Step-Scan IR Spectroelectrochemistry with Ultramicroelectrodes: Nonsurface Enhanced Detection of Near Femtomole Quantities Using Synchrotron Radiation

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latter’s pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed

Femtomole Infrared Spectroscopy at the Electrified Metal–Solution Interface

Characterization of surface adsorbed species using infrared (IR) spectroscopy provides valuable information concerning interfacial chemical and physical processes. However, <i>in situ</i> infrared studies of surface areas approaching the IR diffraction limit, such as micrometer scale electrodes, require a hitherto unrealized means to obtain high signal-to-noise (S/N) spectra from femtomole quantities of adsorbed molecules. A major methodological breakthrough is described that couples the high brilliance of synchrotron-sourced infrared microscopy with attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS). The method is shown to allow the spectral measurement of a monolayer of 4-methoxypyridine (MOP) adsorbed on a surface enhancing gold film electrode under fully operational electrochemistry conditions. A factor of 15 noise improvement is achieved with small apertures using synchrotron IR relative to a thermal IR source. The very low noise levels allow the measurement of high quality IR spectra of 2.5 fmol of molecules confined to a 125 μm² beam spot

Comparing and Correlating Solubility Parameters Governing the Self-Assembly of Molecular Gels Using 1,3:2,4-Dibenzylidene Sorbitol as the Gelator

Solvent properties play a central role in mediating the aggregation and self-assembly of molecular gelators and their growth into fibers. Numerous attempts have been made to correlate the solubility parameters of solvents and gelation abilities of molecular gelators, but a comprehensive comparison of the most important parameters has yet to appear. Here, the degree to which partition coefficients (log <i>P</i>), Henry’s law constants (HLC), dipole moments, static relative permittivities (ε_r), solvatochromic <i>E</i>_T(30) parameters, Kamlet–Taft parameters (<i>β</i>,<i> α</i>, and π), Catalan’s solvatochromic parameters (SPP, SB, and SA), Hildebrand solubility parameters (δ_<i>i</i>), and Hansen solubility parameters (δ_p, δ_d, δ_h) and the associated Hansen distance (<i>R</i>_<i>ij</i>) of 62 solvents (covering a wide range of properties) can be correlated with the self-assembly and gelation of 1,3:2,4-dibenzylidene sorbitol (DBS) gelation, a classic molecular gelator, is assessed systematically. The approach presented describes the basis for each of the parameters and how it can be applied. As such, it is an instructional blueprint for how to assess the appropriate type of solvent parameter for use with other molecular gelators as well as with molecules forming other types of self-assembled materials. The results also reveal several important insights into the factors favoring the gelation of solvents by DBS. The ability of a solvent to accept or donate a hydrogen bond is much more important than solvent polarity in determining whether mixtures with DBS become solutions, clear gels, or opaque gels. Thermodynamically derived parameters could not be correlated to the physical properties of the molecular gels unless they were dissected into their individual HSPs. The DBS solvent phases tend to cluster in regions of Hansen space and are highly influenced by the hydrogen-bonding HSP, δ_h. It is also found that the fate of this molecular gelator, unlike that of polymers, is influenced not only by the magnitude of the distance between the HSPs for DBS and the HSPs of the solvent, <i>R</i>_<i>ij</i>, but also by the directionality of <i>R</i>_<i>ij</i>: if the solvent has a larger hydrogen-bonding HSP (indicating stronger H-bonding) than that of the DBS, then clear gels are formed; opaque gels form when the solvent has a lower δ_h than does DBS