Organic Monolayers as Mimics of Liquid/Liquid Interfaces: Molecular Dynamics Study of Electronic Spectra and Solvent Dynamics

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Hydrogen-Bond Structure and Dynamics at the Interface between Water and Carboxylic Acid-Functionalized Self-Assembled Monolayers

Molecular dynamics computer simulations are used to study hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers (CAFSAMs). Water-water, water-CAFSAM, and internal CAFSAM hydrogen bonds are examined. Roughly half of all adjacent carboxylic acid-terminated hydrocarbon chains are hydrogen-bonded to one another. This is consistent with experimental results reflecting two pKa values for CAFSAMs. Hydrogen-bond dynamics are expressed in terms of hydrogen-bond population autocorrelation functions and are found to be nonexponential. The water-water hydrogen-bond dynamics are slower at the interface than in the bulk, which is similar to what was found at the interface between water and weakly polar liquids such as nitrobenzene. The water-CAFSAM hydrogen bonds are found to be long-lived, on the order of tens of picoseconds. Internal CAFSAM chain-chain hydrogen bonds show almost no relaxation on the simulation time scale.

Structure and organization of hexadecanol isomers adsorbed to the air/water interface

The structure and 2D phase behavior of hexadecanol isomers adsorbed to the air/water interface have been studied using surface tension methods and vibrational sum frequency spectroscopy. Isomers include the linear 1-hexadecanol as well isomers with the alcohol functional group in the 2, 3, and 4 positions. Surface-pressure isotherms highlight how the 2D phase behavior of these monolayers depends sensitively on registry and packing efficiency between the alkyl chains whereas vibrational sum frequency spectroscopy, which is vibrational spectroscopy with surface specificity, reveals details about the molecular structure and orientation of molecules within the monolayer films at their equilibrium spreading pressures. At their equilibrium spreading pressures, both 1- and 2-hexadecanol form compact films having a high degree of conformational order and molecular areas of 18.9 and 21.5 A(2)/molecule, respectively. This result for 2-hexadecanol implies that the isomer remains primarily in an all-trans conformation with the methyl group in the C(1) position buried in the water subphase. This conformation leads to significantly reduced intensity in specific vibrational transitions due to partial destructive interference. In contrast, 3-hexadecanol and 4-hexadecanol form more expanded monolayers at their equilibrium spreading pressures, having areas of 28.7 and 40.3 A(2)/molecule, respectively. In these monolayers, the intensities of selected vibrational bands show less evidence of destructive interference, implying that methyl groups on opposite ends of the adsorbates do not adopt strongly correlated orientations.

Using the dynamic, expanding liquid–liquid interface in a Hele–Shaw cell in crystal growth and nanoparticle assembly

The liquid-liquid interface has been used with considerable success in the synthesis of advanced materials ranging from (bio)minerals to inorganic membranes to nanoparticles. In almost all such cases, the interface is static. The Hele-Shaw cell in which a viscous fluid is displaced by a less viscous one in a constrained manner has been invaluable in the study of dynamic instabilities at interfaces and in the study of viscous fingering pattern formation. However, the potential of the Hele-Shaw cell in carrying out reactions at the interface between the two fluids leading to the formation of inorganic materials has been largely unrecognized and underexploited. Realizing that the dynamic liquid-liquid interface in a Hele-Shaw cell would provide opportunities to control a variety of time-scales associated with material formation, we have started a program on the use of the Hele-Shaw cell in materials synthesis. In this discussion paper, we present some of our recent results on the growth of calcium carbonate crystals in the Hele-Shaw cell by the reaction of Ca2+ ions electrostatically complexed with carboxylate ions pinned to the interface with carbonate ions present in the aqueous part of the biphasic reaction medium. We show that both polymorph selectivity and the morphology of the crystals may be modulated by varying the experimental conditions in the cell. We also discuss the possibility of using the dynamic interface in the Hele-Shaw cell to cross-link gold nanoparticles in water through bifunctional linkers present in the oil phase and investigate the nature of the structures formed.

Reversible light-driven redox switching of multifunctional dipolar ruthenium(III/II) pentaammine(4,4'-bipyridinium) complexes

We report the first example of a molecular switch of multifunctional dipolar ruthenium(III/II) pentaammine-N-methyl-(4,4'-bipyridinium) complexes, exclusively driven by light. This is achieved by using a two-phase (water/benzene) system in which RuIII/II complexes are soluble only in the water phase. The reversible redox switching is triggered by the selective irradiation of the water and the benzene compartments with 254 and 528 nm light, respectively.