Molecular weight determination of macromolecules by vapor pressure osmometry

Abstract

AbstractA new vapor pressure osmometry (VPO) apparatus having a quite good sensitivity was constructed on the basis of a Hitachi 115 molecular weight measurement apparatus. In order to match a pair of thermistors, a conventional Wheatstone bridge circuit was modified by introducing a matching resistor, which enabled us to detect the temperature difference of ca. 6. 10−6°C. By employing this apparatus the measurement of the temperature difference ΔTs between a solution drop and a solvent drop at steady state was carried out as a function of concentration with solutions of the three normal alkanes (C18, C28 and C36) dissolved in the normal alkanes (C6, C8 and C10), and of eight atactic polystyrenes (the number‐average molecular weights ranging from M̄n = 2030 to 4,1.104) in benzene over a wide temperature range. The ratio of the calibration parameter Ksto the corresponding constant Ke(≡(RTV0)/ΔH, R = gas constant, T0 = measuring temperature in K, V0 = molar volume of solvent, ΔH = partial molar heat of condensation), Ks/Ke, which represents the efficiency of measurement was found to be about 0,5, almost independent of the molecular weight of the solute, ranging from 200 to 20.104. The maximum number‐average molecular weight measurable by this apparatus with a precision of ±10% was estimated to be ca. 1·105. The time necessary to reach a steady state was about 9 min regardless of the molecular weight.