Metastable equilibrium contact

Fig. 1. Capillary forces at the edge of a macroscopic drop, (metastable) equilibrium contact angle.

When determining the solubility and dissolution rate of amorphous or partially crystalline solids, the metastability of these phases with respect to the highly crystalline solid must be considered. While the low diffusivity of the molecules in the solid state can kinetically stabilize these metastable forms, contact with the solution, for example during measurements of solubility and dissolution rate, or with the vapor, if the solid has an appreciable vapor pressure, may provide a mechanism for mass transfer and crystallization. Less crystalline material dissolves or sublimes whereas more crystalline material crystallizes out. The equilibrium solubility measured will therefore approach that of the highly crystalline solid. The initial dissolution rate of the metastable form tends to reflect its higher... 

Whatever the composition of the Ni-Si alloy, the adsorption of Ni, r, at the alloy/SiC interface is positive, corresponding to YNi/XNi values of 1.5 to 2. Enrichment of the interface in Ni indicates that interactions between Ni and SiC at the interface are stronger than those between Si and SiC. The work of adhesion of pure Ni on SiC in metastable equilibrium (i.e., for a supposed non-reactive Ni/SiC system), evaluated in Appendix I, is W 1 = 3.17 J/m2. This value is reported in Figure 7.6 along with the corresponding values of work of immersion and contact angle. 

In this equation, Qm is the molar surface area, m i is a structural parameter defined in Section 1.1 (see Figure 1.3) and A is the regular solution parameter of Ni-Si alloy defined by equation (4.3). From the slope of the osL(XNi) curve for XNi— 0, the adsorption energy is found to be E i,(f ) = —8.2 kJ/mole. Thus, in equations (1.2), all the quantities are known (or can be easily estimated), except W and Wf 1 which represent respectively the work of adhesion and the work of immersion of pure liquid Ni in metastable equilibrium with SiC (i.e., for a supposed non-reactive pure Ni/SiC system). The values deduced from equation (1.2) are Wj4 = 3.17 J/m2 and W = —1.35 J/m2 for pure Ni. They are reported in Figure 7.6 along with the corresponding value of contact angle. 

A pattern of this sort does not form directly in the primary Fischer-Tropsch reaction. It does, however, develop when a primary Fischer-Tropsch mixture remains in contact with the catalyst, for a day or so at 35(MOO °C (Fig. 3, bottom), or longer times at lower temperatures (Studier et al., 1968, 1972 Galwey, 1972). Under such conditions, a metastable equilibrium is approached, with methane and aromatic hydrocarbons forming at the expense of ethane and heavier alkanes (Dayhoff et al, 1964 Eck et al, 1966). The kinetics and mechanism of such aro-matization on the catalyst surface has been discussed by Galwey (1972). Of the 61 hydrocarbons in the meteorite, 42 (underlined) are also seen in the synthetic sample, though often not in the same amount. It remains to be seen whether the match can be made more quantitative by changes in the reheating conditions. 

A and B are the imaginary melting-points of pure Pa and P. In equilibrium with the liquid solutions AC there exist the solid solutions Ad and in equilibrium with the liquid solutions BC, there exist the solid solutions Below the eutectic horizontal de lies the metastable equilibrium diagram for the solid solutions ( 2 i 3)> known as white phosphorus, in contact with liquid solutions, 12 x1. LxLi is the curve of internal equilibrium in molten phosphorus. At m, molten phosphorus deposits (if supercooling is excluded) the solid solution n (violet phosphorus). The temperature corresponding to m is about 590 (p. 62). 

Figure 3.7 Stable bulge-like (a) and metastable ridge-like (b) drop morphology on a chemical channel with sharp chemical steps. The equilibrium contact angle on the channel and the reduced fluid volume are 6 eq = 38° and V= 4.0, respectively. The channel edges, at which the contact line is pinned, are indicated by red lines, and the free three-phase contact line on the channel is indicated by the black line. The figure is provided from Ref. [116] (Fig. 4.3) by courtesy of the author.

---