Search results for "Silanol"

1-Hexyl-3-methyl imidazolium tetrafluoroborate: an efficient column enhancer for the separation of basic drugs by reversed-phase liquid chromatograph…

2012

Abstract Ionic liquids are dual modifiers composed by a large anion and a large cation, which interact with both the hydrophobic alkyl-bonded phase and the anionic residual silanols in C18 columns. The deactivation of the silanol groups has important implications on the chromatographic analysis of basic drugs, being the improvement of peak profiles and shorter retention times the most noticeable features. However, other characteristics as selectivity or resolution are not usually considered, or are only examined for selected chromatographic conditions. In this work, the effect of the addition of the ionic liquid 1-hexyl-3-methyl imidazolium tetrafluoroborate to acetonitrile–water mixtures i…

Synthesis and properties of chemically modified dihydroxy-, hydroxyamino- and amino-functional silica packings in adsorption chromatography

1980

1,2-Dihydroxy-3-propoxypropyl (HPP), 1-amino-2-hydroxy-3-propoxypropyl (AHP) and 3-(2-aminoethylamino)propyl (AEAP) silica packings were synthesized by means of both a surface modification and a bulk modification procedure. In surface modification a complete monolayer of bonded groups could be achieved. The modifiers reacted with the silanol groups according to a bifunctional mechanism. Eluotropic series of solvents were established on these polar modified silicas referring to n-heptane having ɛ∘=0. The solvent parameter ratio of modified to bare silica was found to be constant over a wide range of solvent compositions as \(\varepsilon _{{\text{HPP - silica}}}^ \circ = 0.26\varepsilon _{{\t…

Chromatographic Efficiency in Micellar Liquid Chromatography: Should it Be Still a Topic of Concern?

2013

Micellar liquid chromatography (MLC) was first proposed as an attractive alternative to avoid the use of organic solvents. It was soon apparent that pure micellar solutions yield poor efficiencies. This problem was remediated by the addition of a small amount of an organic solvent. However, the general opinion of the poor peak shape has prevailed as a handicap for MLC, in spite of the fact that the hybrid mode often offers similar or even improved efficiencies (for basic compounds) relative to that attained in the hydro-organic mode. Only the efficiencies for apolar non-ionizable compounds are still clearly inferior. This work describes the type of interactions and polarity changes with org…

Retention mechanisms in micellar liquid chromatography.

2008

Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic (RPLC) mode with mobile phases containing a surfactant (ionic or non-ionic) above its critical micellar concentration (CMC). In these conditions, the stationary phase is modified with an approximately constant amount of surfactant monomers, and the solubilising capability of the mobile phase is altered by the presence of micelles, giving rise to diverse interactions (hydrophobic, ionic and steric) with major implications in retention and selectivity. From its beginnings in 1980, the technique has evolved up to becoming a real alternative in some instances (and a complement in others) to classical RPLC with hydro…

Improvement of peak shape and separation performance of beta-blockers in conventional reversed-phase columns using solvent modifiers.

2003

A comparative study of peak shape, elution behavior, and resolution of 16 beta-blockers (acebutolol, alprenolol, atenolol, bisoprolol, carteolol, celiprolol, esmolol, labetalol, metoprolol, nadolol, oxprenolol, pindolol, practolol, propranolol, sotalol, and timolol) chromatographed with hybrid mobile phases of triethylamine (TEA)-acetonitrile and sodium dodecyl sulfate (SDS)-propanol is performed using conventional reversed-phase columns and isocratic elution. Both solvent modifiers (TEA and SDS) prevent the interaction of the basic drugs with the alkyl-bonded phase. However, the protection mechanisms of silanols on the packing are different. Whereas TEA associates with the silanol sites (b…

Impact of the post-treatment conditions of parent silica on the silanization of n-octadecyl bonded silica packings in reversed-phase high-performance…

2001

Native mesoporous silica beads were subjected to a sequence of post-treatment procedure including hydrochloric acid treatment, calcination and subsequent rehydroxylation. The post-treated silica beads were converted into RP-18 silica by silanization with monochloro- and dimethoxy-n-octadecylsilanes, respectively. The influence of post-treatments and silanization conditions on the physico-chemical characteristics and on the chromatographic behaviour of the RP-silicas was studied. Also the changes of the pore structural parameters and the silanol group densities during the post-treatment and silanization were assessed.

New zeolite-like RUB-5 and its related hydrous layer silicate RUB-6 structurally characterized by electron microscopy.

2020

RUB-5 and its related hydrous layer silicate RUB-6 were synthesized in the 1990s, but so far their structures have remained unknown due to their low crystallinity and disorder. The combination of 3D electron diffraction, X-ray powder diffraction, high-resolution transmission electron microscopy, structural modelling and diffraction simulations has enabled a comprehensive description of these two nanomaterials, revealng a new framework topology and a unique silica polymorph.

Water adsorption on amorphous silica surfaces: A Car-Parrinello simulation study

2005

A combination of classical molecular dynamics (MD) and ab initio Car-Parrinello molecular dynamics (CPMD) simulations is used to investigate the adsorption of water on a free amorphous silica surface. From the classical MD SiO_2 configurations with a free surface are generated which are then used as starting configurations for the CPMD.We study the reaction of a water molecule with a two-membered ring at the temperature T=300K. We show that the result of this reaction is the formation of two silanol groups on the surface. The activation energy of the reaction is estimated and it is shown that the reaction is exothermic.

Thermal evolution of the OH-related infrared absorption lineshape in synthetic wet silica

2008

XPS study of leached glass surfaces

1990

Abstract By use of a special deconvolution algorithm, it was possible to decompose the XPS O 1s signal into several components reflecting different oxygen bonds. The influence of exposure in vacuo, environmental atmosphere, distilled water, and in a polish solution as corrosive media on the surface of different glasses has been studied. The O 1s signals of fused silica, Na 2 O· n SiO 2 glasses, a BaO·SiO 2 and a BaOB 2 O 3 SiO 2 glass (Schott SK 16) were analyzed. On the glass surfaces, leached layers were formed and analyzed qualitatively and semiquantitatively by XPS and optical and infrared methods. As a reference, virgin surfaces of samples broken in ultrahigh vacuum have been used. T…