Surface energy is a macroscopic parameter that describes the reactivity of a surface and its capacity to interact through physical forces. Based on the principle of independent actions, the surface energy can be split into various components, based on type of interactions, such as polar, dispersive, hydrogen bonding, etc. Surface energy is a parameter monitored across many industries, from automotive coatings, to floatation in ore separation. For flat macroscopic surfaces the surface energy and its components can be trivially determined by measuring the contact angle of several liquids varying in polarity. At smaller scales, where the contact angle is not possible to measure with standard equipment, developing of new methods is in great demant. For example, for nanoparticles (NPs), the existing methods, such as the Washburn and the wicking technique pose technical difficulties. In the same time, the surface energy and its components can be a valuable parameter to predict the collective behavior of NPs in powders, especially with respect to their dispersibility in air or water, pelleting ability, flowing ability, etc. Here, we introduce a new method, namely Nanoparticles Trapped on Polymerized Pickering Emulsion Droplets (NanoTraPPED) for measuring the surface energy and its components of a series of NPs varying in surface polarity. The method consists in creating Pickering emulsions from monomers, immiscible with water, whereas NPs of interest act as stabilizers, and become trapped at the monomer/water interface. Next, the Pickering emulsions are polymerized, and polymer microspheres, having NPs trapped at the interface, are obtained. With NanoTraPPED the immersion depth and contact angles of NPs into the various polymer surfaces can be measured with the electron microscope. We have used the contact angles as input for the Owens-Wendt-Rable-Kaelble surface energy models, to determine the total surface energy and its polar and disperse components of NPs modified with various functional groups. We demonstrate that NanoTraPPED is able to quantitatively determine the total interfacial energy of NPs with water γNP/water and air γNP, with its polar and disperse components γdNP/water, γpNP/water, γdNP , γpNP and thus can make predictions with respect to their behaviour.

Introducing a new method for measuring the polar and dispersive components of the surface energy of nanoparticles

ACS Spring 2022

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