AFM Study of Surface Nanobubbles on Binary Self-Assembled Monolayers on Ultraflat Gold with Identical Macroscopic Static Water Contact Angles and Different Terminal Functional Groups

Abstract

All experimental findings related to surface nanobubbles, such as their pronounced stability and the striking differences of macroscopic and apparent nanoscopic contact angles, need to be addressed in any theory or model of surface nanobubbles. In this work we critically test a recent explanation of surface nanobubble stability and their consequences and contrast this with previously proposed models. In particular, we elucidated the effect of surface chemical composition of well-controlled solid-aqueous interfaces of identical roughness and defect density on the apparent nanoscopic contact angles. Expanding on a previous atomic force microscopy (AFM) study on the systematic variation of the macroscopic wettability using binary self-assembled monolayers (SAMs) on ultraflat template stripped gold (TSG), we assessed here the effect of different surface chemical composition for macroscopically identical static water contact angles. SAMs on TSG with a constant macroscopic water contact angle of 81 ± 2° were obtained by coadsorption of a methyl-terminated thiol and a second thiol with different terminal functional groups, including hydroxy, amino, and carboxylic acid groups. In addition, surface nanobubbles formed by entrainment of air on SAMs of a bromoisobutyrate-terminated thiol were analyzed by AFM. Despite the widely differing surface potentials and different functionality, such as hydrogen bond acceptor or donor, and different dipole moments and polarizability, the nanoscopic contact angles (measured through the condensed phase and corrected for AFM tip broadening effects) were found to be 145 ± 10° for all surfaces. Hence, different chemical functionalities at identical macroscopic static water contact angle do not noticeably influence the apparent nanoscopic contact angle of surface nanobubbles. This universal contact angle is in agreement with recent models that rely on contact line pinning and the equilibrium of gas outflux due to the Laplace pressure and gas influx due to gas oversaturation in the aqueous medium.

Leakiness of Pinned Neighboring Surface Nanobubbles Induced by Strong Gas–Surface Interaction

“…The stability conditions and expression 1 for the contact angle of a single surface nanobubble at equilibrium were also confirmed by MD simulations 14 and experiments. 15 In most practical scenarios, nanobubbles are however not single, but exist in the neighborhood of other nanobubbles. 1 , 16 − 20 A recent theoretical study 21 shows that two nanobubbles remain stable against Ostwald ripening (mathematically, a linear instability), due to the pinning of the contact line.…”

Unfolding IGDQ Peptides for Engineering Motogenic Interfaces

“…We first characterized the SAMs properties by measuring their wettability, through water contact angle assessment (WCA , Table S2), since this technique is well used to elucidate macroscopically the static and dynamic structural properties of SAMs. 55,57,58,59,60,61,62,63,64 WCA measurements define the range of wettability values for all of the surfaces reported in this study. Specifically, WCA values of the SAMs prepared with only the surface fillers (Table S2, 54.3° has been found for IPS•Au, the latter exposing only the hydrophilic peptide (Table S2, entry 3), while surprisingly low values resulted from the surfaces exposing the hydrophobic one (41.7° for IS•Au, see Table S2 entry 4).…”

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

“…Here the improved resolution of the PTIR serves to analyze nanoscale regions of relatively thin samples. The amplitude of the PTIR signal (S, see eqn (9)) is dependent on the sample thickness (z), absorbed energy per unit area (U abs ), the thermal conductivity of the sample (h) and thermal expansion of the sample a exp (see eqn (9)). The PTIR signal was found to increase linearly with thickness of the sample up to a thickness of 1 mm.…”

“…The atomic force microscope (AFM) rapidly emerged as an invaluable tool for direct measurement of topography and intermolecular forces with atomic resolution for a broad spectrum of applications, such as electronics, [1][2][3] semiconductors, 4 materials and manufacturing, 5 polymers, 6 biology, and biomaterials. 7 An AFM is able to measure a sample at ambient conditions 8 and even in liquid [9][10][11] or buffer solutions, 12 which is in contrast to scanning tunneling microscopy (STM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This feature renders AFM especially useful for in vitro measurements of biological samples.…”