Phase Osmometry

Vapor pressure osmometric (thermoelectric, vaporometric) measurements depend on the following principle A drop of a solution with a nonvolatile solute resides on a temperature sensor, i.e., a thermistor. The surrounding region is saturated with solvent vapor. Initially, the drop and vapor are at the same temperature. Since the vapor pressure of the solution is lower than that of the pure solvent, solvent vapor condenses on the solution drop. Because heat of condensation is released, the temperature of the drop rises until the difference in temperature A Tth between the drop and the solvent vapor again eliminates the difference in vapor pressure, so that the chemical potential of the solvent in both phases is equal. An analogous equation to that which applies in ebulliometry is applicable in this case to the relationship between the temperature difference A Tth and the number-average molar mass Mn)= Mioi the solute. 

like ebulliometry or cryoscopy, the method would have a strong thermodynamic basis if heat transfer other than that due to vapor condensation could be prevented. Vapor and drop are, however, in contact with one another, and the temperature thus tends to equilibrate in time by convection, radiation, and conduction. This again causes renewed condensation of solvent vapor, which proceeds until a final steady state with a temperature difference A r is reached. Equation (9-25) becomes, with AT = A Ttn  

Since ks cannot be derived theoretically, Ke is usually determined by calibration with substances of known molar mass. As with all the other molar-mass-determination methods, only apparent molar masses Afapp are obtained for finite concentrations (see Sections 6.4 and 6.5) because of the effects of the virial coefficients and/or association, A/app must therefore be extrapolated to the concentration a 0. In vapor phase osmometry, low amounts of nonvolatile impurities interfere with the result, but volatile impurities do not, since they pass into the vapor phase. 

This is a widely used technique based on the determination of colligative properties. Despite its name, it is not ait osmotic technique at all, but is actually an indirect method of measuring vapour-pressure lowering. The parameter that is measured is the miniscule temperature difference that is obtained in an atmosphere of saturated solvent vapour between droplets of pure solvent and droplets of polymer solution each experiencing solvent evaporation and condensation. This small temperature difference is proportional to the vapour-pressure lowering of the polymer solution at equilibrium hence, it is also proportional to 

The temperature differences found experimentally are less than expected theoretically because of heat losses within the apparatus. As indicated in the earlier part of this chapter, the experimental approach is to measure these temperature differences at a number of different concentrations and extrapolate to f = 0. The apparatus is calibrated using standard solutes of low relative molar mass, but despite this, the technique can be used on polymers up to Af of about 40 000. 

The technique is useful in that only small amounts of the sample polymer are needed, though experimentally it is time-consuming and may require great patience in use. This is because the technique does not measure equilibrium vapour-pressure lowering, but measures vapour-pressure lowering in a steady-state situation. Thus care must be taken to ensure that time of measurement and droplet size are standardised for both calibration and sample measurement. 

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Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

In which the ratio m/n is close to 3. The silane was produced by free radical copolymerization of vinyltriethoxysilane with N-vinylpyrrolidone. Its number-average molecular weight evaluated by vapour-phase osmometry was 3500. Porous silica microballs with a mean pore diameter of 225 A, a specific surface area (Ssp) of 130 m2/g and a pore volume of 0.8 cm3/g were modified by the silane dissolved in dry toluene. After washings and drying, 0.55% by weight of nitrogen and 4.65% of carbon remained on the microballs. Chromatographic tests carried out with a series of proteins have proved the size-exclusion mechanism of their separation. 

Typically, the characterization of supramolecular systems requires the application of a whole set of analytical techniques. Without going into details, the capsular structure of the tennis ball 1-1 has been confirmed by X-ray crystallography, vapor phase osmometry (VPO), and NMR experiments.1111 The encapsulation of guests can be routinely analyzed by NMR experiments. Recently, a mass spec-trometric method was developed that not only... 

Average of cryoscopy, ebulliometry, 11,000 15,300 18,500 and vapour-phase osmometry Rapid membrane osmometer, first 19,900 22,100 27,900 observable values (5-7 min after sample introduction)... 

Vapour phase osmometry is found satiable for determining Mn of low Molecular weight samples. This... 

In the vapor phase osmometry (VPO) technique, drops of solvent and solution are placed in an insulated chamber close to thermistor probes. Since the solvent molecules evaporate more rapidly from the solvent than from the polymer solution, a temperature difference results that is related to the molarity of the polymer (M), which can be determined if the heat of vaporization per gram of solvent (A) is known using the following relationship ... 

Table XVII. Reproducibility of Molecular Weight Determinations by Vapor-Phase Osmometry (Butarez CTL Lot 18)...

The objective of this present work was to investigate the feasibility of using GPC/DV for absolute molecular weight determination of hydroxypropylated lignins. In order to verify the validity of the universal calibration method, vapor phase osmometry (VPO) was used to provide reference number average molecular weight values. Comparisons with LALLS results have also been made and will be reported in another publication. 

Vapor Phase Osmometry. A Wescan Model 233 vapor phase osmometer was used to obtain number average molecular weights. The lignin solutions were made up with HPLC grade tetrahydrofuran (THF) and shaken manually until the solutions were clear. The experiments were conducted at 30°C. Number average molecular weights were determined by multistandard calibration (41), a procedure found to greatly enhance reproducibility and accuracy of the results. Experiments were conducted immediately after sample preparation and three days later. 

P up to 90 (by vapor-phase osmometry). The structures of the polymers, as indicated by their optical rotation, were strongly influenced by changes in the reaction conditions (especially by the temperature and polarity of the solvent). Lower temperatures (—78°) gave rise to higher yields and greater Pn- No epimerization occurred during polymerization, other than that at the anomeric carbon atom. 

A related technique, vapor phase osmometry, is based on the idea of isothermal distillation. Such an osmometer is shown schematically in Figure 2.4. In effect, the vapor... 

Ru(II) based terpyridine polymers were prepared by Houston et al. in 2003.21 The polymers were prepared by reacting /h.v-terpyridine monomers with Ru(DMSO)4Cl2 in hot ethanol (Fig. 3). To improve the solubility of the coordination polymers, pinene moieties are attached to the monomers. The degree of polymerization of the polymers was studied by elemental analyses, gel permeation chromatography, vapor phase osmometry, STM and electrospray ionization mass spectrometry. However, no precise result was obtained, and the size of the polymers was estimated to be between 40 and 60 repeating units. 

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