Vapor-phase osmometry

Vapor-phase osmometry is based on vapor pressure lowering, which is a colliga-tive property. The method therefore gives M . Combining Raoult s law and Dalton s law we have 

When the solute is nonvolatile, as is the case with high-molecular-weight polymers, the vapor phase consists only of solvent. Therefore, Eq. (4.48) reduces to 

Here AP is the vapor pressure lowering given by difference between the vapor pressure of the solvent above the solution and the vapor pressure of the pure solvent at the same temperature. Rewriting Eq. (4.50) as 

The temperature difference AT, corresponding to the vapor pressure difference AP in Eq. (4.53), can be deduced from the Clausius-Clapeyron equation 

The molecular weight of the solute can thus be determined by measuring AT/ci and extrapolating it to C2 = 0. 

The practical range of molecular weights that can be measured by membrane osmometry is approximately 30,000 to one million. For measurements of less than 30,000 another technique known as vapor-phase osmometry described next is more suitable. 

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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. 

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... 

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. 

Molar mass versus generation low-angle laser light scattering (LALLS) chemical ionization, fast atom bombardment, laser desorption and electrospray mass spectroscopy vapor phase osmometry electrophoresis. 

Fig. 8.1.5. Calibration of styragel columns with polystyrene standards ( ) and with lignin samples (o). The molecular weights of the latter were determined by vapor phase osmometry. (Mansson 1981)...

The ratio of aliphatic protons to aromatic protons for the heavy oil was 4.01/1 and of methylene to methyl 1/1.75. For the asphaltenes, the ratio of aliphatic protons to aromatic protons was 3.49/1 and of methylene to methyl 1/1.1. The asphaltenes were observed to melt at HG C. Molecular weights obtained by vapor phase osmometry (o-xylene solvent) were 407 for the heavy oil and 638 for the asphaltenes. There appears to be little difference in the relative yield of heavy oils and asphaltenes from depolymerized coal as compared with the "as received" coals. Acknowledgements... 

Porphyrin (see Porphyrin) molecules containing peripheral pyridine groups act as comers or 180° linkers depending on the positions of the pyridyl donor atoms. Reaction of (dppp)Pd(OTf)2 with (47) yields the molecular square (14) while a larger square (49) was obtained by employing a modified porphyrin (48) in which the two pyridine units are trans to each other (Scheme 15). The results of these reactions demonstrate that the ability of the molecular components to act as comers or linkers depends on their topologies. Bis-pyridyl-porphyrin (47) reacts with cis- and trani -(PhCN)2PdCl2 to yield the molecular squares (50) and (51), respectively, with varied dimensions. These squares have been characterized by NMR spectroscopy and mass spectrometry combined with UV-visible spectroscopy (see UV- Visible Spectroscopy) and vapor-phase osmometry. 

The sizes determined in this work are the apparent molecular sizes and not necessarily the sizes of the asphaltene and maltene molecules at process conditions. Association efforts for asphaltene molecules have been observed for both vapor-phase osmometry molecular weight and viscosity measurements (14, 15). The sizes reported here were measured at 0.1 wt % in tetrahydrofuran at room temperature. Other solvent systems (chloroform, 5% methanol-chloroform, and 10% trichlorobenzene-chloroform) gave similar size distributions. Under these conditions, association effects should be minimized but may still be present. At process conditions (650-850°F and 5-30% asphaltene concentration in a maltene solvent), the asphaltene sizes may be smaller. However, for this work the apparent sizes determined can be meaningfully correlated with catalyst pore size distributions to give reasonable explanations of the observed differences in asphaltene and maltene process-abilities (vide infra). In addition, the relative size distributions of the six residua are useful in explaining the different processing severities required for the various stocks. Therefore, the apparent sizes determined here have some physical significance and will be referred to just as sizes. 

Two other techniques that are also used to measure M are not colligative properties in the strict sense. These are based on end-group analysis and on vapor phase osmometry. Both methods, which are limited to lower molecular weight polymers, are described later in this chapter. Some general details of the various procedures for measuring M directly are reviewed in this section. 

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