Full description of the orientational statistics of molecules near to interfaces. Water at the interface with CCl4Presented at the 81st International Bunsen Discussion Meeting on ?Interfacial Water in Chemistry and Biology?, Velen, Germany, September 19?23, 2003 (original) (raw)
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Classical density functional theory of orientational order at interfaces: Application to water
The Journal of Chemical Physics, 2004
A classical density functional formalism has been developed to predict the position-orientation number density of structured fluids. It is applied to the liquid-vapor interface of pure water, where it consists of a classical term, a gradient correction, and an anisotropic term that yields order through density gradients. The model is calibrated to predict that water molecules have their dipole moments almost parallel to a planar interface, while the molecular plane is parallel to it on the liquid side and perpendicular to it on the vapor side. For a planar interface, the surface tension obtained is twice its experimental value, while the surface potential is in qualitative agreement with that calculated by others. The model is also used to predict the orientation of water molecules near the surface of droplets, as well as the dependence of equilibrium vapor pressure around them on their size.
Journal of Molecular Liquids, 2013
Molecular dynamics simulations of the water-CCl 4 interface have been done in two different ways. In the first simulation the CCl 4 phase has been frozen in an equilibrium configuration, and only the water molecules have been allowed to move, whilst in the other one no such artificial freezing has been done. This way the effect of the fluid-like structure and fluid-like dynamics of the CCl 4 phase on the surface properties of the aqueous phase could be investigated separately. Due to the separate thermostatting of the two types of molecules in the simulations all the differences seen between the interfacial properties of water in the two systems can indeed be attributed to the rigid vs. fluid nature of the organic phase, and not to the thermal contact with a phase of zero temperature. The obtained results reveal that the rigidity of the opposite phase introduces an ordering both in the layering structure and orientation of the surface water molecules. The enhanced orientational ordering leads to a stronger lateral hydrogen bonding structure of the molecules within the subsequent molecular layers beneath the surface, and hence also to a slower exchange of the water molecules between the surface and the bulk aqueous phase.
Water Structuring at Non-Polar Fluid Interfaces
2017
The structuring of water molecules at the water/vapour interface is an object of scientific interest for decades. After the first successful attempts to explore liquid water with the help of theoretical chemistry, the number of studies on this topic grows progressively. Most of them are focused on bulk water but there is still need of a more detailed research on surface water. In addition, interfaces with alkanes are interesting as being instructive from both biological and industrial perspectives. Since in both bio- and industrial applications water/air and water/oil interfaces are mediated by amphiphiles, the role of a surfactant monolayer on surface water structuring deserves more attention as well. In the present study several atomistic water models were chosen—non-polarisable (SPC, TIP3P, and TIP4P) and polarisable (SW-RIGID-ISO, SWM4-NDP, and COS/G2) and classical molecular dynamics simulations were carried out on bulk water, water/vapour and water/alkane (from pentane to nona...
The Journal of Physical Chemistry C, 2009
Interfaces involving aqueous fluid phases play critical roles in natural and technologically important systems, and the atomic scale differences between interfaces involving hydrophobic and hydrophilic substrates are essential to understanding and manipulating their chemical and physical properties. This paper compares computational molecular dynamics results for the atomic density profiles, H-bonding configurations, and orientational ordering of water molecules at three different and illustrative interfaces. These are the free liquid water surface, which can be considered hydrophobic, and the interfaces of liquid water with talc (001) and muscovite (001) surfaces, which are prototypical hydrophobic and hydrophilic inorganic oxide surfaces, respectively. The results clearly demonstrate the importance of substrate structure and composition in controlling interfacial behavior and illustrate the differences between the vapor interface and those involving solids. The atomic density profiles of water at the solid interfaces show substantial layering, with the details related to the composition and crystal structure of the substrate. In contrast, there is no significant layering at the water-vapor interface. Relative to bulk water, the average density of water at the talc surface is reduced about 9-15% within 6-10 Å from the interface. This is equivalent to a depletion layer about 0.8 Å thick with respect to the similar but hydrophilic mica (001) surface. There is no well-defined vaporlike volume for the talc interface however, and the reduced number of water molecules is spread across the interfacial region. For the free liquid water surface, the results show an asymmetric H-bonding environment and charge density oscillations that provide an additional explanation for the previously observed separation of anions and cations at the surfaces of aqueous solutions. Thus, a delicate imbalance between the accepted and donated H-bonds of interfacial water molecules at this surface, and by inference other hydrophobic and hydrophilic surfaces, determines the preference of charged ions for the interface.
Orientation and motion of water molecules at air/water interface
CHINESE JOURNAL OF CHEMICAL PHYSICS, 2006
Analysis of SFG vibrational spectra of OH stretching bands in four experimental configurations shows that orientational motion of water molecule at air/water interface is libratory within a limited angular range. This picture is significantly different from the previous conclusion that the interfacial water molecule orientation varies over a broad range within the vibrational relaxation time, the only direct experimental evidence for ultrafast and broad orientational motion of a liquid interface by Wei et al. [Phys. Rev. Lett. 86, 4799, (2001)] using single SFG experimental configuration.
Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential
Physical Review Letters, 2018
The orientational distribution of free O-H (O-D) groups at the H 2 O (D 2 O)-air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ~63 o , substantially larger than previous estimates of 30-40⁰. This discrepancy can be traced to erroneously assumed Gaussian/stepwise orientational distributions of free O-H groups. Instead, MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface. Main Text At the interface of water with hydrophobic media, the hydrogen-bond (H-bond) network of water is interrupted, making the O-H groups of the topmost interfacial water molecules dangling (free) from the H-bond network. These free O-H groups are important for determining the energetics of the water surface and are thereby critical for explaining the exceptionally high surface tension of water. Furthermore, free O-H groups provide a unique platform for hydrophobic hydration assembly [1-3], on-water catalysis [4], and growth of aerosol particles [5,6]. As such, there have been many efforts to quantify the number and orientation of free O-H groups at different aqueous interfaces. The free O-H (O-D) groups of interfacial water can be studied experimentally by a sharp peak at ~3700 (~2740) cm-1 in the surface-specific vibrational sum-frequency generation (SFG) spectrum [7-12]. The frequency of the free O-H signal has been examined to determine the interaction strength of the topmost water layer and the hydrophobic medium [13-17]. Moreover, the free O-H SFG response measured with different polarization combinations provides information about their orientation. From the ssp-SFG (shorthand for s, s, and p polarized sum-frequency output, visible input, and infrared input, respectively) and ppp-SFG signals of the free O-H stretch, the averaged angle of the free O-H group at the water-air interface has previously been estimated to 30-40⁰ [17-19]. The orientational distribution of the free O-H group has been concluded to broaden with increasing temperature induced by disordering of the topmost water layer [20,21]. However, to connect the ppp-/ssp-SFG peak amplitudes of the free O-H stretch (A ppp and A ssp , respectively) with the averaged angle of the free O-H group, one needs to assume a functional form for the orientational distribution of the free O-H groups. So far, the distribution has been assumed to be stepwise shaped [18,20,21] or Gaussian shaped [17,19].
A molecular theory of liquid interfaces
Physical Chemistry Chemical Physics, 2005
We propose a site-site generalization of the Lovett-Mow-Buff-Wertheim integro-differential equation for the one-particle density distributions to polyatomic fluids. The method provides microscopic description of liquid interfaces of molecular fluids and solutions. It uses the inhomogeneous site-site direct correlation function of molecular fluid consistently constructed by nonlinear interpolation between the homogeneous ones. The site-site correlations of the coexisting bulk phases are obtained from the reference interaction site model (RISM) integral equation with our closure approximation. For illustration, we calculated the structure of the planar liquid-vapor as well as liquid-liquid interfaces of n-hexane and methanol at ambient conditions.
Structure of Water at Charged Interfaces: A Molecular Dynamics Study
Langmuir, 2014
The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na + vs Cs + ) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na + vs Cs + , the depth (∼1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.
Journal of Molecular Liquids, 2014
Molecular dynamics simulations of the water-CCl 4 interface have been done in two different ways. In the first simulation the CCl 4 phase has been frozen in an equilibrium configuration, and only the water molecules have been allowed to move, whilst in the other one no such artificial freezing has been done. This way the effect of the fluid-like structure and fluid-like dynamics of the CCl 4 phase on the surface properties of the aqueous phase could be investigated separately. Due to the separate thermostatting of the two types of molecules in the simulations all the differences seen between the interfacial properties of water in the two systems can indeed be attributed to the rigid vs. fluid nature of the organic phase, and not to the thermal contact with a phase of zero temperature. The obtained results reveal that the rigidity of the opposite phase introduces an ordering both in the layering structure and orientation of the surface water molecules. The enhanced orientational ordering leads to a stronger lateral hydrogen bonding structure of the molecules within the subsequent molecular layers beneath the surface, and hence also to a slower exchange of the water molecules between the surface and the bulk aqueous phase.
Molecular Orientation near Liquid-vapor Interface of Hydrogen-bonding Systems
1989
Dr. Shuichi Nose kindly advised me about molecular dynamics simulation technique, and Professor Keith E. Gubbins gave me important suggestion and information about their works on interfacial systems. The author heartily thanks them. The simulations and various analyses have been executed in the Kyoto University Data Processing Center and the Computer Center of the Institute for Molecular Science.