Bubble-Point Measurements and Modeling of Binary Mixtures of Linear Siloxanes

The bubble-point pressures of three binary mixtures of linear siloxanes have been measured. The binary mixtures consist of hexamethyldisiloxane (MM) which is mixed with either octamethyltrisiloxane (MDM), decamethyltetrasiloxane (MD₂M), and dodecamethylpentasiloxane (MD₃M). For each mixture, three compositions were measured in which MM was present in approximately 25 mol %, 50 mol %, and 75 mol %. The bubble-point pressures were measured over a temperature range of 270 to 380 K for all mixtures. Large uncertainties are observed for the lower temperatures (below 320 K) due to noncondensable impurities. A detailed analysis is performed to determine the effect of noncondensable gases on the measured bubble-point pressure data. The newly obtained bubble-point pressure data is used to determine new binary interaction parameters for the multicomponent Helmholtz energy model. The data used for the fitting of the binary interaction parameters are weighted by the relative uncertainty; this ensures that data points with high uncertainty contribute less to the final binary interaction parameter. In this work, a description of the experimental apparatus and measurement procedure is given, as well as the measured bubble-point pressure data and newly obtained binary interaction parameters.