Silica molecular properties

There are a number of such relevant correlations to be always considered. Based on both relevance for lead profiling and in silica availability of molecular properties. 

Normal-phase liquid chromatography is thus a steric-selective separation method. The molecular properties of steric isomers are not easily obtained and the molecular properties of optical isomers estimated by computational chemical calculation are the same. Therefore, the development of prediction methods for retention times in normal-phase liquid chromatography is difficult compared with reversed-phase liquid chromatography, where the hydrophobicity of the molecule is the predominant determinant of retention differences. When the molecular structure is known, the separation conditions in normal-phase LC can be estimated from Table 1.1, and from the solvent selectivity. A small-scale thin-layer liquid chromatographic separation is often a good tool to find a suitable eluent. When a silica gel column is used, the formation of a monolayer of water on the surface of the silica gel is an important technique. A water-saturated very non-polar solvent should be used as the base solvent, such as water-saturated w-hexane or isooctane. 

In summary, when we compare the properties of the low and intermediate zeolites with those of the high silica zeolites and silica molecular sieves, we find that their resulting properties allow the low and intermediate zeolites to remove water from organics and to carry out separations and catalysis on dry streams. In contrast, the hydrophobic high silica zeolites and silica molecular sieves can remove and recover organics from water streams and carry out separations and catalysis in the presence of water. 

Classically the term in silica modeling referred to the process of building a model to predict a given endpoint from a set of molecular properties derived purely from the chemical structure of a number of compounds of which the endpoint of interest is known. 

Properties of LZC mesoporous silica molecular sieves with different aging time... 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

Y. Kubota, M.M. Helmkamp, S.I. Zones, and M.E. Davis, Properties of Organic Cations that Lead to the Structure-direction of High-silica Molecular Sieves. Microporous Mater., 1996,6, 213-229. 

P.T. Tanev and T.J. Pinnavaia, Mesoporous Silica Molecular Sieves Prepared by Ionic and Neutral Surfactant Templating A Comparison of Physical Properties. Chem. Mater., 1996, 8, 2068-2079. 

Tanev, P.T. Pinnavaia, T.J. Mesoporous silica molecular sieves prepared by ionic and neutral surfactant templating a comparison of physical properties. Chem. Mater. 1996, 8 (8), 2068-2079. 

Silicalite is a microporous crystalline silica molecular sieve with remarkable hydrophobic properties ( 1) and has been considered to offer practical applications in the clean-up of water contaminated with hydrocarbons and the separation of ethanol from dilute fermentation aqueous solutions (2 ii> 2) Many studies have been reported on the properties of adsorption and diffusion of gases in silicalite (e.g., 6, 8, , HI) However, despite the many potential applica-... 

Using these methods is similar to reconstructing a puzzle. How the retention and vaporization mechanisms can be quantitatively analyzed, and the predicted retention times improved, based on molecular properties calculated in silica, are fundamental questions in chromatography. In gas chromatography, no solvent is used except in special cases where water vapor and ionic gas are mixed with the carrier gas. The basic retention mechanisms depend on the strength of the molecular interaction with the stationary phase, and the vaporization mechanism depends on the properties of the analytes. 

Table 6.1 Molecular properties of pyridine and phenol, and their molecular interaction energies (kcal mol ) with model silica gels.

Table 6.7 Molecular properties of benzoic acid derivatives on a model carbon phase, k values are capacity ratios where ki values were measured on an octadecyl-bonded silica gel phase, k2 the molecular form, and fcji the ionized form. ks values were measured on an octadecyl-bonded polyvinylalcohol phase, k4 and ks were measured on a polystyrene gel (Hitachi 3013 and 3011), and ke values were measured on an ODS silica gel (410ODS). ...

Molecular properties of some common anal3 es. Log k was measured on an ODS silica gel in 80% aqueous acetonitrile at 40 °C. ac represents molecular properties calculated using a model acetonitrile solvent phase. Reproduced by permission of Oxford University Press, ref. 48. 

Molecular properties of standard compounds in a model acetonitrile solvent phase. Log k values were measured on an ODS-bonded silica gel in aqueous 70% acetonitrile containing 0.01% phosphoric acid." HOMO and LUMO energy values (eV) were calculated using the PM5 program. 

Table 13 Molecular properties of some ionized benzoic acid derivatives, fsi, hbi, esi, and vwi are final structure, hydrogen bonding, electrostatic, and van der Waals energy values of ionized benzoic acid derivatives. FSi, HBi, ESi, and VWi are energy values of complexes of ionized benzoic acid derivatives and a pentyl-bonded silica gel (kcal mP ). Reproduced by permission of Elsevier, ref. 15.

Table 15 Molecular properties of some basic drugs and their complexes with a model phase. Phase 1 is a hutyl phase, Phase 2 an octyl phase, Phase 3 a dodecyl phase. Phase 4 is a dimethojgrpentyl-honded silica gel, and Phase 5 is a dimethojgroctyl-honded silica gel. m represents the molecular form. Units kcal moP. Reproduced hy permission of Taylor Franeis, ref. 18.

The curing process takes advantage of the versatile chemical property of silicones. Chemical reactivity is built in the polymer and allows the formation of silicone networks of controlled molecular architectures with specific adhesion properties. The general and inherent molecular properties of the PDMS polymer are conferred to the silicone network. Pure PDMS networks are mechanically weak and do not satisfy the adhesive and cohesive requirements needed for most applications. Incorporation of fillers like silica or calcium carbonate is necessary to reinforce the silicone network (see Composite materials). 

Existence/nonexistence of a compound is certainly an important topic of chemistry, but depends on the state-of-the-art of chemical synthesis and survey of natural occurrence. In Chapters 7 and 8 we shall consider other properties of compounds, including less variable properties. We shall try to deduce experimental properties horn the chemical structure and vice versa. For this purpose it is crucial to generate in silica molecular structures with prescribed structural properties in silica, i.e. to generate virtual molecular libraries. The most important tools for this task will be described in Chapter 5. 

Indeed, the compositional flexibility of mesoporous materials is such that it is possible to prepare mesoporous organosilicas with organic groups (e.g., ethene) inside the pore walls. An unusual property of mesoporous silicas is that they can themselves be used as templates for the formation of mesoporous carbon. The mesoscopi-cally ordered nanoporous (or mesoporous) carbon molecular sieves are prepared by carbonizing sucrose (or other carbon precursors) inside the pores of mesoporous silica molecular sieve. The mesoporous carbon is obtained after subsequent removal of the silica template by dissolution in HF (hydrofluoric acid) or NaOH (sodium hydroxide) solution. 

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