Solid - liquid phase transition under high pressure atmosphere

The melting point of most compounds increases with mechanical or hydrostatic pressure, but some high-pressure fluids can dissolve into the solid sample, causing a decresase in melting temperature. This phenomenon is exploited in a number of applications, such as in polymer industry for easier machining and in the development of controlled size particle producing, such as PGSS (Particles from Gas Saturated Solutions). The design and optimisation of these and similar high-pressure operations can be supported by the knowledge of the pessure and temperature dependence of thermodynamic parameters.

Determining the melting temperature in a high-pressure medium is usually challenging. The most commonly used measurement methods are visual observation-based studies, but high-pressure differential scanning calorimetry (DSC) and transitiometry provide the most information, but their equipment is rare and expensive. In our laboratory we have two devices available to study solid - liquid phase transition.

One is a high-pressure view cell, which has the advantage of being widely applicable. Visual observation is possible by monitoring the processes through a sapphire window. In our research group a method was developed to make measurements possible with multicomponent high-pressure media, i.e. in the presence of co-solvents. In this case, the sample is introduced into the system already under pressure. A specia feature of the apparatus is the possibility to controll the pressure and the temperature independently, wich allows the observation of the melting process under both isobaric and isothermal conditions, The pictures show the steps of a melting point measurement.

The other one is a new device, which was developed in the research group. A schematic picture of the device is shown on the right.

The constant volume device works by measururing the differential pressure as a function of the temperature. The system is capable of detecting the pressure difference between a sampleholder (3) and a reference cell (2) under a continuous gradual heating program by a differential pressure transmitter (1). The solid - iquid transition during heating causes a change in the pressure of the medium above the sampledue to the combined effect of a drop caused by the fluis dissolved in the sample and a pressure change (generally an incrase in pressure) induced by a change in the molar volume of the sample. The time - temperature - pressure difference data detected during the measurement are recorded by a data logger (7) and evaluated (8) by an algorithm developed by the reseach group. The developed method provides a new, easy to automat and precise way to determine the melting temperature in a high pressure medium, allowing a cost and time efficient measurement.

If you find the topic interesting, please contact Dr. Márton Kőrösi for further information or for possible thesis topics.

Research Group of Extraction and High-Pressure Unit Operations

Solid - liquid phase transition under high pressure atmosphere

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