Surface energy, amorphous silica

On the other hand, if ysw Yew. the precipitate tends to form a structurally continuous coating on the substrate grain. The interfacial energy (Eq. 6.16) may even become negative and the activation barrier vanishes. An example reflecting this condition is the growth of amorphous silica on the surface of quartz (Wollast, 1974). 

The surface groups S with the highest free energy should be most efficient in the processes of chemical modification. In this case, one can expect the formation of a great variety of modification products (SR groups). For the compounds with the directional chemical bonds between the neighboring atoms in the main crystalline and amorphous silica modifications, these are the products of bond rupture and/or its rearrangement. 

Some years ago, Kantro, Weise, and I set out to determine the surface energy of tobermorite. We came to grief, because the practice is usually not as simple as the principle. We decided, therefore, to work on some simpler but closely related substances, and we determined the surface energies of calcium oxide, calcium hydroxide, amorphous silica, and hydrous amorphous silica. Even these determinations did not turn out to be simple. We published these in two papers in the Canadian Journal of Chemistry in 1956 (6, 7). After this work, we felt strong enough to tackle tobermorite, and we published the surface energy of tobermorite in the same journal three years later (8). 

The crystal structure of the natural mineral tobermorite was partly worked out by Megaw and Kelsey (22). Taylor and coworkers (15, 17, 26, 27) have carried out detailed investigations of tobermorite prepared synthetically, and correlated their structural features with those of the natural mineral structure. However, little was known about the nature of the surface, except what may be deduced from the surface energy results, the surface energy being approximately the geometric mean of the surface energies of calcium hydroxide and hydrous amorphous silica. 

However, equation (3) still contains two unknowns the work function of the metal and the work function of the gold deposit. A multilaboratory study has shown that neither carbon nor gold provide a suitable internal standard for catalyst supports (34). Alternatively, the binding energy scale has been referenced to a core level of the support metal cation, for example, the Si 2p peak of silica. This is no improvement. The work function of a high surface area, amorphous catalyst support has never been measured. 

The present review discusses the results of the H NMR spectroscopy for a wide range of carbonaceous materials (heat-treated and nongraphitizable activated carbons, carbon blacks, exfoliated and oxidized graphites, porous and amorphous carbonized silicas). This technique made it possible to determine the spectral characteristics of organic molecules with diverse chemical properties, as well as of water molecules adsorbed on the surface. These characteristics are compared with the structural properties of the materials under consideration. The calculations done for the majority of the subjects of inquiry gave the values of their free surface energies in an aqueous medium as well as the characteristics of bound water layers of various types. 

Figure 11 shows that greater free energy changes are required for both the solution and precipitation of quartz than for amorphous silica, based simply on the differences between their solubilities and solution rate constants. This may account in part for the frequent supersaturation of quartz solutions silica monomer can absorb to the quartz surface without actually becoming a part of the quartz structure. This effect is enhanced in solutions of decreasing ionic strength and pH where AG increases as 2 decreases. 

It has been reported (see Moloy et al. (2001) for a summary) that surface free energy terms for amorphous silica have a significant negative temperature dependence, consistent with a positive surface entropy. 

The surface energy (surface tension) of supercooled molten silica above 1000°C has been measured as 0.3 HmT. This is in the same range as values reported for amorphous silica. 

Since the enthalpy difference between quartz and glass is about 9 kJ/mol, and the free energy difference somewhat less than this, a (free) energy crossover at the nanoscale is expected. If one takes the surface enthalpy of quartz as 1 J/m larger than that of amorphous silica (a conservative estimate), the enthalpy crossover will occur at a surface area of 9000 m7mol (150 m/g). This crossover at a relatively small surface area is consistent with the observation that all nanosized silica samples, both natural and synthetic, are amorphous. 

Molodetsky, 1, Navrotsky A, Lajavardi M, Brune A (1998) The energetics of cubic zirconia from solution calorimetry of yttria- and calcia-stabilized zirconia. Z Physik Chem 207 59-65 Molodetsky 1, Navrotsky A, Paskowitz MJ, Leppert VJ, Risbud SH (2000) Energetics of X-ray-amorphous zirconia and the role of surface energy in its formation. J Non-Crystalline Solids 262 106-113 Moloy EC, Davila LP, Shackelford JF, Navrotsky A (2001) High-silica zeolites a relationship between energetics and internal surface area. Microporous Mesoporous Materials (submitted)... 

The establishment of relationships between the surface chemistry and the surface free energy of silicas is important for practical applications of these materials. Inverse gas chromatography, either at infinite dilution or finite concentration, appears to be an effective method for the detection of changes of surface properties induced by chemical or thermal treatments. Silicas of various origins (amorphous or crystalline) with surface chemistries modified by chemical (esterification) or heat treatment were compared. The consequences of these modifications on surface energetic heterogeneities were assessed. 

Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy was used in conjunction with pyridine desorption studies to assess the acidity of a siliceous surface. An amorphous, porous silica substrate was investigated. The results contribute to an understanding of the acidic strength and the distribution of acidic sites on this material. A hydrogen-bonding interaction was observed between pyridine and the surface. Isothermal rate constants and an activation energy for the desorption process are reported and can be used as direct measures of surface site acidity. 

Thermodynamically more stable crystalline phases will be favoured as the nuclei grow in size since the bulk lattice energy terms, rather than the surface energy terms, become more important in stabilising the solid phase, particularly (as in the case of amorphous CaCOs) when the amorphous phase is relatively soluble. In cases where the amorphous phase is relatively insoluble, for example, amorphous calcium phosphate, the transformation to crystalline states may be slow at ambient temperature and pressure. In other cases, for example, biogenic silica, the amorphous phase is metastable and not transformed into a crystalline phase under normal conditions due to the high activation energies required to be overcome for this transformation. ... 

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