Contact angles particle/water interface

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

Figure 4.6. Definition of the contact angle 0 for a particle adsorbed at the water-oil interface. Case where the particle is preferentially wetted by (a) water and by (b) oil.

Paunov VN. Novel method for determining the three-phase contact angle of colloid particles adsorbed at air-water and oil-water interfaces. Langmuir 2003 19 7970-7976. 

The protection against coalescence is direcdy dependent on the energy to expel the particles from the interface. It depends on the contact angle and is easily calculated for the case of spherical particles located at the oil-water interface. The expression is equation 6 in which AH is the energy to expel a spherical particle with radius r from the interface into the phase by which it is predominantly wetted and toward which its contact angle is 0 and Y0/w is the interfacial tension between the oil and water phases. Experimental investigations (32) have shown 90° to be a practical maximum. Higher values probably lead to the expulsion of the particle into the continuous phase. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

Powders often have a stabilizing effect on emulsions [548], To understand the responsible effect we have to remember that a particle assumes a stable position in the liquid-liquid interface if the contact angle is not zero (see section 7.2.2). Upon coalescence of two drops the solid particles would have to desorb from the interface. This is energetically unfavorable. A common example of the stabilizing contribution of solid particles are margarine and butter. Both are water-in-oil emulsions. The water droplets are stabilized by small fat crystals. 

The fluid phase that fills the voids between particles can be multiphase, such as oil-and-water or water-and-air. Molecules at the interface between the two fluids experience asymmetric time-average van der Waals forces. This results in a curved interface that tends to decrease in surface area of the interface. The pressure difference between the two fluids A/j = v, — 11,2 depends on the curvature of the interface characterized by radii r and r-2, and the surface tension, If (Table 2). In fluid-air interfaces, the vapor pressure is affected by the curvature of the air-water interface as expressed in Kelvin s equation. Curvature affects solubility in liquid-liquid interfaces. Unique force equilibrium conditions also develop near the tripartite point where the interface between the two fluids approaches the solid surface of a particle. The resulting contact angle 0 captures this interaction. 

The use of finely dispersed solid particles as stabilisers of O/W and W/O emulsions has been known since last century. In a number of works [73-77] it has been established that there is a close relationship between the type and stability of emulsions, stabilised by solid particles and the value of the contact angle at the solid body/water/organic liquid interface. A... 

Garrett came to the conclusion that most important for the synergy action of an oil-particle antifoam seems to be the ability of the particles to facilitate the appearance of oil droplets into the air/water surface. However, the sizes of the antifoam oil/particle composites should be sufficiently small to ensure a high probability of presence in a given foam film, but not so small to slow down the film drainage and suppress antifoaming effect. It order to possess such properties the particles should be hydrophobic but not completely wetted by the oil. The contact angle 9ow at the oil/water interface should satisfy the condition [20]... 

The present applications have in common that pcirtlcles in a two-phase system can prefer one of the two bulk phases or the interface, depending on the relative hydrophobicity/hydrophilicity. The principle is sketched in fig. 5.44. The sketch is based on the simplification that wetting is the only driving force for the distribution and that equilibrium has been reached. As discussed before, the final situation may depend on the history particles may entrain patches of one liquid into the other. At contact angles of around 90°, the particles are usually found at the interface there is a gradual transition toward preference for the oil/water phase if the water contact angle increases/decreases. 

In water solution containing small particles (i.e., suspended solids or turbidity) and non-surface-active solutes, when air is bubbled through it, little or no particles will be removed by any adsorptive bubble separation process. This is because the particles have virtually no natural affinity for air bubbles and hence there is no adhesion when contact is made. This particular phenomena may be explained by the contact angle between a particle and an air bubble. Consider the case of the three-phase fine of contact between a smooth, rigid, solid phase, a liquid phase and a gas phase. The equilibrium contact angle can be expressed in terms of the average surface tensions (i.e., interfacial tensions, dyne/cm) of the liquid-gas solid-liquid (r j ), and solid-gas (r ) interfaces, by the well-known Young s equation ... 

There is little applicability of this mechanism to stabilization by small particles. For instance, using the values exemplified earlier, the energy required to remove a particle with a diameter of 200 nm (approximate actual size of the particles in the above study) and a contact angle of 150° from a water/toluene interface (interfacial tension = 0.036 N/m) is 4927 kT, while a 5 nm particle in the same system has a binding energy of 3 kT. Therefore, a 200 nm particle will be irreversibly bound to the interface, while a 5nm particle should not be held at the interface and if stabilization occurs, it must take place by a different mechanism. 

Fig. 2 Changes in wettability of solid particles at the oil-water interface at contact angles 6 >90° and < 90°...

In addition to the size of the nanoparticles, the interfacial tension and, therefore, the wettability of a particle surface, also dictates the desorption energy [4]. The wettability is described by the contact angle 0 between the solid and the oil-water interface. The stability of oil-in-water (O/W) emulsions or water-in-oil (W/O) emulsions depends on this contact angle. In general, the less-wetting liquid becomes the dispersed phase. If 0 is lower than 90°, O/W emulsions are more stable at contact angles greater than 90°, W/O emulsions are favored (Fig. 2) [14]. 

Fig. 3 Variation of the desorption energy E (equal to b T) of a spherical particle at a planar oil-water interface as a function of the contact angle 0. The depth of immersion into water is shown as h, the particle radius (R) is 10 nm, and the interfacial tension (yo/w) is 36 mN/m. Reprinted with permission from Langmuir [4]. Copyright (2000) American Chemical Society...

Schulman JH, Leja J (1954) Control of contact angles at the oil-water-solid interfaces - emulsions stabilized by solid particles (Baso4). Trans Faraday Soc 50(6) 598-605... 

Let us now describe the principles of flotation in detail using froth flotation as an example. Let us assume that a particle of radius r is placed at the air-water interface (Fig. HI-31). The interface can be either flat or that of a rather large air bubble. In the absence of gravity equilibrium is reached when the angle between the flat water surface and that of the particle equals the contact angle, 0. The distance between the flat water surface and the plane... 

Let us assume the absolute value of the Stem potential at the water-air interface T, exceeds that of the mineral particle 1 2> I il > Thus, within the r.s.c. F, - -values are higher than in equilibrium. Taking into account the fourth degree in Eq. (10A.6), even a small increase in l, - 4 21 can cause a big increase of the contact angle within the r.s.c. 

Thus, the mechanism of the supporting influence of cationic surfactants on microflotation and flotation can be different. In microflotation the electrostatic barrier can be decreased, in flotation the contact angle can be increased. Naturally, both effects manifest themselves simultaneously. Li Somasundaran (1990, 1992) observed a bubble recharge due to adsorption of multivalent inorganic cations. Thus, their application is recommended in order to increase the contact angle and to stabilise bubble-particle aggregates. Naturally, selective adsorption of multivalent ions at the water-air interface is important. But even in the absence of adsorption selectivity under equilibrium conditions a deviation from equilibrium can happen due to the increase of adsorption within the r.s.c. This is important for the precalculation of increase of the contact angle caused by cation adsorption. 

Because there can be degrees of wetting of particles at an interface, another quantity is needed. The contact angle, 6, in an oil—water—solid system is defined as the angle, measured through the aqueous phase, that is formed at the junction of the three phases. Whereas interfacial tension is defined for the boundary between two phases, the contact angle is defined for a three-phase junction. If the interfacial forces that act along the perimeter of the drop are represented by the interfacial tensions, then an equilibrium force balance can be written as... 

A foam-stabilizing mechanism is operative if the particles are not completely water-wetted. In this case, particles would tend to collect at the interfaces in the foam where they may add to the mechanical stability of the lamellae. On the other hand, quite hydrophobic particles may actually act to destabilize foam. Thus intermediate contact angles (between about 40° and 70°) appear to be optimum for solid-stabilized foams (8). 

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