Thickness, Composition, and Molecular Structure of Residual Thin Films Formed by Forced Dewetting of Ag from Glycerol/D₂O Solutions

Abstract

The thickness, composition, and interfacial molecular structure of residual thin films retained on the surface of polycrystalline Ag substrates after being forcibly dewet from glycerol/D2O solutions are investigated using contact angle measurements, ellipsometry, and polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS). Residual film thicknesses are rationalized on the basis of the relevant long-range van der Waals and structural forces leading to residual film formation along with the interfacial glycerol and D2O structure. Unique interfacial composition, wherein glycerol preferentially segregates to the residual film interfaces, is substantiated by PM-IRRAS. Thus, the residual films possess composition and molecular structure that differ from those of bulk solution. Specifically, in the thinnest residual films, glycerol interacts strongly with the Ag substrate, leading to glycerol that is more ordered than the bulk liquid that coexists with bulk-like D2O. In thicker residual films, the glycerol mole fraction is still enhanced relative to the bulk solution, but both ordered and liquid-like glycerol species are observed along with D2O that is more strongly hydrogen-bonded than in the bulk. The creation of residual films by forced dewetting and their interrogation by spectroscopic methods are thus demonstrated to represent a powerful approach for characterizing interfacial liquid molecular structure near solid surfaces but beyond the first monolayer under ambient conditions.

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report

“…For substrate immersion at velocities below those at which the dewetting process is controlled by hydrodynamic forces, intermolecular interactions hold an ultrathin (i.e., several nanometers) film of fluid at the solid interface which can then be studied spectroscopically. The dewetting process was described, and residual thin film thicknesses were rationalized quantitatively by Pemberton and co-workers in a study, wherein residual thin films of mixed glycerol–water compositions were forcibly dewetted on Ag surfaces and studied using ellipsometry and PM-IRRAS . Interfaces of other simple fluids have also been investigated by this technique.…”

Analysis of fluid film behaviour using dynamic wetting at a smooth and roughened surface

“…The wetting technique is similar to emersion, [14][15][16] and provides direct analysis of molecules that represent interfacial matter that would naturally be present at a fully immersed solid-liquid interface. [17][18][19][20][21][22] The lm interface is analyzed using a combination of polarization modulation infrared reection absorption spectroscopy (PM-IRRAS) and spectroscopic ellipsometry, which provide details on chemical interactions, structure, and lm thickness. Results from these data allow the construction of new chemical insight into the systems in which the uid-solid interface plays an important role.…”

Spectroscopic analysis of molecular fluids at the solid-liquid interface

“…34,35 The Pemberton group at the University of Arizona have undertaken multiple studies using this dynamic wetting technique in conjunction with infrared spectroscopy and ellipsometry. [34][35][36][37][38] In one study, 11-mercapto-1-undecanol-modifieid silver surfaces were rotated through various aqueous and non-aqueous solvents and ellipsometry was used to determine the effect of emersion velocity and fluid viscosity on emersed film thickness.…”

“…Because of the microscopic dimensions of the interface and the proximity of the bulk solution, it is very difficult to exclusively analyze material contained within the solid- The wetting technique is similar to emersion, [31][32][33] and provides direct analysis of molecules that represent interfacial matter that would naturally be present at a fully emersed solidliquid interface. 34,35,37,39,43,45 The film interface is analyzed using a combination of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and spectroscopic ellipsometry, which provide details on chemical interactions, structure and film thickness. Results from these data allow the construction of new chemical insight into the systems in which the solid-liquid interface plays an important role.…”

“…The diagnostic stretching vibrations of the D2O were fit to model line shapes to determine distinct environments of water at the interface. 38,56 Dutta and coworkers report at molecular orientation and shear effects by confining a fluid between a silicon window and a magnetic seal rotating together in a custom sample chamber. The molecular ordering is then examined under the play of shear forces.…”