Specific Hydration on p-Nitroaniline Crystal Studied by Atomic Force Microscopy

Abstract

The molecular-scale structure of water was studied over the (101) surface of p-nitroaniline crystals using advanced atomic force microscopy. p-Nitroaniline contains two polar groups on opposite ends of the nonpolar benzene ring and presents a surface of controlled heterogeneity. The cross-sectional distribution of force applied to the tip was precisely determined and was related to the local density of the structured water. Force modulations were present on the polar end-groups and absent on the benzene ring, suggesting water localization on the polar end-groups.

Mercaptohexanol assembled on gold: FM-AFM imaging in water

“…The structured regions periodically appeared on top of the major protrusions on the surface. Following the assignment proposed in studies by the authors and other researchers [17][18][19][20][21][22][23], the modulated f distribution is ascribed to localized interfacial water. The lateral location of localized water is consistent with the structure proposed in Figure 1(c), where OH end-groups are present at the brightest protrusions.…”

“…Visualization in organic solvents (dodecane, hexadecane and phenyloctane) is also possible [22]. A recent study of water on crystalline p-nitroaniline by the authors [23] showed that water density was modulated on the amino and nitro end-groups exposed on the surface, not on the benzene ring. Mercaptohexanol contains OH and SH end-groups with an n-hexyl chain in between.…”

Simulating Solid-Liquid Interfaces in Atomic Force Microscopy

“…An entirely different set of surfaces is formed by the molecular crystals, with the organic crystals being the vast majority. An example a molecular crystal that has recently been investigated in AFM experiments is p-nitroaniline [93,94]. This organic compound is of interest as it contains two polar groups (amino and nitro) present on opposite ends of a non-polar benzene ring (phenylene), allowing the control of the heterogeneity of the surface.…”

“…Although in some situations the P21/m appears in the AFM experiments, the P21/c surface structure is much more commonly observed [93]. Currently this different surface structure has not been directly observed in crystallography (or any other non-AFM) experiments, as these were mainly focused on the bulk phase instead, but the simulations here point to a possible surface reconstruction, offering different ordering of water on the surface, all in excellent agreement with the majority of experimental results.…”

Advanced Instrumentation of Frequency Modulation AFM for Subnanometer-Scale 2D/3D Measurements at Solid-Liquid Interfaces

“…Since 2010, FM-AFM has been used for visualizing vertical-2D or 3D force distributions at various solid-liquid interfaces [23,[25][26][27][28][29][30][31][32][33][34][35]. For interfaces between water and a rigid inorganic crystal, atomistic molecular dynamics (MD) simulation studies have been started towards detailed understanding of the imaging mechanism [?, [36][37][38][39][40].…”