SFE of particles

Surface free energy (SFE) is the excess energy per unit surface area. It is a quantitative thermodynamic measure of intermolecular and surface forces and therefore governs the hydrophobicity and wettability of a material. For liquid–fluid interfaces, such as air–liquid and liquid–liquid interfaces, SFE is equivalent to surface or interfacial tension, which can be readily determined using established methods such as the Wilhelmy plate and drop shape analysis. In contrast, the SFE of a solid surface cannot be measured directly. Despite longstanding controversies regarding its theoretical interpretation, the contact angle method remains the only established approach for determining the SFE of bulk solid materials.

SFE, or more broadly hydrophobicity, is among the most important physicochemical properties of nanoparticles (NPs), playing a critical role in determining particle dispersion and aggregation, nano–bio interactions, and the fate of NPs in biological and environmental systems. Despite its importance, characterization of NP SFE or hydrophobicity remains highly challenging due to nanoscale heterogeneity and dynamic environmental effects. Numerous methods have been developed to characterize NP hydrophobicity. These methods can generally be classified into two categories: qualitative methods, which rank the relative hydrophobicity of NPs, and quantitative methods, which determine NP SFE. However, none of these approaches has been universally accepted as a standard method for nanoparticle characterization. Therefore, there is an urgent need to develop an accurate, easy-to-use, and cost-effective method for quantitative determination of NP hydrophobicity. 

Over the years, we have developed two completely independent and practical methods for characterizing the SFE or hydrophobicity of micro- and nanoparticles. The first method, termed maximum particle dispersion (MPD), is based on a novel measurement principle that exploits the control of DLVO-governed colloidal stability in nanoparticle suspensions. The second method, termed nonionic dye partitioning (NIDP), was developed based on the classical principle of “like dissolves like.” MPD enables quantitative determination of the SFE of micro- and nanoparticles, whereas NIDP provides a simple means to rank the relative hydrophobicity of diverse particles. Both methods can be readily implemented using routine optical instrumentation, such as a microplate reader. Consequently, these approaches hold considerable promise for development into easy-to-use, standardized methods for quantitative characterization of the hydrophobicity of micro- and nanoparticles.