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The confined spaces provided by colloidal systems, in particular by droplets in emulsions, can be considered as ‘nanoreactors’ in which chemical reactions can be carried out. In this chapter, we cover the possibilities offered by microemulsions and miniemulsions to synthesise inorganic nanostructures, including silica, metals, metal oxides and hydroxides, metal chalcogenides, and more complex inorganic systems. We review specific concepts of colloidal science related to the confinement within droplets and different related synthetic strategies. The formation of the intended inorganic compound can take place either within the nanodroplets, leading to nanoparticles, or at the curved liquid–liquid interface, leading to nanocapsules and hollow structures.

The confinement provided by colloidal systems, in particular by droplets in emulsions, has been extensively exploited in polymer science for the synthesis of organic polymers through so-called emulsion polymerisation techniques.1–5 In an analogous way, emulsions can also be employed for confining inorganic reactions, typically of precipitation, oxidation/reduction or sol–gel processes.6 Several publications in recent years, including some from us, have periodically reviewed the use of droplets and nanodroplets for the synthesis of inorganic materials.6–9 Our aim in this chapter, rather than to provide an exhaustive research review, is to cover in an explanatory fashion the possibilities offered by microemulsions and, especially, by miniemulsions for obtaining nanostructured inorganic materials, and providing thereby examples of different synthetic strategies. Based on a didactic perspective, we will also review some related general concepts of colloidal chemistry, but specifically focused on the field of inorganic reactions within nanodroplets.

Before starting the description of the synthetic strategies, it is convenient to make a few initial remarks about the confusing terminology in the field of emulsions. Traditionally, emulsions have been classified in macroemulsions (or simply emulsions), miniemulsions and microemulsions.1 This nomenclature has been followed by polymer scientists and part of the colloid community.5,10 However, another partially overlapping term is very often found in the literature, especially—although not only—in the fields of food science and pharmacy: nanoemulsions.11,12 We will revisit here the most relevant aspects of the differences between these emulsion terms, which are discussed in more detail elsewhere.5,9

Macroemulsions are thermodynamically unstable emulsions with droplet sizes above 500 nm (usually >1 µm). Microemulsions, in contrast, are characterised by being thermodynamically stable and have droplet sizes below 50 nm. Miniemulsions are in between these two types and are kinetically stabilised emulsions but thermodynamically unstable (like macroemulsions), with droplet sizes in the range between 50 and 500 nm.2,3 The term nanoemulsion (systematically and intentionally hyphenated by some authors13–15 as nano-emulsion) should in principle be used only for miniemulsions with sizes below 100 nm (or 200 nm, at most). Although the term ‘nanoemulsion’ is often used interchangeably with ‘miniemulsion’, nanoemulsions are a subdivision of miniemulsions, corresponding to nanosized (below 100 nm) and thermodynamically unstable emulsions. This classification of emulsions is schematically represented in Figure 1.1. The inclusion of microemulsions within the label ‘nanoemulsions’ is discouraged, because microemulsions are supposed to be thermodynamically stable, by definition. Although some contradictory examples are found from time to time in the literature, we encourage the reader to follow this recommendation, as it is the consensus preference of most of the research groups working in the field.

Figure 1.1

Schematic representation of the different types of emulsions.

Figure 1.1

Schematic representation of the different types of emulsions.

Close modal
When a droplet is formed, there is an intrinsic pressure difference between the inner (Pinn) and the outer (Pext) parts of the curve interface, ΔPL, given by the Young–Laplace equation,
(1.1)

where γ is the surface tension and r is the radius of the droplet. As an example, according to this expression, for water droplets with a radius of 1 mm, the pressure difference is 145.6 Pa = 1.09 mm Hg (considering γ = 72.8 mN m−1 at 20 °C for water). By reducing the radius by a factor of 10 (i.e., 0.1 mm = 100 µm), the pressure is multiplied by the same factor, so that ΔP = 10.9 mm Hg. If we go to droplets with a size of 100 nm (r = 50 nm), the pressure difference would be as high as 2.2 × 105 mm Hg (ca. 29.1 bar). This simple calculation makes very obvious the great effect that the formation of a curved interface has on the pressure of the confined space within a nanodroplet.

In addition to the advantages related to the formation of smaller particles in smaller spaces (higher specific surface area)16 and the possible finite-size effects in crystallisation and selection of polymorphs,17 the high pressure within nanodroplets can also allow the formation of crystal phases of inorganic materials that otherwise would require higher temperatures.18 A shift in the phase diagram through the pressure resulting from the space confinement could even allow the stabilisation of certain polymorphs.9 Also, other properties of the solvent (typically water in inorganic synthesis), which are relevant for solubilisation and crystallisation phenomena (e.g., viscosity, relative permittivity), dramatically change when confined in constrained environments.19–21 Emulsions containing nanodroplets (i.e., miniemulsions and microemulsions) have, therefore, significant potential in inorganic synthesis. It is very important to remark that, in most cases, in miniemulsion and microemulsions, crystallisation of the materials takes place under very mild conditions, often at room temperature or close to it.22 

As mentioned in Section 1.1, the use of emulsions in synthesis has been reviewed in different works. Some representative publications are contained in Table 1.1, with a brief indication of the focus in each case.

Table 1.1

Reviews focusing (at least in part) on the use of emulsions in inorganic synthesis

Authors and reference Year Type of emulsion Focus 
Pileni53  1993 Microemulsions Water-in-oil systems 
Sanchez-Dominguez et al.86  2012 Microemulsions Inorganic nanoparticles (specific for oil-in-water systems) 
Muñoz-Espí et al.6  2012 Miniemulsions Inorganic nanoparticles 
Diodati et al.8  2015 Micro- and miniemulsions General on wet-chemistry and colloidal routes 
Muñoz-Espí et al.7  2013 Miniemulsions and microemulsions Crystalline materials 
Wolf and Feldmann59  2016 Microemulsions Inorganic nanoparticles 
Boutonnet and Sánchez-Domínguez68  2017 Microemulsions Synthesis of catalytic nanoparticles 
Álvarez-Bermúdez and Muñoz-Espí5  2018 Nanoemulsions Nanoparticle synthesis (not only inorganic) 
Muñoz-Espí and Landfester9  2020 Miniemulsions Metal oxides 
López et al.71  2020 Microemulsions Metal and metal oxide nanoparticles 
Elzayat et al.85  2021 Nanoemulsions Biomedical nanocarriers (mostly polymeric, but also inorganic) 
Swisher et al.25  2022 Nanoemulsions Nanoreactors for particle synthesis (not only emulsions) 
van Embden et al.87  2023 Miniemulsions and microemulsions Very specific on colloidal synthesis of ZnO (not only emulsions) 
Authors and reference Year Type of emulsion Focus 
Pileni53  1993 Microemulsions Water-in-oil systems 
Sanchez-Dominguez et al.86  2012 Microemulsions Inorganic nanoparticles (specific for oil-in-water systems) 
Muñoz-Espí et al.6  2012 Miniemulsions Inorganic nanoparticles 
Diodati et al.8  2015 Micro- and miniemulsions General on wet-chemistry and colloidal routes 
Muñoz-Espí et al.7  2013 Miniemulsions and microemulsions Crystalline materials 
Wolf and Feldmann59  2016 Microemulsions Inorganic nanoparticles 
Boutonnet and Sánchez-Domínguez68  2017 Microemulsions Synthesis of catalytic nanoparticles 
Álvarez-Bermúdez and Muñoz-Espí5  2018 Nanoemulsions Nanoparticle synthesis (not only inorganic) 
Muñoz-Espí and Landfester9  2020 Miniemulsions Metal oxides 
López et al.71  2020 Microemulsions Metal and metal oxide nanoparticles 
Elzayat et al.85  2021 Nanoemulsions Biomedical nanocarriers (mostly polymeric, but also inorganic) 
Swisher et al.25  2022 Nanoemulsions Nanoreactors for particle synthesis (not only emulsions) 
van Embden et al.87  2023 Miniemulsions and microemulsions Very specific on colloidal synthesis of ZnO (not only emulsions) 

The title of this chapter contains the term ‘nanoreactor’, which has been very commonly linked to nanodroplets in miniemulsions.3,23,24 The term can also be used for other types of colloidal confinement, such as micelles, colloidal assemblies (see the review by Swisher et al.25) or nano-/mesopores, but we will focus our attention here on nanodroplets. If we move away for a moment from the inorganic synthesis to emulsion polymers, and we compare conventional emulsion polymerisation with miniemulsion polymerisation, the former is characterised by diffusion from monomer droplets, acting as reservoirs, to the growing particles being formed. By contrast, in miniemulsion polymerisation, there is no diffusion between droplets, and the polymerisation reaction takes place (at least ideally) within the droplets, which act as independent ‘nanoreactors’. To maintain the stability of the system, two types of agent are normally used: effective surfactants, which stabilise the droplets (sterically, electrostatically or electrosterically) and minimise the coalescence between them; and osmotic-pressure agents, which help to avoid Ostwald ripening.2–7 For inorganic synthesis, the idea is essentially the same: in an ideal case, the formation of the desired compound should take place within a droplet containing the precursor, while avoiding diffusion of the contained matter towards other ‘nanoreactors’ (see cartoon in Figure 1.2). In a real system, however, this situation is very difficult to reach, and diffusion and interaction between droplets and growing particles occurs in most of cases, and is more likely than in polymerisation reactions for organic polymers.

Figure 1.2

Graphical representation of emulsion droplets acting as nanoreactors for the formation of inorganic matter. Reproduced from ref. 9, https://doi.org/10.1002/chem.202001246, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 1.2

Graphical representation of emulsion droplets acting as nanoreactors for the formation of inorganic matter. Reproduced from ref. 9, https://doi.org/10.1002/chem.202001246, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Close modal
From a thermodynamic point of view, in a first approximation, the change in the Gibbs energy associated with the formation of an emulsion, ΔGform, can be considered as the sum of an interfacial free energy term (ΔGinterf) and a negative configuration entropic term (−TΔSconfig):26 
(1.2)
If temperature, pressure and interfacial chemical potential remain constant, ΔGinterf can be expressed as the change in the contact area between the oil and the aqueous phase (ΔA) multiplied by the interfacial tension between these two phases, γ:
(1.3)

Since both γ and ΔA are positive (the formation of an emulsion involves an increase of the overall surface area), ΔGinterf is always positive and it becomes increasingly positive with decreasing droplet size (the interfacial area increases with decreasing size). By contrast, the configuration free energy, −TΔSconfig, becomes increasingly negative with decreasing droplet size (the number of different ways in which the droplets can be organised increases). For spontaneous formation of an emulsion, ΔGform has to be <0, so that ΔGinterf will oppose the formation of the emulsion, whereas the configuration term will favour it. Since the interfacial term dominates the configuration term, the formation of an emulsion should be thermodynamically less favourable when the droplet size decreases. However, this is only true if we assume that the interfacial tension remains constant with the curvature radius, which is not the case when the size decreases. Indeed, the interfacial tension does not have a trivial dependence on the droplet size. Therefore, the dependence of the overall formation free energy with the size is more complex than initially assumed. For detailed explanations about the contributions of the interfacial and the configuration entropic terms, the work by McClements26 is recommended.

For macroemulsions, the interfacial term clearly dominates (the increase in the interfacial area is much more relevant than the increase in the possible configurations), so that γΔATΔSconfig, and ΔGform > 0. For miniemulsions, the interfacial area increases, but so does ΔSconfig; eventually, the former continues to dominate the latter, and γΔA > TΔSconfig, so that ΔGform > 0 also. Accordingly, macroemulsions and miniemulsions are both thermodynamically unstable and work has to be done to form them. From a kinetic point of view, both need to be stabilised, but miniemulsions achieve much longer stability than macroemulsions.

The situation for microemulsions is more complex. When the droplets become sufficiently small, the interfacial tension of a droplet can no longer be assumed to be the same as for planar interfaces. As mentioned previously, the interfacial tension depends on the curvature of the surfactant monolayer and so at a certain small droplet size (typically <10 nm), the value of the interfacial tension as a function of the droplet radius reaches a minimum. Around that minimum the interfacial free energy term, which usually dominates, is substantially reduced. Under these conditions, γΔA < TΔSconfig and ΔGform becomes negative, so that the formation of microemulsions is spontaneous. In other words, the resulting system is thermodynamically stable.

As for any colloidal system, many processes can affect the stability of emulsions. Figure 1.3 depicts these processes, together with the driving forces in each case. First of all, gravity leads to settling (if the density of the disperse phase is higher than that of the continuous phase) or creaming (if the density of the disperse phase is lower) and, eventually, to phase separation. For spherical particles of radius r and volume Vp, the gravitational force contributing to settling down in a suspension, Fgrav, can be expressed as
(1.4)
Figure 1.3

Factors affecting the stability of emulsions.

Figure 1.3

Factors affecting the stability of emulsions.

Close modal
where m is the mass of the particles, g is the gravitational constant, and ρp and ρm are the densities of the particles and the dispersing medium, respectively. The reaction force opposing gravity is the viscous drag force, Fdrag, given by Stokes’ law:
(1.5)
where r is the radius of the particle, v is the settling rate and η is the viscosity of the medium. By equating eqn (1.4) and (1.5) and clearing v, the following expression for the settling rate is obtained:
(1.6)

According to this eqn (1.6), general ways to reduce sedimentation/creaming are either by decreasing the droplet size or increasing the viscosity of the continuous phase.

In addition to gravity, under certain conditions, temperature (affecting solubility) and concentration of the disperse phase with respect to the continuous one, may be the origin of a phase inversion. Furthermore, attractive forces between droplets, typically from van der Waals interactions or introduced by charges in the system resulting from components of the formulation (charged precursors or additives), can lead to aggregation (flocculation, coagulation). Finally, two other effects are also crucial in the destabilisation of emulsions: coalescence and Ostwald ripening. Since they are especially relevant, we will discuss in more detail the driving forces behind them and what strategies can be used to minimise them.

Coalescence is a direct consequence of the minimisation of surface area and, therefore, the minimisation of the related surface energy. Let us make a simple exemplifying calculation by considering two spherical droplets of equal radius (r1), both with mass m1, volume V1 and area A1 (with ρ being the density of the substance in the droplet):
(1.7)
(1.8)
(1.9)
When the two smaller droplets of radius r1 are combined to form a bigger one of radius r2, the resulting mass and volume, m2 and V2, respectively, will clearly be just the addition of the original two values:
(1.10)
(1.11)
The radius of the resulting droplet is a factor 1.59 bigger than the initial one. Accordingly, the area will be
(1.12)

which is smaller than double the area given by eqn (1.9), 2(4πr12). The new droplet has about 20.9% less area than the sum of the two initial droplets.

With the exception of surfactant-free systems (which are rather unusual) and cases of spontaneous emulsification or the ‘ouzo effect’27,28 (also rare for inorganic compounds), any emulsion strategy implies the use of surfactants, which stabilise the emulsions against coalescence, but also against aggregation and flocculation. Non-ionic surfactants provide a steric stabilisation, while ionic surfactants provide electrostatic or electrosteric (electrostatic and steric simultaneously) stabilisation. The mechanism of emulsification is schematically depicted in Figure 1.4. In a first step, a pre-emulsion is obtained by mechanical or magnetic stirring (droplets with sizes of several micrometres and polydispersed are initially formed). Afterwards, disruption (i.e., comminution) of the droplets is achieved by the application of mechanical energy. The formation of miniemulsions requires high shear forces (ultrasonication or high-pressure homogenisation are the most common methods) to reach stable nanosized droplets. Achieving a stable emulsion will require efficient droplet breakup and minimisation of coalescence.

Figure 1.4

Schematic diagram of droplet breakup and coalescence in emulsification. Reproduced from https://publikationen.bibliothek.kit.edu/1000120408, under the terms of the CC BY-SA 4.0 license https://creativecommons.org/licenses/by-sa/4.0/deed.de.

Figure 1.4

Schematic diagram of droplet breakup and coalescence in emulsification. Reproduced from https://publikationen.bibliothek.kit.edu/1000120408, under the terms of the CC BY-SA 4.0 license https://creativecommons.org/licenses/by-sa/4.0/deed.de.

Close modal

Representative examples of surfactants used in micro- and miniemulsions are shown in Figure 1.5. Poly(ethylene-co-butylene)-block-poly(ethylene oxide) (P(E/B-b-PEO)), often referred to as ‘KLE’ (an acronym comprised of ‘KL’, for Kraton Liquid, the company that produced the ethylene/butylene block, and ‘E’ for ethylene oxide), has been widely used for stabilising, very effectively, water-in-oil systems. Unfortunately, in spite of its great stabilisation ability, this copolymer is not commercially available, requires a complex and tedious synthesis,29 and so is not a very convenient option for most research groups. In this respect, surfactants such as polyglycerol polyricinoleate (PGPR) or sorbitan oleate (commercially known as Span 80) are much more readily available for stabilising inverse systems, even if they are sometimes less effective than P(E/B-b-PEO). The stabilisation of oil-in-water (direct) systems is generally easier. Although ionic (anionic or cationic) surfactants are normally very efficient, they may interact easily with other charges in the systems, leading to coagulation. Therefore, especially when metal complexes are involved as precursors, non-ionic surfactants may be preferred. In this context, polyoxyethylene alcohols linked to apolar chains are a common choice.

Figure 1.5

Schematic representation of how a surfactant stabilises direct and inverse systems (top) and chemical structures of common surfactants used in direct and inverse emulsions (bottom).

Figure 1.5

Schematic representation of how a surfactant stabilises direct and inverse systems (top) and chemical structures of common surfactants used in direct and inverse emulsions (bottom).

Close modal
In general, the suitability of surfactants for stabilising direct (oil-in-water) or inverse (water-in-oil) systems relies on the so-called hydrophilic–lipophilic balance (HLB). The initial HLB index was introduced by Griffin for non-ionic emulsifiers,30 and it was defined as an arbitrary number between 0 and 20 calculated according to the formula
(1.13)

where Mhydro is the molar mass of the hydrophilic segment of the surfactant and Mtotal is the total molar mass, including the hydrophilic and the lipophilic segments. An index between 0 and 8 indicates that the surfactant is oil soluble and suitable for stabilising water-in-oil systems, and surfactants with indices above 11 are water soluble and normally good as oil-in-water emulsifiers. Indices between 8 and 11 indicate poor solubility in both oil and water, and may result in the formation of lamellar structures or vesicles.

In 1957, Davies proposed an alternative method to calculate the HLB index, which could also be applied to ionic surfactants,31 
(1.14)

where m and n are the number of hydrophilic and lipophilic groups, respectively, and Hhydro and Hlipo are arbitrary values assigned to the different chemical groups, of hydrophilic and lipophilic (i.e., hydrophobic) nature, respectively. Table 1.2 lists the H values proposed by Davies for some hydrophilic and lipophilic groups, making some special considerations for ether groups (ethylene oxide is hydrophilic, while propylene oxide is considered lipophilic). It should be noted that eqn (1.14) may sometimes be found in the literature with a negative sign before the lipophilic term; in this case, the values in Table 1.2 for lipophilic groups should be positive instead of negative, so that the final result for the HLB index is the same.

Table 1.2

Values proposed by Davies for some hydrophilic and hydrophobic groups for the calculation of HLB indices31 

Chemical groups Group value (H
–OSO3 Na+ 38.7 
–COO K+ 21.1 
–COO Na+ 19.1 
N (tertiary amine) 9.4 
Ester (sorbitan ring) 6.8 
Ester (free) 2.4 
–COOH 2.1 
–OH (free) 1.9 
–OH (sorbitan ring) 0.5 
–O– 1.3 
=CH– −0.475 
–CH2– −0.475 
–CH3 −0.475 
–(CH2–CH2–O)– +0.33 
–(CH2–CH2–CH2–O)– −0.15 
Chemical groups Group value (H
–OSO3 Na+ 38.7 
–COO K+ 21.1 
–COO Na+ 19.1 
N (tertiary amine) 9.4 
Ester (sorbitan ring) 6.8 
Ester (free) 2.4 
–COOH 2.1 
–OH (free) 1.9 
–OH (sorbitan ring) 0.5 
–O– 1.3 
=CH– −0.475 
–CH2– −0.475 
–CH3 −0.475 
–(CH2–CH2–O)– +0.33 
–(CH2–CH2–CH2–O)– −0.15 

Although HLB values provide indications related to application in direct and inverse systems, in practice the identification of suitable surfactants is slightly more complex, especially for microemulsions. Surfactants such as cetyltrimethylammonium bromide (CTAB), which is ionic and with a HLB index above 20, can be used in water-in-oil microemulsions.32–34 In miniemulsions, in contrast to microemulsions, high or low HLB values are quite a good estimation of the suitability for oil-in-water or water-in-oil systems, respectively.

As a consequence of eqn (1.1), the Laplace pressure for droplets of different size will also be different. Smaller droplets (with higher radius of curvature) will have a higher pressure difference between the inner and the outer part, being energetically less favoured than bigger droplets, for which the pressure difference is lower. Thermodynamically, the system will tend to compensate these differences, and smaller droplets will merge with bigger ones, leading to, in the limiting situation, phase separation and destabilisation of the emulsion. For supressing—or at least minimising—Ostwald ripening, so-called osmotic-pressure agents are used in emulsions. These agents are compounds that are highly soluble in the disperse phase and nearly immiscible in the continuous phase. To understand how osmotic-pressure agents work, one has to take into account the driving force of the Ostwald ripening, which is related to the Laplace pressure difference and the different osmotic pressure of droplets of different size: smaller particles (higher radius of curvature) are more soluble than larger ones (with lower radius of curvature), so that smaller droplets dissolve with time and diffuse to the bulk, to eventually be integrated in larger droplets. This phenomenon, in general when a curved interface is formed, is sometimes known as the Kelvin effect, and is described by the Kelvin equation,35 
(1.15)

where S(r) is the solubility of a particle or droplet of radius r, S(∞) is the solubility of the bulk (r → ∞), γ is the surface tension of the liquid, VmL is the molar volume of the liquid, R is the gas constant and T is the absolute temperature. For efficiency, the added osmotic-pressure agent should have a lower solubility in the continuous phase than the predominant compound of the disperse phase, so that the overall solubility will be slightly lower than without it. When the predominant compound diffuses from smaller droplets to bigger ones, the solubility of the small ones will be further reduced. Consequently, the increased solubility from the Kelvin effect will be balanced by the decrease in solubility when adding the additional compound.36 In this way, the addition of a third component to emulsion systems is able to minimise Ostwald ripening. In the case of inorganic reactions within droplets, inorganic precursors themselves (e.g., inorganic salts in the case of water-in-oil emulsions) may directly have the effect of osmotic-pressure agents.

As an attempt at systematisation of the possible routes for synthesising inorganic nanoparticles from emulsions, we classify the strategies into three groups, which are schematically represented in Figure 1.6: (i) the ‘two-emulsion method’; (ii) external addition of a precipitating agent; and (iii) combination of precursors in the disperse phase, or use of single-source precursors. For completeness, some examples of strategies falling outside this classification will also be discussed at the end of this section.

Figure 1.6

Different strategies of inorganic synthesis in micro- and miniemulsions.

Figure 1.6

Different strategies of inorganic synthesis in micro- and miniemulsions.

Close modal

This method involves mixing two separate miniemulsions, one containing a first reactant (typically a metal precursor) and another containing a second reactant (normally a precipitating agent). Upon application of high shear forces (produced, for example, by ultrasound), the droplets of both components fuse, coalesce and their contents react, leading to the formation of the desired compound, or of an intermediate that eventually converts to it (e.g., hydroxide species that transform to an oxide). This strategy has been used, for example, to prepare ZnO nanoparticles by mixing two emulsions containing a zinc salt and a base, such as NaOH or ammonia.37,38

In this case, the second reactant (precipitating agent) is added externally to an emulsion in which the first reactant (metal precursor) is contained. The added reactant dissolves in the continuous phase and diffuses to the droplets containing the first component, driving the reaction. Since the interface between the droplet and the continuous phase is the first place where the two components meet, hollow structures can also be formed, as discussed in Section 1.5, depending on the reaction kinetics and how fast the diffusion takes place.22,39–41 The concentration of the precursor within the droplets can be crucial for obtaining solid or hollow particles, as reported for Ce1−xCuxO2 systems, in which hollow structures are formed at lower precursor concentrations and solid particles at higher concentrations.40 The chemistry of the precipitating agent (more or less soluble in the disperse phase, depending on polarity), as well as the rate of addition, can also determine the formation, or not, of solid particles.41 

If the reaction leading to the desired compound is sufficiently slow or requires a stimulus, such as temperature, the precursor and the second reactant can be combined in the disperse phase of a single microemulsion/miniemulsion, which is then allowed to react. The formation of some systems by a sol−gel route can take place through this method. For instance, TiO2 and zirconium-doped anatase (ZrxTi1−xO2, 0–7.1 mol% Zr) nanoparticles were prepared under acidic catalysis starting from an inverse miniemulsion containing the precursor bis(2-hydroxyethyl)titanate (C4H12O5Ti) in aqueous solution in the disperse phase.42,43 In some systems, the desired compound can be produced from just one precursor, or so-called single-source precursor. A single-source precursor is typically a metal complex that contains all necessary elements (for instance, the metal and oxygen, in the case of a metal oxide) to obtain an inorganic compound through a thermal- or light-activated decomposition process. The preparation of molybdic acid (hydrated forms of molybdenum trioxide, MoO3·nH2O) from peroxo complexes is one example of this kind of approach:
(1.16)
(1.17)
(1.18)

In the previous reactions, the peroxo species on the left-hand side decompose to produce molybdenum trioxide hydrates. Such processes have been investigated in miniemulsion systems to produce molybdic acid and, analogously, tungstic acid.44 

Another example of the use of a complex single-source precursor in a miniemulsion was reported for the preparation of Au/TiO2 materials starting with a gold-containing titanium peroxo complex with the structure AuCl4(NH4)7[Ti4(O2)4(cit)(Hcit)2]2·12H2O, which was photochemically decomposed in the confined space of the droplets.45 

In addition to the three previously described groups of strategies, a different approach for the formation of inorganic nanoparticles is the use of polymer–inorganic hybrid nanoparticles as precursors that can transform to the targeted material by oxidation or reduction of the metal (e.g., through thermolysis or plasma steps). For example, different organotin compounds were encapsulated in polystyrene nanoparticles by a miniemulsion polymerisation procedure; after thermolysis of the films formed with the resulting suspension of organotin/polystyrene nanoparticles, tetragonal SnO2 (cassiterite) nanoparticles were obtained.46 In a slightly more sophisticated way, plasma treatment can be applied to hybrid nanoparticles containing metal complexes to transform them into highly ordered arrangements of metal nanoparticles.47 In this process, hexagonal close-packed nanoparticles deposited on a surface are initially treated with oxygen plasma to remove the organic parts of the precursor particles, and subsequently thermally annealed to obtain the metal nanoparticles. This approach, depicted graphically in Figure 1.7, has been used for the preparation of platinum47–50 and Fe–Pt alloy49,51 nanoparticles. An analogous process was also reported for the preparation of ZnO nanoparticles from hybrid precursor particles containing zinc complexes.52 

Figure 1.7

Process for the generation of arrays of metal nanoparticles with hexagonal symmetry. Reproduced from ref. 6 with permission from Elsevier, Copyright 2012.

Figure 1.7

Process for the generation of arrays of metal nanoparticles with hexagonal symmetry. Reproduced from ref. 6 with permission from Elsevier, Copyright 2012.

Close modal

Most of the work in the field of inorganic synthesis in emulsions has been conducted in microemulsions. In this context, it is important to mention the pioneering work of Pileni in the application of nanodroplets and colloidal soft templates (micelles and colloidal associations) to prepare inorganic nanoparticles.53–55 Many other groups have also made significant advancements in this area, with especially remarkable contributions from Feldmann34,56–60 and Sánchez-Domínguez61–68 and their respective teams. Both groups have been very active in recent years and have prepared a wide range of materials by using microemulsions.

Since many inorganic precursors are water soluble, it is common to use inverse emulsions, that is, water-in-oil microemulsions in which the disperse phase is aqueous (or based on a polar solvent or mixture of polar solvents) and the continuous phase is apolar. In this case, the inorganic precursor is solubilised in the aqueous phase, with inorganic salts being very common reagents. However, oil-soluble precursors, such as metal acetylacetonates, can also be used in direct (i.e., oil-in-water) emulsions for the synthesis of inorganic compounds.61–63,69,70

As reported in Table 1.3, a large number of inorganic systems have been prepared in microemulsions, including metal oxides, mixed oxides, metal hydroxides, chalcogenides and more complex systems.59,65,71 For the case of miniemulsions, the diversity of inorganic substances prepared is slightly more modest, but it has been increasing in the last few years, in parallel with the development of the technique for inorganic synthesis. Most of the substances prepared have been metal oxides (see Muñoz-Espí and Landfester9), but some chalcogenides (e.g., CuS72 and ZnS73) have also been reported. The preparation of ternary systems is also possible, including, among others, Ca(OH)2 : Ln and Mg(OH)2 : Ln (Ln = EuIII, SmIII, TbIII),74 lanthanide-doped CaF2,75 MFe2O4 (M: CoII, CuII, NiII, ZnII),18 YCrO376 or Ce1−xCuxO2.40 

Table 1.3

Inorganic nanoparticles prepared by using nanodroplets as templates. The works are listed in alphabetical order of the inorganic system

System Precursor Continuous phase Type of emulsion Surfactant Reference 
Ag2AgNO3 and Na2Water and ethanol Microemulsion Linoleic acid 88  
Ag2Se AgNO3 and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
Au/TiO2 Single-source precursora Pentanol/Heptane Miniemulsion SDSb and Triton X100 45  
BaCrO4 Ba(NO3)2 and K2CrO4 Water and Ethanol Microemulsion Linoleic acid 88  
BaO BaCl2 Hexane Miniemulsion CTATosc 89  
CaCO3 CaCl2 and (NH4)2CO3 Cyclohexane Microemulsion CTABd 32  
CaCO3 NaHCO3 and Ca(CH3COO)2 n-Dodecane Microemulsion CTABd 34  
Ca(OH)2 : Ln and Mg(OH)2 : Lne Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Eu(NO3)3·5H2O, Tb(NO3)3·5H2O, C6H9O6Sm·xH2Cyclohexane Miniemulsion Igepal CO630, Brij® 52, Span 80 74  
CdS Cd(NO3)2 and Na2Water and ethanol Microemulsion Linoleic acid 88  
CdSe Cd(NO3)2 and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
CeO2 Ce(NO3)3·6H2Cyclohexane Miniemulsion P(E/B-b-EO)f and PIBSPg 90  
CeO2 Ce[CH3(CH2)3CH(C2H5)CO2]3 Water Microemulsion Brij® 96V or Synperonic® 10/6 61 and 62  
CePO4 Ce(NO3)3·6H2Water and ethanol Microemulsion Linoleic acid 88  
Ce0.5Zr0.5O2 Ce[CH3(CH2)3CH(C2H5)CO2]3 and Zr(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
CoFe2O4 Co2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
CoFe2O4–TiO2 C24H45FeO6, C16H30CoO4, C32H60O8Ti Water Microemulsion Synperonic® 91/5 70  
Co3O4 Co3[Co(CN)6]2 n-Dodecane Microemulsion CTABd 34  
CuxCe1–xO2–δ Ce[CH3(CH2)3CH(C2H5)CO2]3 and Cu[CH3(CH2)3CH(C2H5)CO2]2 Isooctane Microemulsion Synperonic® 91/5 67  
CuFe2O4 Cu2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
CuO Cu(CN) n-Dodecane Microemulsion CTABd 34  
CuS CuCl2 and CS(NH2)2 n-Dodecane Microemulsion CTABd 34  
CuS Cu(NO₃)₂ and Na₂S Cyclohexane Miniemulsion Brij® 52 and Igepal CO630 72  
Fe2O3 FeCl2·4H2Cyclohexane or Isopar M Miniemulsion P(S/EO)h 91  
Fe2O3 FeCl3 n-Decane Miniemulsion Glissopal EM-23 92  
Fe3Mn3O8, Fe3O4, MFe2O4i MCl2·nH2Cyclohexane Miniemulsion PGPRj 18 and 93  
Fe3O4 C24H45FeO6 Hexane Microemulsion Synperonic® 10/6 64  
Fe3O4 FeCl3 and FeCl2 n-Octane Microemulsion CTABd 64  
Fe3O4 or Fe2O3 Fe[CH3(CH2)3CH(C2H5)CO2]3 Water Microemulsion Synperonic® 10/6 62  
HoPO4 Ho(NO3)3 and NaH2PO4 Water and ethanol Microemulsion Linoleic acid 88  
K3[Co(NO2)6CoCl2, CH3COOH and KNO2 n-Dodecane Microemulsion CTABd 34  
MgAl, NiAl, and ZnAl hydrotalcites M(NO3)2·6H2O (M = Mg, Zn or Ni) and Al(NO3)3·9H2n-Butanol and isooctane Microemulsion CTABd 94  
(NH4)Y(C2O4)2 Y(NO3)3 and (NH4)2C2O4 n-Dodecane Microemulsion CTABd 34  
(NH4)3PO4(MoO3)12·4H2NaH2PO4, NH4H2PO4 or organophosphate precursorsl Cyclohexane Miniemulsion PGPRj 16  
NiFe2O4 Ni2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
NiO Ni(OCOCH3)2·4H2Isooctane Miniemulsion Aerosol-OT 95  
NiO Ni(CN)2 n-Dodecane Microemulsion CTABd 34  
PbCrO4 Pb(CH3COO)2·3H2O and K2CrO4 Water and ethanol Microemulsion Linoleic acid 88  
PbS Pb(CH3COO)2·3H2O and Na2Water and ethanol Microemulsion Linoleic acid 88  
PbSe Pb(CH3COO)2·3H2O and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
TiO2 TIPk Isopar M Miniemulsion P(E/B-b-EO)e 42  
TiO2 Ti(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
Y(OH)3 Y(NO3)3 n-Dodecane Microemulsion CTABd 34  
Zn(Mn0.5Zn0.5Fe2O4FeSO4·7H2O, MnSO4·H2O and ZnSO4·7H2Isooctane Microemulsion Synperonic® 13/6.5 66  
ZnO ZnSO4 n-Decane Miniemulsion Glissopal EM-23 96  
ZnO and M : ZnOm Zn(NO3)2·6H2Cyclohexane Miniemulsion Triton X100 37 and 97  
ZnO ZnSO4 n-Decane Miniemulsion PGPRj and Span 80 38  
ZnO Zn(AcO)2·2H2n-Decane Miniemulsion Span 20 98  
ZnO ZnCl2 n-Dodecane Microemulsion CTABd 34  
ZnS Zn(NO3)2·6H2Cyclohexane Miniemulsion Brij® 52 and Igepal CO630 73  
ZnS Zn(NO3)2·6H2O and Na2Water and ethanol Microemulsion Linoleic acid 88  
ZnO2 Zn[CH3(CH2)3CH(C2H5)CO2]2 Hexane Microemulsion Synperonic® 91/6 99  
ZrO2 Zr(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
Zr(HPO4)2nH2ZrOCl2·8H2Cyclohexane Microemulsion Igepal CO520 100  
ZrxTi1–xO2, ZrTiO4 Zr(OiPr)4·nPrOH and TIPj Isopar M Microemulsion P(E/B-b-EO)e 43  
System Precursor Continuous phase Type of emulsion Surfactant Reference 
Ag2AgNO3 and Na2Water and ethanol Microemulsion Linoleic acid 88  
Ag2Se AgNO3 and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
Au/TiO2 Single-source precursora Pentanol/Heptane Miniemulsion SDSb and Triton X100 45  
BaCrO4 Ba(NO3)2 and K2CrO4 Water and Ethanol Microemulsion Linoleic acid 88  
BaO BaCl2 Hexane Miniemulsion CTATosc 89  
CaCO3 CaCl2 and (NH4)2CO3 Cyclohexane Microemulsion CTABd 32  
CaCO3 NaHCO3 and Ca(CH3COO)2 n-Dodecane Microemulsion CTABd 34  
Ca(OH)2 : Ln and Mg(OH)2 : Lne Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Eu(NO3)3·5H2O, Tb(NO3)3·5H2O, C6H9O6Sm·xH2Cyclohexane Miniemulsion Igepal CO630, Brij® 52, Span 80 74  
CdS Cd(NO3)2 and Na2Water and ethanol Microemulsion Linoleic acid 88  
CdSe Cd(NO3)2 and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
CeO2 Ce(NO3)3·6H2Cyclohexane Miniemulsion P(E/B-b-EO)f and PIBSPg 90  
CeO2 Ce[CH3(CH2)3CH(C2H5)CO2]3 Water Microemulsion Brij® 96V or Synperonic® 10/6 61 and 62  
CePO4 Ce(NO3)3·6H2Water and ethanol Microemulsion Linoleic acid 88  
Ce0.5Zr0.5O2 Ce[CH3(CH2)3CH(C2H5)CO2]3 and Zr(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
CoFe2O4 Co2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
CoFe2O4–TiO2 C24H45FeO6, C16H30CoO4, C32H60O8Ti Water Microemulsion Synperonic® 91/5 70  
Co3O4 Co3[Co(CN)6]2 n-Dodecane Microemulsion CTABd 34  
CuxCe1–xO2–δ Ce[CH3(CH2)3CH(C2H5)CO2]3 and Cu[CH3(CH2)3CH(C2H5)CO2]2 Isooctane Microemulsion Synperonic® 91/5 67  
CuFe2O4 Cu2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
CuO Cu(CN) n-Dodecane Microemulsion CTABd 34  
CuS CuCl2 and CS(NH2)2 n-Dodecane Microemulsion CTABd 34  
CuS Cu(NO₃)₂ and Na₂S Cyclohexane Miniemulsion Brij® 52 and Igepal CO630 72  
Fe2O3 FeCl2·4H2Cyclohexane or Isopar M Miniemulsion P(S/EO)h 91  
Fe2O3 FeCl3 n-Decane Miniemulsion Glissopal EM-23 92  
Fe3Mn3O8, Fe3O4, MFe2O4i MCl2·nH2Cyclohexane Miniemulsion PGPRj 18 and 93  
Fe3O4 C24H45FeO6 Hexane Microemulsion Synperonic® 10/6 64  
Fe3O4 FeCl3 and FeCl2 n-Octane Microemulsion CTABd 64  
Fe3O4 or Fe2O3 Fe[CH3(CH2)3CH(C2H5)CO2]3 Water Microemulsion Synperonic® 10/6 62  
HoPO4 Ho(NO3)3 and NaH2PO4 Water and ethanol Microemulsion Linoleic acid 88  
K3[Co(NO2)6CoCl2, CH3COOH and KNO2 n-Dodecane Microemulsion CTABd 34  
MgAl, NiAl, and ZnAl hydrotalcites M(NO3)2·6H2O (M = Mg, Zn or Ni) and Al(NO3)3·9H2n-Butanol and isooctane Microemulsion CTABd 94  
(NH4)Y(C2O4)2 Y(NO3)3 and (NH4)2C2O4 n-Dodecane Microemulsion CTABd 34  
(NH4)3PO4(MoO3)12·4H2NaH2PO4, NH4H2PO4 or organophosphate precursorsl Cyclohexane Miniemulsion PGPRj 16  
NiFe2O4 Ni2[Fe(CN)6n-Dodecane Microemulsion CTABd 34  
NiO Ni(OCOCH3)2·4H2Isooctane Miniemulsion Aerosol-OT 95  
NiO Ni(CN)2 n-Dodecane Microemulsion CTABd 34  
PbCrO4 Pb(CH3COO)2·3H2O and K2CrO4 Water and ethanol Microemulsion Linoleic acid 88  
PbS Pb(CH3COO)2·3H2O and Na2Water and ethanol Microemulsion Linoleic acid 88  
PbSe Pb(CH3COO)2·3H2O and Na2SeSO3 Water and ethanol Microemulsion Linoleic acid 88  
TiO2 TIPk Isopar M Miniemulsion P(E/B-b-EO)e 42  
TiO2 Ti(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
Y(OH)3 Y(NO3)3 n-Dodecane Microemulsion CTABd 34  
Zn(Mn0.5Zn0.5Fe2O4FeSO4·7H2O, MnSO4·H2O and ZnSO4·7H2Isooctane Microemulsion Synperonic® 13/6.5 66  
ZnO ZnSO4 n-Decane Miniemulsion Glissopal EM-23 96  
ZnO and M : ZnOm Zn(NO3)2·6H2Cyclohexane Miniemulsion Triton X100 37 and 97  
ZnO ZnSO4 n-Decane Miniemulsion PGPRj and Span 80 38  
ZnO Zn(AcO)2·2H2n-Decane Miniemulsion Span 20 98  
ZnO ZnCl2 n-Dodecane Microemulsion CTABd 34  
ZnS Zn(NO3)2·6H2Cyclohexane Miniemulsion Brij® 52 and Igepal CO630 73  
ZnS Zn(NO3)2·6H2O and Na2Water and ethanol Microemulsion Linoleic acid 88  
ZnO2 Zn[CH3(CH2)3CH(C2H5)CO2]2 Hexane Microemulsion Synperonic® 91/6 99  
ZrO2 Zr(C8H15O2)4 Water Microemulsion Synperonic® 10/6 62  
Zr(HPO4)2nH2ZrOCl2·8H2Cyclohexane Microemulsion Igepal CO520 100  
ZrxTi1–xO2, ZrTiO4 Zr(OiPr)4·nPrOH and TIPj Isopar M Microemulsion P(E/B-b-EO)e 43  
a

Single-source precursor: AuCl4(NH4)7[Ti4(O2)4(cit)(Hcit)2]2·2H2O.

b

SDS: sodium dodecyl sulfate.

c

CTATos: hexadeciltrimetilamonium-p-toluensulfonate.

d

CTAB: cetyltrimethylammonium bromide.

e

Ln = EuIII, SmIII, TbIII.

f

P(E/B-b-EO): poly(ethylene/butylene-block-ethylene oxide).

g

PIBSP: polyisobutylene succinimide pentaamine.

h

P(S/EO): poly(styrene/ethylene oxide).

i

M: CoII, CuII, NiII, ZnII.

j

PGPR: polyglycerol polyricinoleate.

k

TIP: titanium isopropoxide.

l

Organophosphate precursors: phytic acid sodium salt hydrate, d-glucose-6-phosphate sodium salt, O-phospho-dl-serine.

m

M = AgI, CoII, CuII, EuIII, MgII, MnII.

The majority of work conducted in miniemulsions has applied water-soluble inorganic precursors (typically metal salts). Consequently, inverse (i.e., water-in-oil) miniemulsions have almost always been used. Common apolar solvents in the continuous phase are cyclohexane (b.p. 80.7 °C, which allows for easy evaporation), toluene (b.p. 100.6 °C) or oils, such as n-decane or Isopar M (b.p. >150 °C, which allows for higher synthesis temperatures, but makes it very difficult to remove the solvent).

Table 1.3 contains a collection of the inorganic compounds prepared so far at the nanoscale by using emulsion systems (both microemulsion and miniemulsion), together with a selection of representative references. We include here nanoparticles formed by precipitation within droplets. Particular cases involving interfacial precipitation to form hollow structures are considered in Section 1.5 (see Table 1.4).

Table 1.4

Inorganic nanocapsules prepared at the droplet interface in micro- and miniemulsions. The works are listed in alphabetical order of the inorganic system

System Precursor Continuous phase Emulsion type Surfactant Reference 
Ag2[Ag(Ph3P)4]NO3 Toluene Microemulsion CTABa 57  
AlO(OH) Al[OCH(CH3)C2H5]3 n-Dodecane Microemulsion CTABa 33  
CaCO3 CaCl2/Na2CO3 n-Dodecane Microemulsion CTABa 79  
Ce1–xCuxO2 Cu(NO3)2·3H2O and Ce(NO3)3·6H2Toluene Miniemulsion PGPRb 40  
CeO2 Ce(NO3)3·6H2O and FeCl2 Toluene Miniemulsion PIBSPc, PGPRb and P(S-b-AA)d 22  
CuO Cu(NO3)2·3H2Toluene Miniemulsion PIBSPc and PGPRb 39  
CuS [C6H11(CH2)3CO2]2Cu Toluene Microemulsion CTABa 58  
Cu1.8CuCl(Ph3P)3 Toluene Microemulsion CTABa 58  
Cu2CuCl(Ph3P)3 Toluene Microemulsion CTABa 58  
Fe2O3 FeCl3·6H2Toluene Miniemulsion PIBSPc, PGPRb and P(S-b-AA)d 22  
HfO2, ZrO2, HfZr1–xO2 ZrOCl2·8H2O and HfOCl2·8H2Toluene Miniemulsion PIBSPc 41  
La(OH)3 La(C5H5)3 n-Dodecane Microemulsion CTABa 56  
SiO2 Tetraethoxysilane (TEOS) Cyclohexane, n-hexane, n-octane, n-decane, n-dodecane Microemulsion CA-520 and/or Triton X100 101  
SiO2 Tetraethoxysilane (TEOS) Cyclohexane Microemulsion CTABa 102  
SiO2 Tetraethoxysilane (TEOS) Water Miniemulsion CTMA-Cle 81  
SnO2 Sn(Ot-Bu)4 n-Dodecane Microemulsion CTABa 77  
TiO2 TiCl4 n-Dodecane Microemulsion CTABa 60  
YCrO3 Cr(NO3)3·9H2O and YCl3 Toluene Miniemulsion PIBSPc 76  
ZnO Zn(C5(CH3)5)2 Toluene Microemulsion CTABa 78  
ZrO2 Zr(OtC4H9)4 n-Dodecane Microemulsion CTABa 79  
System Precursor Continuous phase Emulsion type Surfactant Reference 
Ag2[Ag(Ph3P)4]NO3 Toluene Microemulsion CTABa 57  
AlO(OH) Al[OCH(CH3)C2H5]3 n-Dodecane Microemulsion CTABa 33  
CaCO3 CaCl2/Na2CO3 n-Dodecane Microemulsion CTABa 79  
Ce1–xCuxO2 Cu(NO3)2·3H2O and Ce(NO3)3·6H2Toluene Miniemulsion PGPRb 40  
CeO2 Ce(NO3)3·6H2O and FeCl2 Toluene Miniemulsion PIBSPc, PGPRb and P(S-b-AA)d 22  
CuO Cu(NO3)2·3H2Toluene Miniemulsion PIBSPc and PGPRb 39  
CuS [C6H11(CH2)3CO2]2Cu Toluene Microemulsion CTABa 58  
Cu1.8CuCl(Ph3P)3 Toluene Microemulsion CTABa 58  
Cu2CuCl(Ph3P)3 Toluene Microemulsion CTABa 58  
Fe2O3 FeCl3·6H2Toluene Miniemulsion PIBSPc, PGPRb and P(S-b-AA)d 22  
HfO2, ZrO2, HfZr1–xO2 ZrOCl2·8H2O and HfOCl2·8H2Toluene Miniemulsion PIBSPc 41  
La(OH)3 La(C5H5)3 n-Dodecane Microemulsion CTABa 56  
SiO2 Tetraethoxysilane (TEOS) Cyclohexane, n-hexane, n-octane, n-decane, n-dodecane Microemulsion CA-520 and/or Triton X100 101  
SiO2 Tetraethoxysilane (TEOS) Cyclohexane Microemulsion CTABa 102  
SiO2 Tetraethoxysilane (TEOS) Water Miniemulsion CTMA-Cle 81  
SnO2 Sn(Ot-Bu)4 n-Dodecane Microemulsion CTABa 77  
TiO2 TiCl4 n-Dodecane Microemulsion CTABa 60  
YCrO3 Cr(NO3)3·9H2O and YCl3 Toluene Miniemulsion PIBSPc 76  
ZnO Zn(C5(CH3)5)2 Toluene Microemulsion CTABa 78  
ZrO2 Zr(OtC4H9)4 n-Dodecane Microemulsion CTABa 79  
a

CTAB: cetyltrimethylammonium bromide.

b

PGPR: polyglycerol polyricinoleate.

c

PIBSP: polyisobutylene succinimide pentaamine.

d

P(S-b-AA): poly(styrene-block-acrylic acid).

e

CTMA-Cl: cetyltrimethylammonium chloride.

Most studies on the use of micro- and miniemulsions for confining inorganic synthesis have been conducted under ambient pressure. In the last few years, some studies have started to explore the combination of miniemulsions with hydrothermal conditions (i.e., temperatures above room temperature and pressures above the ambient one). In other words, miniemulsions are placed under hydrothermal conditions, so that there is a double pressure effect: the intrinsic pressure resulting from the formation of the curved interface of the droplets (Laplace pressure) and the external one applied during the hydrothermal reaction. Different spinel ferrites, including Fe3MnO8, CoFe2O4, CuFe2O4, NiFe2O4 and ZnFe2O4 have been prepared under hydrothermal conditions by adding NaOH to an inverse emulsion of the metal precursors (metal salts) and ultrasonicating again to allow coalescence and precipitation.18 This study demonstrated that for some of the zinc ferrite systems, the materials obtained by combining miniemulsion and hydrothermal conditions resulted in much more crystalline ferrites than the analogous ones obtained by miniemulsions at ambient pressure or under bulk conditions, either at ambient pressure or under hydrothermal conditions.

Nanocapsules of inorganic materials can be produced when the formation reaction of the target substance takes place at the liquid–liquid droplet interface. A variety of metal oxides, metal hydroxides and metal chalcogenides have been produced in microemulsion in the form of hollow particles.33,56–58,60,77–79

In miniemulsions, one of the most common inorganic capsular systems is silica, prepared by a sol‒gel process starting at the droplet interface. In a typical oil-in-water formulation, a silicon alkoxide, such as tetraethyl orthosilicate (TEOS), is dissolved in an oil (e.g., octane) and dispersed in a continuous aqueous phase.80 The alkoxide groups of the silane precursor are hydrolysed to silanol groups upon contact with water under acidic or basic catalysis. A silica network is produced when the silanol groups condense with each other, leading to the capsule formation. The surface chemistry of the capsule can be tuned by adding functionalised silanes (such as 3-aminopropyltriethoxysilane or 3-mercaptopropyltrimethoxysilane) during the process.81 The strategy is similar to the copolymerisation procedures used to prepare functionalised polymer particles with hydrophilic monomers in miniemulsion polymerisation.82 The silanes with polar functional groups tend to be placed at the surface of the resulting particles.

An analogous strategy for the preparation of silica can also be carried out in inverse systems.83 In this case, the silicon alkoxide is added to the apolar continuous phase, and hydrolyses upon contact with the water droplets of the disperse phase. This situation, together with the formation reactions of silica by hydrolysis and condensation of silanes, is presented in Figure 1.8. By adding silanes with different polarities during the synthesis, the surface hydrophobicity of the resulting capsules can be adjusted.84 

Figure 1.8

Scheme of the formation of silica nanocapsules in inverse miniemulsions. The reactions present the hydrolysis and condensation of a general alkoxide with rest R. Reproduced from ref. 85, https://doi.org/10.1016/j.colsurfb.2021.111764, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 1.8

Scheme of the formation of silica nanocapsules in inverse miniemulsions. The reactions present the hydrolysis and condensation of a general alkoxide with rest R. Reproduced from ref. 85, https://doi.org/10.1016/j.colsurfb.2021.111764, under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Close modal
In a similar fashion to silica, sol–gel processes for group 4 metals (Zr, Hf) have also been carried out in inverse miniemulsions to yield hydrous zirconia (ZrO(OH)2·nH2O) and hafnia (HfO(OH)2·nH2O).41 Metal oxychloride precursors (ZrOCl2·8H2O and HfOCl2·8H2O) are dissolved in water and emulsified in toluene. A schematic representation of the mechanism is depicted in Figure 1.9. Triethylamine (Et3N), a base that is soluble in organic solvents, but also in water to a certain extent, is added to the continuous phase, generating hydroxide ions at the interface and within the droplets:
(1.19)
Figure 1.9

Scheme of the formation of zirconia and hafnia capsules by interfacial precipitation in the presence of triethylamine. Reproduced from ref. 41 with permission from the Royal Society of Chemistry.

Figure 1.9

Scheme of the formation of zirconia and hafnia capsules by interfacial precipitation in the presence of triethylamine. Reproduced from ref. 41 with permission from the Royal Society of Chemistry.

Close modal
The oxychloride precursor reacts with the hydroxide ions to form zirconia or hafnia:
(1.20)
(1.21)

For typical sol–gel systems, such as silica, zirconia, hafnia or titania, the materials produced are amorphous and only crystallise upon calcination. By contrast, crystallisation pathways can occur under mild miniemulsion conditions for various transition metal oxide and lanthanide oxide systems, including γ-Fe2O3, CeO2 and CuO.22,39

Some of the most representative systems reported in the literature for the production of capsules in micro- and miniemulsions are listed in Table 1.4. This compilation complements and completes the inorganic systems given in Table 1.3.

In this chapter we have seen that nanodroplets in miniemulsions and microemulsions can create enclosed areas where chemical reactions can occur. For this reason, nanodroplets can be considered as ‘nanoreactors’, especially for miniemulsion systems, in which the diffusion between droplets is supposed to be ideally suppressed. The formation of the curved interface at the droplet surface provides an intrinsic increase of the inner pressure, which may be advantageous for obtaining certain crystal phases under milder temperature conditions. Unusual crystallisation pathways may be pursued under such conditions, even allowing the potential stabilisation of certain metastable phases. The solubility of the precursor and some properties (e.g., vapour pressure, viscosity, relative permittivity) of the solvent in the disperse phase, most commonly—but not only—water in the case of inorganic systems, may also be significantly affected by the very small size of the droplets and the related confinement. The confinement can be utilised to tune the size of the particles obtained. In the simplest case, the crystalline size and the number of crystalline domains that can aggregate to form polycrystalline particles will be influenced by the restricted space available for crystallisation.

We have classified the methods for producing inorganic materials in emulsions in three groups: (1) the ‘two-emulsion method’, based on the coalescence of droplets containing the two reactants leading to the desired compound; (2) addition of the second reactant to an emulsion containing the first reactant in the disperse phase; and (3) emulsions in which both reactants are simultaneously present in the disperse phase or, alternatively, emulsions containing a single-source precursor that yields, upon thermally or photochemically induced decomposition, the target materials. Furthermore, besides these strategies, we have also seen that highly structured arrangements of metal oxide and metal nanoparticles can be produced by oxidation or reduction of films of polymer/inorganic precursor particles, after thermolysis or plasma treatment to remove the organic matter, to achieve the desired compound.

In addition to the confinement within droplets to form nanoparticles, the space constriction can also take place at the droplet interface, leading to the formation of hollow structures or capsules. The variety of inorganic compounds reported in the literature is very broad, including silica, metal oxides, hydroxides, chalcogenides, phosphates or even ternary or more complex systems.

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