Column chromatography | EPFL Graph Search

Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential adsorption of compounds to the adsorbent; compounds move through the column at different rates, allowing them to be separated into fractions. The technique is widely applicable, as many different adsorbents (normal phase, reversed phase, or otherwise) can be used with a wide range of solvents. The technique can be used on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling. Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column. A thin-layer chromatograph can show how a mixture of compounds will behave when purifie

Ketenimines from Isocyanides and Allyl Carbonates: Palladium-Catalyzed Synthesis of β,γ-Unsaturated Amides and Tetrazoles

Mathias Mamboury, Guanyinsheng Qiu, Qian Wang, Jieping Zhu

The reaction of allyl ethyl carbonates with isocyanides in the presence of a catalytic amount of Pd(OAc)2 provided ketenimines through b-hydride elimination of the allyl imidoylpalladium intermediates. The insertion of the isocyanide into the p-allyl Pd complex proceeded via an unusual h1-allyl Pd species. The resulting ketenimines were hydrolyzed to b,g-unsaturated carboxamides during purification by flash column chromatography on silica gel or converted in situ into 1,5-disubstituted tetrazoles by [3+2] cycloaddition with hydrazoic acid or trimethylsilyl azide.

Wiley-Blackwell2016

Interaction of polyelectrolytes with oppositely charged surfaces

Vesela Malinova

Despite progress during the last decade, the behavior of charged macromolecules, polyelectrolytes, including their interactions is not yet completely understood. In particular, interactions with oppositely charged surfaces, which play a significant role in many practical applications and are of central importance in life processes, need more investigation. This situation has motivated the selection of the topic of this doctoral thesis. The study is dedicated to interactions of positively charged polyelectrolytes with negatively charged surfaces considering two types of surfaces, flat amphiphile monolayers and porous surfaces of microspheres. The research aimed at quantitative evaluation of the influence of macromolecular dimensions, chemical structure, electrostatic parameters, and surface characteristics on the adsorption process. As a prerequisite, a series of poly(vinylbenzyltrialkylammonium chloride) model polyelectrolytes has been synthesized. The characteristics were five defined contour lengths with three different types of substituents at the quaternary ammonium group each in addition to uniform charge density and narrow chain length distribution. The final polyelectrolytes were the quaternization products of precursors synthesized by controlled radical polymerization. Two experimental techniques were employed to quantify the influence of chemical, macromolecular, and electrostatic parameters as well as geometrical characteristics on the adsorption of polyelectrolytes on porous microsphere surfaces. These were adsorption on microspheres suspended in aqueous phase and microspheres packed in a chromatography column. During the first technique the total surface comes immediately in contact with the polyelectrolyte molecules, whereas the second technique followed the rules of chromatography. For the first time, the contour length of the polyelectrolyte, the Debye length, the pore dimensions, as well as the chemical structure were likewise considered in a comprehensive study to describe the interaction of polyelectrolytes with well-characterized porous surfaces. A polyelectrolyte charge to surface charge ratio was defined and served to classify different adsorption behavior. Three models have been proposed to illustrate the adsorption behavior based on experimental findings and model calculations. The first one identifies the Debye length in addition to the contour length as dominating parameter, which decides whether a polyelectrolyte molecule can enter a pore or not. The second quantifies the influence of the contour length. Finally, the third describes the concentration influence on the type of molecule organization, layer formation, on the outer surface or inside of pores with diameters exceeding considerably the contour length. For amphiphile monolayer, for the first time, the influence of chain length and substituent at the quaternary ammonium group on the adsorption process was quantified. Based on the surface pressure, pressure-area and pressure-time isotherms measured, a model describing the monolayer arrangement on polyelectrolyte solutions was proposed. A second model correlates the increase of the area per amphiphile molecule and the decrease of the surface pressure with the size and hydrophobicity of the substituents. In addition chain length influence was evident. The results of the thesis contribute to progress in basic research in the field of polyelectrolytes, while also an impact on technical applications such as surface modification, chromatographic processes or improvement of materials is expected. Precise data for comparison with theoretical model calculations are provided, which were not available before.

EPFL2005

Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal-Organic Framework

Sudabeh Jawahery, Berend Smit

Significant advances have been made in the synthesis of chemically selective environments within metal-organic frameworks, yet materials development and industrial implementation have been hindered by the inability to predictively control crystallite size and shape. One common strategy to control crystal growth is the inclusion of coordination modulators, which are molecular species designed to compete with the linker for metal coordination during synthesis. However, these modulators can simultaneously alter the pH of the reaction solution, an effect that can also significantly influence crystal morphology. Herein, noncoordinating buffers are used to independently control reaction pH during metal-organic framework synthesis, enabling direct interrogation of the role of the coordinating species on crystal growth. We demonstrate the efficacy of this strategy in the synthesis of low-dispersity single-crystals of the framework Co-2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) in a pH 7-buffered solution using cobalt(II) acetate as the metal source. Density functional theory calculations reveal that acetate competitively binds to Co during crystallization, and by using a series of cobalt(II) salts with carboxylate anions of varying coordination strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography to show that crystal morphology has a direct impact on guest diffusional path length for the industrially important hydrocarbon m-xylene. Together, these results provide molecular-level insight into the use of modulators in governing crystallite morphology and a powerful strategy for the control of molecular diffusion rates within metal-organic frameworks.

AMER CHEMICAL SOC2021

Interaction of polyelectrolytes with oppositely charged surfaces

Vesela Malinova

EPFL2005