Face-Selective Additive Adsorption

If additive adsorption occurs very selectively, even one-dimensional nanostructures can be grown, as demonstrated for Ag nanowires with PVP adsorbing onto the side faces of the elongated nanoparticle. Thus the surface energy of these faces was lowered so that the Ag rods only grew at their ends [93]. However, if the polymer adsorption is not homogeneous and has defects, branching may occur as shown later in Fig. 5a. 

Fig. 19 Face-selective adsorption of ions or low molar mass additives (a), steric particle stabilization by polymers (b), and face-selective adsorption and particle stabilization by DHBC (c). Figure reproduced from [57] with permission of the Royal Society of Chemistry...

The changes in morphology seen with the additives in Figure 22b-d with respect to that of the pure crystals (Figure 22a) can be interpreted only in terms of selective adsorption of the R enantiomer at faces (Oil) and... 

Utilized selective anisotropic solvent adsorption on specific crystal faces to favour the growth of morphologically polar crystals. Some additional reports of the study of crystal modification and nonlinear optical activity include those on anhydrous and hydrated sodium / -nitrophenolate (Brahadeeswaran et al. 1999), derivatives of 2-benzylideneindan-l,3-dione (Matsushima et al. 1992), straight-chain carbamyl compounds (Francis and Tiers 1992), benzophenone derivatives (Terao et al. 1990), a 1,3-dithiole derivative (Nakatsu et al. 1990), o -[(4 -methoxyphenyl)methylene]-4-nitro-benzene-acetonitrile (Oliver et al. 1990) and so-called lambda shaped molecules (Yamamoto et al. 1992). Hall et al. (1988) followed the thermal conversion of the centrosymmetric P2 /c) form of 2,3-dichloroquinazirin to the non-centrosymmetric Pc form by monitoring the development of an SHG signal. Consistent with the earlier observation, the centrosymmetric form was obtained under equilibrium conditions, while the non-centrosymmetric one could be obtained under more kinetic conditions. 

Internal structure (unit cell) can be different in crystals that are chemically identical. This is called polymorphism. Polymorphs can vary substantially in physical and chemical properties such as bioavailability and solubility. They can be identified by analytical techniques such as X-ray diffraction, infrared, Raman spectro, and microscopic techniques. For the same internal structure, very small amounts of foreign substances will often completely change the crystal habit. The selective adsorption of dyes by different faces of a crystal or the change from an alkaline to an acidic environment will often produce pronounced changes in the crystal habit. The presence of other soluble anions and cations often has a similar influence. In the crystallization of ammonium sulfate, the reduction in soluble iron to below 50 ppm of ferric ion is sufficient to cause significant change in the habit of an ammonium sulfate costal from a long, narrow form to a relatively chunky and compact form. Additional information is available in the patent literature and Table 18-4 lists some of the better-known additives and their influences. 

If one wishes to study the pure rate of desorption directly one needs to minimize the rate of re-adsorption. This is done by minimizing the concentration CA in the output stream by increasing the rate of flow of the sweeping stream. This in turn creates analytical difficulties in detection of A in the output stream. It will also necessitate an additional control function to implement changes in flow rate during the experiment in order to keep the output concentration below a selected value in the face of a rising rate of desorption as temperature increases. 

Depending on the species of the impurity, the molecule may selectively incorporate into the crystal lattice and may block or inhibit the growth of the face. Docherty and coworkers [24,25] classify additives as disruptive or blocker-type molecules (see Section 4.4). The disruptive effect of an additive is presented in Figure 6.11. It is seen that one part of the molecule can incorporate into the crystal lattice (due to the structural similarity), whereas one part of the molecule is significantly different in structure. The adsorption of the new growth layer will be disturbed here, since incorporation of the structural different moiety of the molecules is not possible. The disruptive effect has impact on the growth rate of the face. 

In order to be effective, it is important that surface modifiers are strongly adsorbed onto an available crystal face of a particle. Nature uses such mechanisms in biomineralization, e.g. in the formation of mussel shells or in the protection of hardy plants against frost. In both cases, additives control the crystal growth of either calcium carbonate or ice through selective adsorption of tailor-made proteins on certain crystal faces . ... 

---