Surface-controlled kinetics, crystal

Sodium bicarbonate Sodium bicarbonate (NaHCOs) is an odorless, white crystalline powder with a saline, slightly alkaline taste. A variety of particle-size grades of powders and granules are available. The carbon dioxide yield is approximately 52% by weight. At RH below approximately 80% (at room temperature), the moisture content is less than 1%. Above 85% RH, it rapidly absorbs an excessive amount of water and may start to decompose. Its solubility in water is 1 part in 11 parts at 20°C, and it is practically insoluble in 95% ethanol at 20°C. When heated to 250-300°C, NaHCOs decomposes and is converted into anhydrous sodium carbonate. However, thisprocess is both time-and temperature-dependent, commencing at about 50°C. The reaction proceeds via surface-controlled kinetics, and when NaHCOs crystals are heated for a short period of time, very fine needle-shaped crystals of anhydrous sodium carbonate appear on the surface. ... 

Surface Spiral Step Control. Many crystals grow faster at small supersaturation than allowed by Equation 7. This lead Frank (17) to suggest that steps may also originate from the presence of a screw dislocation, and that this kind of steps is not destroyed by spreading to the crystal edge, but continues infinitely. The rate law according to this theory is parabolic (7). We shall use the following version of the kinetic equation (10)... 

With surface reaction-controlled kinetics, ion detachment is slow and ion accumulation at the crystal surface cannot keep up with advection and diffusion. In this type of phenomenon, the concentration level next to the crystal surface is tantamount to the surrounding solution concentration. Increased flow rate and stirring have no effect on the rate of surface reaction-controlled rate processes (Berner, 1978, 1983). 

One of the most controversial topics in the recent literature, with regard to partition coefficients in carbonates, has been the effect of precipitation rates on values of the partition coefficients. The fact that partition coefficients can be substantially influenced by crystal growth rates has been well established for years in the chemical literature, and interesting models have been produced to explain experimental observations (e.g., for a simple summary see Ohara and Reid, 1973). The two basic modes of control postulated involve mass transport properties and surface reaction kinetics. Without getting into detailed theory, it is perhaps sufficient to point out that kinetic influences can cause both increases and decreases in partition coefficients. At high rates of precipitation, there is even a chance for the physical process of occlusion of adsorbates to occur. In summary, there is no reason to expect that partition coefficients in calcite should not be precipitation rate dependent. Two major questions are (1) how sensitive to reaction rate are the partition coefficients of interest and (2) will this variation of partition coefficients with rate be of significance to important natural processes Unless the first question is acceptably answered, it will obviously be difficult to deal with the second question. 

If the growth kinetics are controlled by the transport of ions from the bulk solution to the surface of the crystal, it is essential to perform experiments in well-defined hydrodynamic conditions. This is possible using the rotating... 

The hydrothermal treatment of DAY-S zeolites and their decomposition kinetics is studied in more detail. At variance to the modelling of the acid mechanism of NaCaA decomposition [5] in the case of high-silica zeolites the following model assumes that the rate of decomposition is mainly controlled by the size of the shrinking surface of the crystals while for high-alumina zeolites this mechanism fails to predict the experimental data. 

The practical use of these calculations is limited, however, because the kinetics of a reaction can play an important role. This becomes quite obvious for layer compounds such as M0S2. The kinetics may be controlled by adsorption, surface chemistry, surface structure and crystal orientation. According to Fig. 8.15, pEdecomp is close to the conduction band, i.e. M0S2 is rather easily oxidized. In the case of a flat basal surface, it has been observed with several transition metal chalcogenides that the photocurrent onset at n-electrodes occurs with high overvoltages accompanied by a shift of Gfb.(see Section 5.3). Since this is caused by an accumulation of holes at the surface the hole transfer is kinetically inhibited. 

The results of this investigation show that CaCC>3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO2 hydrolysis reaction. 

While the mechanism of BaS04 crystal growth from simple systems is well understood, the influence of growth inhibitors on the growth and deposition process is not. Kinetic studies have shown that BaS04 crystal growth is surface controlled. 

The practical use of these calculations is limited, however, because the kinetics of a reaction can play an important role. This becomes quite obvious for layer compounds such as MoSj. The kinetics may be controlled by adsorption, surface chemistry, surface structure, and crystal orientation. According to Figure 8.15,... 

---