States of Aggregation, Thermodynamic Phases, Phase Transformations, and the Vitreous State

Abstract

From a molecular-kinetic point of view all substances can exist in three different states, as gases, liquids and solids. These three states of aggregation of matter (from the Latin word: aggrego – to unite, to aggregate) are distinguished qualitatively with respect to the degree of interaction of the smallest units of the corresponding substances (atoms, molecules) and, consequently, with respect to the structure and mobility of the system. Gases are characterized, in general, by a relatively low spatial density of the molecules and a relatively independent motion of the particles over distances significantly exceeding their size. The average time intervals, f , of free motion in gases are considerably larger than the times of strong interaction (collisions, bound states) of two or more atoms or molecules. In a first approximation the free volume in a gas is equal to the volume occupied by the system. The molecules can be treated in a such an approximation as mathematical points (perfect gases); however, in more sophisticated models the volume, shape and the interaction of the molecules have to be accounted for. Gases are compressible; with a decreasing volume of the gas the pressure increases as expressed, e.g., for a perfect gas by Boyle-Mariotte's law. Liquids have a significantly higher density than gases and a considerably reduced free volume. Thus, an independent translation of the building units of the liquid is impossible. The molecular motion in liquids and melts gets a cooperative character and the interaction between the particles determines to a large extent the properties of the system. Moreover, the compressibility is much smaller than for gases, simple liquids are practically incompressible. According to an approximation due to Frenkel (1946) [233] liquids can be described in the following way: the motion of the building units in a liquid can be considered as an oscillation around temporary average positions. The temporary centers of oscillations are changed after an average stay time, R . The mean distance between two subsequently occupied centers of oscillation is comparable with the sizes of the molecules. Every displacement of the building units of the liquid requires thus a more or less distinct