Influence of diphenylsiloxane composition on the selectivity of poly(dimethyldiphenylsiloxane) stationary phases for open-tubular column gas chromatography

The solvation parameter model is used to determine the system constants for four poly(dimethyldiphenylsiloxane) and a poly(cyanopropylmethylsiloxane) stationary phase at five equally spaced temperatures in the range 60–140°C. Together with literature data for other poly(dimethyldiphenylsiloxane) stationary phases it is shown that the introduction of from 0 to 65% diphenylsiloxane groups into a poly(dimethylsiloxane) polymer influences selectivity through an increase in dipolarity/polarizability and hydrogen-bond basicity. Changes in cohesion with variation in composition are small and electron lone pair interactions are weak and only significant at the higher end of the temperature range studied. The changes in system constants with composition are pseudo-linear for incorporation of less than 50% diphenylsiloxane groups. Higher incorporation of diphenylsiloxane groups as multimers results in a different contribution to selectivity. The influence of temperature is to decrease the system constants with increasing temperature in an approximately linear manner. The poly(cyanopropyl-methylsiloxane) stationary phase is significantly more cohesive, dipolar/polarizable and hydrogen-bond basic than the poly(dimethyldiphenylsiloxane) stationary phases. A notable feature of this stationary phase is the shallow change in the system constant for dipole-type interactions with temperature compared to the other system constants. A comparison of the properties of the poly(cyanopropylmethylsiloxane) stationary phase with a poly(cyanopropylphenyldimethylsiloxane) stationary phase is made to provide a qualitative picture of the influence of the phenyl group on the selectivity of cyanopropyl-containing stationary phases.