**2.3 Nonionic surfactants**

*Microemulsion - A Chemical Nanoreactor*

obtained by Massart's procedure.

novel nanostructures.

proportion of microemulsion containing Fe2+ to microemulsion containing OH<sup>−</sup> was 2:1. The sample was called S2 [17]. To compare, by Massart's procedure, S3 has been obtained [18]. Magnetic iron oxide NPs (MIONPs) were effectively formed at r.t. with microemulsion method. The MIONPs showed spherical shape. From the results of TEM, AGM, and XRD, the crystallite, physical, and magnetic sizes of the MIONPs were <10 nm. The mean physical size for S1, S2, and S3 is 6.5, 4.2, and 8.7 nm, respectively. The rise of the total number of microemulsion produced smaller particles. Particles produced by microemulsion procedure were smaller in size and showed higher saturation magnetization, compared to the particles

Shen et al. [19] prepare CTAB-stabilized gold nanowires and nanoparticles with a networked structure and different shapes using in situ *n*-butanol reduction in CTAB/n-butanol/n-heptane/HAuCl4(aq) through microwave dielectric heating (XRD, UV-vis, and TEM). The shape of the hydrophobic gold nanocrystals was effectively modulated tuning of CTAB/HAuCl4 ratios. Anisotropic gold nanostructure formation mechanism was deliberated, which confirmed that CTAB (**Figure 3**)

With the decrease of *W*, the shapes of gold nanocrystals obtained by this method changed from sphere-like decahedron nanoparticles to nanowires with a networked structure. It was revealed that AuCl4− cation and ion of surfactant CTAB played key roles on formation and stabilization of the shape of gold nanowires. The attracting force among gold nanoparticles, which also caused an orienting growth of gold nucleus, was caused by the preferential adsorption of AuCl4− and the selective adsorption of CTAB on the facets of preliminary gold particle surface. This method novel and simple for synthesizing gold nanowires with a networked structure is expected to be appropriate to the synthesis of other metals to attain

Sharma et al. [20] reported a detailed and complete study on the growth kinetics of iron oxalate nanorods as they formed throughout several days inside the water pool of CTAB microdroplets (MDs) in isooctane. Authors underlined the novelty of this experimental study on nanostructure preparation in microemulsion-based reactions that could explain the key role of droplet interplay in the reaction kinetics. DLS, TEM, and FCS characterization was completed throughout some days to follow the whole growth kinetics of iron oxalate nanorods beginning from their particle nucleation. Considering the FCS data with an appropriate kinetic model, the droplet-fusion time or dimer lifetime 28 μs was obtained along with the droplet melting rate and the equilibrium constant of the chemical reaction. The droplet association rate exhibited a noteworthy time dependency that straight relates it to the growth mechanism. Combining FCS, DLS, and TEM, three different periods in the complete nanorod growth kinetics appeared: (i) a prolonged nucleationdominant NP growth period, then (ii) a short period where isotropic NPs shifted to anisotropic growth to form nanorods, and (iii) finally the period where dropletfusion-assisted elongation of nanorods was proved. The detailed methodologies discussed in this study could be applied to understand the growth kinetics of nanostructures, which are required for various applications in nanoscience and

perform a preeminent key role in the drummed-up gold nanowires.

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**Figure 3.**

nanotechnology.

*CTAB chemical structure.*

Here I review the literature about the use of nonionic surfactants as stabilizer/ emulsifier of the microemulsion involved on the synthesis of MNPs by microemulsion method.

Sanchez-Dominguez et al. [21] reported a novel and direct approach to synthesize inorganic NPs at ambient conditions using *o/w* microemulsions to dissolve organometallic precursors. Addition of reducing or oxidizing/precipitating agents implies the formation of metallic or metal oxide nanoparticles, respectively. The authors chose nonionic aggregates and several strategic compositions for nanoparticle synthesis at 25°C. HREM shown that small metal NPs (Pd, Pt, and Rh) and nanocrystalline metal oxide NPs (Ce(IV) oxide with cubic crystalline structure confirmed by XRD) of <7 nm can be obtained at atmospheric pressure and 25°C.

Capek reviewed water-in-oil microemulsion method to prepare metal NPs [22]. This technique allows preparation of ultrafine metal particles ranging in the size diameter between 5 and 50 nm. Author reviewed the previous literature on particle synthesis of various metals such as silver, copper, cadmium, cobalt, nickel, cadmium, and gold in the reverse microemulsion systems. The precursor metal salts and reducing agents are mainly water-soluble molecules, and consequently the MNP nuclei formation progresses in the water microdroplets. The rate of the nuclei formation of the particle depends on the percolation degree of microemulsion droplets. Effects of stabilizer nature and concentration, the oil phase nature, reducing agent, and additive on the NP synthesis are shortened and assessed. The impact of numerous factors such as temperature, the metal salt nature, the incident light, and reaction conditions were also revised.

Tan et al. [23] reported the microemulsion application to prepare silica-coated iron oxide NPs using nonionic surfactants (Triton X-100, Igepal CO-520, and Brij-97, their chemical structure can be observed in **Figures 4**–**6**, respectively) for the preparation of microemulsions. Iron oxide NPs are formed by co-precipitation.

**Figure 4.** *Triton X-100 chemical structure.*

**Figure 5.** *IGEPAL CO-520 chemical structure.*

**Figure 6.** *Brij 97 chemical structure.*

**Figure 7.** *1-Tetradecane chemical structure.*

TEM, XRD, and SQUID magnetometry were used to assess both uncoated and silica-coated FeO NPs. Particles displayed magnetic properties near to that of superparamagnetic materials. By using this route, magnetic NPs ranging 1–2 nm and with very monodispersed in size (percentage std. <10%) were obtained. Base-catalyzed hydrolysis and the polymerization reaction of tetraethyl orthosilicate (TEOS) in microemulsion were used to produce a uniform silica coating of 1 nm enclosing the naked NPs.

Khiev et al. [24] reported the synthesis of regular shape and monodispersed nickel sulfide (NiS) NPs were made in *w/o* microemulsion system including sucrose ester (S-1170) as the nonionic surfactants (which is a nontoxic and biodegradable). NiS NPs were characterized by EFTEM, XPS, and UV-Vis-NIR. NPs showed size ranged between 3 and 12 nm.

The typical microemulsion used in their synthesis had a composition of 28, 56, and 16 wt% of tetradecane/1-butanol (see chemical structure in **Figure 7**) and of aqueous solution containing Ni(NO3)2 and Na2S, respectively. The hastened fine particles achieved from centrifugation were cleaned with absolute ethanol and distilled water leastwise fivefold so as to wash away the excess tensioactive byproducts and unreacted reagents. The products were dried in vacuum oven for 16 h at 50°C till a constant weight was attained. It was denoted that the size of the NPs enhanced with the reactants' concentration due to the fusion of nuclei. The presence of quantum confinement effect was apparent for the resultant NPs as the likely bandgap energy exhibited noteworthy increase. Sugar-ester-based nonionic microemulsion offers proper micro-medium to formulate NPs with narrow size distribution and high uniformity.

Pine-Reyes and Olvera reported [25] the use of human and environmentally friendly *w/o* microemulsions to synthesize ZnO NPs based on a not complex procedure (Span 80 and Tween 80 mixture—**Figures 8** and **9**). The method lets achieving NPs with a mean size of approximately 31.2 nm, with low range size variation and pseudo-spherical morphology. XRD, SEM, and TEM confirmed ZnO hexagonal wurtzite phase. The microemulsion organic phase was formed by the mixture of different proportions of emu oil and surfactant (1:1, Span 80, Tween 80) (see their chemical structures in **Figures 8** and **9**, respectively). Next, an aqueous solution of ZnAc (0.5 M) was slowly drop wised to the previous mixture. The systems were mixed under magnetic stirring (1200 rpm, 5 min) and later stored 24 h at r.t. The ZnO powders were synthesized by the drop-wised addition of a (aq) solution of NaOH (1.0 M) to previous prepared microemulsion. NaOH acted as precipitating agent. The mixture was maintained at 60°C and stirred at 1200 rpm for 5 min. In this step, NaOH diffused into the continuous phase achieves the oil-solution interface, which eased the formation of ZnO. Then, mixtures were successively washed by centrifugation at 7500 rpm in water, hexane, and acetone to separate oil and surfactant residues and byproducts from precipitates. Finally, the precipitates were dried during 1 h at 100°C and then calcined by 2 h at 800°C.

Sample M10 with 15% ZnAc, 55% surfactant, and 30% oil exhibits the smallest average particle size, 31.2 nm, and a distribution of 41.2 nm. SEM and TEM micrographs have demonstrated a pseudo-spherical morphology. XRD analysis confirms that samples presented a hexagonal wurtzite phase with preferential growth

**83**

particle.

**Figure 9.**

**Figure 8.**

*Span 80 chemical structure.*

**3. Silica nanoparticles**

*Tween 80 chemical structure.*

silica by means of *w/o* microemulsion method.

*Synthesis of NPs by Microemulsion Method DOI: http://dx.doi.org/10.5772/intechopen.80633*

direction [101]. The results showed that this method is a suitable pathway for ZnO NPs synthesis. Moreover, it is a simple, direct, and cost-effective method for obtaining nanostructured materials with small particle size and narrow size distribution

Here we focus on a metalloid and the use of microemulsion method to obtain silica NPs. Yamauchi et al. [26] reported the synthesis of ultramicroparticles of

Shantz et al. reported [27] synthesis of silicalite-1 nanocrystals in AOT-based microemulsion. They demonstrated that anionic microemulsions drive to essentially distinct crystal patterns than the nonionic [28] or cationic microemulsions formerly explored. The authors concluded that AOT and SDS anions synthesized by microemulsion method were employed to crystallize silicalite-1 nanocrystals with well-defined shapes and structures. Box, disk-like, spherical, and twinned nanocrystals were found under distinct experiential circumstances. It was also shown that tensioactive

*Synthesis of NPs by Microemulsion Method DOI: http://dx.doi.org/10.5772/intechopen.80633*

*Microemulsion - A Chemical Nanoreactor*

TEM, XRD, and SQUID magnetometry were used to assess both uncoated and silica-coated FeO NPs. Particles displayed magnetic properties near to that of superparamagnetic materials. By using this route, magnetic NPs ranging 1–2 nm and with very monodispersed in size (percentage std. <10%) were obtained. Base-catalyzed hydrolysis and the polymerization reaction of tetraethyl orthosilicate (TEOS) in microemulsion were used to produce a uniform silica coating of 1 nm enclosing the

Khiev et al. [24] reported the synthesis of regular shape and monodispersed nickel sulfide (NiS) NPs were made in *w/o* microemulsion system including sucrose ester (S-1170) as the nonionic surfactants (which is a nontoxic and biodegradable). NiS NPs were characterized by EFTEM, XPS, and UV-Vis-NIR. NPs showed size

The typical microemulsion used in their synthesis had a composition of 28, 56, and 16 wt% of tetradecane/1-butanol (see chemical structure in **Figure 7**) and of aqueous solution containing Ni(NO3)2 and Na2S, respectively. The hastened fine particles achieved from centrifugation were cleaned with absolute ethanol and distilled water leastwise fivefold so as to wash away the excess tensioactive byproducts and unreacted reagents. The products were dried in vacuum oven for 16 h at 50°C till a constant weight was attained. It was denoted that the size of the NPs enhanced with the reactants' concentration due to the fusion of nuclei. The presence of quantum confinement effect was apparent for the resultant NPs as the likely bandgap energy exhibited noteworthy increase. Sugar-ester-based nonionic microemulsion offers proper micro-medium to formulate NPs with narrow size distribution and

Pine-Reyes and Olvera reported [25] the use of human and environmentally friendly *w/o* microemulsions to synthesize ZnO NPs based on a not complex procedure (Span 80 and Tween 80 mixture—**Figures 8** and **9**). The method lets achieving NPs with a mean size of approximately 31.2 nm, with low range size variation and pseudo-spherical morphology. XRD, SEM, and TEM confirmed ZnO hexagonal wurtzite phase. The microemulsion organic phase was formed by the mixture of different proportions of emu oil and surfactant (1:1, Span 80, Tween 80) (see their chemical structures in **Figures 8** and **9**, respectively). Next, an aqueous solution of ZnAc (0.5 M) was slowly drop wised to the previous mixture. The systems were mixed under magnetic stirring (1200 rpm, 5 min) and later stored 24 h at r.t. The ZnO powders were synthesized by the drop-wised addition of a (aq) solution of NaOH (1.0 M) to previous prepared microemulsion. NaOH acted as precipitating agent. The mixture was maintained at 60°C and stirred at 1200 rpm for 5 min. In this step, NaOH diffused into the continuous phase achieves the oil-solution interface, which eased the formation of ZnO. Then, mixtures were successively washed by centrifugation at 7500 rpm in water, hexane, and acetone to separate oil and surfactant residues and byproducts from precipitates. Finally, the precipitates were dried during 1 h at 100°C and

Sample M10 with 15% ZnAc, 55% surfactant, and 30% oil exhibits the smallest average particle size, 31.2 nm, and a distribution of 41.2 nm. SEM and TEM micrographs have demonstrated a pseudo-spherical morphology. XRD analysis confirms that samples presented a hexagonal wurtzite phase with preferential growth

*1-Tetradecane chemical structure.*

ranged between 3 and 12 nm.

**Figure 7.**

naked NPs.

high uniformity.

then calcined by 2 h at 800°C.

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**Figure 9.** *Tween 80 chemical structure.*

direction [101]. The results showed that this method is a suitable pathway for ZnO NPs synthesis. Moreover, it is a simple, direct, and cost-effective method for obtaining nanostructured materials with small particle size and narrow size distribution particle.
