Osmotic pressure molecular weight measurements using

Only in the case where the concentration of electrolyte is very large compared with the equivalent concentration of the colloidal ion does the osmotic pressure remain a measure for the molar concentrations of the colloid and can then be used to estimate the molecular weight. 

Thus we have finally established how light scattering can be used to measure the molecular weight of a solute. The concentration dependence of r enters Eq. (10.54) through an expression for osmotic pressure, and this surprising connection deserves some additional comments ... 

Molecular weights of polysaccharides in solution can also be measured by osmotic pressure and light scattering. Osmotic pressure yields the number average molecular weight, which can be usefully used with Mw from sedimentation equilibrium as a measure of polydispersity Preston and Wik [28] have done this for example with hyaluronic acid. The ratio Mw/Mn the polydispersity index is often given as a measure of polydispersity, and can be related to the width of a molecular weight distribution via the well-known Herdan [96] relation ... 

It was Brown and Morris (15) in 1888 who employed Raoult s method. They reported a value of 30,000 for the molecular weight of amylodextrin, a degradation product of the hydrolysis of starch. Subsequently Lintner and Dull (16) also using cryoscopy reexamined amylodextrin, and reported the molecular weight as 17,500. In a third paper, Rodewald and Kattein (17) in 1900, measured the molecular weight of starch by osmotic pressure experiments carried out on aqueous solutions of starch iodide. They obtained somewhat higher molecular weights, 36,700 and 39,700. 

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. 

Why are isotonic drinks useful Osmotic pressure in living systems is incredibly important yet how often is the topic dismissed or merely discussed as a means to measure molecular weight of polymers Why not consider polymers as biological macromolecules and add to the discussion that a balance of osmotic pressure keeps our cells from bursting - which goes back to why the isotonic sport drinks are useful Relevance in the examples used in our courses is possible. 

How does one use osmotic pressure measurements to determine the molecular weight of a solute How does polydispersity in molecular weight affect such a measurment ... 

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