Physics of Surface Tension

Liquid consists of molecules in the state of motion that are kept at relatively small distances from each other by Van der Waals s forces of molecular attraction. Since liquid can be treated as a continuous medium (this is the basic hypothesis of continuum), a small element of this medium can be characterized by the presence of some average molecular field. In this field, the net attractive force acting on an arbitrary molecule in a small element of continuous medium is zero on the average (forces in all directions balance each other). 

This assumption is no longer true at the interface, for example on the boundary between liquid and gas, because molecular motion in a liquid is more constrained than in a gas, where interaction between molecules is relatively weak. The interaction of molecules in a liquid makes it harder for them to escape from it through the interface boundary (evaporation). Therefore, molecules located at the surface of the liquid are acted upon by forces directed along the surface or inward (Fig. 17.1). 

As a result, the interface experiences a tensile force and behaves as membrane seeking to reduce its size. It is necessary to point out that representation of the interface as a smooth surface of the meniscus type where the concentration of molecules suffers a discontinuity is an idealization. Actually, this surface has 

Designate by Z the force acting on a unit length and aspiring to reduce the surface area. Then, introducing the free energy F [12], 

It follows from (17.2) that E 0 when F decreases together with s. The quantity E is called the coefficient of surface tension. The dimensionality of E is N/m. Table 17.1 provides values of S for some pure liquids in equilibrium with their vapor at normal temperature. 

---

One fascinating feature of the physical chemistry of surfaces is the direct influence of intermolecular forces on interfacial phenomena. The calculation of surface tension in section III-2B, for example, is based on the Lennard-Jones potential function illustrated in Fig. III-6. The wide use of this model potential is based in physical analysis of intermolecular forces that we summarize in this chapter. In this chapter, we briefly discuss the fundamental electromagnetic forces. The electrostatic forces between charged species are covered in Chapter V. 

We are naturally interested in connecting a physical constant, like surface tension, with other physical constants, and one such connection is immediately suggested by the decrease in surface tension caused by an increase in temperature. It is only natural to inquire whether there is any parallelism between this and the most obvious change produced in a liquid by increasing temperature expansion. Measurements have shown that this is indeed the case, and that there is marked parallelism between the temperature coefficient of surface tension, i.e., the decrease caused by a rise in temperature of one degree, represented by the constant a in our first equation, and the coefficient of expansion. 

Factors influencing jet breakup may include (a) flow rates, velocities and turbulence of liquid jet and co-flowing gas, (b) nozzle design features, (c) physical properties and thermodynamic states of both liquid and gas, (d) transverse gas flow,[239] (e) dynamic change of surface tension, 1151[2401 (f) swirlj241 242 (g) vaporization and gas compressibility,[243] (h) shock waves,[244] etc. 

Some of the compounds described in this chapter were studied for specific physical properties. Surface tension measurements with solutions of 9-16 in 0.01 M hydrochloric acid demonstrated that these zwitterionic X5Si-silicates are highly efficient surfactants.21 These compounds contain a polar (zwitterionic) hydrophilic moiety and a long lipophilic z-alkyl group. Increase of the n-alkyl chain length (9-15) was found to result in an increase of surface activity. The equilibrium surface tension vs concentration isotherms for 9 and 16 were analyzed quantitatively and the surface thermodynamics of these surfactants interpreted on the molecular level. Furthermore, preliminary studies demonstrated that aqueous solutions of 9-16 lead to a hydrophobizing of glass surfaces.21... 

Both in industry and research, large data are manipulated that could be systemized. Understanding the chemistry and physics of liquid surfaces is important so as to describe interfacial forces as a function of temperature and pressure. The magnitude of surface tension, y, decreases almost linearly with temperature (t) within a narrow range (Birdi, 2002, 2008 Defay et at, 1966) ... 

What is surface tension What are its units Explain the physical significance of surface tension. 

It is instructive to analyze the hydrophobic bond by a simple classical model of surface tension. The model is correct in principle but not in detail. It does, however, illustrate the basic physical principles and is useful in comparing hydrophobic binding in model systems using the transfer of solutes from water to hydrocarbons with proteins and model experiments. The transfer of a hydrocarbon solute from water to a hydrocarbon solvent consists of the following notional steps (Figure 11.4) (1) removal of the hydrocarbon to vacuum (2) creation of a cavity in the hydrocarbon solvent and (3) transfer of the solute to the... 

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.

Surface Tension There is uncertainty regarding the effect of surface tension on tray efficiency. Often, it is difficult to divorce the surface tension effects from those of other physical properties. 

Ono S., and Kondo S. Molecular theory of surface tension in liquids, in "Encyclopedia of Physics" (S. Fliigge, Ed.), Springer, Berlin (1960). 

Single screw extruder. Let us take the case of a single screw extruder section that works well when dispersing a liquid additive within a polymer matrix. The single screw extruder was already discussed in the previous section. However, the effect of surface tension, which is important in dispersive mixing, was not included in that analysis. Hence, if we also add surface tension as a relevant physical quantity, it would add one more column on the dimensional matrix. To find the additional dimensionless group associated with surface tension, as, and size of the dispersed phase, R, two new columns to the matrix in eqn. (4.32) must be added resulting in ... 

The transport phenomenon for any spray material released In the air Is foremost a function of the particle size and size distribution of the released spray. The particle density plays a minor role, the settling rate from Stokes law for example varies as the square root of the density. Further, the density differences between liquids commonly used for pesticides Is very little, varying only slightly from water at density of 1 gm/ml. Other formulation physical factors of surface tension, viscosity and viscoelasticity play significant roles In the atomization process. These are altered by the addition of petroleum and vegetable oil as solvents and carriers as well as a host of adjuvants In varying... 

The micelle formation process and structure can be described by thermodynamic functions (AG°mjc, AH°mjc, AS°mic), physical parameters (surface tension, conductivity, refractive index) or by using techniques such NMR spectroscopy, fluorescence spectroscopy, small-angle neutron scattering and positron annihilation. Experimental data show that the dependence of the aggregate nature, whether normal or reverse micelle is formed, depends on the dielectric constant of the medium (Das et al., 1992 Gon and Kumar, 1996 Kertes and Gutman, 1976 Ward and du Reau, 1993). The thermodynamic functions for micellization of some surfactants are presented in Table 1.1. 

The physical properties of surface active agents differ from those of smaller or nonamphipathic molecules in one major aspect, namely, the abrupt changes in their properties above a critical concentration. This is illustrated in Fig. 1, in which a number of physical properties (surface tension, osmotic pressure, turbidity, solubilization, magnetic resonance, conductivity, and self-diffusion) are plotted as a function of concentration. All these properties (interfacial and bulk) show an abrupt change at a particular concentration, which is consistent with the fact that above this concentration, surface active ions or molecules in solution associate to form larger units. These association units are called micelles and the concentration at which this association phenomenon occurs is known as the critical micelle concentration (cmc). 

Attempts to explain the surface phenomena of pure liquids in terms of a pull parallel to the surface are still rather frequent and two other aspects of these attempts may be noticed. Some writers suggest that the surface molecules have their force-fields so deflected as to form a kind of linked skin in the surface, the attractions of the surface molecules being directed along the surface instead of equally in all directions. Apart from the fact that such a deflexion of the force-fields would be difficult to account for unless the molecules were capable of a very special orientation in the surface, it would seem that this closely knit skin would be a hindrance, rather than a help, to the contraction of the surface for the essential occurrence when a surface contracts is that molecules are expelled from it, and a particularly strong linkage between the surface molecules would tend to keep them in the surface. This is a fair specimen of the contradictory nature of theories which are based on a desire to ascribe a physical reality to the purely mathematical device of surface tension. 

Mar. 22,1874 Semily, then Austro-Hungarian Empire -Apr. 16, 1921, Prague, Czechoslovakia) Since 1912, Professor of experimental physics at Charles University, Prague. Kucera introduced the measurement of surface tension of polarized mercury by applying the dropping mercury electrode [i] rather than the Lippmann capillary electrometer, and he inspired thereby -> Heyrovsky, J. to introduce - polarography. 

It is well known that the effect of surface tension is to minimize the area of a lie surface. From a thermodynamic standpoint, the notion of surface tension can als< applied to a solid surface, although its physical significance is more difficul explain. For our present purpose, we may adopt an analogous definition of surface tension of a clean solid adsorbent to that for a clean liquid surface. Thus,... 

With a liquid-vapor interface, Gibbs [36] has developed a thermodynamic treatment of the variation of surface tension with composition. This derivation comes from the book Physical Chemistry of Surfaces by Adamson [2, p. 340]. This derivation sets the stage for adsorption at the solid—liquid interface, which will be discussed next. 

---