Osmometry, Osmometer

Our primary objective in this section is the discussion of practical osmometry, particularly with the goal of determining the molecular weight of a polymeric solute. We shall be concerned, therefore, with the design and operation of osmometers, with the question of units, and with circumventing the problem of nonideality. The key to these points is contained in the last section, but the details deserve additional comment. 

Attempts were made to determine number average molecular weights (Mn) by osmometry (Mechrolab Model 502, high speed membrane osmometer, 1 to 10 g/1 toluene solution at 37 °C), however, in many instances irreproducible data were obtained, probably due to the diffusion of low molecular weight polymer through the membrane. This technique was abandoned in favor of gel permeation chromatography (GPC). 

Figure 3 Membrane osmometry. Top concentration series with multiple injection of polystyrene (PS, 5,250 g/mol) in toluene. Concentrations 1.07,1.91, 2.94, 4.11, and 5.03 g/L. Multiple solvent injections establish the baseline (subtracted). Inset data evaluation for concentration series of PS (5,250 g/mol) and PS (47,400 g/mol) in toluene. Bottom design of osmometer equipped with flow cell. Reproduced with permission from Lehmann et al. [16]. Copyright 1996 American Chemical Society.

Membrane osmometry measurements were carried out with the capillary osmometer shown in Figure 3. Owing to the short equilibration time of the instrument and the low cut-off molar mass of the membrane, solute permeation through the membrane, which would show up as a drift of the baseline, did not cause problems even for the lowest molar mass fraction. M was obtained from... 

In membrane osmometry the two compartments of an osmometer are separated by a semi-permeable membrane only solvent molecule can penetrate through the semi-permeable membrane which is closed except for capillary tubes. The polymer solute remains confined to one side of the osmometer and the activity of the solvent is different in the two compartments. Because of the thermodynamic drive towards equilibrium a difference in liquid level in the two capillaries results. 

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)... 

There were numerous attempts to apply other qualities proper to macromolecules in the SEC detection, such as turbidimetric titration [304], ebulliometry [305] and osmometry [306], The precise flow-through osmometers would certainly bring enormous progress to polymer HPLC. Their production has been repeatably announced. However, it seems that the technical problems—for example with the preparation of appropriate semi-permeable membranes—connected with the osmometric detection were not yet solved at an acceptable level and the osmometric detectors so far did not appear in the market. 

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. 

Vapor Pressure Osmometry. Number-average molecular weights were evaluated with a vapor pressure osmometer (Knauer) following a previously described method (18). 

Osmometry. Measurements were made in tetrahydronaphthalene at 130 °C by using a membrane osmometer (Mechrolab model 502). The plots of osmotic pressures at different concentrations are shown in Figure 5. Comparing the osmotic pressure results with those of GPC (interpretation 1) it appears that molecules larger than A = 50 may have permeated the membrane. If this is the case, the MN by osmometry is too high. 

Membrane Osmometry and Viscometry. Number-average molecular weights of PMMA were determined with a Mechrolab model 501 highspeed membrane osmometer in toluene at 60 °C except for samples 122-4 and 122-6 which were determined at 40°C. 

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... 

Brzezinski J, Glowala H, Kornas-Calka A (1973) Note on the molecular weight dependence of the calibration constant in vapor pressure osmometry Eur Polym J 9 1251-1253 Burge DE (1979) Calibration of vapor pressure osmometers for molecular weights measurements J Appl Polym Sci 24 293-299... 

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

Based on the theory of colligative properties and the principles of osmometry, it is understood that osmometer will read osmolalities and not osmolarities because colligative properties are directly proportional to the total solute concentration expressed in molality [see Eqs. (1)-(16)]. The relationship between osmolality and osmolarity and its significance can be found in the Remington s Pharmaceutical Sciences and in a review article by Deardorff. However, it is more convenient to use osmolarity because it is based on weight/volume rather than on weight/weight as in... 

Practical Aspects of Osmometry In static osmometers, the heights of liquid in capillary tubes attached to the solvent and solution compartments (Fig. 4.3) are measured. At equilibrium, the hydrostatic pressure corresponding to the difference in liquid heights is the osmotic pressure. The main disadvantage of this static procedure is the length of time required for attainment of equilibrium. 

A full description of various osmometers and the necessary experimental technique can be found in the book edited by Allen. More-recent types of osmometer have been described, but the outstanding problem in osmometry is still the preparation of suitable semi-permeable membranes. Methods of preparing membranes claimed to be suitable for materials of low molecular weight have been described, and there are several reports of comparisons of the behavior of different types of membranes in osmometryThe problem of correcting observed osmotic pressures for any solute diffusion which may occur has been considered theoretically, - and a suitable technique established. ... 

For low molecular weight samples, the preferred method is by vapor pressure osmometry (VPO) (Barth and Mays, 1991 Cooper, 1989). This technique is based on the decrease of vapor pressure of a solvent due to the presence of dissolved polymer. The different equilibrium vapor pressures cause a difference in condensation rate on two matched thermistors, contained in a chamber saturated with solvent vapor. One thermistor is coated with solvent and the other with a solution of the polymer. More solvent condenses on the solution, raising its temperature. The consequent temperature difference is measured, and by calibration, M can be determined. VPO is fast and can yield M as high as about 5 X 10 g/mol. Commercial vapor pressure osmometers are available. 

Measurements became easier with rapidly equilibrating membrane osmometers with servo pressure control. The instrament senses differences in osmotic pressure based on the mass transport through the membrane. The pressure differential between solvent and solution is then controlled by increasing the solution level to hydrostatically counterbalance the osmotic pressure 71. The success of the osmometry... 

The number average molecular weights of the oligomers, obtained by reacting DOMS-SA mixtures at different times, were calculated by means of vapor phase osmometry in chloroform at 37°C with a Knauer osmometer. Monodispersed polystyrene standards were used to generate the calibration curve. 

Vapour pressure depression and membrane osmometry are the most common methods to determine the polyer-solvent interaction parameter. The latter method will be described briefly. In a membrane osmometer a dilute polymer solution has been separated from pure solvent by means of a membrane. The membrane is penneable for solvent molecules but not for polymer molecules. Due to a chemical potential difference solvent molecules will diffuse from the diluted phase to the concentrated phase and this results in a pressure increase which is called the osmotic pressure ti (see also section VI - 2 for a more detaUed description of osmosis). The osmotic pressure is given by... 

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