Octyl-modified Silica Research Articles

Synthesis and Characterisation of Organo-Modified Silica Nanostructured Films for the Water-Repellent Treatment of Historic Stone Buildings

This study presents the facile sol-gel synthesis of nanostructured coatings for use in water-repellent treatment of travertine stone. The synthesized materials combine surface roughness characteristics with particular chemical compositions to give different hydrophobicity results. The influence of the silica particle coating precursor on the hydrophobicity of the polymeric film was investigated, and the octyl-modified silane was selected for further fabrication of the hybrid coatings. The water repellent properties, together with composition and structural properties of the silane-based hybrid material were measured on model glass surface. The coating with the best characteristics was subsequently deposited onto the travertine stone. The potential applicability of the nanostructured material was evaluated considering both the properties of the coating film and those of the travertine stone subjected to the treatment. The surface texture of the film, water repellent properties and uniformity were determined using scanning electron microscopy, atomic force microscopy, dynamic light scattering and contact angle measurements. The coating’s potential for use in stone conservation was evaluated by assessing its impact on the stone’s visual aspect. All the results obtained from the different types of analyses showed that the octyl-modified silica nanostructured material was highly hydrophobic and compatible both with the travertine stone and with the requirements for use on cultural heritage monuments.

Synthesis of Octyl-Quaternary Ammonium Mixed-Mode Stationary Phase by Vapor Deposition Approach and Its Application in Compound Preparation Separation

An octyl-quaternary ammonium (Sil-C8-QA)-modified silica stationary phase was prepared by a two-step vapor deposition method which can substantially reduce the consumption of toxic organic solvents. The structure of the dual-functional ligands stationary phase was characterized by Fourier transform infrared spectroscopy and solid-state 13C CP/MAS NMR. Following elemental analysis, the surface density of octyl and quaternary ammonium bonding amount is 2.13 µmol m−2 and 0.77 µmol m−2, respectively. The efficiency and selectivity of the two-ligand phases were compared with two sole functional ligand phases: quaternary ammonium (Sil-QA)- and octyl-modified silica (Sil-C8). Chromatographic evaluation demonstrated a mixed-mode reversed-phase/anion-exchange retention property on the Sil-C8-QA column. The retention behavior of a homologous series of alkylbenzoic acids on the synthesized phase was systematically studied by changing the pH, ionic, and solvent strength of the mobile phase. A well-established mixed-model mechanism was confirmed. The Sil-C8-QA phase can predominantly enhance the retention of acidic drugs and improve their selectivity. It was also successfully applied to the simultaneous separation and determination of three kinds of compound preparations with large differences in component polarity.

Radial multi-stationary phase thin-layer chromatography for the field-ready fingerprinting of herbal materials

A novel TLC plate design that features multiple screen printed phenyl- and octyl-modified silica gel stationary phases configured as radial sectors was developed for the in-field screening of herbal materials.

Active-site titration analysis of surface influences on immobilized Candida antarctica lipase B activity

Matrix morphology and surface polarity effects were investigated for Candida antarctica lipase B immobilization. Measurements were made of the amount of lipase immobilized and the catalyst's tributyrin hydrolysis activity, along with a determination of the lipase's functional fraction by active-site titration. Soluble, purified lipase had an active fraction of 84%. Immobilization on the hydrophobic surface of macroporous poly(methylmethacrylate) resin resulted in the full retention the lipase active fraction, while immobilization on the hydrophobic surface of mesoporous, amorphous, octyl-modified silica allowed retention of just half of the lipase active fraction. The polar surface of unmodified mesoporous, amorphous silica bound the lipase in such a manner that all of the immobilized enzyme was active. Mesoporous, crystalline SBA-15 silica also bound lipase so that it all was active. The polar, non-porous surface of fumed silica retained only a small fraction (28%) of active lipase. Substantial differences were found among the various supports in their ability to preserve catalytic activity upon vacuum drying. These findings demonstrate that surface polarity alone is not the only determinant for immobilization, as hydrophobic poly(methylmethacrylate) and hydrophilic SBA-15 were equally competent as lipase supports. The ordered-channel mesostructure of SBA-15 may provide a critical balance of interactions with the enzyme to preserve its native conformation.

Effect of Colloidal Fillers on the Cross-Linking of a UV-Curable Polymer: Gel Point Rheology and the Winter−Chambon Criterion

The effect of colloidal silica fillers on the cross-linking behavior of a model UV-curable polymer system (thiol−ene) is studied using in situ rheology and real-time FTIR spectroscopy. The validity of the Winter−Chambon criterion (convergence of the loss tangents at the gel point) is examined for the cross-linking of these filled polymers, some of which are strongly flocculated dispersions (physical gels) prior to chemical cross-linking. Two different types of colloidal silica particles are studied: one with octyl chains tethered to the surface and the other with methyl surface groups. The Winter−Chambon criterion is satisfied for all samples containing the methyl-terminated silica. However, the criterion breaks down for samples containing the octyl-modified silica, with the loss tangents not converging at any single point. This suggests the absence of a self-similar critical gel at the gel point in the latter case. Neither type of silica particles alters the mechanism of the cross-linking reaction, as revealed by FTIR spectroscopy, but they do retard the cross-linking kinetics. An alternate method is suggested for determining the chemical gel point of filled systems that fail to obey the Winter−Chambon criterion. This method involves monitoring the critical strain (limit of the linear viscoelastic region) at various UV exposure times. A dramatic increase is observed in the critical strain at the gel point, indicating a transition from weak, physical bonds to strong, covalent cross-links.

Compositional fractionation of poly(methyl methacrylate)- graft-polydimethylsiloxane by reversed-phase high-performance liquid chromatography

The results are reported of the compositional fractionation of poly(methyl methacrylate)- graft-polydimethylsiloxane (PMMA- graft-PDMS) by high-performance liquid chromatography (h.p.l.c.) of reversed-phase adsorption mode. Graft copolymers with different compositions were prepared by the radical copolymerization of PDMS macromonomer with MMA, using azobisisobutyronitrile as the initiator in benzene at 60°C. The gradient h.p.l.c. measurements were made using a prepacked column of octyl-modified silica gel, with tetrahydrofuran and acetonitrile as eluents. Detection was performed by an evaporative light-scattering detector. The PMMA- graft-PDMS samples were eluted from the components of higher PMMS contents to those of lower PMMA contents, according to reversed-phase adsorption. The present graft copolymers were separated according to their chemical compositions.

A comprehensive method for determining hydrophobicity constants by reversed-phase high-performance liquid chromatography.

The development of a method for determining hydrophobicity constants for small, organic molecules by reversed-phase liquid chromatography (RPLC) is presented. The method uses capacity ratios measured at a number of different compositions of methanol to obtain derived values, denoted log k'w, upon which a new scale of hydrophobicity constants can be developed. This scale eliminates potential problems such as peak inversion that hamper RPLC methods using isocratic data to estimate hydrophobicity. The differential hydrogen bond effect observed in most correlations of RPLC data with logarithms of octanol-water partition coefficients (log Po/w) for compounds of opposite net hydrogen bonding capabilities (noncongeners) was minimized by adding trace quantities of n-decylamine and 1-octanol to the eluent and using an octyl-modified silica gel stationary phase. Values of log k'w are shown to be largely column-independent as long as the hydrophobic properties of columns are similar. The correlation of log k'w values with the logarithms of bovine serum albumin binding constants (log 1/C) is shown to be statistically indistinguishable from the correlation of log 1/C with log Po/w, indicating that this data models log 1/C as well as log Po/w for these compounds. Additionally, the chromatographic system is automatable and thus capable of higher sample throughput than measurements of log Po/w by the shake-flask method.

The use of high performance liquid chromatography for the analysis and monitoring of iron carbonyl complexes in a reaction mixture

The suitability of high performance liquid chromatography (HPLC) as an analytical and preparative tool for the analysis and isolation of the products resulting from a reaction of diironnonacarbonyl [Fe 2(CO) 9] with di-t-butyl-sulfurdiimine [DBSD] was investigated. Reserved phase chromatography with octyl-modified silica as stationary phase and methanol/water mixtures as mobile phase showed a high selectivity for the separation of the products, and by use of gradient elution the separation of 17 products was completed within 30 min, which meant that the progress of the reaction could be monitored, A semi-preparative column (dimensions 250 X 10 mm) was useful in isolating some of the products; with this column 7 mg of reaction products could be injected without loss in efficiency.

Study of the influence of competition and solvent interaction on retention in liquid-solid chromatography by measurement of activity coefficients in the mobile phase

Summary By measuring the activity coefficients of solutes and moderator in the mobile phase and the capacity ratios, it was possible to study separately the dependence of the competition term and the solvent interaction term on the composition of the mobile phase. This was done for mobile phases consisting of 2-propanol and ethyl acetate as moderators in n -heptane, in combination with silica with a high surface area. It was found that both the competition and solvent interaction effects contribute significantly to the change in retention with changing composition of the mobile phase. For a considerable concentration range of the moderator, the stationary phase effects can be understood as a competition for the surface between the moderator and the solutes, while assuming an adsorbed layer of fixed size consisting of moderator and solute. For higher moderator concentrations the competion effect diminishes, and it seems that in this instance solutes can enter the layer without displacing the moderator. The measurement of activity coefficients was also applied to the mobile phases acetonitrile-water and methanol-water used in reversed-phase adsorption chromatography. Comparisons are made with retention data obtained with octyl-modified silica. Preliminary results suggest that the assumption of constant activity of solutes in the stationary phase is not tenable.

Determination of methotrexate in plasma by on-column concentration and ion-exchange chromatography

A method for the concentration of dilute samples on a column and its application to the analysis of biomedical mixtures is described. Two columns are used, one for the concentration step and the other for the separation. The concentration column is used in the position of the loop in an injection valve. Concentration is effected by eluent switching. The influence of the chromatographic parameters such as capacity ratio and void volume on separation and detectability are described and the optimal choice of conditions is discussed. The method has been applied to the analysis of the cytostatic drug methotrexate. An octyl-modified silica was used as the stationary phase in the concentration column and a chemically bonded anion exchanger on silica in the separation column. The procedure shows good selectivity and precision. The detection limit corresponds to 2.10-8 M (9 ppb) in plasma, which is sufficiently low for therapeutic concentrations to be measured. An example of the use of the same stationary phase in both columns is given for the analysis of phenothiazines in blood, using alkyl-modified silica and electrochemical detection.