Dye Aggregates as New Stabilizers for (Mini)emulsions

Keplerate {Mo132}-Stearic Acid Conjugates: Supramolecular Synthons for the Design of Dye-Loaded Nanovesicles, Langmuir-Schaefer Films, and Infochemical Applications

Self-assembly gives rise to the versatile strategies of smart material design but requires precise control on the supramolecular level. Here, inorganic-organic synthons (conjugates) are produced by covalently grafting stearic acid tails to giant polyoxometalate (POM) Keplerate-type {Mo132} through an organosilicon linker (3-aminopropyltrimethoxysilane, APTMS). Using the liposome production approach, the synthons self-assemble to form hollow nanosized vesicles (100-200 nm in diameter), which can be loaded with organic dyes─eriochrome black T (ErChB) and fluorescein (FL)─where the POM layer serves as a membrane with subnanopores for cell-like communication. The dye structure plays an essential role in embedding dyes into the vesicle's shell, which opens the way to control the colloidal stability of the system. The produced vesicles are moved by an electric field and used for the creation of an infochemistry scheme with three types of logic gates (AND, OR, and IMP). To design 2D materials, synthons can form spread films, from simple addition on the water-air interface to lateral compression in the Langmuir bath, and highly ordered structures appear, demonstrating electron diffraction in Langmuir-Schaefer (LS) films. These results show the significant potential of POM-based synthons and nanosized vesicles to supramolecular design the diversity of smart materials.

Investigation of the Mechanism of SiO2 Particle and Capsule Formation at the Oil-Water Interface of Dye-Stabilized Emulsions

In a previous contribution we described the formation of silica nanostructures in dye-stabilized nanoemulsions from tetraethyl orthosilicate droplets in water. Depending on the type of dye, either capsules (crystal violet, CV) or nanoparticles (congo red, CR) are formed. The thorough study of the sol-gel process uses a combination of time- and/or temperature-resolved small-angle X-ray scattering, transmission electron microscopy, and ¹H NMR spectroscopy to elucidate the detailed kinetics and mechanism of structure formation. In both cases, small nuclei of 1.5-2 nm are formed, followed by either a fast cluster-cluster (CV) or a much slower monomer-cluster aggregation (CR). The former leads to a cross-linked network and finally to patchy capsules, while the latter leads to individual nanoparticles (SNPs). From an Avrami plot it can be deduced that the SNPs are formed by an interface-controlled one-dimensional growth process. The mechanisms are based on the different local environments at the oil-water interface, which is either slightly acidic (CV) or fairly basic (CR). The kinetics differ by a factor between 3 and 20 and are presumably caused by the different mobility of the catalyzing species H⁺ or OH^-.

Double in Situ Preparation of Raspberry-like Polymer Particles

Well-defined raspberry-like poly(styrene- co-4-vinylpyridine)-SiO₂ nanocomposite particles with a diameter of around 200 nm were easily prepared by a double in situ process in nanoemulsion with the water-soluble dye Eosin Y as the stabilizer. During radical polymerization of the nanodroplets comprising styrene, 4-vinylpyridine (4-VP), and tetraethoxysilane (TEOS), the silane phase is expelled from the polymer phase to the oil/water (o/w) interface. In the later polymerization stage, SiO₂ nanoparticles with a size of around 25 nm were produced via the in situ sol-gel reaction of TEOS at the o/w interface promoted by the negatively charged dye. The pyridine moieties in the copolymer fix the SiO₂ nanoparticles on the surface of the polymer particles by electrostatic interactions without any sign of free unbound silica particles as proven by transmission electron microscopy.

Green Chemistry in Red Emulsion: Interface of Dye Stabilized Emulsions as a Powerful Platform for the Formation of sub-20-nm SiO2 Nanoparticles

Dye stabilized nanoemulsions offer the unique possibility of creating both silica capsules and sub-20-nm particles with precise control of particle size and narrow dispersity from the same system by the choice of the proper dye. The large o/w interface enhances the kinetics of particle formation significantly over macroscopic interfaces which enables the synthesis of silica nanoparticles without any catalyst or elevated temperatures under static conditions. This is in contrast to syntheses for sub-20-nm silica nanoparticles described until now which can normally not be conducted at neutral pH and/or room temperature without stirring. Furthermore, the synthesis can be run without any additional organic solvent and the dyes can be easily removed from the dispersion which opens the pathway to silica dispersions containing only particles, traces of ethanol and water at neutral pH without centrifugation, washing, or redispersion in accordance with the idea of "green chemistry".

Alizarin Yellow R (AYR) as compatible stabilizer for miniemulsion polymerization

HYPOTHESIS

Many solid particles have been used in Pickering stabilized (mini)emulsions. Stabilizing "particles" can be also formed in situ e.g. by aggregation of dye molecules as reported recently. Among the dyes sodium 2-hydroxy-5-[(E)-(4-nitrophenyl)diazenyl]benzoate (Alizarin Yellow R, (AYR)) is one of the best stabilizers. It is assumed to act as sole stabilizer also in heterophase polymerizations and offers a great potential for applications.

EXPERIMENTS

Aqueous solutions of AYR in varying concentrations (0.3, 0.5, 1.0, 1.5, 2.0mg/mL (dye/water)) were employed as continuous phase in direct miniemulsions. The oil phase comprised ethenylbenzene (styrene) and hexadecane. The effects of AYR concentration and ultrasonication time on size and distribution of the droplets were investigated. The miniemulsions were polymerized with a water-soluble azo-initiator (2,2'-azobis[n-(2-carboxyethyl)-2-methylpropionamidine] n-hydrate, VA-057) and conversion and kinetics were determined.

FINDINGS

The AYR is successfully employed as stabilizer in Pickering-like miniemulsion polymerizations of styrene. The higher the AYR concentrations the more stable the miniemulsions, the smaller the droplet sizes and the narrower the distributions are, ranging from ca. 450 to 180nm and 0.38 to 0.15, respectively. The nucleation mechanism of the polymer particles could be revealed by the number ratio of droplets and particles and follows droplet nucleation. This is confirmed by polymerization kinetics, which is in accordance with classical miniemulsion polymerization, too.

On the Negative Surface Tension of Solutions and on Spontaneous Emulsification

The condition of negative surface tension of a binary regular solution is discussed in this paper using the recently reconfirmed Butler equation (Langmuir 2015, 31, 5796-5804). It is shown that the surface tension becomes negative only for solutions with strong repulsion between the components. This repulsion for negative surface tension should be so strong that this phenomenon appears only within a miscibility gap, that is, in a two-phase region of macroscopic liquid solutions. Thus, for a macroscopic solution, the negative surface tension is possible only in a nonequilibrium state. However, for a nano-solution, negative surface tension is also possible in equilibrium state. It is also shown that nano- and microemulsions can be thermodynamically stable against both coalescence and phase separation. Further, the thermodynamic theory of emulsion stability is developed for a three-component (A-B-C) system with A-rich droplets dispersed in a C-rich matrix, separated by the segregated B-rich layer (the solubility of B is limited in both A and C while the mutual solubility of A and C is neglected). It is shown that when a critical droplet size is achieved by forced emulsification, it is replaced by spontaneous emulsification and the droplet size is reduced further to its equilibrium value. The existence of maximum temperature of emulsion stability is shown. Using low-energy emulsification below this maximum temperature, spontaneous emulsification can appear, which is enhanced with further decrease of temperature. This finding can be applied to interpret the experimental observations on spontaneous emulsification or for the design of stable micro- and nanoemulsions.