Liquid vapor interface, structure

A key concept within the studies of asymptotic decay is the Fisher-Widom (FW) line [199]. The latter is the locus of points in the (p T) plane (or in the (p, T) plane) signaling the crossover of the h (R) decay from monotonic to exponentially damped oscillatory. Although no thermodynamic singularities are related to the FW Une, and despite the fact that it may not exist for some models of fluids (e.g., classical hard spheres), the FW line is very useful in that it marks the change in the decay regime which may be characteristic of certain fluid properties (e.g., monotonic in the vicinity of the critical point) [201]. Furthermore, these two general decay behaviors are connected with interfacial properties, such as the liquid-vapor interface structure, or the wetting transitions at wall-fluid interfaces [188, 200, 201],... 

The simulations described above were performed at constant density, i.e., a volume was imposed on the system irrespective of the resulting pressure or chemical potential. MD simulations performed at constant chemical potential, where the confined liquid is in equilibrium with a vapor or bulk liquid phase, have also been performed. Simulations with free surfaces, i.e., with vapor/polymer interfaces, allow for the study of the equilibrium liquid-vapor interface structure and the calculation of the surface tension, a thermodynamic property fundamental to the understanding of the behavior of a material at interfaces. An MD study of the equilibrium liquid-vapor interface structure and surface tension of thin films of n-decane and n-eicosane (C20H42) has been performed in Ref. 26. The system studied consisted of a box with periodic boundary conditions in all directions. The liquid polymer, however, while fully occupying the x and y dimensions, occupied only a fraction of the system in the z direction, resulting in two liquid-vapor interfaces. The liquid phase ranged from about 4.0 to 7.0 nm in thickness. Simulations were performed at 400 K for both decane and eicosane, with additional decane simulations at 300 K. A similar system of tridecane molecules, using a well calibrated EA force field, has been studied at 400 K and 300 K in Ref 32. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Warren GL, Patel S (2008) Comparison of the solvation structure of polarizable and nonpolarizable ions in bulk water and near the aqueous liquid-vapor interface. J Phys Chem C 112(19) 7455-7467... 

Pohorille, A. Benjamin, I., Structure and energetics of model amphiphilic molecules at the water liquid-vapor interface. A molecular dynamic study, J. Phys. Chem. 1993, 97, 2664-2670... 

The chemical composition, physical structure, and key physical properties or a foam, namely its stability and rheology, are all closely interrelated. Since there is a large interfacial area of contact between liquid and vapor inside a foam, the physical chemistry of liquid—vapor interfaces and their modification by surface-active molecules plays a primary role underlying these interrelationships. 

Figure 2.3 Schematic molecular structure of a liquid-vapor interface.

As the pore liquid is evaporated from a gel network, the capillary pressure associated with the liquid-vapor interface within a pore can become very large for small pores. For example, the capillary pressure with water evaporating from a pore with a radius of 1 nm is on the order of 1.5 x 108 pa. With a distribution of pore sizes in a gel, as is commonly the case, the differential pressure across pores of different sizes leads to a collapse in the pore structure. Thus, strategies that are effective in maintaining the integrity of a gel network aim at minimizing either the differential pressure or the capillary pressure itself. 

The screening is being done in 98 w/o sulfuric acid. Materials samples are placed in the liquid and vapor phases of the test vessel, as well as at the liquid/vapor interface. Typical test results are shown in Figures 10, 11, and 12. As is apparent, commercially available metallic alloys do not appear to be able to survive the operating environment. Silicon containing materials, however, used as structures or protective coatings on metallic alloys, appear to have promise of fulfilling the process needs. 

The structure of the planar liquid-vapor interface [C4mim][PF6] was examined by MD simulations [129], Layering of the ions at the interface was observed as oscillations in the corresponding number density profiles. These oscillations were diminished in amplitude in the electron density profile, due to a near cancellation in the contributions from the anions and the cations. The butyl chains were observed to be preferentially oriented along the interface normal, thus imparting a hydrophobic character. In the densest region of the interface, the imidazolium ring plane was found to lie parallel to the surface normal [129],... 

Stuart s studies of the structure of the liquid-vapor interfaces of metals and alloys can also be related to his previous research. He developed the first theory of transport in dense simple fluids that explicitly recognizes, and accounts for, the different dynamics associated with short-range repulsion and longer-ranged attraction. He has contributed to the theory of the three-molecule distribution function in a liquid and the theory of melting, and he developed the Random Network Model of water and the first consistent... 

In a recent study, a new model of fluids was described by using the generalized van der Waals theory. Actually, van der Waals over 100 years ago suggested that the structure and thermodynamic properties of simple fluids could be interpreted in terms of neatly separate contributions from intermolecular repulsions and attractions. A simple cubic equation of state was described for the estimation of the surface tension. The fluid was characterized by the Lennard-Jones (12-6) potential. In a recent study the dependence of surface tension of liquids on the curvature of the liquid-vapor interface has been described. ... 

Orientational structure at a liquid vapor-interface of diatomic interaction site fluids has been studied extensively by Gubbins and Thompson using both thermodynamic perturbation theory and molecular dynamics simulation, and by Tarazona and Navascues using perturbation theory. Chacon et al. have applied density-functional theories to these systems. The theoretical methodology and results are reviewed in a comprehensive article by Gubbins, to which the reader is directed for more complete details. 

Release stiction has been recognized as a problem since the late 1980s, especially for surface micromachined structures. During drying, surface tension from the liquid-vapor interface causes a downward force on the structural layer. If the layer touches the substrate, it is prone to stick onto the surface. It is hypothesized that etch products and/or contaminants in the rinse water can then precipitate out of solution during drying and cause a bond that is stronger (e.g., a chemical bond)... 

Because of the limited resources on any computer system, simulations are usually restricted to a small, finite set of atoms or molecules. Consequently, a common approach is to use a cluster of atoms in an attempt to represent the entire macroscopic state. However, even for relatively large clusters a significant fraction of the molecules remain close to the surface. For instance, the structuring in liquid water at the liquid-vapor interface under ambient conditions extends at least 5 A, or roughly 2 water diameters into the liquid. So, for a spherical cluster of a thousand water molecules, having a radius of —20 A, only 40% of the water molecules are more than 5 A from the surface. 

M. A. Wilson, A. Pohorille, and L. R. Pratt,/. Chem. Phys., 90,5211 (1989). Comment on Study on the Liquid-Vapor Interface of Water. I. Simulation Results of Thermodynamic Properties and Orientational Structure. ... 

The liquid is then removed by heating the gel in an autoclave to above the critical point, then releasing the pressure. In this way a liquid-vapor interface never forms, so surface tension forces never have an opportunity to collapse the pore structure. This method of production, while effective, is sufficiently expensive to have greatly inhibited significant commercial development of silica aerogels. 

Partay L, Jedloszky P, Vincze A (2005) Structure of the liquid-vapor interface of water-methanol mixtures from Monte Carlo simulations. J Phys Chem B 109 20493-20503 Partay L, Jedloszky P, Wncze A, Horvai G (2008) Properties of free surface of water-methanol mixtures. An analysis of the truly interfacial molecular layer in computer simulation. J Phys Chem B 112 5428-5438... 

Abfaham, F.F. (1975) A theory for the thermodynamics and structure of nonuniform systems, with application to the liquid-vapor interface and spinodal decomposition,/ Chem. Phys. 63, 157. 

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