Solid-liquid interfaces adhesion thermodynamics

Thermodynamically, a relationship of the free energy of adhesion per unit area of a solid-liquid pair is that it is equal to the work required to separate a unit area of the solid-liquid interface, thus creating a unit area of a liquid and solid interface (Figure 7.5(b)). This relationship is the well-known Dupre equation, as follows ... 

Here, Si, S2, S3 are the interfaces of solid-liquid 1, solid—liquid 2, and liquid 1-liquid 2, and v is the rate of separation. When cementing in water and oil products, the adhesive is liquid 1 and the medium is liquid 2. Equation (5.2) indicates that spontaneous wetting (without mechanical work to the interface) is possible when the surface tensions of the adhesive and the liquid are equal, i.e., for crad Hq = although the maximal value of (Tadliq is desirable because it is directly proportional to the adhesion thermodynamic work. 

Negative values imply that the two separate solid-hquid interfaces have together lower energy than the original sohd-sohd interface, and thus separation of the two solid surfaces is favoured. As explained later, polymer adhesion is a very complex phenomenon, depending on both surface phenomena and mechanical properties. Nevertheless it can be, at least qualitatively, understood via the thermodynamic work of adhesion. Example 6.5 presents such an application for the resistance of an adhesive joint in the presence of liquids. 

Adhesion — (a) When two compact materials, be they solid or liquid, are in intimate contact, attractive forces may act between their surface atoms or molecules. These forces are typically - van der Waals forces and electrostatic forces. The work of adhesion W (b)b(a) between the two phases (denoted A and B) is WAB = yA+yB -yAB> where yA and yg are the - interfacial tensions of A and B when each is interfaced only with the vapor phase, and yAB is the interfacial tension of the interface between A and B. In a more rigorous treatment (at thermodynamic equilibrium) each phase is regarded as saturated with the other phase [i]. In the case of liquid phases the equation for the work of adhesion is referred to as the -> Dupre equation. Adhesion forces between particles, or between particles and surfaces, dominate gravity for small particle sizes (pm and sub-pm range). In electrochemistry, increasing attention is being given to various phenomena related to the adhesion of vesicles [ii], particles [iii], droplets [iv], cells [v], etc. to electrode surfaces. 

In principle, an equality between the thermodynamic work of adhesion of liquid-solid systems and the work needed to separate an interface might be expected for simple systems and this has been observed for failure of adhesive-polymer interfaces bonded by van der Waals forces, (Kinloch 1987). Similarly, empirical correlations of interfacial strengths and work of adhesion values of solidified interfaces have been reported for some nominally non-reactive pure metal/ceramic systems. However, mechanical separation of such interfaces is a complex process that usually involves plastic deformation of the lattices, and hence their works of fracture are often at least ten and sometimes one hundred times larger than the works of adhesion, (Howe 1993). Nevertheless, for non-reactive metal/ceramic couples, it is now widely recognised that the energy dissipated by plasticity (and as a result the fracture energy of the interface) scales with the thermodynamic work of adhesion (Reimanis et al. 1991, Howe 1993, Tomsiaet al. 1995). 

Acid-base interactions have found numerous applieations in research dealing with adsorption of molecules of liquids on the surfaees of solids. The main focus of this research is to estimate the thermodynamic work of adhesion, determine mechanism of interactions, analyze the morphology of interfaces and various surfaee coatings, develop surface modifiers, study the aggregation of macromolecular materials, explain the kinetics of swelling and drying, understand the absorption of low molecular weight compounds in polymeric matrices, and determine the properties of solid surfaces. In addition to these, there are many other applieations. 

The work of adhesion is defined as the reversible thermodynamic work that is needed to separate the interface from the equilibrium state of two phases to a separation distance of infinity. Equation (2.4) shows the work of adhesion for a liquid-solid combination. This definition is attributed to the French scientist A. Dupre. 

It is most impressive to find how theoretical knowledge has led to some fascinating developments in the technology. The purpose of this handbook is also to further this development. The molecular description of liquid surfaces has been obtained from surface tension and adsorption studies. The emulsion (microemulsion) formation and stability are described by the interfacial film structures. The surfaces of solids are characterized by contact angle and adsorption studies. The ultimate in interfaces is an extensive description of chemical physics of colloid systems and interfaces. Contact angle and adhesion is described at a very fundamental level. The thermodynamics of... 

The thermodynamic work of adhesion, W, required to separate a unit area of a solid and a liquid phase forming an interface across which secondary forces are acting may be related to the surface and interfacial free energies by the Dupre equation. The reversible work of adhesion, Wa, in an inert medium may be expressed by ... 

It is only the energy for the reversible separation at the very interface that is considered within the framework of thermodynamics. As a starting point, the thermodynamic equations considered in O Chap. 4 for wetting, can be easily adopted by replacing the liquid by a solid adhesive Si and considering the former solid as the adherend S2 now. Hence, the specific energy of adhesion, f, for the solid-solid contact takes the form... 

The thermodynamic work of adhesion, on the other hand, is the work required to pull apart a unit area of a liquid-solid interface, thus creating one solid-vapour and one liquid-vapour interface ... 

In the original Fowkes model [14], only dispersion component of the surface tension was considered, which is caused by London dispersion force. The London dispersion forces arise from the interaction of fluctuating electronic dipoles with induced dipoles in neighboring atoms or molecules [15], It exists in all type of materials and always presents as an attractive force at the liquid-solid interface. The work of adhesion from dispersion interaction has been proved thermodynamically to take the form of the geometric mean according to the Berthelot mixing rule [17, 18]. 

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