Molecular weight determination of high polymers by means of vapor pressure osmometry and the solute dependence of the constant of calibration

Data are presented to show that the calibration constant of the vapor pressure osmometer is in fact not a constant, but rather depends on the nature of the solute. It is shown that the assumption of the constancy of the calibration constant is particularly severe when comparing low molecular weight standards and polymeric materials. A model is presented which visualizes either condensation or evaporation taking place at the solution drop surface, depending on the relative magnitudes of the concentration and Thermistor self-heating. In terms of this model, the solute dependence of the calibration constant is attributed to the formation of a diffusion-controlled surface concentration of the solution drop that differs from the concentration of the drop as a whole. Experimental evidence consistent with a diffusion-controlled surface layer is given. A method based on this model is given for operating the instrument so that the solute dependence of the calibration constant disappears. When the instrument is run in this manner, M_n determinations from vapor pressure and membrane osmometry are in significantly better agreement than when the instrument is operated as recommended by the manufacturer.