Solute rotational dynamics at the water liquid/vapor interface

The rotational dynamics of a number of diatomic molecules adsorbed at different locations at the interface between water and its own vapors are studied using classical molecular dynamics computer simulations. Both equilibrium orientational and energy correlations and nonequilibrium orientational and energy relaxation correlations are calculated. By varying the dipole moment of the molecule and its location, and by comparing the results with those in bulk water, the effects of dielectric and mechanical frictions on reorientation dynamics and on rotational energy relaxation can be studied. It is shown that for nonpolar and weekly polar solutes, the equilibrium orientational relaxation is much slower in the bulk than at the interface. As the solute becomes more polar, the rotation slows down and the surface and bulk dynamics become similar. The energy relaxation (both equilibrium and nonequilibrium) has the opposite trend with the solute dipole (larger dipoles relax faster), but here again the bulk and surface results converge as the solute dipole is increased. It is shown that these behaviors correlate with the peak value of the solvent-solute radial distribution function, which demonstrates the importance of the first hydration shell structure in determining the rotational dynamics and dependence of these dynamics on the solute dipole and location.     

Formation of n-alkane layers at the vapor/water interface

We present a study on the initial wetting behaviors of two low molecular weight alkanes, heptane and octane, at the vapor/water interface using both neutron and X-ray reflectometry. Combined X-ray and neutron reflectivity studies data showed that a uniform film, which has never been reported, was formed continuously at 25 degrees C. As the adsorptive deposition continued, each adsorbed film was saturated at a specific equilibrium thickness: 48 and 36 A for deuterated heptane and octane, respectively, and 21 A for hydrogenated octane. The thickness of the adsorbed layer measured by neutron reflectivity is in agreement with that measured using X-ray reflectivity. Our observations of continuous and saturated adsorption behaviors are analyzed qualitatively using a kinetic adsorption model.     

Dynamic instability at liquid/vapor interface far from equilibrium

Attempt has been made to elucidate the mechanism of electric potential oscillations at oil-aqueous solution interface involving adsorption at oil-vapor interface on a semi-theoretical basis. The mechanism stipulates adsorption of ammonia, amines and pheromones at the liquid-vapor interface followed by transfer of ions through membrane-aqueous solution interface and subsequent interaction of ammonium (amine) ions and carbocations from pheromones with diffusing halide ions from the bulk. Relationship of the above mechanism with sensing mechanism of smell by olfactory nerves has also been pointed out.     

Monte Carlo simulations of Lennard-Jones nonionic surfactant adsorption at the liquid/vapor interface

We present Monte Carlo simulations of nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential using differing interaction parameters. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. Adjacent atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method. We generate quantitative thermodynamic adsorption and surface tension isotherms in addition to surfactant radius of gyration, tilt angles, and potentials of mean force. Surface tension simulations compared to those calculated from the simulated adsorbed amounts and the Gibbs adsorption isotherm agree confirming equilibrium in our simulations. We find that the classical Langmuir isotherm is obeyed for our LJ surfactants over the range of head and tail lengths studied. Although simulated surfactant chains in the bulk solution exhibit random orientations, surfactant chains at the interface orient roughly perpendicular and the tails elongate compared to bulk chains even in the submonolayer adsorption regime. At a critical surfactant concentration, designated as the critical aggregation concentration (CAC), we find aggregates in the solution away from the interface. At higher concentrations, simulated surface tensions remain practically constant. Using the simulated potential of mean force in the submonolayer regime and an estimate of the surfactant footprint at the CAC, we predict a priori the Langmuir adsorption constant, KL, and the maximum monolayer adsorption, Gammam. Adsorption is driven not by proclivity of the surfactant for the interface, but by the dislike of the surfactant tails for the solvent, that is by a "solvophobic" effect. Accordingly, we establish that a coarse-grained LJ surfactant system mimics well the expected equilibrium behavior of aqueous nonionic surfactants adsorbing at the air/water interface.     

A molecular-dynamics study of a model S(N)1 dissociation reaction at the water liquid/vapor interface

The thermodynamics and dynamics of a model S(N)1 reaction: t-BuCl --> t-Bu+ + Cl- is studied at the water liquid/vapor interface using molecular-dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation.     

Monte Carlo simulation of mixed lennard-jones nonionic surfactant adsorption at the liquid/vapor interface

New Monte Carlo simulations are presented for nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent specifically investigating the roles of tail attraction and binary mixtures of different tail lengths. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential. Adjacent atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method. We generate thermodynamic adsorption and surface-tension isotherms and compare results quantitatively to single-surfactant adsorption (Langmuir, 2007, 23, 1835). Surfactant tail groups with attractive interaction lead to cooperative adsorption at high surface coverage and higher maximum adsorption at the interface than those without. Moreover, adsorption and surface-tension isotherms with and without tail attraction are identical at low concentrations, deviating only near maximum coverage. Simulated binary mixtures of surfactants with differing lengths give intermediate behavior between that of the corresponding single-surfactant adsorption and surface-tension isotherms both with and without tail attraction. We successfully predict simulated mixture results with the thermodynamically consistent ideal adsorbed solution (IAS) theory for binary mixtures of unequal-sized surfactants using only the simulations from the single surfactants. Ultimately, we establish that a coarse-grained LJ surfactant system is useful for understanding actual surfactant systems when tail attraction is important and for unequal-sized mixtures of amphiphiles.     

Collective rotations of ferroelectric liquid crystals at the air/water interface

We study the Langmuir monolayers of four different ferroelectric liquid crystals on water surface. Two of them are attached to water surface by their polar groups, and the chiral groups, at the opposite ends of the elongated molecules, remain well above the interface. The other two ferroelectrics have both groups (polar and chiral) at close proximity, and therefore the chiral group is also attached to the surface or even submerged in water. We demonstrate that only when the chiral group of the ferroelectric liquid crystal in Langmuir monolayer is not attached to the interface and stays in the air does the system exhibit the collective rotations induced by evaporation of water (described for the first time by: Tabe, Y.; Yokoyama, H. Nat. Mater. 2003, 2, 806). The isotherms of surface pressure and surface potential versus molecular area of four compounds were measured with simultaneous observations using Brewster angle microscopy. Experimental data of the compression isotherms are described with a van der Waals model with very good accuracy, and the fitted parameters were used for calculations of compressibility coefficients for different phases found in the compounds under investigations. The ability of the two compounds for rotation and the disability of the two others is discussed in a context of thermodynamic properties of the monolayers.     

Discotic liquid crystals at the air water interface

The monolayer properties of two types of discotic liquid crystals, hexasubstituted triphenylenes 1 and azo derivatives of phloroglucinol 5, were examined. First investigations show that these discotic liquid crystals form stable monolayers. It could be shown that electron acceptors insert into the monolayer of 1. Azo discs display a packing behaviour at the monolayer that is ascribed to a side-on packing of the molecules.     

On the uptake of ammonia by the water/vapor interface

The passage of a single ammonia molecule from an infinitely dilute gas through the water/vapor interface is studied by constrained molecular dynamics simulations. The free energy of the system as a function of the distance between the ammonia and the interface has a minimum in the interfacial region. It is found that the preference of the ammonia for the interface is mainly due the disruption of the solvent structure caused by the ammonia in the bulk region, which results in an increase of the solvent internal energy.     

SO2:H2O surface complex found at the vapor/water interface

A weakly bonded SO2:H2O surface complex is found at the vapor/water interface prior to the reaction and dissolution of SO2 into the aqueous phase. The results have important implications for understanding the formation of atmospheric aerosols and understanding the atmospheric sulfur cycle.     

Ions at the air/water interface

In a recent review of this topic [B.C. Garett, Science 303 (2004) 1146] the emphasis was on some recent experiments, in which it was found that some anions accumulate at the air/water interface and not in the bulk, as usually happens to the cations, and on some simulations which explained those positive surface adsorption excesses. Because a large number of these experiments could be explained on the basis of some simple physical models proposed by the authors for the interaction between the ions and the air/water interface [M. Manciu, E. Ruckenstein, Adv. Colloid Interface Sci. 105 (2003) 63; Adv. Colloid Interface Sci. 112 (2004) 109; Langmuir 21 (2005) 11312], those models are reviewed in the present note, the goal being to draw attention to them.     

Investigation of the oxidation of hydroquinone at the liquid/liquid interface

<正>The oxidation of hydroquinone(QH_2) was investigated for the first time at liquid/liquid(L/L) interface by scanning electrochemical microscopy(SECM).In this study,electron transfer(ET) from QH_2 in aqueous to ferrocene(Fc) in nitrobenzene (NB) was probed.The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach curves to the theoretical values.The results showed that the rate constants for oxidation reaction of QH_2 were sensitive to the changes of the driving force,which increased as the driving force increased.In addition,factors that would affect ET of QH_2 were studied.Experimental results indicated ion situation around QH_2 molecule could change the magnitude of the rate constants because the capability of oxidation of QH_2 would be affected by them.     

SECM study of solute partitioning and electron transfer at the ionic liquid/water interface

Molecular partitioning and electron-transfer kinetics have been studied at the ionic liquid/water (IL/water) interface by scanning electrochemical microscopy (SECM). The ionic liquid C8mimC1C1N is immiscible with water and forms a nonpolarizable interface when in contact with it. Partitioning of ferrocene (Fc) across the IL/water interface was studied by SECM and found to be kinetically fast with a partition coefficient CIL/CW of 2400:1. The partition coefficient value was measured by SECM under quasi-steady-state conditions without waiting for complete solute equilibration. To investigate the kinetics of the electron transfer (ET) between aqueous ferricyanide and Fc dissolved in IL, a new approach to the analysis of the SECM current-distance curves was developed to separate the contributions of Fc partitioning and the ET reaction to the tip current. Several combinations of different aqueous and nonaqueous redox species were investigated; however, only the Fc/Fe(CN)63- system behaved according to the Butler-Volmer formalism over the entire accessible potential range.     

Electron spectroscopy and scanning tunneling microscopy study of quasi-two-dimensional freezing at the liquid/vapor interface of Ga-Bi alloys

We have studied the interfacial characteristics of Ga-rich liquid Ga-Bi alloys at various compositions and temperatures up to 300 degrees C by different electron spectroscopies under ultrahigh vacuum conditions. In particular, the thickness and thickness profiles of Bi-rich wetting and surface freezing films have been measured. It is found that at the surface freezing transition the Bi film thickness jumps from approximately 5 A to macroscopic values of over 100 A. This distinct behavior is discussed in comparison with the results in Ga-Pb. In addition, we report first scanning tunneling microscopy observations of the surface structure of a solidified Ga(0.989)Bi(0.011) alloy after passing a surface freezing transition. The topology at room temperature is characterized by extended, atomically flat Bi terraces covered by nanosized triangular two-dimensional Bi islands.     

Molecular theory on dielectric constant at interfaces: a molecular dynamics study of the water/vapor interface

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down. Some computational issues pertinent to Ewald calculations of the dielectric properties are also discussed.     

Topography of bent-core liquid crystals at the air/liquid crystal interface

Bent-core liquid crystals show a variety of novel structures involving the interplay of molecular bend, tilt and polarisation. Here we investigate the microstructures of the B4, B7 and the dark conglomerate (DC) phases at the air/liquid crystal interface. In these phases, bent-core molecules undergo complex self-assembly, forming helical nanofilaments (the B4 phase), layer undulations (the B7 phase) and disordered focal conics (the DC phase) in the bulk. However, due to the fluidity of the phases and the homeotropic alignment at the interface, several different topographies are observed at the air/liquid crystal interface. We will discuss the surface structures discovered so far in the B4, B7 and DC phases and show how they help us to understand the microstructure and the self-assembly of the liquid crystal phases of bent-core molecules.     

磷钼钒杂多阴离子的液/液界面电化学研究

用循环伏安法研究了V^vMo11-V5^vMo7杂多阴离子在水/硝基苯界面的转移行为, 观察了溶液酸度对转移行为的影响及杂多阴离子稳定存在的pH范围, 钒取代的杂多阴离子稳定性较12-磷钼杂多酸显著提高, 但随钒原子数增加离子稳定性降低, 在"电位窗"内转移的离子主要为-4价, 转移过程为扩散控制, 由循环伏安实验得到了杂多阴离子的表观转移电位△0^wψ^0和转移自由能△Gtr^0^w^-^0,对于不同钒数的杂多阴离子, 其半波电位E1/2与pH关系均可用E1/2=A-37pH(mV)描述。     

On the fluctuation theory for the liquid—vapor interface

Using the statistical mechanical formulation of Triezenberg and Zwanzig, we argue that the “intrinsic” or “bare” width of the liquid—vapor interface in the Buff—Lovett—Stillinger capillary model may be identified as the interfacial width described by the “van der Waals-like” theories.