Abstract
General expression for the change with time of temperature difference between a solution drop and a solvent drop in a saturated vapour phase of pure solvent was derived by taking into consideration the condensation of solvent vapour to the solution drop and the heat transmission from the solution drop to the vapour phase and to the thermistor bead on which the drop was added. 01) When the heat transfer except condensation can completely be ignored a thermodynamic equilibrium is realized and the temperature difference between two drop (ΔT) e is given by the well-known equation:
where, Ke= (RT02V0) /ΔH, c, concentration, A2, v, second virial coefficient, R, gas constant, T0, temperature of the solvent (=temperature of the vapour phase), V0, molar volume of solvent, ΔH, heat of condensation.
2) When the heat transfer except condensation can not be ignored, only a steady state is attainable. The temperature difference in such a state (ΔT) s is given by an equation similar to that described above, but in this case, Ks instead of Ke should be used, where, Ks=V0/[{(A1k1+A2k2) / (A1k3ΔHP0 (T0))} +ΔH/RT02], A1, surface area of drop, A2, area of the thermistor bead in contact with the solution drop, k1 and k2, surface heat transfer coefficients corresponding to vapour-solution and solution-thermistor, k3, mass transfer coefficient, P0 (T0), vapour pressure of pure solvent at T0.
Effects of vapour pressure of solvent, total pressure and size of drop on the efficiency Ks/Ke were discussed theoretically and experimentally. The time required for reaching a steady state was expressed as a function of P0 (T0) and compared with experimental data, Hitachi molecular weight measurement apparatus type 115 and Mechrolab 302 (reformed in part) were utilized.
The gradual decrease in (ΔT) s with time can be explained in terms of the change in the concentration due to the condensation of solvent vapour.
