Dissolution anisotropy

Crystal morphology dictates the rate of dissolution along the different crystallographic axes. This dissolution anisotropy can be found in all but cubic crystals, which are isotropic. 

As expected from the anisotropy of chemical etching of Si in alkaline solutions, the electrochemical dissolution reaction shows a strong dependence on crystal orientation. For all crystal orientations except (111) a sweep rate independent anodic steady-state current density is observed for potentials below PP. For (111) silicon electrodes the passivation peak becomes sweep rate dependent and corresponds to a constant charge of 2.4 0.5 mCcm-2 [Sm6]. OCP and PP show a slight shift to more anodic potentials for (111) silicon if compared to (100) substrates, as shown in Fig. 3.4. 

The main difference from the alkaline dissolution scheme is the hole needed to initiate step 1 of the divalent reaction. The polarizing effect on the Si backbonds is the same for Si-F and Si-OH. From this similarity a certain crystal anisotropy is expected for the divalent reaction, too. Faceting along (111) planes in HF electrolytes is observed when the current density is close to JPS and micro PS formation becomes suppressed. This is the case for the electrode surface shown in Fig. 2.4 or at the tips of macropores as shown in Fig. 9.13b. 

The remarkable anisotropy in the dissolution in the magma may have contributed to the irregularity. 

Abstract The objective of this chapter is to present some recent developments on nonaque-ous phase liquid (NAPL) pool dissolution in water saturated subsurface formations. Closed form analytical solutions for transient contaminant transport resulting from the dissolution of a single component NAPL pool in three-dimensional, homogeneous porous media are presented for various shapes of source geometries. The effect of aquifer anisotropy and heterogeneity as well as the presence of dissolved humic substances on mass transfer from a NAPL pool is discussed. Furthermore, correlations,based on numerical simulations as well as available experimental data, describing the rate of interface mass transfer from single component NAPL pools in saturated subsurface formations are presented. 

The effects of aquifer anisotropy and heterogeneity on NAPL pool dissolution and associated average mass transfer coefficient have been examined by Vogler and Chrysikopoulos [44]. A two-dimensional numerical model was developed to determine the effect of aquifer anisotropy on the average mass transfer coefficient of a 1,1,2-trichloroethane (1,1,2-TCA) DNAPL pool formed on bedrock in a statistically anisotropic confined aquifer. Statistical anisotropy in the aquifer was introduced by representing the spatially variable hydraulic conductivity as a log-normally distributed random field described by an anisotropic exponential covariance function. 

When chirality is involved, information on solid-state structures and supra-molecular properties must be obtained by solid-state circular dichroism (CDf spectroscopy, as certain characteristics may be lost upon dissolution. However extreme care is required to obtain artifact-free solid-state CD spectra. This is because CD spectra in the solid state (except for special homogeneous cases [9,10]) are inevitably accompanied by parasitic signals that originate from thd macroscopic anisotropies of a sample such as LD (linear dichroism) and LB (linear birefringence) [11-16]. We have been working in the field of solid-state chirality for the last 30 years and recently developed a novel universal chiroptical spectrophotometer, UCS J-800KCM, for the measurement of true CD and circular birefringence (CB) spectra in the solid state [17]. 

Figure 2 also demonstrates that the CTA is slowly degraded in the TFA-CH2CI2 mixture. As dissolution time increased, the viscosity peak shifted to higher concentrations. Polarized light microscopy indicated the concentration at which anisotropy occurs also increased slightly with dissolution time (Table DC) and correlates with the shift in the viscosity peaks. The concentration at which the viscosity minimum occurs remained essentially constant with time in solution as did the viscosity minimum ( 3.5 x 10 cps). 

When a molecule is partly oriented by dissolution in a liquid crystal solvent it is necessary to consider direct dipolar splittings, nuclear screening anisotropies, and possibly also very large quadrupolar splittings, in order to account for the spectrum. The ways in which this can affect multiple resonance experiments have been considered already. (33, 35-37, 46) In this section we discuss some applications. Naturally, proton decoupling is often used to simplify spectra of oriented molecules, although it is desirable to avoid complete decoupling of the solvent protons. 

In NaOH the dissolution of Si can follow a chemical and an electrochemical route (Fig. 27, top and bottom paths respectively). The chemical path represents 90% of the dissolution [122]. This route underlines the role of water molecules in the dissolution of Si at high pH in accordance with the weak dependence of the etch rate with respect to the OH concentration. Hydroxyl ions are catalysts of the reaction and do not react directly. This is consistent with the fact that Si etching in boiling water also produces atomically flat Si(lll) surfaces, as has been made evident by IR absorption spectroscopy [124] and UHV-STM observations [117]. In this case the temperature is sufficient to overcome the activation energy since no OH ion is available. The remarkable anisotropy of Si etching in strong bases stems from the fact that the chemical route represents about 90% of the dissolution and begins with a hydrolysis step in Fig. 27 [122]. The hydrolysis of Si-H bonds by water molecules... 

In summary, the two models of Figs. 27 and 28 provide new insights into the pH dependence of the surface topography of Si(l 11) in fluoride solutions (see Fig. 21). With increasing pH the uptake of the chemical reaction with water enhances the anisotropy of the dissolution since the chemical route depends critically on the atom coordination in contrast to the electrochemical one [122, 123 b]. 

Under certain conditions, cellulose derivatives possessing the characteristics of cholesteric liquid crystals present cholesteric helical structures dissolution and transition from the cholesteric to the nematic phase [98]. When shear is over, the system is relaxed over a determined time and intense, shifting to a transition state, where the energy of deformation is minimal and the orientation ordering is maintained, causing the appearance of band structures. When the external field is removed, the shear-induced anisotropy is affected by the inevitable relaxation of the macromolecular chains. Structural relaxation after removal of the external field depends on the shear history and relaxation mechanism [99,100]. Moreover, literature suggests a possible competition between the order induced by shear and thermodynamically, and also a correlation between the viscosity peak and the appearance of the anisotropic phase at low shear rates [ 101,102]. 

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