At the application of this method for the preparation of containers, the emulsion droplets serve as soft template particles at whose interface the component forming the shells of the future containers starts to precipitate because of changes in the physical or chemical parameters (composition) in the system. The same droplets contain also the substance to be loaded into containers and function therefore as their future cores. The shell-forming component is at the beginning completely dissolved in the emulsion droplets and both phases of this disperse system (emulsion) remain initially homogeneous. Several changes occurring in the system can disturb the solubility of the shell-building component and lead to start of its precipitation.

In the simplest case, changes in only one physical parameter of the system such as temperature can cause a serious lowering of the miscibility of the shell-forming component with other components in one phase of the two-phase disperse system leading to its precipitation at the droplets interface.

The decrease of the solubility of the shell-forming compound with subsequent precipitation can be also induced by changes in the phase composition—either by removal of the solvent having a good solvency for this compound (further—“solvent”) from the system or by addition of the solvent possessing a poor solvency for it (further—“nonsolvent”) to the system. Since the dispersed phase is a nonautonomous one, the phases are unequal against the independent changes of their compositions. If the shell-forming component is initially dissolved in the dispersion medium, its solubility there could be distorted almost without any effects on the composition of the dispersed phase. Moreover, a precipitation from dispersion medium is additionally favored by the presence of the emulsion droplets according to the heterogeneous nucleation mechanism that proceeds at their interface. Typical processes leading to the interfacial precipitation from the dispersion medium are evaporation of the solvent from this phase or its dilution by the nonsolvent, that is, solvent evaporation-induced precipitation (Figure 8.6), or dilution-induced precipitation.

In the spatially opposite case, precipitation starts from the interior of emulsion droplets, and triggers analogous to these for the dispersion medium can only be activated by the involving of the surrounding dispersion medium. Therefore, the solvent or nonsolvent, respectively, should at least be sparingly soluble also in the dispersion medium of the emulsion to be transferred through it from/to the emulsion droplets with the initially dissolved shell-forming component. Interfacial precipitation from the dispersed phase can additionally be influenced by curvature and confined character of its particles.

In the end, the formation of either containers with the core/shell morphology and liquid core or compact solid containers is attained.

Micro- and nanocontainers for the self-protecting anticorrosion coating containing the water-repelling agent sodium docusate (AOT) were prepared in our group,¹⁰¹ exploiting the solvent evaporation-induced interfacial precipitation in an O/W emulsion. The oil phase of the O/W emulsion was prepared by the mixing of water-immiscible dodecane with ethyl acetate (EA) sparingly soluble in water at the components ratio 5:9. In this composite solvent, ~ 9.5 wt.% of polystyrene (PS, M_w = 34 KDa) and 5 wt.% of sodium docusate, related to the total oil phase composition, were dissolved. Phosphate buffer solution with pH 7 saturated by EA at normal conditions was used as an aqueous dispersion medium of the emulsion. To this solution, 135 mg of nonionic surfactant Triton X-100 was added as emulsifying agent. Then, 10 v/v% rough O/W emulsion was prepared by means of Vortex-shaker (stage 7, for 1 min) and processed finally for 3 min with the high speed rotor-stator homogenizer Ultra-Turrax (IKA Werke, Staufen, Germany). After that, this emulsion was either left for the EA evaporation under the fume hood with continuous stirring at 150 rpm for 12 h or, to accelerate the removal of this solvent, firstly diluted ten times with Milli-Q water and then treated by the dry nitrogen bubbling for 5 h. Volatile EA was completely evaporated and an aqueous dispersion of PS micro- and nanocontainers containing a solution of AOT in dodecane was obtained. The zeta-average size of containers measured by DLS intensity (zetasizer Nano ZS Malvern, UK) was 1570 nm with PDI = 0.8 reflecting the strong polydispersity of containers visible also on the qualitative level in the Figure 8.7. Such a broad containers size distribution is a typical consequence of the emulsion preparation by means of a high speed rotor-stator homogenizer like Ultra-Turrax. Interestingly that the containers obtained only by the gradual evaporation of EA without acceleration showed lower polydispersity with PDI = 0.6 probably because of the Ostwald ripening in the initial emulsion droplet that kept in this case their liquid state longer. Several partially deformed containers with the concavities in the shells observable in the Figure 8.7 can be considered as an indirect argument for their core/shell or multicompartment morphology. On the other hand, this containers structure may be also deduced from the very low miscibility of core material (diluted docusate solution in dodecane) with the aqueous dispersion medium.

In spite of the simplicity and relatively low costs of interfacial precipitation method in its various concrete realizations, there are also a lot of challenges and drawbacks connected with some technical peculiarities of this method and with the properties of containers obtained in this manner related to their embedding into coating matrix. Solvents used for the dissolution of shell-forming component(s) have to be either completely or at least sparingly soluble in the dispersion medium, most often aqueous one. These solvents should be also volatile enough to assure a reasonable time of complete evaporation and formation of the container shell. Therefore, the application of this method in both solvent evaporation-induced precipitation and dilution-induced precipitation types will lead to serious amounts of environmentally dangerous wastes such as volatile organic compounds (VOC) or VOC-contaminated water, especially when upscaled to the industrial extent. Because of such environmental concerns, the industrial realization of the interfacial precipitation method may be additionally complicated by the necessity to install quite expensive recycling set-ups.

Another problem is that polymers used for the formation of containers shells are the preformed polymers, which should be well-soluble in several solvents of intermediate polarity ensuring the good miscibility with both aqueous and oily phases. Thus, the relatively low cross-linking degree of polymers employable for this purpose is required. However, the high solubility of shell-forming polymers in the solvents of moderate polarity excludes automatically the incorporation of containers made of these polymers by interfacial precipitation in the coatings on the basis of organic solvents and restricts their use only to water-borne coating formulations. Moreover, even in this case, the coating formulations with relatively high curing temperature can also be a subject of doubt as the low cross-linking degree of polymer will cause its melting already on the curing step with subsequent destruction of containers.

8.3.5.2 Containers prepared by the use of layer-by-layer (L-b-L) polyelectrolyte interfacial adsorption

Notwithstanding that the L-b-L interfacial adsorption method became well-established worldwide in cases of plane solid¹⁰² and colloidal solid¹⁰³ templates in the last three decades, its extension to the liquid colloidal templates (emulsion droplets) was elaborated not so long ago. Several details of this technique were partially used already in some works issued in the middle 2000s.^104–106

These attempts were, however, seriously restricted either by employing specific chemicals or by application of unique preparation conditions. So, Tjipto and coworkers¹⁰⁶ have used the special substance (4′-pentyl-4-cyanobiphenyl) forming nematic liquid crystals as material for the droplets of O/W emulsion to be encapsulated. At the encapsulation of various food-relevant emulsions, both amounts of hydrophobic bioemulsifiers and the preparation conditions used evidences that the initial solid or solidlike layer was formed at the interface before the container shell formation by L-b-L polyelectrolyte deposition was started.^104,105

Universal approach to the preparation of loaded micro- and nanocontainers based on the direct emulsion encapsulation by polyelectrolyte L-b-L adsorption was developed only recently.¹⁰⁷ Using this approach, the liquid colloidal templates (droplets of O/W emulsion) were composed of water-immiscible substance dodecane. To stabilize the dispersed phase of initial emulsion and to ensure the significant interfacial charge of droplets for the beginning of polyelectrolyte adsorption, the oil phase was doped by small amount of cationic surface-active precursor Dioctadecyldimethylammonium bromide (DODAB). Because of dodecane’s very low solubility in nonpolar solvents, 30 v/v% of chloroform was added to dodecane in order to improve the precursor solubility in the oil phase. The droplets of O/W emulsion, however, did not contain any detectable traces of chloroform during the polyelectrolyte deposition steps because of its considerable solubility in the aqueous media. Colloidal stability of initial emulsion was achieved due to a concentrated monolayer of strongly positively charged DODAB (zeta-potential was about + 90 mV) at the surface of each droplet. The L-b-L assembly was done by the subsequent adsorption of oppositely charged polyelectrolytes from their concentrated (20 mg/ml) aqueous salt-free solutions. The creamed upper layer of the positively charged initial emulsion was added dropwise to the oppositely charged solution of PSS upon continuous stirring, ensuring its good adsorption with the subsequent overcharging of the droplets interface. The excess of the remaining free polyelectrolyte was washed out by pure Milli-Q water. The second encapsulation step was done in an aqueous solution of cationic PE PDADMAC or PAH in the same manner (Figure 8.8). The further repetition of the alternating adsorption steps leads to the formation of containers with desired shell thickness depending on the particular final demand. Resulting containers had Z-average size of 4.2 μm and were quite monodisperse (PDI = 0.32). The dried droplets of O/W emulsion encapsulated by the L-b-L interfacial adsorption technique are presented in the figure. Since the procedure of drying leads, in the case of PE oil-filled containers, to their collapse, especially on the steps connected with the preparation of SEM samples, the oily containers cores cannot be kept intact, and only wrinkled polyelectrolyte containers shells are observable (Figure 8.9), supporting the polyelectrolyte multilayered shell assembly around O/W emulsion droplets.

Following the proposed paradigm for the straightforward preparation of the emulsion-based polyelectrolyte containers¹⁰⁷ further progress in this field was achieved.^108,109 Wackerbarth et al.¹⁰⁸ reported polyelectrolyte microcontainers on the emulsion basis with six layers of biopolyelectrolytes for the potential application in the food industry. The authors of Ref. 109 have presented the successful polyelectrolyte encapsulation of highly unstable polyunsaturated natural oils for the purpose of storage and degradation protection by introducing the natural antioxidant compound (tannic acid) as a shell constituent in alternation with the biocompatible oppositely charged polyelectrolyte.

Container prepared via polyelectrolyte L-b-L direct emulsion encapsulation technique can be envisaged as quite applicable in several types of protective coatings, especially water based, imparting to them some unique functionalities independent of the nature of components encapsulated in the core. This can be achieved due to the many polyelectrolytes available for L-b-L container shell assembly. Purposeful choice of polyelectrolytes for the containers shells allows their opening by numerous environmental or artificially introduced triggers such as temperature,¹¹⁰ pressure,¹¹¹ changes in pH,¹¹² ionic strength, electrochemical potential, and therefore enables the specific stimuli-responsive protective ability of coatings carrying these containers.

Nanometric accuracy provided by polyelectrolyte L-b-L deposition on the each step of the container build-up allows very fine adjustment of their size and the shell thickness that also can be of high practical importance in many research and industrial fields.

On the other hand, some drawbacks of polyelectrolyte containers obtained by the L-b-L interfacial adsorption should be also mentioned here making their utilization in many types of self-protecting coatings problematic. Even at deposition of polyelectrolytes with lowest M_w the permeability of shell for low-molecular agents loaded into containers remains high disabling the possibility of sustained release for these substances and use of such type of containers in the media with good solvency for the core material. Low mechanical stability of the polyelectrolyte shells at any reasonable number of polyelectrolyte layers in the shell is the other source of trouble. It can be significantly improved by the increase of L-b-L deposition steps and subsequent annealing at higher temperature,¹¹⁰ such an overcomplication makes the fabrication costs rise and thus is unacceptable for many potential application fields. At the same time, the use of soft emulsion-based containers with L-b-L polyelectrolyte shells is imaginable in the water-borne coating formulations with the quite mild curing conditions and ductile coating matrices.

8.3.5.3 Containers on the basis of irreversible interfacial attachment—formation of Pickering emulsions

Noticeable phenomenon of emulsions stabilization due to spontaneous attachment of the partially hydrophobic/hydrophilic micro- or nanoparticles at the O/W or W/O interface underlies this method of preparation of emulsion-based nano- and microcontainers.

This “strange” behavior of such particles is known since the beginning of twentieth century^113,114 and can be strictly described in terms of particle energy change at its transition from the bulk location to the interface¹¹⁵:

$\Delta G_{\text{att}}=-\pi r^2{\gamma }_{\alpha \beta }{\left(1\pm cos\theta \right)}^2$

Thermodynamically, even in the case of small nanoparticles, this transition is practically irreversible because of the huge depth of the potential energy of the particle located at the interface and is therefore commonly called not “adsorption” but “attachment.” Due to these reasons, the partially hydrophobic/hydrophilic particles form spontaneously a tight mono- or even multilayer at the emulsion droplets interface and act as building blocks for the containers shells. Again, the emulsion droplets serve as a template for the formation of the particulate shells and work at the same time as reservoirs for the loaded active agent.

The interfacial activity of particles can be an intrinsic property of them connected with their specific affinity to certain types of solvents. In this case even the pristine particles with unmodified surface are able to concentrate at the interface between droplets of this solvent and the external continuous phase.¹¹⁶ However, this peculiarity is more frequently imparted to the particle by its surface modification using various types of surfactants^117,118 bonded electrostatically or covalently to the particle material.

Electrostatic mechanism of the modification does not presume the formation of the proper chemical bonds between surfactant molecules and particle surface, but is rather a result of the equilibration of the complex physical interactions between these molecules, particles, and surrounding medium (solution). Therefore, the small amounts of free surfactant molecules always persisting in the system can occasionally interfere with the corrosion inhibitor or other protective agents in the core-forming emulsion droplets and impair the mechanism of the interfacial particle attachment. To avoid this undesired situation, the thorough selection of appropriate nonreacting and noninteracting surfactants and protective components should be made before fabrication of containers. The indicated problem will, however, not arise at all when inhibitor molecules possess by themselves the certain surface activity and can therefore simultaneously cause the partial hydrophobic modification of the particle surface. Many corrosion inhibitors are weak organic acids, bases, or amphoteric compounds having functional groups partially or fully ionized in a certain pH range and therefore becoming hydrophilic. In the case when residual organic moieties of inhibitor molecules are hydrophobic enough, the entire molecule will demonstrate the amphiphilic behavior in the same pH range, that is, will act as a surfactant. Haase et al. utilized the corresponding ability of the amphoteric corrosion inhibitor 8-hydroxyquinoline (8-HQ)¹¹⁹ to become considerably protonated at the pH < 5.5 (pK_a = 5.13) and to be adsorbed electrostatically on the surface of negatively charged silica nanoparticles (Ludox TMA) in order to make them partially hydrophobized and therefore interfacially active. These particles stabilize the O/W emulsion droplets forming the multilayers at their interface that can be considered as particulate containers shells (Figure 8.10). The droplets of dispersed phase are composed of Diethyl phthalate (DEP) with dissolved 8-HQ and function as containers cores with the already loaded inhibitor. The containers size distribution and the amount of nanoparticles in the containers shells are well-expressed functions of pH in the range from ~ 4.4 to 5.6. At pH 4.4 the monodisperse containers with volumetric average size of 4.5 μm and PDI of about 0.4 were obtained. This pH-dependent behavior can be related to the continuously increased interfacial activity of silica nanoparticles hydrophobized by the gradually growing amount of the adsorbed 8-HQ upon pH drop. Below the pH 4.4, protonation degree of 8-HQ and subsequently its concentration in the dispersion medium attain the values enabling the formation of an 8-HQ bilayer on the surface of silica nanoparticles. The hydrophobe-hydrophobe interaction of the overlapping aromatic rings is suggested to be a driving force for this effect. As a result, the positively charged protonated groups of 8-HQ molecules in the outer part of bilayer are oriented toward dispersion medium and re-hydrophilize the particles in the containers shells. The rapid destruction of the containers is observed in this pH region, leading to the immediate burstlike release of the encapsulated inhibitor. This pH-sensitivity as well as the mechanical rupture of containers could be utilized as triggers for the containers opening and stimulated inhibitor release when the containers become embedded in the matrix of a self-protecting anticorrosion coating.

Emulsion-based containers fabricated by means of interfacial attachment of nano- or microparticulate building blocks forming the container shell reveal several advantages in the preparation routine and in the handling that follows, fashioning them as potential candidates for the use in the various self-protecting coatings. Fabrication of micro- and nanocontainers by means of the interfacial attachment method is usually quite simple procedure that includes as a rule only two steps—(1) preparation of particles as building blocks for the shells of future containers due to their partial hydrophobization (O/W) or hydrophilization (W/O) and (2) addition of them to the two-phase system containing all components of containers during or immediately before the emulsification with subsequent processing leading to the ready containers. Sometimes this procedure can be even further simplified to the practically one-pot process.¹¹⁶ Because of its simplicity, this preparation pathway may often be economically sound and has a high practical relevance. Wide variety of particles of different chemical nature and of immiscible solvents with diverse polarities available facilitates an almost unlimited number of achievable containers types thus making the method very versatile. Comparing with the types of the emulsion-based containers described above, containers prepared by the interfacial attachment of particles demonstrate much higher robustness¹²⁰ and are therefore more suitable for the incorporation into self-protecting coatings with hard coating matrices. However, the main problem in the application of containers prepared by interfacial attachment remains the heterogeneity of their shells comprised of many fine particles joined at the surface of each emulsion droplet to a mono- or multilayer with the lot of gaps between individual building blocks. This discontinuous structure of the container shell reduces considerably its mechanical properties, especially comparing with these of the individual particle in the shell and increases its permeability drastically. The latter could be very critical when the containers loaded with the oil-soluble protective agent are mixed with a solvent-based coating formulation able to wash out the containers load immediately after their addition into coating mixture.

To enhance the mechanical strength of containers and make them more seamless, single particulate elements of the containers shells should be locked at the interface and joined together. It can be done for instance by the local high energy treatment¹²¹ where the high intensity ultrasound was used for this purpose. The second possibility is the interlinkage of particles in the shell by the deposition of additional polyelectrolyte layers atop it. This, on the one hand, complicates the process of the containers preparation by one more step, but on the other hand can significantly improve the integrity and permeability of their shells. Moreover, the polyelectrolyte deposition could impart to shells certain additional valuable features connected with the properties of polyelectrolyte layer like selective pH sensitivity and so on. Figure 8.11 shows containers on the basis of Pickering emulsion droplets made of 1.5 M solution of 8-HQ in dimethyl phthalate (DMP) coated with three polyelectrolyte bilayers composed of pairs PAH/PSS or PAH/PAA. Alternate deposition of polyelectrolytes not only did not disturb the initial containers size distribution with volume-average size of 2.5 μm and PDI = 0.35, but also did lead to the significantly higher stability of containers that remained almost not deformed during the SEM sample preparation.

8.3.6.1 Containers via interfacial polyaddition/interfacial polycondensation

At the containers synthesis by interfacial reaction in an emulsion, at least one of the reactants is distributed outside droplets in the continuous phase of emulsion whereas the others are dissolved in the droplets. Initially, the reactants have sufficiently different polarities and are predominantly soluble only in one of the coexisting phases excluding their interpenetration in the adjacent phase and premature reaction between them. Moreover, reactants have to remain phase-separated all the time when reaction goes on and meet each other only at the interface of emulsion droplets. If the product of such an interfacial reaction is soluble neither in the droplets of the emulsion nor in the medium around them, containers with the core/shell morphology will be synthesized. In the more frequently arising case, however, the resulting product is soluble or swellable in the droplets of dispersed phase, and particles with the compact morphology occur.^122,123 Depending on the release of low-molecular side products, the interfacial polyaddition (without side products) or interfacial polycondensation (with) reactions can be discriminated.

The encapsulation of the mixture of protective agents possessing simultaneously the functionalities of corrosion inhibitors and water repellents was carried out recently¹²⁴ by the interfacial polyaddition technique for the further application in the self-protecting anticorrosion coatings. The oil phase of the future O/W emulsion was composed of mixture of two protective agents, trimethoxy(octadecyl)silane (TMODS) and trimethoxy(octyl)silane (TMOS), Poly[(phenyl isocyanate)-co-formaldehyde] trifunctional pre-polymer participating in the reaction of shell formation as well as solvent (DEP) enabling the miscibility of all components.¹²⁴ This mixture was firstly dispersed in an aqueous medium by means of nonionic polymeric emulsifier polyvinyl alcohol (PVA) under intensive stirring for 3 min with high speed rotor-stator homogenizer Ultra-Turrax (IKA Werke, Staufen, Germany).

Resulting emulsion was added to the phosphate buffer solution containing the second water-soluble reactant glycerol and polyaddition reaction catalyst 1,4-diazabicyclo [2.2.2]octane (DABCO). The formation of polyurethane (PU) containers shells at the interfaces of droplets was occurred according to the following reaction scheme:

$n\text{OCN}-\text{R}-\text{NCO}+n\text{HO}-\text{R}\prime -\text{OH}\to {\left[-\text{R}-\text{NHC}\left(\text{O}\right)\text{OR}\prime -\right]}_n$

The reaction mixture was then left for 1 h at 65 °C under continuous stirring and subsequently overnight without heating and stirring for the completion of containers formation. Suspension of obtained micro- and nanocontainers was dialyzed in order to remove residual glycerol and PVA and then separated by centrifugation at 13,000 rpm. Finally, quite polydisperse microcontainers with the well-expressed core/shell morphology (Figure 8.12) were isolated.

The size distribution of resulting PU containers demonstrated two peaks representing two main fractions of containers in the mixture—one at 1.0 μm and the second one—at 5.5 μm. At the increasing of rate of the high speed rotor-stator homogenizer from 11,000 rpm through 16,000 rpm to 22,000 rpm, the area of the first peak was continuously increased with the subsequent reduction of the second one remaining, however, still bimodal. Such a bimodal particle size distribution can be considered as polymerized replica of an initial O/W emulsion predestined by its droplets size distribution¹²⁵ almost always showing a high polydispersity if the high speed rotor-stator homogenizer was applied for the emulsification process. Utilization of ultrasound for this purpose leads as a rule to narrower and frequently monomodal droplet size distribution in the original emulsion.¹²⁶ Application of this comminution method in the considered case of containers with PU shells was unfortunately impossible because of numerous sonochemical effects accompanying the collapse of cavitation bubbles upon ultrasound propagation. Reactive isocyanate functional groups in the pre-polymer molecules quickly underwent side reaction with water molecules and highly reactive products of the water thermolysis (free radicals, hydroxyl-carrying species, etc.).

The incorporation of a small amount of these containers into the conventional coating (~ 6 wt.% microcontainers in relation to the total mass of the dry coating) provides to itself protecting features: Being mechanically damaged, the modified coating reveals not only recovered corrosion protection at the site of scratch, but also dewetting ability for the aqueous corrosive media.

Figure 8.13a and b demonstrate example of containers obtained by the interfacial polycondensation technique. The containers presented here have silica shells and are filled with the corrosion inhibitor mercaptobenzothiazole (MBT) dissolved in the organic solvent dibenzyl ester. These two compounds along with the shell-forming component TEOS were mixed together to form the oil phase of the future emulsion. An oil phase was dispersed in an aqueous phase with pH = 11.5 using the cationic surfactant CTAB, and the resulting emulsion was then left overnight at 65 °C. Formation of container shell made of silica was achieved due to gradual hydrolysis of TEOS with the silanols as semi-products and their subsequent condensation on the surface of liquid colloidal template¹²⁷:

$\begin{array}{l}\text{Si}{\left(-\text{OEth}\right)}_4\prime +{\text{H}}_2\text{O}\stackrel{-\text{EthOH}}{\to }\left(\text{OH}\right)\text{Si}{(-\text{OEth})}_3\stackrel{+\left(\text{OH}\right)\text{Si}{\left(-\text{OEth}\right)}_3}{\to }\hfill \\ \stackrel{-{\text{H}}_2\text{O}}{\to }{\left(\text{EthO}-\right)}_3\text{Si}-\text{O}-\text{Si}{\left(-\text{OEth}\right)}_3\hfill \end{array}$

At the end, the polydisperse submicrocontainers (zeta-average size 700 nm, PDI = 0.43) loaded with MBT (see EDX spectrum in the Figure 8.13b) were collected.

These containers can be successfully applied in the anticorrosive self-protecting coatings in cases when the corrosion is caused or followed by the pH increase. The dissolution of silica shell in this pH range can lead to the corrosion-triggered release of inhibitor.

Similarly, the containers of other composition with compact morphology were obtained for self-protection purposes by the interfacial polyaddition method. This type of containers made of polyurea and the corrosion inhibitor 2-methylbenzothiazole (MeBT) was used in the self-protecting anticorrosion coatings for the aluminum substrate.¹²³ Formation of the compact morphology can be explained as a result of the high affinity of MeBT to the polymer network. Due to very good solvency of this liquid inhibitor for polyurea, MeBT becomes homogeneously distributed over the whole droplet volume, forming a gel with a high amount of the liquid component. At the same time, crosslinked polymer network can hold the structure of the resulting particles together, ensuring their solid state. These microgel particles combine the attractive properties of both solids and liquids, making them perfect candidates for usage in self-protecting anticorrosion coatings. The solid state of the particles ensures good mechanical stability of the resulting coating system. The liquid component (MeBT), being entrapped in the solid matrix, retains its mobility providing the coating system with self-protecting abilities. The introduction of only a few parts of the particles in an organic coating allowed the significant improvement of its corrosion protection performance, as was shown by scanning vibrating electrode technique and electrochemical impedance spectroscopy.¹²³

Figure 8.14 shows another example of containers obtained by the interfacial polycondensation technique. These containers have polyamide shells and are filled with the corrosion inhibitor Ce(III)-tris-(bis-2-ethylhexyl) phosphate dissolved in the organic solvent dibutylphosphate. These two compounds along with the shell-forming component terephthaloylchloride (TPC) were mixed in the weight ratio 2:13:5 for inhibitor, solvent and TPC, respectively, to form the oil phase of the future emulsion. Oil phase was dispersed in the Milli-Q water using the nonionic emulsifier PVA and the resulting emulsion was then added to the second aqueous phase containing combination of bi- and pentafunctional amines—ethylenediamine and tetraethylenepentamine in the ratio ensuring the trifunctionality of the final mixture. The rapid formation of containers shells according to the reaction:

$n{\text{H}}_2{\text{NRNH}}_2+n\text{ClOCR}\prime \text{COCl}\leftrightarrow {\left[-\text{NHRNHCOR}\prime \text{CO}-\right]}_n+2n\text{HCl}$

was accomplished by the release of HCl as the low-molecular side product and was therefore conducted in the highly basic medium containing fivefold excess of amines relating to the TPC amount. The obtained containers had a mean size of 6 μm and were highly polydisperse (PDI = 0.7) because of very cautious conditions during their preparation (use of high speed rotor-stator homogenizer at lowest stirring rate, short comminution time, etc.).

These containers may be promising for the applications the anticorrosive self-protecting coatings in cases when the corrosion is caused or followed by the pH decrease. The dissolution or strong swelling of the polyamide shell in this pH range will lead to the corrosion-triggered release of inhibitor.

8.3.6.2 Containers through in situ emulsion polymerization

Realization of this technique of containers preparation presumes that all reactants needed for the synthesis are contained in the emulsion droplets from the beginning. These droplets serve as reservoirs for reactants and protective agents and play simultaneously the role of liquid colloidal reactors and are only kinetically stabilized against the premature onset of polymerization reaction. After the O/W or W/O emulsion with desired properties such as stability, concentration, droplet size distribution and so on is prepared, the chemical (initiator substance, catalyst) or physical (T, UV-light etc.) triggers are activated to switch on the reaction and it runs until the attainment of the real equilibrium state corresponding to the completion of the polymerization.

The polyepoxy containers loaded with the corrosion inhibitor MeBT for the use in self-protecting anticorrosion coatings on the aluminum substrates were prepared by in-situ polymerization method¹⁰¹ by the following steps. The mixture of reactants needed for the formation of polyepoxy shells of containers was initially prepared of the 1:1 combination of aromatic and aliphatic epoxy-monomers trimethylolpropane triglycidyl ether and tris(4-hydroxyphenyl)methane triglycidyl ether, respectively. A 20% of MeBT related to the total mass of the mixture was then added. The final step in the preparation of oil phase of the future emulsion was the addition of the second reactant for the in-situ polymerization reaction—diethylenetriamine in the amount ensuring the slight excess of the stoichiometric equivalence with the sum of both epoxy components. The oil phase was dispersed in the 2 wt.% aqueous solution of the nonionic emulsifier PVA using Ultra-Turrax at 16,000 rpm for 5 min. The obtained O/W emulsion was then doped with a small amount of polymerization catalyst tetrabutylphosphonium bromide to increase the reaction rate to a reasonable level. The mixture was left overnight under moderate stirring to complete the polymer formation. The resulting containers are very polydisperse (see Figure 8.15), with the volume-average size of 2.3 μm (PDI = 0.96), and have obviously the core/shell morphology revealing the phase separation in the emulsion droplets in the course of the polymerization reaction leading finally to the segregation of core and shell parts of containers. Again, the significant improvement of the container size distribution by the creation of the more monodisperse initial O/W emulsion via utrasonication was not possible because of the immediate reaction of monomers with water and especially with the products of sonochemical processes. Because of presence of free amino groups in the structure of the epoxy polymer that is obtained, the containers reveal high positive surface charge (zeta-potential of about + 45 mV) and are therefore well dispersible in various aqueous media, for instance, in the water-based liquid coating formulations. Moreover, the introducing of epoxy monomers with the functional groups easily hydrolyzable in the acidic or basic medium (like ester groups) allows their opening in these media and finally acid- or base-triggered release of protective agents from the containers.

Sometimes, the combination of several emulsion-based methods of container fabrication leads to better results than the application of each of these methods alone. Solid micro- and nanocontainers for the self-protecting anticorrosion coatings were prepared via interfacial deposition of droplets-stabilizing SiO₂ nanoparticles with subsequent in-situ polymerization of their liquid cores.¹²⁸ Oil phase of the initial Pickering emulsion was composed either of styrene with dissolved corrosion inhibitor 8-HQ, hydrophobe hexadecane (HD) and polymerization initiator azo-bis-(isobutyronitril) (AIBN) or of styrene, corrosion inhibitor MBT, 4-vinylpyridine (4VP) as a solubilizing agent for MBT, HD, and AIBN. The weight percent ratios of all components were 193:45:10:2 or 255:90:130:20:4, respectively. The silica nanoparticles (Ludox TMA, Sigma) were used for the creation of particulate shells of the future containers. Noteworthy, that no additional steps for making silica partially hyhrophobic and therefore intefacially active were needed: 8-HQ acts as SiO₂ particle hydrophobizing agent and as, simultaneously, the encapsulated inhibitor, 4VP also hydrophobizes silica, acts as a MBT cosolubilizer, and becomes copolymerized with styrene. Variation of the amount of silica introduced in the liquid/liquid system during the comminution step (ultrasonication, VCX 505, Sonics & Materials Inc. Newton, USA) allowed the size controlling of the interim containers with the oily liquid core simply by the almost complete consumption of particles due to their interfacial deposition. The higher was the ratio of nanoparticles mass to the oil volume R, the smaller were the droplets of corresponding Pickering emulsion, independent of the ultrasonication power (W = 40 W) applied to the system. The size distribution of the nanosilica-stabilized emulsion droplets was always satisfactorily narrow: at R = 0.09, the number-average size was 2.3 μm with PDI = 0.45; at R = 0.32, the number-average size was 600 nm with PDI = 0.35; and at R = 0.91, the number-average size was around 260 nm with PDI = 0.2. It is also important to emphasize here that before and during the ultrasonic treatment the system was purged for 7 min by the nitrogen flow and ultrasonication was carried out under continuous cooling in the ice bath. These measures allowed the complete avoiding the undesired premature polymerization on the dispersion step. Finally, the polymerization of the liquid cores was initiated by heating the whole mixture up to 65 °C for 24 h. These solid containers (Figure 8.16) were introduced in the conventional water-based alkyd coating formulation that was then deposited on the aluminum specimens. The coatings with entrapped containers showed considerably better corrosion resistance than unmodified ones, as was confirmed by EIS and SVET measurements (Figure 8.17).