomer, polyol compounds, and, most important, water. When the concentration of these components is increased the AM mechanism is enhanced, leading to higher conversion but at a slower curing rate, and a degradation of the mechanical properties. The presence of water is unavoidable and its concentration in the ink varies in presence of hydrophilic compounds, such as polyol. Furthermore, the water content increases with increasing relative humidity in air. It was shown that the reactivity of the system decreased as the percent of relative humidity increased. Moreover, although a higher overall conversion is achieved, the crosslink density of the film was severely reduced. This lower crosslink density was the result of the predominance of chain transfer reactions, leading to more pendant and short chains which were hydroxyl terminated. Moreover, the extent of conversion was thickness dependent since less water diffuses into the deeper layers occurred in thicker films.

EPFL2007