Interfacial tension wetting thermodynamics

This process involves extraction of fine particles from an aqueous phase into an oil phase. The effectiveness of this technique, as shown in Figure 2, is based on the stability of emulsion droplets with solid particles. If a particle is partially wetted by two immiscible liquids the particle will concentrate at the liquid-liquid interface. The thermodynamic criteria for distribution of solids at the interface of two immiscible liquids is the lowering in the interfacial free energy of the system when particles come in contact with two immiscible liquids. (12) If ygw, yWQ and ygp are the interfacial tensions of solid-water, water-oil and solid-oil interfaces respectively, and if ygQ > y + ygw then the solid particles are preferentially dispersed within the water phase. However, if ygw > ywq + ygQ, the solid is dispersed within the oil phase. On the other hand, if yWQ > ygQ + ysw, or if none of the three interfacial tensions is greater than the sum of the other two, the solids in such case will be distributed at the oil-water interface. 

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]. 

As Table 2.1 shows, the concept of the wetting coefficient has been successfully applied in filled polymer blends containing various fillers, such as carbon black [36], silica [26,27,37], or nano-CaCOs particles [38,39]. Limitations of this criterion include strong discrepancies in interfacial tensions, due to the lack of data in the literature regarding polymer/filler interfaces and issues of extrapolation to the appropriate temperature. Also, this criterion assumes that thermodynamic equilibrium has been reached, which is not always the case experimentally due to the limited processing time. 

The DLVO theory, the principles of polymer-mediated steric interactions, and the wetting approach are aU generic and they should therefore be interconnected. However, the relation between DLVO and steric interactions on the one hand and the interfacial tensions on the other will not be appreciated from the foregoing discussion. In Section 20.5, the apparent disparate approaches are united in a general thermodynamic analysis of particle adhesion. 

In this section, let us eonsider the macroscopic wetting behavior of an axisymmetric meniscus from a thermodynamic viewpoint and discuss the possibility of the measurement of the contact angle and interfacial tension [61]. As in the analysis stated above, the theoretical consideration is based on the assumption described by Eq. (9). 

In 1805 Young established a relationship between the contact angle of a drop of liquid and the interfacial tensions at the three-phase contact line between a solid, a liquid, and its vapor at equilibrium (Eq. 10.1). J. Willard Gibbs in 1928 then derived the contact angle 9, from thermodynamic quantities, the surface free energies for the three interfaces. Substantial refinements in terms of the nature of the forces involved in the wetting process have been made since then, and interested readers can consult fundamental texts on surface forces. ... 

Interfacial and capillary phenomena are present in multiple biological processes. Some examples are duck s feathers impermeability, spiders sticky traps, and Lotus leaf s effect. The last subject is considered in a separate chapter due to its important technological applications. The basis to understand all those processes is the focus of the present chapter, divided into three subsections. The first one addresses the fundamentals of interfacial tension and wetting conditions as thermodynamical concepts. In the second, capillarity effects under dynamical conditions are considered. The third section is devoted to liquid films, their stability, and the spontaneous retraction in simple geometries. 

Notably, all theories for estimating the interfacial tension have been subjected to severe criticism. All theories have limitations and they should be used with care. Despite the problems and uncertainties associated with the component theories, the solid surface energies calculated from contact angles with these theories are often of practical importance in studies of wetting phenomena, surface modification and adhesion, e.g. for developing correlations between the practical work of adhesion with the thermodynamic/reversible work of adhesion, i.e. the work of adhesion calculated from the Young-Dupre equation (or based on interfacial tensions). 

Finally, there is another price to be paid for using (O 4.54) The interfacial tensions )>sl ysv of the soKd have been eliminated, and hence, they cannot be deduced from wetting experiments. This obstacle was the reason for the development of so-called thermodynamic adhesion theories which introduce various proposals for a relationship between the Kquid-soKd interfacial tension ygL the surface tensions lvi Tsv- These theories are considered in Chap. 6. 

In this equation, cOa is the wetting coefficient used for judging the location of filler in different polymer phase in the term of thermodynamics. The symbols represented in the numerator are the different interfacial tensions between filler and polymers 1 and 2 and in the denominator the interfacial tensions between the two blend phases. If > E fillers preferentially distribute in polymer 2 if co < 1, fillers preferentially distribute in polymer 1, and if -1 < co < 1, fillers distribute at... 

If one uses a coarse-grained approach, one has to identify the relevant properties that the description on the coarser scale has to capture. In the following, we specifically consider wetting phenomena in a binary AB polymer blend that exhibits liquid-liquid phase separation between an A-rich and a B-rich phase. The thermodynamics of the surface enrichment layers is dictated by the free energies of the solid in contact with the two coexisting phases. Yaw Ybw> their interfacial tension,... 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

---