Orientation and Motion of Water Molecules at Air/Water Interface

Here we report a quantitative study of the orientational structure and motion of water molecule at the air/water interface. Analysis of Sum Frequency Generation （SFG） vibrational peak of the free O-H stretching band at 3700 cm^-1 in four experimental configurations showed that orientational motion of water molecule at air/water interface is libratory within a limited angular range. The free OH bond of the interracial water molecule is tilted around 33°from the interface normal and the orientational distribution or motion width is less than 15°. This picture is significantly different from the previous conclusion that the interracial water molecule orientation varies over a broad range within the ultrafast vibrational relaxation time, the only direct experimental study concluded for ultrafast and broad orient, ational motion of a liquid interface by Wei et al. （Phys. Rev. Lett. 86, 4799, （2001）） using single SFG experimental configuration.

Simulation of Air-Water Interface Effects for High-speed Planing Hull

High-speed planing crafts have successfully evolved through developments in the last several decades.Classical approaches such as inviscid potential flow–based methods and the empirically based Savitsky method provide general understanding for practical design.However,sometimes such analyses suffer inaccuracies since the air–water interface effects,especially in the transition phase,are not fully accounted for.Hence,understanding the behaviour at the transition speed is of fundamental importance for the designer.The fluid forces in planing hulls are dominated by phenomena such as flow separation at various discontinuities viz.,knuckles,chines and transom,with resultant spray generation.In such cases,the application of potential theory at high speeds introduces limitations.This paper investigates the simulation of modelling of the pre-planing behaviour with a view to capturing the air–water interface effects,with validations through experiments to compare the drag,dynamic trim and wetted surface area.The paper also brings out the merits of gridding strategies to obtain reliable results especially with regard to spray generation due to the air–water interface effects.The verification and validation studies serve to authenticate the use of the multi-gridding strategies on the basis of comparisons with simulations using model tests.It emerges from the study that overset/chimera grids give better results compared with single unstructured hexahedral grids.Two overset methods are investigated to obtain reliable estimation of the dynamic trim and drag,and their ability to capture the spray resulting from the air–water interaction.The results demonstrate very close simulation of the actual flow kinematics at steady-speed conditions in terms of spray at the air–water interface,drag at the pre-planing and full planing range and dynamic trim angles.

Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

The constructive or destructive spectral interference between the molecular groups oriented up and down at the interface in the sum-frequency generation （SFG） spectra provides a direct measurement of the absolute orientation of these molecular groups. This simple approach can be employed to interrogate absolute molecular orientations other than using the complex absolute phase measurement in the SFG studies. We used the -CN group in the p-cyanophenol （PCP） molecule as the internal phase standard, and we measured the phases of the SFG fields of the -CN groups in the 3,5-dimethyl-4-hydroxy-benzonitrile （35DMHBN） and 2,6-dimethyl-4-hydroxy-benzonitrile （26DMHBN） at the air/water interface by measuring the SFG spectra of the aqueous surfaces of the mixtures of the PCP, 35DMHBN, and 26DMHBN solutions. The results showed that the 35DMHBN had its -CN group pointing into the aqueous phase; while the 26DMHBN, similar to the PCP, had its -CN group pointing away from the aqueous phase. The tilt angles of the -CN group for both the 35DMHBN and 26DMHBN molecules at the air/water interface were around 25°-45° from the interface normal. These results provided insights on the understanding of the detailed balance of the competing factors, such as solvation of the polar head groups, hydrogen bonding and hydrophobic effects, etc., on influencing the absolute molecular orientation at the air/water interface.

Dynamic Study of Gemini Surfactant and Single-chain Surfactant at Air/Water Interface

Molecular dynamics (MD) simulation are used to study the properties of gemini surfactant of ethyl-α,ω-bis(dodecyldimethylammonium bromide) (C12C2C12) and dodecyl- trimethylammonium bromide (DTAB) at the air/water interface, respectively. In the two systems, the surfactant concentrations are both 28 wt. %, and other conditions are also the same. After reaching the thermodynamic equilibrium, the concentration profiles, the radial distributions functions (RDF) and the mean squared displacement (MSD) are investigated. The results reveal that the surface activity of C12C2C12 surfactant is higher than DTAB surfactant.

PIV Studies on Turbulence Structure in Air/Water Interface with Wind-Induced Water Waves

When wind appears over the free surface, water waves and turbulence are generated by an interfacial shear stress. In particular, turbulent diffusion promotes significantly mass and momentum transport beneath the interface between the water and air significantly in ocean and lakes, and thus it is very important for global environment problems to reveal such turbulence property and coherent structure. Simultaneous measurements of velocities and free-surface elevation allow us to conduct reasonably the phase analysis of the coherent structure in interfacial shear layer. Furthermore, multi-point measurements such as PIV are very powerful to detect the space-time structure of coherent motions. Therefore, in the present study, we developed a specially designed PIV system which can measure the velocity components and surface-elevation fluctuation simultaneously by using two sets of high-speed cameras to reveal the coherent structure in the interfacial shear layer.

In situ Observation of the Photochromism in the Langmuir Monolayer of a Non-typical Amphiphilic Spiropyran Derivative at the Air/Water Interface

In situ photochromic process in the monolayer of a photochromic spiropyran derivative without long alkyl chain, was investigated. The photochromism at the air/water interface under different surface pressures was studied by surface pressure area isotherms, surface pressure time curves, area time curves and Brewster angle microscopy. Both forms of the compound were found to form monolayers at the air/water interface although it does not have long alkyl chain. A large area expansion in the monolayer corresponding to a zero th order reaction was found at the initial stage of the UV light irradiation. A series of dynamic investigations revealed that at high pressure after phase transition in the monolayer, the surface pressure changes greatly under alternative irradiation of UV and visible light. An obvious morphological change accompanying with the photochromism was observed in situ .

Effects of Calcium Ions on Thermodynamic Properties of Mixed Bilirubin/Cholesterol Monolayers

The mixed monolayer behavior of bilirubin/cholesterol was studied through surface pressure-area （π-A） isotherms on aqueous solutions containing various concentrations of calcium ions. Based on the data of π-A isotherms, the mean area per molecule, collapse pressure, surface compressibility modulus, excess molecular areas, free energy of mixing, and excess free energy of mixing of the monolayers on different subphases were calculated. The results show an expansion in the structure of the mixed monolayer with Ca^2＋ in subphase, and non-ideal mixing of the components at the air/water interface is observed with positive deviation from the additivity rule in the excess molecular areas. The miscibility between the components is weakened with the increase of concentration of Ca^2＋ in subphase. The facts indicate the presence of coordination between Ca^2＋ and the two components. The mixed monolayer, in which the molar ratio of bilirubin to cholesterol is 3：2, is more stable from a thermodynamic point of view on pure water. But the stable 3：2 stoichiometry complex is destroyed with the increase of the concentration of Ca^2＋ in subphase. Otherwise, the mixed monolayers have more thermodynamic stability at lower surface pressure on Ca^2＋ subphase.

Molecular Arrangement of a Bolaamphiphilic Anthrancene Derivative in Langmuir-Blodgett Film

This paper reports the molecular assembly in the LB film of a novel bolaamphiphilic anthracene derivative (BISANS) in comparison with that of one-headed amphiphilic anthracene derivative (ANS). While ANS formed a stretched monolayer at the air/water interface, BISANS underwent aggregation to form crystal film on the water surface. Both of the H-aggregates and J-aggregates exist in the transferred LB films of bolaamphiphilic BISANS, only J-aggregates formed in the LB film of ANS.

Membrane Insertion by Trichosanthin Using the Monolayer Method

A monolayer technique was used to investigate the interaction between the ribosome inactivating protein trichosanthin (TCS) and phospholipid membrane. The adsorption experiments show that the negatively charged 1,2 dipalmitoyl sn glycerol 3 phosphoglycerol (DPPG) causes obvious enrichment of TCS beneath the monolayer, indicating electrostatic attraction between TCS and the negatively charged phospholipid. When TCS was incorporated into the DPPG monolayer at low pH, it could not be completely squeezed out until the monolayer collapsed. The results suggest that the electrostatic attraction and the hydrophobic force are involved in the interaction between TCS and phospholipids at different stages. These findings may be correlated with the membrane translocation mechanism of TCS.