Excess surface modification molecule

Figure 8.6 Stages of washing out of the excess surface modification molecules for the exchange of2C18 molecules on the montmorillonite surface. Reproduced from Ref [15] with permission from Wiley.

As mentioned above that high resolution TGA was used to check the purity of the filler so that no local bilayer of the excess surface modification molecules is present in the filler. The commercially treated fillers, however, are often observed to contain an excess of surface modification molecules [16]. This excess can lead to unwanted interactions with the epoxy prepolymer or can thermally degrade at lower temperatures when composites are subjected to higher temperatures thus, the presence of such excess amount is not required. In order to underline the effect of the excess surface modification molecules on the filler surface on the composite properties, epoxy nanocomposites with a number of commercially pro-... 

TGA is most commonly used for evaluation thermal stability of nanocomposites filled with montmorillonite, clay, or carbon nanotubes. High-resolution TGA is applicable while determining the presence of any excess of surface modification molecules unbound to the surface. It is very important to know this parameter, especially if the nanofiller is to be added to pol5mier at high temperatures. In such a situation, modification molecules may have lower thermal degradation, which will negatively affect the properties of the nanocomposite. The commercially treated filler, in comparison with the second one, clearly exhibits an extra degradation peak at lower temperature, which indicates the presence of modification molecules not bonded with filler surface... 

A qualitatively new approach to the surface pretreatment of solid electrodes is their chemical modification, which means a controlled attachment of suitable redox-active molecules to the electrode surface. The anchored surface molecules act as charge mediators between the elctrode and a substance in the electrolyte. A great effort in this respect was triggered in 1975 when Miller et al. attached the optically active methylester of phenylalanine by covalent bonding to a carbon electrode via the surface oxygen functionalities (cf. Fig. 5.27). Thus prepared, so-called chiral electrode showed stereospecific reduction of 4-acetylpyridine and ethylph-enylglyoxylate (but the product actually contained only a slight excess of one enantiomer). 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

For the Pt/cinchona catalysts only preliminary adsorption studies have been reported [30]. From the fact that in situ modification is possible and that under preparative conditions a constant optical yield is observed we conclude that in this case there is a dynamic equilibrium between cinchona molecules in solution and adsorbed modifier. This is supported by an interesting experiment by Margitfalvi [63] When cinchonine is added to the reaction solution of ethyl pyruvate and a catalyst pre-modified with cinchonidine, the enantiomeric excess changes within a few minutes from (R)- to (S)-methyl lactate, suggesting that the cinchonidine has been replaced on the platinum surface by the excess cinchonine. 

For the same reason as above, excess solvent molecules in the cavitation bubble also seriously limit the applicability of many volatile organic solvents as a medium for sonochemical reactions [2,25,26]. In fact, water becomes a unique solvent in many cases, combining its low vapor pressure, high surface tension, and viscosity with a high yield of active radical output in solution. Its higher cavitation threshold results in subsequently higher final temperatures and pressures upon bubble collapse. Most environmental remediation problems deal with aqueous solutions, whereas organic solvents are mostly used in synthesis and polymer modifications processes. 

To prove the above statement on the determining effect of electric charge of both the CdS colloidal particle surface and the quencher molecule on the adsorption of these molecules from aqueous solution, we have modified the surface of colloidal CdS during its preparation. The most efficient method of such modification consists in changing the surface charge of the colloidal particle via the preparation of nonstoichiometric colloid. In this case, the surface charge is determined by the charge of excessive ion (either S 2 or Cd2+). 

For steric reasons bifunctional or trifunctional silanes can react with either one only or, at most, two silanol groups on the silica gel surface (second reaction in Fig. 1.8A). Thus, some of the functional (Cl or alkoxy) groups remain unreacted and easily hydrolyse to form new silanol groups. If the reaction mixture contains even traces of water, the hydrolysis occurs during chemical modification of silica and the new silanol groups react with excess molecules of reagents to form a polymerised surface layer (Fig. 1.8B). These bonded phases may be more stable and usually show stronger retention than monomeric phases at low pH. However, the reaction is difficult to reproduce and various batches of the same material may have different properties, so that the reproducibility of separation is poorer than with monomeric phases. Polymeric phases are more resistant to penetration of analytes and may show increased mass-transfer resistance and decreased efficiency (plate number) of separation [- 91. 

Monofunctional silane reagents yield efficient stationary phases with flexible furlike or brushlike structure of the chains bonded on the silica surface. When bifunctional or trifunctional silanes are used for modification, Cl or alkoxy groups are introduced into the stationary phase, which are subject to hydrolysis and react with excess molecules of reagents to form a polymerized spongelike bonded phase structure. Stationary phases prepared in that way usually show stronger retention but lower separation efficiency (plate number) than mono-merically bonded stationary phases. 

We cannot just multiply the above data by the ratio of sublimation energies to obtain estimates for the adsorbates like Z1CI4 on bare surface. However, we can expect that, on an incompletely modified silica surface, a closer contact of the tracer molecules with oxygen atoms will result in stronger attractive forces than those of physisorption on the fully modified surface. It might be the most fundamental rationale for the excess of over the sublimation energy, provided that the modification of the surface is incomplete. 

Enantiomeric excess diminished with time on stream when the modification concentration was 0.34mM, but remained constant when the concentration was 3.4mM (Figure 3). It is known for Pt/silica that alkaloid adsorption can occur on both the metal and the support and in our catalysts there may have been diffusion of alkaloid molecules between the support and the active phase during hydrogenations. In such a case, modification at a higher concentration would have provided more extensive adsorption on the support, and hence a larger reservoir of cinchonidine would have been available to sustain a steady state concentration on the Pt surface or to replace any modifier rendered inactive by partial or complete hydrogenation. 

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