Modifier concentration, effect

Furthermore, the pH effect was calculated using reference values. The p Tg values used for the calculations were added the organic modifier concentration effect, Api< a = 0.022 x (% of methanol). The constant 0.022 was experimentally obtained. The predicted k values were calculated from MIFS4m, MIFS4i and the modified reference piSTg values (calculated using eqn (10)). 

In general, retention decreases as the modifier concentration increases because the modifier competes with the analytes for sites on the stationary phase. The effect on retention of changes in modifier concentration seems to be more pronounced for CSPs than for achiral stationary phases in SFC, and peak shapes are apt to degrade rapidly at low modifier concentrations [12]. Efficiency tends to decrease as the modifier concentration increases because analyte diffusion is slowed by the increased viscosity of the eluent [39]. 

Interest has been shown by several groups on the effect of solvent and of added anions upon the oxidation of alcohols. The oxidation of isopropanol proceeds 2500 times faster in 86.5 % acetic acid than in water at the same hydrogen ion concentration . The kinetics and primary kinetic isotope effect are essentially the same as in water. Addition of chloride ion strongly inhibits the oxidation and the spectrum of chromic acid is modified. The effect of chloride ion was rationalised in terms of the equilibrium,... 

The mechanism by which sulphur has these observed effects is as follows. Immersion of native magnesium oxychloride cement in water brings about a slow dissolution which creates pores. When those pores are filled with sulphur, sites of possible stress concentration at points of contact between particles are modified. Similar effects occur when sulphur is used to impregnate hydraulic cements based on Portland cement and silica (Beaudoin, Ramachandran Feldman, 1977). 

This approach is not only very cost-effective, since it adopts the normal commercial processing operation as the reaction step at no additional cost, but has the added advantage also of preparing modified concentrates which are fully compatible with the unmodified polymer. 

Figure 4.7 Effect of high organic modifier concentration on peptide retention. Data obtained using an octadecyl column and 20 mM ammonium acetate acetonitrile mobile phases. (Reproduced from C.T. Wehr and L. Correia, LC at Work LC-121, Varian. With permission from Varian Associates.)...

Mass and heat transfer between the bulk fluid phase and the external catalyst surface can have an affect on reaction rates, and hence the selectivity, because of modified concentration and temperature driving forces. Such effects are unimportant for porous catalysts, but are significant for catalysis by non-porous metallic gauzes (for example, in NH3 oxidation referred to in Sect. 6.1.1). 

Photochemistry in Modified Supercritical Carbon Dioxide—Effect of Modifier Concentration on Diffusion Probed by Triplet-Triplet Energy Transfer (Worrall and Wilkinson, 1996)... 

Catalyst stability. Under the conditions used for preparative experiments, the optical yield remains constant up to complete conversion, suggesting that the modified catalyst is rather stable [62]. However, experiments at low modifier concentration indicate that the cinchona alkaloid deteriorates slowly and its enantioselective effect is lost [33,63]. 

Solvent and additives. Several systems have been studied concerning solvent effects. Fig. 6 shows that quite small changes in substrate, modifier or reaction conditions can lead to rather different results. Generally, very good results are obtained in apolar solvents with dielectric constants of 2-6. But in some cases alcohols can give equally high ee s. An important conclusion is that the optimal modifier concentration is dependent on solvent, modifier and substrate type [33]. The addition of amines and weak acids can affect the enantioselectivity [31,33]. 

Modifier concentration. Preliminary experiments indicated that catalyst and modifier concentration have a strong effect on rate and ee [30], In a detailed investigation this was confirmed (see Section V). 

It was shown in separate experiments that anilines can react with formamidines to give both N-mono- and N,N-diaryl formamidines which no longer are effective inhibitors. Another way of modifier deactivation is the hydrolysis to ineffective formamide by water formed during the hydrogenation of the nitro group. This could explain the rather high modifier concentration needed in order to get good selectivities. Similar observations of instability under reaction conditions have also been reported for dicyandiamide (ref. 4). 

TABLE IV Effect of Modifier Concentration on Extraction Efficiencies for Shale Rock Using On-Line SFE/GC... 

The effect of the percentage of 2-propanol on the enantiomeric resolution of benzetimide is shown in Figure 13a. It may be concluded that retention factor values decreased with an increase in 2-propanol content. According to Wainer et al. [87], the diminution of retention factor values with increases in the content of polar modifiers indicates that competition for the binding sites on the CSP is a saturable process and that a maximum effect on retention factor will be reached at a certain polar modifier concentration. It is interesting to note that although only a... 

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