Energy of adhesion

Energy of Adhesion. The iaterfacial energy between two mutually insoluble saturated Hquids, A and B, is equal to the difference in the separately measured surface energies of each phase ... 

Combination with equation 2 for energy of adhesion of a soHd—Hquid interface (eq. 3) gives equation 5 ... 

The surface forces apparatus (Section 2.3) enables the estimation of a surface energy term, Fq (Eq. 9), providing sufficiently smooth surfaces can be produced. In recent years Chaudhury, Pocius and colleagues have made a valuable contribution to the field of adhesion by developing the technique to study energies of adhesion and of surface energies of polymers [81-85]. These SFA results provide alternatives to values based on traditional destructive tests or contact angle measurements. 

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

Oxidation of polyethylene by sulphuric acid and potassium chlorate [9,10] improves its adhesiveness. The free energy of adhesion of the polymer is found to increase linearly with the surface density of the hydrophyl-lic sites created by oxidation. 

The process of viscoelastic braking just described has certain parallels with the dynamic adhesion of elastomers. When, for example, a rubber strip is peeled from a rigid substrate, the effective, or apparent, work of adhesion, W, is usually much greater than the intrinsic, or reversible, energy of adhesion, Wq, given by the Dupre equation [15] ... 

Thns the strain energy release rate is effectively an instantaneous value of Dupre s energy of adhesion, with 6 = 0(0 instead of the equilibrium value. The sign reversal in the left-hand side of Eq. (18) when compared to Eq. (15) is due simply to the fact that we have a closing crack with a spreading liquid. 

This expression can be roughly interpreted as the difference between the Gibbs energy of adhesion of the two phases and the sum of the Gibbs energies of cohesion for the two phases. 

The properties of the crystal nucleus are a function of its surface energy, its dimensions and the energy of adhesion to the substrate. The equilibrium form of a crystal with the nth plane lying on the substrate is described by... 

E(hg) is negative for adhesive systems and —E(he) is traditionally known as the free energy of adhesion. 

An important consequence resulting from the approach of the interfaces is that Yf, the surface energy of the film, is lowered by the energy of adhesion. When the interfaces are far apart, E(h) is equal to zero and y/ is simply equal to 2yint, where yint is the tension of a single interface. At equilibrium, the surface energy of the film is Yf = 2yint -F E(he). 

This relation is known as the Young-Dupr6 equation. It shows that the energy of adhesion can be determined by measuring the contact angle and the surface... 

Figure 2.35. Energy of adhesion between hexadecane droplets stabilized in water by SDS, at various NaCl concentrations. (Adapted from [111].)...

The energy of adhesion between hexadecane droplets stabilized in water by SDS in the presence of NaCl is shown in Fig. 2.35. It is observed that the adhesion depends strongly on the temperature and on the salt concentration. For a given salt concentration, there is a well defined temperature, T, above which there is no adhesion. As the behavior of the surface energy changes at T, this temperature can be referred to as a wetting transition temperature [109]. The dependence of T versns the salt concentration is plotted on Fig. 2.36. 

Moreover, the adhesion depends strongly on the nature of the salt [105]. A pronounced difference is noted in Fig. 2.37, where the energy of adhesion with KCl, NaCl and LiCl are compared [110]. The system is much more adhesive with KCl. This is the contrary with a carboxylate surfactant [106,110]. This behavior is important because it suggests that the adhesion is linked to the solubility of the... 

The study of inverse adhesive emulsions has revealed the same features as direct emulsions [112,113]. Here again, it was shown that adhesion is favored when the surfactant becomes less soluble in the continuous phase [113]. This can be tested experimentally by using binary mixtures of oils, one in which the surfactant is soluble and another one in which the surfactant is insoluble. For example, water droplets can be stabilized in mineral oil by sorbitan monooleate (Span 80). This surfactant is soluble in dodecane whereas it is not in silicon oil. The affinity of the surfactant for the organic solvent can be tuned by mixing dodecane and silicon oil. As shown in Fig. 2.38, the energy of adhesion between water droplets strongly varies as the ratio of the mixture is changed. A sharp rise is noted as the surfactant... 

Figure 2.38. Energy of adhesion between water droplets stabilized with sorbitan monooleate (Span 80) in a silicon oil-dodecane mixture. The arrow indicates the insolubility threshold of the amphiphUe. (Adapted from [113].)...

Therefore, besides heats of immersion, only the surface energy of the liquid is required to obtain the energy of adhesion. 

Coalescence. The process of coalescence in water-treating systems is more time dependent than dispersion. In dispersion of two immiscible liquids, immediate coalescence seldom occurs when two droplets collide. If the droplet pair is exposed to turbulent pressure fluctuations, and the kinetic energy of oscillations induced in the coalescing droplet pair is larger than the energy of adhesion between them, contact will be broken before coalescence is completed. 

In adhesional wetting, a liquid which is not originally in contact with the solid substrate makes contact and adheres to it. In contrast to spreading wetting, the area of liquid-gas interface decreases. The work (free energy) of adhesion is given by the Dupr6 equation (see equation (4.29) in the form... 

Interfacial Energy of Adhesion. When the polyelectrolyte-grafted nylon surface, in equilibrium with 50% relative humidity, is brought into contact with water or a salt solution, various interactions will occur together they comprise the reversible work of adhesion or free energy of adhesion at the interface of these two phases. This free energy of adhesion should be composed of the following contributions ... 

Of the above contributions, 1, 3, and 4 will tend to lower the free energy of adhesion and favor wetting (lower 6) 2 will raise the free energy and oppose wetting (higher 6) the effects of 5 and 6 may go in either direction. 

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