Surface tension molecular interpretation

Capacitance and surface tension measurements have provided a wealth of data about the adsorption of ions and molecules at electrified liquid-liquid interfaces. In order to reach an understanding on the molecular level, suitable microscopic models have had to be considered. Interpretation of the capacitance measurements has been often complicated by various instrumental artifacts. Nevertheless, the results of both experimental approaches represent the reference basis for the application of other techniques of surface analysis. 

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... 

Throughout this chapter we have dealt with surface tension from a phenomenological point of view almost exclusively. From fundamental perspective, however, descriptions from a molecular perspective are often more illuminating than descriptions of phenomena alone. In condensed phases, in which interactions involve many molecules, rigorous derivations based on the cumulative behavior of individual molecules are extremely difficult. We shall not attempt to review any of the efforts directed along these lines for surface tension. Instead, we consider the various types of intermolecular forces that exist and interpret 7 for any interface as the summation of contributions arising from the various types of interactions that operate in the materials forming the interface. 

The proper choice of a solvent for a particular application depends on several factors, among which its physical properties are of prime importance. The solvent should first of all be liquid under the temperature and pressure conditions at which it is employed. Its thermodynamic properties, such as the density and vapour pressure, and their temperature and pressure coefficients, as well as the heat capacity and surface tension, and transport properties, such as viscosity, diffusion coefficient, and thermal conductivity also need to be considered. Electrical, optical and magnetic properties, such as the dipole moment, dielectric constant, refractive index, magnetic susceptibility, and electrical conductance are relevant too. Furthermore, molecular characteristics, such as the size, surface area and volume, as well as orientational relaxation times have appreciable bearing on the applicability of a solvent or on the interpretation of solvent effects. These properties are discussed and presented in this Chapter. 

Interfacial and surface tensions are the most important chciracteristics of fluid-fluid interfaces and hardly any paper exists in which such tensions do not play a central role. In fact the entire present volume of FICS will be devoted to them. In chapter 2 a molecular Interpretation will be given. Chapters 3 and 4 deal extensively with liquid-fluid interfaces containing spread and adsorbed molecules, respectively and chapter 5 will treat three-phase contacts. For all these applications, measurement is a first and necessary element. Langmuir troughs, to be described in sec. 3.3.1, also involve a kind of interfacial tension determination since... 

As the structure of the surface, and hence U° and S , are unique for each liquid and completely determined by the nature of the molecules and their interactions, it follows that this also applies to ycmd dy/dT. Therefore, it makes sense to search for molecular interpretations of both of these quantities. However, direct relationships between the surface tension and bulk properties, such as energy densities or Hamaker constants, are basically incomplete unless they take the interfacial rearrangements into account. At best one can say that bulk properties and the surface tension are different manifestations of the same interaction. From the data in sec. 1.12 and app. 1, it is concluded that neglecting the TS° term cam lead to errors of several tens of a percent. 

We shall not pursue this approach here because it does not help us much in finding a molecular interpretation, certainly not for U°. Even in the absence of waves (solidified liquids), U° is substantial. Rather, this interpretation deals with a contribution to y than with y itself. However, we recall that the capillary wave connection had already occurred in the technique for measuring surface tensions from surface light scattering, see Mandelstam s equation [1.10.1], from which an explicit formula for y may be derived. 

This group of model interpretations refers typically to interfacial tensions, say between condensed phases a and p, and their relations to the individual surface tensions y and y. Intuitively it is felt that such a relation should exist, since, at a given temp>erature and pressure y and y are unique functions of the composition of phases a and p, respectivety, and so is y fully determined by the Interface that is spontaneously formed upon contact between phases a and p. As, however, the interpretation of y in terms of molecular properties of phase a is not so simple (as proven by the preceding part of this chapter), the relation y (y ,y ) is not as obvious either. Nevertheless, a number of semi-empirical relationships have been put forward, and applied with some success. Many of these contain the geometric mean (y y ) or. where y is the contribution to y of dispersion forces. 

Finally, the scale distinction is also recognized in the interpretation of contact angles. In secs. 2.5c and 2.11b we saw that surface tensions, and hence contact angles, can to a first approximation be Interpreted in terms of an additive contribution of dispersion forces and a non-dispersive contribution (say, hydrogenbridging in water). These forces act across the entire bulk of each phase tmd, at least for London-Van der Waals forces they have a colloided range. On the other hand, a limited adsorption of surfactants, which only act over a molecular range, drastically modifies the wetting behaviour. 

Symmetry is an important but often elusive molecular property when numerical values must be assigned. However, symmetry plays an important role in the quantum mechanical interpretation of atomic and molecular states, NMR spectra and several physico-chemical properties. Symmetry is closely related to those molecular properties which also depend on entropy contributions, such as, for example, melting point, vapour pressure, surface tension, and - dipole moment. Moreover, the nature of overall molecular shape depends on molecular symmetry. 

Classical thermodynamic models of adsorption based upon the Kelvin equation [21] and its modihed forms These models are constructed from a balance of mechanical forces at the interface between the liquid and the vapor phases in a pore filled with condensate and, again, presume a specihc pore shape. Tlie Kelvin-derived analysis methods generate model isotherms from a continuum-level interpretation of the adsorbate surface tension, rather than from the atomistic-level calculations of molecular interaction energies that are predominantly utihzed in the other categories. 

It needs to be noted here that the above results should better be interpreted from a qualitative viewpoint rather than an accurate quantitative viewpoint, since this analysis neglects the influences of dynamic cmitact angle, surface roughness, long-range molecular forces, EDL effects, and the electrical control of surface tension forces. 

In all the preceding discussion of terms having the gAy form, yhas been interpreted as a surface tension, the factor g serving to correct for the molecular-scale curvature effect. But a stuface tension is measured at the macroscopic air-liquid interface, and in the solution case we are actually interested in the tension at a molecular scale solute-solvent interface. This may be more closely related to an interfacial tension than to a surface tension. As a consequence, if we attempt to find (say) g2A2 by dividing by we may be dividing by the wrong munber. 

Give a molecular interpretation of the decrease in surface tension of a liquid with increasing temperature. 

As an example, the role and the effect of alkylammonium ions in the montmorillonite interlayer in relation to the interaction (but also cleavage) energy between the components were investigated by MD simulation. It was confirmed that the organic surface modification of montmorillonites increases the likelihood of exfoliation of high-aspect ratio aluminosilicate layers in polymer matrices. The molecular interpretation on the basis of the simulation is supported by various experimental observations, such as cleavage energies and surface tensions [26,27, 36]. 

The fact that a determined from molecular size coincides with that obtained from surface tension fits (Table 4.5) is very nseful for applications. Thus, when fitting experimental data, we can use the value of a from molecular size, and thus to decrease the number of adjustable parameters. This fact is especially helpful when interpreting theoretically data for the surface tension of surfactant mixtures, such as SDS + dodecanol [54] SDS + CAPB [59], and fluorinated + nonionic surfactant [61]. An additional way to decrease the number of adjustable parameters is to employ the Traube rule, which states that increases with. 025kT when a CH2 group is added to the paraffin chain for details see Refs. [54,55,60]. 

---