Solvent, composition, effect

It has been also reported a study on the threading process of a a - cyclodextrin (a-CD) and polyethylene glycol (PEG), as a function of temperature and solvent composition. This reaction produces a polyrotaxane that eventually precipitates and forms a thick gel. Ceccato et al. [46] have proposed a molecular model for the interpretation of the temperature and solvent composition effect on the threading process. According of this model, the reaction can be depicted as a five - step phenomenon that mainly depend on the threading and sliding of a-CD and PEG. The transition... 

E. Soczewinski, Solvent composition effects in thin-layer chromatography systems of the type sihea gel-electron donor solvent. Anal. Chem. 41 (1969), 179-182. 

E. Bosch, C. Rafols, and M. Roses, Variation of acidity constants and pH values of some organic acids in water-2-propanol mixtures with solvent composition. Effect of preferential solvation, A a/. Chim. Acta 302 (1995), 109-119. 

El-Gholabzouri, O., Cabrerizo, M.A., and Hidalgo-Alvarez, R., Streaming current of polystyrene porous plugs Solvent composition effect, J. Colloid Interf. Sci., 199, 38, 1998. 

J.W. Ho, A Study of the Solvent Composition Effects on the Separation of Seven Clinically Important Porphyrins on Cyclodextrin Bonded Phases, J. Liq. Chromatogr., 13(11)(1990)2193. 

A comparison of rate and product composition of tiie products from reaction of t-butyl chloride with NaOMe in methanol and methanol-DMSO mixtures containing NaOMe has been done. Interpret the effect of the change of solvent composition and NaOMe concentration. 

The authors also investigated the effect of solvent composition on the retention of a series of solutes including a dispersion of silica smoke (mean particle diameter 0.002 pm). The silica smoke was used to simulate a solute of very large molecular size... 

Although it is often possible to predict the effect of the solvent on retention, due to the unique interactive character of both the solvents and the enantiomers, it is virtually impossible to predict the subtle differences that control the separation ratio from present knowledge. Nevertheless, some accurate retention data, taken at different solvent compositions, can allow the retention and separation ratios to be calculated over a wide range of concentrations using the procedure outlined above. From such data the phase system and the column can be optimized to provide the separation in the minimum time, a subject that will be discussed later in the treatment of chromatography theory. 

The Combined Effect of Temperature and Solvent Composition on Solute Retention... 

If the relationship between retention volume and temperature is known, as well as its dependence on solvent composition, then the combined effect of both solvent... 

The numerical constants were obtained over the temperature range of 5°C to 45°C and a concentration range of 0 to 0.5 volume fraction of ethanol inn-hexane.The effect of temperature and solvent composition on solute retention can, again, be best displayed by the use of 3-D graphs, and curves relating both temperature and solvent composition to the retention volume of the (S) enantiomer of 4-benzyl-2-oxazolidinone are shown in Figure 23. Figure 23 shows that the volume fraction of ethanol in the solvent mixture has the major impact on solute retention. 

The effect of temperature, although significant, is not nearly as great as that from the ethanol content and is greatest at low concentrations of the polar solvent. It is clear, that the solute retention is the least at high ethanol concentrations and high temperatures, which would provide shorter analysis times providing the selectivity of the phase system was not impaired. The combined effect of temperature and solvent composition on selectivity, however, is more complicated and to some extent... 

It is seen that the curves in Figure (24) become horizontal between 40°C and 45 °C as predicted by the theory. It is also clear that there is likely source of error when exploring the effect of solvent composition on retention and selectivity. It would be important when evaluating the effect of solvent composition on selectivity to do so over a range of temperatures. This would ensure that the true effect of solvent composition on selectivity was accurately disclosed. If the evaluation were carried out at or close to the temperature where the separation ratio remains constant and independent of solvent composition, the potential advantages that could be gained from an optimized solvent mixture would never be realized. 

Scott and Beesley [2] measured the corrected retention volumes of the enantiomers of 4-benzyl-2-oxazolidinone employing hexane/ethanol mixtures as the mobile phase and correlated the corrected retention volume of each isomer to the reciprocal of the volume fraction of ethanol. The results they obtained at 25°C are shown in Figure 8. It is seen that the correlation is excellent and was equally so for four other temperatures that were examined. From the same experiments carried out at different absolute temperatures (T) and at different volume fractions of ethanol (c), the effect of temperature and mobile composition was identified using the equation for the free energy of distribution and the reciprocal relationship between the solvent composition and retention. 

Bu4NBr in AN/PC in the temperature range 75 °C>6>- 35°C a linear correlation /rmax 41) is found [209], independent of temperature and solvent composition. The use of high-permittivity solvents belonging to the same class suppresses the effects due to strong selective solvation or changing association. 

Fig. 22. The effect of solvent composition on the molecular weight ( ), conversion (O), and molecular weight dispersity ( ) of PaMeSt prepared using the HSi(CH3)2CH2CH2

The effect of changing solvent composition (polarity) on molecular weight dispersity is noteworthy. Mw/Mn is quite low (1.69) in the experiment carried out by the use of 100% CH2C12 and it increases monotonically with increasing n-C6H14 content. It is very difficult to interpret these data at this time. 

Fig. 29. The effect of temperature on the molecular weight of PIB prepared using the HSi(CH3)2CH2CH20>CH2Cl/Me3Al initiating system with solvent composition CH2Cl2/n-QHt = 85/15 ( ) and 65/35 (O) (See Table 11 for reaction conditions)...

VARIATION OF SOLVENT ISOTOPE EFFECT WITH MEDIUM COMPOSITION IN REACTION... 

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