Solvent systems increments

Figure 7. Detailed mapping of the dependence of ion conductivity on salt concentration (m), solvent composition (x), and temperature (7) in a commonly used binary solvent system EC/DMC by surface plots. The orientations, titles, and units of the axes used in the plots are shown in the bottom portion of the figure, and the temperatures for these plots are, in order of their appearance, from 60 to —30 °C in 10 °C increments. The o ranges for the surface plots are, in order of their appearance, (8.49, 16.7), (7.78, 14.6), (7.03, 12.6), (6.23, 10.6), (5.44, 8.7), (4.63, 7.00), (3.14, 5.47), (1.93, 4.14), (1.04, 3.00), and (0.46, 2.06) mS cm (Reproduced with permission from ref 195 (Figure 4). Copyright 2001 The Electrochemical Society.)...

The reverse-phase analysis was carried out on a SUPELCOSIL LC-18, 3-/zm particle size, 150 X 4.6-mm ID column (solvent system A, acetonitrile B, acetonitrile-tetrahydrofuran-chloroform (50 27.5 22.5) linear gradient from 30% to 100% of B in 70 min, flow rate 0.5 ml/min) (Fig. 22). The upper part of Fig. 22 shows that various chain lengths (C12 to C24 with one-carbon increment) of PNB-TBDMS-OHFA separated well enough in 30 min for effective recovery of the components by an absorbance slope-detecting fraction collector-detector combination. The separation of the positional isomers present in the used mixture was only minor, and it did not interfere with the fractionation according to chain length. 

Some interesting conductance studies of protein-nonaqueous solvent systems have been carried out. Greenberg and Larson (1935), for example, found that gelatin, casein, and edestin dissolved in glacial formic acid (D = 56.5 at 16°C) showed marked increases of conductivity over that of the solvent, whereas no conductivity increment was observed with the same proteins dissolved in glacial lactic or acetic (D = 6.15 at 20°C) acids. This is in accord with the conclusions reached in Section IV,B,1, that in solvents of low dielectric constant, ion-pairing (in these cases, of the protein cations to lactate and acetate counterions) is essentially complete. Con-... 

The higher the equilibrium chain rigidity and the increment of refractive index dn/dc in the polymer-solvent system, the more pronounced is this effect. For a rigid-chain polymer the Aa value in Eq. (49) is calculated according to )... 

The solution to this problem has been to attach these catalysts to polymer supports. The ideal polymer-bound catalyst must satisfy a formidable list of requirements. It should be easily prepared from low cost materials. The support must be compatible with the solvent system employed, and be chemically and thermally stable under the reaction conditions. The catalyst should show minimal losses in reaction rate or selectivity when bound to the support, and should be able to be recycled many times without loss of activity. Finally, the interactions between the catalytic site and the support must be either negligible or beneficial. The development of polymer supported rhodium-phosphine catalysts for the asymmetric hydrogenation of amino acid precursors illustrates the incremental process which has led to supports which approach the ideal support. 

The method requires that the Mark-Houwink exponents for the polymer-solvent system and the sample amount injected be known. The concentration at each elution slice is then calculated from the viscometer output the universal calibration curve, and the Mark-Houwink exponent. From the Mark-Houwink equation and the definition of hydrodynamic volume in universal calibration, it can be shown that the concentration at each elution volume increment is given by (18)... 

SEC-LS is used to measure molecular weight distribution directly as a polymer elutes from the SEC without universal calibration. For each polymer-solvent system, the specific refractive index increment dn/dc is required, and for most instruments the solvent refractive index is also needed. Table 3 lists selected papers describing SEC-LS measurements of synthetic polymers, copolymers, polysaccharides, cellulosics, and related polymers. 

The most reliable results of studies of the light scattering and specific refractive increment behaviour of amylose in dimethyl sulphoxide-water are obtained with dialysed solution-solvent systems. Discrepancies in published literature are probably caused by differences in the preparation of the sample and the botanical source of the amylose. 

Using equation 6 a plot of the square of the particle radius versus the sorption time should be linear with a slope dependent on the swelling capacity and on the diffusion coefficient of the polymer/solvent system. In order to test this, a sample of DRYTECH superabsorbent polymer and a sample of Aridall 1078 from American Colloids were individually screened using U.S. Standard Sieves to obtain narrow particle size fractions. The sorption time data Is given in Table 1. The functional relation between the square of the radius and sorption time is shown in Figure 2. The result is consistent with a diffusion controlled absorption rate, and indicates that the simplifications of the diffusion equations are acceptable. Since the particle radius increases as the test proceeds, the initial value should be incremented by a factor equal to (22.8/Wp) -, which is the extent of swelling of the sample at time t, to the one-third power. 

If the poorer solvent is added incrementally to a system which is poly-disperse with respect to molecular weight, the phase separation affects molecules of larger n, while shorter chains are more uniformly distributed. These ideas constitute the basis for one method of polymer fractionation. We shall develop this topic in more detail in the next section. 

As will be seen later (Section V.l), meaningful molecular weights in multicomponent systems can be determined, if the specific refractive index increment appertains to conditions of constant chemical potential of low molecular weight solvents (instead of at constant composition). Practically, this can be realised by dialysing the solution against the mixed solvent and then measuring the specific refractive index increment of the dialysed solution. The theory and practice have been reviewed4-14-1S> 72>. 

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