Assessment of nanoparticle immersion depth at liquid interfaces from chemically equivalent macroscopic surfaces

TL;DR

This study investigates whether macroscopic contact angles can predict nanoparticle wettability at liquid interfaces, revealing that receding contact angles on smooth surfaces are generally good indicators, but not always predictive of nanoparticle behavior.

Contribution

The paper demonstrates that macroscopic contact angles can be used to estimate nanoparticle wettability, with limitations highlighted for certain nanoparticle types and surface roughness conditions.

Findings

Reciprocal contact angles on smooth surfaces estimate nanoparticle wettability.

Macroscopic contact angles do not predict adsorption barriers for some nanoparticles.

Method facilitates assessing nanoparticle wettability for various applications.

Abstract

Hypothesis: We test whether the wettability of nanoparticles (NPs) straddling at an air/water surface or oil/water interface can be extrapolated from sessile drop-derived macroscopic contact angles (mCAs) on planar substrates, assuming that both the nanoparticles and the macroscopic substrates are chemically equivalent and feature the same electrokinetic potential. Experiments: Pure silica (SiO2) and amino-terminated silica (APTES-SiO2) NPs are compared to macroscopic surfaces with extremely low roughness (root mean square [RMS] roughness <= 2 nm) or a roughness determined by a close-packed layer of NPs (RMS roughness about 35 nm). Equivalence of the surface chemistry is assessed by comparing the electrokinetic potentials of the NPs via electrophoretic light scattering and of the macroscopic substrates via streaming current analysis. The wettability of the macroscopic substrates is obtained from advancing (ACAs) and receding contact angles (RCAs) and in situ synchrotron X-ray reflectivity (XRR) provided by the NP wettability at the liquid interfaces. Findings: Generally, the RCA on smooth surfaces provides a good estimate of NP wetting properties. However, mCAs alone cannot predict adsorption barriers that prevent NP segregation to the interface, as is the case with the pure SiO2 nanoparticles. This strategy greatly facilitates assessing the wetting properties of NPs for applications such as emulsion formulation, flotation, or water remediation.