Superstabilization of emulsions by solid particles

Epoxy Phase-Change Materials Based on Paraffin Wax Stabilized by Asphaltenes

The usual problem of meltable phase-change agents is the instability in their form upon heating, which can be solved by placing them into a continuous polymer matrix. Epoxy resin is a suitable medium for dispersing molten agents, but it is necessary to make the obtained droplets stable during the curing of the formed phase-change material. This work shows that molten paraffin wax forms a Pickering emulsion in an epoxy medium and in the presence of asphaltenes extracted from heavy crude oil. Theoretical calculations revealed the complex equilibrium in the epoxy/wax/asphaltene triple system due to their low mutual solubility. Rheological studies showed the viscoplastic behavior of the obtained dispersions at 25 °C, which disappears upon the heating and melting of the paraffin phase. Wax and asphaltenes increased the viscosity of the epoxy medium during its curing but did not inhibit cross-linking or reduce the glass transition temperature of the cured polymer. As a result of curing, it is possible to obtain phase-change materials containing up to 45% paraffin wax that forms a dispersed phase with a size of 0.2-6.5 μm. The small size of dispersed wax can decrease its degree of crystallinity to 13-29% of its original value, reducing the efficiency of the phase-change material.

The chemical (not mechanical) paradigm of thermodynamics of colloid and interface science

In the most influential monograph on colloid and interfacial science by Adamson three fundamental equations of "physical chemistry of surfaces" are identified: the Laplace equation, the Kelvin equation and the Gibbs adsorption equation, with a mechanical definition of surface tension by Young as a starting point. Three of them (Young, Laplace and Kelvin) are called here the "mechanical paradigm". In contrary it is shown here that there is only one fundamental equation of the thermodynamics of colloid and interface science and all the above (and other) equations of this field follow as its derivatives. This equation is due to chemical thermodynamics of Gibbs, called here the "chemical paradigm", leading to the definition of surface tension and to 5 rows of equations (see Graphical abstract). The first row is the general equation for interfacial forces, leading to the Young equation, to the Bakker equation and to the Laplace equation, etc. Although the principally wrong extension of the Laplace equation formally leads to the Kelvin equation, using the chemical paradigm it becomes clear that the Kelvin equation is generally incorrect, although it provides right results in special cases. The second row of equations provides equilibrium shapes and positions of phases, including sessile drops of Young, crystals of Wulff, liquids in capillaries, etc. The third row of equations leads to the size-dependent equations of molar Gibbs energies of nano-phases and chemical potentials of their components; from here the corrected versions of the Kelvin equation and its derivatives (the Gibbs-Thomson equation and the Freundlich-Ostwald equation) are derived, including equations for more complex problems. The fourth row of equations is the nucleation theory of Gibbs, also contradicting the Kelvin equation. The fifth row of equations is the adsorption equation of Gibbs, and also the definition of the partial surface tension, leading to the Butler equation and to its derivatives, including the Langmuir equation and the Szyszkowski equation. Positioning the single fundamental equation of Gibbs into the thermodynamic origin of colloid and interface science leads to a coherent set of correct equations of this field. The same provides the chemical (not mechanical) foundation of the chemical (not mechanical) discipline of colloid and interface science.

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.

Particles adsorbed at various non-aqueous liquid-liquid interfaces

Particles adsorbed at liquid interfaces are commonly used to stabilise water-oil Pickering emulsions and water-air foams. The fundamental understanding of the physics of particles adsorbed at water-air and water-oil interfaces is improving significantly due to novel techniques that enable the measurement of the contact angle of individual particles at a given interface. The case of non-aqueous interfaces and emulsions is less studied in the literature. Non-aqueous liquid-liquid interfaces in which water is replaced by other polar solvents have properties similar to those of water-oil interfaces. Nanocomposites of non-aqueous immiscible polymer blends containing inorganic particles at the interface are of great interest industrially and consequently more work has been devoted to them. By contrast, the behaviour of particles adsorbed at oil-oil interfaces in which both oils are immiscible and of low dielectric constant (ε<3) is scarcely studied. Hydrophobic particles are required to stabilise these oil-oil emulsions due to their irreversible adsorption, high interfacial activity and elastic shell behaviour.

Effects of variables on the dispersion of cationic–anionic organomontmorillonites and characteristics of Pickering emulsion

Montmorillonites were modified with different amounts of cationic and anionic surfactants by variable steps; characteristics of cationic–anionic organomontmorillonites and Pickering emulsion were further investigated.