Osmotic pressure molecular weight effect

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. ... 

A semi-permeable membrane, which is unequally permeable to different components and thus may show a potential difference across the membrane. In case (1), a diffusion potential occurs only if there is a difference in mobility between cation and anion. In case (2), we have to deal with the biologically important Donnan equilibrium e.g., a cell membrane may be permeable to small inorganic ions but impermeable to ions derived from high-molecular-weight proteins, so that across the membrane an osmotic pressure occurs in addition to a Donnan potential. The values concerned can be approximately calculated from the equations derived by Donnan35. In case (3), an intermediate situation, there is a combined effect of diffusion and the Donnan potential, so that its calculation becomes uncertain. 

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. 

How are we to understand this odd result The answer is easy when we remember that osmotic pressure counts solute particles. The macroion cannot pass through the semiperme-able membrane. In the absence of added salt, its counterions will not pass through the membrane either since the electroneutrality of the solution must be maintained. Therefore the equilibrium pressure is that associated with z + 1) particles. Failure to consider the presence of the counterions will lead to the interpretation of a low molecular weight for the colloid. As we already saw, the presence of increasing amounts of salt leads to a leveling off of the ion concentrations on the two sides of the membrane. The effect of the charge on the macroion is essentially swamped out with increasing electrolyte. 

The driving potential for UF - that is, the filtration of large molecules - is the hydraulic pressure difference. Because of the large molecular weights, and hence the low molar concentrations of solutes, the effect of osmotic pressure is usually minimal in UF this subject is discussed in Section 8.5. 

Figure 19.2. Diagram of osmotic behavior and the effect of solute concentration and molecular weight on osmotic pressure, (a) Osmotic-pressure behavior of solutions Ais the excess pressure on the solution required to stop flow of solvent through the semipermeable membrane, (b) Effects of solute concentration and molecular weight on osmotic pressure.

A molecular weight of 6.1(10)6 has been recorded for the acetate of mollusc muscle glycogen by Meyer and Jeanloz48 on the basis of osmotic pressure determinations. Fractions obtained from this glycogen by precipitation with methanol possessed values foi the molecular weight of 6.1(10), 2.1(10)6, and 3.0(10). It was suggested that these represented minimum values because of the effect of impurities of small molecular weight. 

The particle size of rapidly polymerized minidroplets does not or does just weakly depend on the amount of the hydrophobe [34-36]. It was found that doubling the amount of hydrophobe does not decrease the radius by a factor of 2 (as expected from a zero effective pressure), it is just that the effective pressure (pressure difference) has to be the same in every droplet, a mechanism which in principle does not depend on the amount of hydrophobe [19]. However, a minimum molar ratio of the hydrophobe to the monomer of about 1 250 is required in order to build up a sufficient osmotic pressure in the droplets exceeding the influence of the first formed polymer chains. This also explains the fact that a small amount of high molecular weight polymer, e.g., polystyrene, can barely act as an osmotic stabilizing agent here stabilization can only be achieved for the time of polymerization [37,38]. 

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