Surface energy temperature variation

Fig. III-2. Variation of surface tension and total surface energy of CCU with temperature. (Data from Ref. 2.)...

The surface energy of silicones, the liquid nature of the silicone polymers, the mechanical properties of the filled networks, the relative insensitivity to temperature variations from well below zero to very high, and the inherent or added reactivity towards specific substrates, are among the properties that have contributed to the success of silicone materials as adhesives, sealants, coatings, encapsulants, etc. 

In studying interfacial electrochemical behavior, especially in aqueous electrolytes, a variation of the temperature is not a common means of experimentation. When a temperature dependence is investigated, the temperature range is usually limited to 0-80°C. This corresponds to a temperature variation on the absolute temperature scale of less than 30%, a value that compares poorly with other areas of interfacial studies such as surface science where the temperature can easily be changed by several hundred K. This "deficiency" in electrochemical studies is commonly believed to be compensated by the unique ability of electrochemistry to vary the electrode potential and thus, in case of a charge transfer controlled reaction, to vary the energy barrier at the interface. There exist, however, a number of examples where this situation is obviously not so. 

All natural processes are found to be dependent on the temperature and pressure effects on any system under consideration. For example, oil reservoirs are generally found under high temperature (ca. 100°C) and pressure (over 200 atm). Actually, humans are aware of the great variations in both temperature (sun) and pressure (earthquakes) with which natural phenomena surround the earth. Even the surface of the earth itself comprises temperature variation of -50°C to +50°C. On the other hand, the center mantle of the earth increases in temperature and pressure as one goes from the surface to the center of the earth (about 5000 km). Surface tension is related to the internal forces in the liquid (surface), and one must thus expect it to bear a relationship to internal energy. Further, it is found that surface tension always decreases with increasing temperature. 

At the critical temperature, Tc, and critical pressure, Pc, a liquid and its vapor are identical, and the surface tension, y, and total surface energy, as in the case of the energy of vaporization, must be zero (Birdi, 1997). At temperatures below the boiling point, which is 2/3 Tc, the total surface energy and the energy of evaporation are nearly constant. The variation in surface tension, y, with temperature is given in Figure A.l for different liquids. 

Figure 12.3 Variation of surface tension 7 and total surface energy us with temperature for CCI4. Both 7 and us become zero at the critical temperature. Reprinted with permission from A. K. Adamson and A. P. Gast, Physical Chemistry of Surfaces. by John Wiley and Sons, Inc., 1977.

The London dispersive component of the surface free energy, y, of a solid may be shown to be a predominant property for the prediction of behavior of nonpolar adsorbents such as polyolefins or of practically nonpolar adsorbents like some carbon materials (natural graphites and carbon fibers). In this section, we propose a simple approach for the determination of the ys using nonpolar probes such as n-alkanes in inverse GC at infinite dilution. We also discuss the evaluation of the London dispersive component of the surface energy (or enthalpy, / ), starting from the variation of the adsorption characteristics, of a series of long-chain n-alkanes molecules, with temperature. 

---