Osmotic Pressure

Osmosis is the movement of solvent across a semipermeable membrane toward a solution with a higher solute concentration.

As the solution dilutes and expands due to the added solvent, its hydrostatic pressure, which is the pressure exerted by a fluid at equilibrium due to gravity, increases, eventually halting osmosis.

Osmotic pressure, Π, is the pressure that needs to be exerted on a solution to prevent solvent influx.

The Π of an ideal solution is computed using the van't Hoff equation, which correlates Π with the solute's concentration.

Nonetheless, solutions of macromolecules like polymers are non-ideal due to excluded volume effects and polymer-polymer interactions. As a result, their molar masses, M, are calculated using the expanded van't Hoff equation involving the osmotic virial coefficient, B.

To simplify this equation, both sides are divided by the molar concentration of polymer J, which is the ratio of mass concentration, c_mass,J to M.

Plotting Π/c_mass,J versus c_mass,J at various concentrations of J enables the estimation of the M value from the intercept and, then, the B value from the slope.