MOLECULAR ASSOCIATIONS DETECTED BY PHOTOPYROELECTRIC CALORIMETRY

D. Dadarlat, C. Neamtu, X. Filip, M. Streza

National R&D Institute for Isotopic and Molecular Technologies, POB 700, Cluj‑Napoca 5, RO-400293, Romania

In the photopyroelectric (PPE) technique, the temperature variation of a sample, exposed to a modulated radiation, is measured with a pyroelectric sensor. In the last years, the two PPE calorimetric configurations ("back" and "front" one, respectively) have been applied for investigating the behaviour of static (specific heat) and dynamic (thermal diffusivity, conductivity, effusivity) thermal parameters of condensed matter samples.
In the most general case, the complex PPE signal depends on all optical and thermal parameters of the layers of the detection cell. After some mathematical approximations, one can obtain results in which the amplitude or the phase of the signal depends on one or, in a simple way, on two of the sample's related thermal parameters. It is very convenient to measure the phase of the PPE signal, because the phase represents only a time delay of the signal compared with a reference one, and it is not dependent on the power fluctuations of the incident radiation and on the quality of the incident surface.
In the back configuration, the most convenient PPE particular case, largely used in order to find the values of the thermal diffusivity, requests thermally thick sample and sensor, and optically opaque sample. In the back configuration a modulated light impinges on the front surface of a sample and, a pyroelectric sensor, situated in good thermal contact with the sample's rear side, measures the heat developed in the sample due to the absorption of radiation. In order to find the thermal diffusivity of a given sample, the phase of the PPE signal can be measured, by performing either a frequency or a sample's thickness scan.
Concerning the front configuration, the radiation impinges on the front surface of the sensor, and the sample, in good thermal contact with its rear side, acts as a heat sink. Recently, this front configuration, with opaque sensor and thermally thick sensor and sample, was used to measure the thermal effusivity of some liquid samples. The experimental conditions (chopping frequency, number and geometrical thickness of the layers of the detection cell) were set-up in such a way that the information (the value of the thermal effusivity) is contained in the phase of the signal.  
In the paper we combine the two particular cases mentioned above and, using the information contained only in the phase of the PPE signal, we study association processes taking place in associative (ethanol, methanol, isopropanol - water) and non-associative (ethylene glycol, glycerol, acetic acid - water) liquid mixtures. It has been demonstrated that both thermal diffusvity and effusivity are suitable thermal parameters for molecular association studies, the first parameter having an increased sensitivity ( ~ 30%) to associations, compared to the second one ( ~ 8%).