The wettability of solid surfaces

When a drop of liquid is placed on a plane, homogeneous solid surface it assumes a shape which corresponds to a minimum free energy for the system. A representation of the several forces acting on the drop isf shown in Fig. 1.13a. The condition for minimum free energy at equilibrium is that given by Young s equation 

Where the solid is available as a powder, commonly the case with pharmaceutical compounds, experimental determination of the contact angle is difficult. Early methods involved the observation of the liquid penetration into a 

Material Contact angle, Material e (deg) Contact angle, e (deg) 

In general terms wetting is taken to mean the process which occurs when a solid-air interface is replaced by a solid-liquid interface. In practice we may distinguish three distinct types of wetting which are involved when a dry particle is placed into a liquid so that it is completely immersed within the liquid [95]. The wetting processes are shown diagrammatically in Fig. 1.14. 

The work, involved per unit area of surface in this process is given by 

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For a selection of non-polar liquids on a given solid, it follows that 0 should decrease as yLG decreases and become zero below a certain value of tlg- Zisman78 has named this value of yLG the critical surface tension, yc, for the solid. Critical surface tension is a useful parameter for characterising the wettability of solid surfaces (see Table 6.1). 

Ti(>2 is a basic and simple material, and more than thirty years have passed since the discovery of the Ti(>2 photocatalytic reaction. The wettability of solid surfaces is also a basic and familiar property. It is amazing that the excellent characteristics (high hydrophilicity among other properties) of the already well understood TiC>2 were discovered only recently. In this paper, we introduced the surface reconstruction model of hydroxyl groups as the mechanism for the highly hydrophilic conversion. However, areas for further research still remain. 

The wettability of solid surfaces is a veiy important properly of surface chemistiy, which is controlled by both the chemical composition and the geometrical microsttuc-ture of surface [21-23], When a liquid droplet contacts a solid surface, it will sptead or remain as droplet with the formation of angle between the liquid and solid phases. Contact angle (CA) measurements are widely used to characterize the wettability of solid surface. Surface with a water CA greater than 150° is usually called superhydrophobic surface. On the other hand, when the CA is lower than 5°, it is called superhy-drophilic surface. Fabrication of these surfaces has attracted considerable interest for both fundamental research and practical studies [23-25]. 

The wettability of solid surfaces is a very important property, which depends on both chemical composition and surface structure. Generally, the wettability of a surface is associated with its roughness for a given material. There are two well-established models to describe the water dewetting behavior on rough surfaces. When a water droplet can penetrate into the pores, Wenzel gave a wettability description of a rough surface [1] as ... 

Surface modification reactions are used to improve the wettability of glass surfaces by polar stationary phases and to Improve the extent of deactivation by sllylation" [138-146,166]. Miaaiuua procedures have been investigated but only a few are in use. Of these, the most important reactions are etching by hydrogen chloride, leaching with aqueous hydrochloric acid, formation of whiskers and solution deposition of a layer of solid particles. Because of the high purity and thinness of the... 

Contact angles for a variety of liquids on pure amorphous polymer surfaces have been reported by Zisman and co-workers (12, 13). They have also shown (8) that the diffusion of low-molecular weight compounds from within a solid polymer film to its surface results in adsorption and a subsequent change in the wettability of that surface by specific liquids. In a few instances (9, 10), contact angle measurements have been used to show that surface changes in polymers are induced by ionizing radiation. 

The wettability of a solid surface, in terms of both high hydrophilicity and hydrophobicity, is complex and affects various other surface properties. This is a key technology for industries, and further investigations on the control of the wettability of solid materials are expected. 

This equation is valid only for a flat solid surface. 6 is called the contact angle and its value is used to define the wettability of a surface ... 

The capillary water is the result of the action of forces of a molecular character, acting at the soild/liquid phase interface. The capillary forces depend on the water surface tension, on the wettability of solid particles and on the pore size. 

Particle Adhesion in Relation to Wettability of Solid Surfaces. Adhesion is governed by certain properties of the contiguous bodies that also determine the susceptibility of these bodies to wetting by liquids [8, p. 43]. Hence, there is a definite relationship between the contact angle (wetting angle) of the contiguous bodies and particle adhesion [21 ]. 

The wettability of a surface indicates the ease with which a liquid drop spreads on a solid surface. It plays an important role in atmospheric corrosion, and in the application of organic coatings such as paints. The wettability is characterized by the contact angle 0 (Figure 3.5), which in turn depends on the three surface tensions 7si and 7 that act on the line of contact between the solid (s), liquid (1) et gas (g) surfaces. At equilibrium, the sum of the forces parallel to the surface of the solid is equal to zero. This gives the following relationship ... 

Surface-active agents reduce surface tension. This quantity (a) should also have an influence on the wettability of solids. However, this is not a simple correlation and. 

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