PVT measurements of 1,1,1,2-tetrafluoroethane (C2H2F4, HFC-134a) and its bl
end with octofluoropropant (C3F8, FC-218) have been performed in the gas ph
ase near the dew curve. The experimental data were obtained by variable-vol
ume and vibrating tube methods. Discrepancies in the behavior of isotherms
from their classical behavior were experimentally observed. It was Found th
at the phase transition does not go to completion at a single point of the
thermodynamic surface but extends over a limited range of conditions. Obtai
ned results are in accordance with a concept of adsorption of the vapor sam
ple on the surface of the experimental cell. An increase in adsorption unde
r the conditions close to condensation is caused by capillary condensation
of the sample at the walls of the cell that initiates an early phase transi
tion. The ranges of diffuse phase transitions were determined for 1,1,1,2-t
etrafluoroethane as well as for its mixture with octofluoropropane at diffe
rent thermodynamic parameters. The influence of selective adsorption on the
change in the conditions of phase transition of the 1,1,1,2-tetrafluoroeth
ane/octofluoropropane mixture was also experimentally studied.