The Thermodynamics of Wetting

To summarize, while contact angle measurements represent a potentially powerful and practical tool for characterizing the nature and wettability of sohd surfaces, variability leading to errors in interpretation can arise from various sources. That means that proper attention must be focused on experimental conditions, equilibria, sohd and liquid purity, and so on, to ensure the best possible data. Even when all precautions have apparently been taken, interpretation must be done with the above-mentioned caveats in mind. Nevertheless, contact angle data should never be excluded from studies or processes in which wetting and spreading are involved. 

The basic framework for the application of contact angles and wetting phenomena lies in the field of thermodynamics. However, in practical apphcations it is often difficult to make a direct correlation between observed phenomena and basic thermodynamic principles. Nevertheless, the fundamental validity of the analysis of contact angle data and wetting phenomena helps to instill confidence in its apphcation to nonideal situations. 

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In printing, a film of ink is formed by wetting the surface with the compression force of the rollers. This force spreads the ink over the surface and into any capillaries that may be present. Spreading and penetration are controlled thermodynamically and kinetically. Measurement of the contact angle can be used to determine the thermodynamics of wetting. This angle can be used also to determine the contribution that polarity and dispersive forces of the liquid make to the wetting of the surface. 

The foregoing discussions on the thermodynamics of wetting and spreading are limited to systems which arc... 

Thermodynamics of Wetting and Spreading. - The thermodynamics of wetting of a solid by a liquid is well established and discussed in detail in relevant textbooks. The same principles can be applied in the phenomenological treatment of the wetting of one solid by another solid, a phenomenon that also plays a major role in the redispersion of... 

Johnson and Dettre [5] have investigated the thermodynamics of wetting for specific nonplanar surfaces and have shown that both stable and metastable equilibria can be encountered in systems exhibiting incomplete wetting. 

The thermodynamics of wetting dictate that a drop of fluid will spread on a solid only if the surface energy of the solid is greater than the combined... 

Thermodynamics of Wetting. The fundamental objective of flotation is to contact solid particles suspended in water with air bubbles (Fig. 19-65 ) and cause a stable bubble-particle attachment (Fig. 19-65Z ). It is seen that attachment of the particle to an air bubble destroys the solid-water and air-water interfaces and creates air-solid interface. The free energy change, on a unit area basis, is given by... 

Morrison and co-workers (33) measured the heat of wetting of silk fibroin and wool keratin in water as a function of their water content. Their data received proper thermodynamic treatment except that heats of solution are presumed absent. It is likely that solution effects are absent or small if zero heats are found for the immersion of samples equilibrated near the saturation pressure of water. 

Bartell and Fu3 have ingeniously used a comparison of the heat of wetting with the adhesion tension (Chap. V, 20) their thermodynamic reasoning seems open to question, but as the work of adhesion probably differs little from the heat of wetting, and is determined by the method of Bartell and Osterhof (Chap. V, 15) by a method which docs not depend on the surface area, the method may give results no more inaccurate than any of the other methods. 

Ruckenstein [19] also analyzed the stabilities of small crystallites relative to film formation. They showed that there is a minimum crystallite radius rm above which the crystallite state is thermodynamically more stable than a film. This minimum radius depends on the contact angle and hence, on the degree of wetting of the support by the active phase ... 

The study of wet steady-state foams has shown that the foam films at the upper layers rupture at very large thicknesses, i.e. before reaching thicknesses at which specific thermodynamic properties begin to appear [96]. Under these conditions the properties of wet steady-state foams are determined mainly by the effects of Marangoni and Gibbs, which stabilise kinetically the whole system [94-97,116,121,122]. 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

The process of wetting involves the creation of new types of interfaces at the expense of others. For example, when a liquid droplet spreads over a solid, more of the SL and LG interface is created, whereas (psud of the) SG interface has vanished. All these changes are accompanied by chemges in thermodynaunlc characteristics, such as the Gibbs or Helmholtz energies of the system. Here, thermodynamics is useful to... 

In the further elaboration a general phenomenological framework for wetting can be developed. Because of its thermodynamic nature, this framework is macroscopic and static it refers to equilibrium or to reversible processes. So, the kinetics of wetting cannot be analyzed in this way and only one contact angle, the equilibrium angle, can be considered. It remains an issue how this thermodynamic contact angle relates to the one that is physically measurable. Another typical feature is that interfaces are always taken to be at equilibrium with the adjacent... 

I. B. Ivanov, B.V. Toshev and B.P.R. Radoev. On the Thermodynamics of Contact Angles. Line Tension and Wetting Phenomena in Wetting, Spreading and Adhesion, J.F. Padday. Ed., Academic Press (1978) p, 37. 

A close set of equations was formulated in Ref. 16, related to the capillary pressure isotherms determined by the method of standard porosimetry [60], In the latter procedure, the equilibrium amount of the wetting liquid is measured in the porous sample under study. Simultaneously, the amount of the wetting liquid is measured in the standard specimen with a genuine porous structure, in which the capillary equilibrium is established. The standards are kept in thermodynamic equilibrium with the sample. The comparison of the amount of wetting liquid in the membrane with the pore-radius distribution in the standards, enables one to record (with a minimum of theoretical assumptions), the volume-size and surface-size distribution curves, specific pore-space surface area, and absorption isotherm in the membrane of interest, for various wetting liquids. 

Clearly the film thickness, fluid density, and perhaps the orientation of the surfaces [207] are additional thermodynamic variables that may shift or alter phase boundaries. Cases where a single interface stabilizes a different phase than the bulk are central to the field of wetting. The presence of two interfaces separated by only a few nanometers leads to more pervasive phase changes. 

The process of wetting involves replacing the solid/vapour interface (with interfacial tension ygy) with a solid/liquid interface (with interfacial tension yg ). Wetting can be described in equilibrium thermodynamics in terms of the contact angle 0 by Young s equation at the wetting line [5]. 

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