Liquidlike structures

The type of crystalline structure that is formed depends on the concentration of the particles as well as the magnitude of the Debye-Hiickel thickness. For large Debye-Hiickel thicknesses a body-centered cubic crystal is formed, whereas for smaller values a face-centered cubic crystal is preferred. An example of the latter observed experimentally in a dispersion of latex spheres is shown in Figure 13.3. Note that this crystallization phenomenon is analogous to crystallization of simple atomic fluids, as is evident from Figure 13.3a, which shows the coexistence of a crystal with a liquidlike structure. 

When the particle volume fraction / was increased to 0.015, the oscillations of the effective interaction between identical charged particles became larger than those for / = 0.005 at an electrolyte concentration of 10 5 M (Fig. 3). The effective interaction between identical charged particles versus the distance between particles is plotted in Fig. 3 for various values of Z (Z = 300, Z = 600, Z = 1200). As shown in Fig. 4, the colloidal dispersion has a disordered liquidlike structure for Z = 600, but a more ordered structure for Z = 1200. When the electrolyte concentration was increased to 10-4 M, the interaction between identical charged particles became completely screened. As shown in Fig. 5, no oscillations of the effective interaction potential were present for Z = 300,600, and 1200. 

When a liquid supercools (i.e., does not crystallize when its temperature drops below the thermodynamic melting point), the liquidlike structure is frozen due to the high viscosity of the system. The supercooled liquid is in a so-called viscoelastic state. If the crystallization can be further avoided as the ten ierature continues to drop, a glass transition will happen at a certain temperature, where the frozen liquid turns into a brittle, rigid state known as a glassy state. A well-accepted definition for glass transition is that the relaxation time t of the system is 2 X10 s or the viscosity / isio Pas (an arbitrary standard, of course). 

FIG. 19 Schematic representation of the radial pair distribution function g(r) and corresponding solution structure factor S(q) for two cases discussed in the text liquidlike structure (left) and correlation hole effect (right). See text for more details. 

An approach that involves no such perturbations is the use of NMR, from which one may gather information about the conformation of the chains and chain motion. It has been suggested that, on average, the chains tend to be somewhat more extended in the micelle than in pure liquid the measurements of motion also give indication of a liquidlike structure. 

The development of the physical chemistry of rubber was greatly aided by the clear definition of an "ideal" state for this material. An ideal rubber is an amorphous, isotropic solid. The liquidlike structure of rubber was discovered very soon after the technique of X-ray scattering was developed. An isotropic material is characterized by physical properties that do not depend on the orientation of the sample. The deformation of an isotropic solid can be characterized by only two unique moduli the modulus of compression, K, and the shear modulus, G. A solid is characterized by equilibrium dimensions that are functions of temperature, pressure, and the externally imposed constraints. It is convenient to define a shape vector, L, whose components are the length, width, and height of a rectangular parallelepiped. For a system with no external constraints, the shape vector can be expressed as ... 

The behavior with temperature of C14Zn is totally different [127]. The main phase transition at 100 °C is accompanied by an abrupt generation of conformational defects at 99 °C. Above melting, chains have reached almost a liquidlike structure. Correlations between chains do not exist. However, from the temperature dependence of the factor group splitting of the CH2 bending and... 

HjNCHjCOOH, CH3CH(NHj)COOH) with liquidlike structure) ... 

The size and the structure of micelles formed by fluorinated surfactants are discussed in Chapter 7. The interior of typical micelles appears to have a liquidlike structure. Therefore, micellar solutions have solvent characteristics not exhibited by molecular solutions. Aqueous micellar solutions can dissolve water-insoluble substances by incorporating their molecules in or on the micelle (see Section 6.6). 

---