Solvents ranked

For the purpose of polymer/additive analysis it is useful to rank solvents according to their boiling point ranges into low boilers (b.p. <100°C), medium boilers (b.p. 100-150°C) and high boilers (b.p. >150°C). Based on their evaporation numbers, solvents can be subdivided into four groups high volatility (< 10), moderate volatility (10-35), low volatility (35-50) and very low volatility (>50). 

An earlier attempt to rank solvents (144), particularly hydrootganic mixtures, was made by the comparison of the retention factors of 1,4-... 

Several methods have been proposed to guide the solvent replacement process for the many applications described in the chemical literature. These efforts attempt to build an organized framework for this process and provide a substantial improvement over previously ad hoc or trial-and-error approaches to solvent replacement. Joback outlines a methodology for the selection of replacement solvents for various processes such as extraction or cleaning (Joback, 1994). There are basically four steps to this process identify constraints on important solvent properties, compile data for all properties, rank solvents satisfying the target constraints, and evaluate top solvent candidates using simulation. 

Polarity is another property that has been used to tabulate liquid phases. By polarity we mean the electrical field effect in the immediate vicinity of the molecule which depends on the number, nature and arrangement of the atoms and on the type of bond and the groups. Rohrschneider (30) introduced a polarity scale, P, which ranks solvents according to their polarity. 

Two models have been proposed to describe the process of retention in liquid chromatography (Figure 3.3), the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968 and 1983). Both these models assume the existence of a monolayer or multiple layers of strong mobile-phase molecules adsorbed onto the surface of the stationary phase. In the solvent-partition model the analyte is partitioned between the mobile phase and the layer of solvent adsorbed onto the stationary-phase surface. In the solvent-competition model, the analyte competes with the strong mobile-phase molecules for active sites on the stationary phase. The two models are essentially equivalent because both assume that interactions between the analyte and the stationary phase remain constant and that retention is determined by the composition of the mobile phase. Furthermore, elutropic series, which rank solvents and mobile-phase modifiers according to their affinities for stationary phases (e.g. Table 3.1), have been developed on the basis of experimental observations, which cannot distinguish the two models of retention. 

Figure 5 Illustrates the effect of solvent changes on the rJ of the ionophore free acid and its anion. Kosower s Z values proved empirically an effective function for ranking solvents according to their Integrated polar and protic properties (18).

The evaporation rate of a solvent is determined to obtain relative value to some standard, selected solvent. The solvent selection depends on reasons for solvent use and the type of solvent and it is usually agreed upon between interested parties. In Europe, diethyl ether is the most frequently used reference solvent and in the US butyl acetate. The evaporation rate of other solvents is determined under identical conditions and the resultant values are used to rank solvents. The most obvious requirement is that the determination is done without excessive drafts and air currents. The evaporation rate is the ratio of the time required to evaporate a test solvent to the time required to evaporate the reference solvent under identical conditions. The results can be expressed either as the percentage evaporated within certain time frame, the time to evaporate a specified amount, or a relative rate. Relative rate is the most common. 

Numerous books and papers have been published on various effects of solvents on chemical reactions and reactivity. Also many scales were developed to rank solvent effect of reactivity by cation or anion solvation.There are many properties of solvents which affect chemical reactivity. These can be divided into physical and chemical effects. Physical effects of solvents may be generalized as follows ... 

The solvency of a liquid, or its ability to dissolve a certain solute, is often named its solvent power. This expression should, however, be used with care. There is, for instance, a tendency to rank solvents in order of increasing solvent power, but this is only possible for a specific solute. In former days this was usually a widely used binder for paints. For other solutes the order may well decrease rather than increase. The term solvent power, therefore, may lead to confusion and one should always bear in mind for what when it is said that a solvent has a good solvency. A typical example can be given for water and toluene. Water is an excellent solvent for sugar, but not for fat, whereas the reverse is true for toluene. 

An eluotropic series ranks solvents by their abilities to displace solutes from a given adsorbent. Eluent strength in Table 22-2 is a measure of solvent adsorption energy, with the value for pentane defined as 0. The more polar the solvent, the greater its eluent strength. The greater the eluent strength, the more rapidly solutes are eluted from the column. 

A way of providing the description in two dimensions is to rank solvents by both solubility parameter and hydrogen bonding ability (ASTM D3132-84, 1996). Unfortunately the values of the hydrogen bonding index were based on obsolete data, and this reduces the value of this description. 

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