Solid surface energy measurement techniques

Liquid vs Solid Surface Energy Measurement Techniques 

It was shortly noticed before that measuring the surface energy of a solid is a challenging task. We will broadly discuss here this point and compare the solid with the liquid case. 

Liquid—fluid interfacial tension can be directly measured because the interfaces can be deformed through the application of small (and measurable) pressures in bulk and liquids can retain a high degree of mobility at the molecular level. Thus, liquids have the ability to spontaneously reorganize at the surface to minimize the total surface free energy and present a reproducible, smooth, and well-defined surface configuration. 

When a liquid-fluid interface is curved, the pressure on both sides is not the same it is higher in the fluid situated on the concave side. The mean curvature of the interface (C) is related with the pressure difference (AP) and the surface tension (y) through Laplace equation 

The mean curvature is a mathematical description of the deformation of a surface and is deflned as the sum of the inverse of the maximum and minimum radii of curvature of the surface in a given point. For all points of a spherical surface, both radii are equal to the sphere radius R and C = 2/P fora cylindrical surface C = /R (in all points, the maximum radius is infinite and the minimum is the radius of the cylinder, R), and for a plane C = 0. 

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Values of surface energy measured by this technique were found to be 1.140 J/m2 for Ag at 903 °C, 1400J/m2 for Au at 1204 °C, and 1.650 J/m2 for Cu at 1000 °C. The solid-vapor surface energy is relatively independent of the temperature, but the surface energy of a solid-vapor interface depends on its crystallographic orientation, so these measured values must reflect an average value for many orientations. 

There are numerous techniques which provide information related to the surface energy of solids. A large array of high-vacuum, destructive and non-destructive techniques is available, and most of them yield information on the atomic and chemical composition of the surface and layers just beneath it. These are reviewed elsewhere [83,84] and are beyond the scope of the present chapter. From the standpoint of their effect on wettability and adhesion, the property of greatest importance appears to be the Lifshitz-van der Waals ( dispersion) surface energy, ys. This may be measured by the simple but elegant technique of... 

In terms of understanding the mercury/electrolyte interface, it is clear from the above discussion that the measurement of the surface free energy (in terms of the surface tension), is central. If the clectrocapillarity technique could be applied to solid electrodes, then it is capable of supplying information extremely difficult to obtain by any other technique. Sato has indeed developed a technique to measure the surface tension of a metal electrode which he terms piezoelectric surface stress measurement and is based upon the previous work of Gokhshtein (1970). 

X-ray fluorescence is a type of atomic spectroscopy since the energy transitions occur in atoms. However, it is distinguished from other atomic techniques in that it is nondestructive. Samples are not dissolved. They are analyzed as solids or liquids. If the sample is a solid material in the first place, it only needs to be polished well, or pressed into a pellet with a smooth surface. If it is a liquid or a solution, it is often cast on the surface of a solid substrate. If it is a gas, it is drawn through a filter that captures the solid particulates and the filter is then tested. In any case, the solid or liquid material is positioned in the fluorescence spectrometer in such a way that the x-rays impinge on a sample surface and the emissions are measured. The fluorescence occurs on the surface, and emissions originating from this surface are measured. 

Table 4.2. Solid/vapour surface energies of pure metals measured by the zero-creep technique (Eustathopoulos and Joud 1980).

The surface energies of several materials have been determined by measuring the change of the lattice constant (Table 2). One problem of the technique lies in the preparation of the sample. Only a limited number of substances can be prepared as small spherical particles with a defined radius on a carbon support. Often the particles are not spherical, which limits the applicability of the above equation. The surface stress can only be determined for the solid/vacuum interface, not in gas or liquids. In addition, the interpretation of diffraction effects from small particles becomes increasingly difficult with diminishing particle size (43,44],... 

A variation of the technique is to measure the internal surface energies from the heat of solution. When the solid interface is destroyed, as by dissolving, the internal surface energy appears as an extra heat of solution. With accurate calorimetric experiments it is possible to measure the small difference between the heat of solution of coarse and of finely crystalline material (Table 6). The calorimetric measurements need to be done with high precision, since there is only a few joules per mole difference between the heats of solution of coarse and those of finely divided material. A typical example is NaCI. For large crystals the heat of solution in water is 4046 J/mol. Lipsett et al. [951 measured with finely divided NaCI (specific surface area of 125 nr/mol) a heat of solution that was 51 J/mol smaller. 

Techniques to measure the surface tension of solids are notoriously difficult and known for their inaccuracies. Reliable surface tension data requires not only a reliable measurement technique but careful control over parameters such as sample purity and the gaseous atmosphere in which the experiments are conducted. TTie zero creep technique is considered one of the most accurate and reliable of these techniques since it requires only a simple length measurement(8). Samples can be either wires or thin foils. Hondros(9) has postulated that the use of thin foils increases the sensitivity of the technique and thus allows more accurate measurements. The thinner the foil, the more it approximates a surface. Wire gauges are limited due to the loads required to strain the sample. Table I lists some of the results obtained using the zero creep foil technique. It should be pointed out that the terms surface tension and surface free energy are often used interchangeably, though they are not equivalent(9,10). 

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