3.1. Analysis of the Water Droplets in Emulsions
In order to study the effect of space limitation on product crystallization, water droplets in emulsions were first analyzed. In this work, cyclohexane containing Span 80 was used as the continuous phase. Water was used as the dispersed phase. Span 80 was selected as a nonionic emulsifier due to its low hydrophilic−lipophilic balance (HLB = 4.3), which tends to form inverse emulsion [
19]. Ultrasound was employed to generate shear forces to promote the emulsification of the two phases. To resist Ostwald ripening, NaCl was used as co-stabilizer in the water phase of the miniemulsion because of its strong hydrophilia.
The droplet size distributions of macroemulsion and miniemulsion were measured immediately after emulsification (unless mentioned otherwise, the mass fraction of emulsifier for the macroemulsion and miniemulsion was 1 wt % and 3 wt %, respectively). The results are shown in
Figure 2a. The average particle sizes of these two emulsions were 612.3 nm and 251.8 nm, respectively. Further, the PDI’s (Polymer dispersity index) were 0.507 and 0.195, respectively, suggesting that droplets in miniemulsion had a narrower particle size distribution. Besides, the two emulsions differed greatly in stability, as shown in
Figure 2b. The macroemulsion was obviously stratified in 3 h, while the miniemulsion could remain un-stratified for more than 3 days.
3.2. Determination of the Reaction Conditions in Emulsions
The synthesis of PR 146 red 146 is divided into two steps. A diazonium salt KD is formed by adding sodium nitrite to hydrochloric acid containing 3-Amino-4-methoxybenzanilide (
Scheme 1a). Then, PR 146 is synthesized in water by coupling the diazonium salt KD with naphthol AS-LC (
Scheme 1b).
To make sure that the reaction will happen in water droplets, the reactant KD should be confined to the dispersion phase. Therefore, a model experiment consisting of surfactant-free two-phase system was carried out to verify that KD will not diffuse into the continuous phase. As shown in
Figure 3a, the water phase containing KD is yellow before mixing. For the surfactant-free two-phase system, after half an hour of ultrasound and 3 min of rest, the oil phase was almost colorless, indicating that KD was kept in the water phase. Additionally, the color of the macroemulsion is uniform because water droplets were evenly dispersed in the organic phase. The same conclusion can be drawn from the microscopic photograph of macroemulsion containing reactant KD (
Figure 3b). KD is insoluble in cyclohexane and no crystals can be found in the continuous phase. In the meantime, it can be seen from the photograph that diameters of most droplets in the macroemulsion were between 500–2000 nm, which is consistent with the results of dynamic light scattering.
Another reactant Naphthol AS-LC is also insoluble in cyclohexane. It was dispersed in the continuous phase before the reaction. The results of ultrasonic dispersion of naphthol AS-LC in cyclohexane are shown in
Figure 4. The particles were evenly distributed, which is conducive to diffuse into water droplets under stirring.
3.3. Confined Crystallization within Emulsion Droplets
To explore the effect of space confinement on product crystallization, PR 146 products were prepared from three different crystallization methods: The bulk solution crystallization, macroemulsion crystallization, and miniemulsion crystallization. The particle sizes of the obtained PR 146 by different methods were analyzed, and the results are shown in
Figure 5. It can be found that the particle size distribution of PR 146 obtained from miniemulsion was the smallest while most uniform, and all particles were below 500 nm. Most particles of PR 146 obtained from macroemulsion were in the range of 300–1000 nm. Additionally, PR 146 obtained from the bulk solution had the biggest particles with size ranging between 500 and 6000 nm. It could be found that the size distribution of PR 146 was related to the crystallization space. PR 146 products, obtained from the miniemulsion, can meet the size demand of inkjet ink.
X-ray diffraction (XRD) data can provide information not only about crystal form and crystallinity of the crystalline materials, but also about their crystallite size. The diffractograms of pigments produced from bulk solution, macroemulsion, and miniemulsion are shown in
Figure 6. It can be found that all the products have the same XRD patterns, indicating that all products crystallized from different methods were the same crystal form. Furthermore, the crystallinity of the products obtained from macroemulsion or miniemulsion is nearly the same with the crystallinity of product from bulk solution. This is one advantage of macroemulsion or miniemulsion over microemulsion, which tends to form an amorphous product for its minuscule space.
The values of crystallite size, determined by the Scherrer formula [
20], are shown in
Table 1. It can be found that crystallite sizes of the products obtained by the three ways differed little. By comparing crystallite sizes and particle sizes determined by dynamic light scattering, it can be concluded that the particles crystallized from bulk solution were composed of hundreds of crystallites, whereas the size of the particles crystallized from macroemulsion and miniemulsion were comparable with ten to twenty times the crystallite size.
The morphology and particle size of PR 146 prepared from different solutions were also analyzed by SEM, and the results are shown in
Figure 7. The morphology of the three products were all flaky or rod-like. Compared with the other two products, PR 146 products obtained from miniemulsion were smaller in size and more uniform in size distribution. Additionally, PR 146 products obtained from bulk solution obviously had a larger size.
3.4. Process Mechanism for Crystallization in Confined Space
The processes mechanism of PR 146 crystallization confined in miniemulsion is described in
Scheme 2. Inverse emulsion was formed under the function of ultrasound. The water droplets were covered by surfactants to prevent them from coalescing. The reactant KD and co-stabilizer NaCl were dissolved and confined to water droplets. Then, the organic phase containing naphthol was added. Because of its high solubility in water, the reactant naphthol AS-LC can cross the liquid-liquid interface and dissolve into the water droplets. With the diffusion of naphthol, the precipitation could occur at the liquid–liquid interface or inside the droplets. Nucleation and growth of the crystal were confined by the small space, thus forming nano-scale PR 146. The separation of the product was accomplished by subsequent centrifugation steps. The crystallization processes in the macroemulsion were similar with that shown in
Scheme 2, except that no co-stabilizer was used.
In the process of emulsification, the high shear forces can generate small water droplets in the presence of emulsifier. However, the Laplace pressure (i.e., the pressure difference between the inner and outer part of a curved surface caused by the surface tension at the liquid-liquid interface) is higher for small droplets than for bigger ones, which results in the disappearance of small droplets (i.e., Ostwald ripening). The immediate cause is that water molecules diffuse across the interface from small droplets to larger ones. As a result, the droplet size distribution of the macroemulsion eventually widens and becomes larger. In our work, the additional sodium chloride was used for the formation of the miniemulsion to resist Ostwald ripening. The function mechanism of co-stabilizer is described in
Scheme 3. For the miniemulsion, the hydrophilic substance (i.e., sodium chloride) can counteract the pressure difference as an osmotic pressure agent and prevents the migrating of water from small droplets into big droplets [
21]. When two water droplets come into contact, the interface is equivalent to a semipermeable membrane due to the presence of a surfactant. Water molecules diffuse from small to large droplets under the Laplace pressure described above. This increases the concentration of sodium chloride in the smaller droplets, allowing water molecules to diffuse from the larger droplets to the smaller ones under osmotic pressure. The two effects are balanced to keep the droplet size at the nanometer scale. These nanometer scale spaces act as nanoreactors and allow the control of both the nucleation and growth of products due to the confinement space of the crystallization process.
3.5. Effect of Emulsifier and Co-Stabilizer Dosage
The sizes and distributions of droplets in macroemulsions containing different dosages of emulsifier (i.e., Span 80) were analyzed, and the results are shown in
Figure 8. The amounts of Span 80 in the three groups were 1 wt %, 5 wt % and 10 wt %, respectively. It could be found that size distributions of these three macroemulsions were similar and inhomogeneous. They all have some droplets of 4–6 micron in size, caused by the Ostwald ripening. The average sizes were 612.3 nm, 711.4 nm, and 595.4 nm, respectively. The difference of average particle size was mainly caused by inhomogeneous size distribution. It seems that the dosage of emulsifier had little effect on the particle size distribution of macroemulsion. It should also be noticed that all three groups of macroemulsions were unstable during the experiments.
The size distributions of PR 146 products prepared from macroemulsions containing 1 wt %, 5 wt % and 10 wt % Span 80 were also analyzed, and the results are shown in
Figure 9. It could be found that there is no significant difference in particle size distribution between the three experiments, and the average particle sizes of the obtained products were 378.2 nm, 361.4 nm, and 384.5 nm, respectively. Since the amount of emulsifier in the range of 1–10 wt % had little effect on the water droplet size of macroemulsion, it is reasonable that it had little effect on the size of the obtained particles.
Similarly, the size distributions of droplets in miniemulsions containing different dosages of emulsifier were analyzed, and the results are shown in
Figure 10. The amounts of Span 80 in the three groups were 1 wt %, 3 wt %, and 5 wt %, respectively. The amount of co-stabilizer (i.e., NaCl) were all 1.5 wt %. It could be found that the size distributions of the three miniemulsions had little difference, although the size of the droplets from the system containing 3 wt % emulsifier was smaller. The average sizes of different droplets were 299.8 nm, 251.8 nm, and 271.4 nm, respectively.
Additionally, the particle size distributions of PR 146 products prepared from miniemulsions containing different dosages of emulsifier were analyzed, and the results are shown in
Figure 11. The amount of emulsifier and co-stabilizer was the same as that in the droplets experiment. It could be found that the particle size distribution of pigment prepared with 3 wt % Span 80 was uniform, with most particle sizes ranging from 70 nm to 400 nm, and the average particle size was 172.5 nm. However, when the dosage of Span 80 changed into 1 wt % or 5 wt %, the average particle size increased by about 100 nm, and the number of large particles with a size over 300 nm was also increased obviously. Compared with the conclusion of miniemulsion droplet size, it can be inferred that the small change of miniemulsion droplet size can significantly affect the product size. The results indicated that the emulsifying effect was the best when the dosage of emulsifier was 3 wt %, and the pigment with the best particle size distribution can be obtained.
According to the formation mechanism of miniemulsion, the amount of co-stabilizer could also influence the size of droplets of miniemulsion and hence influence the properties of the final product. Therefore, the effect of different doses of co-stabilizer on the droplets in miniemulsion was also investigated. In all experiments, the amount of emulsifier was set constantly at 3 wt %. The results are shown in
Figure 12. It could be found that the co-stabilizer effect was significant. A content of only 0.5 wt % could reduce the droplet’s average particle size below 400nm, and the droplet’s particle size was further decreased with the increasing of the content. It could be inferred from the formation mechanism that increased co-stabilizer content will result in higher osmotic pressure under the same conditions, which will counteract Laplace pressure between large and small droplets and therefore prevent droplets from becoming larger.
Next, the effect of different doses of co-stabilizer on the product was also investigated, and the results are shown in
Figure 13. With the increasing of the amount of co-stabilizer, average particle size of products decreased firstly and reached the minimum value as the amount of co-stabilizer was 1.5 wt %. Then, average particle size increased significantly after the amount of co-stabilizer exceeded 1.5 wt %. Co-stabilizer NaCl can stabilize the water droplets by counteracting the pressure difference and allows the crystallization process to occur in nanoscale space. When the amount of NaCl was increased, the water droplets became smaller and more stable, which will help to suppress the growth of crystals. However, when the amount of NaCl was too high, the solubility of KD in water decreased, and part of the reactant precipitated. In the mixing process, part of the reaction will then occur in the continuous phase, and the space confinement effect will be weakened, resulting in larger particle size.