Contact angle and adsorption energies of nanoparticles at the air–liquid interface determined by neutron reflectivity and molecular dynamics

Abstract

Understanding how nanomaterials interact with interfaces is essential to control their self-assembly as well as their optical, electronic, and catalytic properties. We present here an experimental approach based on neutron reflectivity (NR) that allows the in situ measurement of the contact angles of nanoparticles adsorbed at fluid interfaces. Because our method provides a route to quantify the adsorption and interfacial energies of the nanoparticles in situ, it circumvents problems associated with existing indirect methods, which rely on the transport of the monolayers to substrates for further analysis. We illustrate the method by measuring the contact angle of hydrophilic and hydrophobic gold nanoparticles, coated with perdeuterated octanethiol (d-OT) and with a mixture of d-OT and mercaptohexanol (MHol), respectively.The contact angles were also calculated via atomistic molecular dynamics (MD) computations, showing excellent agreement with the experimental data. Our method opens the route to quantify the adsorption of complex nanoparticle structures adsorbed at fluid interfaces featuring different chemical compositions.

Influence of the liquid helium meniscus on neutron reflectometry data

“…In the case of 4 He the coherent scattering length is 3.26 fm, with no incoherent scattering or significant neutron absorption. For 3 He, on the other hand, the situation is significantly different with a coherent scattering factor of 5.74 À 1.483i fm, where the imaginary number represents a resonant absorption term; it also has a significant incoherent scattering factor of À2.5… þ 2.568i fm that leads to a neutron scattering background.…”

“…In later experiments with neutron reflection from the free surfaces of commercially pure 4 He and of a 3 He- 4 He mixture with a 3 He concentration of 0.5% it was found that the addition of 3 He isotopic impurities could be leading to the formation of Andreev levels 7 at low temperatures (around 0.3 K) resulting in the formation of a diffusive 3 He nanolayer with a thickness of a few hundred Å on the bulk 4 He liquid surface. 8,9 Unfortunately, useful numerical analysis of the experimental data turned out to be impossible.…”

“…The results are also applicable to reflectometry from liquids more generally, wherever there is the possibility of a meniscus. He on the surface of nanoseparated 3 He/ 4 He liquid mixtures. 16 Neutron reflectometry offers a unique opportunity for direct observation of nanostratification/interdiffusion of the 3 He/ 4 He mixture in the ultra-low temperature limit.…”

“…More than four decades ago NR was used for the first time at low temperatures (at 1.23 K) for measuring the real part of the bound-atom coherent neutron scattering length of 3 He by comparing the reflectivities of quartz-liquid- 3 He and quartzliquid- 4 He interfaces. This parameter was found to have the value (6.1 6 0.6) 10 À13 cm.…”

“…We thus investigated the surface properties of liquid 3 He/ 4 He mixtures by neutron reflection for temperatures in the range 0.08 K < T < 2.2 K (Refs. 8 and 9) both for commercial helium with $0.3 ppm (3 Â 10 À5 %) of 3 He, and for a stronger 0.1% 3 He/ 4 He mixture.…”