Hydrophobic Silica Surface

The largest silicate species in the extracts of the concentrated TEA system after 45 min, detected with GPC as a shoulder on the main peak, has a retention volume characteristic for dimer of tetracyclic undecamer (Fig.2). This indicates that the first steps of the TEOS polycondensation processes till the formation of (4) and (4 ) occur with TEA as well. This is in agreement with the proposed model in which TEOS hydrolyses at the TEOS-aqueous interfaces in the vicinity of the alkyl groups of the tetraalkylammonium cations (Fig.l). These alkyl groups favor the formation of the hydrophobic silica surfaces encountered already in the smallest species (l)-(3). The absence of trimers... 

One of the difficulties of the technique is the specific nature of the surfaces that are used, predominantly mica and graphite. Hence, the ability to compare directly with the results from other techniques, such as ellipsometry and neutron reflectivity, is not straightforward. Furthermore, the degree to which the crystalline nature of those surfaces influences the observed structures is not established. This has been partially addressed in recent studies by Ducker and Wanless [43] on the adsorption of Ci6TAB onto modified mica surfaces and by Wolgemuth et al. [44] with a range of surfactants adsorbed onto a TMCS-coated hydrophobic silica surface. In these studies, changes in the surface properties appear to have a dramatic effect upon the surface structures that are... 

A possible reason for the decreased adsorption of some nonionic materials has been suggested by Claesson et al. [42]. In a study of the adsorption of sodium dodecyl sulfate and ethyl(hydroxyethyl) cellulose onto hydrophobized silica surfaces, they found that the amount of adsorbed cellulosic derivative was reduced when sodium dodecyl sulfate is also adsorbed. A possible reason for this is again the effect of hydrophobic associations. The sodium dodecyl sulfate, being more surface-active, will tend to adsorb onto the hydrophobic surface more strongly. It will also associate with the ethyl(hydroxyelhyl) cellulose in solution, rendering this nonionic material more hydrophilic. With both the adsorbing surface and the... 

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

The influence of adsorption on the structure of a -chymotrypsin is shown in Fig. 10, where the circular dichroism (CD) spectrum of the protein in solution is compared with that of the protein adsorbed on Teflon and silica. Because of absorbance in the far UV by the aromatic styrene, it is impossible to obtain reliable CD spectra of proteins adsorbed on PS and PS- (EO)8. The CD spectrum of a protein reflects its composition of secondary structural elements (a -helices, / -sheets). The spectrum of dissolved a-chymotrypsin is indicative of a low content of or-helices and a high content of //-sheets. After adsorption at the silica surface, the CD spectrum is shifted, but the shift is much more pronounced when the protein was adsorbed at the Teflon surface. The shifts are in opposite directions for the hydrophobic and hydrophilic surfaces, respectively. The spectrum of the protein on the hydrophilic surface of silica indicates a decrease in ordered secondary structure, i.e., the polypeptide chain in the protein has an increased random structure and, hence, a larger conformational entropy. Adsorption on the hydrophobic Teflon surface induces the formation of ordered structural elements, notably an increase in the content of O -helices (cfi, the discussion in Sect. 3.1.4). 

The enzymatic activities of O -chymotrypsin in solution and adsorbed at the different surfaces are presented in Fig. 11, where the specific enzymatic activity (defined as activity per unit mass of protein) is plotted as a function of temperature. The enzyme loses activity due to adsorption. On the hydrophobic Teflon and PS surfaces, the activity is completely gone, whereas on the hydrophilic silica surface, or-chymotrypsin has retained most of its biological function. These differences are in agreement with the adsorption isotherms and the circular dichroism spectra. The influence of the hydrophobicity of the sorbent surface on the affinity of the protein for the sorbent surface, as judged from the rising parts of the adsorption isotherms (Fig. 8), suggests that the proteins are more perturbed and, hence, less biologically active when adsorbed at hydrophobic surfaces. Also, the CD spectra indicate that adsorption-induced structural perturbations are more severe at hydrophobic surfaces. 

Many other types of solid phase adsorbents, including those based on conventional and specialty materials like restricted access media (RAM), can increase analysis speed and improve assay performance. These types of materials, also known as internal reversed-phase packings, are especially useful for assaying target compounds in biological samples such as serum and plasma. They are chemically modified porous silicas that have hydrophilic external surfaces and restricted-access hydrophobic internal surfaces. The ratio of interior to external surface areas is large. Macromolecules such as proteins cannot enter the pores of the RAM (they are excluded from the hydrophobic internal surface) and they elute quickly through the column. However, the smaller analyte molecules that can enter the pores are retained via interactions with the hydrophobic bonded phase within... 

Hydrophobic polar surfaces, adsorption of ionic surfactants on, 24 140-141 Hydrophobic precipitated silica, 22 399 Hydrophobic solvents, 16 413 Hydrophobic surfaces, 1 584-585... 

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

V. Hlady, C. Golander, and J. D. Andrade, Hydrophobicity gradient on silica surfaces A study using total internal reflection fluorescence spectroscopy, Colloids Surf. 33, 185-190... 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

Figure 1.5 A non-wetting water droplet on the surface of methylated, hydrophobic silica.

Modifications of monodisperse colloidal silica, of 10 or 500 nm in diameter, were carried out using trialkoxysilane-terminated polymer in a low polar solvent, such as acetone or 1,2-dimethoxyethane. without coagulation during the coupling reaction (35,37-42). In this modification, the hydrophobic polymer can be efficiently bound to hydrophilic colloidal silica surface. The reaction mechanism of the binding... 

Precipitated and pyrogenic silicas are widely employed in antifoams used in industry. To be effective, the silica surface must be rendered hydrophobic by reaction with an agent such as polydime thylsiloxane. The hydrophobic particles are dispersed in a carrier fluid such as mineral or silicone... 

In all the cases of poly acetylene, polythiophene, and polypyrrole coating, the amount of plasma-film deposition was different, caused by the difference in the structure of the three different monomers and their reactivity during the plasma process. PPy- and PTh-silica are more hydrophobic than PA-silica, probably due to the presence of different chemical moieties in the complex film structure deposited onto the silica surface. 

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