Surface activity and molecular organization of metallacarboranes at the air-water interface revealed by nonlinear optics

Because of their amphiphilic structure, surfactants adsorb at the water-air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases, and surface activity. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.

Phase-transfer energetics of small-molecule alcohols across the water-hexane interface: molecular dynamics simulations using charge equilibration models

We study the water-hexane interface using molecular dynamics (MD) and polarizable charge equilibration (CHEQ) force fields. Bulk densities for TIP4P-FQ water and hexane, 1.0086±0.0002 and 0.6378±0.0001 g/cm(3), demonstrate excellent agreement with experiment. Interfacial width and interfacial tension are consistent with previously reported values. The in-plane component of the dielectric permittivity (ɛ(||)) for water is shown to decrease from 81.7±0.04 to unity, transitioning longitudinally from bulk water to bulk hexane. ɛ(||) for hexane reaches a maximum in the interface, but this term represents only a small contribution to the total dielectric constant (as expected for a non-polar species). Structurally, net orientations of the molecules arise in the interfacial region such that hexane lies slightly parallel to the interface and water reorients to maximize hydrogen bonding. Interfacial potentials due to contributions of the water and hexane are calculated to be -567.9±0.13 and 198.7±0.01 mV, respectively, giving rise to a total potential in agreement with the range of values reported from previous simulations of similar systems. Potentials of mean force (PMF) calculated for methanol, ethanol, and 1-propanol for the transfer from water to hexane indicate an interfacial free energy minimum, corresponding to the amphiphilic nature of the molecules. The magnitudes of transfer free energies were further characterized from the solvation free energies of alcohols in water and hexane using thermodynamic integration. This analysis shows that solvation free energies for alcohols in hexane are 0.2-0.3 kcal/mol too unfavorable, whereas solvation of alcohols in water is approximately 1 kcal/mol too favorable. For the pure hexane-water interfacial simulations, we observe a monotonic decrease of the water dipole moment to near-vacuum values. This suggests that the electrostatic component of the desolvation free energy is not as severe for polarizable models than for fixed-charge force fields. The implications of such behavior pertain to the modeling of polar and charged solutes in lipidic environments.

Interfacial structure, thermodynamics, and electrostatics of aqueous methanol solutions via molecular dynamics simulations using charge equilibration models

We present results from molecular dynamics simulations of methanol-water solutions using charge equilibration force fields to explicitly account for nonadditive electronic interaction contributions to the potential energy. We study solutions across the concentration range from 0.1 to 0.9 methanol mole fraction. At dilute concentrations, methanol density is enhanced at the liquid-vapor interface, consistent with previous molecular dynamics and experimental studies. Interfacial thickness exhibits a monotonic increase with increasing methanol mole fraction, while surface tensions display monotonic decrease with methanol concentration, in qualitative agreement with experimental data and previous molecular dynamics predictions using polarizable force fields. In terms of interfacial structure, in keeping with predictions of traditional force fields, there is a unique preferential orientation of methanol molecules at the interface. Moreover, there is a free energetic preference for methanol molecules at the interface as evidenced by potential of mean force calculations. The pmf calculations suggest an interfacial state with 0.8 kcal/mol stability relative to the bulk, again in qualitative agreement with previous simulation and experimental studies. Interfacial potentials based on double integration of total charge density range from -610 to -330 mV over the dilute to concentrated regimes, respectively. The preponderance of methanol at the interface at all mole fractions gives rise to a dominant methanol contribution to the total interfacial potential. Interestingly, there is a transition of the water surface potential contribution from negative to positive upon the transition from methanol mole fraction of 0.1 to 0.2. The dipole and quadrupole contributions to the water component of the total interfacial potential are effectively of equal magnitude and opposite sign, thus cancelling one another. We compute the in-plane component of the dielectric permittivity along the interface normal. We observe a nonmonotonic behavior of the methanol in-plane dielectric permittivity that tracks the methanol density profiles at low methanol mole fractions. At higher methanol mole fractions, the total in-plane permittivity is dominated by methanol and displays a monotonic decrease from bulk to vapor. We finally probe the nature of hydration of water in the bulk versus interfacial regions for methanol mole fractions of 0.1 and 0.2. In the bulk, methanol perturbs water structure so as to give rise to water hydrogen bond excesses. Moreover, we observe negative hydrogen bond excess in the vicinity of the alkyl group, as reported by Zhong et al. for bulk ethanol-water solutions using charge equilibration force fields, and positive excess in regions hydrogen bonding to nearest-neighbor methanol molecules. Within the interfacial region, water and methanol density reduction lead to concomitant water hydrogen bond deficiencies (negative hydrogen-bond excess).

Revisiting the hexane-water interface via molecular dynamics simulations using nonadditive alkane-water potentials

We present results addressing properties of a polarizable force field for hexane based on the fluctuating charge (FQ) formalism and developed in conjunction with the Chemistry at Harvard Molecular Mechanics (CHARMM) potential function. Properties of bulk neat hexane, its liquid-vapor interface, and its interface with a polarizable water model (TIP4P-FQ) are discussed. The FQ model is compared to a recently modified alkane model, C27r, also based on the CHARMM potential energy function. With respect to bulk properties, both models predict bulk density within 1%; the FQ model predicts the liquid vaporization enthalpy within 2%, while the C27r force field underestimates the property by roughly 20% (and in this sense reflects the quality of the C27r force field across the spectrum of linear and branched alkanes). The FQ hexane model realistically captures the dielectric properties of the bulk in terms of a dielectric constant of 1.94, in excellent agreement with experimental values in the range of 1.9-2.02. This behavior is also in conformity with a recent polarizable alkane model based on Drude oscillators. Furthermore, the bulk dielectric is essentially captured in the infinite frequency, or optical, dielectric contribution. The FQ model is in this respect a more realistic force field for modeling lipid bilayer interiors for which most current state-of-the-art force fields do not accurately capture the dielectric environment. The molecular polarizability of the FQ model is 11.79 A3, in good agreement with the range of experimental and ab initio values. In contrast to FQ models of polar solvents such as alcohols and water, there was no need to scale gas-phase polarizabilities in order to avoid polarization catastrophes in the pure bulk. In terms of the liquid-vapor and liquid-liquid interfaces, the FQ model displays a rich orientational structure of alkane and water in the respective interfacial systems, in general conforming with earlier simulation studies of such interfaces. The FQ force field shows a marked deviation in the interfacial dipole potentials computed from the charge densities averaged over simulation trajectories. At the liquid-vapor interface, the FQ model predicts a potential drop of -178.71 mV in contrast to the C27r estimate of -433.80 mV. For the hexane-water interface, the FQ force field predicts a dipole potential drop of -379.40 mV in contrast to the C27r value of -105.42 mV. Although the surface dipole potential predicted by the FQ model is roughly 3.5 times that predicted by the C27r potential, it is consistent with reported experimental potentials across solvated lipid bilayers in the range of 400-600 mV.

Grid-based dynamic electronic publication: a case study using combined experiment and simulation studies of crown ethers at the air/water interface

Conventional paper-based scientific publications are limited in the amount of data and interaction they can provide. Simply placing an electronic copy of such a publication on the web makes for easier distribution, but more can be achieved by making use of the technology to navigate through the paper, to show the links between calculated parameters and the data, and provide access to the chain of calculated results all the way back to the raw data and ultimately the laboratory notebook. While the paper version of this publication is in a relatively conventional form, an interactive demonstrator version (available at http://epaper.combe.chem.soton.ac.uk and as an installable package) illustrates the concepts of publication@source, whereby all the figures and data presented in the paper are linked back to the original raw data together with a description of the processes by which the raw data were analysed. This level of interactivity is achieved using semantic technologies, which have the additional advantage of making the final document subsequently available and navigable by automated techniques. We present the combined information from experimental studies of surface tension and second harmonic generation (SHG) on the behaviour of benzo-15-crown-5 at the solution/air interface, together with a molecular dynamics computer simulation, to demonstrate how the simulation aids the interpretation of the SHG experiment. The adsorption isotherm was determined using SHG and fitted to a Langmuir isotherm giving Delta ads G0=26 kJ mol-1.

Application of Transient Evanescent Grating Techniques to the Study of Liquid/Liquid Interfaces

Transient grating experiments performed with evanescent fields resulting from total internal reflection at an interface between a polar absorbing solution and an apolar transparent solvent are described. The time evolution of the diffracted intensity was monitored from picosecond to millisecond time scales. The diffracted signal originates essentially from two density phase gratings: one in the absorbing phase induced by thermal expansion and one in the transparent solvent due to electrostriction. A few nanoseconds after excitation, the latter grating is replaced by a thermal grating due to thermal diffusion from the absorbing phase. The speed of sound and the acoustic attenuation measured near the interface are found to be essentially the same as in the bulk solutions. However, after addition of a surfactant in the polar phase, the speed of sound near the interface differs substantially from that in the bulk with the same surfactant concentration. This effect is interpreted in terms of adsorption at the liquid/liquid interface. Other phenomena, which are not observed in bulk experiments, such as acoustic echoes and a fast oscillation of the signal intensity, are also described.