**3. Process to stabilization**

### **3.1. Zeta potential and** *in situ* **hydrophobization**

Colloids are suspensions or liquid foams that are generally thermodynamically unstable. The instability arises due to their high gas-liquid interfacial areas, which raise the free energy of the system. To achieve a stable system, free energy must be minimized. The electrokinetic properties of a colloidal system can be described using the zeta potential (Figure 4(a)). Higher charges on the particles' surfaces stabilize a colloidal suspension by preventing the particles from coming into contact and coalescing. Colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate as shown in Table 1. A suspension's pH affects its charge distribution, and hence its zeta potential. The isoelectric point (IEP) is the pH at which a colloid's zeta potential is zero; it can be used to derive information about the pH ranges in which a colloid is stable. A suspension's pH can be modified to allow dissociated surfactant to adsorb electrostatically as counter ions onto oppositely charged alumina hydroxyl surface groups [39]. The suspension's inorganic particles can be stabilized *in situ* by the particles' hydrophobization with different colloidal particles containing predominantly -OH2 + , -OH, and -O surface groups. Surfaces with predominantly -OH2 <sup>+</sup> and -OH groups can be achieved on inorganic alumina particles at pH 4.5 and pH 9.5, respectively. This could be derived from the zeta potential data for bare alumina particles (Fig. 4(b)), which confirm that the surface exhibits mainly -OH2 <sup>+</sup> (positive net charge) and -OH (neutral net charge) groups under those conditions [26, 31, 20]. Amphiphiles of short chain carboxylic acids and gallates are expected to adsorb well onto alumina particles. Propyl gallate has been used to modify the surfaces of particles by ligand exchange reactions [15]. The surface hydroxyl groups (-OH or -OH2 + ) were replaced by one or more of the molecule's hydroxyl groups (-OH or -O- ). Therefore, the adsorption of gallate molecules does not necessarily require oppositely charged surfaces and amphiphiles and can be used at pH values at which the surface groups and the molecules exhibit the same charge polarity. Hydrophob‐ izing adsorption can change the wettability of particles at the interface of two immiscible phases, and the system is stabilized by the neutral forces between the particles and the amphiphilic coatings. Therefore, the choice of amphiphile depends upon the IEP and the zero net charge of the oxide. Surface hydrophobization can be accomplished by choosing amphi‐ philes with functional groups that react with the surface hydroxyl groups. Pyrogallol groups can efficiently adsorb on oxide surfaces via ligand exchange reactions [14, 16] and thus can be used with a short hydrocarbon tail to modify the surfaces of particles with intermediate IEPs. The selection of amphiphiles with suitable head groups and tail lengths allows the surface hydrophobization of particles of various compositions.

#### **3.2. Destabilizing suspension**

**Figure 3.** *In situ* hydrophobization of particles and solid foam formation by direct foaming.4

Fig. 2. Currently available methods of forming porous ceramics.2

template, as opposed to the positive morphology obtained from replication. The method of the sacrificial material's extraction from the consolidated composite depends primarily on the type of pore former employed [33]. A wide variety of sacrificial materials can be used as pore formers, including natural and synthetic organics, salts, liquids, metals, and ceramics. This technique is flexible and can employ various chemical compositions. Various oxides have been used to fabricate porous ceramics using starch particles as sacrificial templates [9, 10]. Nonoxide porous ceramics have also been produced using pre-ceramic polymers and various template materials [34, 35]. Since this method produces a ceramic to the negative of the original template, the removal of the sacrificial phase does not lead to flaws in the struts as can occur using positive replicas. The microstructures obtained by this technique reflect directly the pattern of the sacrificial phase and higher mechanical strengths are generally

achievable than by using positive replicas [36, 37].

60 Advanced Ceramic Processing

**Figure 2.** Currently available methods of forming porous ceramics.2

6

Colloidal dispersions can be thermodynamically unstable, with long-term kinetic stability determining their self-life. The main destabilization mechanisms are drainage (creaming and sedimentation), coalescence, and flocculation (Fig. 5). Creaming and sedimentation are caused by gravity: lighter particles float and heavier particles settle. They are reversible in that

**Figure 4.** (a) The distribution of charges in a colloidal suspension; higher charges at the particles' surfaces can stabilize the system. (b) Zeta potential of raw Al2O3 and SiO2 colloidal particles.


**Table 1.** Zeta potential as key indicator of the stability of colloidal dispersions

mechanical agitation (homogenization or simple shaking) will redisperse the suspension. Coalescence and flocculation are not reversible and so affect a suspension's stability. Floccu‐ lation is the clustering of colloidal particles via attractive van der Waals forces. It can be overcome or prevented by higher-energy ultrasonification or by generating particles with repulsive interactions [40]. Coalescence is the greatest destabilizing mechanism. It involves smaller particles collapsing into each other, forming larger particles with different properties. Many dispersion techniques have been developed to prevent coalescence [41].

#### **3.3. Suspension stability**

The foams require the adsorption of particles on the surfaces of air bubbles upon their formation. Alumina particles can be hydrophobized by modification with short-chain carbox‐ ylic acids: the carboxylate groups adsorb to the alumina's surface [42], leaving the hydrophobic tail in contact with the aqueous solution. This has been shown to stabilize the dispersion [43].

**Figure 5.** The destabilization of colloidal suspensions.

mechanical agitation (homogenization or simple shaking) will redisperse the suspension. Coalescence and flocculation are not reversible and so affect a suspension's stability. Floccu‐ lation is the clustering of colloidal particles via attractive van der Waals forces. It can be overcome or prevented by higher-energy ultrasonification or by generating particles with repulsive interactions [40]. Coalescence is the greatest destabilizing mechanism. It involves smaller particles collapsing into each other, forming larger particles with different properties.

**Figure 4.** (a) The distribution of charges in a colloidal suspension; higher charges at the particles' surfaces can stabilize

**Zeta potential [mV] Stability behaviour of the colloid** From 0 to ±5 Rapid coagulation or flocculation

From ±10 to ±30 Incipient instability From ±30 to ±40 Moderate stability From ±40 to ±60 Good stability More than ±61 Excellent stability

the system. (b) Zeta potential of raw Al2O3 and SiO2 colloidal particles.

**Table 1.** Zeta potential as key indicator of the stability of colloidal dispersions

The foams require the adsorption of particles on the surfaces of air bubbles upon their formation. Alumina particles can be hydrophobized by modification with short-chain carbox‐ ylic acids: the carboxylate groups adsorb to the alumina's surface [42], leaving the hydrophobic tail in contact with the aqueous solution. This has been shown to stabilize the dispersion [43].

Many dispersion techniques have been developed to prevent coalescence [41].

**3.3. Suspension stability**

62 Advanced Ceramic Processing

The hydrophobicity imparted by the first layer of depronated amphiphiles adsorbed onto the surface leads to an energetically unfavorable exposure of hydrophobic species to the aqueous phase. This favors the adsorption of additional molecules from the aqueous phase onto the particles' surfaces to decrease the system's free energy, which determines the stability of a suspension or wet foam. Particles attached to foam and mists' gas-liquid interfaces lower the overall free energy by replacing part of the interfacial area rather than reducing the interfacial tension, as in the case of surfactants [5]. The energy of the attachment, i.e., the Gibbs free energy (G), gained by the adsorption of a particle of radius *r* at the interface can be calculated using simple geometrical arguments that lead to the following equation (Fig. 6).

$$G = \pi r^2 \Upsilon\_{\rm LG} \left( 1 - \cos \theta \right) \text{ for} \theta < \Psi 0^\circ \text{ \AA}$$

where *θ* is the contact angle and *LG* is the gas-liquid interfacial tension. While the maximum energy gain can only be achieved at *θ* = 90º, contact angles as low as 20º can yield attachment energies in the order of 103 kT in systems of 100 nm particles [2]. The high energy associated with the adsorption of particles at interfaces contrasts to low adsorption energies of surfactants and leads to foams stabilized by particles being more stable than those stabilized with surfactants. It also leads to steric layers which strongly hinder bubbles' shrinkage and expansion, minimizing Ostwald ripening for very long periods of time [46].

The particle systems described in Fig. 6 had adsorption achieved by ligand exchange, whereby a surface hydroxyl group is exchanged for another group. This occurred because of the favorable change in the surface charge by the removal of (OH2 + ), a better leaving group, and replacement with (-OH) [44, 45].

**Figure 6.** Foams produced through the adsorption of colloidal particles at the gas-liquid interface.
