Osmotic molecular weight determination results

The osmotic pressure can be measured accurately for colloidal solutes, and one molecular parameter of interest that is readily determined by osmometry is the number average molecular weight of the solute. Molecular weights determined by osmometry are absolute values no calibration with known standards or any assumed theoretical models is required. Even the assumption of solution ideality is not a problem, since results are extrapolated to zero solute concentration before calculations are made. 

The advantage of the osmotic-pressure difference method of determining the molecular weights of macromolecules over alternative methods, such as the freezing-point depression method, is evident from a comparison of the magnitudes of the effects to be measured. In Illustration 12.3-1 it will be shown that the addition of 0.01 g/naL of a 60 000 molecular-weight protein results in a freezing-point depression of water of only... 

The recent studies on light scattering in polymer solutions lead to principally the same results as the osmotic pressure method. Compare the chapter on molecular weight determinations, p. 146. 

These results show more clearly than Fq. (8.126)-of which they are special cases-the effect of charge and indifferent electrolyte concentration on the osmotic pressure of the solution. In terms of the determination of molecular weight of a polyelectrolyte by osmometry. ... 

V, is the molar volume of polymer or solvent, as appropriate, and the concentration is in mass per unit volume. It can be seen from Equation (2.42) that the interaction term changes with the square of the polymer concentration but more importantly for our discussion is the implications of the value of x- When x = 0.5 we are left with the van t Hoff expression which describes the osmotic pressure of an ideal polymer solution. A sol vent/temperature condition that yields this result is known as the 0-condition. For example, the 0-temperature for poly(styrene) in cyclohexane is 311.5 K. At this temperature, the poly(styrene) molecule is at its closest to a random coil configuration because its conformation is unperturbed by specific solvent effects. If x is greater than 0.5 we have a poor solvent for our polymer and the coil will collapse. At x values less than 0.5 we have the polymer in a good solvent and the conformation will be expanded in order to pack as many solvent molecules around each chain segment as possible. A 0-condition is often used when determining the molecular weight of a polymer by measurement of the concentration dependence of viscosity, for example, but solution polymers are invariably used in better than 0-conditions. 

Carbon-14 labelled co-catalysts have been used on a number of occasions to measure the numbers of active sites. Usually, however, osmotically determined number average molecular weights, were not reported. Thus it was not possible on these occasions to demonstrate unequivocally that nc< l which is a prime requirement for meaningful results. Recently, Ayrey and Mazza (80) have examined the titanium trichloride-triethyl aluminium catalysed polymerization of styrene at 60° and have found values of ne 3—10. They also obtained indirect evidence for the formation of poiyethylene-14C during the preparation of the catalyst. Similar observations have been reported previously (92). On the strength of these observations the use of labelled co-catalysts to measure active centres must be regarded as a somewhat suspect procedure. This conclusion is borne out by the recent kinetic work of Coover et al. (81). 

The number and weight average molecular weights were determined for two samples of linear polyethylenes distributed by the Macromolecular Division of IUPAC. The methods used were GPC, osmotic pressure, infrared analysis, melt viscosity and intrinsic viscosity. Data interpretations are discussed for each method. By comparing the results the average molecular weights were obtained for one sample, STN = 10,500 to 11,000 and Mw = 150,000 to 165,000 for another sample, MN = 13,600 to 18,500, and Mw = 40,000 to 48,000. 

To examine the potential of this new approach, we analyze the experimental data for the osmotic pressure of bovine serum albumin (BSA) in 0.15 mol dm-3 sodium chloride [112] and human serum albumin (HSA) solution in 0.1 molx dm-3 phosphate buffer [111]. According to a previous experimental and theoretical study [111] the two solutions differ substantially in the degree of protein association. The theoretically determined osmotic coefficient can be fitted to the experimental results to obtain the fraction of dimers in the solution. The results of our analysis are presented in Figs. 11 and 12. The protein molecular weights used in these calculations were 69,000 g/mol for BSA and 66,700 g/mol for HSA. The hard-sphere diameter of spherical proteins was assumed to be 6.0 nm. For the case of the multicomponent model, the ions of the low-molecular weight +1 — 1 electrolyte were modelled as charged hard spheres with diameter 0.4 nm. 

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