Equilibration time

Fig. XI-8. Adsorption of BaDNNS on TiOi at 23°C from n-heptane solution. , x, A, D, O, adsorption points for indicated equilibration times. , desorption points following 12-hr and 20-min equilibrations, respectively. (From Ref. 124.)...

Typical runs consist of 100 000 up to 300 000 MC moves per lattice site. Far from the phase transition in the lamellar phase, the typical equilibration run takes 10 000 Monte Carlo steps per site (MCS). In the vicinity of the phase transitions the equilibration takes up to 200 000 MCS. For the rough estimate of the equihbration time one can monitor internal energy as well as the Euler characteristic. The equilibration time for the energy and Euler characteristic are roughly the same. For go = /o = 0 it takes 10 000 MCS to obtain the equilibrium configuration in which one finds the lamellar phase without passages and consequently the Euler characteristic is zero. For go = —3.15 and/o = 0 (close to the phase transition) it takes more than 50 000 MCS for the equihbration and here the Euler characteristic fluctuates around its mean value of —48. 

Following the completion of the polymerization process, the beaded polymer is recovered from the suspension mixture and freed from the stabilizer, diluents, and traces of monomers and initiators. For laboratory and small-scale preparation, repeated washings with water, methanol, or acetone are appropriate. Complete removal of the monomer diluent, solvents, and initiator, especially from macroporous resin, may require a long equilibration time with warm methanol or acetone. In industry, this is usually accomplished by stream stripping. 

The Importance of Equilibration Time for Equilibrium Between Two Ligands... 

The fact that the aliosterically preferred conformation may be relatively rare in the library of conformations available to the receptor may have kinetic implications. Specifically, if the binding site for the modulator appears only when the preferred conformation is formed spontaneously, then complete conversion to alios terically modified receptor may require a relatively long period of equilibration. For example, the allosteric p38 MAP kinase inhibitor BIRB 796 binds to a conformation of MAP kinase requiring movement of a Phe residue by 10 angstroms (so-called out conformation). The association rate for this modulator is 8.5 x 105 M-1 s-1, 50 times slower than that required for other inhibitors (4.3 x 107 M 1 s-1). The result is that while other inhibitors reach equilibrium within 30 minutes, BIRB 376 requires 2 full hours of equilibration time [8],... 

In summary, the Third Law predicts that ordering processes are favored as the temperature is lowered, so that eventually perfect order should be obtained in any solid as its temperature approaches 0 K. But kinetic effects are such that the equilibration times needed to achieve this order are sometimes very long. 

The most common approaches to sulfonylurea determinations involve high-performance liquid chromatography (HPLC). The earliest reported methods utilized normal-phase liquid chromatography (LC) with photoconductivity detection this type of detector demonstrated undesirably long equilibration times and is no longer... 

As in the 1,2-dichloroethane case too, transient EMF and SHG responses to KSCN were observed for the nitrobenzene membranes without ionic sites. This suggests that here too not only SCN but also K ions are transferred into the nitrobenzene phase. Salt extraction into the bulk of the organic phase, in analogy to similar observations previously reported for neutral ionophore-incorporated liquid membranes without ionic sites [55], was indeed independently confirmed by atomic absorption spectrometry. Figure 15 shows the concentration of K in nitrobenzene equilibrated at room temperature with a 10 M aqueous solution of KSCN as a function of equilibration time. The presence of the ion exchanger TDDMA-SCN efficiently suppresses KSCN extraction into the organic phase but in its absence a substantial amount of KSCN enters the nitrobenzene phase. The trends of the EMF and the SHG responses are therefore very similar in spite of the different polarities of the plasticizers. 

The practices of isocratic and gradient sorptive chromatography are very different. Isocratic chromatography tends to be very sensitive to the details of mobile phase preparation, temperature, pump speed, and sample composition. Gradient chromatography is usually more tolerant of small variations in these factors but may be extremely sensitive to column history, equilibration time, and gradient preparation. 

Parameters influencing the performance of headspace methods include sample preparation, sample temperature, equilibration time, carrier gas pressure, pressurisa-tion time, sampling time and transfer line temperature. For validation of headspace instrumentation, see Kolb and Ettre [207],... 

The pH-metric method, which also requires no phase separation, has been used to determine dmg-liposome partitioning [149,162,385-387], The method is the same as that described in Section 4.14, except that FAT-LUV-ET liposomes are used in place of octanol. SUV liposomes have also been used [385,386]. To allow for pH gradients to dissipate (Section 5.6) in the course of the titration, at least 5-10 min equilibration times are required between successive pH readings. 

Solubility measurement at a single pH [37-39] under equilibrium conditions is largely a labor-intensive procedure, requiring long equilibration times (12h-7 days). It s a simple procedure. The drug is added to a standard buffer solution (in a flask) until saturation occurs, indicated by undissolved excess dmg. The thermostated saturated solution is shaken as equilibration between the two phases is established. After microfiltration or centrifugation, the concentration of the substance in the supernatant solution is then determined using HPLC, usually with UV detection. If a solubility-pH profile is required, then the measurement needs to be performed in parallel in several different pH buffers. 

The initial titration aliquots were added automatically on the basis of the rate of change of EMF, mode (i), and the resulting time between aliquot additions was usually shorter than the 10 second equilibration time allowed in the mode (ii) titrations described above. The time differences were especially significant when the transmittance change per 0.05 cm3 aliquot was small, for example when the hyamine was present in excess at high salt concentrations. This means that the mode (i) titrations are more influenced by kinetic effects and so the measured curves are less distinctive as may be seen by comparing the results at 1.46% salt in Figures 6 and 7. 

FIGURE 8 Titration of 35 cm brine and 10 cm chloroform against SDBS 230 seconds initial equilibration time and 10... 

An example of these effects is shown in Figure 9 where a mode (ii) surfactant titration has been performed in the absence of salt and allowing a 10 seconds equilibration time between each aliquot. This gives a more clearly defined equivalence point when compared to the mode (i) titration in Figure 5. 

You could first wash the column with methanol, then trichloromethane, then heptane (or methanol, ethyl ethanoate, heptane). You cannot go directly from methanol to heptane because the two are only partly miscible. The column needs to be washed with about 20 dead volumes of each solvent (about 50 cm3 of each solvent for a 25 cm x 4.6 mm column). To get back to CH3OH/H2O 50 50 you would have to go through the sequence of solvents in reverse. If buffer solutions or ion-pairing reagents have been used in the mobile phase, very much longer equilibration times may be needed. 

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