NanoTraPPED—A New Method for Determining the Surface Energy of Nanoparticles via Pickering Emulsion Polymerization

Abstract

Surface energy with its polar and disperse components describes the physicochemical state of nanoparticles’ (NPs) surfaces, and can be a valuable parameter for predicting their bulk behavior in powders. Here, we introduce a new method, namely, Nanoparticles Trapped on Polymerized Pickering Emulsion Droplets (NanoTraPPED), for measuring the surface energy of a series of silica NPs bearing various surface functional groups. The method consists in creating Pickering emulsions from vinyl bearing monomers, immiscible with water, whereas NPs of interest have a stabilizing role, and in the process, become trapped at the monomer/water interface of emulsion droplets. The Pickering emulsion is polymerized, and polymer microspheres (colloidosomes) decorated with NPs are obtained. NanoTraPPED relies on measuring contact angles from the immersion depth of nanoparticles at the interface of various polymer colloidosomes with the electron microscope. The contact angle values are used as input for the Owens-Wendt-Rabel-Kaelble (OWRK) model, to quantitatively determine the total surface energy with water γNP/water, air γNP, and the corresponding polar and dispersive interaction components of NPs carrying -NH2, -SH, -OH, -CN and -C8 surface functional groups, ranking these according to their polarity. Our findings were confirmed independently by calculating the interfacial desorption energies of NPs from contact angles.

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

“…In all cases, we kept the same concentration of NPs and the same water to oil ratio in emulsion preparation, Table S2 . The preparation procedure for the NP-CN and NP-SH was reported previously [ 35 ]. The synthesis of NP-Gly is given in the experimental methods and SEM images are given in the Figure S1 .…”

“…We believe that an indication that this mechanism is at play is that the surface of these particles is covered by a compact monolayer of NPs, which adsorbed and self-assembled at the interface during emulsion formation prior to polymerization. This hypothesis stood at the base of developing a new method, called NanoTraPPED, for measuring the contact angle and surface energy of nanoparticles [ 35 ].…”

“…The measurement of the contact angle for NP-Gly was done according to the recently reported NanoTraPPED method [ 35 ]. The interfacial immersion depth of the nanoparticles trapped at the polymer-water interface during the Pickering emulsion formation and polymerization were studied with a Verios G4 UC Scanning Electron Microscope (SEM) from Thermo Fischer Scientific (Eindhoven, The Netherlands) in field immersion mode, using either a Through Lens Detector (TLD) or Mirror Detector (MD), using a stage bias of 1000 mV, a beam energy between 500 eV and 1 KeV, and an aperture of 500 pA-1 nA, to minimize the surface charging effects and also the impact of the electron beam on polymers.…”

“…Another particularly attractive aspect related to Pickering emulsions is the possibility to polymerize them, when the oil phase is a water immiscible vinyl bearing monomer; this way a plethora of new polymeric nano- and micromaterials can be obtained, including molecular imprinted polymers [ 6 , 22 , 31 ], colloidal particles with structural complexity [ 32 ], conductive nanostructured microcapsules [ 33 ], organic-inorganic composite particles [ 34 ], etc. In a recent report, we have shown that the Pickering emulsion polymerization technology can be used to measure the surface energy and surface energy components of the emulsion stabilizing particles [ 35 ].…”

“…It is also implied in these works that an emulsion phase inversion to take place, from w / o to o / w and vice versa, must pass through a contact angle of 90°. Thus far, to the best of our knowledge there has not been a quantitative experimental confirmation for these theoretical predictions, because measuring the contact angle of the particles has been an ongoing challenge for fundamental science [ 35 , 40 , 41 ]. Further, there are no current studies linking the contact angle with the emulsification ability of NPs in terms of size of the emulsion droplets generates, nor the curvature of the oil/water interface upon adsorption of NPs at the interface.…”