Surface tension values, pure substances

It was made clear in Chapter II that the surface tension is a definite and accurately measurable property of the interface between two liquid phases. Moreover, its value is very rapidly established in pure substances of ordinary viscosity dynamic methods indicate that a normal surface tension is established within a millisecond and probably sooner [1], In this chapter it is thus appropriate to discuss the thermodynamic basis for surface tension and to develop equations for the surface tension of single- and multiple-component systems. We begin with thermodynamics and structure of single-component interfaces and expand our discussion to solutions in Sections III-4 and III-5. 

Although the change in state of the heat bath, hence the value of Q, usually is determined by measuring a change in temperature, this is a matter of convenience and custom. For a pure substance the state of a system is determined by specifying the values of two intensive variables. For a heat bath whose volume (and density) is hxed, the temperature is a convenient second variable. A measurement of the pressure, viscosity, or surface tension would determine the state of the system equally as well. This point is important to the logic of our development because a later dehnition of a temperature scale is based on heat measurements. To avoid circularity, the measurement of heat must be independent of the measurement of temperature. 

The situation is somewhat different for a solution, particularly if a surface-active component is present. The measured surface tension is strongly influenced by adsorption at the liquid-vapor surface (16). Yet adsorption does not influence the values of the Hamaker coefficients that must be used in Equations 3 and 10, which are related to those of the pure substances by the volume fraction weighted averages. Thus, for solution /... 

Inserting the values for surface tension cr and molar volume of pure substances at the above temperature in Equations (15a) and (15b). 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Over wide ranges of temperatures, parachor values for pure substances are nearly constant, but for solutions they depend on concentration of solutes. Using surface tensions and densities from 10 to 50 °C temperature range, which were determined in the Silesian University, the parachor values for aqueous solutions of citric acid can be expressed by... 

We recognize y as the force per unit length exerted by the surface. For this reason it is called the surface tension. The SI units of surface tension can be given as N (newtons per meter) or as J m (joules per square meter). Table A. 10 in Appendix A gives values of the surface tension for several pure substances in contact with a vapor phase of the same substance. 

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