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Diffusion, membrane selectivity, and nonequilibrium effects in rapid membrane osmometry
Author(s) -
Ghosh Kalyan K.,
Swenson Harold A.
Publication year - 1968
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1968.070120704
Subject(s) - membrane , osmometer , diffusion , chemistry , selectivity , polymer , osmotic pressure , analytical chemistry (journal) , solvent , molecular diffusion , semipermeable membrane , non equilibrium thermodynamics , membrane structure , chromatography , thermodynamics , organic chemistry , metric (unit) , biochemistry , economics , catalysis , physics , operations management
Rapid membrane osmometry, narrow distribution polystyrenes in the range 4000–700,000, and three membrane types of different porosity were used to study the effects of diffusion and of Staverman membrane selection in osmometry. It was found that equilibrium was not obtained in the rapid measurement with the two densest membranes which led to grossly low values for the high molecular weight sample but had small effect at low molecular weights. Because of membrane selectivity effects at lower, and of nonequilibrium effects at higher molecular weights, the effective range was about 4000–350,000 for the densest membrane, 90,000–500,000 for the medium porosity membrane and from 145,000 to an unrestricted upper limit for the grossest membrane. A unique characteristic of the Mechrolab 501 instrument used in the study is the small constant volume on the solvent side of the membrane which allows determination of the osmotic pressure of the diffused material and its concentration when the molecular weight is known. Diffusion can be avoided if the pressure reading is taken quickly enough but membrane selectivity effects cannot be avoided if diffusible solute is present. Enormous selectivity effects were found when mixtures of diffusible and nondiffusible molecules were measured. Experimental values of the selectivity coefficient S n of the mixtures were found to agree closely with those calculated from the Staverman equations. Estimates of the coil size of the polymers are given to provide an estimate of the effective porosities of the membranes.