Interfaces, crystal/liquid structure

Because experimental study of the structure of crystal/liquid interfaces has been difficult due to the buried nature of the interface and rapid structural fluctuations in the liquid, it has been investigated by computer simulation and theory. Figure B.3 provides several views of crystal/liquid (or amorphous phase) interfaces, which must be classified as diffuse interfaces because the phases adjoining the interface are perturbed significantly over distances of several atomic layers. 

Crystal/crystal interfaces possess more degrees of freedom than vapor/crystal or liquid/crystal interfaces. They may also contain line defects in the form of interfacial dislocations, dislocation-ledges, and pure ledges. Therefore, the structures and motions of crystal/crystal interfaces are potentially more complex than those of vapor/crystal and liquid/crystal interfaces. Crystal/crystal interfaces experience many different types of pressures and move by a wide variety of atomic mechanisms, ranging from rapid glissile motion to slower thermally activated motion. An overview of crystal/crystal interface structure is given in Appendix B. 

On the other hand, a diffuse interface possesses a significantly wider core that extends over a number of atomic distances. A diffuse crystalline/amorphous phase interface is shown in Fig. B.3. Similar structures exist in crystal/liquid interfaces [5]. 

Lastly, the mass transport processes at the crystal-liquid interface play a central role in crystallization. The influence of solvent and impurities on the structure and growth rates of faces is considered in this chapter along with its effect on the incorporation of impurities. The solvent solute-impurities interactions in solution will also be shown to interact in subtle, but important, ways with the interface during the crystallization process. With appropriate thermodynamic analysis it is shown how these interactions ultimately affect crystallization as a purification process. 

The relationship between the chemical structure of the ionic additive and the structural characteristics at the crystal-liquid interface is known to be of importance in determining the relative efficacy of many of these additives (Davey 1982b). An approximately equal spacing between crystal surface cations and ionizable... 

Fundamentally, the solvent can influence crystal structure, crystal size, morphology, and purity by modifying solution properties (i.e., density, viscosity, and component diffusivities), solute solubility, as well as the structure of the crystal-liquid interface. The influence of the solvent on the first three factors are well known especially morphology (e.g., Srinivisan et al. 2000 Maruyuma et al. 2000 Wang et al. 1999 Walker 1997 Khoshkhoo et al. 1996 Blagden et al. 1998 Roberts et al. 1994 Geertman et al. 1992 Docherty et al. 1991 Shimon et al. 1990 van der Voort et al. 1990). The solvent properties affecting heat and mass transport... 

The nature of the interface the solid-water interface (or solid-liquid interface in general) in systems involving particles (e.g., minerals and ceramics) or the air-water interface or liquid-liquid interfaces in systems having bubbles or oil droplets, respectively the surface charge, its hydrophobicity, and the nature of adsorption sites at the interface (e.g., exposed metal ions at the interface providing sites for chelation at interfaces). In the case of crystalline solids, the surface crystal structure of the interface plays an important role in surfactant adsorption. 

One of the earliest methods for reducing coalescence is to use mixed surfactant films. These will increase the Gibbs elasticity and/or interfacial viscosity. Both effects reduce film fluctuations and, hence, reduce coalescence. In addition, mixed surfactant films are usually more condensed and hence diffusion of the surfactant molecules from the interface is greatly hindered. An alternative explanation for enhanced stability using surfactant mixture was introduced by Friberg and coworkers [67] who considered the formation of a three-dimensional association structure (liquid crystals) at the oil/water interface. These liquid crystalline structures prevent coalescence since one has to remove several surfactant layers before droplet-droplet contact may occur. 

As discussed above, for emulsion stabihzation in food systems lamellar liquid crystalline structures are ideal. At the interface, the liquid crystals serve as a viscous... 

The macroscopic topology of lyotropic or liquid crystal phases involving segregation is determined by the curvature of the interface a lamellar structure has zero curvature, while micellar phases or hexagonal phases exhibit interfacial curvature. An interface is defined by the segregation of different molecules or molecular subunits. Deformation of this interface may occur in a variety... 

Ferroelectric liquid crystals with very fast switching times have been prepared for evenmal use in various sensing and switching devices. Other areas of materials research include the synthesis of spin-labeled polymers for sensing composite interface properties, the structure and function of polymer-stabilized synthetic membranes, and polymers for various sensor applications. 

It has been emphasized that the validity of these kinetic models as descriptions of crystal growth has yet to be provided. The advance of a crystal-liquid interface should depend critically on the detailed structure (on a molecular scale) of the interface. In considering crystal growth it is in general important to know the number and distribution of solid-like atoms on an interface plane as well as the population and form of molecular groups in the liquid. Unfortunately such structural details are very difficult to calculate on a statistical basis because the relevant statistics relate to small cluster size and limited numbers of configurations. 

It is therefore not possible to analyze liquid crystalline structures in the same terms than those used for classical molecular crystals. These structures are to be described as ordered entanglements of two disordered liquids separated by an interface, and their element of structure is not the individual molecule but the... 

For an inhomogeneous system such as an interface, the principal structural measure is the single-particle density, p(r). In a crystal, the molecules are restricted to small regions around the lattice sites, yielding a single-particle density that is inhomogeneous and spatially periodic. In a liquid, however,... 

There have been several liquid-solid interface simulations on the LJ system. These are reviewed in some detail in Ref. 3. Of these, by far the most extensive are those of Broughton and Gilmer. These studies of the structure and thermodynamics of fee [100], [110] and [111] LJ crystal-liquid interfaces were part of a six-part series on the bulk and surface properties of the LJ system. Like most of the earlier simulations, these were done under triple-point conditions. The numbers of particles for the [111], [100] and [110] simulations were 1790, 1598 and 1674, respectively. Analysis of diffusion profiles, various layer-dependent trajectory plots, pair correlation functions, nearest-neighbor fractions and angular correlations yield a width of about three atomic diameters for all three interfaces. The density profiles indicate an interface width that is larger... 

Early theories of crystal-liquid interfaces were based on phenomenological models of interface structure that were extensions of models for liquid-vapor or crystal-vapor interfaces. At one end are models, such as that due to Jackson, that view the interface as being relatively sharp with a clear distinction between liquid and crystal particles even at the interface. At the other are theories of diffuse structureless interfaces that, while useful for liquid-vapor interfaces, are not ideal for the highly structured crystal-liquid case. 

---