Interfacial energy silica-water

In the case of contact of hydrated silica surface with air (water vapor), the structure of adsorbed water is determined not only by the adsorbent surface (i.e., phase boundary of silica/water) but also by the phase boundary of water/air. Appearance of these phase boundaries leads to reduction of the free energy of the interfacial water accompanied by lowering of its freezing temperature. The information on a structure of the adsorption complexes at a surface of oxide adsorbents can be obtained from temperature dependences of chemical shift of protons of interfacial water molecules. It is necessary to take into account that 5h is defined by strength of the hydrogen bonds between water molecule and active surface sites and depends on the amounts of hydrogen bonds per water molecule. 

FIGURE 7.8 Dependence of changes in (a) Gibbs free energy of water in yeast S. cerevisiae cells on the amounts of unfrozen water and (b) interfacial free energy of cell-water on concentration of total amount of water in cells (in air). (Adapted from/. Colloid Interface ScL, 283, Turov, VY., Gun ko, V.M., Bogatyrev, V.M. et al., Structured water in partially dehydrated yeast cells and at partially hydrophobized fumed silica surface, 329-343, 2005, Copyright 2005, with permission from Elsevier.)... 

The theory of homogeneous nucleation has apparently not yet been developed on a quantitative basis, but some relationships have been considered between degree of supersaturation, interfacial energy of silica to water, and the critical size of nuclei. 

A theory of nucleation was developed, based on an interfacial surface energy of the silica-water interface of about 45 ergs cm" in good agreement with the values obtained from solubility studies (Chapter 1). Fluoride ion at 10-100 ppm accelerated nucleation and particle growth. 

Figure 14. Characteristics of interfacial water in aqueous suspensions of A-300 sonicated (US) or treated in a ball-mill (MCA) at different concentration of silica (a) amounts of unfrozen water as a function of temperature at T < 273 K (b) relationship between the thickness of unfrozen water layer and temperature and changes in Gibbs free energy of interfacial water versus (c) pore radius, (d) pore volume, and (e) amounts of water unfrozen in these pores (f) interfacial Gibbs free energy as a function of silica concentration in suspensions differently treated.

The variation of the desorption energy with the contact angle is displayed in Fig. 3. Binks and Lumsdon investigated a toluene-water system with constant interfacial tension of 36mN/m by using silica nanoparticles of constant radius of lOnm and various wettabilities [4]. At a contact angle of 90°, a maximum in desorption... 

In order for a silica sol to lower its specific surface free energy by particle growth or aggregation to form a sol of lower surface area, it is necessary to reverse the ionization and return the adsorbed ions to the intermicellar liquid. According to Yates the free energy change will be yS, where y is the value of the specific interfacial free energy between the sihca surface and water. The latter is a function of particle size and thus of S. 

Changes in the Gibbs free energy of the interfacial water in the aqueous suspensions of modified silica are maximal at a medium concentration of MAPTMS (0.153 mmol/g) grafted onto A-175, but at the maximal Cmaptms value, these changes are lower than those of... 

FIGURE 38.13 Relationships between changes in the Gibbs free energy of the interfacial water in the frozen aqueous suspension of silicas of the first series and (a) amounts of unfrozen water (b) the volume and (c) the radius of pores filled by unfrozen water. 

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