Crystalline state surface tension

Molecular Theory of Surface Tension (Harasima) Molecules, Barriers to Internal Rotation in (Wilson) Molecules, Convex, in Gaseous and Crystalline States (Kihara). ... 

Contrary to a fluid in the gaseous state, a liquid has a surface, and is characterized by a surface tension. For water, the surface tension is 72 mN m" at 25°C [1,2]. Again, contrary to the fluid in the gaseous state, the volume of a liquid does not change appreciably under pressure it has a low compressibility and shares this property with matter in the solid (crystalline, glassy, or amorphous) state. For water, the compressibility is 0.452 (GPa)" at 25°C [1,2]. These are macroscopic, or bulk, properties that single out the liquid state from other states of aggregation of matter. 

Due to the fact that the extrapolation of surface tensions of melts to room temperature leads to reliable values for the solid polymer, the surface tension of solid polymers may be calculated from the parachor per structural unit by applying Eq. (8.5). The molar volume of the amorphous state has to be used, since semi-crystalline polymers usually have amorphous surfaces when prepared by cooling from the melt. We have found that the original group contributions given by Sugden show the best correspondence with experimental values for polymers. 

The region of jump-like changes in the dAsurface tension (see Fig. 3.77). This jump [364,365] is explained with a phase transition in the adsorption layer. Other authors have also noticed the flexion in Ao(C) isotherm and have considered it to be a transition from liquid-crystalline to gel state of the adsorption layer, e.g. in solutions of dodecylamine hydrochloride [374]. This transition can be found experimentally also from AV(C) dependence. As it is seen from Fig. 3.77 the minimum of AV coincides with the flexion point of Ao(lgC) isotherm. 

Reactivity in the solid-state is always connected with specific motions which allow the necessary contact between the reacting groups. In most cases solid-state reactions proceed by diffusion of reactions to centers of reactivity or by nucleation of the product phase at certain centers of disorder. This leads to the total destruction of the parent lattice. If the product is able to crystallize it is highly probable that nucleation of the crystalline product phase at the surface of the parent lattice will lead to oriented growth under the influence of surface tension. In such topotactic reactions certain crystallographic directions of parent and daughter phases will coincide. Typical examples for this behaviour are the solid-state polymerizations of oxacyclic compounds such as trioxane, tetroxane or 3-propiolactone... 

Of the three states of matter, the liquid is the least understood at the molecular level. Because of the randomness of the particles in a gas, any region of the sample is virtually identical to any other. As you ll see in Section 12.6, different regions of a crystalline solid are identical because of the orderliness of the particles. Liquids, however, have a combination of these attributes that changes continually a region that is orderly one moment becomes random the next, and vice versa. Despite this complexity at the molecular level, the macroscopic properties of liquids are well understood. In this section, we discuss three liquid properties— surface tension, capillarity, and viscosity. 

Solid materials have a cohesive structure, which depends on the interaction between the primary particles. The cohesive structure leads indispensably to a void space, which is not occupied by the composite particles such as atoms, ions, and line particles. Consequently, the state and population of such voids strongly depends on the inter-particle forces. The interparticle forces are different from one system to another chemical bonding, van der Waals force, electrostatic force, magnetic force, surface tension of adsorbed films on the primary particles, and so on. Even the single crystalline solid, which is composed of atoms or ions has intrinsic voids and defects. Therefore, pores in solids are classified into intra-particle pores and inter-particle pores (Table 3.6) [80]. 

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