Glassy mixture

Quantum yields for photochemically induced low-temperature reactions are also sensitive to intermolecular effects (i.e., the structure of the solid). For example, the quantum yield for reaction (9.15) in glassy mixtures of methane and chlorine at 20 K varies from 0.1 to 0.001, depending on the method of sample preparation [Benderskii et al., 1983]. The lowest values are obtained for samples prepared by thermal or ultrasonic annealing prior to UV photolysis. 

The pseudo-equilibrium state of the glassy mixture is desaibed by the usual state variables (temperature, pressure and composition) plus the polymer density p2 that accounts for the departure from equilibrium frozen into the glass. 

Fig. 6 Dependencies of the reduced vapor pressures of TL (squares) and of CH (circles) on the volume fraction of PS at 40°C [41]. Closed symbols liquid mixtures, open symbols glassy mixtures...

Fig. 7 Reduced vapor pressure of TL as a function of the volume fraction of PS measured at 20°C with either HS-GC closed circles liquid solutions, open circles glassy mixtures), or at 30°C, by means of the flow-through method [32] (squares). According to [32], the crossed squares refer to compositions inside the range of glassy solidification no information is given for the open squares. Lines are guides for the eye...

By considering the solid glassy mixture as a homogeneous phase, the apparent qualitative differences between the solubility of alkanes and alcohols in PTMSP may be easily interpreted as simple quantitative differences in energetic and entropic contributions to the free energy of mixing. Indeed, the chemical potentials of the penetrant species in the mixture show similar isotherms for every case considered. The difference in the value of the chemical potential for alkanes and alcohols in PTMSP is mainly due to differences in energetics of the sorption process, and not to entropic effects. 

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