The development of the physics of "simple liquids" was greatly helped by the use of computerized calculation methods, by "computer experiments." The methods of computer modeling greatly enrich the possibilities of theory, increase its yield of results, on the other hand, they create the feeling of a certain dissatisfaction on account of the deviation from the purely theoretical deductive path. One way or another, it cannot be denied that the basic properties of monatomic simple liquids have lately become ever better known and understood. The situation is much more complicated as regards the molecular physics of polyatomic liquids. Here it is not a priori obvious that it is possible to devise a theory ensuring the description of sufficiently broad classes of substances. In fact, there exists an enormous variety of compositions and structures of molecules multiplied by the existence of conformers, and there are complications due to the deformability of the interatomic bonds, the distribution of static charges, the nonadditivity of the interaction forces, not to speak of the additional complications such as the manifestation of the chemism of interaction. This abundance of complications and peculiarities naturally brings to mind the fact that the properties of polyatomic substances, too, have to be just as variegated, and that it is difficult to expect the existence of regularities that are sufficiently general. Such considerations generate pessimism whose extreme expression is the opinion that each substance has to have its own regularities and relationships that are uniquely its own. To overcome this pessimism and skepticism is possible only with facts, with purposeful analysis of the empirical data, the search for really existing regularities. At present it is becoming more or less clear that in the properties of large classes of substances, a very limited amount of this variety of characteristics of the molecules is coming to the fore, that the behavior of a substance is determined by only some, the most fundamental qualities, and that the role and importance of the other details are secondary, as a rule. Below we will reveal the content of our assertions, and then we will turn to the works published in recent years and dealing with the development of the molecular-kinetic of polyatomic liquids, and we will examine the principal trends of the theoretical investigations. A large group consists of works examining models of molecules that represent hard cores. It is no accident that these models arouse great interest. This is due chiefly to the important role of purely geometric factors in the physics of liquids and the corresponding important role ascribed to hard globular molecular models in the physics of simple monatomic liquids. Regardless of its extreme simplicity, it is a good basis for describing important aspects of the nature of liquids, chiefly their structure; the model of globular molecules is the zero approximation in the methods of the theory of perturbations [l]. The following were examined as models of polyatomic molecules: ellipsoids [2], spheroids [3-9], dumbbells [10-15] including asymmetric (heteroatomic) ones [16-19], systems of three, *The article is based on a paper of the same name read by the author at the plenary session of the Sixth All-Union Conference on the Thermophysical Properties of Substances (Minsk, November, 1978); both the review part and the original part of the paper were considerably enlarged.
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Oct 14, 2024
O I Topor + 2
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