Surfaces and interfaces

Role of Surface Energy of Nanoparticle Stabilizers in the Synthesis of Microspheres via Pickering Emulsion Polymerization

“…This can be explained by the polymerization mechanism of Pickering emulsions, which is rather different from the polymerization mechanism of the classic emulsions stabilized by surfactants. On one hand, the mechanism in conventional emulsion polymerization stabilized by surfactants is well-understood, whereas the standard theory notes the existence of three different loci for the nucleation mechanism, micellar nucleation, homogeneous coagulative nucleation, and emulsion droplet nucleation [ 43 , 44 ]. Although, almost always, the three mechanism are at play in the emulsion polymerization, the dominant mechanism depends on the emulsion type, such that the polymerization loci can shift out of the emulsion droplet toward micelles or the water phase driven by many factors, such as emulsion droplet curvature (emulsion droplet curvature 1/R, R is the radius, increases in the order: emulsions, miniemulsions, microemulsions), by the oil–water interfacial tension, solubility of the monomer in water, the solubility of the polymerization initiator in water or oil, etc.…”

“…Although, almost always, the three mechanism are at play in the emulsion polymerization, the dominant mechanism depends on the emulsion type, such that the polymerization loci can shift out of the emulsion droplet toward micelles or the water phase driven by many factors, such as emulsion droplet curvature (emulsion droplet curvature 1/R, R is the radius, increases in the order: emulsions, miniemulsions, microemulsions), by the oil–water interfacial tension, solubility of the monomer in water, the solubility of the polymerization initiator in water or oil, etc. [ 43 ] On the other hand, in the case of Pickering emulsions, in the absence of surfactants, the micellar nucleation is excluded, therefore Dai et al [ 34 ] proposed that only two possible mechanisms are present in the initial stage of Pickering emulsion polymerization, namely (i) the homogeneous coagulative nucleation, and (ii) nucleation in the emulsion droplet. In the first case, the oligomers produced in the water phase coagulate to form nuclei that are subsequently swollen by incoming of the monomers from the emulsion droplet reservoir.…”

“…However, a vanishingly small interfacial energy of the dispersive component γ d NP/water ≈ 0 mJ/m 2 , such as it is the case for NP-Gly and NP-CN, indicates that the dispersive adhesion energy per unit area (adhesion forces per unit length) between the solid NP and water is comparable with the cohesion energies arising due to the dispersive interactions, in the two phases [ 43 ].…”

“…Further, the NP/w interfacial energies γ NP/water , , in Table 1 , are converted into surface energy of nanoparticles in air, NP/air, , , using the combining rules [ 35 , 43 ] formula: …”

“…The free energy of adsorption and desorption of the NPs at the interfaces are related . The desorption energy or the detachment energy of an NPs can be calculated from the contact angle at the interfaces and is given by the following expression [ 35 , 43 ]: where γ PW is the P/W interfacial energy. It is important to note that the sign in the parenthesis is taken negative when the NP detaches/desorbs from the oil/water interface into water and the sign is taken positive when the NP detaches from the oil/water interface into the oil.…”