Surface free energy of silica

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. 

Various methods, either chemical (4-8) or physical (9-15), can be used for the determination of these surface groups, and their number and type can be easily modified by chemical (e.g., esterification upon reaction with alcohols) or heat treatment. However, for heat treatment, as shown by Fripiat (16), the modification of the surface chemical properties is much more complex than would be expected when only considering the curves relating weight loss to temperature. Thus it should be of interest to relate the evolution of surface silanol groups to the surface free energy of silica samples. 

Figure 1. Evolution of the dispersive component of the surface free energy of silicas versus heat treatment temperature.

London Component of the Surface Free Energy of Heat-Treated Silicas. Figure 1 shows the evolution of 7sd for the different types of silicas versus heat-treatment temperature. The origin of the sample as well as the thermal treatments applied are important in determining 7sd. 

Specific Component of the Surface Free Energy of Heat-Treated Silicas. Specific interaction capacities of heat-treated silicas, that is, their ability to interact with polar molecules, were examined with chloroform (Lewis acid probe) and toluene and benzene (amphoteric molecules). Figure 2 provides examples of the evolution of the specific interaction parameter Zsp of the different silicas with chloroform as a probe. 

London Component of the Surface Free Energy of Heat-Treated Silicas. 381... 

Two methods can be used for the assessment of y and its components contact angle measurements and inverse gas chromatography (IGC) [31]. Chibowski and Perea-Carpio [32] reviewed the problems encountered when attempting to determine the surface free energy of powered solids, like silica particles, using the contact angle technique. Wu reviewed the different techniques that can be employed to measure the surface tension of polymer melts [30]. These techniques are based on the pendant and sessile drop techniques that require density data or contact angle measurements. 

FIGURE 12.3 Results of the analysis of total surface free energy of fillers (a) Silica, (b) Wollastonite, (c) Kaolin (L - total surface free energy, - dispersive part, L - polar part). 

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