Silica octanol

Triple-exponential monomer and excimer fluorescence decays were reported for the following systems Py on Si-C g (38), 1Py(3)1Py adsorbed on silica/octanol and on reversed-phase octa-decylsilica Si-C g (38), as well as PPS and PDS chemically bound to silica in contact with solvents (42,43). 

Silica/i-Octanol. The monomer and excimer fluorescence decays of 1Py(3)1Py in the system silica/octanol were fitted with three exponentials (38), double-exponential fits giving unacceptable results. The decay times at 25°C (38) for the monomer (20, 43 and 146 ns), have values in the same range as those of the excimer (27, 51 and 106 ns). As was noted in studies with 1Py(3)1Py and related compounds in homogeneous solution (20,62), the monomer decay often contains a contribution from an impurity with a lifetime similar to that of e.g. 1-methylpyrene (x ), becoming more important with increasing fluorescence quenching. This then leads to the difference observed in the longest decay times of excimer and monomer. 

Although it is not surprising that unresolvable distributions of decay times are encountered for systems such as Py on dry silica, this conclusion should not be carried over to fluid-like media such as silica with adsorbed alcohols and reversed-phase Si-C g. For 1Py(3)1Py in fluid solution, the concept of three discrete molecular states (one monomer and two excimers. Scheme (II)) has been shown to be valid (22-25). Similarly, the use of such a discrete state model can very well lead to interpretable results for the photophysical behaviour of Py and 1Py(3)1Py in media such as silica/octanol and Si-C g, see Section 4.3. 

Miyake, K. Kitaura, F. Mizuno, N., Phosphatidylcholine-coated silica as a useful stationary phase for high-performance liquid chromatographic determination of partition coefficients between octanol and water, J. Chromatogr. 389, 47-56 (1987). 

Data collected with a simple RPLC procedure has been found to be in good agreement with 1-octanol shake-flask partition or distribution coefficients over a 3.5 log range [64], A chemically bonded octadecylsilane support is coated with 1-octanol. With 1-octanol-saturated buffers as mobile phases, a stable baseline (compared to 1-octanol absorbed on silica) is... 

The general relationship between the type of solute and its retention can be seen by comparing the retention factors, k, of a set of standard compounds with their octanol-water partition coefficients, i.e. the logP value (listed in Table 4.1), as a measure of their relative solubility in water. The logarithm of the retention factor, log k, of these compounds measured in 50% aqueous acetonitrile on an octadecyl-bonded silica gel column shows a close linear relationship (Figure 4.1). 

Direct chromatographic determination of octanol-water partition coefficients has been attempted using a Corasil I column impregnated with octanol (287) or a reversed-phase column which had been coated with octanol (288). In either case, the sample was injected into an aqueous mobile phase. When the octanol-coated octyidecyl silica support was used, the... 

Among the large number of existing lipophilicity parameters [31], the descriptor frequently estimated by in silica methods is the partition coefficient of a solute between 1-octanol and water, expressed as log Poet [32]. However, lipophilicity determination in different solvent systems, such as alkane/water system, proved its utility in (Q)SAR studies and therefore some predictive methods also emerged in this field. Many publically available databases include numerous experimental values collected through the literature the quality of the experimental data represents the cornerstone of most of the models developed to predict lipophilicity. 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

Similar equipment for applications on the laboratory scale has been reported (and has recently been commercialized) (69-72). Most of the reported applications had the aim of investigating kinetics of chemical reactions as indicated by changes in liquid-phase concentrations. The equipment can typically be used at elevated temperatures and pressures. Applications to heterogeneous catalytic reactions include investigations of the enantioselective hydrogenation of exocyclic a,p-unsaturated ketones catalyzed by Pd/C in the presence of (A)-proline (73) and the esterification of hexanoic acid with octanol catalyzed by a solid acid (the resin Nafion on silica) (74). 

Fio. 11. Concentration profiles as measured with the equipment depicted in Fig. 10 (open symbols) and by GC analysis (closed symbols) for an esterification reaction between octanol and hexanoic acid. Conditions 200 ml, of reactant, 0.4mol/L of octanol, 0.4mol/L of hexanoic acid, l.Og Nafion resin/ silica, 447 K. The profiles were constructed from signals at 1720 and 1745 cm , and the spectra were corrected for solvent, octanol, and catalyst (74). 

This latter interpretation would mean that with the approach depicted in Fig. 10, the catalyst itself could be monitored. The authors reported that the silica-supported Nafion could not be observed in the beginning of their experiments and appeared in the spectra only after the catalyst interacted with octanol. This observation may indicate that the octyl groups promote the sticking of the catalyst particles onto the ATR probe, within the evanescent field. However, the example also shows that this approach may not be without problems, because it depends on the adsorption of the particles from the slurry reactor onto the ATR element. This process is accompanied by the adsorption of molecules on the catalyst surface and complicates the analysis. More important, as also indicated by the work of Mul et al. (74). this adsorption depends on the surface properties of the catalyst particles and the ATR element. These properties are prone to change as a function of conversion in a batch process and are therefore hardly predictable. 

Reversed phase HPLC methods have many supporters who insist that careful application of this technique can deliver log Poct values very reliably (Klein, 1988). When the stationary support is octanol-saturated silica, the process most nearly imitates the completely solvated distribution between phases (Mirrlees, 1976), but great care must be taken to avoid "channeling" in the solid support, especially for hydrophobic solutes where column length is short. 

Figure 4-8. Logarithm of retention factors of -hexanol and -octanol on octadecyl-silica at different water/organic compositions. (Reprinted from reference 47, with permission.)...

A fluorodensitometric assay was developed by Montague and co-workers 140) to analyze cultures from Ochrosia elliptica for ellipticine (1), 9-methoxy-ellipticine (2), and 9-hydroxyellipticine (3) by thin-layer chromatography (TLC) without the need for prior purification. Using silica gel impregnated with dimethyl sulfoxide and a mobile phase of EtOAc-water-l-octanol (17 2 2), these workers were able to achieve good separation of these alkaloids and to assay the resulting chromatograms by fluorodensitometry (40-300 fmol detection limits of alkaloid). 

One application for such differentially functionalized nanotubes is as smart nanophase extractors to remove specific molecules from solution. Nanotubes with hydrophUic chemistry on their outer surfaces and hydrophobic chemistry on their inner surfaces are ideal for extracting hpophihc molecules from an aqueous solution. The hydrophobic molecule 7,8-benzoquinoline (BQ), which has an octanol/water partition coefficient of 10 [3,8] was used as a model compound for such nanophase solvent extraction experiments. Figure 24.4A shows the UV-Vis spectmm of control solution containing 10 M aqueous BQ. A 5 mg of the silica-outer/Cig-inner nanotubes were suspended into 5 mL of 1.0 x 10 M aqueous BQ. The suspension was stirred for... 

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