Transition temperature-pressure measurement methods

Cryoscopy. Souchay (40) has summarized the application of fused salt cryoscopy to ionic solutes. Obviously two limitations are inherent in this method. Under ordinary pressures, measurements are possible at only one temperature—namely that of the transition point (e.g., ca. 32.38°C. in the case of Na2S04 10 H20). Secondly, the solute is being examined in solutions of high ionic strength only. Isopiestic vapor pressure measurements have been used as a variation, which, in principle, eliminate both limitations. However, it does not appear that it is as yet possible to analyze such data to yield equilibrium constants (33). Furthermore, Tobias has cast doubt upon the inherent accuracy of the method when the polyions contain more than 3 or 4 metal ions (41). 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

Just as one may wish to specify the temperature in a molecular dynamics simulation, so it may be desired to maintain the system at a constant pressure. This enables the behaviour of the system to be explored as a function of the pressure, enabling one to study phenomena such as the onset of pressure-induced phase transitions. Many experimental measurements are made under conditions of constant temperature and pressure, and so simulations in the isothermal-isobaric ensemble are most directly relevant to experimental data. Certain structural rearrangements may be achieved more easily in an isobaric simulation than in a simulation at constant volume. Constant pressure conditions may also be important when the number of particles in the system changes (as in some of the test particle methods for calculating free energies and chemical potentials see Section 8.9). 

Polymeric materials can absorb considerable amounts of gas, for example CO2 especially at elevated pressures (p) and temperatures (T) above the so-called glass transition temperature [5.19]. This often causes changes in size and volume of the polymer, which have to be taken into account in industrial processing situations, for example in gas separation processes using polymeric sorbent materials [5.20]. Sorption phenomena of swelling polymers cannot be measured adequately by either gravimetric or volumetric methods. 

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