Second harmonic generation measurements of molecular orientation and coadsorption at the interface between two immiscible electrolyte solutions

Chemical Reaction Dynamics at Liquid Interfaces: A Computational Approach

Recent advances in experimental and theoretical studies of liquid interfaces provide remarkable evidence for the unique properties of these systems. In this review we examine how these properties affect the thermodynamics and kinetics of chemical reactions which take place at the liquid/vapor interface and at the liquid/liquid interface. We demonstrate how the rapidly varying density and viscosity, the marked changes in polarity and the surface roughness manifest themselves in isomerization, electron transfer and photodissociation reactions.

Fabrication of Supramolecular Chirality from Achiral Molecules at the Liquid/Liquid Interface Studied by Second Harmonic Generation

We present the investigation into the supramolecular chirality of 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) at water/1,2-dichloroethane interface by second harmonic generation (SHG). We observe that PARC18 molecules form supramolecular chirality through self-assembly at the liquid/liquid interface although they are achiral molecules. The bulk concentration of PARC18 in the organic phase has profound effects on the supramolecular chirality. By increasing bulk concentration, the enantiomeric excess at the interface first grows and then decreases until it eventually vanishes. Further analysis reveals that the enantiomeric excess is determined by the twist angle of PARC18 molecules at the interface rather than their orientational angle. At lower and higher bulk concentrations, the average twist angle of PARC18 molecules approaches zero, and the assemblies are achiral; whereas at medium bulk concentrations, the average twist angle is nonzero, so that the assemblies show supramolecular chirality. We also estimate the coverage of PARC18 molecules at the interface versus the bulk concentration and fit it to Langmuir adsorption model. The result indicates that PARC18 assemblies show strongest supramolecular chirality in a half-full monolayer. These findings highlight the opportunities for precise control of supramolecular chirality at liquid/liquid interfaces by manipulating the bulk concentration.

The Nonlinear Optical Response of Pt(111) Electrodes in Perchloric Acid Solution: Implications for the Potential of Zero Charge

Second harmomic and sum frequency generation (SHG, SFG) are used to investigate the second-order non-linear optical response of a Pt(111) single-crystal electrodes in perchloric acid solution. In the potential window between hydrogen evolution and surface oxidation the SHG signal shows a pronounced minimum at 760 mVRHE and rises linearly with decreasing electrode potential. The potential of the minimum as well as the magnitude of the signal in the potential range of hydrogen adsorption depend on the pH of the electrolyte. The dependence of the SHG signal on excitation frequency in the range of 9000–1200 0cm−1 shows a continuous SHG signal increase to higher frequencies without characteristic surface resonances. The SHG signal is assigned to the excitation of a continuum of electronic levels. The maximum of the signal intensity is observed at potentials close to 0 VRHE, where hydrogen evolution takes place and the surface has a maximum of negative charge. Sum-frequency spectra of CO adsorbed on Pt(111) exhibit the known vibrational signature of terminal and bridge-like coordination and an additional resonant signal. On the same surface, the SHG signal is characterized by a high signal intensity which remains constant up to the CO oxidation potential. The potential dependence of the nonlinear response of the Pt(111)/CO surface as well as of the neat surface in perchloric acid indicates a high sensitivity to the surface charge. As a consequence, a negatively charged surface up to a potential of 600 mVRHE is deduced. Our results are at variance with a value for the potential of zero charge of 0.34 VRHE which was derived from the CO charge displacement method, but in agreement with the value based on the immersion technique (U. W. Hamm et al. , J. Electroanal. Chem. 414 (1996) 85).

Self-Aggregation of the SDS Surfactant at a Solid−Liquid Interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.

Mixtures of sodium dodecyl sulfate/dodecanol at the air/water interface by computer simulations

Molecular dynamics simulations of monolayers of surfactant mixtures at the air/water interface were performed where the binary mixture was composed of sodium dodecyl sulfate (SDS) and dodecanol molecules. At the same ratio of SDS and dodecanol molecules, two monolayer mixtures were prepared. In the first monolayer, all the dodecanol molecules were placed together in the center of the simulation box, whereas in the second monolayer, those molecules were uniformly distributed in the surface area in such a way that they were far from each other. Simulations of both systems indicate that the dodecanol tails in the first monolayer are straighter and more ordered than those in the second monolayer. From the present results, we observed new insights of how the different molecules should array or distribute at the interface in real systems. Finally, studies of the interfacial water around the different surfactants were also analyzed, showing that they are closer to the polar headgroups of dodecanol than to the SDS headgroups.

Computer simulation studies of surfactant monolayer mixtures at the water/oil interface: charge distribution effects

Charge distribution effects on polar head groups for a mixture of amphiphilic molecules at the water/oil interface were studied. For this purpose a model which allowed us to investigate the charge effects exclusively was created. As a molecular model we used the structure of sodium dodecyl sulfate. Then we prepared molecules with the same molecular structure but with different charge distributions in order to have one cationic and one nonionic molecule. So, in this way, we were able to focus only in the charge effects. The monolayer mixtures were composed of anionic/nonionic and cationic/nonionic surfactants. Simulations of these systems show that the location of the different surfactants at the interface is determined by the interaction and the charge distribution of the molecules. Due to the difference in the charge distribution of the surfactant monolayers, the water molecules present distinct orientations in the mixture. Finally, it was found that the electrostatic potential difference across the interface depended on the interactions (charge distribution) of the anionic, cationic, and nonionic molecules in the mixture.

Total internal reflection fluorescence and electrocapillary investigations of adsorption at a H2O-dichloroethane electrochemical interface. 1. Low-frequency behavior

Total internal reflection fluorescence and electrocapillary measurements are employed to provide complementary potential-dependent information about the mechanical and photophysical properties of the interface between two immiscible electrolyte solutions, 1,2-dichloroethane-H2O. Adsorption of the zwitterionic amphiphile, di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate (I) produces an interface with mechanical (interfacial tension) and charge transport properties qualitatively like the unmodified interface. Addition of dilauroylphosphatidylcholine (DLPC) to the organic phase produces an interface dominated by DLPC adsorption and drastically alters the potential dependence of the interfacial tension, gamma, the interfacial excess populations, GammaI, the charge transport, and the fluorescence response from I. This result is explained in terms of a potential-dependent protonation of the DLPC at the interface, which causes it to desorb, and a competition for interfacial sites between DLPC and protonated and unprotonated dye I. Protonation of DLPC results in a rise in gamma, which is correlated with an increase in transport of the organic-phase anion tetraphenylborate, TPB-, and an increase in interfacially excited fluorescence from I. Both results are explained by a model in which the mechanical properties of the interface, as determined by the interfacial DLPC population, direct the ability of other species to transfer across TPB- or adsorb to I the interface.

Molecular structure and dynamics at liquid-liquid interfaces

The structural, dynamical, and electrical properties of the interface between two immiscible liquids are described. The adsorption of solute molecules and the processes of ion transfer across the interface and of electron transfer at the interface are discussed. The microscopic perspective is emphasized by focusing on selected recent experimental results and on results obtained from molecular dynamics and Monte Carlo computer simulations. The validity of some of the existing models of the interface is examined. A proper account of the molecular structure of the interface is important for understanding solvation and charge transfer processes at the interface.