HEAT-CAPACITY ANOMALY IN A WIDE VICINITY OF THE CRITICAL POINT OF THE TRIETHYLAMINE-WATER PHASE TRANSITION

摘要

The behavior of the isobaric heat capacity in the triethylamine-water binary system has been investigated experimentally by the adiabatic calorimetry method. The possible physical models of the observed effects are discussed. Investigation of the behavior of liquid demixing systems in the metastable region and determination of its boundaries are needed for understanding the nature of the critical phenomena and developing a modern theory of phase transitions. The difficulty of such investigations is connected with the wide variety of concrete conditions under which the processes proceed in solutions and the diversity of critical phenomena in them. Despite the development of the experimental basis and theory, some questions connected with the structural transient states in liquid systems remain open. And the nature of the phase transitions in systems with a weak disorder or localized impurities (LI) is still a subject of intensive investigations. This is evidenced by the recent reviews, where the macroscopic effects of disorder that are always present in real systems are considered. Investigations of the influence of LIs on the universal properties of the critical behavior of substances are of practical importance and are also of interest for the development of basic science. It is in the range of temperatures close to the critical one that even a negligible quantity of impurities can strongly affect the properties of a substance compared to a perfect system. The presence of impurities leads to the appearance of microinhomogeneities responsible for the appearance of phases that are thermodynamically advantageous only in a limited temperature range, usually covering the interval between the critical temperatures of a perfect and a disordered system. In liquid systems, a weak disorder can result from the formation of compact regions with a higher concentration of liquid molecules surrounded by regions with a low concentration. Under definite conditions, e.g., at phase transitions, the LIs begin to interact with concentration fluctuations. They can lead to limited sizes of fluctuations, markedly changing the critical behavior of the system. Despite the numerous investigations of the phase transitions in soft systems, the issues connected with the interactions of LIs with concentration fluctuations or collective order parameters are still not clearly understood. In many biological systems and process liquids, membranes, colloids, and polymer solutions in which molecular complexes are formed, dissolution occurs with a temperature decrease, i.e., they have a lower critical dissolution point. Therefore, in the present paper we focus on the investigation of the influence of LIs on the behavior of the heat capacity of the binary triethylamine-water demixing liquid. As is known, in this system complexes with a hydrogen bond are formed, and segregation into two phases occurs not under cooling but under heating. This is due to the fact that the Brownian mixing intensity and the entropy effects differently depend on temperature. The domination of one effect over another determines the critical behavior of the system.