Using Diffusion To Characterize Interfacial Heterogeneity

Abstract

We report on the use of molecular diffusional motion over a range of length scales to characterize compositional heterogeneity in monolayer structures. This work focuses on the diffusional motion of perylene in two types of films supported on functionalized silica surfaces: single-component (stearic acid) and two-component (hydrocarbon/fluorocarbon) films. Langmuir-Blodgett (LB) monolayers were deposited directly on silica or were bound to surface-modified silica by means of metal ion complexation. The LB films were characterized by their π-A isotherms and by Brewster angle microscopy (BAM) during formation and deposition. Chromophore mobility and monolayer structural heterogeneity were evaluated by comparing rotational diffusion data (fluorescence anisotropy decay imaging) and translational diffusion data (fluorescence recovery after photobleaching) on the same LB films. Our results indicate that the mobility of the chromophore depends sensitively on both metal ion identity and film composition.

Effects of Cu(II) on the Formation and Orientation of an Arachidic Acid Langmuir–Blodgett Film

“…These metal ions function as binding and potentially as ordering agents due to the inherent charge of the ion and its ability to coordinate polar organic functionalities. The presence of metal ions tends to increase the LB monolayer stability. ,,,, In addition, metal-ion complexation can be used to mediate monolayer fluid properties on a solid support, , depending on metal-ion identity and charge. Divalent cations have been investigated most extensively, with Cd 2+ being the ion studied most thoroughly.…”

“…Divalent cations have been investigated most extensively, with Cd 2+ being the ion studied most thoroughly. With arachidic acid as the amphiphile, cadmium arachidate has become the reference system for transition metal–amphiphile LB monolayers. ,,− …”

“…The presence of metal ions tends to increase the LB monolayer stability. 1,28,39,42,43 In addition, metal-ion complexation can be used to mediate monolayer fluid properties on a solid support, 43,44 depending on metal-ion identity and charge. Divalent cations have been investigated most extensively, with Cd 2+ being the ion studied most thoroughly.…”

Spectroscopic Analysis of Cu(II)-Complexed Thin Films to Characterize Molecular-Level Interactions and Film Behavior

“…The amphiphile chain tilt angle and organization were characterized by using −CH 2 symmetric and asymmetric stretching frequencies and band intensity ratios. The film fluidity was measured with fluorescence recovery after photobleaching (FRAP) measurements of a chromophore confined within the aliphatic chain region of the monolayer. ,, We find that Cu 2+ –amphiphile interactions affect the diffusional behavior of the chromophore, and these interactions depend on the subphase pH. These findings point to the central role of the Cu 2+ –arachidic acid complex in controlling the properties of the monolayer.…”

“…A variety of methods exist to deposit monolayer films, including self-assembly of thiols on coinage metals, vapor deposition, dip coating, and chemical bonding. − Langmuir–Blodgett (LB) deposition is one of the oldest methods, and it provides a facile means of forming and depositing monolayers under conditions of controlled pressure and organization. ,, LB deposition affords control over amphiphile identity, the incorporation of metal ions and/or chromophores, the number of layers deposited, and the orientation of each adlayer. Metal ion complexation, where metal ions reside between the substrate and amphiphilic adlayer, has been shown to enforce monolayer organization and affect the mobility of monolayer constituents. − The effects of monovalent ions (e.g., Li + , Na + , and K + ) and divalent ions (e.g., Mg 2+ , Ca 2+ , Ba 2+ , and Zn 2+ ) on film properties have been studied in detail and shown to enhance amphiphile packing and film robustness. − ,,,, Such enhancements of film structure can broaden the range of applications for which these interfaces are used.…”